DOCTOR AL DISSERTATION IN ODONTOLOGY
EVAGGELIA PAPIA
MICROMECHANICAL
RETENTION AND
CHEMICAL BONDING
TO POLYCRYSTALLINE
DENTAL CER AMICS
Studies on aluminum oxide and stabilized zirconium dioxide
MICROMECHANICAL RETENTION AND CHEMICAL BONDING
T O P O LY C RYS TA L L I N E D E N TA L C E R A M I C S
Malmö University
Faculty of Odontology Doctoral Dissertations 2014
© Evaggelia Papia, 2014
Photographs and illustrations: Evaggelia Papia
ISBN 978-91-7104-539-3 (print)
ISBN 978-91-7104-540-9 (pdf)
Holmbergs i Malmö, 2014
EVAGGELIA PAPIA
MICROMECHANICAL
RETENTION AND
CHEMICAL BONDING
TO POLYCRYSTALLINE
DENTAL CER AMICS
Studies on aluminum oxide and stabilized zirconium
dioxide
Malmö University, 2014
Department of Materials Science and Technology,
Faculty of Odontology
Malmö, Sweden
This publication is also available in electronic format at:
se www.mah.se/muep
To my family
TABLE OF CONTENTS
LIST OF PUBLICATIONS ....................................................................... 9
THESIS AT A GLANCE ....................................................................... 11
ABSTRACT ......................................................................................... 12
POPULÄRVETENSKAPLIG SAMMANFATTNING ................................ 14
ABBREVIATIONS AND DEFINITIONS ................................................ 16
INTRODUCTION ............................................................................... 18
Treatment planning ...........................................................................................18
Choosing materials ...........................................................................................19
Dental ceramics .........................................................................................19
Aluminum oxide and stabilized zirconium dioxide ..................................22
Process technology ....................................................................................24
Clinical use and issues .....................................................................................25
What do we know today? – Final remark ......................................................27
HYPOTHESES..................................................................................... 28
AIMS ................................................................................................. 29
Specific aims .....................................................................................................29
MATERIALS AND METHODS ............................................................. 31
Laboratory Procedures......................................................................................31
Study I .......................................................................................................31
Study II .......................................................................................................37
Study IV .......................................................................................................40
Systematic review ..............................................................................................45
Study III .......................................................................................................45
RESULTS ............................................................................................ 48
In vitro studies ...................................................................................................48
Study I .........................................................................................................48
Study II ........................................................................................................50
Study IV .......................................................................................................54
Systematic review ..............................................................................................59
Study III .......................................................................................................59
DISCUSSION .................................................................................... 70
Methods: In vitro studies...................................................................................70
The choice of included materials ...............................................................71
The choice of processing and surface treatment ......................................74
The choice of tests.......................................................................................79
Methods: Systematic review .............................................................................82
Study design ...............................................................................................83
Results ................................................................................................................83
Surface treatments ......................................................................................84
Shear bond strength: How does surface treatment
affect bond strength? ..................................................................................85
Types of Failure ...........................................................................................86
Shear bond strength: How do adhesive cement systems
affect bond strength? ..................................................................................87
Biaxial flexural strength: How does surface treatment
affect flexural strength? ..............................................................................88
Surface roughness and chemical surface composition:
How does the surface treatment affect material composition
and properties? ...........................................................................................89
Clinical significance ...................................................................................90
Future investigations ..........................................................................................91
CONCLUSIONS ................................................................................ 93
ACKNOWLEDGEMENTS ................................................................... 95
REFERENCES ..................................................................................... 98
PAPERS I – IV ..................................................................................111
LIST OF PUBLICATIONS
This thesis is based on the following articles, referred to in the text
by their Roman numerals. All articles are reprinted with permission
from the copyright holders and appended to the end of the thesis.
I.
Papia E, Vult von Steyern P. Bond strength between different
bonding systems and densely sintered alumina with sandblasted
surfaces or as produced. Swed Dent J 2008; 32: 35–45.
Contribution of E. Papia: Planning the study, executing all tests,
and writing the article (all steps with support).
II. Papia E, Zethraeus J, Ransbäck P-Å, Wennerberg A, Vult von
Steyern P. Impaction-modified densely sintered yttria-stabilized
tetragonal zirconium dioxide (Y-TZP): Methodology, surface
structure and bond strength. J Biomed Mater Res Part B Appl
Biomater 2012; 100: 677-84.
Contribution of E. Papia: Planning the study; specimen preparation, bond strength testing and surface analysis (with support);
and writing the article.
III. Papia E, Larsson C, du Toit Madeleine, Vult von Steyern P.
Bonding between oxide based ceramics and adhesive cement
systems: A systematic review. J Biomed Mater Res Part B Appl
Biomater 2014; 102: 395-413.
Contribution of E. Papia: Planning the study; literature search
and data interpretation (with support); and writing the article.
9
IV. Papia E, Jimbo R, Chrcanovic BR, Andersson M, Vult von Steyern P. Surface structure and mechanical properties of impaction-modified Y-TZP. Submitted
Contribution of E. Papia: planning the study, specimen preparation, biaxial flexure strength testing; surface analysis (with support); and writing the article.
10
Thesis at a glance
Thesis
at a glance
Study
I
Bond strength between
different bonding systems
and densely sintered alumina
with sandblasted surfaces or
as produced.
Aim
Evaluate the shear bond
strength between various
adhesive cement systems and
densely sintered aluminum
oxide, as-‐produced or
sandblasted.
II
Impaction-‐modified densely
sintered yttria-‐stabilized
tetragonal zirconium dioxide
(Y-‐TZP): Methodology,
surface structure and bond
strength.
Methods
Shear bond strength test
Describe a method for producing
bondable Y-‐TZP structures using
impaction modification with two
different mediums: glass
granules and polymer granules.
Investigate and describe the
surface structures of surface-‐
modified Y-‐TZP and evaluate the
shear bond strength.
Make an inventory of existing
III
methods for achieving bondable
Bonding between oxide-‐
based ceramics and adhesive surface on oxide ceramics and
cement systems: A systematic evaluate which methods might
provide sufficient bond strength.
review.
Shear bond strength test
Surface analysis:
Interferometry (IFM)
IV
Surface structure and
mechanical properties of
impaction-‐modified Y-‐TZP.
Flexural strength test
Surface analysis:
IFM,
AFM,
SEM,
EDS,
XRD
11
Investigate and describe the
chemical surface composition of
surface-‐modified Y-‐TZP and
evaluate the flexural strength of
Y-‐TZP, with or without surface
modification, with various
pretreatments: etching before or
after sintering – alone and in
combination with an adhesive
cement system.
Illustration
Systematic literature review
Main findings
No general recommendation can
be made whether to use densely
sintered alumina, either as-‐
produced or surface treated with
airborne particle abrasion,
because bond strength depends
on the adhesive cement system
used.
Impaction modification with
either glass granules or polymer
granules can create a bondable
cementation surface suitable for
Y-‐TZP-‐based reconstructions,
resulting in rougher surface
structure compared to
unmodified surfaces.
There is no universal surface
treatment for clinically sufficient
bonding based on the oxide
ceramics tested by the studies
included in this review.
Considerations should be given to
the specific materials to be
cemented and the adhesive
cement systems to be used.
The surface structure and the
chemical composition of glass-‐
modified Y-‐TZP differ from
unmodified Y-‐TZP. The flexural
strength decreased with glass
modification, but increased after
cementation. The glass
modification creates a bondable
cementation.
ABSTRACT
Researchers are constantly developing new dental materials to
replace missing teeth. One material group receiving major focus
is ceramic materials; more specifically, oxide ceramics; and, in
particular, yttrium dioxide-stabilized tetragonal polycrystalline
zirconium dioxide (Y-TZP). In addition, one of the major challenges
is to ensure retention of oxide ceramic-based restoration in the
mouth, in a tissue preserving way.
Success in traditional cementation of dental restorations relies on
a geometric form that establishes the macromechanical retention,
the surface structure of the dental restoration, the tooth substance
(micromechanical retention) and the cement itself. In clinical
situations when macromechanical retention is insufficient, it may
be necessary to use an adhesive cementation technique. Reliable
adhesive bonding between the restoration, the cement, and the
tooth substance requires micromechanical retention and cement
that achieves chemical retention. In oxide ceramics, chemical
retention has been difficult to achieve and unpredictable. Various
techniques have been proposed for modifying the surface of oxide
ceramic-based restorations making adhesive cementation technique
a possible treatment option.
The overall aim of this thesis is to evaluate and develop techniques
for modifying the surface of oxide ceramics that enable durable
bonding between the restorations and adhesive cement systems.
Additionally, the thesis will inventory existing methods for achieving
a bondable surface on oxide ceramics and how these methods affect
the materials.
12
Study I evaluated bond strength between several adhesive cement
systems and densely sintered aluminum oxide. Two of six of the
cement systems studied showed acceptable bonding to densely
sintered aluminum oxide. The choice of surface treatment for the
oxide ceramic should be based on the cement system to be used.
Study II described a modified-additive technique for producing
bondable Y-TZP and evaluated the resulting surface structure and
bond strength. Surface-modified Y-TZP showed a rougher surface
structure and higher bond strength than unmodified Y-TZP. Study
IV extended these evaluations with additional surface analysis
and flexural strength testing. The results showed increased surface
roughness, with a chemical composition of glass and with a content of
monoclinic phase. Compared to unmodified Y-TZP, glass-modified
Y-TZP showed lower flexural strength values that increased with
the use of cement.
Study III was a systematic literature review to inventory existing
methods for achieving a bondable surface on oxide ceramics. This
study also evaluated which methods provide clinically relevant
bond strength and classified the various surface treatments into
seven main groups: as-produced, grinding/polishing, airborne
particle abrasion, surface coating, laser treatment, acid treatment,
and primer treatment. Abrasive surface treatment, as well as silicacoating treatment, combined with the use of a primer treatment can
result in sufficient bond strength for the bonding of oxide ceramics.
This conclusion, however, needs to be confirmed by clinical studies.
There is no universal surface treatment; the choice should be based
on the specific materials.
Together, the results of this thesis demonstrate that different
surface treatments/modifications of oxide ceramics increase the
bond strength between ceramics and adhesive cement systems.
Surface modification with a glass medium was particularly effective.
All surface treatment, however, affects the material properties and
the resulting dental restoration. Choice of surface treatment should
be made based on the restoration materials: the oxide ceramics and
the adhesive cement systems.
13
POPULÄRVETENSKAPLIG
SAMMANFATTNING
Det pågår en ständig utveckling av olika dentala material avsedda för
att ersätta förlorad tandsubstans. En materialgrupp som är särskilt
intressant är höghållfasta keramiska material, oxidkeramer så som
yttriumdioxidstabiliserad tetragonal polykristallin zirkoniumdioxid
(Y-TZP). Förutom utveckling av material, är en av de större
utmaningarna att få tandersättningar att sitta fast i munnen på ett
vävnadsbesparande sätt.
Vid traditionell cementeringsteknik, fästs tandstödda ersättningar
med vattenbaserade cement, vars vidhäftning dels är beroende
av att tänderna slipas i syfte att skapa en geometrisk form för att
åstadkomma makromekanisk retention och dels av ytstrukturen
på tand och tandersättning som skapas under processen och utgör
mikromekanisk retention. I kliniska situationer med otillräcklig
makromekanisk retention kan det vara nödvändigt att använda
adhesiv cementeringsteknik. En förutsättning för en tillförlitlig
adhesiv bindning mellan tandersättning, cement och befintlig
tand är mikromekanisk retention och resinbaserade cement som
möjliggör en kemisk bindning. Det senare har visat sig vara svårt
och oförutsägbart att uppnå för oxidkeramer. Olika tekniker för
modifiering av oxidkeramers cementeringsyta har föreslagits för att
möjliggöra adhesiv cementeringsteknik.
Övergripande mål med föreliggande avhandlingsarbete var att
utveckla och utvärdera metoder för att modifiera polykristallina
keramers yta och därigenom möjliggöra kombinerad mekanisk och
kemisk bindning mellan oxidkeramer och adhesiva cementsystem.
14
I delarbete I utvärderades bindningsstyrkan mellan olika adhesiva
cementsystem och en tätsintrad aluminiumoxidbaserad keram. Två
av sex undersökta cementsystem uppvisade acceptabel bindning till
aluminiumoxid. Valet av ytbehandling på oxidkeramen bör baseras
på vilket cementsystem som ska användas.
I delarbete II presenterades och utvärderades en ny framställningsteknik för ytmodifierad Y-TZP, lämpad för adhesiv
cementeringsteknik. Ytmodifieringen visade ökad mikrostruktur och
högre bindningsstyrka jämfört med obehandlad Y-TZP. Uppföljning
gjordes i delarbete IV med ytterligare ytanalyser och hållfasthetstest.
En kemisk sammansättning med glas och monoklin fas identifierades
med ökad ytråhet. Ytmodifieringen med glasmedium resulterade i
lägre hållfasthet som dock ökade i samband med cementering.
Delarbete III var en systematisk litteraturöversikt med syfte
att inventera olika metoder för ytbehandling/modifiering av
oxidkeramer och utvärdera vilka av dessa som ger kliniskt relevant
bindningsstyrka. Indelningen av de olika ytbehandlingarna var:
fabriksproducerad, slipad/polerad, sandblästrad, ytmodifierad med
olika typer av täckande lager, laser-, syra- och primerbehandlad.
Sandblästring eller kiseltäckning av cementeringsytan kombinerat
med primer utmärkte sig med högre värden på bindningsstyrkan,
något som dock ännu inte blivit bekräftat i kliniska studier. Det
finns ingen universell ytbehandling. Valet av ytbehandlingar bör
baseras på vilket material som ska användas.
Sammanfattningsvis visar resultaten i avhandlingen att olika
ytbehandlingar av oxidkeramer, i synnerlighet ytmodifiering
med glasmedium, kan öka bindningsstyrkan mellan keram och
adhesivt cementsystem. All ytbehandling påverkar dock materialets
egenskaper och slutligen tandersättningen. Valet av ytbehandling
bör göras utifrån specifika materialval, avseende både keram och
respektive cementsystem.
15
ABBREVIATIONS AND DEFINITIONS
AFM
Bis-GMA
CAD
CAM
CIP
EDS
FDP
HF
HIP
IFM
LTD
MDP
MPa
µSBS
µTBS
N
PFM
RBCB
RDP
RT
TBS
TC
TEC
SBS
SEM
XRD
16
Atomic force microscopy
bisphenol-A-diglycidyl-methacrylate
Computer-aided design
Computer-aided manufacturing
Cold isostatic pressing
Energy-dispersive X-ray spectroscopy
Fixed dental prosthesis
Hydrofluoric acid
Hot isostatic pressing
Interferometry
Low temperature degradation
10-methacryloyloxydecyldihydrogen-phosphate
Megapascal
Micro-shear bond strength
Micro-tensile bond strength
Newton
Porcelain fused to metal
Resin-bonded all-ceramic bridges
Removable dental prosthesis
Room temperature
Tensile bond strength
Thermocycling
Coefficient of thermal expansion
Shear bond strength
Scanning electron microscopy
X-ray diffraction
Y-TZP
Alumina
Yttria
Zirconia
Yttrium oxide-stabilized tetragonal zirconium dioxide
polycrystals
Aluminum oxide
Yttrium oxide
Zirconium dioxide
17
INTRODUCTION
Loss of teeth lowers self-esteem and impairs oral function. Treating
tooth loss is very important for those who are affected. Patients who
receive treatment experience increased self-esteem and improved oral
function (1) and quality of life (2). There are a variety of treatment
options to replace missing teeth, with fixed dental prostheses (FDPs)
or removable dental prostheses (RDPs). Many patients prefer FDPs,
either tooth- or implant-supported (3).
Treatment planning
The increased demand for aesthetic and biocompatible materials,
together with the development of high-strength ceramics and new
technological process, has made the use of all-ceramic materials
for FDP treatment a common choice for the patient and dentist (4,
5). But treatment decisions include more than choosing the dental
material for the restoration. It also involves patient preference and
the prevailing clinical conditions, which can affect the design of the
tooth preparation and restoration, and subsequently the choice of
cementation technique (6, 7). Tooth preparation depends on the
quality and quantity of the tooth substance remaining, the space
needed for the intended restorative material, and the expected load
under function in the oral cavity (6). Both the prepared tooth and
the restoration should have smooth and rounded contours to avoid
concentrations of stress (8). Replacement of missing tooth tissue
should restore function with minimal biological cost (i.e., tooth
preserving treatment) while establishing retention and resistance,
providing strength and internal and marginal fit between the
supporting tooth substance and the restoration. Tooth preparation
18
has the risk of injuring the pulp and surrounding gingival tissues.
Excessive tooth reduction may lead to endodontic complications
from increased temperature and dehydration. Inadequate removal of
tooth substance, however, may cause an over-contoured restoration,
which could affect aesthetics and lead to biological complications
because of difficulties maintaining oral hygiene (6).
Restorations are retained with either traditional non-adhesive
water-based cements (often referred to as conventional cements)
or resin-based adhesive polymerizing cements (9). The traditional
cementation technique is based on the macromechanical retention
gained from the geometry of the preparation, which provides
retention and marginal fit, and influences the durability of the
restoration (8,10). The most commonly used water-based cement
is zinc phosphate, which is considered the criterion standard due
to its successful long-time clinical use. Adhesive cementation
techniques promote preservation of dental tissue because they
rely on micromechanical and chemical retention, and not on
macromechinal retention. Well-established bonding between the
interfaces of the restoration and cement, and the cement and the
tooth, improves the retention and marginal seal in comparison to
cement that relies on macromechanical retention (7, 9, 11). It is
preferable for the preparation site to be completely in enamel to
achieve and maintain an optimal bond. Preparation, however, often
exposes considerable amounts of dentine. The dentine bond is more
complicated than the bond to enamel because of the characteristics
of dentine, which include its lower inorganic content, its tubular
structure, and variations in this structure. Therefore, bond strength
to enamel is higher and more predictable than to dentine (12).
Choosing materials
Dental ceramics
Ceramics are used in dentistry because they closely mimic the
optical properties of enamel and dentine, in addition to their
chemical and mechanical properties such as biocompatibility, high
elastic modulus, low thermal expansion coefficients, and good wear
resistance (13). All these properties arise from the strong covalent
and ionic interatomic bonds of ceramics. Unfortunately, some of
their mechanical properties are undesirable, such as brittleness. If
19
they are deformed more than 0.1-0.3 %, they will fracture (14,
15). Another disadvantage is that ceramic materials are sensitive
to pre-existing flaws and defects, both on the surfaces and within
the material. Flaws and defects can act as starting points for crack
formation and, under load, lead to crack propagation that affects
the strength of the material (16, 17).
According to standards set by the International Organization
for Standardization (ISO), ISO 6872:2008 Dentistry - Ceramic
materials (18), dental ceramics have two classifications: ceramic
products that are provided from powder (Type I), and all other
forms of ceramic products (Type II). Other classification methods
involve intended use, chemical composition, process technology (19)
or sensitivity to hydrofluoric acids (HF) (20). This thesis classifies
ceramics according to their chemical composition: porcelain, glass
ceramics, hybrid ceramics, and oxide ceramics (Table 1) and their
intended use.
Due to their high glass content, porcelain and glass ceramics
are esthetic materials with desirable optical properties, but limited
strength. These two dental ceramics can be used monolithic (fullcontour) restorations or as (surface) porcelain/ceramic veneer
material in metal- or oxide ceramic-based restorations. Monolithic
restorations made of porcelain are used only for laminate veneers
in the anterior region. Glass ceramics have a wider application. The
indications for glass ceramics differ depending whether they are
leucite-based or lithium disilicate-based. The latter shows higher
strength (21). Leucite-based glass ceramics can be used for laminate
veneers, crowns in the anterior region and as onlays and inlays in
any regions of the mouth. Glass ceramics based on lithium disilicate
have these same indications but can also be used as monolithic
restorations for FDPs of up to three units in the anterior and
premolar regions (4, 21). The material shows approximately half
the strength compared to yttrium-stabilized tetragonal zirconium
dioxide polycrystals (Y-TZP), but it has three times the strength
of the veneering porcelain that is used in combination with Y-TZP
(22, 23). Even if glass ceramics have greater strength and toughness
compared to porcelain, they both need to be etched and adhesively
cemented to reinforce the ceramic restoration to withstand loads
during function (24, 25).
20
Table 1. Overview of ceramic materials. Classification, mechanical properties
and examples of material systems.
Type of ceram and Classifications
Flexural
Fracture
Example of
strength
toughness
material systems
(MPa)*
(MPa m1/2)*
Feldspathic porcelain
Porcelain
50-120
~1.0
Duceram® Plus
Leucite-reinforced
Glass
120-180
~1.5
IPS Empress®
glass ceramic
ceramic
350- 400
~3.0
IPS e.max® Press
Lithium-disilicate
reinforced
IPS e.max® CAD
glass ceramic
Glass-infiltrated
Hybrid
aluminum oxide
ceramic
Glass-infiltrated
450-500
3.5-4.0
In-Ceram® Alumina
650-700
4.5-5.0
In-Ceram® Zirconia
420-650
3.0-4.0
Procera® Alumina
900-1200
6.0-8.0
Procera® Zirconia
zirconia-toughened
aluminum oxide
Densely sintered
Oxide
aluminum oxide
ceramic
Y-TZP
Denzir®
Y-TZP= Yttrium oxide-stabilized tetragonal zirconium dioxide polycrystals
*from: Vult von Steyern P. Dental ceramics in clinical practice. In: Nilner K, Karlsson S, Dahl B L
(ed). A textbook of fixed prosthodontics: the Scandinavian approach. Stockholm: Gothia 2013: 205222 (13). Rekow E.D et al. Performance of dental ceramics: challenges for improvements J. Dent.
Res.2011;90: 937-952 (26). Miayazaki T. et al. Current status of zirconia restoration. J. Prosthodont.
Res. 2013;57:236-261 (5).
The hybrid ceramics consists of glass-infiltrated aluminum oxide
(Al2O3, alumina) or glass-infiltrated zirconium dioxide- (ZrO2,
zirconia) toughened alumina (4). Hybrid ceramics have mainly
been used for single crowns and FDPs with up to three units as
substructures, veneered with porcelain (8, 27). Use of this ceramic
group has decreased due to their technique sensitive and timeconsuming fabrication process and the increased use of lithium
disilicate glass ceramics and oxide ceramics (4).
Oxide ceramics, often defined as polycrystalline, high-strength
oxide ceramics, lack a glass phase and are acid resistant. Densely
sintered aluminum oxide (high purity alumina) and Y-TZP with no
glass phase are the two main oxide ceramics in dentistry (24, 28, 29).
21
Due to its mechanical properties, Y-TZP has a wide range of uses;
crowns, FPDs, posts, and implant abutments (11, 30-32). Y-TZP is
used for substructures, with veneer material consisting of porcelain
or glass ceramics. In many cases it has replaced dental appliances in
aluminum oxide (4). Aluminum oxide is still used, but mainly as a
substructure for single crowns. Since the substructure, i.e. the oxide
ceramic, is what determines the strength, the restorations do not
need to be adhesively cemented if mechanical retention is provided
(24, 32-34). Research is constantly developing stabilized zirconia
materials to improve its clinical use and outcomes, and to meet
future demands. Recent developments include translucent Y-TZP,
allowing monolithic crowns to be made that maintain the favorable
mechanical properties of Y-TZP (35).
Aluminum oxide and stabilized zirconium dioxide
Densely sintered aluminum oxide consists of high purity (99.9%)
aluminum oxide. The aluminum oxide powder is pressed and
sintered (4, 36). During the sintering the aluminum oxide particles
grow together to a grain-like structure, having a mean grain size of
4 µm. The sintering shrinkage of densely sintered aluminum oxide is
approximately 20%, which is controlled during the manufacture of
an individual substructure (36-38).
Unlike aluminum oxide, zirconium dioxide is a polymorphic
material that shows three different crystal structures, depending on
the temperature. From room temperature (RT) up to approximately
1170°C the crystals have a monoclinic structure. Above 1170°C,
phase transformation occurs and the monoclinic crystals become
tetragonal. At about 2370°C, the crystals have a cubic structure. And
if the temperature is further raised, the material will melt at around
2680 °C. During cooling, reversed transformation occurs back to
tetragonal and then monoclinic structure, increasing the crystal
size (volume) with approximately 3% to 5%. This increase causes
undesirable concentrations of stress and, sometimes, spontaneous
crack formation within the material at RT. Hence, pure zirconium
dioxide is unsuitable for dental use. By adding a stabilizing oxide,
a dopant, the tetragonal phase can be stabilized at RT. Examples of
such dopants are yttrium (Y2O3), magnesium (MgO), cerium (Ce2O3)
and calcium (CaO) oxide. In dental contexts, zirconium dioxide is
22
most frequently stabilized with 3 mol% yttrium oxide, (3Y-TZP).
The Y-TZP will be metastable, i.e. the tetragonal zirconium dioxide
is stabilized at RT but can transfer to monoclinic structure under
local stress (4, 29, 31, 39, 40).
Flaws and cracks play an important role in the fracture mechanism
of dental ceramics. Fractures often originate from such defects,
which act as starting points for slow crack growth (41). When a
crack starts to propagate in the Y-TZP, a local stress-initiated phase
transformation will occur at the crack tip. This transformation from
tetragonal to monoclinic phase causes a local increase in volume of
the crystals (approximately 3%) close to the tip of the crack, resulting
in a localized compressive stress that will prevent or delay further
crack propagation. This mechanism is known as transformation
toughening, and it is the underlying reason for the high fracture
toughness of Y-TZP. However, this metastability means that Y-TZP
is likely to age in the presence of water, hot vapor, steam sterilization,
body fluids like saliva, or mechanical treatments such as grinding or
airborne particle abrasion (39, 42-44). Water can “catalyze” stress
corrosion, breaking the bond between the atoms at the crack tip,
leading to slow crack growth where the transformation of tetragonal
to monoclinic phase takes place. The process starts at the surface
grain boundaries and the transformation continues layer by layer
through the whole body, resulting in microcracks, grain pullout,
and decreased strength. This degradation or aging of Y-TZP is also
known as low temperature degradation (LTD), which is defined as
the spontaneous transformation from tetragonal to monoclinic at
low temperature over time. Y-TZP is prone to LTD in the presence
of water (43, 44).
Y-TZP is less translucent than other dental ceramic materials,
so development has focused on increasing its translucency (5, 35).
The translucency of stabilized zirconium dioxide is related to the
amount and type of dopants; the amount and size of crystals and
pores; and the sintering process, including the sintering temperature,
heat rating, and atmosphere during the sintering process (45, 46).
Few clinical studies of translucent Y-TZP are available, however,
and further investigations are needed to evaluate the performance
(5, 35).
23
Process technology
The properties of oxide ceramics largely depend on its processing,
including the ceramic powder used, the fabrication technique, and
final treatment of the restoration. The powder should be as pure
as possible and the particle size sufficiently small to allow the final
packing and sintering that creates optimal mechanical properties
(39, 40, 47, 48).
Oxide ceramics for dental appliances are commonly produced
by pressing ceramic granules into a body. These are subsequently
sintered and milled by computer-aided design/computer-aided
manufacturing (CAD/CAM). CAD/CAM techniques involve
producing a digital model of the dental arches, the prepared tooth/
teeth, and designing a restoration using the CAD-software (31, 49).
There are three compaction techniques used to increase the
density of the ceramic granules: uniaxial pressing, cold isostatic
pressing (CIP), and hot isostatic pressing (HIP). In the uniaxial press
technique, the pressure is applied in one direction, yielding a green
body with low material pack, especially in the outer contours due
to friction against the walls of the mold. The material will have a
somewhat lower density and may have inherent stresses (50, 51).
Isostatic press techniques make the material more homogenously
packed because pressure is applied in all directions. This creates
higher density, fewer pores and voids, and isotropic properties (50).
There are two methods for CIP, wet bag and dry bag. In wet bag
process, the powder is enclosed in a waterproof mold with flexible
walls and then placed in a container filled with liquid and put under
pressure. Dry bag, however, is performed with very little liquid.
The mold is connected to channels filled with pressurized liquid
that compress the material. HIP includes isostatic pressing with presintering, followed by a temperature increase to between 1400°C
and 1500°C under high pressure in an inert gas atmosphere. This
creates a very dense material (99% of the theoretical density) (50).
Y-TZP produced by HIP has been reported to be less sensitive to
LTD (32). However, the milling procedure will affect the surface
and the crystalline structure, causing some degree of tetragonal to
monoclinic phase transformation in the Y-TZP (31).
With stabilized zirconium dioxide, milling is performed either by
soft machining the oxide ceramic in its green stage (pressed only) or
24
white stage (pressed, partially sintered), or by hard machining the fully
sintered stage (pressed and completely sintered). When a restoration
is milled from a blank, the production technique is subtractive.
When the restoration is built up by adding ceramic granules to
achieve the desired design, the production technique is additive (31,
49). The two techniques can be combined by pressing the ceramic
granules on an enlarged die and then subsequently milling the outer
contours to the desired shape (36, 37). The machining process of
subtractive production leaves a certain structural roughness in the
surface, depending on the milling tools used (11, 31, 52). With
additive production, however, the ceramic granules are pressed to
green stage on the surface of a prefabricated die. Subsequently, the
machining process shapes the outer contours whereby the surface
structure of the cementation surface mirrors the surface structure of
the die, which is often quite smooth (36, 37).
Clinical use and issues
The mechanical properties of oxide ceramics allow for cementation
of crowns and FDPs with conventional cements, such as zinc
phosphate cement (24, 32-34). However, in some clinical cases, such
as those with compromised retention, it might be beneficial to apply
adhesive cementation and establish a durable bond between the
restoration and the tooth substance (34). Some restorations, such as
resin-bonded bridges which are commonly made of porcelain fused
to metal (PFM), rely on adhesive cementation. Disadvantages of
metal-framed, resin-bonded bridges include decreased translucency
and a greyish appearance of the abutment teeth. With resin-bonded
all-ceramic bridges (RBCBs) the same minimally invasive approach
can be applied. RBCBs, on the other hand, do not have the same
strength as the metal-framed and may have higher risk of fracture.
Oxide ceramics, with well-dimensioned design and beneficial
mechanical properties compared to other ceramics, might not
encounter this issue. Longitudinal clinical studies of oxide-based
RBCBs are lacking (53, 54).
Compared to silica-based ceramics, which can be bonded after
HF etching and silanization (24), densely sintered oxide ceramics
have surface structures without glass phase. They require alternative
techniques for adhesive cementation (33, 55). The reliability
25
of the long-term bond to oxide ceramics is determined by the
micromechanical and chemical retention between the adhesive
cement system and the surface of the ceramic restorations (24, 5557). Macromechanical retention, however, is important to avoid
unacceptably high stress levels that may affect the interfacial bond
between restoration/cement/tooth, potentially reducing the clinical
performance of the restoration (9).
Micromechanical retention is determined by the structure of the
restoration’s cementation surface. The cementation surface will differ
in surface roughness, depending on the manufacturing technique,
thereby influencing bond strength (20, 48, 58-61). With a rougher
surface, the size of the surface area will increase and, in turn, affect
wettability. This allows the cement to flow into the microretentions,
thus creating a stronger micromechanical interlock (61, 62).
The literature describes several methods of surface treatment
and modification (33, 55). These can be divided into: (i) abrasive
techniques, such as grinding/polishing and airborne particle
abrasion also known as sandblasting (20, 63-65) and (ii) additive
surface treatments, such as tribochemical silicon dioxide (silica)
coating (64,66-68), silanization (64), plasma spraying, (69) fusion
sputtering, (70) laser irradiation, (71-73) selective infiltration
etching, (74, 75) ceramic powder (67-69, 76, 77) or nanostructured
alumina coating, (78) and primers (55, 66, 79-81). Some of these
techniques, however, are not yet commercially available (33). There
are some drawbacks to the various surface treatments. Different
surface modification techniques e.g. abrasive techniques can result
in structural damage, such as grain pullout and material loss,
creating flaws that can decrease strength (57, 76, 82-85). Other
coating techniques do not induce any surface flaws, but might
instead affect the fit of a restoration as the coating adds material
to the surface (69, 86).
To promote chemical retention, different cement systems have been
proposed for adhesive cementation of oxide ceramics in an attempt
to attain durable bonding to the ceramic restoration (5, 55). Hence,
silica/silane bonds are not the only way to achieve a stable bond.
Several primers with reactive monomers have been evaluated in
vitro in order to investigate their bond to oxide ceramics (11, 52, 8791). The monomer 10-methacryloyloxy-decyl dihydrogenphosphate
26
(MDP) was originally designed to bond to metal oxides, and its use
has been extended to oxide ceramics (92). MDP-containing resin
cements seem to be the most appropriate for oxide ceramics due
to the chemical interaction between the hydroxyl groups of the
passive zirconium dioxide surface and the phosphate ester group of
the MDP. Some suggest that a chemical bond might be established
between MDP and oxide ceramics (20, 34, 56, 57, 64). Regular
bisphenol-A-diglycidyl-methacrylate (bis-GMA) resin cements,
which do not contain MDP, might improve the bond strength to
aluminum oxide and stabilized zirconium dioxide if combined with
an MDP-containing primer (55, 87). Chemical retention alone is
difficult and unpredictable to obtain for oxide ceramics (52, 74,
78, 91, 93). Several studies have concluded that surface treatment
that creates micromechanical retention is beneficial to chemical
retention, thereby allowing a durable bond between oxide ceramics
and adhesive cement systems (34, 57, 91, 94, 95). Nevertheless,
the literature indicates that establishing a strong and reliable
bond between oxide ceramics, particularly Y-TZP, and a bonding
component is both difficult and unpredictable (9, 24, 55).
What do we know today? – Final remark
Achieving long-term bond strength of oxide ceramics to adhesive
cement systems could result in wider applications for oxide-based
ceramic restorations, especially those that rely heavily on bonding.
By using adhesive cementation, it would also be possible to decrease
the need for excessive tooth preparation, thus preserving tooth
substance. However, there is no conclusive evidence or consensus
regarding the most suitable materials and techniques for creating a
durable bond to oxide ceramics. Hence, further studies are needed
to investigate different surface treatments for oxide ceramics and
their effect on bond strength and material properties.
27
HYPOTHESES
•
There is no difference between the shear bond strength
achieved on densely sintered aluminum oxide with different
adhesive cement systems or with different surfaces treatments.
•
A modified-additive impaction technique can achieve a
bondable surface on Y-TZP.
•
The surface structure of surface-modified Y-TZP will differ
from an unmodified surface of Y-TZP. Surface roughness will
increase when granules are added to the surface.
•
The chemical surface composition of surface-modified Y-TZP
will differ from an unmodified surface of Y -TZP. Glass phase
remnants and various phases will be present after sintering
on the surface of the Y-TZP produced with modified-additive
production.
•
There is no difference for either unmodified or surfacemodified Y-TZP between the shear bond strength achieved
with different adhesive cement systems.
•
There is no difference in flexural strength, regardless of surface
modification or pretreatments, either with or without an
adhesive cement system.
28
AIMS
The overall aim of this thesis was to evaluate and develop techniques
for modifying the surface of oxide ceramics that enable durable
bonding between the restorations and adhesive cement systems.
Additionally, the thesis inventories existing methods for achieving a
bondable surface on oxide ceramics and how these methods affect
the materials.
Specific aims
•
Evaluate whether sufficient and durable shear bond strength
(>20 MPa) can be established between various adhesive cement
systems and densely sintered aluminum oxide (as-produced or
sandblasted).
•
Describe a method for producing a bondable Y-TZP surface
using impaction modification with two different mediums:
glass granules and polymer granules.
•
Investigate and describe the surface structures of surfacemodified Y-TZP produced using modified-additive technique
and compare them to an unmodified Y-TZP surface.
•
Investigate and describe the chemical surface composition of
surface-modified Y-TZP produced using modified-additive
technique and compare them to an unmodified Y-TZP surface.
•
Evaluate the shear bond strength (>20 MPa) between various
adhesive cement systems and both unmodified and surfacemodified Y-TZP.
29
•
Evaluate the flexural strength of Y-TZP, with surface
modification and without, with various pretreatments (etching
either before or after sintering, both with an adhesive cement
system and without).
•
Make an inventory of existing methods for achieving a
bondable surface on oxide ceramics and evaluate which
methods might provide sufficient bond strength.
30
MATERIALS AND METHODS
This thesis comprises three in vitro studies and a systematic review.
Table 2 summarizes the various materials and methods that are
explained in this section. For further details, see the “Materials and
methods” sections of each individual study.
Laboratory Procedures
Studies I, II and IV
These studies evaluated the bond and flexural strength of the oxide
ceramics aluminum oxide and stabilized zirconium dioxide with
surface treatment of the cementation surface and various adhesive
cement systems.
Study I
Specimen preparations
Study I used 120 pairs of industrially manufactured specimens – one
block and one cylinder (Ø 6.0 mm, thickness of 2.00 mm) of densely
sintered aluminum oxide (Procera® Alumina, Nobel Biocare AB,
Gothenburg, Sweden). The specimens were divided into 12 groups
(n=10) depending on the adhesive cement system used (six different
systems were tested) and on the artificial aging procedure (water
storage only or storage including thermocycling [TC]).
Preparation of the cementation surface of the blocks involved
cleaning the blocks with acetone (Apoteksbolaget AB, Gothenburg,
Sweden) and then performing airborne particle abrasion with
110 µm aluminum oxide particles (Aloxcobra, Renfert GmbH,
Hilizingen, Germany) for 10 seconds at an air pressure of 5 bars
and a distance of 100 mm between the surface of the sample and the
31
32
Table 2. Summary of materials and methods.
Paper
Type
Specimens/
Studies
Oxide ceramic
Adhesive cement
system
Artificial aging
I
In vitro
120 blocks + cylinders
II
In vitro
48 blocks + cylinders
Procera® Alumina
Unmodified Y-TZP,
Glass- or polymer-modified Y-TZP
Variolink®II
Panavia™F 2.0
Variolink® II
BISCO CHOICE™
BISCO ILLUSION™
NEXUS 2™
3M™ ESPE™ RelyX™ Veneer Cement
Panavia™ F
Water storage (1 week in 37°C water)
TC (5 000 cycles 5°C to 55°C)
or including TC (5 000 cycles 5°C to
55°C)
Test method
Shear bond strength test
Shear bond strength test
Surface analysis
Light microscopy
Light microscopy, IFM
Statistic method
Student’s t-test, one-way ANOVA,
Tukey’s test and Fisher´s exact test
One-way and two-way ANOVA, Tukey’s
test, and Fisher´s exact test
III
Literature review
127 studies for Q1
23 studies for Q2
12 brands of Y-TZP
2 brands of Alumina
*
IV
In vitro
248 discs
Storage of test specimens, or
other methods with mechanical
cyclic loading (>500 cycles) in
combination with water/liquid
All test methods for bond
strength without the influence of
macromechanical retention
*
Preloading (10,000
cycles, 10 N to 100 N)
and TC (5 000 cycles
5°C to 55°C)
Biaxial flexural strength
test
*
Unmodified Y-TZP,
Glass-modified Y-TZP
Panavia™F 2.0
Light microscopy, IFM,
AFM, SEM, EDS, XRD
One-way ANOVA,
Tukey’s test
*not evaluated, Q= addressed question in the literature search, TC= Thermocycling, IFM= Interferometry, AFM= Atomic force microscopy, SEM= Scanning electron microscopy,
EDS= Energy-dispersive X-ray spectroscopy, XRD= X-ray diffraction.
blasting nozzle. The nozzle of the blaster was moved gently in circles
at a 70° angle to the surface. After sandblasting, the samples were
ultrasonically cleaned in 96% isopropanol (Apoteksbolaget AB,
Gothenburg, Sweden) for 5 minutes. The cylinder-shaped samples
were not sandblasted, but stored as-received from the manufacturer
and subsequently cleaned, first with acetone and then ultrasonically
in 96% isopropanol for 5 minutes.
Study I tested six different adhesive cement systems. Table 3 lists
the various systems and their characteristics. Prior to cementation
all cementation surfaces were treated, irrespective of group, with
the primer recommended for each adhesive cement system by the
cement’s manufacturer. The untreated cylinders were cemented
to the prepared blocks using an alignment that applied a seating
load at 15 newtons (N) during polymerization. The apparatus
standardized the seating load and ensured that the axes of the
cylinder were perpendicular to the surface of the block. Disposable
brushes (Top Dent, DAB Dental AB, Sweden) were then used to
remove excess resin from the margin. If it was part of the adhesive
cement system, an oxygen-blocking gel was applied according
to the manufacturer’s instructions (Table 3). The cement were
light-cured with dental curing lamp (Optilux 400, Model VCL
401, Demetron Research Corporation, Danburry, CT, USA) with
a mean light intensity of 300 mW/cm2.. The curing time was 40
seconds from four directions, 90° apart, and then 60 seconds in
one direction with the seating load removed. All excess resin was
removed with a surgical blade (AESCULAP® no. 12, Aesculap AG
& Co, Tuttlingen, Germany) after polymerization. In a final step,
the specimens were rinsed with water for 1 minute to remove any
residues of the oxygen-blocking gel.
Each bonding group of 20 samples was randomly divided into
two subgroups (n=10) for further treatment: water storage only or
storage including TC. Both subgroups were stored in 37°C distilled
water for 1 week. During this week, the subgroups that included TC
received further artificial aging.
33
34
Table 3. Adhesive cement systems used in Study I.
System and components
Abbreviation
Type
Manufacturer
VARIOLINK II
VA
Bis-GMA based resin*
Ivoclar Vivadent AG/ FL-9494 Schaan/
Lichtenstein
®
Batch No./LOT
Monobond-S, primer agent
Dual-cured
D51336
Heliobond, porcelain primer
Two-paste system
D68417
Varolink® II, Base + Catalyst, (T)
D20542 + D17352
Liquid Strip
BISCO CHOICETM
D50843
BC
Bis-GMA based resin
BISCO PORCELAIN PRIMER
Dual-cured
ONE-STEP Univ. dental adhesive
Two-paste system
®
Bisco Inc/ Schaumburg, IL 60193/ USA
0200007636
0100012867
CHOICETM Porcelain Adhesive
0100014106 +
0100014108
Paste Base (translucent) + Catalyst
BISCO ILLUSIONTM
BI
Bis-GMA based resin
BISCO PORCELAIN PRIMER
Dual-cured
ONE-STEP Univ. dental adhesive
Two-paste system
®
ILLUSIONTM Porcelain Adhesive
Paste Clear Base
+ Clear Catalyst
Bisco Inc/ Schaumburg, IL 60193/ USA
0200007636
0100012867
0100014248 +
0100013914
(Table 3 continues on next page)
(Table 3 cont.)
NEXUS 2TM
NE
Bis-GMA based resin
Kerr Corporation/ Orange, CA 92867/ USA
Silane Primer
Dual-cured
010381
NEXUS 2TM Base Clear + Catalyst
Two-paste system
010435 + 010440
3MTM ESPETM RelyXTM Veneer Cement
RV
TEGDMA**/ Bis-GMA
based resin
3M ESPE/ St. Paul, MN 55144/ USA
3MTM RelyXTM Ceramic Primer
Light-cured
20020716
3M Scotchbond 1, adhesive
One-paste system
20020620
TM
TM
3MTM ESPETM RelyXTM Veneer Cement (TR)
PANAVIA F
TM
20011019
PF
Phosphatemonomer
containing
KURARAY MEDICAL INC/ Okayama 7108622/ Japan
CLEARFILTM PORCELAIN SE BOND
ACTIVATOR
resin (MDP)***
0563B
CLEARFILTM SE BOND PRIMER
Dual-cured
00120B
PANAVIATM F A Paste + B Paste (TC)
Two-paste system
00050A + 00029A
PANAVIA F OXYGUARD II
TM
00433A
*Bis-GMA= Bisphenol-A-diglycidylmethacrylate, **TEGDMA =Triethilene glycol-dimetrhacrylate, ***MDP =10-methacryloloyloxydecyl-dihydrogenphosphate
35
Artificial ageing – TC
All specimens underwent TC at 5 000 cycles in a specially constructed
thermocycling device with two water baths: one at 5°C and the other
at 55°C. Each cycle lasted 60 seconds (20 seconds in each bath and 10
seconds for transfer between the baths).
Bond strength test
Following pretreatment and artificial aging, a universal testing
machine (Instron model 4465, Instron®, Canton, MA, USA)
measured the shear bond strength of the samples using a knifeedged blade parallel to the bonded surfaces, in accordance with
ISO/TS 11405:2003 Dental materials –Testing of adhesion to tooth
structure (96) and previous studies (63, 97). The blocks were placed
in a brass holder fixed to the testing device to maintain their position
during testing. The crosshead speed was 0.5 mm/minute. The load
at fracture was recorded in newton (N). The shear bond strength in
megapascals (MPa) was calculated by dividing the recorded load by
the area of the cementation surface, measured individually in mm2,
and determined the mean and standard deviation for each group.
Failure type
The fracture surfaces were examined in a light microscop (Wild M3,
Wild Heerbrugg, Heerbrugg, Switzerland at ×6.4 magnification) to
classify the type of failure of the debonded area: adhesive, cohesive,
or a combination of the two.
Surface analysis and characterization
Microscopy. During all steps in the fabrication process of the
specimens, two types of light microscope were used to analyze
the cementation surfaces and fracture surfaces (Wild M3, Wild
Heerbrugg, Heerbrugg, Switzerland at ×5 to x10 magnification and
Leica DM 2500M, Leica Microsystems CMS, Wetzlar, Germany at
×500 magnification).
Statistics
Student’s t-test and one-way ANOVA, Tukey’s test determined
differences in bond strength within and between the groups (IBM
SPSS Statistics 15.0, SPSS Inc., Chicago, IL, USA). The level of
significance was set to α = 0.05.
36
Study II
Specimen preparations
In this study, 48 pairs of specimens were fabricated: one Y-TZP
cylinder and one block made of feldspathic porcelain that was
adhesively cemented together. The specimens were divided into six
groups (n=8/group) depending on the cementation surface of the
Y-TZP cylinder and adhesive cement system (three different surfaces
and two different systems).
A special dry-press punching tool made of stainless steel was made
to fabricate the Y-TZP cylinders. To fabricate the specimens, the tool
was filled with 0.33 ±0.01 g of Y-TZP granulated powder (grade
TZ-3YSB-C, batch S306269B, Procera Zirconia, Nobel Biocare™
AB, Gothenburg, Sweden) mechanically compressed uniaxially with
125 MPa using a cuvette press. Prior to compaction, the bottom
punch surfaces of the pressing tool (which define the cementation
surface of the Y-TZP) was modified by applying a thin layer of one
of two different mediums onto the surface (98). The three groups
varied depending on their surface modifications:
•
Surface G: Glass granules (Experimental Impaction Medium,
Cerasci AB, Malmö, Sweden) with a particle size of 40 µm or
less covering the bottom punch surface prior to compaction.
•
Surface P: Polymer granules (Cystrip U type 2 20/30,
Blästerprodukter Köping AB, Köping, Sweden) with a particle
size of 40 µm or less covering the bottom punch surface prior
to compaction.
•
Control (C). No medium was added to the bottom punch prior
to compaction.
The tool was cleaned with 95% ethanol prior to each compaction.
The cylinders, regardless of group, were sintered in a sintering
furnace (Everest Therm 4180, KaVo Everest, Biberach®, Germany)
according to the manufacturer’s instructions. The cylinders were
measured after sintering, which ranged in diameter from Ø 4.97 to
5.11 mm, with a height of 3 mm ± 0.1 mm. Measurement was made
using a digital caliper (Powerfix® Electronic digital caliper, Paget
Trading, London, UK).
37
Feldspathic porcelain blocks were manufactured by using
porcelain (Duceram® Plus dentin A3.5, Degudent, Hanau-Wolfgang,
Germany) shaped and fired in a calibrated porcelain furnace
(Programat P500, Ivoclar-Vivadent, Schaan, Liechtenstein) using one
dentin and one self-glaze firing, with all steps performed according
to the manufacturer’s recommendations.
The cementation surfaces of both the porcelain blocks and Y-TZP
cylinders were treated with 9.6% HF (Top Dent 9.6%, DAB Dental,
Upplands Väsby, Sweden), thoroughly rinsed with water, cleaned
with 35% phosphoric acid (Ultra-Etch 35%, Ultradent Products,
South Jordan, UT, USA) and again thoroughly rinsed with water.
After manufacturing, the specimens (n=8/group) were randomly
divided into subgroups depending on the adhesive cement system
used: either Variolink®II (Ivoclar-Vivadent AG, FL-9494 Schaan,
Liechtenstein) or Panavia™F 2.0 (Kuraray Medical, Osaka, Japan).
Prior to cementation, all cementation surfaces were treated with a
primer appropriate to the adhesive cement system, according to the
cement manufacturer’s recommendations. Study II used the same
cementation procedures as Study I, apart from the light-curing.
Light-curing in Study II used a different curing lamp (IvoclarVivadent Bluephase, Scaan, Liechtenstein). Polymerization light
intensity was 1100 mW/cm2 and curing time was 20 seconds in each
of four directions, 90° apart, and then 60 seconds in one direction
with the seating load removed. All specimens were stored for 10
hours in a humid environment to avoid desiccation during storage
before the additional treatment of artificial aging.
Artificial ageing – TC
All specimens were thermocycled at 5 000 cycles in the same manner
as Study I.
Bond strength test
Following pretreatment and artificial aging, a universal testing
machine (Instron model 4465, Instron®, Canton, MA, USA)
measured shear bond strength using a knife-edged blade parallel to
the bonded surfaces as in Study I. From the data collected the mean
and standard deviation for each group were calculated.
38
Failure type
The fracture surfaces were examined in a light microscope (Wild M3,
Wild Heerbrugg, Heerbrugg, Switzerland at ×6.4 magnification) to
classify the type of failure in the debonded area: adhesive, cohesive,
or a combination of the two.
Surface analysis and characterization
Microscopy. Two types of a light microscope were used to analyze
the cementation surfaces and fracture surfaces (Wild M3, Wild
Heerbrugg, Heerbrugg, Switzerland at ×5 to x10 magnification and
Leica DM 2500M, Leica Microsystems CMS, Wetzlar, Germany at
×500 magnification).
Interferometry. The samples were examined with interferometry
(IFM) using a MicroXAM™ instrument (ADE Phase shift Technology,
Inc., Tuczon, USA), in order to characterize the surface roughness
at the micrometer level. The IFM had a maximum resolution of
0.05 nm in the vertical direction and 0.3 µm in the lateral direction.
The scanned area of the specimens was 200 x 260 µm. The images
obtained by IFM were subjected to leveling and Gaussian filtering
(size 50 μm x 50 μm), and the roughness parameters were calculated
using the software MapVue (MetaMAP, Lexington, KY, USA).
Three parameters were selected according to proposed guidelines
for biomaterial surface characterization (99): one height descriptive,
Sa = the arithmetic average height deviation from mean plane (μm),
one spatial descriptive, Sds = the density of summits (μm-2), and
one hybrid parameter Sdr = the developed surface ratio (%). Three
specimens from each group and three measurements per specimen
(n=9/group) were made.
Statistics
One-way ANOVA, Tukey’s test (IBM SPSS Statistics 17.0, SPSS Inc.,
Chicago, IL, USA) determined differences in bond strength between
the groups. Two-way ANOVA, Tukey´s test provided statistical
analysis of the surface roughness between groups. Fisher’s exact test
tracked differences in the type of failure between each group. The
level of significance was set to α = 0.05.
39
Study IV
Specimen preparations
Study IV performed surface analysis (n=48) and a biaxial strength
test (n=200 on Y-TZP discs. The specimens were divided into
groups depending both on the cementation surface of Y-TZP (either
unmodified Y-TZP [C], or glass-modified Y-TZP surfaces [G]) and
on the production process (either etching before sintering [CE/
GE], sintering [CS/GS], sintering followed by etching [CSE/GSE],
sintering followed by sandblasting [CSS], sintering and sandblasting
followed by etching [CSSE]) all tested with or without the use of
cement, Panavia™F 2.0 (Kuraray Medical, Osaka, Japan) Table 4.
The method of specimen fabrication was the same as for Study II.
The tool was filled with 0.75 -1.15 g ±0.01 g of Y-TZP granulated
powder that was mechanically compressed uniaxially with 140 MPa
for five minutes, using a cuvette press. The bottom punch surface
of the pressing tool, which defines the cementation surface of the
Y-TZP, had either no medium added prior to compaction (C) or glass
granules with a particle size of 40 μm or less (G), Table 4. The tool
was cleaned with 95% ethanol (Ethanol 95%, batch nr. SE10016023,
Kemetyl AB, Haninge, Sweden) before each compaction.
Prior to sintering, the specimens with unmodified surface (CE)
and those with glass-modified surface (GE, GEC,) were etched for
two minutes using 9% HF. Then the specimens were neutralized for
two minutes before rinsing them thoroughly with water and storing
them at room temperature 24 hours. After this, all specimens,
regardless of group, were sintered in a sintering furnace according
to the manufacturer’s instructions. After sintering, specimens with
unmodified surface (CS) either underwent no further treatment or
they were sandblasted with 110 µm Al2O3 particles for 10 seconds
with an air pressure of 2 bars at a distance of 10 mm with gentle
movements of the blasting nozzle perpendicular to the surface before
being thoroughly rinsed with water (CSS, CSSC). The remaining
specimens with the unmodified surface (CSE), sandblasted surface
(CSSE) and with the glass-modified surface (GSE, GSEC) were
etched with 9% HF according to the etching procedure described
in Table 4.
40
Table 4. Overview of the groups, materials, pretreatments used and the
analysis performed.
Groups
(Abbreviation)
Materials used
Pretreatment/
Sintering
CE
Y-TZP*+ HF#
CS
Y-TZP
Etched# before
sintered**
Sintered
CSE
Y-TZP + HF
CSC
Y-TZP + PanaviaF™
2.0##
Y-TZP +
Al2O3 100µm***
CSS
CSSE
Y-TZP +
Al2O3 100µm+ HF
CSSC
Y-TZP + Al2O3 100µm
+ PanaviaF™ 2.0
GE
Glass-modified YTZP### + HF
Glass-modified Y-TZP
+ HF + PanaviaF™
2.0
Glass-modified Y-TZP
GEC
GS
GSE
GSEC
Glass-modified Y-TZP
+ HF
Glass-modified Y-TZP
+ HF + PanaviaF™
2.0
Sintered +
etched
Sintered +
cement##
Sintered
+ sandblasted***
Sintered +
sandblasted +
etched
Sintered +
sandblasted +
cement
Etched before
sintered
Etched + sintered + cement
Sintered
Sintered +
etched
Sintered +
etched +
cement
Surface
analysis
(n=6)
IFM, AFM,
SEM, EDS
IFM, AFM,
SEM, EDS
IFM, AFM,
SEM, EDS
-
Flexural strength
test (n=25)
Yes
Yes
IFM, AFM,
SEM, EDS
Yes
IFM, AFM,
SEM, EDS
-
-
Yes
IFM, AFM,
SEM, EDS
-
Yes
IFM, AFM,
SEM, EDS
IFM, AFM,
SEM, EDS
-
-
Yes
Yes
Yes
*
Y-TZP: grade TZ-3YSB-C, (batch S306269B, Procera Zirconia, Nobel BiocareTM AB, Gothenburg,
Sweden). # HF: etched with 9% hydrofluoric acid (Ultradent® Porcelain Etch, 9%, LOT B7814,
Ultradent Products, Inc., South Jordan, Utah, USA) for two minutes, neutralized (IPS Ceramic,
Neutralizing powder, LOT M04796 Ivoclar Vivadent, Schaan, Liechtenstein) for two minutes, followed
by thorough rinsing with water according to the cement manufacturer’s recommendation. ** Sintered:
sintered in a sintering furnace (Everest Therm 4180, KaVo Everest®, Biberach, Germany) according
to the manufacturer’s instructions. ## PanaviaF™2.0: treated with a primer (Clearfil Ceramic Primer,
LOT 0025AA, Kuraray Noritake Dental Inc., Okayama, Japan) prior to application of the adhesive
cement, Panavia™F 2.0 (Panavia™F 2.0, base paste LOT 0112AA and catalyst paste LOT 0575AA,
Kuraray Noritake Inc., Okayama, Japan) and oxygen-blocking gel (Panavia™F 2.0 Oxyguard II,
LOT 00655A, Kuraray Medical Inc., Okayama, Japan) according to the cement manufacturer’s
recommendation. *** Al2O3 100 µm: sandblasted with 110 µm Al2O3 particles for 10 seconds with an
air pressure of 2 bar and at a distance of 10 mm, then thoroughly rinsed in water. ### Glass-modified
YTZP: modified with glass granules (Experimental Impaction Medium, Cerasci AB, Malmö, Sweden).
41
Based on a power analysis consisting of a two-sample test with a
significance level of 5% with true difference of means set at 50 N
and with a power of 95%, 200 specimens of Y-TZP were prepared
for the biaxial test. This included a total of four subgroups, each
with or without an adhesive cement system, Panavia™F 2.0 (n=25),
Table 4. The fabrication process was the same as for the fabrication
of the specimens for surface analysis and the final size of the discs,
Ø 12.8±0.2 mm and a height of 1.29±0.1 mm, accorded with ISO
6872:2008 (18). All discs were subjected to heat treatment to simulate
the firing cycles of a recommended veneering porcelain (GC Initial
ZR-FS, GC Europe N.V., Leuven, Belgium). Firing cycles proceeded
in a calibrated furnace and each disc underwent four firings: frame
modifier, dentin 1, dentin 2 and self-glaze firing, all according to the
manufacturer’s recommendations.
All subgroups with the surface covered by cement (CSC, CSSC, GEC,
GSEC) were etched with HF, according to the same etching procedure
described above; etching prior sintering. All these discs were treated
with a primer before application of the adhesive cement Panavia™F
2.0 according to the cement manufacturer’s recommendation, Table
4. The cement was applied to the cementation surface of the Y-TZP
and a 0.12 mm thick plastic film was placed between an alignment
apparatus with a seating load of 15 N during polymerization. The
excess resin was removed from the margin using disposable brushes
and the cement was light-cured with a curing lamp (Heraeus
Translux® Power Blue®, Hereaus Kulzer GmbH, Hanau, Germany).
The polymerization light intensity was 1000 mW/cm2 and the curing
time was 20 seconds for each of four directions, 90° apart, and
then 60 seconds in one direction with the seating load removed.
Subsequently, an oxygen-blocking gel was applied for 3 minutes,
followed by a thorough rinse with water for 1 minute to remove any
residue of the oxygen-blocking gel, Table 4. After polymerization,
all the excess of resin was removed with a surgical blade and
then stored the discs in water at RT (22°C) for 24 hours to avoid
desiccation during storage.
42
Artificial ageing – Cyclic preload and TC
Before the biaxial flexure strength test, all specimens underwent
artificial ageing with cyclic preloading and TC. The specimens were
subjected to cyclic preload at loads between 10 and 100 N at 1 Hz
in a wet environment for 10,000 cycles using a specially constructed
preloading device (MTI Engineering AB, Lund, Sweden/Pamaco
AB, Malmö, Sweden). The discs were placed on three supporting
steel balls (Ø 2.5 mm) while a centrally placed stainless punch (Ø
1.4 mm) applied the load at the center, perpendicular to the discs.
A 0.12 mm thick plastic film was placed both between the three
supporting balls and the disc, and between the punch and the disc.
Subsequently, all specimens were thermocycled at 5 000 cycles in
two baths; one at 5°C and the other at 55°C, as described previously
in Studies I and II.
Biaxial flexural strength
The specimens were placed in a universal testing machine (Instron
model 4465, Instron®, Canton, MA, USA) according to the cyclic
preloading, and loaded to the point of fracture in a wet environment.
The crosshead speed was 0.5 mm/min. Load at fracture (N) was
registered when a visible fracture occurred. The flexural strength in
MPa was calculated according to ISO 6872:2008 Dentistry – Ceramic
materials (18) and with the Poisson’s ratio of 0.25. Throughout the
test period, whenever the specimens were not being actively tested,
they were stored in a humid environment at RT.
Surface analysis and characterization
Microscopy. During all steps in the fabrication process of the
specimens, the cementation surfaces were analyzed under light
microscopes, as described in Study II.
Interferometry. To define and characterize the surface, IFM, was
used as described in Study II. Three specimens from each group
and three measurements per specimens (n=9/group) were analyzed,
Table 4.
43
Atomic force microscopy. To further characterize the surface,
atomic force microscopy AFM analysis (XE-100, Park Systems,
Suwon, Korea) was performed in intermittent-contact mode using
etched silicon probes with cantilever lengths of 125 nm and nominal
resonance frequencies of 270-310 KHz. A scanning area of 10 x 10
µm with a resolution of atomic level in the vertical direction and
2 nm in the lateral direction was used and the measurements were
performed at a scan rate of 0.2-0.4 Hz. Three specimens from each
group, with three measurements per specimen (n=9/group) were
performed. The same three parameters selected for the IFM, also
served for data collection and analysis of the AFM.
Scanning electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDS). Morphological examination was performed
with scanning electron microscopy (SEM) using a LEO Ultra 55 FEG
high resolution SEM (Leo Electron Microscopy Ltd, Cambridge, UK)
equipped in combination with an Oxford Inca EDS system (Oxford
Instruments Nano Analysis, Bucks, UK) operated between 5 and 10
kV at ×5 000 to ×100,000 magnification and with high vacuum.
The samples were examined after surface sputtering for 60 seconds
at 10 mA, resulting in a 30 nm thick gold layer. To describe the
atomic composition, EDS analysis at 15 kV and WD 10 at ×10,000
magnification was performed. One specimen from each group was
analyzed.
X-ray diffraction (XRD). XRD was performed to analyze the
crystalline structure of the samples using a Bruker D8 Advance
X-ray diffractometer (Bruker BioSpin Corp., Billerica, MA, USA),
collecting a 2θ scanning range of 27-60°, with a step size of 0.05°
and with Cu Ka1 radiation (λ=1.5405 Å) as the diffraction light
source.
Statistics
The one-way ANOVA, Tukey’s test (IBM SPSS Statistics 20, SPSS
Inc., Chicago, IL, USA) with the level of significance set to α =
0.05 exposed differences in surface structure and flexural strength
between the groups.
44
Systematic review
Study III
Search strategy
Study III sought to obtain an overview of existing methods to achieve
bondable surfaces on oxide ceramics, and determine which methods
might provide sufficient bond strength, through a systematic review
using PubMed (National Center for Biotechnology Information,
U.S. National Library of Medicine). The literature search addressed
the following questions:
1. What different methods of surface treatments are available
to achieve a bond between oxide ceramics and adhesive
cement systems?
2. Do any of these methods provide sufficient bond strength
for retention of dental restorations without the need for
macromechanical retention?
Definitions:
• Oxide ceramics are defined as polycrystalline aluminum oxide
and yttrium oxide stabilized tetragonal zirconium dioxide (33).
•
A bondable surface is a treated oxide ceramic surface that
provides micromechanical interlocking and/or activation for
chemical bonding (33).
•
Macromechanical retention is defined as the retentive
geometric shape of the tooth preparation with the
corresponding shape of the dental reconstruction (10).
•
Sufficient bond strength is defined by test values >20 MPa,
regardless of the testing methods used (100).
Inclusion and exclusion criteria
To address Questions 1 and 2, the literature review included
only the following: original article with abstract included, based
on oxide ceramics/polycrystalline oxide ceramics, evaluating
the bond strength between oxide ceramics and adhesive cement
systems, oxide ceramic bonded to ceramics, composite, enamel or
dentine (human or animal), and all test methods for bond strength
45
performed without the influence of macromechanical retention. For
Question 2, inclusion criteria also required: inclusion of a control
group, well-defined artificial aging procedure or fatigue simulation
artificial aging with TC comprising a minimum of 500 cycles in
water with a minimum of 50°C difference between the baths, or
long-term storage of 6 months in water at 37°C according to ISO/
TS 11405:2003 Dental materials –Testing of adhesion to tooth
structure (96), or other test methods with mechanical cyclic loading
(>500 cycles) in combination with water/liquid, and verifying the
surface of the oxide ceramics before and after testing with more
than light microscopy. The exclusion criteria for the addressed
questions and aims eliminated: reviews, studies based on glassinfiltrated ceramics or metals or posts, orthodontic brackets made
of oxide ceramics, and cement other than adhesive cementation
systems (e.g., not resin-modifies glass ionomer cement).
The literature search involved three successive steps (Figure 5).
It was conducted from August 2011 to the 1st of January 2012.
Free-text words were used as search terms, with English set as the
language filter:
(oxide ceramics OR high strength ceramics OR alumina OR aluminum
oxide OR zirconia OR zirconium dioxide OR Y-TZP) AND (dental
cementation OR dental cements OR adhesive cements OR dental
bonding OR resin bond OR resin cements OR luting OR adhesive
retention) AND (bond strength)
The literature searches proceeded with snowballing, where
handsearch of the reference lists of included studies and systematic
reviews yields additional relevant articles. Two authors read the
titles and further examined any potentially relevant publications and
their abstracts, focusing on bond strength analyses between oxide
ceramics and adhesive cementation. If at least one author found an
article relevant, it was selected for full-text reading using a protocol
that indicated inclusions and exclusions criteria (Appendix 1).
46
Appendix 1. Instructions for in vitro studies:
•
The alternative “unclear” is used when the data are not possible to
obtain from the text.
•
The alternative “not applicable” should be selected when the issue is
not relevant.
•
When clarifying comments are needed, indicate in footnotes
Question
Comments
1. Study population
Yes
No
Unclear
Not applicable
a) Is it clear how many specimens where
included in the study?
b) Were any specimens excluded?
c) If any specimens were excluded,
is it clear how many?
d) and why the specimens were excluded?
2. Comparability between groups
a) Were the groups equal in sample size?
b) Were the groups equally treated in
an artificial aging procedure/fatigue
simulation of the interface (for example,
TC)?
Supplementary questions
a) If TC was done, how many cycles
did the specimens undergo and in what
temperature range?
Temperature range
Numbers of cycles:
b) Were the groups stored long-time?
c) If long-time storage was done, for how
long were the specimens stored?
Storage time:
3. Material and method
a) Are all subgroups well-defined?
b) Were there any control groups?
Supplementary questions
a) Is the surface treatment commercial?
b) Is the surface treatment experimental?
4. Results and precision
a) Are the results presented in a way that
support and relates to the conclusion?
b) What test method was used to measure
bond strength?
Test method:
c) How were the surfaces described before
and after testing?
Surface analysis:
Supplementary questions
a) Is it clear that the significance level was
defined in advance?
b) Was a power analysis performed?
c) Were the results statistically analysed?
47
RESULTS
In vitro studies
Study I
Bond strength between different bonding systems and
densely sintered alumina with sandblasted surfaces or
as produced.
Bond strength
The highest bond strengths, which two of the six adhesive cement
systems achieved, were significantly higher (p<0.001) compared to
the tested adhesive cement systems, irrespective of treatment (artificial
aging). There were some differences between the groups with water
storage only and those with storage including TC. The Variolink® II
group showed a higher bond strength after TC (p<0.05). Table 5a
shows shear bond strengths and significant differences between the
two subgroups within each group and Table 5b shows them between
all the groups (the six adhesive cement systems).
Failure type
Table 6 describes type of failure in the debonded areas in each
group. The predominant failure type for both treated (sandblasted)
and untreated surfaces were adhesive. All groups, irrespective of
pretreatment, showed indications of complex failure.
48
Table 5a. The results for shear bond strength (MPa): mean values, standard
deviation (SD), in the water storage only and thermocycled subgroups of each
adhesive cementation system. The statistical difference between the subgroups
in each system was determined with Student’s t-test.
Groups
Water storage only
Mean bond
strength (SD)
Thermocycled
Mean bond
strength (SD)
Variolink® II
27 MPa (7.0)
34 MPa (5.0)
(p<0.05)
Bisco Chioce™
14 MPa (5.5)
11 MPa (7.6)
(p>0.05)
Bisco lllusion™
10 MPa (3.9)
9 MPa (4.4)
(p>0.05)
Nexus 2™
10 MPa (4.4)
5 MPa (3.6)
(p<0.05)
RelyX™ Veneer
Panavia™ F
Statistical significance
between the subgroups
within the adhesive
cement group
4 MPa (1.7)
2 MPa (1.4)
(p<0.05)
31 MPa (4.9)
36 MPa (5.8)
(p>0.05)
Water storage only (storage condition 37°C H2O), Thermocycled (storage condition
37°C H2O incl. thermocycling)
Table 5b. Differences in mean shear bond strengths between the water storage only and thermocycled subgroups. The statistical difference between all
the subgroups was determined with one-way ANOVA, Tukey’s test.
Variolink® II
***
***
***
Bisco Choise™ (BC)
***
***
NS
NS
***
NS
NS
*
***
***
NS
***
***
NS
***
***
NS
NS
RelyX™ Veneer (RV)
NS
***
NS
Nexus 2™ (NE)
Panavia TM F
***
***
NS
Bisco Illusion™ (BI)
RV
NE
BI
Thermocycled
BC
Water storage only
NS
***
***
Upper /Lower = Water storage only /Thermocycled
NS= Non-statistical significance,***p<0.001, **p<0.01, *p<0.05
49
Table 6. Mean percentage of the various failure types in each group according to
storage conditions.
Groups
Failure types
(Mean percentage)
Water storage only
Failure types
(Mean percentage)
Thermocycled
Variolink® II
α 93% β 1% γ 5% δ 1%
α 92% β 2% γ 4% δ 2%
Bisco Choise™
α 5% β 50% γ 42% δ 0%
α 3% β 56% γ 41% δ 0%
Bisco Illusion™
α 95% β 0% γ 1% δ 4%
α 91% β 2% γ 2% δ 5%
Nexus 2™
α 21% β 60% γ 19% δ 0%
α 31% β 53% γ 10% δ 6%
RelyX™ Veneer
α 74% β 4% γ 20% δ 2%
α 44% β 3% γ 50% δ 3%
Panavia™ F
α 35% β 46% γ 10% δ 9%
α 28% β 65% γ 6% δ 1%
Water storage only (storage condition 37°C H2O), Thermocycled (storage condition 37°C H2O incl.
thermocycling)
α= Adhesive failure from the untreated surface, only, β= Adhesive failure from the
treated surface, only, γ= Adhesive failure from both the untreated and treated surfaces,
δ= Complex cohesive failure type.
Study II
Impaction-modified densely sintered yttria-stabilized tetragonal zirconium dioxide (Y-TZP): Methodology, surface
structure and bond strength.
Surface analysis and characterization
Microscopy. The cementation surface of feldspathic porcelain
showed a rough surface after etching with HF. The control group
(unmodified Y-TZP) showed no visible change in the surface
structure. The surface-modified Y-TZP specimens, however, revealed
a visibly increased surface structure (Figure 1 a-d).
Figure 1 (a-d). Cementation surfaces after etching (x31 magnification). (a): feldspathic
porcelain: the glass phase of the cementation surface is removed, leaving a typical surface with
micromechanical retention. (b): Y-TZP control: no visible change in surface structure. (c): Y-TZP
surface G: the glass residues are removed, leaving a surface with increased surface roughness and
enlarged cementation surface (compared to the control group). (d): Y-TZP surface P: an increased
surface roughness, and enlarged cementation surface (compared to the control group).
50
Interferometry. The results from all three parameters (Sa, Sds and Sdr)
showed a significant difference between the groups that had been
surface-modified with glass granulates and all the other groups,
both after sintering and after etching, Table 7. Figure 2 (a-f) shows
images of the various surfaces, including unmodified surfaces and
those modified with glass or polymer.
Table 7. Mean values and standard deviation (SD) of tridimensional roughness parameters (Sa, Sds
and Sdr) as determined by IFM (scanning area of 200 × 260 μm2).
Surface treatment
Sa (μm)
(SD)
Mean
Sds (μm-2)
(SD)
Mean
Sdr (%)
(SD)
Mean
Control sintered
0.15a
(0.02)
174611a
(8555)
2.85a
(0.34)
Control etched
0.10b
(0.01)
143196a,b
(9286)
0.92b
(0.23)
Surface G sintered
5.03
(0.48)
293188
a,b,c
(8694)
754
(114.5)
Surface G etched
4.07a,b,d,e
(0.57)
249765a,b,d
(6822)
142a-d
(40.9)
Surface P sintered
0.26c,d
(0.11)
158313a,c,e
(29185)
11.9c
(12.8)
Surface P etched
0.28
(0.33)
179152
(7949)
7.62
(15.09)
a-c,e
e
a,b,d,e
a-d
d
Groups labeled with the same superscripted letter indicate significant statistical difference in
surface roughness (p < 0.05) between the groups.
a, b, c
Figure 2 (a-f). Cementation surfaces after sintering a,c,e and after etching b,d,f: (a): Y-TZP control:
clearly defined grain boundaries. (b): Y-TZP control: small changes in surface structure can be
observed after etching. (c): Y-TZP surface G: creates a rough surface structure. (d): Y-TZP surface G:
the glass residues are removed leaving a surface with increased surface roughness (compared to the
control group) but not as rough as before etching the same surface. (e): Y-TZP surface P: creates a
rough surface structure. (f): Y-TZP surface P: the surface structure is rougher compared to the same
non-etched surface, but is not statistically significant.
51
Bond strength
All groups with modified cementation surfaces showed significantly
higher bond strength compared to their corresponding control group.
The glass-modified surface in combination with the Variolink® II
cement system, showed the highest bond strength. Table 8 lists the
results.
Failure type
The surface-modified groups showed mainly cohesive failure (24 of
32) in which the failure occurred in the feldspathic porcelain, while
all failures in the two control groups were adhesive. There were
no significant differences between groups with the same surfaces,
irrespective of cement system, Table 8.
52
Table 8. The results for bond strength (MPa): mean values, standard deviation (SD), maximum and minimum values and failure types between the groups, the number (n) of
adhesive and cohesive failures and statistical differences.
Groups
Abbreviation
Bond strength
Mean value SD
Maximum/Minimum value
Failure type
Adhesive/Cohesive(n)
Variolink® II + surface G
VA-G
34.9 a
(5.8)
42.4
25.8
2/6
Variolink® II + surface P
VA-P
30.9 b
(7.7)
40.2
20.6
0/8a
Variolink® II + control
VA-C
20.5 a-c
(4.2)
27.8
15.5
8/0a
Panavia™ F + surface G
PA-G
26.1 a,d
(4.0)
31.8
21.6
2/6
Panavia™ F + surface P
PA-P
29.6 a,c,e
(3.8)
35.8
24.7
4/4
Panavia™ F + control
PA-C
17.8 a,b,d,e
(3.5)
25.1
13.1
8/0a
Adhesive denotes debonding between the cement and the Y-TZP or porcelain. Cohesive denotes fracture in the feldspathic porcelain.
a, b, c
Groups labeled with the same superscripted letter indicate significant statistical difference in bond strength and fracture type (p > 0.05) between the groups.
53
Study IV
Surface structure and mechanical properties of impaction-modified
Y-TZP.
Topographical surface characterization by IFM and AFM
The surface structure of sandblasted Y-TZP differed from glassmodified Y-TZP. In both resolutions investigated, micrometer and
nanometer, the glass-modified surface was rougher. Table 9 shows
the parameters calculated from the IFM and AFM and Figure
3 (a-h) shows images of the surface structures. The results of the
IFM, indicate a significant difference (p<0.05) between both the
unmodified Y-TZP (CE, CS, CSE) and sandblasted with or without
the etching procedure (CSS, CSSE), and the glass-modified groups,
which showed a rougher surface. Among the glass-modified groups,
those that were sintered (GS) or sintered and etched (GSE) had the
roughest surfaces.
Results from the AFM found no significant differences between
the groups for the parameter Sa. The Sds parameter showed a
significant difference between the group that were etched before
sintering (CE), as well as the unmodified surface with or without the
etching procedure (CSE, CS), and the groups that were sandblasted
and etched (CSSE) or glass-modified (GS, GSE). However, there was
no significant difference found between the sandblasted and glassmodified groups. The Sdr parameter showed significantly higher
values for the group with the sandblasted surface than for the groups
that were etched before sintering (CE) or just sintered (CS).
54
Table 9. Mean values and standard deviation (SD) of tridimensional roughness parameters (Sa, Sds and Sdr) as determined by IFM (scanning area of 200 × 260 μm) and AFM
(scanning area of 10 × 10 μm).
Groups
IFM
Sa (µm)
AFM
Mean (SD)
Sds (µm-2)
Mean (SD)
Sdr (%)
Mean (SD)
Sa (nm)
Mean (SD)
Sds (µm-2)
Mean (SD)
Sdr (%)
Mean (SD)
CE
0.24 (0.21)a
178971 (14550)a
8.3 (1.9)a
37.0 (5.4)
6.6 (0.98)a
7.4 (1.6)a
CS
0.27 (0.02)b
161326 (5761)b
8.7 (0.98)b
35.3 (3.5)
7.5 (1.6)b
6.9 (1.2)b
CSE
0.29 (0.29)
c
a-c
140941 (6902)
11.0 (1.9)
41.5 (16.8)
7.6 (3.4)
9.0 (5.5)
CSS
d
0.47 (0.02)
b-d
191903 (1613)
15.8 (0.44)
58.4 (12.8)
15.5 (5.7)
CSSE
0.50 (0.04)
e
24.6 (2.0)
51.1 (12.4)
22.5 (6.2)
GE
4.2 (0.70)
227139 (19254)
a-f
207 (65)
41.5 (10.3)
13.5 (5.1)
GS
4.3 (0.29)a-f
278845 (18650)a-f
368 (13.4)a-f
40.2 (13.8)
23.8 (11.6)a-d
15.8 (15.2)
GSE
3.8 (0.57)a-f
297974 (21567)a-f
322 (90)a-f
62.2 (50.1)
21.0 (7.6)a-c
22.6 (27.5)
e
a-e
170396 (4733)
c-e
a-f
c
d
c
27.3 (14.9)a,b
a-c
d
14.5 (9.1)
11.6 (5.8)
CE: Control; etched before sintering, CS: Control; sintered, CSE: Control; sintered , etched, CSS: Control; sintered, sandblasted, CSSE: Control; sintered, sandblasted, etched, GE:
Glass-modified; etched before sintering, GS: Glass-modified; sintered, GSE: Glass-modified; sintered, etched.
a, b, c
Groups labeled with the same superscripted letter indicate significant statistical difference in surface roughness (p > 0.05) between the groups.
55
Figure 3 (a-p). AFM images (10 x 10µm) and SEM electromicrographs (× 30,000
magnification) of each group: CE: Control; etched before sintering (a and i), CS:
Control; sintered (b and j), CSE: Control; sintered , etched (c and k), CSS: Control;
sintered, sandblasted (d and l), CSSE: Control; sintered, sandblasted, etched (e and m),
GE: Glass-modified; etched before sintering (f and n), GS: Glass-modified; sintered (g
and o) and GSE: Glass-modified; sintered, etched (h and p).
56
Surface morphology and chemical surface characterization by SEM
and EDS
Figure 3 (i-p) depicts SEM images of surface morphology. The
chemical composition of glass-modified Y-TZP differed from
unmodified Y-TZP. A phase transformation was identified in the
glass-modified Y-TZP. While the EDS analysis found oxygen (O),
zirconium (Zr), yttrium (Y) or hafnium (Hf), carbon (C), boron
(B), and aluminum (Al) on the surface of both the unmodified and
sandblasted Y-TZP, the components on the glass-modified Y-TZP
surface were O, Zr, C, B and silicon (Si), sodium (Na), calcium (Ca),
and fluorine (F). Figure 4a summarizes the chemical compounds
identified.
(at%)
* Yttria was primary identified, however it could also be hafnium, which also has been suggested as to be present.
Figure 4a. Mean values and standard deviation (SD) of atomic concentration (at %) of elements
according to the sample groups, as determined by EDS analysis.
*Yttria was identified primarily, although it could also be hafnium which has been suggested might
be present.
Identification of crystalline structure by XRD
The XRD results showed that two crystalline structures dominate,
the monoclinic phase and the tetragonal phase, Figure 4b.
Unmodified Y-TZP samples showed only the tetragonal phase. After
sandblasting the Y-TZP, monoclinic phase was identified too. The
glass-modified Y-TZP showed increase monoclinic structure and less
of the tetragonal phase. Etching procedures showed no differences,
regardless of group. Thus etching will not affect the crystalline
structure.
57
Figure 4b. XRD diffractograms of samples from each group.
*m: peak indicating the monoclinic phase, #t: peak indicating the tetragonal phase. CE: Control;
etched before sintering, CS: Control; sintered, CSE: Control; sintered , etched, CSS: Control; sintered,
sandblasted, CSSE: Control; sintered, sandblasted, etched, GE: Glass-modified; etched before sintering, GS: Glass-modified; sintered and GSE: Glass-modified; sintered, etched.
Biaxial flexural strength
The groups with no glass modification showed significantly higher
flexural strength compared to the glass-modified groups (p<0.001).
They also showed increased flexural strength after sandblasting
(p<0.001). Adding cement to the sandblasted surface increased
flexural strength even more (p<0.01). After TC, however, the cement
layer on both the unmodified and the sandblasted surfaces had air
pockets, and part of the cement was loose. This was not seen in the
glass-modified groups. Table 10 summarized the results.
58
Table 10. The results for flexural strength (MPa): mean values, standard deviation (SD), maximum
and minimum values and statistical differences.
Groups
Abbreviation
Mean (SD)
Maximum
Minimum
Control, sintered
CS
443 (64)a
549
252
Control, sintered + cement
CSC
476 (63)b
600
327
Control, sintered +
sandblasted
CSS
574 (53)a-c
686
488
Control, sintered +
sandblasted + cement
CSSC
624 (68)a-d
782
492
Glass-modified, etched before
sintering
GE
237 (49)a-e
420
160
Glass-modified, etched before
sintering + cement
GEC
250 (37)a-d,f
325
192
Glass-modified, sintered +
etched
GSE
187 (20)a-g
222
147
Glass-modified, sintered +
etched + cement
GSEC
235 (33)a-d,g
346
193
Groups labeled with the same superscripted letter indicate significant statistical difference in
fracture strength (p > 0.05) between the groups.
a, b, c
Systematic review
Study III
Bonding between oxide-based ceramics and adhesive
cement systems: A systematic review.
Literature identification
Figure 5 is a flow diagram of the number of retrieved, potentially
relevant publications and the publications included and excluded in
each step of the data extraction. The total number of publications
included was 127 publications, of which 23 examined the question
“Do any of the methods provide sufficient bond strength for the
retention of dental restorations without the need for macromechanical
retention?”.
59
Figure 5. Search strategy and results of the literature review
60
Interpretation of data on surface treatment of oxide ceramics and
bond strength
Figure 6 presents the various methods of surface treatment that
aims to create a bondable cementation surface in oxide ceramics.
The surface treatments are divided into seven main categories: asproduced, grinding/polishing, airborne particle abrasion, surface
coating, laser treatment, acid treatment, and primer treatment. Each
group has own specific surface treatments, followed by a variety of
cleaning procedures with water, ethanol, acetone or phosphoric acid
and then various cement systems. The evaluation did not consider
cleaning and cementation procedures.
Oxide ceramics
Concerning Question 2, the 23 studies included 12 different brands
for stabilized zirconium dioxide (28, 61, 72, 76, 78, 88, 89, 101114) and two of aluminum oxide (61, 89, 94, 113).
Artificial aging
Artificial aging procedure differed in the various studies. For TC, the
number of cycles ranged from 500 to 100,000 at water temperatures
of 5°C to 60°C (72, 78, 88, 89, 101, 102, 104-107, 110-112, 115)
or TC (20,000 or 37,500 cycles) with long-term water storage
(37°C) for 90 or 150 days (28, 61, 76, 94, 108, 109). Other artificial
aging procedures included long-term water storage, varying from 24
weeks to 104 weeks, (103, 104, 110, 113) and cyclic impaction and
compressive load in water with up to 1,000,000 cycles (114).
Test method
The most common test method (16 of 23) to evaluate bond strength
was the shear bond strength test (SBS/µSBS) (61, 72, 76, 78, 88, 89,
101, 102, 105-107, 111-115) (Table 11).
61
62
Figure 6. A schematic illustration and overview of the various surface treatments from the literature review.
The reference number is in accordance with the original published study (see Study III in the appendix).
Table 11. The results from the included studies (n=23). Ceramic material, surface treatment, cement system, type of artificial aging and test methods.
Article
Materials
Cementation systems
Artificial aging
TC/ LT storage
Test method
Phark JH, et al. 2009 (61)
Procera Alumina, Nobel Biocare
Procera Zirconia, Nobel Biocare
bonded to composite resin
Clearfil Esthetic + Clearfil Ceramic
primer
RelyX ARC + RelyX Ceramic primer
90 days of water storage
including 20,000 cycles (5°60°C)
SBS
Phark JH, et al. 2009 (76)
Procera Zirconia, Nobel Biocare
bonded to composite resin
Panavia F2.0
RelyX ARC
RelyX Unicem
90 days of water storage (RT)
including 20,000 cycles (5°60°C)
SBS
Jevnikar P, et al. 2010 (78)
TZ-3YB-E zirconia powder, Tosoh
bonded to composite resin
RelyX Unicem
12,000 cycles (5°-55°C) after 24
hours of water storage (37°C)
SBS
Matinlinna JP, et al. 2006
(88)
Procera AllZircon, Nobel Biocare
bonded to cements
Experimental Bis-GMA, Röhm
RelyX ARC
6 000 cycles (5°-55°C) after 24
hours of storage in a desiccator in RT.
SBS
Aboushelib MN. 2011 (110)
Procera Zirconia, Nobel Biocare
bonded to composite resin
PanaviaF 2.0
10,000 cycles (5°-55°C)
or 4 -26- 52-104 weeks in water
(37°C)
µTBS
Moon JE, et al. 2011 (111)
ZrO2 Rainow, Dentium bonded
Clearfil SA luting cement
Zirconite
RelyX Unicem
Superbond C&B
Multilink
5 000 cycles (5°-55°C) after 24
hours of water storage (37°C)
SBS
to composite resin
63
(Table 11 continues on next page)
64
(Table 11 cont.)
Article
Materials
Cementation systems
Artificial aging
TC/ LT storage
Koizumi H, et al. 2010 (115)
Alumina, Furuuchi Chemical
Corp. bonded to cement with 8
different primers
Unprimed as control
Eight primers: 1)Alloy Primer 2)Eye
Sight Opaque primer 3)Estenia
Opaque Primer 4)MR. Bond 5)M.L.
Primer 6)Super-Bond Liquid 7)All
Bond II Primer B 8)Acryl Bond
Luting agent: Super-Bond Cat. +
Liquid + Opaque powder
100,000 cycles (5°-55°C) after
24 hours of water storage
SBS
Kulunk S, et al 2011 (112)
ICE Zircon translucent,
Zirconzahn bonded to
composite
PanaviaF 2.0
6 000 cycles (5°-55°C) after 24
hours of water storage (37°C)
SBS
Akyil MS, et al. 2010 (72)
Zirconia, Copran Zirconia
blank, White Peaks Dental
system bonded to composite
Clearfil Esthetic Cement
5 000 cycles (5°-55°C) after 24
hours of water storage (37°C)
SBS
Yun JY, et al. 2010 (107)
ZrO2 Rainow, Dentium bonded
Alloy primer+ PanaviaF2.0
V-primer + Superbond C&B
Metaltite + M bond
5 000 cycles (5°-55°C) after 24
hours of water storage (37°C)
SBS
PanaviaF 2.0
10,000 cycles (5°-55°C) or 4
weeks in water (37°C) or 20
µTBS
to cements
Aboushelib MN, et al. 2010
(104)
LAVA Zirconia, 3M ESPE Dental
Products bonded to composite
weeks in water (37°C)
(Table 11 continues on next page)
(Table 11 cont.)
Nakayama D, et al 2010
(105)
KATANA™ Zirconia, Noritake
bonded to cement
Unprimed as control
Eight primers: 1)Alloy Primer 2)Eye
Sight Opaque primer 3)Estenia
Opaque Primer 4)MR. Bond 5)M.L.
Primer 6)Super-Bond Liquid 7)All
Bond II Primer B 8)Acryl Bond
Luting agent: Super-Bond Cat. +
Liquid + Opaque powder
10,000 cycles (5°-55°C) after 24
hours of water storage (37 °C)
SBS
Heikkinen TT, et al. 2007
(89)
LAVA Zirconia, 3M ESPE Dental
Products. Procera Alumina or Procera
Zirconia, Nobel Biocare bonded to
composite
3M Multipurpose resin
6 000 cycles ( 5°-55°C)
SBS
Matinlinna JP, et al. 2007
(102)
Procera AllZircon, Nobel Biocare
bonded to cement
RelyX ARC
6 000 cycles (5°-55°C)
SBS
Kumbuloglu O, et al. 2006
(101)
DCS Zirconia bonded to cement
Panavia F
RelyX Unicem
2 000 cycles (5°-55°C) after 24
hours of water storage (37°C)
SBS
Takeuchi K, et al. 2010
(106)
KATANA™ Zirconia , Noritake
bonded to Ti, JIS grade2
RelyX ARC
30,000 cycles (5°-60°C) after 24
hours of water storage (37°C)
SBS
Zhang S, et al. 2010 (109)
ZrO2, Cercon bonded to
composite resin
PanaviaF 2.0
150 days of water storage
(37°C) including 37,500 cycles
(5°-55°C)
TBS
65
(Table 11 continues on next page)
66
(Table 11 cont.)
Article
Materials
Cementation systems
Artificial aging
TC/ LT storage
Yang B, et al 2010 (108)
Zirconia, Cercon ceramic
bonded to cement
Multilink (automix)
150 days of water storage
(37°C) including 37, 500 cycles
(5°-55°C)
TBS
Kern M, et al 2009 (28)
Zirconia, Cercon ceramic
bonded to cement
Multilink (automix)
150 days of water storage
(37°C) including 37,500 cycles
(5°-55°C)
TBS
Hummel M, Kern M. 2004
(94)
Procera Alumina, Nobel Biocare
bonded to Clearfil F2
Variolink II + Heliobond, = System
Variolink II system
Alloy primer + Variolink system
Monobond-S + Variolink II system
Superbond C&B
Panavia 21
150 days of water storage
(37°C) including 37,500 cycles
(5°-55°C)
TBS
Foxton RM, et al. 2011
(113)
Procera Alumina or Procera Zirconia,
Nobel Biocare bonded to cements
NAC-100
Variolink II
6 months of water storage (37°C)
µSBS
Oyague RC, et al . 2009
(103)
Zirconia Cercon bonded to
composite resin
Clearfil Esthetic Cement
RelyX Unicem
Calibra
6 months of water storage (37°C)
µTBS
Kawai N, et al. 2011 (114)
YPSZ, Nikkato bonded to each
other
SuperBond C&B
Panavia Fluora Cement
Fuji Luting (RRGC)
Cyclic impaction
Compressive cyclic load or
Shear cyclic impaction load
numbers of cycles 1-106
SBS
Table 12. Mean values from tests of various surface treatments.
Surface treatment
As-produced
Grinding/polishing
Airborne particle
abrasion
Tribochemical
silica-coating
NobelBond
SIE
AlN
Laser etching
Primer treatment
Included studies
Question 1 and 2
Q1 (34/127)
Q2 (10/23)
Q1 (56/127)
Q2 (8/23)
Q1 (94/127)
Q2 (15/23)
Q1 (55/127)
Q2 (10/23)
Q1 (8/127)
Q2 (2/23)
Q1 (7/127)
Q2 (2/23)
Q1 (2/127)
Q2 (2/23)
Q1 (8/127)
Q2 (2/23)
Q1 (98/127)
Q2 (9/23)
TBS
Test methods (range of mean values, MPa)
μTBS
SBS
μSBS
No. of studies with
mean values >20 MPa
0
0-8
0-17
11-20
No study
0
*
0-36
*
1 study (115)
0-30
0-27
0-31
9-26
9 studies (28, 72, 94, 104, 108, 110-113)
25
0-15
0-45
*
7 studies (72, 89, 94, 101, 106, 112, 114)
*
*
1-41
*
1 study (61)
*
43-53
*
*
2 studies (104, 110)
9-33
*
24-28
*
2 studies (78,109)
*
*
15-22
8-17
1 study (72)
0-38
*
0-45
*
5 studies (28, 94, 106, 108, 115)
67
*Values not available from the specific test method
SIE: Selective infiltration etching, AlN: Coating with nanostructured alumina powder.
Definition of the different surface treatments
Table 12 shows the mean test values for the surface treatments
described below.
As-produced: Studies that have kept the cementation surface asproduced, some milled from pre-sintered or fully sintered blocks and
other pressed, according to the production process of the respective
manufacturer and material.
Grinding/polishing: Studies examining ground and/or polished
surfaces obtained using diamond wheels and/or various grit silicon
carbide paper (grit size range: 120-1,500 grit).
Airborne particle abrasion (sandblasting): This can be performed
using aluminum oxide, diamond, or boron nitride particles, with
particle sizes ranging from 25 to 250 µm, and various air pressures
and distances during blasting. It is one of the most common
mechanical surface roughening treatments.
Surface coating: The literature described a variety of coating
processes. Tribochemical silica-coating is a surface treatment that
forms a silica layer by means of airborne particle abrasion using
special silica-coated aluminum oxide particles. It is used either as a
combined surface treatment, for mechanical roughening and surface
coating. Tribochemical coating was one of the most widely used
surface treatments.
Other surface treatments included plasma spraying, glass fusing,
selective infiltration etching (SIE), and coating with nanostructured
alumina powder (AlN). Plasma spraying can be carried out using
various coating techniques, such as chemical or physical vapor
deposition, to apply plasma coatings, for example fluorine,
chlorosilane, hexamethyldisiloxane, or tin oxide.
Glass fusing is an internal coating technique which can be carried
out using a number of different materials, including commercial
porcelain, ceramics or experimental overglaze material, before being
finished with porcelain fusing.
The infiltration technique (SIE) is a coating technique followed
by an etching procedure. A thin layer of glass conditioning agent
is sprayed onto the cementation surface of Y-TZP and then heated.
The molten glass diffuses at the grain boundary regions. Excess glass
is subsequently removed with HF prior to the cementation.
68
AlN technique, in brief, involves coating the restoration with
nanostructured alumina powder (nitride of aluminum) by immersing it
in a solution in which the dispersed AlN powder begins to decompose,
forming a nanostructured boehmite coating on the surface.
Laser treatment: Laser etching can be done with carbon dioxide
(CO2), neodymium-doped yttrium aluminum garnet (Nd:YAG) or
erbium-doped yttrium aluminum garnet (Er:YAG). The surface of
the oxide ceramic is irradiated with lasers to roughen it in order to
create micromechanical interlocking.
Acid treatment: HF or phosphoric acid treatment prior to
cementation is the most common acid treatment. It is included in
the cementation procedure as a cleaning method and is not always
specified as a surface modification per se. Some studies evaluated
an experimental etching with heat or chemical etching with alkaline
hydroxylation.
Primer treatments: The majority of the reviewed studies used an
adhesive cementation system that included a primer. Some studies
evaluated uses of primer, including commercially available primers
and experimental primers, in combination with heat or flame
treatment.
69
DISCUSSION
Three in vitro studies and a systematic review examined both
general and specific knowledge regarding surface treatment of oxide
ceramics, as well as the bond strength between them and adhesive
cement systems.
Methods: In vitro studies
In vitro studies are often used to evaluate materials or designs of
restorations and are relatively efficient in terms of time and cost
compared to in vivo studies. The purpose of in vitro studies is to
be able to test dental materials and specific factors in a controlled
laboratory environment, with methods that are standardized and
mimic clinical situations as closely as possible (116). The goal of
laboratory testing the materials is to be able to correctly predict
clinical performance before materials are tested and evaluated
in a clinical situation. In vitro studies have several limitations,
however, arising from the fact that material testing in a laboratory
is simplified and does not fully capture the complexity of the
physical and mechanical properties of substructures, the design of
the restoration, the biological environment of the oral cavity, or
the loads of chewing. This makes direct comparison of laboratory
results to clinical situations difficult. Rather, in vitro studies should
be seen as a step in predicting clinical performance (57, 117, 118).
70
The choice of included materials
Oxide ceramics - aluminum oxide and yttrium oxide stabilized
tetragonal zirconium dioxide
The clinical applications of oxide ceramics have increased in
comparison to PFM. Due to material and process developments
of oxide ceramics, aluminum oxide-based restorations have been
replaced by stabilized zirconium dioxide-based restorations. Looking
ahead, there will likely be further development of translucent Y-TZP
and its dental applications, for instance in multiple FDP units (35)
and RBCBs (53).
The blocks and the cylinders that were the specimens in Study
I were all made of densely sintered aluminum oxide. This choice
was made to simplify the study design. By having the same oxide
ceramic material, both ceramic surfaces could be tested equally
(119). The literature shows that it is more common to use different
materials, such as composite cylinders bonded to oxide-based
ceramic substrate (58, 61, 76, 78, 87, 89, 94, 103, 104, 109, 111,
112). One advantage of having composite is that the bond in the
interface between adhesive cement system and composite is well
established. Another benefit is that the composite is easy to handle
and specimens can be produced easily. One of the drawbacks is that
each material has a different elastic modulus, which influences the
interfaces and the received load, and affects the bond strength and
the type of failure. Bond strength tends to increase with the use of
materials with high elastic modulus (119, 120).
Study II presents a novel method for creating a bondable
cementation surface on Y-TZP. The method uses modified-additive
production in conjunction with two different impaction mediums.
The use of the glass medium was based on the knowledge that a
bondable surface is created by etching glass-containing ceramics (24,
25). Using HF on feldspathic porcelain and glass ceramics creates a
rough, mainly crystalline surface with pits and microlacunas. The
surface glass is almost completely removed but the crystal phases
are not markedly affected by the acid and hence remain substantially
unchanged after etching. The etched surface enhances retention
by interlocking with the cement, thus creating micromechanical
retention while the small portion of glass residues on the surface
enhances the chemical bond between the cement and the ceramic
71
surface (24, 48). The glass granules themselves can be etched and the
glass-modified Y-TZP surface can be treated as a glass-containing
ceramic. To evaluate if micromechanical retention was sufficient
or if chemical retention was needed as well, polymer granules were
used as another medium. The specimens consisted of blocks made
of feldspathic porcelain and cylinders made of Y-TZP. The choice
of feldspathic porcelain was based on its large proportion of glass,
which can be etched to create better conditions for micromechanical
retention and thereby a strong adhesive bond (24). The feldspathic
porcelain was intended to act as a reference for the tested surfaces
of Y-TZP. Where the feldspathic porcelain fractured, however,
there were cohesive failures. Thus, the achieved bond strength was
higher than the cohesive strength of the porcelain. Both parts of the
specimen could have been made of the same material, as in Study I.
Study IV followed up and further developed the results from
Study II, evaluating surface-modified Y-TZP, in particular the
glass-modified Y-TZP. The study investigated the influence of the
glass granules and the effect of etching the cementation surface on
chemical composition and strength of the surface-modified Y-TZP.
Using glass granules embedded prior to sintering meant the glass
would melt but still be present after sintering. This glassy surface
must be removed before cementation to allow micromechanical
retention. The etching with HF may leave small amounts of glass
residues in the surface structure where the etch fails to act. This
residues promotes chemical retention between the glass-containing
Y-TZP surface and the primer, and thus further increases bond
strength beyond the micromechanical retention created (24, 48, 93).
The glass residues may also influence the physical properties of the
Y-TZP material, especially if phase transformation of the material’s
crystals occurs (31, 40, 42). The etching procedure was done both
before and after the sintering of the untreated and glass-modified
Y-TZP. With etching before the sintering of the glass-modified
Y-TZP, the sintering process could be more beneficial because of
a more homogeneous sintering of the ceramic powder. It resulted
in a dense material for which no regard to the difference in the
coefficient of thermal expansion (TEC) between the glass medium
and the Y-TZP needed to be taken. The difference in the TEC of the
materials might influence the properties of the final restorations by
creating inherent stresses and weaken its mechanical properties (29).
72
Adhesive cement systems
Study I investigated six adhesive cement systems which, at the time
of the study, were recommended by various cement manufactures for
adhesive bonding to oxide ceramics. Adhesive cement systems vary
in composition (one-paste or two paste systems), curing (chemical,
light-cured, or dual-cured), type and amount of fillers (nano, micro,
and hybrid) and viscosity (low, medium, or high). Other differences
include the physical and chemical properties of the cement, which in
turn depend upon degree of conversion in the cement (how many of
the reactive sites on the polymer chains were activated as the cement
set) (121). Five of the chosen adhesive cement systems were dualcured, of which four were Bis-GMA-based and one, Panavia™ F,
was a MDP-containing resin cement. The sixth, RelyX™ Veneer
Cement, differed from the other cement systems in that it was lightcured (Table 3).
Study II tested the two cements, Variolink® II and Panavia F™2.0,
that showed superior results in Study I. Both cements are well
documented and have shown stable bond strength to oxide ceramics
compared to other adhesive cements (52, 55, 94). Variolink® II is
considered to be primarily for use with feldspathic porcelain or glass
ceramics (122). The blocks of feldspathic porcelain combined with
the use of Variolink® II could serve as reference for the surfaces to
be tested. The hypothesis that the surface of glass-modified Y-TZP
contains glass also suggested choosing Variolink® II. The phosphate
monomer-containing Panavia F™2.0 system, primarily developed
for metal restorations, has shown that it bonds chemically to metal
oxides and oxide-based ceramics (5, 55, 56, 64).
The choice of cement systems in Study IV was based on previous
studies, which showed that the MDP-containing cement Panavia
F™2.0 bonds to oxide-based ceramics (55, 56, 64). This cement
was used partly to investigate whether an adhesive bond could be
achieved on an unmodified surface compared to a modified surface,
and partly to seal the surfaces, in particular the glass-modified
surface, in order to reinforce the material (7, 101). Since LTD starts
at the surface, surface cementation might slow the aging process and
maintain the mechanical properties of the oxide ceramic (29).
73
The choice of processing and surface treatment
Processing technology
The specimens in Study I were industrially manufactured by CIP
technology under controlled conditions with the optimal material
properties for a dental restoration. In Studies II and IV, the
specimens were manufactured by uniaxial pressure in a modified
additive production, since the production was a methodological
development, with different mediums embedded into the surface
of Y-TZP, aiming to create a bondable surface of oxide ceramics.
During uniaxial pressing, the distribution of pressure on the ceramic
powder might have been uneven, especially against the walls of the
mold, where areas of low particle packing result in an anisotropic
material (50, 51). This could have been avoided with isostatic
pressing, in which the pressure is applied from all directions, resulting
in a more homogenous and dense material. Probably, the mechanical
properties would have been better and the biaxial flexural strength
higher (123).
Pretreatment of the cementation surface and cleaning process
In Study I, the cleaning process and surface treatment of the
cementation surface of the blocks involved first cleaning with
acetone, then sandblasting with 110µm Al2O3 particles, and finally
ultrasonic cleaning in 96% isopropanol. Only the cylinders were
subjected to the cleaning process. This was done for all specimens;
for specimens in the groups with the Panavia™ F system, the cleaning
process also included phosphoric acid treatment. In Studies II and
IV, all cementation surfaces of the specimens (including blocks,
cylinders and discs) were etched with HF. This replaced previous
cleaning process from Study I, partly because it is the standard
procedure for etching feldspathic porcelain and partly because HF
has been shown to be effective in cleaning the cementation surface
of ceramic restorations. Choosing HF cleaning also allowed all
cementation surfaces to be etched and cleaned in the same way,
whether feldspathic porcelain, glass-modified Y-TZP, or Y-TZP.
In Study II, after etching with HF, the cementation surfaces were
cleaned with phosphoric acid before cementation. The reason for
this cleaning was to mimic the clinical situations, where restorations
need to be cleaned of contaminations after try-in in the mouth
74
(124-127). At the time of testing, using phosphoric acid to clean the
cementation surface was the clinical procedure recommended by the
manufacturer. Phosphoric acid cleaning was excluded from Study
IV, however, because of its potential negative effect. Phosphoric acid
might react irreversibly with the zirconium dioxide surface, creating
a layer of zirconium phosphate that might inhibit adhesion of the
primer to the ceramic. Another explanation is the difficulties in
eliminating acid residues from the cementation surface (127).
Air borne abrasion
To achieve a durable bond between a ceramic restoration and cement,
it is necessary to enlarge the cementation surface for micromechanical
retention (24, 55). With unmodified oxide ceramics the etching
technique is insufficient to accomplish this. Techniques to increase
the surface roughness, and consequently the bond strength, of oxide
ceramics are often restricted to machining, grinding or airborne
particle abrasion. Air borne particle abrasion with Al2O3 particles
is the most widely used surface treatment method in dentistry and
is considered an effective method of achieving micromechanical
retention in oxide ceramics to the cement or to the veneering
porcelain (5, 55, 56, 90).
The literature suggests two main retentive mechanisms for stronger
bonding between ceramics and adhesive cement systems. First, if a
pore is close to the surface, the material between the pore and the
surface may break during sandblasting, creating a surface structure
that enhances mechanical interlocking (128). Second, sandblasting
removes contaminations from the surface, which also increases
bond strength (94, 125). Sandblasting also has disadvantages, as it
may tear the surface and cause surface damages, grain pullout and
material loss (57, 76, 83-85). If such flaws induce surface cracks,
they could act as fractural impressions and cause an insidious
weakening of the ceramic (129). In the case of Y-TZP, a local phase
transformation may occur which initially increases fracture strength
but lacks transformation toughening over time, reducing the strength
(84, 85, 123, 130, 131).
In Study I the blocks were blasted with 110µm Al2O3 particles
for 10 seconds with an air pressure of 5 bars and a distance of 100
mm, with gentle movements of the blasting nozzle perpendicular
75
to the surface, in accordance with the manufacturer’s instructions.
Some studies have suggested that blasting ceramic surfaces with 50
µm particles would allow sufficient micromechanical retention (20,
63, 87, 94, 132). Results from other studies, however, indicate that
a smaller particle size, such as 50 µm, polishes rather than enlarges
the surface (133, 134). Several investigations have tested air pressure
between 2.5 and 2.8 bars at a distance of 10 mm between the blasting
tip and the object (20, 63, 87, 94, 132). Having an air pressure of 5
bars may have influenced the results by creating a rougher surface
and a larger area that might improve the bond strength. On the
other hand using > 4 bars air pressure when sandblasting may tear
the surface and induce flaws and cracks (131, 135, 136). By having
a distance of 100 mm however, the force of sandblasting may be
similar to the force applied when using lower air pressure closer to
the surface. In study IV the specimens were sandblasted with 110
µm Al2O3 particles for 10 seconds with an air pressure of 2 bars at
a distance of 10 mm, with gentle movements of the blasting nozzle
perpendicular to the surface, in accordance with the manufacturer’s
instructions. The findings of the systematic review also supported
this methodology.
Surface modification with additive impaction technique
Both the surface modification mediums tested in Study II, glass
granules and polymer granules, were embedded in the ceramic
surface during green-stage pressing, thus aiming to create a basis
for a surface structure suitable for bonding (98). When using glass
granules embedded prior to sintering, the glass will melt and partly
remain on the surface after sintering. This glassy surface must be
removed prior to cementation, which is done by etching the surface
with HF. The possible residues of glass left on the cementation surface
may enhance the chemical retention between the glass-containing
zirconium dioxide surface and the adhesive cement system, thus
increasing bond strength in addition to the micromechanical
retention created by etching. Surface modification may also affect
the physical properties of the Y-TZP material, especially if phase
transformation of the crystals occurs in the material. (84, 85, 123,
130, 131, 137, 138). Study IV further investigated these issues of
surface modification.
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When polymer granules are used, the granules burn out
during sintering, leaving a surface structure with ready-made
micromechanical retention. The burn-out process is not clean,
however, since contaminants could be seen on the treated surface
after sintering. Those contaminants may have remained after etching
with HF, which could affect subsequent bonding. The results in
this study however, did not verify this assumption. The choice to
investigate only the glass medium in Study IV was because it likely
enables chemical retention, while the polymer medium only provides
micromechanical retention.
Cementation process
All in vitro studies used the same cementation procedures, in
accordance with the suggested methods of the manufacturer. Using
an alignment apparatus with an applied seating load of 15 N
during polymerization ensured that the axes of the specimens were
perpendicular and that the load for all specimens was standardized
during polymerization (139). The cement was light-cured with
different curing lamps in each study. Each study used the curing lamp
available at the facility at the time of the study. The polymerization
light intensity in Study I was 300 mW/cm2 (mean light intensity)
and the curing time was 40 seconds in each of four directions, 90°
apart, and a final 60 seconds in one direction with the seating load
removed. In Study II, the polymerization light intensity was 1100
mW/cm2 and in Study IV, 1000 mW/cm2. In both these studies, the
curing time was set to 20 seconds in each of four directions, 90°
apart, and a final 60 seconds with the seating load removed. An
overly short curing time may give the impression that the cement has
completely set when only the outermost has initially hardened, and
thus fail to fully utilize the properties of the cement system (121).
The sufficient light intensity and curing time for each study was
determined according to the manufacturer’s recommendations
The maximum accepted clinical value for cement thickness of
adhesive cement systems has been debated for many years. In theory,
the thickness should be from 30 to 50 µm, but clinical studies of
accuracy, combined with longevity data, indicate that the cement
may seal wider internal gaps between the tooth and restoration and
function for many years. Improper cement thickness, may trigger
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technical or biological complications that limit the durability of a
restoration (6, 25). No measurements of cement thickness were made
which means that this is an unknown variable. However, because all
specimens underwent the same cementation procedure, there was
likely little variation in cement thickness between specimens treated
with the same cement system.
Artificial aging process (Water storage, TC, mechanical pre-load)
It is generally acknowledged that environmental influences in the
oral cavity can affect the durability of the bond strength. Artificial
aging processes attempt to mimic the degradation that orally placed
restorations undergo by decreasing the strength of the materials and
their bond (56, 57, 64, 93, 140, 141). There is, however, no consensus
regarding an appropriate procedure for aging (93, 140). Studies I
and II conducted artificial aging using TC. Between tests, Study I
specimens were stored in water at 37°C while Study II specimens
were stored in water at RT (approx. 20°C). This water storage was
primarily to prevent desiccation of the cement and was not aimed
to be an artificial aging process. TC is preferred when the interfaces
can be influenced by thermal stress and reduction in bond strength
can be observed (24, 58, 64, 142). However, the extent to which
TC mimics clinical situations has been questioned. Temperature (5°55°C), dwell time, and the size, number and materials composition
of specimens in the TC are also some topics that have been discussed
in the literature (57, 140). The difference of approximately 50°C
between the two baths may not be great enough to affect the
ceramics, but affect the interfaces of the included materials (141).
Nevertheless, a wet environment may affect the ceramics, which
are susceptible to mechanical degradation in the presence of water
when combined with stress corrosion at a crack tip. Fatigue and
subcritical crack growth reduces the strength of the material and
can result in fracture loads that are lower than the original strength
of the material (55, 143). Other artificial aging methods, such as
fatigue with dynamic cyclic loading, have been suggested and tested
when evaluating bond strength (57, 114, 144).
The dynamic cycling fatigue method, or mechanical preload, is
preferred when evaluating the flexural and fracture strength of a
material (143, 145). In fatigue testing, it is important to describe the
78
applied load, the number of cycles, and the rate of application of
the load. Study IV achieved its fatigue process by cyclic preloading
in a wet environment, followed by TC, which are accepted and
commonly used methods in in vitro studies (93, 131, 143, 145).
Dental ceramics are susceptible to slow crack growth, and cements,
to degradation. During cyclic preloading, cracks tend to propagate
even under a small load due to stress corrosion at the crack tip (143,
145). The applied load and the time of the test cycle can vary (131).
The load interval used in Study IV was 10 to 100 N. This interval
was chosen to avoid the risk of fracture during preload.
The choice of tests
Bond strength definition and tests
Bond strength can be tested using various test methods, each with
advantages and disadvantages. Recommendations for choice of
adhesive cementation of oxide ceramics and adhesive bonding
systems are mainly based on in vitro tests (117, 118, 146). These
test methods vary and include tensile/micro-tensile (TBS/µTBS)
bond strength test, shear/micro-shear (SBS/µSBS) bond strength
test, and pullout tests (61). The most commonly used methods for
testing bond strength, including micro-tensile and micro-shear tests
(120, 146). Variations in bond strengths are large, with mean values
ranging from debonding to above 50 MPa, and depend on the chosen
test method. Results are not directly comparable, but they give an
indication on the obtained bond strength (117, 120, 146). Studies
I, II and III defined bond strength as clinically sufficient, regardless
of the test method used, when the values were ≥ 20 MPa. From a
clinical perspective, it is difficult to set a standard and define what
constitutes sufficient, adequate, and durable bond strength. The
value of 20 MPa has proved clinically sufficient, compared to the
lower limit of the bond strengths between resin and enamel (100).
The advantage of the SBS test are that it is suitable for evaluating
adhesive and restorative materials and is considered to be a user
friendly test method (120). The test’s reliability, however, is
questionable (117, 119, 120). Stresses developing at the bonded
interface have been shown to be more complex than the calculated
load at failure. In the shear test, stresses close to the loading area are
much higher and more complex than pure shear load. The test load
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involves compressive as well as tensile stresses (117, 119, 120). The
SBS test generally shows lower bond strength values than the TBS
tests (118).
The advantage of the TBS test include that little material is
consumed when the test is performed and that the stresses may be less
complex, that is, allowing for a more even distribution of stress. One
of the drawbacks is the production of the specimens and attachment
during testing, since it may produce uneven stress distribution and
affect the results (120). The use of microspecimens and the µSBS or
µTBS tests have been developed to overcome these drawbacks (147).
Results from previous studies indicate that a smaller bonding area
provides higher bond strength values (117, 118, 120). This can be
explained by the Weibull distribution, which shows that an increase
in bond area increases the probability of encountering strengthlimiting flaws and that a specimen’s flaws are size dependent (116).
There are many different testing parameters, with different
designs and materials of crosshead and loading conditions (117,
118). Studies I and II used a crosshead shaped like a knife-edge
chisel for their SBS test to evaluate bond strength. The knifeedge introduces a bending moment and, in cylinders made of low
modulus material, failure typically starts at the load site (148, 149).
The interface between the ceramic, the adhesive cement system and
the composite does not deform plastically to any great extent during
such tests. In Studies I and II, the cylinders were made of a high
modulus ceramics, where a bending moment also was present, but
deformation during loading will occur in the cementation interfaces
(149). The use of a knife-edge chisel crosshead concentrates stress
at the load application area. Crossheads with a wire loop design
have shown better stress distribution at the edge of the bonding
area. There is other crosshead design as well, such as a notchededge shear bond strength test that may produce more even stress
distribution at the interfaces. Stress concentration with the use of
the chisel may explain the small areas of cohesive failure that was
found close to the loading point. It may also explain the lower bond
strengths when the chisel crosshead is compared to crossheads with
larger contact areas. Crosshead speed does not seem to influence
bond strength values (120). The crosshead speed used, was within
the limits suggested by the ISO/TS 11405:2003 (96).
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Types of Failure
Study I provided a workable definition of the different types of
failure. Adhesive failures occur between the different interfaces e.g.
ceramic-cement-ceramic. Cohesive failures occur within either the
ceramic materials or cement systems. In mixed failures, both adhesive
and cohesive failures are found on the bonded area (117). In Studies
I and II, the type of failure in debonded areas was determined by
whether or not the ceramic surface was free of cement. However,
classifying types of failure is difficult, especially for mixed failures.
Classification of failure type remains an issue as there is no uniformly
accepted classification system (118).
Surface analysis and characterization
Microscopy. In the in vitro studies (I, II and IV), the cementation
surfaces of the specimens were examined under a light microscope
during every step of the fabrication process. This examination
checked if there were any remnants or other particles, such as dust,
and whether the surface looked homogenous. Studies I and II also
used light microscopy to perform fracture surface analyses of the
bonded areas.
Interferometry (IFM) and Atomic force microscopy (AFM).
Topographical analysis using IFM and AFM characterized the
surface roughness at the micrometer level (Studies II and IV) and
nanometer level (Study IV) (42, 48, 62, 99, 118). Neither IFM nor
AFM require specific sample preparation and neither are destructive
in comparison to SEM (42). There are several parameters available,
and three were selected according to the proposed guidelines for
biomaterial surface characterization (99). The surface modification
technique aiming to modify the surface at micrometer level would
likely affect topography at the nanometer level as well. Therefore
the surfaces were examined using both IFM and AFM.
Scanning electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDS). In addition to characterizing the surface
topography at the micrometer and nanometer levels, it was also
performed a morphological examination in Study IV that used SEM
in combination with EDS to describe the atomic composition.
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X-ray diffraction (XRD). In Study IV, XRD was performed to analyze and identify crystalline structure. Using EDS and XRD technologies made it possible to analyze the chemical composition of
the ceramic surface and detect any remnants of the impaction medium or possible phase transformations (tetragonal → monoclinic)
in the Y-TZP that could weaken the material (123, 131, 137, 138).
Biaxial flexural strength test- load until fracture
One possible drawback of the surface treatments is that mechanical
properties, such as strength, of the oxide ceramics may be affected.
The impaction-modified technique may leave remnants of the
impaction medium and induce production-related flaws that may
act as fractural impressions (82-85). This was one of the issues
investigated and evaluated in Study IV using a biaxial flexural strength
test, conducted in accordance with ISO 6872:2008 (18). This type
of test can, in a controlled environment, provide information on
basic mechanical properties, such as fracture strength, and compare
different materials. Flexural strength can be measured in a threepoint flexure test, a four-point flexure test, or a biaxial flexure test.
In all cases, the load applied increases until fracture. The values
of flexural strength obtained with the four-point flexure test are
generally lower because surface cracks or critical flaws are more
likely to develop between the two loading pistons than in the more
limited area of a three-point flexure test. In the biaxial flexure test,
in which a disc is loaded in the center, the probability of edge failure,
which also depends on surface finish of the specimen, is reduced
compared to three- or four-point flexure tests (41, 145).
Methods: Systematic review
Systematic literature reviews should systematically collect and
summarize data, and provide results and conclusions in a transparent
way based on well-defined questions, and set criteria, protocols, and
evaluation processes (116, 150). Lack of consensus was found in
in vitro studies evaluating bond strength regarding the appropriate
testing methodology, interpretation of result, and application of
clinical outcomes. Shared protocols for in vitro studies could ease
comparison between study results (116, 118, 120).
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Study design
In a literature review, it is important that the aims and questions
are well defined. This makes inclusion and exclusion criteria easier
to set, streamlines summary of results, and allows more meaningful
conclusions. To address the aims and questions of the literature
search, Study III sought literature on all possible surface treatments
of oxide ceramics to promote bonding and the bond strength results
from different test set-ups. The inclusion criteria in Study III were
set to include oxide ceramics, with no consideration given to the
adhesive cement system used or whether the materials and surface
treatments were commercially available or purely experimental.
These criteria made a general inventory possible.
To address Question 2 (Do any of the methods provide sufficient
bond strength for the retention of dental restorations without
the need for macromechanical retention?), it included studies on
artificial aging and evaluating the ceramic surface before and after
bond strength testing. Initial bond strength tests often show higher
strength values than after artificial aging (56, 64). If no artificial
aging has been conducted, then it is difficult to evaluate and apply
the bond strength values to bond strength in vivo. The surface
analysis and the failure types are also an evaluation of the bond
strength test method, and light microscopy analysis only was not
considered sufficient (117, 118).
The literature search was limited to only one database, PubMed.
It may have been advantageous to perform the literature search
in other databases, such as ScienceDirect (Elsevier), as well.
Furthermore, each of the two aims and questions should preferably
have been answered by two separately literature searches. Clinical
relevance is lacking because the included studies are all in vitro
studies. Advantages of Study III include its design: two investigators,
calibrated in accordance with set criteria, conducting the literature
search, reviewing the literature, and choosing the studies to include.
Results
The results of the systematic review showed that there is a wide variety
both of surface treatments for oxide ceramics and of test methods
that evaluate the bond strength between oxide ceramics and adhesive
cement systems. This makes it difficult to compare studies and make
recommendations of optimal surface treatment of oxide ceramics.
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Surface treatments
Seven categories of surface treatments were defined from the
literature: as-produced, grinding/polishing, airborne particle
abrasion, surface coating, laser treatment, acid treatment, and primer
treatment. Each surface treatment category included several specific
surface treatments, some of which are commercially available and
others experimental. In the included studies, three surface treatments
showed bond strength values exceeding 20 MPa: airborne particle
abrasion with aluminum oxide particles; (28, 72, 94, 104, 108, 110113) airborne particle abrasion in combination with silica-coating,
such as tribochemical silica-coating; (72, 89, 94, 101, 106, 112,
114) and a primer in combination with an abrasive or silica-coating
surface treatment (28, 94, 106, 108, 115) (Table 12). The results
from Study III indicate that micromechanical retention is necessary
to achieve a durable bond between oxide ceramics and an adhesive
cement system.
In almost all the studies included in Study III, airborne particle
abrasion used aluminum oxide particles with a particle size ranging
from 50 to 125 µm. This may be because it is an accessible, timeefficient, and economical method to use (55). Several studies show
that it is an effective cleaning method and influences higher bond
strength (125-127, 151, 152). Alternative airborne abrasion particles,
such as synthetic diamond particles, have also been evaluated and
show high bond strength values (112).
Silica-coating, more specifically tribochemical silica-coating,
suitable for glass-infiltrated oxide ceramics, but it appears to be less
suitable for oxide ceramics such as aluminum oxide and stabilized
zirconium dioxide, resulting in reduced bond strength after
artificial aging (33, 55). However, in Study III, surface treatment
with tribochemical silica-coating showed sufficient bond strength
values, regardless of test method (72, 89, 94, 101, 106, 112, 114).
This indicates that tribochemical silica-coating also was a surface
treatment that could be used to achieve a durable bond between
oxide ceramics and adhesive cement systems.
The majority of the included studies used an adhesive cementation
system that included a primer. The literature indicated that there
is interest in finding a mechanical or chemical surface treatment
that combines with a primer (55). The rapid development of new
adhesive materials means that there are a variety of primers and
84
various ways of applying them to the cementation surfaces (91). The
components and compositions of the primers found in the included
studies were not record.
The production process of the glass-modified surface in Studies
II and IV did not seem to induce any of the surface damage that
has been identified after machining or after airborne particle
abrasion. The modified surfaces showed irregularities, probably
as a result of uneven distribution of the medium prior to pressing.
This implies that the coating procedure could be refined, though
the bond strength achieved already appears to be sufficient. When
using the impaction-modified technique, the cementation surface
becomes more comparable to glass ceramic. Even if microcracks
and flaws are present after sintering, they are likely reduced by the
etching procedure. HF seems to dissolve the glass components and
produce a more smoothed, but still irregular surface, with both
micromechanical and chemical retention that facilitates adhesion of
the adhesive cement (24, 93).
In Study IV the cement remained intact on the glass-modified
surface after the artificial aging procedure, indicating a stable bond
between Y-TZP and adhesive cement. Furthermore, the results show
that the cement had an enhancing effect, probably due to the sealing
of the surface, reinforcing the material (11). Also, after the artificial
aging procedures, the unmodified Y-TZP and the sandblasted Y-TZP
showed areas with loose cement and air pockets throughout the
cementation surface. This was not found in the glass-modified Y-TZP
groups. These findings suggest that the cementation surface of the
restorations needs a surface treatment that enhances more than just
micromechanical retentions (138). The benefit of the modified-additive
production technique is that it seems to achieve both micromechanical
and chemical retention, producing durable bond strength between the
surface-modified Y-TZP and adhesive cement system.
Shear bond strength: How does surface treatment
affect bond strength?
The mean values for the strength test from the studies (n=23)
included in Study III, range from debonding to 45 MPa for the SBS
test, from 8 to 26 MPa for the µSBS test, from debonding to 38 MPa
for the TBS test, and from debonding to 53 MPa for the µTBS test
(Table 12). It is difficult to make any statements based on the results
85
of the studies because different materials were tested with differing
methods and scattered results.
Even though the results from reviewed studies with the same
design and materials as Studies I and II were widely spread, the
achieved results fall within a same range (76, 78, 89, 101, 102, 106,
107, 112). The values achieved for Variolink® II and Panavia™ F
in Study I agree with other studies (94, 113) and indicate that these
might be sufficient for adhesive cementation. Some clinical studies
have used these cement systems for adhesive cementation of densely
sintered aluminum oxide or Y-TZP (153-156). In Study II the
bond strength in the unmodified groups was reasonably high (1820 MPa), considering that no surface modification was done. But
bond strength in the surface-modified groups was higher, regardless
of which cement that was used. The cementation surface that was
modified with glass granules and cemented with Variolink® II showed
the highest mean bond strength (35 MPa). This suggests that it may
be a glassy phase left on the glass-modified Y-TZP surface, as for
feldspathic porcelain and glass ceramics, that enhances chemical
bonding between the cement and the ceramic surface (104, 110).
The results from Study II are sufficient for clinical use if compared
to conclusions reached in previous studies, where the values for
SBS range between 15 and 30 MPa (76, 78) . Even though it is not
possible to compare numerical values from different studies, the
results, if restricted to shear bond strength only, support the use
of the investigated method for restorations such as veneers, onlays
and RBCBs. Clinical studies, however, are needed to confirm the
findings of in vitro studies. There are limited data on the clinical
outcome of adhesive bonding to oxide ceramics (32) and only a
few studies on oxide-based RBCB (54). Independent of the adhesive
cement system, Sasse et al. showed promising results, during the 3
first year with a small number (n=25) of single-retained stabilized
zirconium dioxide- based RBCBs. (53).
Types of Failure
When there are cohesive failures in the substrate materials, the
validity of the reported bond strength is questionable. Only adhesive
failures or mixed failures with small areas (less than 10%) should
be considered (117, 118). Cohesive failure is explained by the
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mechanics of the test and the brittleness of the materials used and is
more frequent for macro- than micro-bond strength tests (118, 120).
In Study I, the predominant failure type of some cements (Variloink®
II, Bisco Illusion™, RelyX™ Veneer) was adhesive at the untreated
surface. It is possible that higher bond strength could have been
achieved if the untreated surfaces of the cylinders had been treated
as the blocks. For the remaining groups (Bisco Choice™, Nexus™ 2
and Panavia™ F) the failure type was adhesive at the treated surface
or at both surfaces. It may be that surface treatment is irrelevant
for some adhesive cement systems and important for other systems.
The cohesive failures in Study II can be explained by tensile stress
in the feldspathic porcelain that caused a propagation of the crack
into the substrate, leading failure in the feldspathic porcelain (120).
The surface-modified groups showed less debonding between the
cement and the Y-TZP surface than the two control groups, where
all the failures were adhesive. It is noteworthy that the groups with
adhesive failures had lower bond strength and surface roughness.
The classification of the various failure types, in particular
complex failures, might have been more accurate if the examination
had been done under greater magnification, such as SEM-analysis,
revealing remnants that are undetectable with microscopy (117). It
may be that the fracture types in Studies I and II were sometimes
more complex than classified. If that is the case, the measured values
are dependent on the mechanical strength of the cement rather than
the bond strength between oxide ceramic/cement or cement/oxide
ceramic or feldspathic porcelain.
Shear bond strength: How do adhesive cement systems
affect bond strength?
The use of adhesive cement systems including a primer appears to
have improved bond strength in Study I. Coupling agents such as
adhesive cement consist of bifunctional molecules that function
between the inorganic ceramic surface and the organic polymer
matrix of the resin. The lack of glass or silica-containing matrix on
the material unable the chemical bond created by silanization (55,
57). The exact bonding mechanisms of silane to oxide ceramics are
still unclear. A durable bond between densely sintered aluminum
oxide or stabilized zirconium dioxide and adhesive cement systems
87
(combined with their corresponding primers) can be achieved,
especially with MDP-containing adhesive cement systems (20, 58,
64, 94, 157). Some authors recommend that adhesive cements
containing MDP can be used without primer (87), whilst others
suggest that the use of a primer enhances the wetting of the surface
and increases the cooperative interaction between the interfaces (20,
58, 93-95). All in vitro studies (I, II and IV) used the corresponding
primers for each cement system. The use of a primer and its action as
wetting agent, may explain some of the results in Study I, where the
MDP-containing cement Panavia™ F, and Variolink® II, a Bis-GMAbased cement, had the highest bond strength values, irrespective of
pretreatment (20, 63, 94, 158). Of the six different bonding systems,
only two Nexus™ 2 and RelyX™ Veneer, decreased significantly
(p<0.05) in bond strength after TC. Variolink® II showed higher
bond strength after TC, where TC may have acted as a catalyst
and promoted continued curing of the adhesive cement. A possible
explanation of the low bond strength values for the adhesive cement
system RelyX™ Veneer may be that it is a one-paste system that is
light cured only. If the aluminum oxide cylinders partly hindered
light from reaching the cement, there might have been poorly cured
areas. The quality of the transmitted light could have influenced the
conversion and, consequently, bond strength (121). Differences in
compositions between the various cements, such as filler particles
and other chemical additives, could also have influenced the level of
conversion (121).
Biaxial flexural strength: How does surface treatment
affect flexural strength?
The results for flexural strength of unmodified and sandblasted
Y-TZP in Study IV were in accordance with ISO values for the
substructure of non-adhesively cemented restorations (18), but lower
than in other studies (41, 51, 131, 137, 159). The specimens were
fabricated with uniaxially applied force and the results are not directly
comparable with other studies due to variations in study design, such
as the material tested, fabrication process, use of artificial aging,
and loading conditions (41, 51, 131, 159). Glass-modified Y-TZP
showed a decrease in flexural strength compared to the unmodified
groups. The presence of a low-modulus glass phase on the surface
88
of Y-TZP could explain the results in Study IV. Glass-infiltrated
aluminum oxide and zirconium dioxide have shown comparable
results, where the glass matrix will give lower mechanical properties
(41, 47). The groups submitted to etching before sintering showed
higher values than the groups submitted to etching after sintering.
This may be because the glass medium that was etched away before
sintering enabled the grains to sinter together. In the group with
etching after sintering, the glass medium seemed to prevent sintering,
leaving voids between the grain boundaries, which weakens the
material. The results from the surface analysis in Study IV amplify
this assumption, with voids/porosities apparent in the SEM images.
Comparing the unmodified and modified Y-TZP, the group that was
sintered and sandblasted showed the highest values, with an increase
in flexural strength when cement was included. Sandblasting may
have induced small amount of stress concentration at the surface,
causing phase transformation (83, 130, 131, 136, 160, 161) and
resulting in compressive stress that increased flexural strength (93,
130, 159-162). Sandblasting of the Y-TZP surface seems to provide
high initial value, (130, 131), but further investigations of how the
material will behave over time are needed.
Surface roughness and chemical surface composition:
How does the surface treatment affect material composition
and properties?
The combined use of IFM, AFM, SEM and XRD analyses is a
procedure employed to characterize a surface and the effect of
surface treatments (42, 48, 62, 131, 136, 137, 163). In Study II,
the analysis of the cementation surfaces made by microscopy and
IFM showed that each of the surface-modified groups had a surface
with increased roughness after sintering followed by etching in
comparison to unmodified Y-TZP. Study IV found similar results
when AFM analysis was also included. Unmodified Y-TZP showed
no change in microstructure, either before or after the sintering and
etching procedure. Airborne particle abrasion increased surface
roughness, creating microretentive grooves, but the surface also
showed a damaged and deformed surface as found in other studies
(84, 85, 123, 131, 136, 160). The SEM analysis exposed defects
(i.e., voids/porosities) likely caused by the fabrication technique.
89
Most of the fabrication techniques and every type of ceramic
have varying degrees of defects. The material should be handled
carefully to minimize these defects (48, 85). In the groups with glass
modification, the degree of sintering of the ceramic powder and the
formation of grain boundaries were different than for unmodified
Y-TZP. The glass between the grain boundaries prevented the grains
from sintering together, resulting in a porous material. This may
have happened because the glass melted during sintering and settled
in the intergranular areas. After the sintering process is complete, it
forms a void/porosity (50, 123). The presence of voids on the surface
increases surface roughness, which enhances micromechanical
retention. On the other hand, voids might also weaken the material,
depending on the location, size, shape and distribution of such
defects (47, 85, 123, 138).
Based on the XRD analysis results, two crystalline structures,
monoclinic and tetragonal, were predominant. Monoclinic phase
was identified after sandblasting the unmodified Y-TZP and even
more monoclinic phase in the glass-modified Y-TZP. This indicates
that, when glass is present, the Y-TZP becomes unstable and phase
transformation occurs (47). The same applies for the sandblasted
Y-TZP surface, although not in the same degree. These findings
agree with other studies (84, 131, 135, 159).
Clinical significance
There are few clinical trials available on adhesively cemented oxidebased ceramic restorations and their conclusions differ as to whether
they are a valid alternative to metal frameworks (32, 164, 165).
According to a systematic review, the survival rate of tooth-supported
and implant-supported stabilized zirconium dioxide-based crowns
is comparable to PFM crowns. One of the most common reasons
for technical failures or complications of tooth-supported crowns
was loss of retention (165). However, there is often no information
available on the possible causes of loss of retention, and several
different cement systems have been used in the trials, which makes it
difficult to draw general conclusions.
The development and use of oxide ceramics and adhesive cement
systems may provide an extended treatment option with toothsupported crowns and FDPs. Possible indications for use of adhesive
cementation techniques with oxide ceramics are for tooth preserving
90
treatment options, such as RBCBs, or minimally invasive treatments
in clinical situations with limited retention, such as moderate tooth
wear. (53, 54). In those cases the risk for unnecessary preparation of
the tooth and surrounding tissues is less. It may also benefit young
patients who are not yet eligible for implant treatment (53).
When choosing cement system, a dual or chemically curable
adhesive cement system may be the best choice. Several studies
suggest that MDP-containing adhesive cement systems enhance the
bond strength between oxide ceramics and cement, with or without
any surface treatment (55, 58, 59, 95, 163). Well-design longitudinal
studies with large patient groups are required before extending the
indication of treatment options.
Future investigations
Commercially available materials are usually tested according to a
set of standard tests to evaluate their suitability and performance as
dental materials. These standards are developed and published by
the International Organization for Standardization (ISO). The most
common test methods are in vitro, but in vivo studies are needed to
make clinical recommendations and statements. Test set-ups differ
currently, so the development and standardization of test methods is
an important component of future research. Studies that investigate
the effect of different artificial aging process on the various materials
and interfaces, that improve categorization of failure types, and
that validate findings for a reliable clinical application will all be
necessary in the future.
All-ceramic dental restorations have improved considerably in their
performance, and Y-TZP is the one with most preferable material
properties (5). Still, there is much to learn about oxide ceramics and
production of restorations. The main focus of development of surface
treatment for oxide ceramics seems to be for stabilized zirconium
dioxide. At present, the development of translucent Y-TZP and its
application could be of great interest (35). The basic composition
between traditional oxide ceramics and translucent Y-TZP is similar
(45), which means that the surface treatments that enhance the
adhesive bond between oxide ceramic and adhesive cement system
work for booth materials. Before making clinical recommendations
regarding glass-modified Y-TZP, it is necessary to conduct further
clinical studies to investigate and evaluate both the influence of the
91
medium on the cementation surface and its mechanical properties
throughout the entire fabrication process of the restoration. Any
surface modification or pretreatment can affect the properties of the
material, and those effects should be assessed.
92
CONCLUSIONS
Sufficient bond strength can be achieved to densely sintered
aluminum oxide by using a Bis-GMA based dual-cured resin cement
system or a phosphate-monomer containing resin cement system.
(Study I)
Recommendations on how to achieve a sufficient bond between
densely sintered aluminum oxide and adhesive cement system should
be based on the adhesive cement system used since bond strength
will depend on the specific system. (Study I)
The surface structure and the chemical composition of Y-TZP
ceramics produced using a glass-modification technique differ from
that of unmodified Y-TZP. The differences include a rougher surface
structure, superficial glass remnants and the presence of monoclinic
phase. (Study II and IV)
Etching the glass-modified Y-TZP before sintering creates a more
homogenous surface than etching after sintering. (Study IV)
Impaction modification with either glass granules or polymer
granules in an additive production technique can create a bondable
cementation surface suitable for Y-TZP-based restorations with a
shear bond strength exceeding 20 MPa. (Study II, IV)
As-produced oxide ceramics needs to be surface treated to create
durable bond strength. (Study III and IV)
93
Airborne particle abrasive surface treatment, with or without the
use of primer treatment, can create sufficient bond strength for the
bonding of oxide ceramics. The same applies for treatment with
silica-coating, but only with the use of a primer treatment. This
conclusion, however, is not yet confirmed by clinical studies. (Study
III)
For each specific ceramic material, consideration should be given to
surface treatment and choice of adhesive cement system in order to
achieve sufficient bond strength. No general recommendations can
be given. (Study III)
The flexural strength of Y-TZP decreased after glass modification,
but increased after cementation. Further studies are needed before
recommending the technology for clinical applications. (Study IV)
94
ACKNOWLEDGEMENTS
This thesis would not have been possible without all the inspiring
and generously supporting people around me. I wish to express my
sincere gratitude to everyone, and in particular I wish to thank:
Associate professor Per Vult von Steyern, my supervisor; for your
expert guidance in academic thinking, writing and performing. With
your never-ending enthusiasm and patience, you have motivated me
during all my years on the faculty to start and complete this work
and to continue the journey.
Professor Ann Wennerberg, my co-supervisor; for sharing your
knowledge with me and for encouraging and guiding me. Thank
you also for always making me feel welcome.
Associate professor Erik Strandman; who encouraging me to start
the journey.
Professor emeritus Krister Nilner; for always making time for me
and my questions, and giving me wise advice.
My co-authors:
Johan Zethreaus and Per-Åke Ransbäck; for your catching curiosity
and contribution.
Christel Larsson; for sharing your knowledge and for your
engagement. I look forward to future collaborations and challenges.
Madeleine du Toit; for sharing your knowledge, for your curiosity
95
on life, and for your engagement, from small issues to large, but
above all for being a real friend.
Associate professor Ryo Jimbo; for sharing your knowledge, for
all your support, and making me feel welcome and that I was one
of you, at work and in our spare time. I look forward to future
collaborations.
Bruno Chrcanovic and Associate professor Martin Andersson; for
sharing your knowledge, for your hard work and contribution.
Professor Ulf Örtengren; for valuable comments on the research
program during my half-time seminar.
All my dear colleagues, past and presents, at the Faculty of
Odontology, Malmö University, and especially the entire staff
of the Department of Materials Science and Technology and the
Department of Prosthodontics; thank you for all your help and care.
Among dear colleagues I would like to send a special thank you
to: Lars Olsson, Zdravko Bahat, Camilla Johansson and Ewa
Linderoth; for taking the time to support me and follow along on
my journey, and also for sharing your knowledge, work-related or
not. Especially Camilla and Lars for cheering me on during the hard
times with your notes, and for watching the TC. Håkan Fransson;
for helping me to check, re-check and double check as someone wise
taught me.
All great women who have inspired me, especially Camilla Ahlgren,
Christina Diego-Löfgren; thank you for sharing your experience and
knowledge, for all your support, for sharing ups and downs and for
being my friends. Kerstin von Bahr and Eva-Lotta Rymark; for your
thoughtful and precious comments on all subjects that matters in
life, and, of course all nice dinners!
Per Egevad; for valuable assistance with computer management of the
references and making the systematic review figures understandable.
96
Jenny Widmark; for valuable help in the field of publication rights.
Ulf Persson, Daniel Bengmark and Jonas Finnhult, starting up as the
group of “After Drill” which developed into great friendship; each
of you, thank you for all your support, cheering me on and saving
me in my panic - we will always have the Christmas baking spirit!
The staff at Serviceenheten: Peter, Roger, Ingvar, Rickard, Richard,
Steve and Ingemar; for always making things work in time!
Thanks to all present and past students at the undergraduate
education program in Dental technology; for your catching curiosity
and questions that encourage one to seek greater knowledge.
My fantastic friends; for all your love and support, especially to my
dear friend Caroline Linné Erixon; for our endless phone calls where
we discuss issues of all kinds and your encouraging comments and
advice.
My family, my brother Apostolos and his Kristina; for always
being there for me and being real friends, giving me distance from
my thoughts with insightful and encouraging advice. My parents;
for their endless love and making me believe in myself,: Μαµµα,
µπαµπα, σας ευχαριστω για την αγαπη και την υποστηριξη σας
και ο,τι προσφερατε για να φτασω µεχρι εδω.
Research grants were received from:
The Faculty of Odontology, Malmö University
97
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