Fracture resistance of different Zirconia three-unit posterior all-ceramic Fixed Partial Dentures

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1 Aus der Universitätsklinik für Zahn-, Mund- und Kieferheilkunde der Albert-Ludwigs-Universität Freiburg Abteilung Poliklinik für Zahnärztliche Prothetik (Ärztl. Direktor: Prof. Dr. J. R. Strub) Fracture resistance of different Zirconia three-unit posterior all-ceramic Fixed Partial Dentures INAUGURAL-DISSERTATION zur Erlagung des Zahnmedizinischen Doktorgrades der Medizinischen Fakultät der Albert-Ludwigs-Universität Freiburg Vorgelegt 2006 Von Kassiani Stamouli Geboren in Aigio, Achaia, Griechenland 1

2 Dekan: Prof. Dr. Christoph Peters 1. Gutachter: Prof. Dr. J. R. Strub 2. Gutachter: Prof. Dr. J. Hausselt Jahr der Promotion:

3 Index 1 Introduction 6 2 Literature review Ceramics Historical perspectives of ceramics Composition, properties and limitations of dental ceramics Mechanisms of increasing the fracture resistance of ceramics: Zirconia Origin and applications Properties Zirconia products Aging of zirconia ceramics Classification of high-strength all-ceramic systems: Glass-ceramics Leucite Reinforced Glass-Ceramics Lithium Disilicate Glass-Ceramics Glass-infiltrated ceramics In-Ceram Alumina (Vita, D-Bad Säckingen) In-Ceram Spinell (Vita, D-Bad Säckingen) In-Ceram Zirconia (Vita, D-Bad Säckingen) Polycrystalline ceramics (CAD-)/CAM Systems: Definition/Historical Background CAD-CAM Components Open/Closed (CAD-)/CAM systems Materials Yttrium Tetragonal Zirconia Polycrystals (Y-TZP) The Cerec System CAM Technologies Marginal fit of CAD-CAM restorations Clinical and technical aspects of all-ceramic FPDs Clinical aspects Preparation design Translucency/Esthetics Fracture resistance testing Marginal Fit 32 3

4 Occlusal forces Cementation Technical aspects for all-ceramic FPDs Connector dimensions Thermal Expansion Coefficient (TEC) Survival rates of all-ceramic FPDs 36 3 Aim of the study 39 4 Outline of the study (Fig. 4.1) 40 5 Materials and Methods MATERIALS Abutment Teeth Materials used for the fabrication of the all-ceramic FPDs Materials used for the cementation procedure 43 Impression and die materials Additional Materials (Table 4): METHODS Representative model Artificial periodontal membrane Embedding models in the sample holders Tooth preparation Impression procedure Fabrication of master models Fabrication of all-ceramic FPDs Manufacturing the framework Veneering procedures: Cementation of the FPDs Dynamic loading of the test samples Survival rate Fracture resistance test Statistics 55 6 Results Survival rate of all-ceramic FPDs after aging Fracture resistance tests Fracture patterns 58 4

5 6.3.1 Procera Zirconia group DCS group Vita CerecInLab group 61 7 Discussion Methods The use of natural teeth as abutments Artificial periodontal membrane The antagonistic material Preparation design and connector dimensions Clinical relevance of fracture resistance tests Clinical relevance of the artificial aging process Results Survival rate after the chewing simulation Fracture resistance tests Influence of the chewing simulation on the fracture resistance Influence of the veneering process on the fracture resistance of zirconiabased frameworks Fracture patterns 71 8 Conclusions 73 9 Summary Zusammenfassung Appendix Fracture resistance values of the Procera group 76 Without aging (Tab. 11.1) Fracture resistance values of the DCS group 76 Without aging (Tab. 11.3) Fracture resistance values of the Vita group 77 Without aging (Tab. 11.5) References Curriculum vitae Acknowledgements 100 5

6 INTRODUCTION 1 Introduction Most cultures throughout centuries have acknowledged teeth as an integral facial structure for health, youth, beauty, and dignity. Unexpected loss of tooth structure and, particularly, missing anterior teeth creates not only physical and functional problems, but often psychological and social disturbances as well (Kelly et al, 1996). The treatment alternatives for the replacement of a single missing tooth have expanded during recent times, so that the choice of a proper treatment plan is no longer a simple decision. A missing mandibular first molar is a relatively frequent dental problem. Treatment options to replace a single missing molar include the Removable Partial Denture (RPD), the Fixed Partial Denture (FPD), the Resin-Bonded Fixed Partial Denture (RBFPD) and the implant supported crown (Priest, 1996a). In making the proper choice of the most appropriate restoration type and material, one should consider both patient s priorities and scientific objectives (Priest, 1996a). It is widely assumed that if posterior edentulous spaces are not treated, the adjacent teeth ultimately will be lost. There is no sound scientific study, however, that describes the loss of teeth associated with the inevitable disarrangement of one or both dental arches, triggered by an unreplaced missing tooth (Shugars et al, 1998). According to the previous study, the vast majority of untreated spaces did not result in loss of adjacent teeth within the study period. Treatment with a removable partial denture did not increase the likelihood of adjacent tooth survival, while treatment with a fixed partial denture resulted in an improved survival of adjacent teeth. In spite of their good survival rates of 86% after 5 years of function (Shugars et al, 1998), 84% and 59% after 5 and 10 years, respectively (Dietze, 2003), and low fabrication costs, the indication of RPDs should be strictly limited because of high plaque accumulation, high risk of caries and periodontitis progression and more frequent repair demand (Kerschbaum, 2004). Therefore, replacing missing teeth by means of an RPD should only be applied when all other treatment options would not be selected (Kerschbaum, 2004). The RBFPD has gained in popularity over the years due to the rapid progress of adhesive technology. The survival rates given in the literature are very divergent ranging from 65% (maxillary region) and 40% (mandibular region) over 5 years (De Kanter et al, 1998) to 29% over 6.2 years (Rijk et al, 1996) and 10% over 11 years of use (Priest, 1996b). The data indicate that RBFPDs could be utilized as ideal interim restorations, offering a conservative, quick and cost effective treatment option to the patient for a short or longer period of time. 6

7 INTRODUCTION Different studies have reported survival rates ranging between 93-98% for implants and between 83-98% for crowns with observation periods between 4 and 11 years, respectively in partially edentulous cases (Scheller et al, 1998; Creugers et al, 2000; Naert et al, 2000; Gibbard and Zarb, 2002; Mayer et al, 2002). The implant-supported single crown is considered as a conservative (adjacent teeth remain intact), biocompatible and potentially excellent esthetic treatment alternative. However, its application may be contraindicated due to esthetic limitations, health considerations or negative patient compliance. The traditional porcelain-fused to-metal fixed partial denture (PFM) is the most popular treatment option for the majority of dentists because of the familiar fabrication techniques, the acceptable esthetic outcome and the high survival rates ranging between 74% and 85% after 15 years of service (Creugers et al, 1994; Scurria et al, 1998; Walton, 2002). The frequent gingival discoloration around the metal margins of PFMs (Christensen, 1994), together with some allergic reactions by metal alloys, are still a weak point of these restorations that dissatisfies both patients and dentists (Shepard et al, 1983; Hansen and West, 1997). The development of facial porcelain margins is one significant modification that enhances esthetics by eliminating the display of metal and allowing a more natural transmission of light (Hobo and Shillingburg, 1973; Shillingburg et al, 1973; Goodacre et al, 1977; Chiche et al, 1986). Using this technique, the framework is shortened by 1 to 3 mm in the shoulder area. Other authors suggested that a minimum facial metal reduction of 2 mm is necessary in order to obtain proper light transmission after cementation, which may compromise the fracture strength (O'Boyle et al, 1997). It was further concluded that collarless metal-ceramic crowns having up to 2 mm of unsupported porcelain could resist the same axial pressure as restorations with complete metal strength, provided that a 90-degree shoulder tooth preparation is used (Lehner et al, 1995). Since the application of PFMs has proven to be successful over the years, these restorations still remain the gold standard in terms of predictability. Despite this success, however, the demands for more esthetic materials with biocompatible properties is increasing. The use of metal in the oral cavity has come under dispute in recent years due to its eventual biological incompatibility risks (Pfeiffer and Schwickerath, 1989; Reuling et al, 1990; Lucas and Lemons, 1992; Rechmann, 1993). Therefore, all-ceramic restorations are considered as an alternative of high importance and clinical value. After the introduction of feldspathic porcelain reinforced with alumina (McLean and Hughes, 1965), researchers have been developing new high-strength ceramic materials that can be used for the fabrication of FPDs for use in both the anterior and posterior regions. In vitro and in vivo investigations of these 7

8 INTRODUCTION newly-developed all-ceramic systems should be undertaken before introducing them into routine clinical use. The aim of the present study was to compare the fracture resistance and mode of failure of different zirconia three-unit posterior all-ceramic fixed partial dentures and to evaluate the effect of fatigue loading on the fracture resistance. 8

9 LITERATURE REVIEW 2 Literature review 2.1 Ceramics Historical perspectives of ceramics During the 18 th century, the candidate materials for replacing teeth were human teeth, carved animal teeth, ivory, and mineral or porcelain teeth. In 1723, Piere Fauchard was credited with recognizing the potential of porcelain enamels and initiating research with porcelains to imitate the color of teeth and gingival tissues (Jones, 1985). In 1774, Alexis Duchateau and Nicholas Dubois de Chemant fabricated the first successful porcelain dentures. Dubois de Chemant, who improved porcelain formulations continually during his scientific career, was awarded both French and British patents. In 1808, in Paris, Giuseppangelo Fonzi introduced individually-formed porcelain teeth that contained embedded platinum pins. Their esthetic and mechanical versatility provided a major advance in prosthetic dentistry. As early as at the end of the 19 th century, all-ceramic restorations, called jacket crowns, were fabricated by firing a feldspathic ceramic material on a die prepared with platinum foil. Jacket crowns were the only fixed esthetic restorations available at that time (Freese, 1959). Despite their esthetic advantages, the restorations failed to gain widespread popularity because of their high probability of fracture, low strength and poor marginal seal. This technique went out of fashion once the metal-ceramic era began (Jones, 1985). A noteworthy development occurred in the 1950s, with the addition of leucite to porcelain formulations that elevated the coefficient of thermal expansion to allow their fusion to certain gold alloys to form complete crowns and FPDs (Freese, 1959; Weinstein, 1962). 9

10 LITERATURE REVIEW Composition, properties and limitations of dental ceramics Dental ceramics consist of a compound of metals (aluminium, calcium, lithium, magnesium, potassium, sodium, tin, titanium, and zirconium) and nonmetals (silicon, boron, fluorine, and oxygen) that may be used as a single structural component, such as when used for a CAD- CAM inlay, or as one of several layers used for the fabrication of a ceramic-based restoration. Conventional dental porcelain is a vitreous ceramic based on a silica (SiO 2 ) network and potash feldspar (K 2 O Al 2 O 3 6SiO 2 ), soda feldspar (Na 2 O Al 2 O 3 6SiO 2 ) or both. Pigments, opacifiers and glasses are added to control the fusion temperature, sintering temperature, thermal contraction coefficient, and solubility. The feldspars used for dental porcelains are relatively pure and colorless. Therefore, pigments must be added to produce the hues of natural teeth (Anusavice, 2003). Most of the ceramics are characterized by their refractory nature, hardness, and chemical inertness. A hardness of a ceramic similar to that of enamel is desirable to minimize the wear of resulting ceramic restorations, and reduce the wear damage that can be produced on enamel by the ceramic restoration. Chemical inertness ensures that the surface of dental restorations does not release potentially harmful elements, and reduces the risk for surface roughening and an increased susceptibility to bacterial adhesion to insure excellent biocompatibility over time. Furthermore, ceramics demonstrate excellent insulating properties, such as low thermal conductivity, low thermal diffusivity, and low electrical conductivity. Their most attractive property is their potential for matching the appearance of natural teeth, offering great esthetic results (Anusavice, 2003). On the other hand, the susceptibility of ceramics to brittle fracture is a drawback, particularly when flaws and tensile stresses coexist in the same region of the restoration. The flaw can be a microcrack on the surface (e.g. created during occlusal adjustment with a diamond stone), or it can be a subsurface porosity (e.g. from a processing error during the build-up and baking of the porcelain) (Rosenblum and Schulman, 1997). When tension stress is applied, small flaws tend to open up and propagate cracks (crack propagation theory) (O' Brien, 2002). Irregularities in a bulk of the material, such as discontinuities and/or abrupt changes in shape or thickness in the ceramic contour, act as stress raisers, making the restoration more prone to failure. Stress around a stress raiser is higher than the average stress in the body of the material. The amount of this increased stress depends on the shape of the stress raiser (e.g. stress at the tip of a sharp notch would be greater than that of a semicircular groove). Because of the stress concentration at surface scratches and other defects (brittleness), ceramics tend to fail at stress levels that are much lower than the theoretical strength to be tolerated. Compared 10

11 LITERATURE REVIEW to metals, which can yield to high stress by deforming plastically, ceramics tend to have no mechanism for yielding to stress without fracture (O' Brien, 2002). Therefore, cracks may propagate through a ceramic material at low average stress levels. As a result, ceramics and glasses have lower tensile strengths than compressive strengths (O' Brien, 2002) Mechanisms of increasing the fracture resistance of ceramics: 1. Development of residual compressive stresses: The thermal expansion coefficient (TEC) of the core ceramic is slightly greater than that of the veneering ceramic. This mismatch allows the core material to contract slightly more upon cooling from the firing temperature to room temperature, and leave the veneering ceramic in residual compression while offering additional strength (Mackert, 1988). 2. Minimize the number of firing cycles: Firing procedures sinter the particles densely together and produce a relatively smooth surface. In addition, they increase the concentration of leucites in the porcelain, which in turn leads to an increase of the TEC and a further mismatch between core/veneering porcelain. This mismatch will cause immediate or delayed crack formation in the porcelain (Fairhurst et al, 1980; Mackert, 1988; Mackert and Evans, 1991; Fairhurst et al, 1992). 3. Minimize tensile stress through optimal design of ceramic restorations Dental restorations containing ceramics should be designed in a way to overcome their weaknesses. The design should avoid exposure of the ceramic to high tensile stresses (Anusavice, 2003). In the case of a crown, tensile stresses can be reduced by using strong core materials with appropriate thickness, since these stresses are distributed on the inner surface (core material is in tension) (Kelly et al, 1989; White et al, 1994; Zeng et al, 1996; Wakabayashi and Anusavice, 2000; Lawn et al, 2001). In the case of a FPD, high tensile stresses develop at the gingival surface of the connector and a larger radius of curvature at the gingival embrasure reduces the concentration of tensile stresses, thus affecting the fracture resistance of the FPD (Oh et al, 2002; Oh and Anusavice, 2002). To promote achieving the required connector dimensions without compromising the health of the supporting tissues, it 11

12 LITERATURE REVIEW was suggested to fabricate the gingival and lingual aspects of the connectors exclusively out of the framework material (McLaren, 1998). 4. Ion Exchange (or chemical tempering): This process involves the exchange of larger potassium ions for the smaller sodium ions (a common constituent of a variety of glasses)(anusavice et al, 1992). If a sodium-containing glass article is placed in a bath of molten potassium nitrate, potassium ions in the bath exchange places with some of the sodium ions on the surface of the glass particles. The potassium ion is about 35% larger than the sodium ion. Squeezing of the potassium ion into the place formerly occupied by the sodium ion creates large residual compressive stresses in the surfaces of the glasses subjected to this treatment. However, the depth of the compression zone is less than 100 µm, so that this effect would be easily worn out after long term exposure to certain inorganic acids (Southan, 1970; Jones, 1983; Seghi et al, 1990; Anusavice et al, 1992; Anusavice et al, 1994). 5. Thermal Tempering: This is a process of creating residual surface compressive stresses by rapidly cooling the surface of the object while it is hot and in the softened (molten) state. This rapid cooling produces a skin of rigid glass surrounding a soft (molten) core. As the molten core solidifies it tends to shrink, but the outer skin remains rigid. The pull of the solidifying molten core, as it shrinks, creates residual tensile stresses in the core and residual compressive stresses within the outer surface, inhibiting the initiation and the growth of cracks (Anusavice et al, 1989; Anusavice and Hojjatie, 1991; DeHoff and Anusavice, 1992) 6. Dispersion strengthening: This involves the reinforcement of ceramics with a dispersed phase of a different material that is capable of hindering a crack from propagating. Dental ceramics containing primarily a glass phase can be strengthened by increasing the crystal content of leucite, lithium disilicate, alumina, magnesia-alumina spinel, zirconia and other types of crystals (McLean and Hughes, 1965). 12

13 LITERATURE REVIEW When a tough, crystalline material such as alumina (Al 2 O 3 ) is added to a glass, the glass is toughened and strengthened, because the crack cannot pass through the alumina particles as easily as it can pass through the glass matrix (McLean and Hughes, 1965; Jones, 1983). The amount of toughening depends on the crystal type, its size, its volume fraction, the interparticle spacing, and its relative thermal expansion coefficient to the glass matrix. In most instances, the use of a dispersed crystalline phase to disrupt crack propagation requires a close match between the thermal contraction coefficients of the crystalline material and the surrounding glass matrix (Jones, 1983). 7. Transformation toughening: The dispersion strengthening process relies on the toughness of the particle to absorb energy from the crack and deplete its driving force for propagation. The transformation toughening process relies on a crystal structural change of a material under stress to absorb energy from the crack (Morena, 1986). Zirconia (ZrO 2 ) ceramic is a good example for this mechanism. The material is polymorph occurring in three forms: monoclinic (M), tetragonal (T) and cubic(c). Pure zirconia is monoclinic in room temperature. This phase is stable up to 1170 C. Above this temperature it transforms into tetragonal and then into a cubic phase at 2370 C. When ZrO 2 is heated above 1170 C, the transformation from the monoclinic to the tetragonal phase is associated with a 5% volume decrease. Reversely, during cooling, the transformation from the tetragonal to the monoclinic phase is associated with a 3% volume expansion. These phase transformations, however, induce stresses which result in crack formations. The inhibition of these transformations can be achieved by adding stabilizing oxides (CaO, MgO, Y 2 O 3 ), which allow the existence of tetragonal-phase particles at room temperature. When sufficient stress develops in the tetragonal structure and a crack in the area begins to propagate, the tetragonal grains transform to monoclinic grains. The associated volume expansion results in compressive stresses at the edge of the crack front and extra energy is required for the crack to propagate further (Tateishi, 1987). 13

14 LITERATURE REVIEW Zirconia Origin and applications Zirconia, the metal dioxide (ZrO 2 ), was identified in 1789 by the German chemist Martin Heinrich Klaproth in the reaction product obtained after heating some gems. It was used for a long time, blended with rare earth oxides, as pigments for ceramics. The first biomedical application of Zirconia, was carried out in 1969 by Helmer and Driskell (Helmer, 1969), while the first use of zirconia in orthopedics was introduced by Christel (Christel, 1988) to manufacture ball heads for total hip replacements. Its application over the years was further expanded in dentistry; including the fabrication of brackets in orthodontics (Keith et al, 1994), post and core systems (Edelhoff and Sorensen, 2002; Heydecke et al, 2002) and ceramic implants/implant abutments offering improved esthetic alternatives (Glauser et al, 2004; Kohal et al, 2004). 14

15 LITERATURE REVIEW Properties The composition and properties of alumina and ZrO 2 based biomaterials are listed in Table 1: PROPERTY UNITS ALUMINA MG-PSZ TZP Chemical composition 99.9% Al 2 O 3 + MgO ZrO mol % MgO Density Gcm >6 ZrO mol % Y 2 O 3 Porosity % <0.1 - <0.1 Bending strength Compression strength MPa > MPa Young modulus GPa Fracture toughness K ic Thermal Expansion Coeff. Thermal conductivity -1 MPa m K -1 8 x x x 10-6 W mk Hardness HV Table 1 (given from Piconi, 1999 ) Table 1 shows that zirconia ceramic exhibits higher bending strength (Wagner and Chu, 1996; McLaren, 1997) and fracture toughness (Wagner and Chu, 1996) than alumina ceramics. Additionally, its Young modulus is much lower than that of alumina, in the same order of magnitude of stainless steel alloys (CoCr alloy 230 GPa), pointing out its interesting elastic deformation capability. Fracture toughness is a very important physical property since it represents the ability of a material to resist crack growth. Clinically, lots of subcritical loads are applied on the materials by chewing, leading to the growth of subcritical cracks. Therefore, materials with higher fracture toughness are more ideal clinically, since it takes more energy to cause crack growth (McLaren and Terry, 2002). 15

16 LITERATURE REVIEW Zirconia products Partially Stabilized Zirconia (PSZ) is a product consisting of pure zirconia and stabilizing oxides like CaO, MgO, CeO 2, Y 2 O 3. Its microstructure at room temperature consists of cubic zirconia as the major phase, with monoclinic and tetragonal zirconia precipitates as the minor phase (Subbarao, 1981). It has been observed that tetragonal metastable precipitates, finely dispersed within the cubic matrix, were able to transform into the monoclinic phase when the constraint exerted on them by the matrix was relieved (i.e. by a crack advancing in the material). In that case, the stress field associated with expansion due to the phase transformation acts in opposition to the stress field that promotes the propagation of the crack. An enhancement in toughness is obtained, because the energy associated with the crack propagation is dissipated both in the T-M transformation and in the process of overcoming the compression stress due to the volume expansion (Garvie, 1972; Garvie, 1975). Several PSZs, like Y 2 O 3 -ZrO 2 or MgO-ZrO 2, were tested as ceramic biomaterials. Mg-PSZ (8% mol MgO in ZrO 2 ) showed favorable results, but its application diminished rapidly due to the rather coarse grain size (in the range 30-40µm), the resultant high residual porosity, and the higher sintering temperatures compared to that for TZP materials (Tetragonal Zirconia Polycrystals). Additionally, difficulties in obtaining Mg-PSZ precursors free of SiO 2, Al 2 O 3 and other impurities (Leach, 1987) together with the increase in SiO 2 contents due to the wear of the milling media during powder processing before firing (Rühle, 1984), have contributed to the shift in interest towards TZP materials. Ceramics containing MgO and magnesia silicates, such as MgSiO 3 and Mg 2 SiO 4, may form at the grain boundaries, lowering the MgO contents in the grains and promoting the formation of the monoclinic phase, which in turn leads to a further reduction of the mechanical properties and stability of the material in a wet environment (Leach, 1987). Tetragonal Zirconia Polycrystals (TZP) ceramic is composed mostly out of the T-phase at room temperature and contains approximately 2-3% Y 2 O 3 as stabilizing factor. The fraction of T-phase retained at room temperature is dependent on the size of the grains, on the yttria content and on the grade of constraint exerted on them by the matrix (Rieth, 1976; Gupta, 1978). The tetragonal grains show a metastable nature. A critical grain size exists, linked to the yttria concentration, above which spontaneous T-M transformation of grains takes place; whereas this transformation would be inhibited in a grain structure that is too fine 16

17 LITERATURE REVIEW (Theunissen, 1992). Surface tetragonal grains are not constrained by the matrix, and can transform to monoclinic spontaneously or because of abrasive processes that induce compressive stresses at a depth of several microns under the surface (Reed, 1977). Aluminosilicate glasses in the grain boundaries scavenge yttrium ions from TZP grains, leading to a loss of stability of the tetragonal phase (Lin, 1990). Moreover, mullite (3Al 2 O 3 2SiO 2 ) pockets were detected in the aluminosilicate glass, which lead to a loss of material stability in a wet environment Aging of zirconia ceramics The mechanical property degradation in zirconia, known as aging, is due to the progressive spontaneous transformation of the metastable tetragonal phase into the monoclinic phase. This behavior is well known at a temperature range above 200 C and in the presence of water or vapor (Sato, 1985a, b). The aging steps of TZP as given by (Swab, 1991) are: 1. The most critical temperature range is C. 2. Aging reduces strength, toughness and density of the material, and increases the monoclinic phase content. 3. Degradation of mechanical properties is due to the T-M transition, which takes place with micro and macro cracking of the material. 4. T-M transition starts on the surface and progresses into the bulk of the material. 5. Reduction in grain size and/or increase in concentration of stabilizing oxide reduce the transformation rate. 6. T-M transformation is enhanced in water or in vapor. The variability in aging behavior among different zirconia materials is related to the differences in equilibrium of the microstructural parameters, such as concentration and distribution of yttria grain size, and population and distribution of flaws (Lilley, 1990). Stable performances of TZP ceramics in a wet environment were reported by several authors (Chevalier, 1977; Swab, 1991; Shimizu et al, 1993; Burger, 1995; Fujisawa, 1996; Burger, 1997; Geis-Gerstorfer and Fässler, 1999). Hence, there is experimental evidence that TZP stability can be controlled acting on several parameters, such as stabilizing oxide concentration, distribution, grain size and residual stresses in the ceramics (Lepistö, 1992), or the presence of the cubic phase (Chevalier et al, 2004). 17

18 LITERATURE REVIEW The degradation resulting from aging is characterized by surface roughening and microcracking at the surface (Chevalier, 2006). Garvie (1975) first pointed out that grinding increases the strength of ceramics containing metastable tetragonal zirconia compared with fine polishing. Another recent study showed that the grinding of 3Y-TZP ceramics induced no monoclinic phase formation, but only a rhombohedral zirconia and a strained tetragonal zirconia phase formation (Denry and Holloway, 2006). This led to a significant increase in mean flexural strength and increased resistance to crack propagation, but was also associated with surface and subsurface damage, with formation of microcraters and grain pullout. Although annealing successfully reversed the zirconia transformation, the surface and subsurface damage created by grinding remains and could lead to failure by crack propagation (Denry and Holloway, 2006). Similarly, another group of researchers tested the influence of surface and heat treatments on the flexural strength of Y-TZP ceramics (Guazzato et al, 2005b) and In Ceram Zirconia (Guazzato et al, 2005a). In both studies it was concluded that sandblasting and wet grinding did increase the flexural strength of the ceramics, due to the monoclinic transformation, but also led to microcracking and strength degradation. Hence, it was suggested that any surface treatment performed on In-Ceram Zirconia should always be followed by heat treatment to avoid strength degradation (Guazzato et al, 2005a), while in the case of Y-TZP ceramics, an initially weaker (with no surface treatment) but in the long-term more stable (no strength degradation) material may be more desirable (Guazzato et al, 2005b). The aging sensitivity of Y-TZP is directly linked to the type (compressive or tensile) and amount of residual stresses. Rough polishing produces a compressive surface stress layer beneficial for the aging resistance, while smooth polishing produces preferential transformation nucleation around scratches, due to elastic/plastic damage and the tensile residual stresses occurred (Deville et al, 2006). Another relevant aspect for the stability of the material in a biological environment is the presence of glassy phases formed by SiO 2, Al 2 O 3, TiO 2 or CaO impurities in grain boundaries. These impurities may come from the chemical precursors, from the milling bodies used in powder processing, or may be added to powders as sintering aids. Their presence leads to a loss of stability of the tetragonal phase, as it was demonstrated that aluminosilicate glassy phases in grain boundaries are able to scavenge yttrium ions from TZP grains (Lin, 1990). 18

19 LITERATURE REVIEW Biological safety of zirconia: An in vitro study reported that Y-PSZ shows a dose dependent cytotoxicity; its toxic effect is similar to that of alumina, and both lower than that of TiO 2 (Dion, 1994). In-vivo studies have shown an absence of local or systemic toxic effects after the implantation of zirconia ceramics into muscles or bones of different animals or after powder injection in mice (Bukat, 1990; Richter, 1994; Walter, 1994). During tests, especially in the early postoperative phase, connective tissue is frequently observed at the bone-ceramic interface (Tateishi, 1994). 2.2 Classification of high-strength all-ceramic systems: High-strength ceramic core materials may be classified according to their chemical structure into 3 major groups (Raigrodski, 2005): Glass-ceramics They are multiphase materials that contain an amorphous, glassy phase and crystalline constituents Leucite Reinforced Glass-Ceramics The main representatives of this category are the IPS Empress (Ivoclar Vivadent, FL- Schaan) and the Optec OPC (Jeneric Pentron, D-Kusterdingen). These core materials use crystalline filler to reinforce glass-ceramic structures. Copings may be fabricated by using either a heat-pressing procedure or via CAD/CAM technology. The restorations are highly translucent (Heffernan et al, 2002b, a) providing the potential for a highly esthetic restoration. Therefore, they are not recommended for cases where the underlying abutment is a discolored tooth, a metallic-core built up, or a metal implant abutment. The reported flexural strength of this core material ranges between MPa, and the fracture toughness from 1.5 to 1.7 MPa x m 1/2 (Campbell, 1989; Seghi et al, 1990; Seghi et al, 1995; Seghi and Sorensen, 1995). The strength of these restorations depends on a successful bond to the tooth structure and, therefore must be adhesively cemented. Their indication is restricted only for veneers or crowns at the front region giving survival rates up to 95% after 11 years of clinical service (Fradeani and Redemagni, 2002). 19

20 LITERATURE REVIEW Lithium Disilicate Glass-Ceramics The main representative of this category is the Empress II (Ivoclar, Schaan, Liechtenstein) core material. The framework can be fabricated either with the lost-wax and heat-pressure technique, or can be milled out of prefabricated blanks. Its flexural strength ranges from 300 to 400 MPa (Schweiger, 1999) and its fracture toughness between 2.8 and 3.5 MPa/ m 1/2 (Schweiger, 1999; Quinn et al, 2003). It is recommended that these restorations should be etched and adhesively luted to enhance their strength and longevity (Sorensen, 1999). The material is indicated not only for the fabrication of anterior FPDs, but also for short-span posterior FPDs (pontic not wider than a premolar) extending up to the second premolar (Sorensen, 1999; Holand et al, 2000). Esquivel-Upshaw et al (2004) reported a survival rate of 93% for posterior Empress II FPDs after 2 years. Marquardt and Strub (2006) reported a survival rate of 100% for single crowns and 70% for FPDs extending up to the second premolar after 5 years of function Glass-infiltrated ceramics These products consist of infiltrating molten glass to partially sintered oxides. The main representatives of this category are In-Ceram Alumina, In-Ceram Spinell and In-Ceram Zirconia (Vita, D-Bad Säckingen) In-Ceram Alumina (Vita, D-Bad Säckingen) The material is composed of a highly sintered-alumina glass-infiltrated core and the veneering porcelain. The fabrication of the core/framework can be carried out either with the slip-cast technique or by the milling out of prefabricated partially sintered blanks through CAD-CAM technology. The flexural strength of the material ranges between 236 and 600 MPa (Giordano et al, 1995; Guazzato et al, 2002) and the fracture toughness between 3.1 and 4.61 MPA/m 1/2 (Seghi et al, 1995; Wagner and Chu, 1996). It is recommended for anterior and posterior crowns, as well as for 3-unit anterior FPDs (Sorensen, 1992; McLaren, 1998). Because of its semiopaque core, the ceramic does not allow full transmission of light and provide therefore limited esthetic results (Heffernan et al, 2002b, a). 20

21 LITERATURE REVIEW In-Ceram Spinell (Vita, D-Bad Säckingen) The In-Ceram Spinell consists of a MgAl 2 O 4 core infiltrated with glass. The fabrication procedures are the same as those for In-Ceram Alumina. Its flexural strength is lower than that of In Ceram Alumina ranging between 283 and 377 MPa (Magne and Belser, 1997; McLaren, 1998; Schweiger, 1999), but its translucency is twice as high. Therefore, it is indicated for anterior crowns, where esthetic demands are higher (Fradeani and Redemagni, 2002) In-Ceram Zirconia (Vita, D-Bad Säckingen) The In-Ceram Zirconia core consists of glass-infiltrated alumina with 35% partially stabilized zirconia. Its flexural strength ranges from 421 to 800 MPa and its fracture toughness from 6 to 8 MPa x m 1/2 (McLaren, 2000; Chong et al, 2002; Guazzato et al, 2002). The fabrication may be carried out either with the slip-casting technique or with CAD/CAM technology. The high opacity of its core (Heffernan et al, 2002b, a) restricts its application only for the fabrication of posterior FPD s, resulting in successful short-term data (Suarez et al, 2004) Polycrystalline ceramics This category contains materials with densely packed particles and no glassy components. They cannot be processed into shapes without the use of Computer-Assisted- Design/Computer-Assisted-Machining (CAD/CAM) technologies (CAD-)/CAM Systems: Definition/Historical Background The term CAD/CAM, which comes from machine-tool technology and stands for Computer- Aided-Design / Computer-Aided-Manufacturing, designates the three-dimensional planning of a workpiece on the screen of a computer with subsequent automated production by a computer controlled machine tool (Tinschert et al, 2004a). In 1971, Francois Duret 21

22 LITERATURE REVIEW introduced CAD-CAM technology to the field of dentistry (Duret et al, 1988). His idea was based upon the assumption that the technologies established in industry could be easily transferred to dentistry. The industrial use of CAD-CAM allows the production of any number of similar workpieces automatically, while saving time and manual effort. In dental medicine, however, this philosophy can not be applied due to the demands of the individual adaptation of the restoration design (one-of-a-kind production) to the patient (Tinschert et al, 2004a) CAD-CAM Components The contemporary CAD/CAM systems consist of three components (Luthardt, 2001a, b): 1. The scanner, which scans the dental preparation provided by the dentist either intraorally or extraorally by reference to tooth models. For inlays and single crown frameworks, just the surface data of the prepared teeth need to be digitized. For FPD frameworks or additional occlusal characterization, further data from the neighboring teeth and antagonists, as well as from the spatial relation of the prepared teeth to one another, are required. 2. The software CAD consists of a computer unit used for the three-dimensional planning and design of restorations on the computer screen. The software programs available today offer a high level of intervention and permit the design and production of an individually adapted restoration. Systems not offering a full CAD component are not considered as CAD/CAM systems but just as CAM systems. Therefore, we can refer to them as (CAD-)/CAM systems (Witkowski, 2005). 3. The hardware CAM covers different production technologies for converting the virtual restoration into a dental material. At present, computer-controlled milling or grinding machines are mainly used. They machine the restoration from the full material block consisting of prefabricated metal or ceramic. As a rule, after the CAM production, some manual corrections and final polishing or individualization of the restoration with staining colors or veneering materials are required to be carried out by the dental technician (Luthardt, 2001a, b). 22

23 LITERATURE REVIEW Open/Closed (CAD-)/CAM systems Most CAD-CAM systems in dental technology operate as closed data systems, i.e., all components, such as the scanner, the CAD and CAM units, are linked by the specific data format of the user. The materials used for producing the restorations are also part of this compound, in the sense that code systems are used. On the other hand, more and more CAD-CAM systems operating with an open data exchange are being introduced in the dental market. In this case, the 3-D volume model of the design is transferred from CAD to CAM in a neutral data format. This language is an industrially compatible format (such as stereolithography language [STL]), which allows free choice among different production centers and CAM systems (Witkowski, 2005) Materials The material groups available for the various CAD-CAM systems are as follows: Silicate ceramics; glass-infiltrated aluminium oxide ceramics; densely sintered aluminium oxide ceramics; densely sintered zirconium dioxide ceramics ( ZrO 2 Y-TZP Zirconia, Yttria- Tetragonal-Zirconia-Polycrystal), manufactured as green stage, presintered stage and completely sintered stage; titanium; precious alloys; nonprecious alloys; acrylics of improved strength and castable acrylics (Witkowski, 2005). The Procera AllCeram (Nobel Biocare, S-Göteborg) is a polycrystalline ceramic consisting of a densely sintered high-purity aluminium-oxide core (Oden et al, 1998). It has a flexural strength between 500 and 650 MPa (White et al, 1996; Zeng et al, 1996)and a fracture toughness of MPa x m 1/2 (Christel et al, 1989; Wagner and Chu, 1996). It is recommended for the fabrication of anterior and posterior crowns, but its use for 3-unit FPDs is still questionable (Raigrodski, 2005). Zirconium dioxide has been introduced into dentistry as a framework material for various indications. The ZrO 2 frameworks for crowns and FPDs are made by milling in the green stage (diamonds with cooling liquid) (Filser, 1997), the presintered stage(dry carbide burs), and the completely sintered stage (diamonds with cooling liquid) (Witkowski, 2005). ZrO 2 that belongs to the green stage group can be individualized by coloring of the framework 23

24 LITERATURE REVIEW according to the Vita shade concept. Erdelt (2004) showed no changes in the physical properties of the materials when colored by an oxide liquid prior to the sintering process Yttrium Tetragonal Zirconia Polycrystals (Y-TZP) Y-TZP is a glass-free, high-strength polycrystalline ceramic material with a flexural strength of 900 to 1200 MPa and fracture toughness of 9 to 10 MPa x m 1/2 (Christel et al, 1989). It is indicated for anterior and posterior crown copings and FPD frameworks (Luthardt et al, 2004). The majority of the Y-TZP based (CAD-)/CAM systems use CAM of partially sintered Y-TZP blanks (Lava, 3M Espe Dental AG, Seefeld; Cercon, DeguDent, Hanau; Cerec InLab, Sirona Dental Systems, Bensheim; Procera AllZirkon, Nobel Biocare, S- Göteborg). The size of partially-sintered infrastructures is increased during the milling stage to compensate for prospective shrinkage (20-25%) occurring during final sintering (Raigrodski, 2005). The milling of these blanks is faster and results in less wear and tear to the hardware (Raigrodski, 2004b). With fully sintered blanks, such as DC-Zirkon (DCS- Precident, DCS Dental AG, CH-Allschwill), there is no shrinkage involved in the milling process, but microcracks may be introduced to the infrastructure (Luthardt et al, 2004). Product examples of ZrO 2 materials and the groups according to the milling/grinding technology are: (Witkowski, 2005) Milling at green stage: Cercon Base (Cercon), Lava Frame (Lava), Hint-Els Zirkon TZP-G (DigiDent), ZirkonZahn (Steger), Xavex G 100 Zirkon (etkon) Grinding at presintered stage: In Ceram YZ-Cubes (Cerec InLab), ZS-Blanks (Everest), Hint-Els Zirkon TZP-W (DigiDent), DC-Shrink (Precident DCS) Grinding at completely sintered stage: DC-Zirkon (Precident DCS), Z-Blanks (Everest), Zirkon TM, Pro 50 (Cynovad), Hint-Els Zirkon TZP-HIP (DigiDent), HIP Zirkon (etcon) Only a few CAD-CAM systems offer the possibilities of using different materials and fabricating occlusal surfaces. Even if a complete reconstruction of the occlusal surface (framework production only) is not wanted, a framework design according to anatomical aspects with inclusion of the contact relations should be a primary goal. In this so called intelligent framework design, the construction is strengthened in all areas with sufficient 24

25 LITERATURE REVIEW clearance from the antagonists, the neighboring teeth and the gingiva, so that the veneering ceramic can be fired on with uniform thickness (Rudolf, 2003). This procedure should ensure that the veneering ceramic receives sufficient support while avoiding the occurrence of too thick veneer layers and material stresses because of layer thickness fluctuations. In employing this method, the risk of veneer spalling off is also reduced (Tinschert et al, 2004a) The Cerec System In 1985, the Cerec I (Brains, Zürich, Switzerland) CAD-CAM system was introduced to the dental market. In 1994, Siemens (Bensheim, Germany) introduced the Cerec 2 unit. Due to the restricted efficiency of the computer at that time, the full effects of the correlation and function construction modes were limited (Mörmann et al, 1999). The Cerec 3 system (Sirona, Bensheim, Germany) was introduced in After 1 year of use, hardware and software improvements were implemented in early The chairside Cerec 3D system is an improved version of the Cerec 2; including the intraoral 3D scanning camera, image processing, computing power and a form-grinding unit. With this advance in computer efficiency, the two-impression correlation and function modes for designing partial and full crowns are able to proceed as desired, using occlusion and preparation optical impressions without loss of time (Mörmann and Bindl, 2002). Consequently, the occlusion and the preparation images can be used alternately to fit design suggestions arising from the morphologic data bank to the individual situation. The separate form-grinding unit, working true to morphologic detail and with fine surface quality, is connected to the optical unit by radio control. The form-grinding unit receives data from the control unit, independent of its location in the office. The next restoration can be designed while the first is being milled (Mörmann and Bindl, 2002). The form-grinding unit is fitted with a laser scanner (Cerec Scan, CerecinLab Sirona) and can be used by itself with a standard personal computer for indirect application in the dental laboratory. In April 2001, the application was expanded for the fabrication of three-unit fixed partial denture frames (Mörmann and Bindl, 2002) CAM Technologies The CAM technologies can be divided in three groups according to the technique used (Witkowski, 2005): 25

26 LITERATURE REVIEW 1. Subtractive Technique from a Solid Block The CAM technique most commonly applied in manufacturing frameworks for single crowns and FPDs is to cut the contour out of an industrially prefabricated, solid block of different materials (Andersson et al, 1989; Witkowski, 2002). The size of the material blocks available for the milling units limits the size of the FPD. When industrial prefabricated zirconium dioxide blocks are used, the restoration can be shaped both before and after the block is sintered. Consequently, we can have the green machining process of presintered ceramic blocks and the hard machining process of densely sintered ceramic blocks. In regard to the green machining, it offers the benefit of saving time and grinding tools for the labor, but the sintering shrinkage that occurs is difficult to be computercontrolled for extensive restorations. Further, it has not been proven whether or not the grinding dust arising in green machining leads to damage of the milling unit in the long run. The hard machining on the other hand is time-consuming, leads to greater wear of the grinding tools, and there is a risk of introducing unwanted surface or structural defects into the ceramic during the machining (Tinschert et al, 2004a). The DCS Precident system (Allscwill, Switzerland) is based on the hard machining process, using a laser scanner to scan multiple units at once, and software which suggests connector sizes and pontics for frameworks. The system uses a variety of materials including porcelain, glass-ceramic, In Ceram, densely sintered Zirconia (DC-Zirkon), metals and fiber reinforced composite. There s no shrinkage or sintering involved after milling (Giordano, 2003). The Cercon system (Degudent, Hanau, Germany) is not a CAD/CAM system. It requires a wax-up of the desired bridge framework. This wax-up is then scanned and through software manipulation and CAM processing an oversized coping of partially sintered Zirconia is milled out. This oversized coping (compensation for the 25-30% sintering shrinkage) will afterwards be fired for 6-8 hours at high temperature in order to produce a fully sintered zirconia. The Lava system (3M/ESPE Dental AG, Seefeld, Germany) is an offsite system. The central unit uses an optical scanner to scan multiple units at once. The software automatically finds the margin, suggests pontics and designs the desired framework (Giordano, 2003). Afterwards, with the milling machine, an oversized coping from partially sintered zirconia is milled out to compensate for sintering shrinkage. An additional feature is the ability to color the zirconia by dipping it in various solutions prior 26

27 LITERATURE REVIEW to dense sintering. The entire procedure from scanning to milling is completed at the center and then returned to the labor (Giordano, 2003). 2. Additive Technique by Applying Material on a Die There are three different systems that apply the framework material on a die of a prepared tooth (Witkowski, 2005): Procera (Nobel-Biocare AB, Göteborg, Sweden) The first system that was based on the knowledge of exact dimensional changes that take place during sintering was the Procera system (Nobel Biocare), which Andersson and Oden introduced in 1993 (Andersson and Oden, 1993). The system was also the first to introduce an industrialized process in which the framework is manufactured in a remote production unit (Anusavice, 1989; Andersson et al, 1998). The scanner in the dental laboratory scans the working die, and stores the information in a computer. After scanning, the technician marks the preparation margins on the computer screen and indicates the desired material (alumina or zirconia) framework thickness, and, in some instances, different opacities. This information is then compressed and transferred via a modem line to the production unit, which uses the information to calculate the anticipated shrinkage and fabricate an enlarged die. Alumina or zirconia is dry pressed against the enlarged die, and the temperature is raised to a temperature similar to the presintering stage. At this point in the process, the enlarged and porous coping is stable. Its outer surfaces are milled to the desired shape and the coping, removed from the enlarged die, and sintered into the furnace for firing to full sintering. During this cycle, the coping shrinks to fit the dimensions of the original working die. The completed coping is then sent back to the laboratory, where it is veneered with the compatible silica-based ceramic (Sadan et al, 2005). The second system (EPC 2019, Wol-Ceram System, Wol-Dent, Ludwigshafen, Germany) (Wolz, 2002) of this group generates the ceramic powder (In-Ceram Alumina and Zirconia, Vita Zahnfabrik, Bad Säckingen, Germany) directly on the die of the master 27

28 LITERATURE REVIEW model. The ceramic material (slurry stage) is generated by an electrophoretic dispersion method within a few minutes on the die(s) for single copings and FPDs. Overextended margin contours can be manually trimmed and the outside contour is shaped by a CAM process. Then, the coping is removed from the die glass-infiltrated at high temperature (1140 C) (Witkowski, 2005). The third system involves the solid direct form fabrication technique, which generates copings and frameworks for FPDs of pure Al 2 O 3 and ZrO 2 ceramics in a production center (ce.novation, Inocermic, Hermsdorf Germany). The dispersed super-fine nanoceramic powders consist of particles well below 100 nm in diameter. With this technology, the frameworks attain high strength and calculable sintering shrinkage (Brick, 2003). These new technologies are relatively new and need further development (Witkowski, 2005). 3. Solid free form fabrication This category includes new technologies originating from the area of rapid prototyping, which have been adapted to the needs of dental technology (Gebhardt, 2000; Wohler, 2003). The first system applying this technology for dental use was the wax plotter technique, which works according to the ink jet principle. The machine builds (solid free form) frameworks and full crowns in wax for the casting technique in alloys and titanium (Wax Pro 50, Cynovad, Montreal, Canada) (Witkowski, 2005). A second technology originating from rapid prototyping is the stereolithography (Perfactory, Delta Med, Frieberg, Germany). In this technique, the restoration is produced from light sensitive plastic, which can be converted into any desired alloy with the casting technique (Witkowski, 2003). Occlusal splints and diagnostic templates for oral implantology can also be produced with this technique. The third technique is the selective laser sintering, where sinterable powder materials are built up to form 3-D restorations. Every applied powder layer is fused by means of a laser (Medifacturing, Bego Medical, Bremen, Germany) (Strietzel, 2001). Only metals can be processed currently, whereas laser sintering of ceramics is still in the testing phase. 28

29 LITERATURE REVIEW Marginal fit of CAD-CAM restorations The fitting accuracy of a CAD-CAM produced restoration depends primarily upon the quality of the digital data acquisition and processing, the filters matched to the measuring system and used for removing measuring-induced noise and stray points; and upon the position of the preparation border, equator and undercuts. Therefore, the number of measuring points is not an absolute criterion for the quality of the data record (Rudolf, 2003). However, precise tooth preparation and careful design are still prerequisites for good fitting accuracy of a CAD-CAM produced restoration (Luthardt, 1998; May et al, 1998; Hertlein, 2001; Tinschert et al, 2001b; Beuer, 2003; Nakamura et al, 2003). 2.3 Clinical and technical aspects of all-ceramic FPDs Clinical aspects Preparation design Although there is no standard, the preparation design for all-ceramic FPDs requires an occlusal reduction of 2 mm (Banks, 1990) and a deep chamfer or circular shoulder with an inner rounded angle between 1.0 and 1.5 mm wide. The occlusal angle of convergence should not be greater than 10 degrees (Friedlander et al, 1990; Sorensen et al, 1998; McLaren and White, 1999, 2000). While 1.4 to 1.7 mm of facial reduction is necessary to achieve an esthetic result with metalceramic systems (Sorensen, 1999; Sorensen et al, 1999), a reduction of 1.0 to 1.5 mm is required for ceramic systems (Chiche, 1994; Rosenstiel, 1995; Shillingburg, 1997) Translucency/Esthetics All ceramic restorations offer improved esthetic results because of their natural reflection and translucency of light. A recent study (Heffernan et al, 2002a, b) compared the translucency of 6 all ceramic system core materials at clinically appropriate thicknesses. In order of decreasing translucency, the ranges were as follows: Vitadur Alpha Dentin(standard)>In- Ceram Spinell>Empress, Procera, Empress II>In-Ceram Alumina>In-Ceram Zirconia, 52 SF alloy. The authors demonstrated that all systems showed an increase in translucency after the 29

30 LITERATURE REVIEW veneering process and glazing cycle, with the exception of In-Ceram Zirconia and the metalceramic specimens, which showed no improved opacity after the glazing cycle. Another study evaluated the light transmission through different types of ceramic frameworks with different cements (zinc phosphate/adhesive cement). The light transmission coefficients of the infrastructures (0.9 mm) are listed as follows: In-Ceram Spinell>Empress II>Procera>Y- TZP>In-Ceram Alumina>In-Ceram Zirconia (Edelhoff, 2002). When the thickness of a material increases from 0.5 to 1.5 mm, the opacity rises from 65 to 85% (Hauptmann, 2000). The knowledge of the opacity limitations of each system is of great importance, since it allows us to successfully determine, especially for the anterior region, which system achieves better esthetics Fracture resistance testing The physical properties of newly developed dental ceramics must be tested before they can be recommended for clinical application. Generally, a study design which uses fixed abutments produces fracture loads that are much higher than those reported in studies which employ mobile abutments (Kappert et al, 1991; Kelly et al, 1995). The fracture loads thus obtained should be viewed critically in terms of their clinical relevance. In vitro investigations, finite element analyses (FEA) of FPDs and fractographic studies conducted on clinically failed FPDs have shown that the stress distribution, fracture origin and mode of failure of all-ceramic FPDs are substantially different from those of crowns (White et al, 1994; Kelly et al, 1995; Zeng et al, 1998; Proos et al, 2000; Thompson, 2000; Wakabayashi and Anusavice, 2000; Chong et al, 2002). In the case of crowns, tensile stresses are distributed at the inner surface, while all-ceramic FPDs tend to fail at the connector area where there is a peak of tensile stress. Consequently, the properties of the porcelain which veneers the stronger core material control the failure fate of the connector (Kelly et al, 1995; Proos et al, 2000). Therefore, some authors concluded that all areas of the restoration that are subjected to tensile stress should not be veneered with porcelain (White et al, 1994; Zeng et al, 1998). Another study showed that increasing the connector height from 3 to 4 mm dramatically reduced the stress levels within the connectors (Kamposiora et al, 1996). More recent studies carried out by Guazzato et al (2004a, b, c, d) confirm the previous findings indicating that the material subjected to the peak of tensile stress dictates the ultimate strength of the restoration. Some fractures, observed at the interface between the core ceramic and the 30

31 LITERATURE REVIEW veneering porcelain, have been related to the stress enhancement arising from large differences in elastic modulus between the veneer and the core ceramic materials. Contact loading, a new approach in the testing of dental ceramics, has been suggested by Lawn et al (2001) to evaluate the physical properties of ceramics (Peterson et al, 1998a; Peterson et al, 1998b; Jung et al, 1999a, b; Kim, 1999; Jung et al, 2000). A summary of the contact loading research studies has indicated the following: 1. Contact damage mode analysis for fine-grain, low-toughness dental porcelain found that the fracture mode was conventional hertzian cone fracture. Fractures in high crystalline, higher toughness, coarse-grain ceramics were caused by quasi-plastic deformation. 2. Stress-indentation curves of hertzian contact damage confirmed the ranking from flexural strength data (i.e., micaceous glass-ceramic<porcelain<alumina<zirconia). 3. For cyclic contact tests in dry versus wet environments, and for any given contact load, the strength of a ceramic degraded with an increase in the number of loading cycles. Most ceramic materials degraded significantly between to cycles. The presence of water enhanced damage accumulation in cyclic indentation. 4. Machining effects were shown to cause surface damage and reduce surface strength but to have only a secondary effect on the initiation and evolution of cone cracking and quasi-plasticity. In regard to the mode of fracture and its origin, new generations of multilayered coreveneer ceramics are of increased interest. Jung et al (1999b) reported that the substrate has a profound influence on the damage evolution, which leads to ultimate failure in the bilayered systems. Thompson (2000) showed that the different failure modes and failure origins by ceramic testing are dependent on testing methodology, relative layer heights, and very likely, the ceramic system being investigated. White et al (1994) demonstrated that layered ceramics made of strong cores veneered with weaker feldspathic porcelain may be prone to failure when the feldspathic porcelain surfaces are subjected to tensile force. Wakabayashi and Anusavice (2000) found that as the elastic modulus of the metal substrate increased, there was an increase in the load to cause failure, but no change in fracture origin. As the thickness ratio of ceramic core to veneer increased, the site of crack initiation shifted from the veneer porcelain to the core porcelain. The mean fracture strength increased as the core-to-veneer thickness ratio increased, but it did not exceed that of the core material. 31

32 LITERATURE REVIEW Marginal Fit The accuracy of fit of all-ceramic restorations is important for the integrity of dental and periodontal tissues, dissolution of luting agents and the fracture resistance of the restorations. Poorly fitted dental restorations are believed to be closely associated with the development of secondary caries and periodontitis (Gardner, 1982; Lang et al, 1983; Knoernschild and Campbell, 2000; Becker and Kaldahl, 2005). Up to this date, the dental research has demonstrated a wide variety of values for clinically acceptable marginal gaps of dental restorations. These gaps vary from 20 to 200 µm (Dreyer, 1958; McLean and von Fraunhofer, 1971; Rehberg, 1971; Marxkors, 1980; Fransson et al, 1985; Spiekermann, 1985; Kerschbaum, 1995). In 1985 Spiekermann estimated a marginal gap up to 100 µm as clinically acceptable. In 1995, Kerschbaum supported a marginal gap value of up to 200 µm as clinically acceptable for dental restorations. In vitro results demonstrating marginal gaps of µm in CAD/CAM-generated allceramic restorations are quite promising (Sulaiman et al, 1997; Lin et al, 1998; Bindl et al, 1999; Boening et al, 2000; Tinschert et al, 2001b). An in vivo study by Procera titanium crowns yielded gap widths that were µm wider in the bucco-lingual direction and µm wider in proximal locations than gap widths measured in vitro (Karlsson, 1993). Mean values between µm were reported for zirconia multi-unit frameworks produced by the DCS CAD/CAM system (DCS, Allschwil, Switzerland) (Tinschert et al, 2001b). Another study reported mean marginal gaps of 75 µm for the DigiDent (Girrbach, Pforzheim, Germany) CAD/CAM system (In Ceram Zirkonia blanks) and 65 µm for both the Lava (3M ESPE, Seefeld, Germany) system (yttrium-stabilized Zirkonia blanks) and the Cerec InLab (Sirona, Bensheim, Germany) (In Ceram Zirkonia blanks) system (Reich et al, 2005). The authors concluded that the accuracy of CAD/CAM-generated three-unit FPDs is satisfactory for clinical use Occlusal forces The normal physiological chewing forces on posterior teeth range between N (Eichner, 1963; Bates et al, 1976; De Boever et al, 1978; Jäger, 1989). Maximum biting forces that may occur in the posterior area vary between 300 and 880 N (Bates et al, 1976; Gibbs et al, 32

33 LITERATURE REVIEW 1986; Kiliaridis et al, 1993). Males show higher bite values than females. For a male, the maximal bite force is 382 N for the molar region and 108 N for the incisor region, while for females the values are 216 N and 108 N, respectively (Helkimo et al, 1977). In several simulating clinical conditions, a load of 49 N is set for the chewing simulator to simulate the physiological biting force (Krejci et al, 1990; Behr et al, 1999; Kern et al, 1999; Koutayas et al, 2000). Further literature data indicate that the maximum biting forces given for natural teeth and short-span FPDs in the posterior region range between 50 N and 400 N (Helkimo et al, 1977; Körber, 1983; Hagberg, 1986). In case of bruxism, the biting forces can rise up to 500 N or 880 N (Kelly, 1995, 1997; Kikuchi et al, 1997). Hence, a mean value of 500 N is considered as the minimum-demanded fracture resistance of a material for application at the posterior region. Dental ceramics, due to their subcritical crack growth, exhibit 50% of their initial fracture resistance after fatigue loading. This degradation should be taken into consideration in determining the minimum demanded fracture resistance and further ensuring the long-term clinical stability of the restorations (Sorensen et al, 1998; Geis-Gerstorfer and Fässler, 1999; Marx et al, 2001). Hence, it appears reasonable that an initial fracture resistance of 1000 N should be demanded for posterior FPDs Cementation The surface treatments indicated for glass-infiltrated ceramics are either airborne particle abrasion with Al 2 O 3 ( µm at 2.5 bar) with the use of a phosphate-modified resin cement (Panavia 21, Kuraray, Osaka, Japan) (Isidor et al, 1995; Kern and Thompson, 1995; Kern and Wegner, 1998; Madani et al, 2000), or tribochemical surface treatment (Rocatec System, 3M ESPE, Seefeld. Germany) in combination with conventional Bis-GMA resin cement (Sadoun and Asmussen, 1994; Kern and Thompson, 1995; Ozcan et al, 2001). Some authors recommend Panavia without a silane or bonding agent (Kern and Thompson, 1995), whereas others suggest a silane coupling agent to increase the saturation properties of the ceramic substrate (Madani et al, 2000; Ozcan et al, 2001). The Rocatec System (3M ESPE, Seefeld. Germany) is an effective and user friendly silicacoating method (Piotrowski, 2001). It includes 2 steps of airborne particle abrasion and the application of a silane coupling agent (ESPE-Sil; 3M ESPE, Seefeld, Germany) that bonds to the silica coated surface and to resin. The treatment with the Rocatec Sytem is not effective 33

34 LITERATURE REVIEW for glass-phase-free and densely sintered alumina and zirconia ceramics (Kern and Wegner, 1998; Friederich and Kern, 2002). For densely sintered alumina- and zircionia-based ceramics, such as Procera All Ceram or Y- PSZ ceramics, a durable resin bond can only be achieved with airborne particle abrasion ( µm Al 2 O 3 at 2.5-bar) and a phosphate-modified resin cement (Panavia or Panavia 21) (Kern and Wegner, 1998; Wegner and Kern, 2000; Friederich and Kern, 2002; Hummel and Kern, 2004). The high-strength aluminium- and zirconium-based core ceramics do not require adhesive cementation techniques to strengthen the restoration (Sorensen et al, 1998; Sorensen, 1999; Fradeani, 2000; McLaren, 2000), whereas acid-etching and cementation with adhesive resins are recommended for silica-based all-ceramic systems (Wohlwend, 1990; Fradeani and Aquilano, 1997; Sorensen et al, 1998). With Y-TZP-based materials, adhesive cementation may be used, but is not mandatory; and traditional luting agents, such as glassionomer cements, may be applied (Besimo et al, 2001; Filser et al, 2001; Suttor et al, 2001). However, some clinical situations, such as compromised retention and short abutment teeth, require resin bonding and, therefore, adequate ceramic surface conditioning (Burke et al, 2002; Blatz et al, 2003). While the elimination of postcementation sensitivity remains a clinical objective, all types of dental cements cause side effects (Sorensen et al, 1998). Johnson recorded a 32% incidence of immediate postcementation sensitivity for zink phosphate cement and 19% for glass-ionomer cement (Johnson et al, 1993). A 9.1% incidence of postcementation symptoms by using the adhesive technology compares favorably to the incidence of symptoms for conventional cements (Sorensen et al, 1998) Technical aspects for all-ceramic FPDs Connector dimensions The maximum strain in fixed partial dentures is systematically located in the connectors (Kelly et al, 1995). The law of beams is the basis for the proper design of the connectors and pontic. The deflection of a beam increases as the cube of its length is inversely proportional to its width, and is inversely proportional to the cube of its height (Dupont, 1968). When the underlying structure is deformable, as in the case of dental and periodontal structures, the 34

35 LITERATURE REVIEW resultant strains bring the anchoring systems together and open the angle of the axis formed by abutment teeth. The more deformable a structure is, the closer the tensile strength approaches the cervix. The tensile strength is mainly induced on the cervical side of the connectors. The 0.15% strain threshold, beyond which rupture is possible, is only reached when the connector surface is minimal (3.3 mm 2 ). Extending the surface of the connectors, which is less consistent with periodontal clinical requirements, is not necessary to ensure resistance to rupture (Augereau et al, 1998). Kamposiora et al (1996) showed that increasing the connector height from 3 to 4 mm dramatically reduced the stress levels within the connectors. Apart from the size of the connectors, their design is also crucial. Generally, connectors with a round design do not develop stress peaks, and therefore withstand higher pressures. Rounding off the connector in the mesiodistal axis to provide an adequate radius also prevents stress peaks, and thereby allows higher pressures to be applied before fracture occurs (Filser, 2003). Dimension requirements of 3x3 mm have been suggested for all-ceramic FPD coreconnectors (Futterknecht, 1990; McLean, 1993). Another suggestion is 4x4 mm (Kappert, 1990) or 3 mm buccolingually and 4 mm occlusogingivally (McLaren, 1998). Furthermore, it was suggested that a diameter of 4 mm in case of a molar replacement and 3 mm in other cases should be applied (Vult von Steyern et al, 2001). In order to minimize the risk of fracture for all-ceramic FPDs clinically, we should always follow manufacturer recommendations in regard to the minimum critical dimensions demanded for the connectors Thermal Expansion Coefficient (TEC) Metals and porcelains should be selected with a slight mismatch in their TEC s (metal slightly larger), so that the metal contracts more than the porcelain on cooling from the firing temperature to room temperature. This mismatch leaves the porcelain in residual compression and provides additional strength to the restoration (Anusavice, 2003). This philosophy has been successfully applied for the fabrication of many multilayered all-ceramic restorations. Considering the fact that the core is put under tensile stress, a relative high flexural strength and a minimum critical material thickness are required in order to achieve a stable bond between the core and the veneering material. In the Empress II system, a minimum core (TEC=10.6x10-6 /K, 350 MPa flexural strength) thickness of 0.8 mm is required, in order to achieve a reliable bond-strength (10% TEC difference) with its corresponding veneering 35

36 LITERATURE REVIEW material (TEC=9.7x 10-6 /K, 100 MPa flexural strength). In-Ceram Alumina shows a slight TEC-difference ( 5%) between its core (TEC=7.4 x 10-6 /K, 500 MPa flexural strength) and its veneering material (TEC=7.0 x 10-6 /K, 84 MPa flexural strength), so that a minimum 0.4 mm core thickness is required for a long-term stability (Kappert, 2001). Thermal expansion coefficients of different Y-TZP framework values given by the manufacturer range from 10x10-6 /K (DC-Zirkon, DCS Precident, Allschwill, Switzerland and Lava, 3M ESPE, Germany) to 10.4x10-6 /K (Procera Zirconia, Nobel-Biocare AB, Göteborg, Sweden) and 10.5 x10-6 /K (Cercon, DeguDent, Frankfurt, Germany and Vita In Ceram 2000 YZ Cubes, Vita Zahnfabrik, D-Bad Säckingen). The corresponding company, for each of these framework materials, also develops their respective compatible veneering materials. 2.4 Survival rates of all-ceramic FPDs Clinical Studies Empress II: In a short term clinical study, 4 out of 61 Empress II fixed partial dentures (anterior and posterior region) failed to fracture, showing a failure rate of 6.7% after an observation period of 1 year (Sorensen et al, 1999). Three of the four restorations that failed had occlusogingival connector heights that failed to achieve the recommended design standards. Esquivel-Upshaw et al (2004) showed a 93% success rate for Empress II posterior FPD s. One of the two fractures was associated with short connector height (2.9 mm). A further twoyear follow-up for Empress II crowns and FPD s (replacing up to the 1 st premolar) reported a 100% success rate for single crowns, but 50% survival rate for the FPD s, which tended to fracture at the connector regions (Taskonak and Sertgoz, 2005). Marquardt and Strub (2006) reported a 100% survival rate for Empress II crowns and 70% for FPDs (anterior and premolar region) after an observation period of 5 years. Three out of the six failures did not follow manufacturer recommendations in regard to the connector dimensions. In-Ceram Alumina: In a 3-year clinical study, In-Ceram Alumina FPDs yielded a 0% failure rate for anterior FPDs, 11% for FPD s with premolar pontics and 24% for FPD s with molar pontics. All FPDs were cemented with glass-ionomer cement. This study confirmed the indication given 36

37 LITERATURE REVIEW from the manufacturer recommending the use of In-Ceram Alumina only for anterior FPD s (Sorensen et al, 1998). Another clinical study on posterior In-Ceram Alumina FPDs (replacing premolar and molar teeth) showed a 90% success rate after an observation period of 5 years, demonstrating that the In-Ceram technique can also be a viable treatment alternative for the posterior region (Vult von Steyern et al, 2001). In a long-term retrospective study with anterior and posterior In-Ceram FPDs, the survival rates were 93% and 83% after 5 years and 10 years, respectively (Olsson et al, 2003). In-Ceram Zirconia: In a 3-year clinical observation, the survival rate of In-Ceram Zirconia posterior FPDs was 94.5 % (Suarez et al, 2004). Because of the esthetic limitations, resulting from the high opacity of its core, this material is recommended only for the fabrication of posterior FPDs (McLaren and White, 1999). Y-TZP-based restorations: In a 1 year clinical evaluation, 22 adhesively cemented 3-unit posterior ZrO 2 FPDs, milled out with the DCM-System, showed a success rate of 100% (Sturzenegger et al, 2000). Another 1-year clinical study reported a success rate of 100% for 20 3-unit and 6 4-unit posterior ZrO 2 FPDs, fabricated with the DCS Precident- System and cemented using zinc-phosphate (Tinschert et al, 2001c). Twenty-one posterior ZrO 2 FPDs, milled out with the Lava CAD- CAM system and cemented with glass-ionomer cement, showed a 100% success rate after an observation period of 8 months (Pospiech et al, 2002). After a 15.5-month mean evaluation period, 36 posterior and 10 anterior DC-Zirkon FPDs, cemented with zinc-phosphate cement, showed no framework fractures (100% survival rate) but only one chip-off defect (2.5%) of the veneering ceramic material (Tinschert, 2002). Fifty-eight 3- to 5-unit ZrO 2 FPDs (replacing premolars and molars), milled out with the Cercon (Cercon Smart Ceramics, DeguDent, Germany) system gave a survival rate of 93% after 2 years (Zembic et al, 2002) and 83% after 3 years of clinical service (Sailer et al, 2003). In these studies, 18% of the restorations showed no clinically acceptable marginal discrepancies, indicating that a further refinement of the manufacturing techniques is demanded. Bornemann et al (2003) examined 44 3-unit and 15 4-unit posterior ZrO 2 FPDs, milled out with the Cercon System and cemented with zinc-phosphate cement. Their results showed no framework fractures but a 3.5% chip-off of the veneering ceramic after 6 months in service. Molin (2003) reported a survival rate of 67% for 18 posterior and one anterior ZrO 2 Denzir FPDs after an observation 37

38 LITERATURE REVIEW period of 2 years. Fifty posterior DC-Zirkon FPDs (33 three-unit, 14 four-unit and 3 fiveunit), milled out with the DCS Precident- System and cemented with zinc-phosphate cement, showed no framework fracture, but 6% demonstrated chip-off veneering defects, after 3 years of clinical service (Tinschert et al, 2004b). In a 1-year clinical observation, 21 conventionally cemented Cercon ZrO 2 cantilever-fpds with maximum extension up to 8 mm showed a survival rate of 100% (Rinke, 2004). In another study, no framework fractures but only a 15% chip-off veneering defect rate for 20 conventionally cemented 5-unit (minimum 3 retainers) posterior and anterior DC-Zirkon FPDs were reported after an observation period of 2 years (Vult von Steyern et al, 2005). Although the presented short-term clinical results are quite promising, longitudinal clinical studies are necessary for the assessment of long-term success and for the establishment of more specific guidelines for the use of zirconia as a material for the fabrication of FPDs (Raigrodski, 2004b). Considering that metal ceramic FPDs, which are our current standard of care, demonstrate a cumulative survival rate of 96% after 5 years and 87% after 10 years (Walton, 2002), all-ceramic FPDs should demonstrate at least a similar survival rate in clinical studies if they are to be considered as a predictable restorative alternative (Raigrodski, 2004a). 38

39 AIM OF THE STUDY 3 Aim of the study The aim of the present in vitro study was: 1. To compare the fracture resistance and mode of failure of posterior zirconia all ceramic fixed partial dentures, fabricated with 3 different CAD/CAM systems. 2. To evaluate the effect of fatigue loading in the chewing simulator on the fracture resistance of the tested materials. 39

40 OUTLINE OF THE STUDY (FIG. 1) 4 Outline of the study (Fig. 1) Figure 1. shows the outline of the study: 96 teeth (48 premolars + 48 molars) 32 teeth 32 teeth 32 teeth 16 samples 16 samples 16 samples Procera + DCZirkon + Vita YZ + e max Ceram e max Ceram e max Ceram 16 FPDs 16 FPDs 16 FPDs 8 aged FPDs 8 nonaged FPDs 8 aged FPDs 8 nonaged FPDs 8 aged FPDs 8 nonaged FPDs Fracture strength test Figure 1 40

41 MATERIALS AND METHODS 5 Materials and Methods 5.1 MATERIALS Abutment Teeth Forty-eight caries-free human lower premolars and molars, without fractures and/or hypoplastic defects, were used as abutments. The teeth were obtained directly after extraction and stored in 0.1% thymol solution throughout the study. The root surfaces were cleaned from concrements and desmodontal rests ultrasonically and with handscalers Materials used for the fabrication of the all-ceramic FPDs The all-ceramic FPDs were divided into 3 groups of 16 specimens each. Each group was fabricated with a different framework material. All frameworks of different groups were veneered with the same ceramic veneering material (e max Ceram, Ivoclar Vivadent AG, Schaan, Liechtenstein). The composition and properties of the materials used for the fabrication of the frameworks of the FPDs is listed in Table 2. 41

42 MATERIALS AND METHODS COMPOSITION (WEIGHT %) PROPERTIES FRAMEWORK MATERIAL Procera Zirconia DC-Zirkon Vita In Ceram YZ Cubes ZrO 2 (HfO 2 ) approx. 95%, <5% HfO 2 approx. 95% Approx. 95%, <3% HfO 2 Y 2 O % <5% 5% Al 2 O 3 +other oxides <0.5% <1% (Na 2 O) <1% (Si O 2 ) Al 2 O 3 or other oxides Not provided <0.5 Na 2 O Not provided Volumetric weight >6.05 g/cm 3 >6.08g/cm g/cm 3 Particle size <0.5µm <0.6 µm 300 nm Vickers Hardness 1200 HV 1200 HV 1200 HV Melting temperature 2700 C Not provided 2706 C Bending strength 1121MPa 900 MPa >900 MPa Elasticity modulus 210 GPa 210 Gpa 210 GPa Fracture toughness 10 Mpa m 7 Mpa m 5.9 MPa m Compression resistance Not provided 2000 MPa Not provided Fatigue strength (ring Not provided <0.002mm 3 /h Not provided on disk) Corrosion resistance in Not provided <0.01(mg/m 2 x24h) Not provided Ringer solution 37 C Flexural strength Not provided 1200 MPa Not provided Chemical solubility Not provided Not provided <20 µg/cm 2 (ISO 6872) TEC ( C) 10.4x10-6 /K 10 x10-6 /K 10.5 x10-6 /K Sintering stage Presintered Densely sintered Presintered Table 2: Overview of the framework materials (Data provided by manufacturers) 42

43 MATERIALS AND METHODS The composition of the e max Ceram veneering material (Ivoclar Vivadent AG, Schaan, Liechtenstein) used for the veneering of all the frameworks are listed in Table 3: Composition (weight%) Veneering Material e max Ceram (Ivoclar, Vivadent) SiO Al 2 O K Na F Zr ZnO 2-3 TiO TEC (10-6 K -1 ) 9.5 ± 0.25 Table 3: Composition of the e max Ceram (emc) veneering material (Data provided by manufacturer) Materials used for the cementation procedure Ketac TM Cem Maxicap TM (3M ESPE) is a glass-ionomer cement, which consists of a powder and a liquid. The powder is composed of glass powder and pigments. The liquid is composed of polycarbonacids, tartaric acid, water and conservation agent. The cement is provided in a capsule with standardized doses of powder and liquid ensuring a controllable quality of the cement properties. The system also provides a Maxicap activator to activate the capsules for 2 seconds; a high-frequency mixer with 4300 oscillations/min, such as Capmix (8-15 sec mixing time), or a rotation mixer Rotomix (7-12 sec) for mixing; and a Maxicap applier for application. The times applied for an ambient temperature of 23 C are as follows: Activation time: 2 sec Mixing in Rotomix: 8 sec Mixing in high-speed mixer: 10 sec Working time (from beginning of mixing): 3 min Setting time (from beginning of mixing): 7 min 43

44 MATERIALS AND METHODS An overextended working time causes the loss of adhesion to enamel and dentin. Impression and die materials Twinduo (Picodent, Wipperfürth, Germany) The material is an additional cross-linking high viscosity silicone impression material presenting a 30-second mixing time, 1.5-minute working time and 6- to 7-minute setting time. It presents 0.9% deformation under load, 99.2% elasticity after deformation and 0.1% linear dimensional change. Dimension Garant L (Espe, Seefeld, Germany) This is a hydrophilic low consistency additional polymerization silicone material. It presents 4.5% deformation under load (ISO), 99.9% elasticity after deformation (ISO) and 0.20% linear dimensional change (ISO, after 24h). The setting time is 5.5 minutes after mixing. Firmer Set Putty (Espe, Seefeld, Germany) The material is an additional cross-linking high viscosity silicone impression material with a 0.05% dimensional change and a 0.3% compression set. The mixing time is 45 seconds and the setting time is 5 minutes. GC Fujirock (GC Belgium) This is a Type 4 dental stone with improved physical properties and workability. The recommended water/powder ratio is 20 ml/100gr. The setting expansion is 0.08% and the compressive strength is 53 Mpa. 44

45 MATERIALS AND METHODS Additional Materials (Table 4): Material/Equipment Anti-Rutsch Lack Artificial oral environment/ Thermocycling System Steatite ceramic ball Diamond Burs No , No No 837KR.012, No 8837KR.012 Technovit 4000 Occlu Plus-Spray FG-Diabolo burs Zwick Z010/TN2S Dentona esthetic-base gold dental stone Company Wenco-Wenselaar GmbH, Hilden, Germany Willytech, Munich, Germany Gebrüder Haake GmbH, Karlsruhe, Germany Hoechst Ceram Tec, Wunsiedel, Germany Gebr. Brasseler, Lemgo, Germany Zwick, Ulm, Germany Hager & Werken GmbH &Co. KG, Duisburg, Germany Bredent, Senden, Germany Zwick, Ulm, Germany Detmold, Germany Table 4: Overview of the additional materials used for the experiment 5.2 METHODS Representative model The representative model consists of two abutments (a second premolar and a second molar) and one pontic, which functions to replace the first missing molar. In a study by Stambaugh and Wittrock (1977), who reported that the average mesiodistal width of the mandibular first molar is mm, the mesiodistal width of the pontic used in the study was 11 mm. The selected model was embedded in a sample holder of an artificial oral environment (Wilytech, Munich, Germany) using a silicone putty material (Twinduo, Picodent, Wipperfürth, Germany). The occlusal table of the model was set to be parallel to the horizontal plane. A silicon mold (Twinduo, Picodent, Wipperfürth, Germany) of the model was then fabricated covering at least 2 mm above the edge of the sample holder. This mold was also used as the negative form for fixing the abutments in the sample holder. The 45

46 MATERIALS AND METHODS abutment teeth were set in place in the mold with wax, which covered their cervical area to an extent of 2 mm apically from the cemento-enamel junction (CEJ). The 2 mm represent the biological width, considering that the margin of the preparation was set approximately to the height of the CEJ Artificial periodontal membrane It has been demonstrated that abutment mobility has a big influence on the fracture resistance of FPDs (Kelly et al, 1995). Therefore, in order to imitate the physiological tooth mobility, all roots were covered with an artificial periodontal membrane made out of a gum resin (Anti- Rutsch-Lack, Wenko-Wenselaar GmbH, Hilden, Germany) (Kern et al, 1993; Koutayas et al, 2000). This resin covered the root starting 2 mm apically from the CEJ to the root tip (biological width principle) Embedding models in the sample holders After the gum resin has dried, the silicon mold with the abutments in place was attached to the sample holder, which was previously isolated with Vaseline (Weises Vaselin Lichtenstein, Winthrop, D-Fürstenfeldbruck). A self-curing polyester resin (Technovit 4000, Kulzer, Wehrheim, Germany) was then mixed and poured into the sample holder. After the resin had set, the silicon mold was removed and the abutments were cleaned. The resin specimens were then stored in 0.1% thymol solution Tooth preparation (Fig. 2) The abutment teeth were prepared with a 1.2-mm circular deep chamfer, an occlusal reduction of 1.5 mm, a facial reduction of 1.2 mm and a convergence angle of 6. The heights of the premolars and the molars were 6 mm and 5 mm, respectively. All transitions from the axial to the occlusal surfaces were rounded. Homogeneous and smooth surfaces were achieved. 46

47 MATERIALS AND METHODS Figure 2: Representative tooth preparation Impression procedure The impression technique used is known as the simultaneous Putty/Wash-Technique. First, a thin coat of 3M ESPE tray adhesive was brushed on the mini-tray and left to air dry for a minimum of 5 minutes. After the prepared abutments were thoroughly dried, Dimension Garant L was placed around them using a syringe. Equal volumes of Firmer Set Putty base and catalyst were then mixed, put on the mini-tray and placed parallel to the tooth line. It was held in position without pressure until the material was set (5 minutes). The impression was then removed from the abutments, and after one hour the master model was poured Fabrication of master models In order to avoid introduction/encapsulation of bubbles into the master model, the impressions were first conditioned with a silicone surfactant. Then, GC Fujirock type 4 dental stone was added to the water within 10 seconds and mixed uniformly for 60 seconds by mechanical spatulation under vacuum. The recommended water/powder ratio is 20ml/100gr. After a setting time of at least 45 minutes, the master model was removed from the impression. Exceptionally, for the fabrication of the Vita In Ceram 2000 YZ Cubes master models, a special scannable dental stone (Dentona esthetic-base gold, Detmold, Germany) was used. 47

48 MATERIALS AND METHODS Fabrication of all-ceramic FPDs Manufacturing the framework The frameworks of all groups had a connector height of 3 mm, a connector width of 3 mm, a 0.7 mm occlusal thickness and an axial thickness of 0.5 mm. Procera Zirconia, Nobel-Biocare AB, Göteborg, Sweden (Fig. 3) Figure 3: Procera Zirconia frameworks After the fabrication of the stone die, a tactile scanner (Procera Forte, Procera, Nobel Biocare, Göteborg, Sweden) was used to scan the dies in order to duplicate the shape and to identify the margins with precision. This scanner is indicated not only for single units but also for bridges with their soft tissues, neighboring teeth, and bite registrations. The whole scanning process takes from 6-10 minutes. The data were transmitted to the Procera Software CAD- Application, where the design of the restoration took place. A 0.7 mm occlusal thickness, 0.5 mm circumferential thickness and 9 mm 2 connector surface were taken into consideration for the design of the frameworks. Then, the computer-designed framework was oversized by 20% to compensate for contraction which occurs during the final sintering. After completion of the design-procedure, the enlarged dies were fabricated. Zirconia is dry-pressed against the enlarged dies, and the temperature is raised to a temperature similar to the presintering stage. At this point, the enlarged and porous copings are stable. Their outer surfaces were milled to the desired shape and the copings, now removed from the enlarged dies, were placed into the furnace and fired to full sintering. During this cycle, the copings shrank to fit the dimensions of the original working dies. Subsequent fitting adjustments were made after sintering using 48

49 MATERIALS AND METHODS rotary diamond instruments with water cooling (FG-Diabolo burs, Bredent, Senden, Germany). The completed copings were then ready for the veneering process. DC-Zirkon (DCS President System, Allschwill, Switzerland) (Fig. 4) Figure 4: DC-Zirkon frameworks The DCS President system consists of three main parts: I) the Preciscan, a fully automatic laser scanner, II) the DCS Dentform software and III) the Precimill machining center. The Preciscan laser scanner measures preparation dies (stone dies) as well as the whole cast (up to 14 single abutments or entire FPDs). After measuring, the collected data were then digitized and transmitted to a computer where the FPD framework was designed and calculated (CAM process). Data on the framework were subsequently forwarded to the Precimill machining center, where a framework (0.5 mm axial thickness, 0.7 mm occlusal thickness and 3x3 mm connector dimensions) was milled out of a densely sintered zirconia block. This ZrO 2 block was manufactured under optimized industrial conditions by the sintering and hot isostatic post-compaction (HIP) of tetragonal zirconia polycrystals (TZP) (TKT Metoxit AG, Thayngen, Switzerland). With this system there was no need to confront any dimensional changes because the material was already sintered. Subsequent fitting adjustments were made using rotary diamond instruments with water cooling (FG-Diabolo burs, Bredent, Senden, Germany), and the frameworks were ready for further veneering. 49

50 MATERIALS AND METHODS Vita In Ceram 2000 YZ Cubes (Vita Zahnfabrik, Bad Säckingen, Germany) (Fig. 5) Figure 5: Vita In Ceram YZ Cubes frameworks Dies of the Vita group were scanned using the Cerec InLab scanner (Sirona, Bensheim, Germany). The data were then digitized and transmitted to a computer where the framework was designed and calculated. Presintered Y-TZP blocks (Vita In Ceram 2000 YZ Cubes, Vita Zahnfabrik, D-Bad Säckingen) were milled in the CAM unit (Cerec InLab, Sirona, Bensheim, Germany). The unit mills an enlarged framework (0.5 mm axial thickness, 0.7 mm occlusal thickness and 3x3 mm connector dimensions) out of a presintered block to compensate for the later sintering shrinkage. The milled frameworks were then carefully removed from the machine and separated from the block holders at the milled side using diamond cutting instruments. The frameworks were subsequently postsintered using a high temperature furnace (Vita ZXrcomat, Vita Zahnfabrik, Bad Säckingen, Germany). The temperature in the firing chamber would not exceed 1600 C. Subsequent fitting adjustments after sintering were made using rotary diamond instruments with water cooling (FG-Diabolo burs, Bredent, Senden, Germany) Veneering procedures (Fig. 6) All frameworks of the test groups were veneered with the same ceramic veneering material (e max Ceram, Ivoclar-Vivadent, Schaan, Liechtenstein) in the Programat 100 oven (Ivoclar- Vivadent, Schaan, Liechtenstein). Silicon keys were made in order to control the thickness of the veneering material. The veneering process of all three types of zirconia-frameworks with e max Ceram is listed in Table 5: 50

51 MATERIALS AND METHODS Procedure Heating Holding High Preheating Heating Vacuum Vacuum rate time temp. ( C) rise (min) in ( C) out ( C) ( C/min (min) ( C) ) Opaque Dentin Glaze Table 5: Firing chart of the e max Ceram (emc) (Ivoclar-Vivadent, Schaan, Liechtenstein) Figure 6: Procera Zirconia framework veneered with the e max Ceram veneering material Cementation of the FPDs The Ketac TM Cem Maxicap TM (3M ESPE, Seefeld, Germany) glass-ionomer cement was used for the cementation of all FPDs. The abutments were thoroughly cleaned with 0.1% chlorhexedine and air-dried so that the surface has a matte shiny appearance. Similarly, the inner surfaces of the copings of the FPDs were cleaned with chlorhexedine 0.1% and completely air-dried. A capsule of Ketac TM Cem Maxicap TM was then activated for 2 sec in the Maxicap activator and further mixed for 10 sec in the Rotomix. The capsule was placed in 51

52 MATERIALS AND METHODS the Maxicap applier and further applied on a glass-slab. Afterwards, the cement was evenly applied on the inner surfaces of the copings with a small brush and the FPDs were placed on the abutments and held in place under finger pressure for 2-3 minutes. After it was set (7 minutes), excessive cement was removed using a scalpel Dynamic loading of the test samples The artificial mouth (Figure 7) allows the evaluation of dental restorative systems under clinically relevant conditions (Krejci et al, 1990). The artificial oral environment (Willytec, Munich, Germany ) consists of 8 identical sample chambers, two stepper motors controlling the vertical and horizontal movement of the samples against an antagonist, a hot and cold water circulation system (Haake, Karlsruhe, Germany) and a computerized control unit (Kern et al, 1999). Half of the samples of each group were subjected to 1.2 million chewing cycles by a reproducible dynamic occlusal load, which corresponds to five years of clinical function (Krejci et al, 1990). The applied load was 50 N (De Boever et al, 1978; Krejci et al, 1990), and the thermocycling was 5 C to 55 C for 60 seconds each, with an intermediate pause of 12 seconds, maintained by the thermostatically-controlled liquid circulator (Haake, Karlsruhe, Germany) (Fig. 8). A 6-mm diameter ceramic antagonist Steatit ball (Höchst Ceram Tec, Wunsiedel, Germany) was applied vertically onto the occlusal surface of the pontic of the FPDs. During dynamic loading, all samples were examined twice a day and any fracture of the teeth or the porcelain was recorded as a failure. Cold/hot bath temperature Dwell time Vertical movement Horizontal movement Descending speed Rising speed Forward speed Backward speed 5 C/55 C 60 sec 6 mm 0.5 mm 60 mm/s 55 mm/s 60 mm/s 55 mm/s Applied weight per sample 5 kg (49 N) Cycle frequency 1.6 Hz 52

53 MATERIALS AND METHODS Table 6: Test parameters of the artificial mouth Figure 7: Schematic drawing of the dual-axis chewing simulator with eight sample chambers (Willytech, Munich, Germany) (Kern, 1999) (1) upper crossbeam, (2) lower crossbeam, (3a) water reservoir (in), (3b) water reservoir (out), (4) filter for cold water, (5) filter for warm water, (6) pump for removal of cold water, (7) pump for removal of warm water, (8) pump for application of cold water, (9) pump for application of warm water, (10) motor table, (11) table 53

54 MATERIALS AND METHODS Figure 8: Schematic drawing for one chewing chamber (Kern, 1999) 54

55 MATERIALS AND METHODS Survival rate During the dynamic loading, all samples were examined twice a day. Fractures of the tooth or the porcelain were recorded as failure Fracture resistance test All samples of all test groups were loaded until fracture occurred using a universal-testing machine (Zwick Z010/TN2S, Ulm, Germany). Tin foil of 1mm thickness (Dentaurum, Ispringen, Germany) was placed over the occlusal surface of the first molar (pontic) to achieve a homogeneous stress distribution. A perpendicular load was applied to the occlusal surface of the first molar (pontic), under a stroke control of 2 mm/min. The loads required to fracture the samples were recorded with the Zwick testxpert V 7.1 software Statistics The statistical analysis of the fracture resistance tests was performed by Dr T. Gerds, Institute of Medical Biometry and Medical Informatics, Albert Ludwigs University, Freiburg, Germany. Fracture resistance data were analyzed by nonparametric ANOVA using Kruskal- Wallis and Wilcoxon rank sum tests (R Development Core Team 2005, R: A language and environment for statistical computing, R Foundation for statistical computing, Vienna, Austria. ISBN with a significance level of Boxplots were used for visualizing the data. 55

56 RESULTS 6 Results 6.1 Survival rate of all-ceramic FPDs after aging All specimens subjected to aging survived 1,200,000 cycles of dynamic loading. No chipping of the veneering ceramic or decementation of the FPDs was recorded. 6.2 Fracture resistance tests (Table 7, Fig. 9) The smallest fracture resistance value was observed in the aged Procera group (1044 N), whereas the highest value was observed in the non-aged DCS group (1993) (Table 7). The highest median fracture resistance value without aging occurred in the Vita CerecInLab group (1702 N), followed by the DCS (1683 N) and Procera (1522 N) groups. After aging, the highest value was found in the DCS group (1618 N) followed by the Vita CerecInlab (1556 N) and Procera (1256 N) groups. Group Minimum 1 st Quartile Median Mean (± S.D.) 3 rd Quartile Maximum Procera Zirconia ± initial Procera Zirconia ± aged DC-Zirkon Initial ± DC-Zirkon aged ± Vita CerecInLab ± initial Vita ±

57 RESULTS CerecInLab aged Table 7: Results of the f racture resistance values of different groups in N (fracture resistance values of individual samples are listed in the appendix): Fig. 9 Box plot of the results of the fracture resistance test in N The central box shows the data between the 1 st -Quartile and the 3 rd -Quartile, the median is represented by a line. Compared to the values at the initial stage, artificial aging reduced the fracture resistance by 4.76%, 13.3% and 7% for groups DCS, Procera and Vita CerecInLab, respectively. This reduction, however, was not statistically significant for any of the groups tested. Similarly, no significant differences were found for the fracture resistance comparisons between different groups before artificial aging (Kruskal-Wallis test: p=0.3). After artificial aging, the Kruskal- Wallis test showed a significant group effect (p=0.03). Wilcoxon test revealed significantly smaller fracture resistance of Procera compared to Vita CerecInLab (Wilcoxon test: p=0.015) and to DCS (Wilcoxon test: p=0.038). 57

58 RESULTS 6.3 Fracture patterns The location and mode of failure of the specimens after the load-to-fracture test are summarized in Table 8. Most of the fractures before and after aging occurred either at the distal connector or at both connectors. Group Abutment fracture Distal connector Mesial connector Both connectors DCS/initial DCS/aged Procera/initial Procera/aged Vita CerecInLab/initial Vita CerecInLab/aged Table 8: Location and mode of failure after the load-to-fracture test 58

59 RESULTS Procera Zirconia group In the group without artificial aging, 5 FPDs fractured at both connector sides; 1 FPD at the mesial connector (premolar side) and 2 FPD at the distal connector (molar side) followed by decem entation as well. In the group after artificial aging, all 8 FPDs fractured at the distal connector (molar side). Decementation w as observed in 2 specimens. Although it was difficult to assess whether the fractures started at the loading point or at the connectors, the f ractures were perpendicular to the mesial-distal axis of the frameworks in a smooth curve between the loading point and the gingival side of the connector (Fig. 10) Figure 10 Representative figure of the fracture pattern of Procera group. 59

60 RESULTS DCS group In the group without artificial aging, 7 FPDs fractured at the distal connector (molar side) and 1 FPD showed a fracture of the premolar tooth combined with a total fracture at the distal connector (molar side) In the group after artificial aging, 4 FPDs fractured at the distal connector (molar side), 3 FPDs showed fractures at both connector sides and 1 FPD showed a fracture of the self-curing resin (Technovit 4000, Zwick, Ulm, Germany) at the premolar region combined with connector fracture at this area. Similar to the Procera group, the fractures were perpendicular to the mesial-distal axis of the frameworks in a smooth curve between the loading point and the gingival side of the connector (Fig. 11). Fig. 11 Representative figure of the fracture pattern of DCS group 60

61 RESULTS Vita CerecInLab group In the group without artificial aging, 3 FPDs demonstrated fractures at the mesial connector (premolar side), 4 FPDs at the distal connector side (molar region) and 1 FPD fractured at both connector sides. In the group after artificial aging, 3 FPDs fractured at both connector sides, 3 FPDs at the distal connector side (molar region) and the rest 2 showed fractures of the self-curing resin (Technovit 4000, Zwick, Ulm, Germany) around the premolar abutments, in combination with fractures at both connector sides. As in the previous test groups, it was difficult to determine the origin of the fracture. The fractures were perpendicular to the mesial-distal axis of the frameworks in a smooth curve between the loading point and the gingival side of the connector (Fig. 12). Fig. 12 Representative figure of the fracture pattern of the Vita CerecInLab group. 61

62 DISCUSSION 7 Discussion The design and tests conducted in this study were chosen to better simulate clinical conditions. The number of the specimens tested and the use of water rather than artificial saliva during testing are limitations that may affect the interpretation of the results. 7.1 Methods The use of natural teeth as abutments In this study, extracted human teeth were used as abutments because their modulus of elasticity, bonding characteristics, thermal conductivity and strength are closer to the clinical situation than those of metal, plastic and animal teeth. Human teeth have also been used in other studies (Dietschi et al, 1990; Haller, 1990; King and Aboush, 1999; Chitmongkolsuk et al, 2002; Rosentritt et al, 2006). The extracted human teeth were stored in 0.1% thymol solution, preventing them from drying out and thereby becoming brittle (Helfer et al, 1972), and also inhibiting microbial activity (Sparrius and Grossman, 1989). In several studies, metal abutments were used to test the fracture strength of bridges (Kappert, 1990; Bieniek, 1994; Ludwig, 1994; Kappert, 1995; Rosentritt et al, 2006). Different types of metals were used for testing, such as Ni-Cr-Fe alloy (Wiskott et al, 1996), stainless steel (Wilson, 1994), and brass (Yamashita et al, 1997b). Their advantages are that the metal abutments have identical physical properties and dimensions. However, the elastic and bonding properties of these abutments cannot be compared to those of natural teeth. Plastic abutments made out of composite or epoxy resin have been also used as abutments for fracture resistance testing of all-ceramic crowns (Scherrer and de Rijk, 1993; Yoshinari and Derand, 1994; Neiva et al, 1998) and post and core systems (Schmeißner, 1977; Wegmann, 1987). Their advantages are similar to those of metal abutments, including a modulus of elasticity similar to that of human dentin. They can be etched with 34% phosphoric acid (Neiva et al, 1998), but they lack bonding characteristics as they do not consist of water and organic substances. In a comparative in vitro study, Rosentritt et al (2006) investigated the influence of the abutment material, using human, polymer and alloy abutments, on the fracture resistance of all-ceramic FPDs. They found out that the application of the alloy 62

63 DISCUSSION abutments clearly lead to an overestimation of the fracture resistance of the ceramic FPDs. Human and polymeric abutments showed similar influence, and the combination with an artificial periodontium lead to a 70% reduction of the fracture resistance after artificial aging. Bovine teeth have similar bonding characteristics, modulus of elasticity and tensile strength to human teeth (Sano et al, 1994). They have also been used for testing the fracture strength of ceramic restorations (Mesaros et al, 1994), the bonding strength of luting materials (Phrukkanon et al, 1998; Hosoya and Tominaga, 1999) and the fracture resistance of different post systems (Isidor et al, 1999). Because of the great size discrepancy between bovine and human teeth, however, the use of bovine teeth for testing the fracture resistance of all-ceramic FPDs is difficult Artificial periodontal membrane In the present study, a thin layer of gum resin (Anti-Rutsch-lack ) was painted on the roots of the abutments to imitate physiological tooth mobility during both chewing simulation and fracture resistance testing. Kern et al (1993) and Koutayas et al (2000) showed that it can mimic tooth mobility similar to physiological tooth movement (Mühlemann, 1951). Abutment mobility has been demonstrated as a decisive factor in the evaluation of fracture resistance, and when a small abutment rotation is allowed, failure is more likely to occur (Kelly et al, 1995). A series of in vitro studies tested the fracture resistance of In Ceram Alumina FPDs using variable materials to simulate the abutment mobility and variable aging methods. The results after chewing simulation, with and without artificial periodontal membrane, were 523 N and 337 N (DeLong and Douglas, 1983), 676 N and 256 N (Rosentritt et al, 2000) and 919 N and 305 N (Scherrer et al, 1996), respectively. Similarly, after thermocycling in artificial saliva, the values obtained were 2225 N for groups without artificial periodontium versus 703 N for groups with periodontal membrane (Kappert et al, 1991). These observations are in agreement with the reports of another recent study showing that the fracture strength of Empress II FPDs (Ivoclar-Vivadent, Schaan, Liechtenstein) was significantly influenced by the use of an artificial periodontium device (1 mm Impregum, 3M Espe, G-Seefeld)(Rosentritt et al, 2006). Resigning on the periodontium during artificial aging caused fracture strength values almost twice as high as the ones with periodontium. More distinct was the influence of the artificial periodontium by the non-aged FPDs: rigid teeth showed results that were three-times higher than those with the polyether layer. 63

64 DISCUSSION The antagonistic material In this in vitro study, ceramic balls (Steatite, Hoechst Ceram Tec, Wunsiedel, Germany) were used as antagonists during the artificial aging in the chewing machine. These antagonists have Vickers hardness similar to that of enamel. Few studies have investigated the influence of the antagonistic material on the fracture resistance of FPDs. In an in vitro study, the use of human antagonists instead of ceramic balls significantly decreased the loading capacity of FPDs (Condon and Ferracane, 1996). In another study, the antagonistic material and design showed a similar effect on the FPDs, reducing insignificantly the fracture strength by 175 N (Rosentritt et al, 2006) Preparation design and connector dimensions Although there are no standards in the literature in terms of preparation design for zirconiabased all-ceramic FPDs, it is recommended that an occlusal reduction of mm, a facial reduction of mm, a deep chamfer or circular shoulder with inner rounded angle between 1.0 and 1.5 mm wide, and an occlusal angle of convergence not greater than 10 degrees be employed (Friedlander et al, 1990; Chiche, 1994; Shillingburg, 1997; Sorensen et al, 1998; McLaren and White, 1999; McLaren, 2000). In the present study, a circular 1.2-mm deep chamfer preparation with a 6 -convergence angle, 1.5-mm occlusal reduction and smooth, rounded transitions from the axial to the occlusal surfaces was the aim. The increased fracture resistance of the new high-strength ceramics compensates for the decreased thickness of the material required. The maximum strain in FPDs is located in the connector area. In the present study, all connectors had 3x3-mm well rounded dimensions. Variable connector dimensions have been suggested in the literature: 4x4 mm (Kappert, 1990), 3x3 mm (Futterknecht, 1990; McLean, 1993), 3 mm buccolingually and 4 mm occlusogingivally (McLaren, 1998) and a 4 mm diameter in the case of a molar replacement and 3 mm diameter for other cases (Vult von Steyern et al, 2001). Other studies showed that the stress concentration on the connectors of the FPDs is reduced with a connector of at least 4 mm in height (Kamposiora et al, 1996; Pospiech et al, 1996). A study using strain gauges in posterior FPDs indicated that the strain distribution in-vivo is different from that in-vitro. In-vitro, the marginal portion under the 64

65 DISCUSSION cusp during loading was greatly strained. In-vivo, however, the whole portion of the FPD strained, and the strain values were greatest on the buccal and lingual portions of the posterior retainer, and on the distal connector (Yamashita et al, 1997a). Enlarging the connector crosssection reduces the peak stresses and hence the probability of fracture for conventional systems. Oversized connectors, however, limit the clinical range of use, compromise plaque control and lead to unaesthetic results. Under clinical conditions, the occlusal contact and the gingival tissue define the limits of the connector dimensions. If the minimum vertical dimension required for the connectors is not available, the clinician may consider performing electrosurgery to remove the soft tissue to gain space for the connector height, although the extent of tissue removal is limited and the biologic width must be respected (Sorensen et al, 1999). Because the core ceramic is significantly stronger than the veneering porcelain, it may be advisable in the case of a FPD not to veneer the core material at the tissue side of the connectors and in areas where esthetic considerations are not crucial (Sorensen et al, 1999; McLaren and White, 2000; Guazzato et al, 2004d). In contrast to the former study, Sundh et al (2005) showed no significant difference in fracture resistance of Y-TZP FPDs between the veneered specimens and those heat-treated without veneering. As the few available studies are controversial, further studies are necessary to investigate the design and influence of the environment of restorations with the core material exposed (Guazzato et al, 2004c) Clinical relevance of fracture resistance tests In order to assess the suitability of new experimental ceramic materials for dental application, a number of in-vitro studies testing the properties and behavior of the materials must be carried out. Taking into consideration the brittleness of ceramics, the fracture strength of experimental ceramic materials should be comparable to those of other accepted materials to validate their use for further clinical testing. Fracture strength tests of ceramic materials are important for determining or maximizing the expected lifetime with an acceptable low probability of failure (Ritter, 1995b). Considering the practical difficulties in the preparation of specimens, the most commonly used testing methods for ceramic materials are four-point bending, three-point bending, and biaxial bending; which include the ring-on-ring (Kao, 1971), ball-on-ring (McKinney, 1970), and piston-on-three-ball tests (Kirstein, 1967). It is important to note that the failure stresses derived from different testing methods are significantly different, and a direct comparison is not valid. The ring-on-ring biaxial bend test offers a larger specimen area or volume subjected 65

66 DISCUSSION to the maximum stress compared to the three-point bending and the piston-on-three ball tests. It is also unaffected by edge failure, and resembles the clinical condition as it generates the greatest number of interfacial failures (Zeng et al, 1996; Guazzato et al, 2004c). Furthermore, the disk-shaped specimens are preferable because they have an area similar to dental restorations (Ban and Anusavice, 1990; Thompson, 2000). Thompson (2000) investigated the effect of testing method and variation of the core-thickness ratio of bilayered ceramics on the mode and origin of failure, and found a positive effect. In this study, 250 out of 270 specimens delaminated; and for these testing configurations and relative layer heights, the flexural strength of the core material generally proved greater than interface toughness. In regard to the origin of failure, none of the clinically similar specimens (core/veneer ratio 1:2) failed at the interface with any testing method. When the ratio becomes 1:2, which resembles the geometry of a FPD connector, the interface failure increases to 43%. This is in agreement with the findings of Kelly et al (1995), who reported that the most interface failure origins (failure rate 70-78%) occurred at the gingival side of the FPD connectors. Another study, carried out by Wakabayashi and Anusavice (2000), investigated the effect of the core/veneer thickness ratio and of the elastic modulus of the substrate on the crack initiation of bilayered ceramic disks. They found that the core/veneer thickness ratio is the dominant factor that controls the failure initiation site (shifts from the veneer to the core as the ratio increases) in bilayered ceramics; but the increase in elastic modulus did not affect the crack initiation site. Additional factors that need to be considered when trying to compare the results gained from different studies are the use of mobile or immobile abutments, the static or dynamic mode of load, and the wet or dry environment. It has been shown that the values obtained when using immobile abutments are higher than those when using mobile abutments (Tinschert et al, 1999), whereas dynamic loading reduces the flexural strength of dental ceramics compared to their static values (Geis-Gerstorfer and Fässler, 1999). Furthermore, water and temperature changes reduce the fracture strength of ceramics. For the strength to accurately reflect the variability and time dependency of a ceramic, the test environment must be the same as the service environment, and the strength-controlling flaw population must be the same as that responsible for failure in service. Therefore, it is generally recommended that test samples and mode of loading be chosen to closely simulate the actual components in service (Ritter, 1995a, b; Kelly, 1999). Thus, a direct comparison between the results obtained in the different studies is difficult, as is extrapolation of in-vitro results to clinical situations. 66

67 DISCUSSION Clinical relevance of the artificial aging process In the oral environment, the forces applied on dental restorations are of a cyclic nature (Lundgren and Laurell, 1984). In addition, the presence of moisture and temperature changes leads to slow flaw propagation and degradation of the mechanical properties of dental ceramics (Kelly, 1999). Therefore, instead of monotonic static loading, it is more clinically relevant to test the specimens under fatigue load in a chewing simulator (DeLong and Douglas, 1983; Krejci et al, 1990; Kelly, 1999). Various chewing simulators have been used in several in vitro studies to simulate clinical conditions and evaluate dental restorative systems under clinically relevant conditions (DeLong and Douglas, 1983; Krejci et al, 1990; Strub and Beschnidt, 1998; Behr et al, 1999; Kern et al, 1999; Koutayas et al, 2000). After 100,000 fatigue loading cycles, the flexural strength for In Ceram, IPS Empress and Dicor was approximately 50% of the initial strength (Schwickerath, 1996). The chewing simulator used in this study was developed to reproduce the in vivo environment by adding moisture and a controlled temperature to the test conditions (Kern et al, 1999). The artificial chewing cycle in the artificial oral environment is designed to correspond to physiologic conditions. The magnitude, duration and frequency of the force applied are comparable to values reported in the literature (Bates et al, 1975, 1976; Bradley, 1996). Krejci et al (1990) indicated that the chewing machine fulfils the parameters concerning chewing motion and thermal changes reported in the literature. Fatigue produced by 240, ,000 cycles in the chewing simulator corresponds to a period of 1 year of clinical service. Therefore, 1,200,000 chewing cycles correspond to a five-year clinical service (Kern et al, 1999). In a recent study, the influence of different simulation parameters on the properties of dental restorations was compared (Rosentritt et al, 2006). It was found that the duplication of chewing frequency, lateral sliding or continued force increased during aging did not significantly influence the fracture resistance of FPDs. A temperature gradient of 50 C led to a significant reduction of fracture resistance (400 N), compared to constant cycling of water with 25 C. Duplication of the mouth opening distance from 2 to 4 mm did not significantly influence the fracture resistance. Increasing the loading force from 50 to 150 N significantly reduced the loading capacity. A staircase load-increase to 150 N showed no significant differences compared to a test with a constant load of 150 N. 67

68 DISCUSSION 7.2 Results Survival rate after the chewing simulation In the present in vitro study, one-half of all samples were exposed to the artificial oral environment to simulate a five-year clinical service, before the fracture resistance test was performed. All the exposed samples survived, exhibiting no fractures or chipping off defects Fracture resistance tests To date, conventional porcelain-fused-to-metal fixed partial denture (PFM) is still considered the gold standard, in terms of predictability, for the rehabilitation of edentulous spaces at the posterior region. Therefore, the resistance of PFMs may serve as a guideline for the new tested ceramic materials. A direct comparison among the studies, however, seems reasonable only if the test methodology is the same. Chitmongkolsuk et al (2002), following the same test parameters with the ones in the present study, reported fracture resistance values for posterior PFMs of 3500 and 2800 N, before and after artificial aging, respectively. In the present study, the mean fracture resistance values for Procera were 1496 N and 1297 N, for DCS 1659 N and 1580 N and for Vita CerecInLab 1713 N and 1593 N, before and after artificial aging, respectively. The artificial aging lead to a degradation of the fracture resistance behavior in all groups tested, but this influence was not stastically significant. All values (before and after artificial aging) were much lower than the corresponding ones reported for PFMs (3500, 2800 N) but still much higher than 1000 N, which is considered as the minimum demanded resistance of a material for application at the posterior region (Schwickerath, 1986, 1994; Geis-Gerstorfer and Fässler, 1999; Marx et al, 2001; Tinschert et al, 2001a). The fracture resistance of Y-TZP-based all-ceramic FPDs was evaluated in several in vitro studies. Tinschert et al (2001a) reported mean values of 2289 N (veneered) and 1900 N (only substructure) for DC Zirkon-based (DCS President System, Allschwill, Switzerland) FPDs, when using fixed metal abutments and without dynamic loading processes. Another research group investigated the fracture resistance of green stage Y-TZP Lava (3M ESPE, G) FPDs, placed on human abutments, and reported mean values of 1300 N after exposure to dynamic loading (Rosentritt et al, 2003). The fracture resistance testing of Vita CerecInLab (Vita CerecInLab Zahnfabrik, Bad Säckingen, Germany) posterior FPDs, also investigated in the present study, revealed values ranging from 68

69 DISCUSSION 900 N (only substructure) to 1900 N (veneered) when using fixed metal dies, 3x3 mm connector dimensions and exposure to dynamic loading (Sundh and Sjogren, 2006). Following a like experimental design, the same study group reported values for HIPed Y-TZP Denzir (Cad.esthetics AB, Skelleftea, Sweden) FPDs ranging from 3291 N (only substructure) to 2237 N and 1973 N, depending on the veneering material applied (Sundh et al, 2005). Because of the variety of the experimental designs followed, including connector dimensions, abutment selection/mobility and exposure to fatigue loading, a direct comparison between the values reported in previous studies may be misleading Influence of the chewing simulation on the fracture resistance In the present study, the exposure to fatigue loading reduced the fracture resistance of all tested groups. The Procera group showed a 13.30% reduction of the fracture resistance, followed by 7% for the Vita CerecInLab group and 4.76% for the DCS group. This reduction, however, was not statistically significant (P>0.05) for all groups. This outcome is in agreement with previous findings showing that cyclic loading in water did not significantly affect the fracture resistance of Y-TZP Denzir (Cad.esthetics AB, Skelleftea, Sweden) FPDs (Sundh et al, 2005). In addition, Y-TZP ceramics have previously been identified to be high strength materials, which were not influenced by repeated loading of up to 3000 N (Jung et al, 2000). However, when the stresses induced by repeated loading do not initially exceed the flexural strength, the growth of subcritical flaws may result in the occurrence of delayed catastrophic failure following 5x10 5 cycles (Chevalier, 1999b; Jung et al, 2000; Rauchs, 2001). Variations in the reduction rate of fracture resistance between different groups may be explained by different fabrication techniques for each system. Milling a framework out of a presintered Y-TZP blank and subsequently postsintering will probably produce surface flaws and residual/compressive stresses different than those produced in a Y-TZP framework milled out of a fully sintered blank (Tinschert et al, 2004a; Deville et al, 2005; Sundh et al, 2005; Chevalier, 2006; Deville et al, 2006). This will in turn lead to differences in the low temperature degradation resistance between different systems (Chevalier, 2006).Other comparative studies, which employ surface quality and structure detecting methodologies such as X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM) and Optical Interferometer (OI), may elucidate the effect of different fabrication techniques on the aging sensitivity of Y-TZP ceramics. 69

70 DISCUSSION Moisture contamination has been identified to be detrimental to the fracture resistance of ceramic-based dental restorative materials, routinely resulting in a 20% decrease in mean fracture strength (Sherrill, 1974; Morena et al, 1986; Addison et al, 2003). Schwickerath (1986) reported that the fracture strength of ceramic specimens was lowered by approximately 50% after 10 6 cycles. Similarly, combined thermal and mechanical loading significantly reduced the fracture resistance of Empress II FPDs from 1832 N to 410 N (Rosentritt et al, 2006). In the same study, after testing the influence of diverse stress parameters on the fracture resistance of all-ceramic FPDs, it was concluded that significant artificial aging, combining thermal cycling with mechanical loading, should be performed to obtain clinically relevant results. In the literature, however, there is some controversy regarding the influence of moisture on Y-TZP ceramics (Dauskardt, 1987; Shimizu et al, 1993; Chevalier, 1995). In an in vitro study, it was found that artificial aging lead to a statistically significant decrease of the fracture resistance of 3-unit CAD-CAM Lava FPDs, but not 4-unit FPDs (Rountree et al, 2001). In this study, even after chewing simulation, all test specimens of different groups exhibited fracture resistance values in the range of 1297 to 1593 N, which are far higher than the minimum expected loading capacity of 1000 N for application in the posterior region. Thus, the results indicate that Y-TZP ceramic is a material to be considered for all-ceramic FPDs in premolar and molar regions Influence of the veneering process on the fracture resistance of zirconiabased frameworks In order to improve their esthetic appearance, milled frameworks are veneered with compatible veneering materials. During the veneering procedure, the frameworks are exposed to moisture at relatively high temperatures. Y-TZP ceramic, however, are unstable over time due to the spontaneous transformation of the tetragonal phase (t) into the monoclinic phase (m), leading to mechanical property degradation (Chevalier, 1999a; Piconi and Maccauro, 1999). Since the t-m transformation is affected by temperature and vapor, the possibility that the mechanical properties of Y-TZP ceramics are affected during veneering cannot be excluded. Additionally, it has been suggested that grinding by machining introduces residual compressive stresses on the surface, which influences the mechanical properties of zirconia ceramics, and that subsequent heat treatment/veneering relaxes these residual stresses (Reed, 1977; Kosmac et al, 1999; Sundh and Sjogren, 2006). In the literature, however, conflicting 70

71 DISCUSSION results are reported regarding the effects of surface treatment on the strength of dental ceramics (Zhang et al, 2004; Guazzato et al, 2005b; Sundh et al, 2005). In a study of Zhang et al (2004), sandblasting of Y-TZP and alumina ceramics significantly reduced their strength, while in another study sandblasting increased the strength of Y-TZP ceramics (Guazzato et al, 2005b). Moreover, the fracture resistance of 3-unit Y-TZP frameworks was significantly reduced after veneering with a feldspar-based ceramic or a glass ceramic or after heat treatment in a way similar to veneering (Sundh et al, 2005). Similarly, the fracture resistance of an Mg-PSZ (Denzir-M ) ceramic was significantly reduced by heat treatment or veneering, while that of a Y-TZP (Vita CerecInLab ) ceramic considerably increased after veneering (Sundh and Sjogren, 2006). Although the reported results differ, the mechanical properties of zirconia ceramics are clearly affected by surface treatments (Zhang et al, 2004; Guazzato et al, 2005b; Sundh et al, 2005; Sundh and Sjogren, 2006). In the present study, all Y-TZP frameworks were veneered with a compatible feldspar-ceramic, and then tested to fracture. There has been no control group of as-sintered (without veneering) frameworks. Therefore, the influence of the veneering process on fracture resistance of the tested materials could not be evaluated Fracture patterns A goal of prosthetic dentistry is to preserve the remaining tooth structure even if the restoration fails. Failures of a bridge can occur because of technical, biophysical and biological problems. Technical and biophysical failures include fracture of the metal and/or ceramics, abutment fracture, and loss of retention (Strub et al, 1988). For most of the specimens in the present study, the fractures were located at the loading point and through one or both of the connectors. One specimen of the non-aged DC-Zirkon exhibited a tooth fracture; while 3 specimens, 1 of the aged DC-Zirkon group and 2 of the aged Vita CerecInLab- group, showed fractures of the self-curing resin (Technovit 4000, Zwick, Ulm, Germany) surrounding the premolar abutments. In general, the fracture patterns observed were similar to those reported in previous in vitro studies of 3-unit partiallystabilized zirconia FPDs (Filser et al, 2001; Tinschert et al, 2001a; Sundh et al, 2005). It was difficult to assess the exact origin of the fractures occurred; whether they start at the loading point or at the connectors. The fractures, however, were perpendicular to the mesial-distal axis of the frameworks in a smooth curve between the loading point and the gingival side of the connector. The failure mode is in agreement with findings of previous studies showing 71

72 DISCUSSION that the exclusive mode of failure in vitro and in vivo for all-ceramic FPDs was a fracture of the connectors, and that the gingival side of the connectors can be the area where high tensile stresses are located (Campbell and Sozio, 1988; Kelly et al, 1995; Filser et al, 2001; Tinschert et al, 2001a; Vult von Steyern et al, 2001; Sundh et al, 2005). To ensure long-term success of PFMs, the minimal critical dimensions recommended for the connectors are 2.5 x 2.5 mm (Miller, 1977; McLean, 1982). For all-ceramic FPDs, due to their primary mode of failure and their brittleness, the required connector dimensions are larger. This may be a major contributing factor in restricting the versatility of their use. Therefore, appropriate diagnosis, patient selection and conception of the requirements, such as anatomical limitations, hygenic reasons and esthetic expectations of proper ceramic framework design, are crucial for the success of these restorations (Raigrodski, 2004a). In the present study, connector dimensions of 3x3 mm were used. Several in vitro studies, investigating the fracture resistance of Y-TZP based all-ceramic FPDs with connector dimensions of 3x3 mm, have yielded good results (Vult von Steyern, 2005; Vult von Steyern et al, 2005) In a clinical study, Y-TZP all-ceramic FPDs with connector dimensions of 4x4 mm showed a success rate of 100% after an observation period of 2 years (Vult von Steyern et al, 2005). No clinical studies are available on the clinical success of Y-TZP all-ceramic FPDs with connector dimensions of 3x3 mm. Therefore the long-term success of these restorations should be evaluated before recommending them for clinical application. 72

73 CONCLUSIONS 8 Conclusions Within the limitations of this in vitro study, the following conclusions may be drawn: 1. All tested Y-TZP all-ceramic FPDs have the potential to withstand physiological occlusal forces applied in the posterior region, and appear therefore to be a viable alternative to replace conventional posterior PFMs. 2. Critical issues such as connector shape and size, veneering ceramics, aging behavior and long-term clinical performance need to be further assessed before recommending Y-TZP allceramic FPDs for daily practice. 73

74 SUMMARY 9 Summary Objective: The purpose of this in vitro study was to evaluate the fracture resistance of different zirconia three-unit posterior all-ceramic fixed partial dentures (FPDs) before and after fatigue loading in the chewing machine. Material and methods: 96 teeth (48 mandibular premolars and 48 molars) were prepared, covered with an artificial periodontal membrane, in order to simulate the physiological abutment mobility, and fixed into models representing a 3-unit FPD. After impression taking and the fabrication of master models, 48 frameworks from 3 different zirconiumdioxide materials (Procera Zirconia, Nobel-Biocare AB, S-Göteborg; DC-Zirkon, DCS Dental AG, CH-Allschwill; Vita In-Ceram YZ Cubes,Vita Zahnfabrik, D-Bad Säckingen) were fabricated for the 3-unit FPDs (3 groups, 16 FPDs each). All frameworks were veneered with the e-max Ceram ceramic (Ivoclar, Vivadent, FL-Schaan). All FPDs were subsequentally cemented with glasionomer-cement (Ketac Cem, GC Europe, B-Leuven). One-half of the specimens (n=24) were artificially aged in the chewing simulator (1,2 million cycles, 49 N). All of the samples were then loaded until fracture occurred using a universal testing machine (Zwick Z010/TN2S, Zwick, Ulm, Germany). Results: All specimens, subjected to artificial aging, survived with no failures. The median fracture resistance values (min, max) before artificial aging were for DCS: 1683 N (1278,1993); Procera: 1522 N(1105, 1800); Vita: 1702 N (1472, 1946) - and after aging for DCS:1618 N (1175, 1804); Procera:1256 N (1044, 1783); and Vita: 1556 N (1394, 1854). The effect of artificial aging was not statistically significant between the test groups. Similarly, no significant differences were found for the fracture resistance comparisons between different groups before artificial aging. After artificial aging, Procera showed significantly smaller fracture resistance than Vita (Wilcoxon test: p=0.015) and DCS (Wilcoxon test: p=0.038). Conclusions: All tested restorations have the potential to withstand occlusal forces applied in the posterior region and may thus represent viable alternatives for use as posterior all-ceramic restorations. Other issues such as connector size and shape, ceramic veneering materials and methods, aging behavior, and long-term clinical performance need to be further assessed before recommending such restorations for daily practice. 74

75 ZUSAMMENFASSUNG 10 Zusammenfassung Zielsetzung: Das Ziel der vorliegenden in vitro Studie war es die Bruchfestigkeit von verschiedenen dreigliedrigen vollkeramischen Zirkondioxid Seitenzahnbrücken vor und nach künstlicher Alterung im Kausimulator zu evaluieren. Material und Methode: Es wurde 96 Zähne (48 Unterkieferprämolaren und 48 Molaren) beschliffen und mit einem künstlichen parodontalen Ligament, welches die physiologische Mobilität des Zahnes simuliert, in Modelle, entsprechend einer dreigliedrigen Brücke, fixiert. Nach Abformung und Arbeitsmodellherstellung wurden 48 Gerüste für dreigliedrige Seitzenzahnbrücken (3 Gruppen à 16 Brücken) aus drei verschiedenen Zirkoniumdioxid Materialien angefertigt: Procera Zirconia (Nobel-Biocare AB, S-Göteborg), DC-Zirkon (DCS Dental AG, CH-Allschwil), Vita In-Ceram YZ Cubes (Vita Zahnfabrik, D-Bad Säckingen).Alle Gerüste wurden mit e-max Ceram Keramik (Ivoclar, Vivadent, FL-Schaan) verblendet. Die Seitenzahnbrücken wurden mit Glasionomer-Zement (Ketac Cem, GC Europe, B-Leuven) zementiert. Jeweils eine Hälfte (n=24) der Prüfkörper wurde im Kausimulator künstlich gealtert (1,2 Millionen Zyklen, F= 49 N). Alle Prüfkörper wurden bis zum Bruch belastet (Universal-Prüfmaschine: Zwick Z010/TN2S, Zwick, Ulm, Deutschland). Ergebnisse: Alle Prüfkörper hielten der Kausimulation stand. Die mediane Bruchfestigkeit (Min, Max) betrug ohne Kausimulation für Procera 1256 N (1105, 1800), DCS 1522 N (1278, 1993), Vita 1618 N (1472, 1946), und nach künstlicher Alterung für Procera 1256 N (1044, 1783), DCS 1618 N (1175,1804), Vita 1556 N (1394, 1854). Innerhalb einer Gruppe, der Einfluß der künstlichen Alterung war nicht statistisch signifikant. Es gaben ebenfalls keine signifikante Unterschiede für die Bruchfestigkeit zwischen den Gruppen ohne Alterung. Nach künstlicher Alterung, Procera zeigte signifikant niedrigere Bruchfestigkeit als DCS (Wilcoxon test: p=0.038) und Vita (Wilcoxon test: p=0.015). Schlussfolgerung: Alle getesteten Restorationen hielten den im Seitenzahnbereich Belastungskräften stand und könnten als interessante Alternative zur konventionellen metallkeramischen Versorgungen im Seitenzahnbereich in Betracht gezogen werden. Zusätzliche Parameter, wie Verbindergestaltung, Verblendmassen, Alterung und langzeitiges klinisches Verhalten sollten noch weitergeprüft werden, bevor diese Restorationen als alltägliches Verfahren empfohlen werden können. 75

76 APPENDIX 11 Appendix 11.1 Fracture resistance values of the Procera group Without aging (Tab. 11.1) Sample # F (N) Table 11.1 With aging (Tab. 11.2) Sample # F (N) Table Fracture resistance values of the DCS group Without aging (Tab. 11.3) Sample # F (N) Table

77 APPENDIX With aging (Tab. 11.4) Sample # F(N) Table Fractu re resistance valu es of th e Vita g roup Without aging (Tab. 11.5) Sample # F(N) Table 11.5 With aging (Tab. 11.6) Sample # F(N) Table

78 REFERENCES 12 References Addison O, Fleming GJ, Marquis PM (2003). The effect of thermocycling on the strength of porcelain laminate veneer (PLV) materials. Dent Mater 19: Andersson M, Bergman B, Bessing C, Ericson G, Lu ndquist P, Nilson H ( 1989). Clinical results with titanium crowns fabricated with machine duplication and spark erosion. Acta Odontol Scand 47: Andersson M, Oden A (1993). A new all-ceramic crown. A dense-sintered, high-purity alumina coping with porcelain. Acta Odontol Scand 51: Andersson M, Razzoog ME, Oden A, Hegenbarth EA, Lang BR (1998). Procera: a new way to achieve an all-ceramic crown. Quintessence Int 29: Anusavice K (1989). Dental Ceramics and Metal Ceramics. In: Transactions, International Congress of Dental Materials: Honolulu, Hawaii. Anusavice KJ (2003). Dental Ceramics. In: Phillips' Science of dental ceramics. ed t, (ed). Saunders. Anusavice KJ, De Hoff PH, H ojjatie B, Gray A (1989). Influence of tempering and contracti on mismatch on crack development in ceramic surfaces. J Dent Res 68: Anusavice KJ, Hojjatie B (1991). Effect of thermal tempering on strength and crack propagation behavior of feldspathic porcelains. J Dent Res 70: Anusavice KJ, Hojjatie B, Chang TC (1994). Effect of grinding and fluoride-gel exposure on strength of ion-exchanged porcelain. J Dent Res 73: Anusavice KJ, Shen C, Vermost B, Chow B (1992). Strengthening of porcelain by ion exchange subsequent to thermal tempering. Dent Mater 8: Augereau D, Pierrisnard L, Barquins M (1998). Relevance of the finite element method to optimize fi xed partial denture desi gn. Part I. Influence of the size of the connector on the magnitude o f strain. Clin Oral Investig 2: Ban S, Anusavice KJ (1990). Influence of test method on failure stress of brittle dental materials. J Dent Res 69: Banks RG (1990). Conservative posterior ceramic restorations: a literature review. J Prosthet Dent 63: Bates JF, Stafford GD, Harrison A (1975). Masticatory function--a review of the literature. 1. The form of the masticatory cycle. J Oral Rehabil 2: Bates JF, Stafford GD, Harrison A (1976). Masticatory function - a review of the literature. III. Masticatory performance and efficiency. J Oral Rehabil 3:

79 REFERENCES Becker CM, Kaldahl WB (2005). Current theories of crown contour, margin placement, and pontic design J Prosthet Dent 93: Behr M, Rosentritt M, Leibrock A, Schneider-Feyrer S, Handel G (1999). In-vitro study of fracture strength and marginal adaptation of fibre-reinforced adhesive fixed partial inlay dentures. J Dent 27: Besimo CE, Spielmann HP, Rohner HP (2001). Computer-assisted generation of all-ceramic crowns and fixed partial dentures. Int J Comput Dent 4: Beuer FF, K; Erdelt, KJ; Gernet, W (2003). Marginale und innere Passung von CAM- Dtsch Zahnarztl Z 58: gefrästen Zirkoniumoxid-Einzelkronen-gerüsten bei unterschiedlichen Prparationswinkeln. Bieniek KM, R (1994). Die mechanische Belastbarkeit neuer vollkeramischer Kronen- und Brückenmaterialien. Schweiz Monatsschr Zahnmed 104: Bindl A, Windisch S, Mormann WH (1999). Full-ceramic CAD/CIM anterior crowns and copings. Int J Comput Dent 2: Blatz MB, Sadan A, Kern M (2003). Resin-ceramic bonding: a review of the literature. J Prosthet Dent 89: Boening KW, Wolf BH, Schmidt AE, Kastner K, Walter MH (2000). Clinical fit of Procera AllCeram crowns. J Prosthet Dent 84: Bornemann G, Rinke S, Hüls A (2003). Prospective clinical trial with conventionally luted zirconia-based fixed partila dentures-18-month-results. In: IADR 2003: Goteborg. Bradley M (1996). Physiologie des stomatognathen Systems 1. ed. UllsteinMosby: Berlin. Brick ER, H; Luthardt, RG; Johannes, M; Sandkuhl, O (2003). Einsatz von Nanokeramik für Kronengerüste. ZWR 112: Bukat A (1990). GSP-made Ca-PSZ for biomedical applications. In: Bioceramics 3. Hulbert JH, SF, (ed). Rose-Hulman Institute of Technology: Terre Haute, Indiana, USA, pp Burger WB, M; Cittadini, A; Piconi, C; Richter, HG; Ricci, GA (1995). Yttria stabilised zirconia (Y-TZP) for biomedical applications by an innovative approach. In: 4th Euro- Ceramics vol 8-Bioceramics. Ravaglioli A, (ed). Faenza Editrice Publ.: Faenza, Italy, pp Burger WR, HG, Piconi, C; Cittadini, A; Boccalari, M; Vaterroni, R (1997). New TZP powders for medical grade zirconia. J Mat Sci Mater Med: Burke FJ, Fleming GJ, Nathanson D, Marquis PM (2002). Are adhesive technologies needed to support ceramics? An assessment of the current evidence. J Adhes Dent 4:

80 REFERENCES Campbell SD (1989). A comparative strength study of metal ceramic and all-ceramic esthetic materials: modulus of rupture. J Prosthet Dent 62: Campbell SD, Sozio RB (1988). Evaluation of the fit and strength of an all-ceramic fixed partial denture. J Prosthet Dent 59: Chevalier J (2006). What future for zirconia as a biomaterial? Biomaterials 27: Chevalier J, Deville S, Munch E, Jullian R, Lair F (2004). Critical effect of cubic phase on aging in 3mol% yttria-stabilized zirconia ceramics for hip replacement prosthesis. Biomaterials 25: Chevalier JC, B Drouin, JM; (1999a). Low temperature aging of Y-TZP ceramics. J Am Ceram Soc 82: Chevalier JD, JM; Cales, B (1977). Low temperature aging behavior of zirconia hip joint heads. In: Bioceramics. Sedel LR, C, (ed). Elsevier: Amsterdam, pp Chevalier JO, C; Cales, B (1995). Crack propagtion behaviour in Y-TZP ceramics. J Am Ceram Soc 78: Chevalier JO, C; Fantozzi, G; (1999b). Subcritical crack propagation in 3Y-TZP ceramics: static and cyclic fatigue. J Am Ceram Soc 82: Chiche G, Radiguet J, Pinault A, Genini P (1986). Improved esthetics for the ceramometal crown. Int J Periodontics Restorative Dent 6: Chiche GP, A;, (ed). (1994). Esthetics of anterior fixed prosthodontics. Carol Stream (IL) Quintessence. Chitmongkolsuk S, Heydecke G, Stappert C, Strub JR (2002). Fracture strength of all-ceramic lithium disilicate and porcelain-fused-to-metal bridges for molar replacement after dynamic loading. Eur J Prosthodont Restor Dent 10: Chong KH, Chai J, Takahashi Y, Wozniak W (2002). Flexural strength of In-Ceram alumina and In-Ceram zirconia core materials. Int J Prosthodont 15: Christel P (1988). Biomechanical compatibility and design of ceramic implants for orthopaedic surgery. Bioeramics: material characteristics vs in vivo behavior. Ann NY Acad Sci: Christel P, Meunier A, Heller M, Torre JP, Peille CN (1989). Mechanical properties and short-term in-vivo evaluation of yttrium-oxide-partially-stabilized zirconia. J Biomed Mater Res 23: Christensen GJ (1994). Ceramic vs. porcelain-fused-to-metal crowns: give your patients a choice. J Am Dent Assoc 125: 311-2,

81 REFERENCES Condon JR, Ferracane JL (1996). Evaluation of composite wear with a new multi-mode oral wear simulator. Dent Mater 12: Creugers NH, Kayser AF, van 't Hof MA (1994). A meta-analysis of durability data on conventional fixed bridges. Community Dent Oral Epidemiol 22: Creugers NH, Kreulen CM, Snoek PA, de Kanter RJ (2000). A systematic review of single- tooth restorations supported by implants. J Dent 28: Dauskardt RY, W; Ritchie, RO (1987). Fatigue crack propagation in transformation toughened zirconia ceramic. J Am Ceram Soc 70: De Boever JA, McCall WD, Jr., Holden S, Ash MM, Jr. (1978). Functional occlusal forces: an investigation by telemetry. J Prosthet Dent 40: De Kanter RJ, Creugers NH, Verzijden CW, Van't Hof MA (1998). A five-year multi-practice clinical study on posterior resin-bonded bridges. J Dent Res 77: DeHoff PH, Anusavice KJ (1992). Analysis of tempering stresses in bilayered porcelain discs. J Dent Res 71: DeLong R, Douglas WH (1983). Development of an artificial oral environment for the testing of dental restoratives: bi-axial force and movement control. J Dent Res 62: Denry IL, Holloway JA (2006). Microstructural and crystallographic surface changes after grinding zirconia-based dental ceramics. J Biomed Mater Res B Appl Biomater 76: Deville S, Chevalier J, Gremillard L (2006). Influence of surface finish and residual stresses on the ageing sensitivity of biomedical grade zirconia. Biomaterials 27: Deville S, Gremillard L, Chevalier J, Fantozzi G (2005). A critical comparison of methods for the determination of the aging sensitivity in biomedical grade yttria-stabilized zirconia. J Biomed Mater Res B Appl Biomater 72: Dietschi D, Maeder M, Meyer JM, Holz J (1990). In vitro resistance to fracture of porcelain inlays bonded to tooth. Quintessence Int 21: Dietze SK, Th; Teeuwen, R (2003). Langzeitschicksal von Restgebiss und 1474 klammerverankerten Modelguss-prothesen in einer zahnärztlechen Praxis. Dtsch Zahnarztl Z 58: Dion I (1994). Physico-chemistry and cytotoxicity of ceramics. J Mater Sci Mater Med: Dreyer J (1958). Prüfungsergebnisse zahnärztlicher Gussverfahren. Dtsch Zahnarztl Z 13: Dupont R (1968). Large ceramo-metallic restorations. Int Dent J 18: Duret F, Blouin JL, Duret B (1988). CAD-CAM in dentistry. J Am Dent Assoc 117:

82 REFERENCES Edelhoff D, Sorensen JA (2002). Retention of selected core materials to zirconia posts. Oper Dent 27: Edelhoff DS, J (2002). Light transmission through all-ceramic framework and cement combinations. J Dent Res: 81. Eichner K (1963). Messung der Kaukräfte bei Kauvorgängen. Dtsch Zahnarztl Z 18: Erdelt KB, F; Schweiger, J; Eichberger, M; Gernet, W (2004). Die Biegefestigkeit von weißkörpergefrästem Zirkoniumdioxid. Quintessenz Zahntech 30: Esquivel-Upshaw JF, Anusavice KJ, Young H, Jones J, Gibbs C (2004). Clinical performance of a lithia disilicate-based core ceramic for three-unit posterior FPDs. Int J Prosthodont 17: Fairhurst CW, Anusavice KJ, Hashinger DT, Ringle RD, Twiggs SW (1980). Thermal expansion of dental alloys and porcelains. J Biomed Mater Res 14: Fairhurst CW, Lockwood PE, Ringle RD, Thompson WO (1992). The effect of glaze on porcelain strength. Dent Mater 8: Filser F, Kocher P, Weibel F, Luthy H, Scharer P, Gauckler LJ (2001). Reliability and strength of all-ceramic dental restorations fabricated by direct ceramic machining (DCM). Int J Comput Dent 4: Filser FK, P; Lüthy, H; Schärer, P; Gauckler, L (1997). All-ceramic dental bridges by direct ceramic machining (DCM). In: Bioceramics. Sedel LR, C, (ed). Pergamon Press/Elsevier Science: Oxford, pp Filser FK, P; Lüthy, H; Schärer, P; Gauckler, L (2003). Posterior all-ceramic bridgework Assessment of fracture load and reliability of materials. Quint Dent Technol 1: Fradeani M, Aquilano A (1997). Clinical experience with Empress crowns. Int J Prosthodont 10: Fradeani M, Redemagni M (2002). An 11-year clinical evaluation of leucite-reinforced glass- Dent Technol 23: ceramic crowns: a retrospective study. Quintessence Int 33: Fradeani MB, G (2000). Lithium disilicate glass-ceramic restorations: Indications and guidelines. Quint Fransson B, Oilo G, Gjeitanger R (1985). The fit of metal-ceramic crowns, a clinical study. Dent Mater 1: Freese A (1959). Porcelain fused to iridio-platinum crowns. J Prosthet Dent 9: Friederich R, Kern M (2002). Resin bond strength to densely sintered alumina ceramic. Int J Prosthodont 15:

83 REFERENCES Friedlander LD, Munoz CA, Goodacre CJ, Doyle MG, Moore BK (1990). The effect of tooth preparation design on the breaking strength of Dicor crowns: Part 1. Int J Prosthodont 3: Fujisawa AS, T; Makinouchi, K (1996). The development of zirconia balls fo THR with a high strength, low phase transformation. In: Biocaramics 9. Kokubo TN, T; Miyaji F, (ed). Elsevier Science: Amsterdam, pp Futterknecht NJ, V (1990). Renaissance in der Vollkeramik. Quintessence-Zahntech 16: , Gardner FM (1982). Margins of complete crowns--literature review. J Prosthet Dent 48: Garvie R (1972). stucture and thermodynamical properties of partially stabilised zirconia in the CaO-ZrO 2 system. J Amer Ceram Soc: Garvie RH, RH; Pascoe, RT (1975). Ceramic steel. Nature: Gebhardt A (2000). Rapid Prototyping. Hanser Verlag: München. Geis-Gerstorfer J, Fässler P (1999). Untersuchungen zur Ermüdungsverhalten der Dentalkeramiken Zirkondioxid-TZP und In-Ceram. Dtsch Zahnarztl Z 54: Gibbard LL, Zarb G (2002). A 5-year prospective study of implant-supported single-tooth replacements. J Can Dent Assoc 68: Gibbs CH, Mahan PE, Mauderli A, Lundeen HC, Walsh EK (1986). Limits of human bite strength. J Prosthet Dent 56: Giordano R (2003). CAD/CAM: An overview of Machines and Materials. J Dent Technol: Giordano RA, 2nd, Pelletier L, Campbell S, Pober R (1995). Flexural strength of an infused ceramic, glass ceramic, and feldspathic porcelain. J Prosthet Dent 73: Glauser R, Sailer I, Wohlwend A, Studer S, Schibli M, Scharer P (2004). Experimental zirconia abutments for implant-supported single-tooth restorations in esthetically demanding regions: 4-year results of a prospective clinical study. Int J Prosthodont 17: Goodacre CJ, Van Roekel NB, Dykema RW, Ullmann RB (1977). The collarless metalceramic crown. J Prosthet Dent 38: Guazzato M, Albakry M, Quach L, Swain MV (2005a). Influence of surface and heat treatments on the flexural strength of a glass-infiltrated alumina/zirconia-reinforced dental ceramic. Dent Mater 21:

84 REFERENCES Guazzato M, Albakry M, Ringer SP, Swain MV (2004a). Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part I. Pressable and alumina glassinfiltrated ceramics. Dent Mater 20: Guazzato M, Albakry M, Ringer SP, Swain MV (2004b). Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part II. Zirconia-based dental ceramics. Dent Mater 20: Guazzato M, Albakry M, Swain MV, Ironside J (2002). Mechanical properties of In-Ceram Alumina and In-Ceram Zirconia. Int J Prosthodont 15: Guazzato M, Proos K, Quach L, Swain MV (2004c). Strength, reliability and mode of fracture of bilayered porcelain/zirconia (Y-TZP) dental ceramics. Biomaterials 25: Guazzato M, Proos K, Sara G, Swain MV (2004d). Strength, reliability, and mode of fracture of bilayered porcelain/core ceramics. Int J Prosthodont 17: Guazzato M, Quach L, Albakry M, Swain MV (2005b). Influence of surface and heat treatments on the flexural strength of Y-TZP dental ceramic. J Dent 33: Gupta TBJKRCLR, BR (1978). Stabilisation of tetragonal phase in polycristalline zirconia. J Mater Sci: Hagberg C (1986). Electromyography and bite force studies of muscular function and dysfunction in masticatory muscles. Swed Dent J Suppl 37: Haller BT, R; Klaiber, B; Schmitz, A (1990). Höckerstabilisierung durch Adhäsivinlays in MOD-Kavitäten. Dtsch Zahnarztl Z 45: Hansen PA, West LA (1997). Allergic reaction following insertion of a Pd-Cu-Au fixed partial denture: a clinical report. J Prosthodont 6: Hauptmann HS, D; Frank, S; Hoescheler, H (2000). Material properties of all ceramic zirconia prostheses. J Dent Res 79: 507. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, Haselton DR, Stanford CM, Vargas MA (2002a). Relative translucency of six all-ceramic systems. Part I: core materials. J Prosthet Dent 88: 4-9. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, Haselton DR, Stanford CM, Vargas MA (2002b). Relative translucency of six all-ceramic systems. Part II: core and veneer materials. J Prosthet Dent 88: Helfer AR, Melnick S, Schilder H (1972). Determination of the moisture content of vital and pulpless teeth. Oral Surg Oral Med Oral Pathol 34: Helkimo E, Carlsson GE, Helkimo M (1977). Bite force and state of dentition. Acta Odontol Scand 35:

85 REFERENCES Helmer JDT (1969). Research on bioceramics. In: Symposium on Use of Ceramics as Surgical Implants: South carolina, Clemson University. Hertlein GH, S; Frank, S; Suttor, D (2001). Marginal fit of CAD/ CAM manufactured all ceramic zirconia prostheses. J Dent Res 80: 42. Heydecke G, Butz F, Hussein A, Strub JR (2002). Fracture strength after dynamic loading of endodontically treated teeth restored with different post-and-core systems. J Prosthet Dent 87: Hobo S, Shillingburg HT, Jr. (1973). Porcelain fused to metal: tooth preparation and coping design. J Prosthet Dent 30: Holand W, Schweiger M, Frank M, Rheinberger V (2000). A comparison of the microstructure and properties of the IPS Empress 2 and the IPS Empress glass-ceramics. J Biomed Mater Res 53: Hosoya Y, Tominaga A (1999). A comparison of five adhesive systems to primary enamel. Pediatr Dent 21: Hummel M, Kern M (2004). Durability of the resin bond strength to the alumina ceramic Procera. Dent Mater 20: Isidor F, Brondum K, Ravnholt G (1999). The influence of post length and crown ferrule length on the resistance to cyclic loading of bovine teeth with prefabricated titanium posts. Int J Prosthodont 12: Isidor F, Stokholm R, Ravnholt G (1995). Tensile bond strength of resin luting cement to glass infiltrated porous aluminium oxide cores (In-Ceram). Eur J Prosthodont Restor Dent 3: Jäger KG, G; Schrutt, L (1989). Aufbau und Test einer Messkette zur Kaukraftmesung. Schweiz Monatsschr Zahnmed 99: Johnson GH, Powell LV, DeRouen TA (1993). Evaluation and control of post-cementation pulpal sensitivity: zinc phosphate and glass ionomer luting cements. J Am Dent Assoc 124: Jones D (1983). The strength and strengthening mechanisms of dental ceramics. In: Dental Ceramics Proceedings of the First Intarnational Symposium on Ceramics. JW M, (ed). Quintessence: Chicago, pp Jones DW (1985). Development of dental ceramics. An historical perspective. Dent Clin North Am 29: Jung YG, Peterson IM, Kim DK, Lawn BR (2000). Lifetime-limiting strength degradation from contact fatigue in dental ceramics. J Dent Res 79:

86 REFERENCES Jung YG, Peterson IM, Pajares A, Lawn BR (1999a). Contact damage resistance and strength degradation of glass-infiltrated alumina and spinel ceramics. J Dent Res 78: Jung YG, Wuttiphan S, Peterson IM, Lawn BR (1999b). Damage modes in dental layer structures. J Dent Res 78: Kamposiora P, Papavasiliou G, Bayne SC, Felton DA (1996). Stress concentration in allceramic posterior fixed partial dentures. Quintessence Int 27: Kao RP, N; Capps, W (1971). Large-deflection solution of the coaxial-ring-circular-glassplate flexure problem. J Am Ceram Soc 54: Kappert HF, Knode H, Schultheiss R (1991). [Strength of the In-Ceram system under mechanical loads and thermocycling in artificial saliva]. Dtsch Zahnarztl Z 46: Kappert HFK, H; Manzotti, L (1990). Metallfreie Brücken für den Seitenzahnbereich. Dent Labor (Munch) 38: Kappert HFK, M (2001). Keramiken-eine Übersicht Dentalkeramiken und zahntechnische Verfahren zur Herstellung von keramischem Zahnersatz. Quintessenz Zahntech 27: Kappert HK, U; Wehrstein, A; Kmitta, M; Knipp, J (1995). Festigkeit von Zirkonoxidverstärkten Vollkeramikbrücken aus In Ceram. Dtsch Zahnarztl Z 50: Karlsson S (1993). The fit of Procera titanium crowns. An in vitro and clinical study. Acta Odontol Scand 51: Keith O, Kusy RP, Whitley JQ (1994). Zirconia brackets: an evaluation of morphology and coefficients of friction. Am J Orthod Dentofacial Orthop 106: Kelly J (1995). Clinical failure of dental ceramic structures: Insight from combined fractography, in vitro testing and Finite Element Analysis. Ceramic Transactions 48: Kelly J (1997). Ceramics in restorative and prosthetic dentistry. Ann Rev Mat Science 27: Kelly JR (1999). Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 81: Kelly JR, Campbell SD, Bowen HK (1989). Fracture-surface analysis of dental ceramics. J Prosthet Dent 62: Kelly JR, Nishimura I, Campbell SD (1996). Ceramics in dentistry: historical roots and current perspectives. J Prosthet Dent 75: Kelly JR, Tesk JA, Sorensen JA (1995). Failure of all-ceramic fixed partial dentures in vitro and in vivo: analysis and modeling. J Dent Res 74:

87 REFERENCES Kern M, Douglas WH, Fechtig T, Strub JR, DeLong R (1993). Fracture strength of allporcelain, resin-bonded bridges after testing in an artificial oral environment. J Dent 21: Kern M, Strub JR, Lu XY (1999). Wear of composite resin veneering materials in a dual-axis chewing simulator. J Oral Rehabil 26: Kern M, Thompson VP (1995). Bonding to glass infiltrated alumina ceramic: adhesive methods and their durability. J Prosthet Dent 73: Kern M, Wegner SM (1998). Bonding to zirconia ceramic: adhesion methods and their durability. Dent Mater 14: Kerschbaum T (1995). Normierter Kronenrand? Eine Analyse aus klinischer Sicht. In: Qualitätssicherung in der Zahnheilkunde-Anspruch und Wirklichkeit. Walther WH, M, (ed). Hüthig: Heidelberg. Kerschbaum T (2004). Langzeitüberlebensdauer von Zahnersatz. Eine Übersicht. Quintessenz 55: Kikuchi M, Korioth TW, Hannam AG (1997). The association among occlusal contacts, clenching effort, and bite force distribution in man. J Dent Res 76: Kiliaridis S, Kjellberg H, Wenneberg B, Engstrom C (1993). The relationship between maximal bite force, bite force endurance, and facial morphology during growth. A crosssectional study. Acta Odontol Scand 51: Kim DJ, YJ; Peterson, IM; Lawn, BR (1999). Cclic fatigue of intrinsically brittle ceramics in contact with spheres. Acta Mater 99: King ML, Aboush YE (1999). Effect of adhesive resin cement on design of partial veneer crowns. J Prosthet Dent 82: Kirstein AW, RM; (1967). symmetrical bending of thin circular elastic plates of equally spaced point supported. J Res Natl Bur Stds 71: Knoernschild KL, Campbell SD (2000). Periodontal tissue responses after insertion of artificial crowns and fixed partial dentures. J Prosthet Dent 84: Kohal RJ, Weng D, Bachle M, Strub JR (2004). Loaded custom-made zirconia and titanium implants show similar osseointegration: an animal experiment. J Periodontol 75: Körber KL, K (1983). maximale Kaukraft als Berechnungsfaktor zahntechnischer Konstruktionen. Dent Labor (Munch) 31: 55. Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L (1999). The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater 15:

88 REFERENCES Koutayas SO, Kern M, Ferraresso F, Strub JR (2000). Influence of design and mode of loading on the fracture strength of all-ceramic resin-bonded fixed partial dentures: an in vitro study in a dual-axis chewing simulator. J Prosthet Dent 83: Krejci I, Reich T, Lutz F, Albertoni M (1990). [An in vitro test procedure for evaluating dental restoration systems. 1. A computer-controlled mastication simulator]. Schweiz Monatsschr Zahnmed 100: Lang NP, Kiel RA, Anderhalden K (1983). Clinical and microbiological effects of subgingival restorations with overhanging or clinically perfect margins. J Clin Periodontol 10: Lawn BR, Deng Y, Thompson VP (2001). Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review. J Prosthet Dent 86: Leach C (1987). Sintering of magnesium partially stabilised zirconia-behavior of an impurity silicate phase. Mater Sci Technol: Lehner CR, Mannchen R, Scharer P (1995). Variable reduced metal support for collarless metal ceramic crowns: a new model for strength evaluation. Int J Prosthodont 8: Lepistö TM, TA (1992). Degradation of TZP ceramics in humid atmospheres. In: Corrosion of glass, ceramics and ceramic superconductors. Clarke DZ, BK, (ed). Noyes Publ: Park Ridge (New Jersey, USA), pp 492. Lilley E (1990). Review of low temperature degradation in zirconia. In: Corrosion and corrosive degradation of ceramics. Tressler RM, M, (ed). Ceramic Trans. Lin MT, Sy-Munoz J, Munoz CA, Goodacre CJ, Naylor WP (1998). The effect of tooth preparation form on the fit of Procera copings. Int J Prosthodont 11: Lin Y-JA, P; McArtney, ML (1990). Microstructural and chemical influences of silicate grain-boundary phases in yttria-stabilised zirconia. J Amer Ceram Soc: Lucas LC, Lemons JE (1992). Biodegradation of restorative metallic systems. Adv Dent Res 6: Ludwig KJ, K (1994). Untersuchungen zur Bruchfestigkeit von IPS-Empress-Kronen in Abhängigkeit von den Zementiermodalitäten. Quintessenz Zahntech 20: Lundgren D, Laurell L (1984). Occlusal forces in prosthetically restored dentitions: a methodological study. J Oral Rehabil 11: Luthardt RG, Holzhuter MS, Rudolph H, Herold V, Walter MH (2004). CAD/CAMmachining effects on Y-TZP zirconia. Dent Mater 20:

89 REFERENCES Luthardt RGH, V; Sandkuhl, O; Reitz, B; Knaak, JP; Lenz, E (1998). Kronen aus Hochleitungskeramik. Zirkondioxid-Keramik, ein neuer Werkstoff in der Kronenprothetik. Dtsch Zahnarztl Z 53: Luthardt RGR, H; Sandkuhl, O; Walter, M (2001a). Aktuelle CAD/CAM Systeme zur Herstelleung von keramischem Zahnersatz. Teil 1. ZWR 110: Luthardt RGR, H; Sandkuhl, O; Walter, M (2001b). Aktuelle CAD/CAM Systeme zur Herstelleung von keramischem Zahnersatz. Teil 2. ZWR 110: Mackert J (1988). Effects of thermally induced changes on porcelain-metal compatibility. In: Perspectives in dental ceramics Proceedings of the Fourth International Symposium on Ceramics. JD P, (ed). Quintessence Publishing Co: Chicago, pp Mackert JR, Jr., Evans AL (1991). Effect of cooling rate on leucite volume fraction in dental porcelains. J Dent Res 70: Madani M, Chu FC, McDonald AV, Smales RJ (2000). Effects of surface treatments on shear bond strengths between a resin cement and an alumina core. J Prosthet Dent 83: Magne P, Belser U (1997). Esthetic improvements and in vitro testing of In-Ceram Alumina and Spinell ceramic. Int J Prosthodont 10: Marquardt P, Strub JR (2006). Survival rates of IPS empress 2 all-ceramic crowns and fixed partial dentures: results of a 5-year prospective clinical study. Quintessence Int 37: Marx R, Fischer H, Weber M, Jungwirth F (2001). Rissparameter und weibullmodulle: unterkritisches Risswachstum und Langzeitfestigkeit vollkeramischer Materialien. Dtsch Zahnarztl Z 56: Marxkors R (1980). Der Randschluss der Gusskronen. Dtsch Zahnarztl Z 35. May KB, Russell MM, Razzoog ME, Lang BR (1998). Precision of fit: the Procera AllCeram crown. J Prosthet Dent 80: Mayer TM, Hawley CE, Gunsolley JC, Feldman S (2002). The single-tooth implant: a viable alternative for single-tooth replacement. J Periodontol 73: McKinney KH, CM (1970). Effect of surface finish on structural ceramic failure. J Am Ceram Soc 53: McLaren EA (1998). All-ceramic alternatives to conventional metal-ceramic restorations. Compend Contin Educ Dent 19: 307-8, 310, 312 passim; quiz 326. McLaren EA, Terry DA (2002). CAD/CAM systems, materials, and clinical guidelines for all-ceramic crowns and fixed partial dentures. Compend Contin Educ Dent 23: , 644, 646 passim; quiz

90 REFERENCES McLaren EA, White SN (1999). Glass-infiltrated zirconia/alumina-based ceramic for crowns and fixed partial dentures. Pract Periodontics Aesthet Dent 11: ; quiz 996. McLaren EA, White SN (2000). Survival of In-Ceram crowns in a private practice: a prospective clinical trial. J Prosthet Dent 83: McLaren ES, JA (1997). Flexural strength of ceramic materials with different surface treatments. J Dent Res 76. McLaren EW, SN (2000). Glass-infiltrated zirconia/alumina-based ceramicfor crowns and fixed partial dentures. Quint Dent Technol 23: McLean J (1982). Bridge design and laboratory procedures in dental ceramics. In: The science and art of dental ceramics. Quintessence: Chicago. McLean JW (1993). Designing high strength ceramic crowns and bridges. Excellence: McLean JW, Hughes TH (1965). The reinforcement of dental porcelain with ceramic oxides. Br Dent J 119: McLean JW, von Fraunhofer JA (1971). The estimation of cement film thickness by an in vivo technique. Br Dent J 131: Mesaros AJ, Jr., Evans DB, Schwartz RS (1994). Influence of a dentin bonding agent on the fracture load of Dicor. Am J Dent 7: Miller LL (1977). Framework design in ceramo-metal restorations. Dent Clin North Am 21: Molin M (2003). A 2-year clinical study of ceramic fixed partial dentures. In: IADR 2003: Goteborg. Morena R (1986). Toughening of dental porcelain by tetragonal Zr0 2 additions. J Am Ceram Soc 69. Morena R, Beaudreau GM, Lockwood PE, Evans AL, Fairhurst CW (1986). Fatigue of dental ceramics in a simulated oral environment. J Dent Res 65: Mörmann W, Bindl A, Richter B, Apholt W, Toth R (1999). Cerec computer-aided-design: full ceramic crowns. In: CAD-CIM library. Mörmann W, (ed). FACD Publ: Zürich. Mörmann WH, Bindl A (2002). All-ceramic, chair-side computer-aided design/computer- Mschr Zahnheilk 93: aided machining restorations. Dent Clin North Am 46: , viii. Mühlemann H (1951). Die physiologische Zahnbeweglichkeit. Schweiz Naert I, Koutsikakis G, Duyck J, Quirynen M, Jacobs R, van Steenberghe D (2000). Biologic outcome of single-implant restorations as tooth replacements: a long-term follow-up study. Clin Implant Dent Relat Res 2:

91 REFERENCES Nakamura T, Dei N, Kojima T, Wakabayashi K (2003). Marginal and internal fit of Cerec 3 CAD/CAM all-ceramic crowns. Int J Prosthodont 16: Neiva G, Yaman P, Dennison JB, Razzoog ME, Lang BR (1998). Resistance to fracture of three all-ceramic systems. J Esthet Dent 10: O' Brien W (2002). Dental Materials and their Selection, 3d edn. Quintessence Publishing Co. O'Boyle KH, Norling BK, Cagna DR, Phoenix RD (1997). An investigation of new metal framework design for metal ceramic restorations. J Prosthet Dent 78: Oden A, Andersson M, Krystek-Ondracek I, Magnusson D (1998). Five-year clinical evaluation of Procera AllCeram crowns. J Prosthet Dent 80: Oh W, Gotzen N, Anusavice KJ (2002). Influence of connector design on fracture probability of ceramic fixed-partial dentures. J Dent Res 81: Oh WS, Anusavice KJ (2002). Effect of connector design on the fracture resistance of all- GE (2003). A long-term retrospective and ceramic fixed partial dentures. J Prosthet Dent 87: Olsson KG, Furst B, Andersson B, Carlsson clinical follow-up study of In-Ceram Alumina FPDs. Int J Prosthodont 16: Ozcan M, Alkumru HN, Gemalmaz D (2001). The effect of surface treatment on the shear bond strength of luting cement to a glass-infiltrated alumina ceramic. Int J Prosthodont 14: Peterson IM, Pajares A, Lawn BR, Thompson VP, Rekow ED (1998a). Mechanical characterization of dental ceramics by hertzian contacts. J Dent Res 77: Peterson IM, Wuttiphan S, Lawn BR, Chyung K (1998b). Role of microstructure on contact damage and strength degradation of micaceous glass-ceramics. Dent Mater 14: Pfeiffer P, Schwickerath H (1989). [Solubility of dental alloys in saliva]. Dtsch Zahnarztl Z 44: Phrukkanon S, Burrow MF, Tyas MJ (1998). Bonding of amalgam and a gallium alloy to bovine dentin. Oper Dent 23: Piconi C, Maccauro G (1999). Zirconia as a ceramic biomaterial. Biomaterials 20: Piotrowski PR (2001). Comparative studies on the adhesion of a silicone elastomer to a chromium-cobalt dental alloy. Eur J Prosthodont Restor Dent 9: Pospiech P, Rammelsberg P, Unsold F (1996). A new design for all-ceramic resin-bonded fixed partial dentures. Quintessence Int 27: Pospiech P, Schweiger J, Meinen J (2002). Vom Zirkoniumoxidgerüst zur Lava-Vollkeramik. Dental Labor 1:

92 REFERENCES Priest GF (1996a). Esthetic comparisons of alternatives for replacement of a single missing tooth. J Esthet Dent 8: Priest GF (1996b). Failure rates of restorations for single-tooth replacement. Int J Prosthodont 9: Proos K, Steven G, Swain M, Ironside J (2000). Preliminary studies on the optimum shape of dental bridges. Comput Methods Biomech Biomed Engin 4: Quinn JB, Sundar V, Lloyd IK (2003). Influence of microstructure and chemistry on the fracture toughness of dental ceramics. Dent Mater 19: Raigrodski AJ (2004a). Contemporary all-ceramic fixed partial dentures: a review. Dent Clin North Am 48: viii, Raigrodski AJ (2004b). Contemporary materials and technologies for all-ceramic fixed partial dentures: a review of the literature. J Prosthet Dent 92: Raigrodski AJ (2005). All-ceramic full-coverage restorations: concepts and guidelines for material selection. Pract Proced Aesthet Dent 17: ; quiz 258. Rauchs GF, T; Munz, D; Oberacker, R; (2001). Tetragonal-to-monoclinic phase transformation in CeO2-stabilised zirconia under uniaxial loading. J Eur Ceram Soc 21: Rechmann P (1993). Nachweiss metallischer Restaurationsmaterialien in klinisch unauffälliger Gingiva. Dtsch Zahnarztl Z 48: Reed JL, AM (1977). Effect of grinding and polishing on near surface phase transformations in zirconia. Mat Res Bull: Rehberg H (1971). Exakter Randschluss-was ist das? Dtsch Zahnarztl Z 26: Reich S, Wichmann M, Nkenke E, Proeschel P (2005). Clinical fit of all-ceramic three-unit fixed partial dentures, generated with three different CAD/CAM systems. Eur J Oral Sci 113: Reuling N, Wisser W, Jung A, Denschlag HO (1990). Release and detection of dental corrosion products in vivo: development of an experimental model in rabbits. J Biomed Mater Res 24: Richter HG (1994). Zirconia for medical implants-the role of strength properties. In: Bioceramics. Andersson OY-U, A, (ed). Butterworth-Heinemann: London, pp Rieth PRJNA (1976). Fabrication and flexural strength of ultra-fine grained yttria-stabilised zirconia. Bull Am Ceram Soc: 717. Rijk WG, Wood M, Thompson VP (1996). Maximum likelihood estimates for the lifetime of bonded dental prostheses. J Dent Res 75:

93 REFERENCES Rinke S (2004). Klinische Bewährung von vollkeramischen Extensionsbrücken:1-Jahres- Ergebnisse. Dtsch Zahnarztl Z 59: Ritter JE (1995a). Critique of test methods for lifetime predictions. Dent Mater 11: Ritter JE ( 1995b). Predicting lifetimes of materials and material structures. Dent Mater 11: Rosenblum MA, Schulman A (1997). A review of all-ceramic restorations. J Am Dent Assoc 128: Rosenstiel SFL, MF; Fujimoto, J (1995). The metal-ceramic crown preparation. In: Contemporary Fixed Prosthodontics. Mosby: St Louis, pp Rosentritt M, Behr M, Gebhard R, Handel G (2006). Influence of stress simulation parameters on the fracture strength of all-ceramic fixed-partial dentures. Dent Mater 22: Rosentritt M, Behr M, Handel G (2003). Fixed partial dentures: all-ceramics, fibre-reinforced composites and experimental systems. J Oral Rehabil 30: Rosentritt M, Plein T, Kolbeck C, Behr M, Handel G (2000). In vitro fracture force and marginal adaptation of ceramic crowns fixed on natural and artificial teeth. Int J Prosthodont 13: Rountree P, Nothdurft F, Pospiech P (2001). In vitro investigation on the fracture strength of all ceramic posterior bridges of ZrO 2 -ceramic. J Dent Res 80: 57. Rudolf HQ, S; Luthardt, RG (2003). CAD/CAM-Neue Technologien und Entwicklungen in Zahnmedizin und Zahntechnik. Dtsch Zahnarztl Z 58: Rühle MC, N; Heuer, AH (1984). Microstructural studies of Y 2 O 3 -containing tetragonal ZrO 2 polycrystals (Y-TZP). In: Advances in ceramics/science and technology of Zirconia II. Rühle MC, N; Heuer, AH, (ed). Colombus, American Ceramic Society: OH, USA, pp Sadan A, Blatz MB, Lang B (2005). Clinical considerations for densely sintered alumina and zirconia restorations: Part 1. Int J Periodontics Restorative Dent 25: Sadoun M, Asmussen E (1994). Bonding of resin cements to an aluminous ceramic: a new surface treatment. Dent Mater 10: Sailer I, H, Lüthy H, Feher A, Schumacher M, Schärer P, Hammerle CHF (2003). 3-year Clinical Results of Zirconia Posterior Fixed Partial Dentures Made by Direct Ceramic Machining (DCM). Sano H, Ciucchi B, Matthews WG, Pashley DH (1994). Tensile properties of mineralized and demineralized human and bovine dentin. J Dent Res 73: Sato TS, M (1985a). Control of the tetragonal-to-monoclinic phase transformation of yttria partially stabilised zirconia in hot water. J Mater Sci:

94 REFERENCES Sato TS, M (1985b). Transformation of yttria-doped tetragonal ZrO 2 polycrystals by annealing in water. J Amer Ceram Soc 68: Scheller H, Urgell JP, Kultje C, Klineberg I, Goldberg PV, Stevenson-Moore P, Alonso JM, Schaller M, Corria RM, Engquist B, Toreskog S, Kastenbaum F, Smith CR (1998). A 5-year multicenter study on implant-supported single crown restorations. Int J Oral Maxillofac Implants 13: Scherrer SS, de Rijk WG (1993). The fracture resistance of all-ceramic crowns on supporting structures with different elastic moduli. Int J Prosthodont 6: Scherrer SS, De Rijk WG, Belser UC (1996). Fracture resistance of human enamel and three all-ceramic crown systems on extracted teeth. Int J Prosthodont 9: Schmeißner H (1977). Über das Verhalten von Stiftkernaufbauten mit und ohne zirkuläre Stumpffassung im Belastungsexperiment. Dtsch Zahnarztl Z 32: Schweiger MH, W; Frank, M et al (1999). IPS Empress II. A new pressable high-strength glass-ceramic for esthetic all-ceramic restorations. Quint Dent Technol 22: Schwickerath H (1986). [Fatigue resistance of ceramics]. Dtsch Zahnarztl Z 41: Schwickerath H (1994). Neue Keramiksysteme unter Dauerbeanspruchung. Quintessenz Zahntech 20: Schwickerath H (1996). Werkstoffprüfung von Vollkeramiksystemen. In: Vollkeramik: Werkstoffkunde-Zahntechnik-klinische Erfahrung. Kappert HF, (ed). Quintessenz Verlag: Berlin, pp Scurria MS, Bader JD, Shugars DA (1998). Meta-analysis of fixed partial denture survival: prostheses and abutments. J Prosthet Dent 79: Seghi RR, Crispin BC, Mito W (1990). The effect of ion exchange on the flexural strength of feldspathic porcelains. Int J Prosthodont 3: Seghi RR, Denry IL, Rosenstiel SF (1995). Relative fracture toughness and hardness of new dental ceramics. J Prosthet Dent 74: Seghi RR, Sorensen JA (1995). Relative flexural strength of six new ceramic materials. Int J Prosthodont 8: Shepard FE, Moon PC, Grant GC, Fretwell LD (1983). Allergic contact stomatitis from a gold alloy--fixed partial denture. J Am Dent Assoc 106: Sherrill GOBJ (1974). Transverse strength of aluminous and feldspathic porcelain. J Dent Res 53: Shillingburg HT, Jr., Hobo S, Fisher DW (1973). Preparation design and margin distortion in porcelain-fused-to-metal restorations. J Prosthet Dent 29:

95 REFERENCES Shillingburg HTH, S; Whitsett, LD; Bracket, SE, (ed). (1997). Fundametals of fixed prosthodontics. Quintessence. Shimizu K, Oka M, Kumar P, Kotoura Y, Yamamuro T, Makinouchi K, Nakamura T (1993). Time-dependent changes in the mechanical properties of zirconia ceramic. J Biomed Mater Res 27: Shugars DA, Bader JD, White BA, Scurria MS, Hayden WJ, Jr., Garcia RI (1998). Survival rates of teeth adjacent to treated and untreated posterior bounded edentulous spaces. J Am Dent Assoc 129: Sorensen J (1999). The IPS Empress II system. Defining the possibilities. Quint Dent Technol 22: Sorensen JA, Cruz M, Mito WT, Raffeiner O, Meredith HR, Foser HP (1999). A clinical investigation on three-unit fixed partial dentures fabricated with a lithium disilicate glass- ceramic. Pract Periodontics Aesthet Dent 11: ; quiz 108. Sorensen JA, Kang SK, Torres TJ, Knode H (1998). In-Ceram fixed partial dentures: three- Technol 15: year clinical trial results. J Calif Dent Assoc 26: Sorensen JAK, H; Torres, TJ; (1992). In Ceram aa-ceramic bridge technology. Quint Dent Southan DE (1970). Strengthening modern dental porcelain by ion exchange. Aust Dent J 15: Sparrius O, Grossman ES (1989). Marginal leakage of composite resin restorations in combination with dentinal and enamel bonding agents. J Prosthet Dent 61: Spiekermann H (1985). Kronenersatz aus perioprothetischer Sicht. In: Parodontologie, Implantologie und Prothetik im Brennpunkt von Praxis und Wissenschaft. Lange D, (ed). Quintessenz: Berlin. Stambaugh RV, Wittrock JW (1977). The relationship of the pulp chamber to the external surface of the tooth. J Prosthet Dent 37: Strietzel R (2001). FutureDent-PreisgünstigerZahnersatz mit Hilfe eines CAD/CAM-Systems. Quintessenz Zahntech 27: Strub JR, Beschnidt SM (1998). Fracture strength of 5 different all-ceramic crown systems. Int J Prosthodont 11: Strub JR, Stiffler S, Schärer P (1988). Ursachen von Mißerfolgen bei der oralen Rehabilitation: biologische und technische Faktoren. Quintessenz:

96 REFERENCES Sturzenegger B, Feher A, Luthy H, Schumacher M, Loeffel O, Filser F, Kocher P, Gauckler L, Scharer P (2000). [Clinical study of zirconium oxide bridges in the posterior segments fabricated with the DCM system]. Schweiz Monatsschr Zahnmed 110: Suarez MJ, Lozano JF, Paz Salido M, Martinez F (2004). Three-year clinical evaluation of In- Ceram Zirconia posterior FPDs. Int J Prosthodont 17: Subbarao E (1981). Zirconia-an overview. In: Advances in ceramics/science and Technology of Zirconia. Heuer AH, LW, (ed). Elsevier: Amsterdam, pp Sulaiman F, Chai J, Jameson LM, Wozniak WT (1997). A comparison of the marginal fit of In-Ceram, IPS Empress, and Procera crowns. Int J Prosthodont 10: Sundh A, Molin M, Sjogren G (2005). Fracture resistance of yttrium oxide partially-stabilized zirconia all-ceramic bridges after veneering and mechanical fatigue testing. Dent Mater 21: Sundh A, Sjogren G (2006). Fracture resistance of all-ceramic zirconia bridges with differing phase stabilizers and quality of sintering. Dent Mater. Suttor D, Bunke K, Hoescheler S, Hauptmann H, Hertlein G (2001). LAVA--the system for all-ceramic ZrO2 crown and bridge frameworks. Int J Comput Dent 4: Swab J (1991). Low temperature degradation of Y-TZP materials. J Mater Sci 26: Taskonak B, Sertgoz A (2005). Two-year clinical evaluation of lithia-disilicate-based allceramic crowns and fixed partial dentures. Dent Mater. Tateishi T (1987). Research and development of advanced biocomposite materialsand application to the artificial hip joint. Bull Mech Eng Lab Jap 45: 1-9. Tateishi T (1994). Simulator test of artificial joints. Mat Sci Eng: Theunissen GB, JS; Winnbust, AJA; Burggraa,f AJ (1992). Mechanical properties of ultra- fine grained zirconia ceramics. J Mater Sci: Thompson GA (2000). Influence of relative layer height and testing method on the failure mode and origin in a bilayered dental ceramic composite. Dent Mater 16: Tinschert J (2002). Erste klinische Langzeiterfahrungen mit vollkeramischen brücken aus DC-Zirkon. 31 Jahrestagung der Arbeitsgemeinschaft Dentale Technologie. Tinschert J, Natt G, Dose B, Fisher H, Marx R (1999). Seitenzahnbrücken aus hochfester Strukturkeramik. Dtsch Zahnarztl Z 54: Tinschert J, Natt G, Hassenpflug S, Spiekermann H (2004a). Status of current CAD/CAM technology in dental medicine. Int J Comput Dent 7:

97 REFERENCES Tinschert J, Natt G, Mautsch W, Augthun M, Spiekermann H (2001a). Fracture resistance of lithium disilicate-, alumina-, and zirconia-based three-unit fixed partial dentures: a laboratory study. Int J Prosthodont 14: Tinschert J, Natt G, Mautsch W, Spiekermann H, Anusavice KJ (2001b). Marginal fit of alumina-and zirconia-based fixed partial dentures produced by a CAD/CAM system. Oper Dent 26: Tinschert J, Natt G, Schulze K, Spiekermann H (2004b). Three-year clinical results of zirconia-based all-ceramic bridges. In: ISPRD/ Poster No 17. Tinschert J, Natt G, Spiekermann H (2001c). Aktuelle Standortbestimmung von Dentalkeramiken. Dental-Praxis 18: Vult von Steyern P (2005). All-ceramic fixed partial dentures. Studies on aluminum oxideand zirconium dioxide-based ceramic systems. Swed Dent J Suppl: Vult von Steyern P, Carlson P, Nilner K (2005). All-ceramic fixed partial dentures designed according to the DC-Zirkon technique. A 2-year clinical study. J Oral Rehabil 32: Vult von Steyern P, Jonsson O, Nilner K (2001). Five-year evaluation of posterior all-ceramic three-unit (In-Ceram) FPDs. Int J Prosthodont 14: Wagner WC, Chu TM (1996). Biaxial flexural strength and indentation fracture toughness of three new dental core ceramics. J Prosthet Dent 76: Wakabayashi N, Anusavice KJ (2000). Crack initiation modes in bilayered alumina/porcelain disks as a function of core/veneer thickness ratio and supporting substrate stiffness. J Dent Res 79: Walter A (1994). Wear screening of materials combinations for hip joint replacements. In: Trans 11th ESB: Pisa, Italy, pp Walton TR (2002). An up to 15-year longitudinal study of 515 metal-ceramic FPDs: Part 1. Outcome. Int J Prosthodont 15: Wegmann UG, M (1987). Die Stabilität von Wurzelstiftsystemen im Wechsellastversuch. Dtsch Zahnarztl Z 42: Wegner SM, Kern M (2000). Long-term resin bond strength to zirconia ceramic. J Adhes Dent 2: Weinstein MW, AB (1962). Porcelain-covered metal-reinforced teeth. USA. White SN, Caputo AA, Li ZC, Zhao XY (1996). Modulus of rupture of the Procera All- Ceramic System. J Esthet Dent 8: White SN, Caputo AA, Vidjak FM, Seghi RR (1994). Moduli of rupture of layered dental ceramics. Dent Mater 10:

98 REFERENCES Wilson PR (1994). The effect of delayed placement of capsulated cements on crown seating. Aust Dent J 39: Wiskott HW, Nicholls JI, Belser UC (1996). The relationship between abutment taper and resistance of cemented crowns to dynamic loading. Int J Prosthodont 9: Witkowski S (2002). (CAD-)/CAM in der Zahntechnik: Buyer's Guide Zahntech Mag 6: Witkowski S (2005). (CAD-)/CAM in Dental Technology. Quintessence Dent Technol: Witkowski SL, R (2003). Stereolithographie als generatives Verfahren in der Zahntechnik. Schweiz Monatsschr Zahnmed 113: Wohler T (2003). Rapid prototyping and tooling: State of the art of the industry. Fort Collins, CO. Wohlwend AS, P (1990). Die Empress-Technik. Quintessenz Zahntech 16: Wolz S (2002). Das Wol-Ceram--EPC-CAM-System. Teil 2. Dent Labor (Munch) 49: Yamashita J, Shiozawa I, Takakuda K (1997a). A comparison of in vivo and in vitro strain with posterior fixed partial dentures. J Prosthet Dent 77: Yamashita J, Shiozawa I, Takakuda K, Miyairi H (1997b). Surface strain on crown and luting cement fractures. Int J Prosthodont 10: Yoshinari M, Derand T (1994). Fracture strength of all-ceramic crowns. Int J Prosthodont 7: Zembic I, Lüthy H, Schumacher M, Schärer P, Hämmerle CHF (2002). 2- and 3-year results of zirconia posterior fixed partial dentures, made by direct ceramic machining (DCM). European Cells and Materials. In: 8th General Meeting of the Swiss Society for Biomaterials: Central Medical University Geneve. Zeng K, Oden A, Rowcliffe D (1996). Flexure tests on dental ceramics. Int J Prosthodont 9: Zeng K, Oden A, Rowcliffe D (1998). Evaluation of mechanical properties of dental ceramic core materials in combination with porcelains. Int J Prosthodont 11: Zhang Y, Lawn B, Rekow E, Thompson V (2004). Effect of sandblasting on the long-term performance of dental ceramics. J Biomed Mater Res B Appl Biomater 15:

99 CURRICULUM VITAE 13 Curriculum vitae Date of Birth 16. October 1977 Place of Birth Aigio, Achaia, Griechenland Parents Marital Status Nationality Vasileios Stamoulis Maria Roumana-Stamouli Single Greek Education Secondary School, Aigio, Greece Secondary School, Patras, Greece Gymnasium, Patras, Greece Gymnasium, Wuppertal, Germany Lyzeum, Wuppertal, Germany University Chemical engineering, Metsoveio Polytechnik Institute, Athens, Greece Dental School, University of Athens, Greece Employment Assistant in private dental office, Athens, Greece Assistant in private dental office, London, England Posttgraduate student, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany 99

100 ACKNOWLEDGEMENTS 14 Acknowledgements I would like to express my most sincere gratitude to Prof. Dr. J. R. Strub, Chair, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany for offering me the opportunity to carry o ut research under his supervision. I would also like to thank: Prof. Dr. J. Hausselt, Forschungszentrum Karlsruhe GmbH in der Helmholtz-Gemeinschaft, Materials Research Department, for the review of the manuscript. Dr. W. Att, The Weintraub Center for Reconstructive Biotechnology, UCLA School of Dentistry, L os Angele s, California, USA, for his continuous advise, help and support throughout the study. ZTM S. Witkowski, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany, for his help and technical support throughout the experiment. Dr. M. Tomic, Department of Prosthodontics, Albert Ludwigs University, Freiburg, Germany for the conception of the idea of the present study. Dr. T. Gerds, Institute of Medical Biometry and Medical Informatics, Albert Ludwigs University, Freiburg, Germany, for the statistical analysis of the data. The dental technicians of Nobel Biocare, Goetheborg, Sweden; Mr Ahlmann, Chief, Dental Laboratory, Kelkheim, German y and Mr G. Lombardi, Chief, Dental laboratory, Zurich, Switzerland, for the fabrication of the frameworks of the test samples. The dental laboratory Woerner Zahntechnik, Freiburg, Germany for the veneering of the test samples. I would like to add a special thank to my parents for their continuous moral and financial support throughout my studies. I would also like to thank Jörg for his patience, support and understanding even under difficult circumstances. 100

101 ACKNOWLEDGEMENTS 101