Influence of Material and Technique on Occlusal Chipping of All-Ceramic Molar Crowns

Transcription

1 Influence of Material and Technique on Occlusal Chipping of All-Ceramic Molar Crowns By MAJED ALSARANI A thesis submitted in conformity with the requirements for the degree of Master of Science Graduate Department of Restorative Dentistry Faculty of Dentistry University of Toronto Copyright by Majed Alsarani (2014)

2 Influence of Material and Technique on Occlusal Chipping of All- Ceramic Molar Crowns Majed M. AlSarani Master of Science, 2014 Graduate Department of Restorative Dentistry, Faculty of Dentistry University of Toronto Abstract: Objective: To evaluate the effect of material and fabrication technique on chipping behavior of all-ceramic molar crowns. Materials & methods: A molar tooth with crown preparation was used to make fifty identical replicas from epoxy resin (n=10). Porcelain-fusedto-metal crowns (PFM) constituted the control group. There were four experimental groups: monolithic CAD/CAM lithium disilicate glass-ceramic crowns (LDG); Zirconia core 0.5 mm thick veneered with feldspathic porcelain added by hand-layering (ZVL), by heat-pressing (ZVP) and CAD/CAM milled lithium disilicate glass-ceramic veneer (CAD-on). All crowns were subjected to compressive cyclic loading at mesio-buccal cusp at 30 angle in universal testing machine. Results: All LDG and CAD-on crowns survived fatigue test; while all specimens of PFM, ZVP and ZVL groups failed at different stages of the 500,000 cycles fatigue test (P<.001). Conclusion: Use of lithium disilicate glass-ceramic as a monolithic crown and as a veneering material significantly improved chipping resistance of all-ceramic crowns. ii

3 Table of Contents 1. Introduction Background Classification of Dental Ceramics Classification by Fusion Type Classification by Fabrication Method Classification by Crystalline Phase Dental Zirconia (Zirconium Dioxide) Zirconia Transformation Low temperature degradation (Ageing of zirconia) Machining methods of the available zirconia blocks Zirconia/veneering porcelain complications and survival rates Factors that influence chipping of veneering porcelain Monolithic Restorations Surface pre-treatment and luting cement for all-ceramic restorations: CAD/CAM Technology Metal-Ceramic Restorations Rationale, Objectives and Hypotheses Materials & Methods Sample size Specimens Preparation Restorations designing and fabricating Porcelain-fused-to-metal (PFM) group Monolithic lithium disilicate glass ceramic (LDG) group Zirconia coping veneered by hand layering technique (ZVL) Zirconia coping veneered by heat pressing technique (ZVP) iii

4 2.3.5 Zirconia core & CAD veneering technique (CAD-on) Cementation: Compressive cyclic loading test Results Discussion Conclusions: Reference iv

5 List of Figures No. Description Page Figure 1: Die replica (Reprinted with permission from Zahran et al., J Prosthodont 2008) Figure 2: Prepared tooth replica inserted into the dentoform Figure 3: Application of a thin layer of contrast spray powder prior to the optical impression Figure 4: Fabrication steps for porcelain-fused-to-metal (PFM). A: Designing the coping with CEREC. B: The metal Core is finished. C: Porcelain application. D: Molar crown finalized Figure 5: A: Optical impression. B: Designing crown in virtual occlusion. C: Milling the restoration. D: Crowns mounted and placed on firing tray Figure 6: A: IPS e.max ZirCAD blocks. B: Milling chamber. C: Milled coping. D: Finishing the zirconia coping with fine diamond bur Figure 7: IPS e.max ZirLiner application prior to the porcelain veneering Figure 8: silicon impression was taken to minimize crown s shape and size variation Figure 9: A: Coping with wax-ups and sprued ready for investing; B: Pressing ceramic ingots; C: The crown after finishing and glazing Figure 10: Fabrication process for CAD-on crowns. A: Designing core and veneer. B: Milled zirconia core and lithium disilicate veneer. C: After connecting the core and veneer Figure 11: Length of specimen (tooth + crown) was measured with a digital caliper Figure 21: Hydrofluoric acid application on the internal surface of e.max CAD crowns Figure 13: Panavia F 2.0 (Kuraray) pastes A&B were mixed together Figure 14: The crown was seated under a 10N static load Figure 15: Preparing the sample in the correct inclination Figure 16: The red tube filled with the resin material and the sample embedded into it at 30 angle Figure 17: A: The sample mounted in acrylic resin for the cyclic loading; B: The angle was measured (30 ±2 ) Figure 18: The specimen positioned for the compressive cyclic loading test Figure 21: Schematic illustration of the load application Figure 20: Sectioning the specimen mesio-distally Figure 21: Some specimens after sectioning v

6 Figure 22: Box-plot of the fatigue test results (Number of cycles) Figure 23: survival line of Porcelain-fused-to-metal specimens Figure 24: Survival line of ZVL specimens Figure 25: Survival line of ZVP specimens Figure 26: Survival results for all groups Figure 27: Veneer chipping of zirconia crown veneered by press-on (Cohesive failure) Figure 28: Adhesive (A) and cohesive (B) failure of PFM crowns Figure 29: Manually veneered zirconia crowns presenting cohesive (A) and adhesive (B) fracture Figure 30: Monolithic crown that survived the cyclic fatigue. (Arrow indicates superficial damage) Figure 31: SEM pictures of sectioned specimens of (A) PFM and (B) ZVL. The circles indicate where the measurement was made Figure 32: SEM images of sectioned crowns (A) ZVP, (B) LDG and (C) CAD-on. The circles indicate where the measurement was made vi

7 List of Tables No. Description Page Table 1: Experimental groups employed in the present study Table 2: Measurements of the epoxy resin die replicas Table 3: Firing program for metal ceramic crowns Table 4: Firing parameter for monolithic lithium disilicate glass ceramic (IPS e.max CAD) Table 5: Firing program for hand-layered zirconia crowns Table 6: Firing program for pressed-on zirconia crowns Table 7: Crystallization and fusion Firing Program of CAD-on crowns Table 8: Compositions of different materials utilized in the study according to manufacturers data Table 9: Materials properties according to the manufacturer s instructions Table 10: One-way ANOVA and P-value for angle measurement Table 11: The number of crowns in each group whether survived or fractured Table 12: Chi-square test and P-value for Fisher s exact test Table 13: The number of cycles at which samples failed during the cyclic loading Table 14: Type of failure of the failed samples Table 15: Marginal opening gaps (µm) vii

8 1. Introduction 1.1 Background In the ongoing trend toward more aesthetically-pleasing and biologically-compatible restorations, all-porcelain crowns have gained significant popularity over the last decade (1). This popularity can be attributed to a combination of technical improvements and the physical appearance of porcelain materials. Despite these advantages, there have been limitations to the use of ceramics in the posterior region, such as inadequate mechanical reliability due to the materials high brittle nature and tendency to cause excessive wear of the opposing natural teeth (2, 3). Nonetheless, the introduction of computer-aided-design and computer-aided-manufacturing (CAD/CAM) technology allows high strength ceramic materials such as alumina and zirconia to be used (4). With the advantages that all-ceramic restorations offer, the drive to overcome the current limitations of these materials has led to significant improvements in their physical properties as well as development of new techniques for fabrication of dental restorations. 1.2 Classification of Dental Ceramics Classification of dental ceramics may be made according to fusion temperature, fabrication technique and crystalline phase Classification by Fusion Type There are four classifications of ceramics according to fusion type: Ultra-low fusing (< 870 ), Low-fusing ( ), Medium-fusing ( ). Porcelain denture teeth are made with medium-fusing and high-fusing types. The low- 2 ), and High-fusing (1315

9 fusing porcelain type is utilized for constructing metal-ceramic and all-ceramic restorations while with titanium and titanium alloys, ultra-low fusing porcelain is used (5, 6) Classification by Fabrication Method Classifications according to fabrication method are numerous, and consist of conventional layering, hot pressing, slip casting and machining. All of these are described below Conventional layering Conventional layering refers to the technique that is typically employed to layer veneer onto the prosthetic core. Porcelain powder and modeling liquid are mixed according to the manufacturer s instructions and applied to the core with a brush in multiple applications and firings. Because of these processes, there is an high possibility that the materials may be compromised by the inclusion of voids or contaminants (7). Some examples of the many commercially available products used with the manual layering technique are IPS e.max Ceram (Ivolcar Vivadent), Lava Ceram (3M ESPE) and Vita D (Vita Zahnfabrik) Hot pressing This technique is also called the lost wax technique. It is used to fabricate monolithic allceramic crowns, inlays, onlays and veneers. Interestingly, the coping of all-ceramic restorations also can be produced by this technique where the veneering porcelain is applied manually. Pressing veneer onto the core involves building a wax contour on the core. After attaching a sprue to the wax contour, the complex is then invested and heated to remove the wax. Molten ceramic ingots are then heat pressed into the mold and directly onto the core. Firing shrinkage that may result from the multiple firings in the layering technique is minimized by way of press veneering (8, 9). It has been demonstrated that the press-on veneering technique minimizes 1

10 chipping in the zirconia prosthesis compared to the conventional layering technique (9, 10). This is because the voids and variations during the manual layering technique are eliminated by the over-pressed technique. Examples of materials that are available on the market that utilize the hot pressing technique are IPS Empress and IPS Empress II, which are used to fabricate monolithic restorations (full anatomic). These provide restorations with high translucency. After further enhancement, IPS e.max Press was introduced with advanced mechanical and physical properties and improved aesthetics. Clinically, IPS e.max press crowns showed excellent survival rate at 96.6% after three years (11, 12) Slip casting Slip casting is a fabrication technique that involves the utilization of a ceramic slip that is poured into a negative mold of the desired framework, which is typically made of gypsum. When the walls of the mold material wick water away from the slip, powder particles near the walls of the mold become compacted and form a layer of ceramic that will become the framework. After the remaining slip is discarded the framework is removed and infused with molten glass. Veneering porcelain is then applied (13, 14). However, the literature has indicated that fit errors of dental restorations are frequently encountered by way of slip casting. Due to the number of steps and the level of complexity involved in this fabrication method, slip casting has never gained popularity (15, 16). Some examples of ceramics that can be used for slip casting are zirconia toughened alumina, alumina-based, and spinel based ceramics. In-Ceram Alumina, a slip-cast ceramic, was introduced in 1989 as an all-ceramic system for single crowns and 3 unit anterior bridges. With flexural strength of 600 MPa, it is considered a strong material, however, due to its opacity, In-Ceram Alumina is not considered a highly 3

11 aesthetic material (17, 18). For this reason, In-Ceram spinel was introduced in 1994 to solve this issue. Adding a magnesia and alumina mixture to the framework improved the translucency, but the flexural strength decreased compared to In-Ceram Alumina. The low flexural strength (250 MPa) limited its use to the anterior crowns (19). In-Ceram Zirconia was introduced as a modified material for In-Ceram Alumina. It is considered a strong material because of its flexural strength of 700 MPa, however, the opaqueness of the zirconia limits its use to posterior crowns and bridges (20, 21) Machining (CAD/CAM) All-porcelain crowns may be fabricated using computer-aided designing and computeraided manufacturing (CAD/CAM). The ceramic blocks used for CAD/CAM fabrication are milled using tools that are controlled with a computer which will be discussed in the CAD/CAM section below (22). The porosity of the pre-sintered ceramic blocks, zirconia blocks as an example, allows for quick milling. This means the material requires further sintering after milling process in order to eliminate the porosity and render it stronger for better clinical performance Classification by Crystalline Phase Predominantly glassy ceramics Feldspathic porcelain, so named because of the presence of silica- and alumina-based feldspar, has the most life-like appearance and the optical properties of natural enamel, making it the most aesthetically-pleasing ceramic for dental restorations. The primary weakness of feldspathic porcelain is its low flexural strength of 90 MPa (23, 24). 4

12 Particle-filled glasses It is possible to improve mechanical properties of feldspathic porcelain-based glass ceramics by adding crystalline fillers such as leucite, crystalline mica or lithium disilicate crystals to the glass. Leucite, such as IPS Empress, is added at a concentration of between 35 and 45% mass. IPS Empress has a flexural strength of ~180 MPa (25), and has been shown to have a survival rate that ranges from 92% to 99% over 3 to 3.5 years for anterior crowns (26). In a systematic review, Heintze and Rousson et al (2010) stated that the fracture rate for posterior IPS Empress crowns was 6.7% over an observation period of approximately 5 years which was higher than anterior crowns (27). The higher fracture rate at the posterior region might be related to the higher forces that are applied during mastication. Crystalline mica, as in Dicor, is added at a concentration of 55% mass. Dicor was the first commercial glass ceramic used for crowns and bridges which was introduced in 1984 (28). However, the high incidence of fracture and difficult fabrication procedure led to the discontinuation of this system. IPS Empress II, a lithium disilicate crystal additive, was introduced in The addition of lithium disilicate to glass ceramics led to a higher strength of 360 MPa. Furthermore, with the introduction of IPS e.max Press, superior physical properties and translucency were achieved. Following IPS e.max Press, IPS e.max CAD, also a lithium disilicate glass ceramic, was designed for CAD/CAM processing (29) Polycrystalline ceramics Polycrystalline ceramics are stronger and tougher than glassy ceramics because of the lack of glassy components. Aluminum and zirconium-oxide are examples of polycrystalline 5

13 ceramics. Of the polycrystalline ceramics, yttrium stabilized zirconia polycrystal (Y-TZP) is considered to be suitable for fabrication of strong frameworks and has a reportedly low failure rate compared to alumina frameworks due to its superior mechanical properties (29, 30). Because of its outstanding mechanical properties, Y-TZP use has spread from cores for single crowns to multi-unit frameworks for bridges (31). Despite its strength, the high crystalline content makes zirconia rather opaque, and this material is best limited to substructures (cores and frameworks). Therefore, when using a zirconia framework, these structures must be veneered with a highly translucent dental porcelain to create a more natural appearance (32). 1.3 Dental Zirconia (Zirconium Dioxide) Polycrystalline zirconium dioxide, or zirconia, was introduced in the medical field for hip replacements in Twenty years later, computer-aided technology allowed the introduction of this material into the field of restorative dentistry (33). Zirconia is a popular material for forming the core of ceramic dental restorations due mostly to its mechanical properties and strength. Interestingly, the growing trend towards zirconia prostheses is estimated to be 12% annually (34). The flexural strength of zirconia ranges between 800 and 1000 MPa which is significantly higher than most ceramic materials (35) (36) Zirconia Transformation Above all, the phase transformation property of zirconia is what makes it unique. Zirconia may take on any of 3 crystallographic forms, which are determined by temperature. From room temperature to 1170 C, it will take a monoclinic form; from 1170 C to 2370 C, it will take a tetragonal form; and from 2370 C to its melting point (2715 C), it will take a cubic 6

14 form (35). The tetragonal (t) zirconia structure can be retained at room temperature when it is stabilized with oxides such as magnesium oxide, cerium oxide, calcium oxide and yttrium oxide. This metastable (t) phase can be transformed to the more stable monoclinic (m) phase by way of external stresses, a transformation which has an associated volume expansion of 3-5%, and that can lead to catastrophic failure (35). Nonetheless, it is possible to stabilize the transformed particles in order to increase strength and crack resistance by adding stabilizing oxides (yttria) (37). Compressive stresses that are created around the transformed particles successfully stop the initial formation of the cracks, and increase resistance to crack propagation. Despite these advantages, zirconia-based restorations still have a higher failure rate (chipping and fracture) than metallic restorations, mainly in the veneering porcelain layer (10, 38, 39) Low temperature degradation (Ageing of zirconia) Another drawback of zirconia restorations is the slow transition from the tetragonal to the monoclinic phase in the presence of a watery environment (33). This phenomenon is known as low temperature degradation. This low temperature degradation begins on the surface of the material and continues into the bulk of the zirconia and stresses initiate around grains leading to microcrack formation. The water molecules penetrate into the cracks and pull out the grains, this subsequently results in the roughened surface of the zirconia and leads to degradation of its strength (40). In addition to environmental features, grain size, concentration of stabilizers and residual stresses are also significant factors in this phenomenon (41). It is reported that annealing at 900ºC for one hour may induce reverse transformation from the monoclinic to the tetragonal phase, causing a relaxation of compressive stresses, an increased resistance to ageing with accompanying decrease in strength (42). 7

15 1.3.3 Machining methods of the available zirconia blocks Only three of the many zirconia-based ceramics available are commonly used in restorative dentistry: Yttrium stabilized tetragonal zirconia, where 3mol% yttria is used as a stabilizer (3Y-TZP), magnesium partially stabilized zirconia (Mg-PSZ) and zirconia-toughened alumina (ZTA). Yttrium stabilized tetragonal zirconia: in the last thirty years 3Y-TZP has been used for orthopedic applications in the medical field. Early in the 1990s, this material was introduced to dentistry and is used for post and core, orthodontic brackets, implant abutments and all-ceramic restorations (43). Magnesium partially-stabilized zirconia: Due to the presence of porosities and large grain size, this material has not been utilized successfully in dentistry (44). The processing of Mg-PSZ involves a high sintering temperature that requires careful controlling. An example of magnesium partially-stabilized zirconia is Denzir-M (Dentronic AB, Skelefta, Sweden) which is provided as fully sintered blocks (45). Zirconia-toughened alumina contains 12 mol % of ceria stabilized zirconia (12 Ce-ZTA) added into In-ceram Alumina. This material is available for CAD/CAM processing as a pre-sintering stage and for slip-cast processing (46). However, the amount of porosities in Ce-ZTA is large compared to Y-TZP (47). These materials are available as blocks for fabricating crowns and fixed partial denture prostheses. Pre-sintered blocks are processed via soft machining, and fully sintered blocks of zirconia are processed via hard machining. Soft machined blocks suffer shrinkage (~25%) during final sintering which needs to be compensated for at the milling stage. The fully sintered zirconia 8

16 blocks have superior fit with no shrinkage involvement in the process because the shrinkage took place during manufacturing. However, these blocks require robust milling systems to be processed (33, 35). 1.4 Zirconia/veneering porcelain complications and survival rates The veneering porcelain on the top of a Y-TZP core becomes the weak link of the restoration and a high incidence of occlusal chipping is the most frequently reported type of failure (48, 49). The failure rate observed is between 3% and 50% for both tooth-supported and implant-supported zirconia crowns as reported in different studies (25, 35, 50, 51). In a 5-year clinical prospective study conducted by Vigolo and Mutinelli (2012), 60 single posterior molar crowns fabricated by two CAD/CAM systems, Procera (Nobel Biocare) and Lava (3M ESPE), were compared to PFM restorations as control. All crowns were veneered manually with their respective veneering material. Investigators found that the most frequent failure type was veneer chipping. The Kaplan-Meier survival rate after five years was 95%, 79%, and 85% for PFM, Procera, and Lava, respectively (52). The high incidence of cohesive failure (chipping) has been seen in the majority of zirconia brands that are used in the profession. The failure rate in the literature varies and depends on the zirconia types and the layering technique. For example, the failure rate for Cercon (Densply) after one, two and 5 years was 3%, 3%, and 15%, respectively (53, 54) whereas this percentage was 3%, 3%, and 25% when utilizing Lava (3M EPSE) after 1, 2 and 2.5 year (55, 56). Also the chipping incidence was 13% after one year when IPS e.max ZirCAD was used as the all-ceramic core while Procera prostheses after 3 years of insertion showed the lowest chipping and fracture rate (2%) among the tested zirconia brands (50, 57). In a recent systematic 1

17 review study, researchers concluded that the failure rate (4.5%) was acceptable over a five-year period regardless of the material used (58). Therefore, higher failure rates not only place the reputation of the practitioner at a high risk but it is also time and money-consuming. It is noteworthy that the incidence of veneer fracture increases in the molar region compared to the anterior and premolar areas. A relation between the fracture and crown location has been reported: 3% has been revealed in anterior crowns, 7% in premolar crowns and 21% in molar crowns, respectively (59). Kassem et al (2010) concluded that the failure rate of all- Ceramic molar crowns was 10.2% after five years service with a higher incidences of failure in the molar area than the premolar (60). In a recent systematic review, this result was confirmed where molar crowns showed a higher fracture rate at 8.1% than premolars at 3% (58). Beside the chipping and fracture observations, zirconia-based restorations showed other complications such as loss of retention, caries and need for endodontic treatment. These ranged from 0% to 5% (61), while other studies reported 21% for these complications (59, 62). In a retrospective clinical study, researchers investigated more than 1300 different allceramic restorations irrespective of the materials used, the type of the restoration and the fabrication technique. The mean observation time was 102 ± 60 months with 184 restorations observed for 15 years and 21 restorations for 20 years. In that study, characteristics of different restorations were examined such as the aesthetic matching, veneer surface and marginal adaptation. Not surprisingly, the authors found that veneer fracture (chipping) was the major cause of failure at 34%. The estimated survival rate was 93.5%, 85.8% and 78.5% after 10, 15 and 20 years, respectively. The investigators concluded that the reasons for the higher failure were bruxism, loss of vitality, and cement used (63). 21

18 1.5 Factors that influence chipping of veneering porcelain According to numerous studies, there are several known primary causes of failure in allceramic bi-layered restorations, as follows: Core Design Prosthesis geometry plays an important role in the veneer porcelain chipping of allceramic restorations. Even core thickness was frequently utilized when high strength ceramic materials, such as Y-TZP, were first introduced(64). The thickness variations of the core/coping are based on the manufacturers recommendations and are usually between 0.5mm and 0.7mm. Modifying the design of the coping was suggested in an attempt to enhance the overall strength as well as to improve aesthetics. In many studies that compared the conventional core design and different modified ones, the chipping in the porcelain layer was observed for both systems (65-67). Silva et al (2011) pointed out that modifying the all-ceramic core design has benefits to support the porcelain compared with maintaining conventional designs of molar crowns. For example, they found that by increasing the thickness of the core in the proximal area to 1mm and maintaining the height of the lingual wall which extended proximally by 2.5 mm, the support of the veneer was increased and thereby minimized, but didn't eliminate, the possibilities of fracture in the lingual cusp (66). Silva et al (2012) used the same methodology of construction modified Y-TZP molar crowns and compared it the conventional PFM crown. It has been found that Y- TZP core design modification resulted in fatigue reliability comparable to PFMs and fracture modes matched to clinical failure scenarios (68). Similarly, a recent in-vitro study found that anatomically shaped copings better supported the porcelain veneer at the cusp tip and hence reduced the probability of chipping compared to veneers supported by an even-core design irrespective of the veneering technique (69). 22

19 In contrast with the previous studies, Lorenzoni et al (2010) evaluated the effect of the coping modification for three bi-layered crown systems (PFM, glass infiltrated alumina, and zirconia) on the fatigue life and failure mode. The core design modification involved a 0.5mm even thickness with the thickness increased by adding 1 mm at the lingual surface and maintaining 2 mm of height. They found that the modified core had no beneficial effect in improving the fatigue life and reliability. Cohesive fracture of the porcelain layer was observed in the modified and standard zirconia coping groups. Interestingly, the chipping behavior was the sole difference between the two all-ceramic groups where larger chipping was seen for the standard Y-TZP group compared to the modified zirconia core (70). Bulk fracture was reported only in the glass-infiltrated alumina particularly in the standard coping design Veneer/core thickness Porcelain chipping is one of the complications affecting the longevity of bi-layered allceramic as well as metal-ceramic restorations; thus, the veneer to core thickness ratio plays a pivotal role in determining the probability of veneer chipping (71). In the dental literature, 1.5 mm is suggested as the minimum recommended thickness for all-ceramic prostheses (72). In order to provide strength, aesthetics and fabrication tolerance, a 1-to-1 ratio of core to veneer thickness should be taken into consideration (72). As veneering porcelain is naturally brittle; when exposed to different loads during mastication it needs to be supported with proper core design and sufficient thickness to minimize failure occurrence. The thickness of the veneering porcelain depends on the occlusal space and anatomical features of the tooth. It has been demonstrated that the application of a minimal veneering porcelain layer, that is aesthetically and functionally acceptable, increases the strength of all-ceramic restorations (32). A higher thickness ratio of the coping layer to the veneering porcelain might generate low thermal residual 21

20 stresses during the firing procedures. Therefore, a bi-layered prosthesis with a higher porcelain thickness ratio may generate a higher thermal residual stress in comparison to a lower thickness ratio of porcelain, and consequently may be subject to fracture more easily (73) Coefficient of thermal expansion (CTE) mismatch The incompatibility of CTE between the veneering porcelain and the core substrate is a crucial factor for causing veneer chipping. This mismatching could lead to tensile or compressive stresses, depending on whether the mismatch of CTE of the veneering ceramic is higher or lower than that of the zirconia core (74). Because dental ceramics have low tensile strength, the buildup of minimal compressive stresses is desired to increase the veneering porcelain resistance to fracture. Thus, the theory for having slight positive CTE mismatch is widely employed when fabricating all-ceramic restorations to create desirable residual compressive stresses and to increase the chipping resistance. Therefore, manufacturers have introduced veneering porcelain materials that have slightly lower CTE than zirconia cores (75). Thermal expansion mismatch between the veneering porcelain and zirconia has a significant effect on the bond strength and strongly influences the resistance ability of the veneering porcelain to fracture (9). Delamination and extensive microcrack formation in the veneer have been found when layering porcelain with a higher CTE than that of the zirconia framework. Debonding, for example, has been reported when a / C CTE mismatches existed (76). However, the shear bond strength of zirconia/veneer ceramics did not differ with a CTE mismatch that ranged from 0.75 to / C (77). Furthermore, other studies found no correlation between shear bond strength and CTE mismatch of zirconia and veneering porcelain (78). 23

21 1.5.4 Veneering Application Technique The application of the porcelain layer on top of the zirconia coping and the veneering material used influences chipping behavior. Traditionally, veneering ceramics are manually layered on the metal or zirconia core, although the press veneering ceramics technique is an alternative (7, 79). Because of the multiple process steps in the conventional layering technique, there is an increased possibility that the materials may be compromised by the inclusion of voids or contaminants (7). Firing shrinkage that may result from the multiple firings in the layering technique was minimized by way of press veneering. It was shown that the pressing veneering technique minimized chipping in zirconia crowns compared to the conventional layering technique (9, 10). Veneering techniques also have a direct effect in terms of the bond strength between the zirconia core and veneering porcelain. The bond strength of press-on veneering ceramic to Cercon framework was found to be substantially higher than when layering veneering porcelain to the same zirconia framework (80). Nonetheless, in spite of the superior mechanical properties and bond strength of press-on veneers, aesthetically they are less desirable than that of the layered ceramics. To achieve an excellent aesthetic outcome, a double veneer method has been introduced for single restorations. The high bond strength and superior interface quality of press-on veneer ceramics could be combined with the aesthetics of layered veneer ceramics by utilizing a double veneer technique (81). The effect of the veneering technique on the reliability of the zirconia substrate has been investigated and the fatigue values, interestingly, were similar between the pressed and layered techniques (79). Similarly, in terms of the influence of veneering techniques with modified (anatomically supported) zirconia core, manual and pressed veneering techniques have also been 24

22 tested. The reliability of all-ceramic prostheses was found to be higher for the modified zirconia coping either veneered conventionally or by the press-on technique than the even-thickness zirconia coping (69). Nonetheless, in one study, although it is known that the heat-pressed technique has advantages over the hand-layering method, it has been reported that the hand veneering technique on conventional core design has higher reliability than the over-pressing technique on the same core design but not with an anatomical core. Thus, the authors attributed this result to the fabrication techniques since the mechanical and physical properties of the two veneering materials were similar (69). Furthermore, a new technique for fabricating all-ceramic crowns has been used recently, when a combination of a zirconium oxide core and lithium disilicate veneer were united in a technique called CAD-on. Simply, this technique combines a milled zirconia core and milled a veneer that could be connected by a fusing glass ceramic or resin cement (1, 82). In a recent study comparing CAD-on crowns with conventional manual layering crowns under thermocycling and chewing simulation for 1.2 million cycles and at 30 o inclination, the load was applied on the mesiolingual cusp. The investigators found that approximately 88% of manually veneered crowns failed while no failure was observed in the CAD-on veneered crowns (82). This technique shows superior chipping and fracture resistance over conventional and pressable veneering methods Core/Veneer Bonding Interface Inadequate bond strength between the porcelain veneer and the zirconia core could be the underlying factor that causes veneer chipping (83). Some factors reported to lead to inadequate bond strength are flaws during layering such as bubbles and voids, liner material application, and zirconia surface preparation (84, 85). Rekow et al (2011) noted that the bond strength of the core 25

23 to the veneer in all-ceramic crowns is affected by laboratory and clinical procedures such as intra-oral occlusal adjustment and air abrasion to create an adequate bonding. Studies have been conducted to evaluate the bond strength of zirconia bi-layered ceramic, and it was found to be lower than that of metal ceramic restorations (77, 86). The build-up of increased tensile and compressive stresses in the veneer layer may cause either chipping or delamination (83, 87). The bond strength of bi-layered restorations has been evaluated using shear, tensile and micro-tensile testing. In the dental literature, the shear bond strength test has been widely used to evaluate the strength between the zirconia core and porcelain veneer (75, 88, 89). For metal ceramic restorations, the estimated adequate bond strength is 25 MPa, however, no similar values have been advocated for the bond strength of bilayered ceramic restorations. Mean shear bond strength values of different commercial zirconia cores and their recommended veneering materials have been determined as: DC-Zirkon MPa, Lava MPa, Zirkonzahn MPa and Ceron MPa. The fracture mode was either combined (cohesive & adhesive) or adhesive failure but no cohesive failure was found (80). The application of a liner ceramic on the zirconia coping is commonly utilized to mask the opaque color of the zirconia core and improve the bond strength between the veneering porcelain and zirconia. However, it has a negative effect that has been reported in some studies (76) (89). In one study, liner materials recommended for layered veneering ceramic were found to weaken the bond strength between the veneering porcelain and zirconia coping when used with the press-on veneering system (89). In addition to the liner application, the zirconia surface treatment also affects the bond strength of the veneering layer. Airborne particle abrasion seems to be a useful way to improve bond strength (90). In contrast, investigators found that airborne 26

24 particle abrasion had no effect in increasing bond strength of veneering porcelain to zirconia (91). These conflicting results could be explained by the use of different materials, testing design, and study setup among the different studies. 1.6 Monolithic Restorations As an alternative to Y-TZP cores with veneering porcelain, monolithic crowns (fullanatomic) made of lithium disilicate glass-ceramic may be employed due to its high flexural strength and better translucency. In a prospective clinical study, Marguardt and Strub (2006) evaluated the survival rate of IPS Empress II by placing 27 crowns in the molar and premolar regions and 31 three-unit FPDs in anterior and premolar areas. After five years the Kaplan-Meire test revealed a 100% survival rate for the single crowns but 70% for the three-unit bridges (92). The combination of monolithic and CAD/CAM technology, in a system named IPS e.max CAD, has been advocated to fabricate reliable and relatively strong monolithic crown restorations. One research group examined the clinical performance of monolithic crowns (IPS e.max CAD), and after 24 months observation, they found that the survival rate was 97.4% (93). In an in-vitro study, Guess et al (2010) observed that bulk fracture occurred at higher load levels for lithium disilicate (IPS e.max CAD) compared to bi-layered zirconia crowns with manual veneer application which were susceptible to chipping of the veneer (29). In one clinical report, 62 monolayer crowns (IPS e.max CAD) were inserted in premolar and molar regions of 43 patients. Two adhesive systems were used for the cementation: 23 crowns were cemented with Multilink Automix (Ivoclar Vivadent) used as control and 39 crowns were cemented with an experimental dual-curing cement (EC) developed by Ivoclar Vivadent. The postoperative sensitivity for the crowns cemented with MA and EC was 13% and 27

25 10.3%, respectively one week after insertion. No sensitivity was detected at the first and second year; however, discoloration on one-half of the buccal and lingual surfaces staring from the margin occurred in 20.5% of the crowns cemented with EC. After two years of clinical service, there was no fracture or chipping observed for either MA or EC (94). 1.7 Surface pre-treatment and luting cement for all-ceramic restorations: In order to achieve the desired bonding between the intaglio surface of zirconia crowns and the tooth structure, a strong resin bond is required. The strong resin bond can be achieved through micromechanical interlocking achieved mechanically by roughening the surface or chemically by using acid etching. Silica-based ceramics may be treated with hydrofluoric acid prior to cementation to provide a rough and clean surface. This increases the surface area for mechanical interlocking (83). However, it was reported that etching the internal surface of e.max CAD for more than four minutes will lead to defects and will change the material structure (95). On the other hand, high strength polycrystalline materials, or non-silica based ceramics such as zirconia will not be affected by this treatment, which makes it difficult to roughen the surface. Also, the lack of silica prevents the chemical bond which is necessary for silanization (96). Due to the difficulties encountered in establishing mechanical or chemical bonding in zirconia, alternative methods have been advocated. Commonly, zirconia may be mechanically ground with a diamond bur or through airborne particle abrasion on the intaglio surface of the crowns with m aluminum oxide powder, or other abrasive particle, followed by a silane application. This process creates microretentive features and removes residual stresses on the internal surface; however, it also creates microcracks on the surface which decrease the zirconia s flexural strength (87, 97). In the case of grit-etching the zirconia s intaglio surface, it is reported that this process may cause transformation from the tetragonal to the monoclinic 28

26 phase, which produces compressive stresses within the restoration and subsequently weakens the structure (42, 98). Application of a tribochemical silica coating is another approach to create a chemical bond between zirconia and resin cement. In this technique, the intaglio surface is airborneparticle abraded with aluminium oxide particles modified with silica. Some studies reported high bond strength after silicoating and silanization. Unfortunately, the bond strength values significantly decreased when artificial aging was conducted (99). Moreover, a new method to acquire a high strength bond to polycrystalline ceramics by heat treatment has been proposed which is the selective infiltration etching (SIE) technique. This technique transforms the zirconia from a non-retentive and low energy surface to a highly retentive and active surface. The procedures involve coating the surface of the zirconia with glass containing conditioning agents which is heated above its glass transition temperature. Then, in an acidic bath, the glass is dissolved to create a porous surface necessary to achieve the bond strength to resin cement (100). It has been reported that the microtensile bond strength values increased when utilizing SIE on zirconia compared to zirconia treated by particle air-abrasion (101). The success of all-ceramic restorations strongly depends upon marginal adaptation. A well-fitted margin is expected to reduce plaque accumulation and minimize the chance of recurrent caries, and potentially increase the longevity of the restoration (102). Another factor that will determine the success of the ceramic crowns is the compatibility between the ceramic composition and the cement used. There is lack of information in the dental literature on effect of the resin cement thickness on the fracture resistance of porcelain crowns. It is assumed that a thicker cement layer causes higher stresses and these stresses might lead to restoration failure 21

27 (103, 104). Therefore, it has been suggested to keep a minimal film thickness, approximately 50µm, to reduce the water sorption and its consequences on the properties of the cement (96). 1.8 CAD/CAM Technology CAD/CAM technology (Computer-assisted Design/ Computer-aided Manufacturing) has become a very important development in the dental field. CAD/CAM technology in dentistry was introduced in 1971 when Dr. Duret of France utilized an optical impression and a numerically controlled milling machine (22). Following this introduction, Dr. Moermann of Switzerland developed CEREC for use in dental offices in the 1980s (105). Later, Dr. Andersson introduced the Procera system, which is also known as NobelProcera by Nobel Biocare, a labbased system used for fabrication of coping for all porcelain crowns (106). As this technology has evolved, several CAD/CAM systems have been introduced, and others discontinued. Currently, a number of CAD/CAM based systems are available on the market. The first operational CAD/CAM system used in dental offices was the CEREC system. In 1985, Moermann fabricated the first chair-side inlay using CEREC I, and in 1994, the improved CEREC II was launched (107). The CEREC system was initially limited because of poor marginal adaptation and limited computer efficiency, but these limitations were overcome with the CEREC III version. With further improvements, and the addition of 3D capabilities to the design software, all-ceramic restorations milled with CEREC 3D have shown superior marginal adaptation over its predecessors (22). CEREC 3D is made up of a recording and designing unit, as well as a milling machine unit. Time wastage in the design phase is eliminated with an easy-to-use designing program and integrated efficient computer. An intra-oral camera allows users to record the cavity or preparation from a variety of angulations. The time needed for this has been reduced between 11

28 this generation and the CEREC II. CEREC AC, an even more innovative design, allows for digital impressions (BlueCAM) that are highly precise for the production of impeccable veneers, crowns, and bridges (108). Among the many advantages of using the CEREC system is the resultant aesthetic appearance and high quality of the product due to precision measurements. Further, this system eliminates the lab procedures such as waxing up, investing, and casting with all its potential errors. This means that CAD/CAM also saves time, and ultimately reduces the cost of impression materials, which allows the fabrication of restorations that have better structural reliability by eliminating the effects of different variables involved in traditional procedures. It has been reported that 40 seconds is enough to take an impression of each half arch (108). Nonetheless, accompanying the major advantages of CAD/CAM technology, there are disadvantages of this system which are mainly the cost of both the device and the software. Dentists also need time to be trained adequately so that they are able to use the system efficiently (109). When fabricating restorations with the CEREC system, the following steps should be taken into account. First, for the tooth preparation, the general principles of all-ceramic crown preparation are applied for the CEREC. The external margin of the preparation should be well defined and all line angles rounded off. The restoration can be fabricated with only a single optical impression, but other impressions can be used to improve the occlusal result (110). After taking the optical impression, the restoration design can be generated by the software program. Then, a porcelain block of choice is inserted in the milling unit and the restoration is milled with different types of cutting diamonds. An alternative method could be used if the dentist does not have access to a milling machine by sending the optical impression to the dental laboratory for the crown to be designed and milled. The most well-known laboratory CAD/CAM technology is 12

29 InLab CEREC, which is able to design and mill zirconia core and full anatomical crowns and bridges (108). Another chair-side system besides CEREC is the E4D dentist system (D4D technologies, Richardson, Texas), which was introduced in The system comes with laser scanner, design center and milling machine and it works similarly to the CEREC system. It is not necessary to use the reflection powder spray in order to take the impression for all cases. After taking the impression, finish lines are automatically detected by the system and marked on the monitor. When the dentist accepts the markings, the E4D proposes a restoration of the prepared tooth. From the ceramic blocks of choice, the milling machine will fabricate the restoration Interestingly, the systems allows the dentist to work on up to sixteen restorations at the same time (111). Different ceramic blocks are available to be used for the chair-side systems: IPS e.max CAD (Ivoclar Vivadent) is a partially crystallized lithium disilicate glassceramic. At the pre-sintering stage, the e.max CAD blocks are supplied in a bluish color and they exhibit a flexural strength of 120 MPa to 150 MPa. This will allow both that the milling process to be faster as well as the performance of intraoral adjustments. After crystallization, the flexural strength increases to 360 MPa and that makes it suitable for fabricating anterior as well as posterior crowns. Studies show that this material is promising for the dental field (112, 113). A feldspathic porcelain that can be used for veneers, inlays, onlays, and partial as well as full crowns is Vita Mark II (Vita Zahnfabrik). The CTE of this material is comparable to 11

30 that of the tooth structure, with a flexural strength near 110MPa, and it boasts excellent machinability and abrasion properties that are equal to enamel (114). A unique and durable nano ceramic resin product is Lava Ultimate (3M ESPE). Lava Ultimate is fracture resistant to a degree comparable to IPS e.max CAD, with a flexural strength of 200MPa. This material is fabricated quickly compared with other CAD/CAM materials, and is unique in that it does not require firing (92). ProCad (Ivoclar, Vivadent) is a leucite reinforced feldspathic porcelain that is used for the fabrication of veneers, inlays, onlays, and partial and full crowns. This material has good aesthetic value and translucency due to the fact that the leucite crystal has a reflection index equal to feldspathic glass (113, 115). A highly aesthetic CAD/CAM product for monolithic single crowns, as well as for veneers, inlays, onlays, anterior, and posterior restorastions, is IPS Empress CAD (Ivoclar Vidadent), which is the second generation of ProCad (115). 1.9 Metal-Ceramic Restorations Metal-ceramic restorations (MCR) have been the superior choice for crown fabrication because of their long term structural performance and reasonable aesthetic appearance as well as its clinically acceptable fit accuracy (70). Furthermore, porcelain-fused-to-metal (PFM) crowns and bridges have found widespread popularity over time because of the use of precious alloys which demonstrate the best structural performance, with a survival rate of 78% over 20 years in single crowns ( ). In spite of the wide utilization of PFM crowns, researchers continue to pursue metal-free option because there continues to be some concern in terms of aesthetics and biological compatibility (119). Aesthetically, PFM crowns do not always result in life-like appearances 13

31 because of the metallic opacity of the core (120). This results in a restoration with opacity that differs from the reflection and translucency of natural teeth. Different studies reported that most crown failure is in the anterior maxillary region is due to the aesthetic appearance (118). In addition to the aesthetic considerations, there are a number of reported biological complications, such as periodontally-related ones as well as allergic reactions and caries (121, 122). These biological complications might increase when base metal alloys are used. Poor gingival health has been reported when Ni-Cr alloy metal crowns were utilized while using gold metal crowns is less harmful to the surrounding periodental tissues (123). Of greater concern is the issue of chipping of the veneering porcelain. Pjectursson et al (2007) stated that the rate of porcelain chipping in PFM crowns reached 5.4% in a five-year systematic review (121). Walton (2003) evaluated 515 PFM restorations and calculated the cumulative survival rate of these prostheses at 5, 10 and 15 years as 96%, 87% and 85%, respectively (124). When mechanical failure of PFM crowns takes place, it is never the metal coping that undergoes the breakage but rather the weaker veneering porcelain. Depending on the degree of porcelain chipping, the crown may need to be replaced (38). Despite the fact that CAD/CAM technology is used in the dental field to produce allceramic crowns and bridges, metal-ceramic restorations took advantage of this technology. Conventionally, the metal coping is produced by using casting metal alloys such as nickelchromium and cobalt-chromium. Due to the difficulty of milling metal alloys in the same manner of milling polycrystalline oxide ceramic, the laser-sintering technology allows fabricating metal copings for the metal-ceramic restorations with more ease and efficiency (125, 126). The first system using the laser sintering of cobalt-chromium was the MP100 dental system which achieved major advantages over the conventional casting method (127). 14

32 The clinical outcome of PFM crowns produced with laser-sintering technology was evaluated in an in vivo study. Thirty-nine participants were given 60 crown restorations in the posterior region. A noble alloy (Gold-Platinum) and base-metal alloy (Cobalt-Chromium) were utilized with the laser-sintering technique and crowns were then veneered manually (IPS d,sign, Ivoclar Vivadent). Over a mean observation period of 47 months, the Kaplan-Meier analysis revealed a survival rate of 98.3%. Interestingly, these findings are promising because technical complications such as chipping of the porcelain layer were not reported. The authors concluded that there are comparable results between this new technology and cast metal-ceramic crowns (126). Akova et al (2008) compared the bond strength of cast nickel-chromium and cobaltchromium to laser-sintered cobalt-chromium. There was no significant difference of the bond strength between laser-sintered cobalt-chromium and cast cobalt-chromium whereas the highest shear bond strength value found was for cast nickel-chromium. Mixed cohesive and adhesive failures were observed for the cast metal alloys while half of the laser-sintered cobalt-chromium samples showed adhesive failure and the other exhibited both adhesive and cohesive failure (127). Another research group evaluated the internal fit of the same materials employed in the previous study (cast nickel-chromium and cobalt-chromium) and laser-sintered cobaltchromium. Two techniques were utilized to evaluate the fit of the crowns either by weighing light-body addition silicone used between tooth and crown or measuring the internal gap width on the die for longitudinally sectioned specimens. They found a significant difference between the laser-sintered Co-Cr and the other groups for weighing the light-body addition silicone. 15

33 However, no significant differences were detected for the internal gap width among all groups (125). Base metal alloys are widely used for fabricating porcelain-fused-to-metal crowns and bridges. Nickel has long been known to be common cause of contact allergies (128). Data indicated that about 5% to 28% of the population has a hypersensitivity to nickel. The incidence of allergic sensitivity to nickel has been reported to be significantly higher for females than males. A cobalt-chromium alloy or a nickel free alloy should be utilized on patients with a history of allergic response to nickel. Hence, due to the high incidence of allergy to nickel in the general population, it is not recommended that it be used(129). 16

34 1.10 Rationale, Objectives and Hypotheses The increasing aesthetics demanded by clinicians and patients was behind the ongoing trend toward all-ceramic restorations. In spite of this movement toward aesthetically pleasing restorations, the high rate of chipping failure in bilayer zirconia crowns is a major drawback. The purpose of this study was to evaluate the effect of materials and techniques on resistance to chipping of porcelain crowns when made with different materials and layering techniques. The objectives of this study were: a) To determine the effect of material and fabrication techniques on the fatigue resistance of all-ceramic crowns subject to compressive cyclic loading. b) To compare the chipping resistance of monolithic and bi-layered all-ceramic prostheses to PFM crowns. The null hypotheses were: a) Material and fabrication technique do not influence the fatigue resistance of allceramic crowns. b) The chipping resistance of all-ceramic crowns is similar to that of porcelainfused-to-metal crowns. 17

35 2. Materials & Methods 2.1 Sample size Five experimental groups (Table I) were formed (n=10). The number of specimens per group was determined through power analysis considering the values suggested by Norman et al. (2000) at α = 0.05 and β = 0.2 and the calculation reported by a previous study (Schmitter et al., 2012). It compared the fracture strength of zirconia crowns fabricated using conventional layered veneer porcelain to zirconia crowns fabricated employing CAD-on technique. The effect size (f) was calculated by using the following formula, developed by Cohen (1999). ( ) Where k is the number of groups and d is ( ). d = ( ) / (( ) / 2) = 2.4 f = ( ) ( ) 18

36 Table 1: Experimental groups employed in the present study. Group Coping Veneering Porcelain-fused-to-metal (PFM) Metal core with 0.5 mm uniform thickness IPS d.sign Conventional layering technique Lithium disilicate glass monolithic crowns Monolayer crown made from IPS e.max CAD (LDG) Zirconia veneered by layering technique (ZVL) Zirconia veneered by press-on technique (ZVP) Zirconia Veneered digitally with lithium disilicate CAD-on technique (CAD-on) Y-TZP core with 0.5 mm uniform thickness Y-TZP core with 0.5 mm uniform thickness Y-TZP core with 0.5 mm uniform thickness IPS e.max Ceram Conventional layering technique IPS e.max ZirPress Press-on layering technique IPS e.max CAD Digital veneering technique 2.2 Specimens Preparation A porcelain crown preparation with the following characteristics was made on an epoxy resin mandibular first molar: axial walls reduction of 1.5 mm, occlusal surface reduction of 2 mm and gingival margin of 1 mm width circumferential shoulder. The angle of convergence was 6-8 and all line angles were beveled in order to prevent stress concentration. The prepared tooth was used to make fifty replicas from a highly filled epoxy resin material (Viade Products Inc. 11

37 Camarillo, CA, USA) (figure 1). The modulus of elasticity of these replicas was comparable to that of the human dentin (130). To verify the accuracy and to ensure standardization of the replicas, their dimensions were verified mesio-distally and bucco-lingually from the occlusal and cervical points (Table 2). 2008). Figure 1: Die replica (Reprinted with permission from Zahran et al., J Prosthodont Table 2: Measurements of the epoxy resin die replicas. Replicas Dimensions Mesio-distal measurements Bucco-lingually measurements Occlusal Cervical Occlusal Cervical Mean 6.36 mm 6.90 mm 6.90 mm 7.51 mm Standard Deviation 0.01 mm 0.02 mm 0.01 mm 0.01 mm 31

38 2.3 Restoration design and fabrication The replicas were positioned in a dentoform with adjacent teeth in order to take the optical impression as the first step (figure 2). The prepared tooth, adjacent teeth and opposite teeth were coated with thin layer of optical reflection powder (figure 3). To capture the optical impression, the CEREC 3D intra-oral camera was utilized (CEREC 3.84, Sirona). Figure 2: Prepared tooth replica inserted into the dentoform. 32

39 Figure 3: Application of a thin layer of contrast spray powder prior to the optical impression. Crowns were made in accordance with the experimental groups (Table II) as follows: Porcelain-fused-to-metal (PFM) group Ten conventional porcelain-fused-to-metal crowns (PFM) constituted the control group. The metal coping was designed digitally (CEREC 3.84, Sirona) with 0.5mm uniform thickness and fabricated with laser-sintering technology (Argeloy NP Supreme, IDENTalloy, USA). Metal copings were seated into dies to ensure fit accuracy and also the thickness was measured with Iwanson caliper to verify accuracy. Metal cores were then veneered manually (IPS d.sign, Ivoclar, Vivadent) by an experienced dental technician with minimum 1.5mm occlusal thickness, following the manufacturer s recommendations (figure 4). 31

40 A B C D Figure 4: Fabrication steps for porcelain-fused-to-metal (PFM). A: Designing the coping with CEREC. B: The metal Core is finished. C: Porcelain application. D: Molar crown finalized. Table 3: Firing program for metal ceramic crowns. Step Drying temperature ( C) Holding Time (min) Temperature increase rate ( C/min) Top temperature Holding time (min) Vacuum 1 ( C) Vacuum 2 ( C) Oxide firing Wash opaque Opaque layer Dentin Glaze

41 2.3.2 Monolithic lithium disilicate glass ceramic (LDG) group This group consisted of monolithic CAD/CAM lithium disilicate crowns (IPS e.max CAD, Ivoclar Vivadent). The prepared tooth, adjacent teeth and opposing teeth were coated with thin layer of optical reflection powder (IPS Contrast Spray, Ivoclar, Vivadent). In order to capture the optical impression the CEREC 3D intra-oral camera was utilized (figure 5a). Subsequently, a full contour crown was designed (CEREC 3.84, Sirona), then IPS e.max CAD blocks (size C14) (shade LT A1) were milled with CAD/CAM milling machine, followed by crystallization firing and glazing (Programat P500, Ivoclar Vivadent) according to the manufacturer instructions (Table 4). A B C D Figure 5: A: Optical impression. B: Designing crown in virtual occlusion. C: Milling the restoration. D: Crowns mounted and placed on firing tray. 34

42 Table 4: Firing parameter for monolithic lithium disilicate glass ceramic (IPS e.max CAD). Step Crystallization & glaze Drying temperature ( C) Holding Time (min) Temperature increase rate ( C/min) Top temperature Holding time (min) Vacuum 1 ( C) Vacuum 2 ( C) Zirconia coping veneered by hand layering technique (ZVL) In this group, each prepared tooth replica was digitally-scanned and the coping was designed. Pre-sintered Y-TZP blocks were milled with 0.5 mm uniform thickness (IPS e.max ZirCAD, Ivoclar Vivadent) (Size C15) (Shade MO 0) by the CAD/CAM system (CEREC 3.84, Sirona). Milled cores were then cleaned with running water to remove any remaining debris. Drying was performed for 15 minutes under 70 C followed by the sintering step in a sintering furnace. Sintered copings were slightly ground and finished with fine diamond bur (Figure 6). In the ceramic furnace (Programat P500, Ivoclar Vivadent), regeneration firing for the zirconia cores was conducted to eliminate any micro-cracks that might have occurred during the grinding step. The conventional hand-layering technique (IPS e.max Ceram, Ivoclar Vivadent) (A1 shade) was utilized for the veneer application by an experienced technician. The first applied layer was IPS e.max ZirLiner to establish bonding with the zirconia framework (Figure 7). After firing the first layer, veneering porcelain was sequentially built up until the final step, which is glazing. Firing cycles and materials application followed manufacturer's recommendations (Table 5). The thickness of the crowns was verified with a digital caliper to ensure uniformity among all specimens. 35

43 Step Table 5: Firing program for hand-layered zirconia crowns. Drying temperature ( C) Holding Time (min) Temperature increase rate ( C/min) Top temperature Holding time (min) Vacuum 1 ( C) Vacuum 2 ( C) ZirLiner First Dentin Second Dentin Glaze A B C D Figure 6: A: IPS e.max ZirCAD blocks. B: Milling chamber. C: Milled coping. D: Finishing the zirconia coping with fine diamond bur. 36

44 Figure 7: IPS e.max ZirLiner application prior to the porcelain veneering Zirconia coping veneered by heat pressing technique (ZVP) Y-TZP copings were prepared as described for the ZVL group. The veneering porcelain was applied by using the pressed-on veneering technique (IPS e.max ZirPress, Ivoclar Vivadent) (Shade LT A1). IPS e.max Ceram ZirLiner was applied to the zirconia copings as mentioned above. After firing ZirLiner, a full-contour wax pattern was built up onto the copings. The thickness and shape of the wax pattern was maintained the same for all specimens using a silicon impression of a fully anatomical crown (Figure 8). After adding sprue, both core and wax up were invested (IPS PressVEST, Ivoclar, Vivadent) then burn out procedure followed IPS e.max Zirpress ingots were pressed into the mold using a press furnace at 700 C (Pro press 100, Whip Mix Corporation). Crowns were finished with a glaze layer following the manufacturer s recommendations (Figure 9) (Table 6). 37

45 Figure 8: silicon impression was taken to minimize crown s shape and size variation. A B C Figure 9: A: Coping with wax-ups and sprued ready for investing; B: Pressing ceramic ingots; C: The crown after finishing and glazing. Step Table 6: Firing program for pressed-on zirconia crowns. Drying temperature ( C) Holding Time (min) Temperature increase rate ( C/min) Top temperature Holding time (min) Vacuum 1 ( C) Vacuum 2 ( C) ZirLiner ZirPress Glaze

46 2.3.5 Zirconia core & CAD veneering technique (CAD-on) The multilayer option was chosen in the CEREC 3D to design and mill both zirconia coping and digital veneer. To do this, a layer of reflection powder first was applied (IPS Contrast Spray, Ivoclar, Vivadent) then optical impression was taken for the prepared, adjacent and opposing teeth. Separate core and veneer were generated with a minimal occlusal thickness of 1.5mm for the two components together. The milled core and milled veneering porcelain were joined together using a ceramic fusing agent (IPS e.max CAD crystal/connect). The fusion ceramic agent was applied into the interior surface of the veneer and on top of the coping. A vibration device was used to ensure full seating of the veneer onto coping. The final step of fabricating the specimens in this group was the crystallization firing followed by the glazing according to the manufacturer recommendations. A B C Figure 10: Fabrication process for CAD-on crowns. A: Designing core and veneer. B: Milled zirconia core and lithium disilicate veneer. C: After connecting the core and veneer. Table 7: Crystallization and fusion firing program of CAD-on crowns. Step Drying temperature ( C) Holding Time (min) Temperature increase rate ( C/min) Top temperature Holding time (min) Vacuum 1 ( C) Vacuum 2 ( C) Crystallization & fusion Glaze :

47 Table 8: Compositions of different materials utilized in the study according to manufacturers data. Material Composition Argeloy NP Supreme Cobalt (Co) 61% Silicon (Si) 1% Chromium (Cr) 27% Manganese (Mn) <1% Molybdenum (Mo) 6% Iron (Fe) <1% Tungsten (W) 5% Carbon (C) <1% IPS e.max ZirCAD IPS e.max CAD IPS e.max Ceram Zirconium dioxide ZrO % Yttrium oxide Y 2 O % Hafnium oxide HfO % Aluminum dioxide Al 2 O % Other oxides < 0.2% Silicon dioxide SiO Lithium oxide Li 2 O % Potassium oxide K 2 O % Phosphorus pentoxide P 2 O % Zirconium dioxide ZrO % Zinc oxide ZnO % Aluminum dioxide Al 2 O % Magnesium oxide MgO % Coloring oxides % Silicon dioxide SiO % Aluminum dioxide Al 2 O % Zinc dioxide ZnO 2 < 6% Sodium oxide Na 2 O % Potassium oxide K 2 O % Zirconium dioxide ZrO 2 Calcium oxide CaO < 6 % 41

48 Phosphorus pentoxide P 2 O 5 < 6 % fluoride and pigments % IPS d.sign Silicon dioxide SiO % Aluminum dioxide Al 2 O % Potassium oxide K 2 O % Zinc dioxide ZnO 2 < 6% Sodium oxide Na 2 O % Zirconium dioxide ZrO 2 Calcium oxide CaO Phosphorus pentoxide P 2 O 5 Pigments IPS e.max ZirPress Silicon dioxide SiO % Aluminum dioxide Al 2 O % Zinc dioxide ZnO 2 < 6% Sodium oxide Na 2 O % Potassium oxide K 2 O % Zirconium dioxide ZrO 2 Calcium oxide CaO < 6 % Magnesium oxide MgO % Phosphorus pentoxide P 2 O 5 < 6 % Other oxides % IPS e.max CAD Crystall./Connect Silicon dioxide SiO % Aluminum dioxide Al 2 O % Zinc dioxide ZnO % Sodium oxide Na 2 O % Potassium oxide K 2 O % Other oxides and pigments % 42

49 2.4 Cementation: All crowns were tried-in onto their corresponding tooth replicas in order to confirm proper marginal fit. Length of specimen (tooth + crown) was measured with a digital caliper (Figure 11). Prior to cementation process, the marginal gap was checked for each specimen at four points under a microscope. If any specimen had a marginal gap greater than 150 µm, it was considered as a defective sample and excluded from the test; however, no specimen was excluded. Crowns were then cemented to their corresponding replicas with dual-polymerized phosphate modified resin cement (Panavia F2.0, Kuraray, America, Inc). The replicas surfaces were first prepared by applying 37 % phosphoric acid etchant (Total etch, Ivoclar Vivadent) for 1 minute to remove contaminants and create micro-roughness. The surfaces were thoroughly rinsed with water and air-dried. Mixed A&B ED primer II (Kuraray) was applied with a brush in two consecutive layers, which were slightly air-dried. Cementation then followed. For metal and zirconia copings, the intaglio surfaces were grit-etched with 50µm aluminum oxide particles under 1 bar pressure for 5 seconds, followed by cleaning with distilled water in an ultrasonic bath. The inner surfaces of zirconia crowns were then coated with a layer of silane-coupling agent (Mono-bond plus, Ivoclar Vivadent). For lithium disilicate monolithic crowns the intaglio surface of crowns were etched for 20 seconds with hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar, Vivadent) (Figure 12). After cleaning the crowns ultrasonically for 60 seconds and air-drying the surfaces were then coated with a layer of silane-coupling agent (Mono-bond Plus, Ivoclar, Vivadent). Panavia F 2.0 (Kuraray) pastes A&B were mixed together for 20 seconds and then applied to the inner surface of the crown with a plastic instrument (Figure 13). The crown was seated onto its corresponding replica and excess cement removed. A 10N load was applied to the 41

50 occlusal surface for 15 min (Figure 14). Light-curing was performed for 20 seconds on each surface followed by Oxyguard II (Kuraray) application to the crowns margins for 3 minutes. Length of specimen (tooth + crown) was re-measured with a digital caliper to ensure complete seating of the crowns. Any specimen that differed by more 50 µm than the initial measurement before cementation was to be excluded from the study. No specimens were excluded based on this criterion. Figure 11: Length of specimen (tooth + crown) was measured with a digital caliper. Figure 21 : Hydrofluoric acid application on the internal surface of e.max CAD crowns. 43

51 Figure 13: Panavia F 2.0 (Kuraray) pastes A&B were mixed together. Figure 14: The crown was seated under a 10N static load. 44

52 Table 9: Materials properties according to the manufacturer s instructions. Material Classifications Manufacturing Flexural CTE (10-6 ) & LOT # technique strength (MPa) IPS e.max Ceram Nano-fluorapatite glassceramic Manual (S00837) application IPS e.max ZirPress Fluroapatite glass-ceramic Pressing (P76153) technique IPS e.max CAD Lithium disilicate glassceramic CAD/CAM (R67755) IPS e.max ZirCAD Yttrium stabilized CAD/CAM (R71099) zirconium oxide Argely NP Supreme (Co) 61% (Cr) 27% Laser sintering ( /10) (Mo) 6% (W) 5% technique IPS d.sign (R73590) Leucite glass-ceramic Manual 80± application Panavia F 2.0 (061124) Resin cement 79 - IPS e.max CAD crystal/connect (R66132) Fusion glass-ceramic Manual application CTE: Coefficient of thermal expansion; 2.5 Compressive cyclic loading test Compressive cyclic-loading was applied uniaxially to each specimen in a universal testing machine (Instron). The samples were positioned in a 30 incline with a special holder by using resin material (SR Ivolen, Ivoclar Vivadent). To do this, the specimen was attached to a vertical metal arm and then was tilted until the desired 30 angle was achieved (Figure 15). The holder was filled with the resin material and the sample embedded into the resin (Figure 16). The angle was also measured after mounting the sample by trimming the acrylic base form the mesial surface (Figure 17). A virtual line was drawn along the sample (red line - Figure 17-B). From the 45

53 y-axes to the virtual line the angle was measured. The data were analyzed statistically using oneway ANOVA. As a result, there was no significant difference between the groups (P-value = 0.76) (table 10) and the angle was confirmed to be 30 ±2. Lastly, all specimens crowns were stored in distilled water at 37ºC for one week. Figure 15: Preparing the sample in the correct inclination 30. Figure 16: The red tube filled with the resin material and the sample embedded into it at 30 angle. 46

54 A B Figure 17: A: The sample mounted in acrylic resin for the cyclic loading; B: The angle was measured (30 ±2 ). Table 10: One-way ANOVA and P-value for angle measurement. Angle ANOVA Sum of Squares Df Mean Square F Sig. Between Groups Within Groups Total N Mean Std. Deviation Std. Error 95% Confidence Interval for Mean Minimum Maximum Lower Bound Upper Bound PFM ZVL ZVP CAD-on LDG Total

55 One week after the cementation specimens were placed in a special jig for the compressive cyclic loading. The chamber of the jig was filled with water to enable conducting the test under water, and the cyclic loading was applied with a cone shaped stainless steel stylus on the mesio-buccal cusp incline (Figure 18). Up to 500,000 compressive load cycles (50-450N) at 20 Hz frequency were applied to each specimen. At 250,000 cycles a light microscope was used to examine each specimen to determine presence of fractures, chipping or crack lines. If no defects were detected another 250,000 cycles were applied and the specimens were subsequently microscopically re-inspected. After the cyclic loading test, five crowns from each group were sectioned mesio-distally to determine internal fit tolerance through measuring cement film thickness and margin gap under scanning electron microscopy (SEM) (Figure 20). 48

56 Figure 18: The specimen positioned for the compressive cyclic loading test. 41

57 30 Figure 21 : Schematic illustration of the load application. 51

58 Figure 20: Sectioning the specimen mesio-distally. 52 f

59 Figure 21: Some specimens after sectioning Data was statistically analyzed with the Chi-square test followed by Fisher s exact test to compare the chipping resistance of different crowns designs. The overall statistical significance was pre-set at 5%. A log rank test was performed for comparing Kaplan-Meier survival curves for the fractured samples during the cyclic loading test. 51

60 3. Results All CAD/CAM lithium disilicate glass ceramic (LDG) specimens and CAD-on specimens survived the fatigue test with no failure, while PFM, ZVL and ZVP group specimens failed during the cyclic loading test at different levels. The Chi-square test revealed a significant difference between the groups (P < 0.001). Fisher s exact test is suitable due to the expected proportion, more than 50% of the cells, was less than 5. The Fisher s exact test showed a statistically significant difference between the tested groups. Therefore, the null hypothesis is rejected. Table 11: The number of crowns in each group whether survived or fractured. Groups Sample Status Survived Fractured or chipped PFM % 100% LDG % 0% ZVL % 100% ZVP % 100% CAD-on % 0% 53

61 Table 12: Chi-square test and P-value for Fisher s exact test. Chi-Square Tests Value D f Asymp. Sig. (2- Exact Sig. (2- Exact Sig. (1- Point Probability sided) sided) sided) Pearson Chi-Square a Likelihood Ratio Fisher's Exact Test Linear-by-Linear Association b N of Valid Cases 50 a. 5 cells (50.0%) have expected count less than 5. The minimum expected count is b. The standardized statistic is Zirconia crowns veneered by hand layering technique as well as zirconia crowns veneered by press-on technique presented inferior chipping resistance compared to metalceramic crowns. Kaplan-Meier survival analysis revealed higher survivability for LDG specimens and CAD-on specimens than specimens of PFM, ZVL and ZVP (Fig: 22). Cohesive failure within the veneering porcelain was observed without exposing the zirconia substructure (Fig: 24) while two samples of ZVL group had adhesive fracture where the core was exposed. Similarly, four of the PFM crowns had adhesive fracture and the rest of the samples had cohesive failure of the veneering porcelain (Fig: 25). Bulk fracture was not detected in any group. In contrast, no failure was observed for LDG or CAD-on crowns either as chipping or bulk fracture. 54

62 ZVL ZVP PFM Mean SD Table 13: The number of cycles at which samples failed during the cyclic loading. 55

63 Figure 22: Box-plot of the fatigue test results (Number of cycles). 56

64 Figure 23: survival line of Porcelain-fused-to-metal specimens. 57

65 Figure 24: Survival line of ZVL specimens. 58

66 Figure 25: Survival line of ZVP specimens. 51

67 Figure 26: Survival analysis results for all groups. Overall Comparisons Chi-Square Df Sig. Log Rank (Mantel-Cox) Test of equality of survival distributions for the different levels of experimental groups. Significance difference was found by Log rank test. 61

68 Table 14: Type of failure of the failed samples. Cohesive Adhesive PFM 6 4 ZVL 8 2 ZVP

69 Figure 27: Veneer chipping of zirconia crown veneered by press-on (Cohesive failure). A B Figure 28: Adhesive (A) and cohesive (B) failure of PFM crowns. 61

70 A B Figure 29: Manually veneered zirconia crowns presenting cohesive (A) and adhesive (B) fracture. Figure 30: Monolithic crown that survived the cyclic fatigue. (Arrow indicates superficial damage). 63

71 The cement film thickness was measured at the margin from SEM pictures for the crowns made of different materials as listed below. Table 15: Marginal opening gaps (µm). Groups PFM LDG ZVL ZVP CAD-on Mean Standard Deviation A B Figure 31: SEM pictures of sectioned specimens of (A) PFM and (B) ZVL. The circles indicate where the measurement was made. 64

72 A B C Figure 32: SEM images of sectioned crowns (A) ZVP, (B) LDG and (C) CAD-on. The circles indicate where the measurement was made. 65