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Keywords:

  • Zirconia;
  • abutment angulations;
  • implant;
  • core thickness

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Background:  An in vitro study was performed to assess the effect of three implant abutment angulations and three core thicknesses on the fracture resistance of overlaying computer-aided manufacturing (CAM) milled zirconia (Cercon® system) single crowns.

Methods:  Three groups, coded A to C, with different implant abutment angulations (group A/0°, group B/15° and group C/30° angulation) were used to construct 15 crowns for each angulation. Forty-five overlay restorations were milled using the Cercon® system with zirconium core thicknesses of 0.4, 0.6 and 0.8 mm using five crowns for each angulation. The final restorations were prepared and stored in distilled water at mouth temperature (37 °C) for 24 hours prior to testing. The restorations were cemented using Temp Bond®. The load required to break each crown and the mode of failure were recorded. All the results obtained were statistically analysed by the ANOVA test (level of significance p < 0.05). Tested crowns were examined using a stereomicroscope at 40X and selected crowns (five randomly selected from each group were further examined by scanning electron microscopy) to reveal the zirconia–ceramic interface and to determine the fracture origin.

Results:  Implant abutment angulations significantly (p < 0.05) reduced the fracture resistance of overlaying CAM-milled zirconia single crowns. The fracture loads of Cercon® crowns cemented onto abutment preparations with a 30° angulation were the lowest of the groups tested. The core thickness (0.4 to 0.8 mm) did not significantly (p > 0.05) affect the fracture resistance of the CAM-milled zirconia single crowns. SEM showed that the origin of the fracture appeared to be located at the occlusal surfaces of the crowns and the crack propagation tended to radiate from the occlusal surface towards the gingival margin.

Conclusions:  The implant angulation of 30° significantly (p < 0.05) reduced the fracture resistance of overlaying CAM-milled zirconia single crowns. Reducing the core thickness from 0.8 mm to 0.4 mm did not affect (p > 0.05) the fracture resistance of overlaying CAM-milled zirconia single crowns.


Abbreviations and acronyms:
CAD

computer-aided design

CAM

computer-aided manufacturing

SEM

scanning electron microscopy

Y-TZP

yttria stabilized tetragonal zirconia polycrystals

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Patients’ assessment of their dental aesthetics appears to indicate that a proportion of the population are unhappy with the appearance of their teeth.1 Increasing patient expectations regarding the appearance of restorations continues to test the ingenuity and skill of the dental team. To this end the quest for sufficiently strong, metal-free, all-ceramic restorations to function in all areas of the mouth continues apace.1 For any form of treatment or material introduced for use in the dental field, there are a number of characteristics that need to be considered and assessed including the physical and biological properties, and most importantly, the clinical performance of the restored tooth. Other characteristics that also need to be assessed include the fracture resistance of restorations, associated aesthetics, marginal adaptation and the effects of the restoration on pulpal and periodontal health.2

Implant dentistry is an aspect of oral healthcare involving oral surgery, restorative dentistry and prosthodontics that is revolutionizing clinical practice and totally transforming peoples’ lives. This approach has been growing exponentially in recent years and is advocated for single-tooth replacement through to restoration of the fully edentulous mouth. The physiological functional loads applied to the prostheses are transferred through the implant to the surrounding bone. The bone is able to withstand a broad range of strains and stresses which differ in both tension and compression and stimulate remodelling. When these forces are excessive, bone modifications inducing resorption may occur. However, bone loading per se has been questioned as the lone source of resorption.3 Nevertheless, biomechanical considerations play a role in the planning of fixed prostheses supported by osseointegrated implants where design of crown contour as well as occlusal form should be planned to achieve appropriate implant location.3 Pre-angled abutments may be used to overcome non-ideal implant location due to lack of bone.4 The high stresses induced through pre-angled abutments at the cervical zone of the implant could be a dominant factor influencing the success of the restoration.3 In a recent review it was reported that identical vertical loads applied to pre-angled abutments produced higher stresses at the coronal zone of the implant compared with straight abutments. Pre-angled abutments have been found to produce different stress distributions compared to straight abutments.3 In addition, the types of restorative materials used to construct the overlaying crowns are significant factors in determining the amount and distribution of the stresses loaded onto the superstructure and implant under functional forces.5 The development of computer-aided design/computer-aided manufacturing (CAD/CAM) technology has focused on precise and consistent manufacturing of zirconia ceramics with high strength and toughness.6 CAD/CAM technology relies on exact dimensional predictions to compensate for sintering shrinkage, and is an economical and highly reproducible method for manufacturing complex and individual geometrics from a green or pre-sintered ceramic material.7 Zirconia has recently been introduced as a promising metal-free core structure for fixed prostheses.7–9 Recent studies reported that since 1998, 15 studies have demonstrated a 90% or greater success rate using zirconia-based materials.10,11 Cercon® ceramics is a CAM system designed for processing zirconia. As the overall crown thickness may be of primary importance in resisting fracture, a minimum overall thickness of 1.5 mm has been recommended.12 The stiffness, or elastic modulus of the core material is also influential. A stiffer core will better resist flexure under load. This is important because ceramics have low critical strains and are poorly supported by flexible dentine.13 However, stiffer cores may also be more vulnerable to radial cracks originating from their internal surfaces.13 Zirconia is stronger, tougher and more flexible than alumina14 and therefore zirconia-based crowns might be expected to differ from alumina-based crowns in clinical failure mode and overall clinical performance. The superior mechanical properties of zirconia allow clinicians to reconsider established preparation guidelines for the design of single anterior teeth copings, variations such as reducing the coping thickness from 0.5 mm to 0.3 mm and changing finish line margins from chamfer to minimally-invasive knife edge margins. In vitro research evaluating the influence of processing variables on the fracture resistance of all ceramic restorations has revealed highly divergent failure loads of 450 to 1600 N for zirconia single crown copings, depending on coping thickness, marginal design, and luting agent.15,16 In a study where zirconia was used with a 0.5 mm layer thickness as a framework for primarily posterior 3 to 5-unit fixed partial dentures, failures were attributed to cracking or crazing of the veneering porcelain, while bulk fractures were uncommon.10 Fracture appears to be the most common clinical failure mechanism affecting all ceramic crowns.17,18 However, there is inadequate data published investigating the effect of different core thickness and implant abutment angulations on the fracture resistance of overlaying zirconia-based restorations.

The objectives of this study were: (1) to test the hypothesis that pre-angled abutments produce different stress distributions compared with straight abutments; (2) that these different angulations may have an effect on the fracture resistance of the overlaying restorations produced using the Cercon® system; and (3) to provide useful information on the effect of three core thicknesses on the fracture resistance of the overlaying restoration using an experimental biomechanical model.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Implants and abutments were supplied by Dentsply using the ANKYLOS® plus system (ANKYLOS® FRIADENT GmbH Mannheim/Germany, Lot 20029618). The three different abutment angulations chosen were: 0° (balance Posterior, Lot 20031417), 15° (balance Posterior, Lot 20035459) and 30° (balance Posterior, Lot 20037419). A total of 45 overlaying restorations were milled using the Cercon® system, with three groups of 15 samples constructed for each abutment angulation of 0°, 15° and 30° and with three subgroups, each of five samples with zirconium core thicknesses of 0.4 mm, 0.6 mm and 0.8 mm. Cores were fabricated using Cercon’s CAD/CAM based technology, Cercon® Brain, comprising Cercon® Eye, a laser scanner and milling device. Cercon® Art 2.2 software was used for the CAD and construction of the copings. An opaque spray was used prior to scanning the abutments and after milling, the cores were sandblasted to remove any excess zirconium and the margins were refined using a water-cooled turbine and suitable rotary instruments to reduce stress fractures in the zirconia cores and prevent localized overheating.

Diagnostic wax-ups of the crowns were performed using a preformed putty key where liquid wax was injected into the putty key, and vented out the other side. In this way, a detailed reproduction of the wax pattern was obtained with full anatomical contour and as the putty key was used for each restoration, variables were reduced. Wax sprues, 3.5 mm in diameter and 5 mm in length, were attached at an angle of approximately 45° to the pattern. All junctions were smoothed to allow the high-viscosity ceramic material to flow from thick to thin areas. Four specimens were attached to each Cercon® plastic base to be pressed using one pellet. The Cercon® plastic base was attached to the muffle mould and sprayed with a thin coating of oil-free silicon spray to facilitate detachment of the plastic base from the investment. The investment material used was IPS® PressVEST Speed (Ivoclar Vivadent technical), and was mixed following the manufacturer’s instructions. A powder:liquid ratio of 100 g:27 ml distilled water was used and the investment material was then vacuum mixed for 2.5 minutes at approximately 350 rpm at room temperature. During investing, the muffle mould was placed on a dental vibrator while pouring the material into the ring to ensure the mix was free from bubbles and that the waxed units were completely covered by the investment. The muffle mould was then left at room temperature in order to set for 30 minutes. After setting, the plastic muffle form was removed and placed inside a preheating furnace at 850° for 60 minutes. The muffle was taken from the preheating furnace after 60 minutes, and the pressing programme activated on the Cercon® Ceram Press. The pellets chosen were Cercon® Ceram Press, shade A2. The Cercon® pellet was placed in the muffle chamber, and the press stamp applied. The muffle thus equipped was placed into the pressing furnace (Cercon® press) and the pressing programme started. Once the pressing was finished, the flasks were removed from the pressing furnace immediately and allowed to cool and the restorations were carefully divested using glass beads for sandblasting (50 μm, 2 bar). The restorations were then trimmed of excess material occlusally where sprued, and given a final firing, before testing.

Each specimen was stored under distilled water at mouth temperature (37 °C) for 24 hours prior to testing. The restorations were cemented using Temp Bond® NE unidose (SDS Kerr, USA), and allowed 2–3 minutes to set before removing excess cement. The abutment, all-ceramic crown, and implant analogue complexes were secured in a prefabricated steel jig. The specimens were then subjected to compressive loading at a crosshead speed of 1 mm/minute in a universal testing machine (Shimadzu Autograph AG-50 kNE, Shimadzu Co., Ltd, Japan). Compressive force was applied by means of a 4 mm diameter steel bar placed along the midline fissure of the upper premolar crown. The force required (N) to cause fracture of the crown, and the mode of failure were recorded using a classification designed for the investigation, according to Ellakwa et al.19 (Table 1). The means and standard deviations for each group were calculated and compared. One-way analyses of variance (ANOVA) and companion post hoc Tukey pair group comparison tests (p ≤ 0.05) were used for fracture resistance data analysis. The broken fragments were examined using an optical microscope under low magnification (50–100X) to identify the areas of interest. Representative failed specimens from each fracture type were selected and sputter gold coated (EMITECH K550x, Kent, UK) before fractographic analysis using a scanning electron microscope (Phillips XL30 CP) to identify crack propagation patterns. Special attention was focused on the loading surface, veneer core interface and crown margin.

Table 1.   Classification of modes of failure modified from Ellakwa et al.19
CodeDescription
IMinimal fracture capable of refinishing and repair
IILess than half of crown lost
IIICrown fracture through midline, half of crown displaced or lost
IVMore than half of crown lost
VSevere fracture of the core as well as crown

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

The force at fracture of the three test groups (A = 0°, B = 15° and C = 30° angulations) and mode of failure at each of the three subdivided groups (1 = 0.4 mm, 2 = 0.6 mm, 3 = 0.8 mm zirconia core thickness) are presented in Tables 2–5. Statistical analysis by two-way analysis of variance followed by a post hoc Tukey test showed that the implant angulations significantly (p < 0.05) reduced the fracture resistance of overlaying CAM-milled zirconia single crowns. Those samples constructed on the 15° angled abutments showed the highest fracture resistence, although not significantly different from the results recorded for the straight abutments, while the fracture resistance of crowns cemented onto the 30° angled abutments were the lowest of the groups tested. The different core thicknesses (0.4 to 0.8 mm) did not significantly (p > 0.05) affect the fracture resistance of the overlaying CAM-milled zirconia single crowns. The results showed that 39 out of the 45 cores were intact following fracture. Of the six fractured cores, four cores belonged to test group B1 (15° angulation/0.4 mm thickness) and the remaining two belonged to test group A1 (0° angulation/0.4 mm thickness).

Table 2.   The fracture force (N) and description of mode of failure of group A (0° angulation)
Core thickness = 0.4 mm
Sample #ForceMode of failureCore was intact?
1911.25IIIYES
21420.6III to IVYES
31458.75IV–VYES
4487.25IYES
51668.75IIINO
Core thickness = 0.6 mm
Sample #ForceMode of failureCore was intact?
1403.125IYES
21013.12IIIYES
3493.75IYES
41058.87IIIYES
5492.5IIIYES
Core thickness = 0.8 mm
Sample #ForceMode of failureCore was intact?
12229.37VNO
21236.87IYES
3565.00IYES
4740.00I–IIYES
5463.125I–IIYES
Table 3.   The fracture force (N) and description of mode of failure of group B (15° angulation)
Core thickness = 0.4 mm
Sample #ForceMode of failureCore was intact?
11720.62VNO
2927.5VNO
31183.12VNO
4525.625IVYES
51994.37VNO
Core thickness = 0.6 mm
Sample #ForceMode of failureCore was intact?
1376.25IIIYES
21917.7IIIYES
3568.12IIIYES
4994.35IIIYES
5554.37IIIYES
Core thickness = 0.8 mm
Sample #ForceMode of failureCore was intact?
11748.12IVYES
21578.12IIIYES
3504.75IIIYES
4563.57IVYES
51817.50VYES
Table 4.   The fracture force (N) and description of mode of failure of group C (30° angulation)
Core thickness = 0.4 mm
Sample #ForceMode of failureCore was intact?
1404.37IIYES
2401.87IIYES
3700.625IIYES
4525.00IIYES
5469.375IIIYES
Core thickness = 0.6 mm
Sample #ForceMode of failureCore was intact?
1480.625IIIYES
2492.500IIIYES
3468.750IIIYES
4409.375IIIYES
5428.75IIIYES
Core thickness = 0.8 mm
Sample #ForceMode of failureCore was intact?
1693.75IIIYES
2633.125IIYES
3508.125IIIYES
4483.75IIIYES
5686.25IIIYES
Table 5.   The mean fracture strength N (SD) of groups A to C with three different core thicknesses (1–3)
Abutment angulationsThickness of the core
1 ≡ 0.4 mm2 ≡ 0.6 mm3 ≡ 0.8 mm
  1. Groups with same superscripts in the same row are not significantly (p > 0.05) different from each other.

  2. Groups with same number under the same column are not significantly (P > 0.05) different from each other.

A ≡ 0 degree1189.32 (481.19)a,1692.27 (316.33)a,11046.87 (724.83)a,1
B ≡ 15 degrees1270.25 (593.03)a,1882.16 (621.88)a,11242.41 (652.71)a,1
C ≡ 30 degrees500.25 (123.07)a,2456.00 (35.42)a,2601.00 (99.09)a,2

With regards to the fracture mode in group A (0° angulation), 5 crowns presented with minimal fractures that could be considered suitable for refinishing and repair and 5 crowns fractured through the midline with half the crowns displaced or lost. In group B (15° angulation), 7 crowns fractured through the midline, with half the crowns displaced or lost, and 5 crowns fractured severely. In group C (30° angulation), 10 crowns fractured through the midline with half the crowns displaced or lost.

The different modes of fracture are shown in Fig 1 (A–D). Macroscopic examination of the fractured surfaces of the crowns revealed four major types of failures: (I) fracture of the veneering porcelain only; (II) delamination of the veneering porcelain with exposed intact zirconium coping; (III) delamination of the veneering porcelain with core fracture; and (IV) catastrophic fracture of both the veneering porcelain and core into small pieces. In samples where the crack type involved fracture of the veneering porcelain producing very small irregularities in a flat plane, it is difficult to determine the fracture origin because of the luminous surface of the porcelain. However, by using SEM, specific large fractographic patterns such as arrest lines, Wallner’s lines, and large twist and wake hackle lines can be detected. When examining these characteristics, the fracture origin appears to be located at the occlusal surface of the crowns (Figs 2–4) and radiates towards the gingival margin.

image

Figure 1.  Photographic representation depicting the different modes of fracture. (A) shows a minimal fracture of the crown that can be easily refinished and repaired. (B) fracture is classified as mode III; crown fracture through the midline, with half the crown displaced or lost. (C) failure is classified as mode VI, with more than half the crown lost. (D) shows severe fracture of the core as well as the crown.

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image

Figure 2.  (A) depicts the surface delamination of the veneering porcelain of a 0.4 mm core thickness and 0° abutment angled sample. (B) SEM of the fracture site demonstrated radial cracks projecting from the occlusal surface (arrow).

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image

Figure 3.  (A–B). Photographic and SEM depiction of one crown in the group of 0.4 mm core thickness with 0° abutment angulation in which the core fracture and crack propagation can be clearly traced originating from the occlusal surface.

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image

Figure 4.  (A–B). SEM representation of a fan-like pattern of crack propagation originating from the occlusal surface of the opposite fragment, from a specimen in group A1 with 0.4 mm core thickness, 0° abutment angulation.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

The importance of experimental biomechanical models such as the one tested in this study is to provide information related to the clinical situation. For obvious biomechanical reasons, slanted straight implants are not ideal, implants with angled abutments are preferred for functional and aesthetic reasons.20

Porcelain fracture is one of several common failure modes affecting all-ceramic crowns.21 Like most brittle materials, porcelain has a high compressive and low tensile strength.22 Therefore, copings for crowns must be designed to minimize tensile loading of the veneering porcelain. Appropriate porcelain and core thickness may decrease internal stress, reduce mechanical failure and optimize aesthetics.22–25

Zirconia has become an established and indispensable material in all-ceramic fixed prosthodontics over the past few years, having good mechanical properties, with a load to fracture reported to be between 900 to 1200 MPa.26,27 It offers the highest fracture strength and the best fracture resistance of all dental materials currently available on the market.28 Cercon® Smart Ceramics was selected for the current study because it is a CAM system designed for processing zirconia, which allows Y-TZP (yttria stabilized tetragonal zirconia polycrystals) to be processed in dental laboratories. Y-TZP is commonly used as a core material due to its enhanced fracture toughness. The underlying mechanisms that allow for additional energy dissipation at the developing crack tip include stress-induced transformation of the crystalline phase, followed by a 3–5% volume expansion and nucleation of microcracks.6In vitro studies have reported that saliva and blood can degrade the bond strength of a zirconia-based restoration,29 as can surface flaws in zirconia that are induced during grinding. Additionally, grinding using a coarse high-speed diamond rotary cutting instrument removes tens of microns of material in each pass. The associated high stresses and temperatures can induce surface cracks that lower the strength and reliability of the material.30 The results of the current study suggest that if an abutment overlaying crown is going to fail, it will be through its crown material and not related to core thickness (Table 5). However, 30° abutment angulations (group C) allowed more stresses to be generated on the overlaying ceramic restoration, frequently creating a mode of fracture that would allow for refinishing and repair. However, it is difficult to compare failure loads reported in the literature to those found in this study due to different experimental variables. It is important to recognize that this in vitro study aims to reproduce a complex biomechanical environment under complex loading to failure. The primary limitation of this study was that fracture strength testing of the overlaying abutment restoration was placed under vertical loading, and thus results must be interpreted bearing this in mind. Clinically, restorations are subjected to more dynamic complex forces in a biological environment with saliva, which is considerably different from conditions simulated in our experimental biomechanical model.

The results obtained provide valuable knowledge to aid in the clinical setting and indicate that pre-angled abutments (Group C) may produce different stress distributions compared to their straight counterparts which affect fracture resistance of the overlaying restorations. In fact, the 30° implant abutment angulations significantly (p < 0.05) reduced fracture resistance of the overlaying CAM-milled all ceramic zirconia single crowns. Reich et al.6 investigated the fracture performance of high-strength zirconia copings, comparing finish line preparations and coping layer thickness, and observed that crack propagation within a coping is accelerated by high shear stresses under vertical loading, reducing the catastrophic fracture forces. These observations support the idea that prosthesis fractures are sensitive to mechanical strength and hardness of the selected restorative materials. It was noted that for bi-layer all-ceramic restorations, attention should be given to strengthening the core ceramic, such as using stiffer ceramic cores, like zirconia, to provide superior protection for the underlying dentine as well as the veneering porcelain.6 Overall, crown thickness may be of primary importance in resisting fracture; a minimum overall thickness of 1.5 mm has been recommended.12 However, relative layer thickness (core to overlaying veneering porcelain) is also important.31 Relative layer thickness influences strength, stress distribution, and failure mode. It has been suggested that a 1:1 ratio of core to veneering porcelain thickness may provide reasonable strength, aesthetics, and fabrication tolerance.32 However, the importance of adequate core thickness may be paramount.33 The stiffness, or elastic modulus, of the core material is also influential. A stiffer core will better resist flexure under load. This is important because ceramics have low critical strains and are poorly supported by flexible dentine.34 However, stiffer core materials may also be more vulnerable to radial cracks originating from their internal surfaces.35

In the current study, reducing the zirconia core thickness from 0.8 mm to 0.4 mm did not affect the fracture resistance of the tested crowns. Further research may be needed to assess the effect of reducing the core thickness from 0.4 mm to 0.1 mm on the fracture resistance of zirconia-based crowns. Fracture resistance evidence can effectively be provided by microscopic fracture analysis. Fractography represents an effective analytical tool for assessing fractured surfaces in order to locate fracture origins and elucidate fracture patterns. Reich et al.6 reported that, regardless of the margin preparation, a reduction in the thickness of a single crown coping from 0.5 mm to 0.3 mm resulted in a 35% reduction in fracture resistance. These results are not consistent with our results and this difference may be attributed to the difference in the core thickness and the composition of the materials tested. For the all-ceramic prosthesis which uses a ceramic framework, it has been reported that, similar to metal ceramic restorations, veneering porcelain fracture remains one of the primary complications affecting longevity.35 While a certain number of fractures are expected due to fatigue after long-term service, Shirakura et al.35 reported that an improperly designed core/framework which required the application of an excessively thick layer of veneering porcelain may result in a higher incidence of failure for both metal ceramic and all-ceramic prostheses. Interestingly, the thickness of the porcelain was inversely related to the failure load in metal ceramic crowns, while it did not significantly affect that of all-ceramic crowns, implying that all-ceramic crowns show significantly higher success and survival rates under loading compared to metal ceramic crowns.36

In addition to zirconia thickness, the elastic modulus of the core material is also influential. Reasons for fracture can be assessed from different crack propagation modes, such as cone, radial or fatigue crack pathways. Marchack et al.31 reported that a stiffer core will better resist flexure under load. However, this core type may also be more vulnerable to radial cracks originating from the internal surfaces.

In the current study, SEM results showed that cracks are propagated from the occlusal towards the gingival margin (Figs 2–4) and this may be attributed to different stresses generated according to the degree of abutment angulation and the direction of occlusal compressive load. Therefore, appropriate porcelain and core thickness may decrease internal stress, reduce mechanical failure and optimize aesthetics. With a zirconia layer thickness of 0.5 mm serving as a framework for primarily posterior 3–5 unit fixed partial dentures, failures were attributed to cracking or crazing of the veneering porcelain, but bulk fractures were uncommon.6

The current results indicated that a 30° implant abutment angulation significantly (p < 0.05) reduced the fracture resistance of the overlaying Cercon® CAM-milled zirconia single crowns and this may be attributed to the differences in stress generated underneath the cusp tips. The results also showed that 39 out of 45 cores were intact after applying force to the crowns. Nearly 50% of crowns failed through the midline, with half of the crown displaced or lost leaving the core intact except in only one case. Every effort should be made to achieve ideal implant location. However, more importantly, the material of the overlaying restoration should be chosen meticulously in order to achieve long-term success. Craig and Powers36 reported that the average biting force is 665 newtons (or approximately 150 pounds of force) for natural teeth in the molar region. The results of our study showed that after cementation, the load to fracture was well above the average occlusal force applied in the molar region except for group C.

The general conclusion requires careful consideration of the present model system. First, this model tested a dental material in a static manner. Second, every effort was made to standardize the occlusal surface of the overlaying Cercon® crown but because of the difference in implant abutment angulation, this was difficult to achieve and more research is needed to assess the effect of the cusp angle on the scatter of compressive failure loads recorded using the model.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Within the limitations of the current study, the following can be concluded: (1) the implant abutment angulation of 30° significantly (p < 0.05) reduced the fracture resistance of the overlaying CAM-milled zirconia single crowns; and (2) core thickness did not significantly (p > 0.05) affect the fracture resistance of the overlaying CAM-milled zirconia single crowns.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

This study was generously supported by an Australian Dental Research Foundation Undergraduate Research Grant and Dentsply, Australia. We also thank Fisodent Pty Ltd, especially Franz Flintrop for the technical support and allowing us to use the Cercon® system.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
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