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