Failure of all-ceramic fixed partial dentures in vitro and in vivo: analysis and modeling

Evaluation of bond strength between zirconia milled ceramic material and veneered dental porcelain

This study aimed at examining the bond strength between zirconia and ceramic veneer, following the ISO 9693 guidelines. A total of fifty specimens of zirconia/ceramic-veneer system were produced using two commercial zirconias (VITA YZ-HTWhite and Zolid HT+ White, referred to as Group A and Group B, respectively) and a ceramic-veneering material (Zirkonia 750). The microstructure (via x-ray diffraction analysis, XRD and Secondary Electron mode, SEM) and the mechanical properties (via 3-point bending tests) of the two groups were assessed. Then, experiments were conducted according to the ISO 9693 and conventional protocols applied for producing zirconia/ceramic-veneer restorations. Bond strength values, measured by 3-point bending tests, were 34.42 ± 7.60 MPa for Group A and 31.92 ± 6.95 MPa for Group B. SEM observations of the cohesively fractured surfaces (on the porcelain side) and the examination for normality using the Shapiro-Wilk test suggested the use of Weibull statistical analysis. Median strength (σ50%) for Group A and Group B was 34.76 and 32.22 MPa, while the characteristic strength (σ63.2%) was 35.78 and 33.14 MPa, respectively. The Weibull modulus disparity between groups (12.69 and 13.07) was not significant. Bond strength exceeded the ISO 9693 minimum of 20 MPa, suggesting satisfactory strength for clinical use.

Biomechanical behavior and Weibull survival of CAD-CAM endocrowns with different marginal designs: A 3D finite element analysis

OBJECTIVES

To evaluate and compare the effect of tooth preparation designs and different CADCAM. materials on stress distribution and Weibull survival probability of endocrowns. applied to root canal-treated lower first permanent molar using the 3D finite element. analysis method.

METHODS

A root canal-treated lower first permanent molar was prepared for endocrowns with a. butt joint or with a ferrule design by placing, circumferentially, a 1-mm wide shoulder. finish line. The prepared molar was scanned for the two designs and modeled on a 3D. Finite element model. Monolithic zirconia (IPS e.max ZirCAD MT, FCZ), lithium. disilicate (IPS e.max CAD, EX), and nano-ceramic resin composite (Lava Ultimate, LU). CAD-CAM materials were used for each preparation to design the virtual endocrown. A. total of six models were built according to the different tooth preparation designs and. endocrown materials. An occlusal load of 600 N and a transverse load of 20 N in. magnitude that simulates the average occlusal load was directed toward the occlusal. surfaces. von Mises and maximum principle values were evaluated Weibull risk-ofrupture. analysis was used to analyze the survival probability of the restorations and. tooth in the different models.

RESULTS

The highest von Mises were found in the butt joint design for FCZ, EX, and LU (45.3. MPa, 35.2 MPa, and 24.2 MPa, respectively) compared to the ferrule design for the. same materials (42.6 MPa, 31.2 MPa, and 23.6 MPa, respectively). For von Mises. stress distribution in the remaining part of tooth structure (dentin), the highest stresses. were found in LUFerrule which was closely similar to LUButt joint (135.4 MPa and. 134.7 MPa, respectively), followed by EXFerrule and FCZFerrule (132.2 MPa and. 131.7 MPa, respectively), while the lowest stresses were found in EXButt joint and. FCZButt joint (129.0 MPa and 128.4 MPa, respectively). Shear stresses within the. resin cement were the highest in FCZ and EX compared to LU. EX was found to be the. most reliable material with the highest survival probability, while FCZ showed the. lowest survival probability according to the Weibull risk-of-fracture results.

SIGNIFICANCE

Materials with high elastic modulus transfer more stresses to the endocrown and less. to the remaining tooth structure. Endocrown tooth preparation with ferrule design has. better stress distribution and magnitude compared to the butt joint design.

Effect of connector configuration on the fracture load in conventional and translucent zirconia three-unit fixed dental prostheses

PURPOSE

The purpose of this study was to determine the effect of the connector configuration on the fracture load in conventional and translucent zirconia of three-unit fixed dental prostheses (FDPs).

MATERIALS AND METHODS

Six different three-unit FDPs were prepared (n = 6) from three types of zirconia (3Y-TZP (Katana ML®), 4Y-TZP (Katana STML®), and 5Y-TZP (Katana UTML®)) in combination with two connector configurations (4 × 2.25, 3 × 3 mm). The Co-Cr master models were scanned, and the FDPs were designed and fabricated using CAD-CAM. The FDPs were cemented on the metal model and then loaded with a UTM at a crosshead speed of 1 mm/min until failure. Two-way ANOVA and Tukey's test were used for statistical analysis (α = .05).

RESULTS

Fracture loads of 3Y-TZP (2740.6 ± 469.2 and 2718.7 ± 339.0 N for size 4 × 2.25 mm and 3 × 3 mm, respectively) were significantly higher than those of 4Y-TZP (1868.3 ± 281.6 and 1663.6 ± 372.7 N, respectively) and 5Y-TZP (1588.0 ± 255.0 and 1559.1 ± 110.0 N, respectively) (P < .05). No significant difference was found between fracture loads of 4Y-TZP and 5Y-TZP (P > .05). The connector configuration within 9 mm2 was found to have no effect on the fracture loads on all three types of zirconia (P > .05).

CONCLUSION

Fracture loads of three-unit FDPs were affected by the type of zirconia. The fracture loads of conventional zirconia were higher than those of translucent zirconia. However, it was not affected by the connector configuration when the connector had a cross-sectional area of 9 mm2.

Biomechanical properties of different endocrown designs on endodontically treated teeth

OBJECTIVE

To compare the biomechanical properties of different endocrown designs on endodontically treated teeth with an extensive defect in the mesial wall using a three-dimensional finite element method (3D FEM).

METHODS

Four finite element analysis models were designed and built up based on different endocrown configurations in a mandibular molar. One model was designed as a butt joint preparation with 2 mm occlusal thickness(control), the other three models were butt joint designs with different distances between the bottom of the mesial wall preparation and the cemento-enamel junction (CEJ): 2 mm, 1 mm and 0 mm respectively. A vertical load parallel with the longitudinal axis of the tooth and an oblique load with a 45°angle to the longitudinal axis were applied to the occlusal surfaces. The maximum Von Misses (VM) stresses and stress distribution patterns were calculated and compared. Weibull risk-of-rupture analysis was used to analyze the survival probability of the restorations and tooth in the different models.

RESULTS

For the restoration, the model with a mesial wall destruction at the level of CEJ showed much higher risk of failure than other models. Overall, none of the four models showed failure. Under oblique loading, VM stress in the cement layer of the models with a mesial wall defect was higher than in the control model. In the dentin, the highest VM stresses were found in the peri-cervical dentin. Under the oblique loading, the model with the mesial wall destruction at the level of CEJ restored by endocrown showed the highest risk of failure.

CONCLUSION

Under the oblique loading, with the increase of the simulated defect in the mesial wall, the peak VM stress values in the cement layer increased accordingly. In the model with a mesial wall defect up to the level of CEJ risk of failure was highest in the cervical dentin.

Structural Integrity of Anterior Ceramic Resin-Bonded Fixed Partial Denture: A Finite Element Analysis Study

This study was conducted as a means to evaluate the stress distribution patterns of anterior ceramic resin-bonded fixed partial dentures derived from different materials and numerous connector designs that had various loading conditions imposed onto them through the utilization of the finite element method. A finite element model was established on the basis of the cone beam computed tomography image of a cantilevered resin-bonded fixed partial denture with a central incisor as an abutment and a lateral incisor as a pontic. Sixteen finite element models representing different conditions were simulated with lithium disilicate and zirconia. Connector height, width, and shape were set as the geometric parameters. Static loads of 100 N, 150 N, and 200 N were applied at 45 degrees to the pontic. The maximum equivalent stress values obtained for all finite element models were compared with the ultimate strengths of their materials. Higher load exhibited greater maximum equivalent stress in both materials, regardless of the connector width and shape. Loadings of 200 N and 150 N that were correspondingly simulated on lithium disilicate prostheses of all shapes and dimensions resulted in connector fractures. On the contrary, loadings of 200 N, 150 N, and 100 N with rectangular-shaped connectors correspondingly simulated on zirconia were able to withstand the loads. However, two of the trapezoidal-shaped zirconia connectors were unable to withstand the loads and resulted in fractures. It can be deduced that material type, shape, and connector dimensions concurrently influenced the integrity of the bridge.

Finite Element Analysis of an Implant-Supported FDP with Different Connector Heights

All-ceramic fixed dental prostheses (FDPs) tend to fracture in the connector areas, due to the concentration of tensile stresses. This study aimed to evaluate the role of connector height on the stress distribution of a posterior three-unit implant-supported all-ceramic FDP using finite element analysis (FEA). Two titanium dental implants, their abutments, screws, and a three-unit all-ceramic FDP were scanned using a micro-CT scanner. Three 3D models with altered distal connector heights (3, 4, and 5 mm) were generated and analyzed on ABAQUS FEA software. The maximum principal stress values in MPa observed for each model with different connector heights and their respective locations (MA = mesial abutment; DA = distal abutment; F = framework; V = veneer) were: 3 mm—219 (MA), 88 (DA), 11 (F), 16 (V); 4 mm—194 (MA), 82 (DA), 8 (F), 18 (V); 5 mm—194 (MA), 80 (DA), 8 (F), and 18 (V). All the assembled models demonstrated the peak stresses at the neck area on the mesial abutments. The connector height had a significant influence on the stress distribution of the prosthesis. The models with higher distal connectors (4 and 5 mm) had a lower and more uniform distribution of maximum principal stresses (except for the veneer layer) when compared with the model with the smallest distal connector.

Adhesion of Resin-Resin and Resin-Lithium Disilicate Ceramic: A Methodological Assessment

The aim of this study was to evaluate four test methods on the adhesion of resin composite to resin composite, and resin composite to glass ceramic. Resin composite specimens (N = 180, Quadrant Universal LC) were obtained and distributed randomly to test the adhesion of resin composite material and to ceramic materials (IPS e.max CAD) using one of the four following tests: (a) Macroshear SBT: (n = 30), (b) macrotensile TBT: (n = 30), (c) microshear µSBT: (n = 30) and (d) microtensile µTBT test (n = 6, composite-composite:216 sticks, ceramic-composite:216 sticks). Bonded specimens were stored for 24 h at 23 °C. Bond strength values were measured using a universal testing machine (1 mm/min), and failure types were analysed after debonding. Data were analysed using Univariate and Tukey’s, Bonneferroni post hoc test (α = 0.05). Two-parameter Weibull modulus, scale (m), and shape (₀) were calculated. Test method and substrate type significantly affected the bond strength results, as well as their interaction term (p < 0.05). Resin composite to resin composite adhesion using SBT (24.4 ± 5)^a, TBT (16.1 ± 4.4)^b and µSBT (20.6 ± 7.4)^a,b test methods presented significantly lower mean bond values (MPa), compared to µTBT (36.7 ± 8.9)^b (p < 0.05). When testing adhesion of glass ceramics to resin composite, µSBT (6.6 ± 1)^B showed the lowest and µTBT (24.8 ± 7)^C,D the highest test values (MPa) (SBT (14.6 ± 5)^A,D and TBT (19.9 ± 5)^A,B) (p < 0.05). Resin composite adhesion to ceramic vs. resin composite did show significant difference for the test methods SBT and µTBT (resin composite (24.4 ± 5; 36.7 ± 9 MPa) vs. glass ceramic (14.6 ± 5; 25 ± 7 MPa)) (p > 0.05). Among substrate–test combinations, Weibull distribution presented the highest shape values for ceramic–resin in µSBT (7.6) and resin–resin in µSBT (5.7). Cohesive failures in resin–resin bond were most frequently observed in SBT (87%), followed by TBT (50%) and µSBT (50%), while mixed failures occurred mostly in ceramic–resin bonds in the SBT (100%), TBT (90%), and µSBT (90%) test types. According to Weibull modulus, failure types, and bond strength, µTBT tests might be more reliable for testing resin-based composites adhesion to resin, while µSBT might be more suitable for adhesion testing of resin-based composites to ceramic materials.

Long-term clinical performance and complications of zirconia-based tooth- and implant-supported fixed prosthodontic restorations: A summary of systematic reviews

OBJECTIVES

To present an overview on systematic reviews on prosthodontic zirconia restorations and to discuss long-term complications as well as information on anatomical and functional changes to the masticatory system.

DATA/SOURCES

MEDLINE, EMBASE, Trip medical, and Cochrane Library databases were searched for systematic reviews up to February 2021. Bias was assessed and clinical survival and complications were analyzed.

STUDY SELECTION

38 eligible articles published between 2006 and 2021 were included. The reviews were based on 128 in vivo studies on approximately 10,000 zirconia restorations. 5-year cumulative survival rates varied between 91.2% and 95.9% for tooth-supported (TS) single crowns (SC), 89.4% and 100% for TS multi-unit fixed dental prostheses (FDP), 97.1% and 97.6% for implant-supported (IS) SCs and 93.0% and 100% for IS FDPs. Chipping was the most often technical complication, followed by framework fracture, loss of retention, marginal discrepancies/discoloration, occlusal roughness and abutment/screw loosening. Color mismatch was the only esthetic complication. Biological complications were caries, endodontic complications, tooth fracture, periodontal disease, abrasion/attrition, persisting pain, high sensitivity, peri‑implantitis and soft tissue issues. Patients with bruxism were only examined sporadically.

CONCLUSIONS

5-year results for zirconia restorations were satisfactory. The predominant technical problem of veneering fractures could be overcome with adapted design or fabrication and application of monolithic restorations, but reviews of clinical studies on this subject are rare. The impact of zirconia restorations on the masticatory system remains unclear.

CLINICAL SIGNIFICANCE

Zirconia restorations are experiencing a rapidly increasing use in dental practice. Being highly wear-resistant, hard and durable, it can be assumed that they do not follow natural abrasion and changes in the masticatory system. Possible long-term effects on the stomatognathic system as a whole should therefore be considered.

Mechanical performance of Anatomic-Functional-Geometry dental treatments: A computational study

In this paper the biomechanical response of a novel dental preparation technique, referred to as the Anatomic-Functional-Geometry treatment (AFG), is investigated through a 3D nonlinear finite-element modelling approach. A comparative investigation against a standard technique employed in dental clinical practice is carried out, by simulating typical experimental mechanical tests and physiological functional conditions. Failure mechanisms of treated tooth models are investigated through a progressive damage formulation implemented via a displacement-driven incremental approach. Computational results clearly show that AFG-treated teeth, as a consequence of a more conservative morphological preparation of the tooth, are characterized by more effective crown-dentin loading transfer mechanisms, higher fracture strength levels and more homogeneous stress patterns than the standard-treated ones, thereby opening towards widespread clinical application.

Effect of Fabrication Method on Fracture Strength of Provisional Implant-Supported Fixed Dental Prostheses

There has been an increase in utilizing 3D printers in dental restorations. The purpose of the study is to compare mechanical properties of 3D-printed prostheses to those of self-cured and/or computer-aided design-computer-aided manufacturing (CAD-CAM) restorations. A metal master typodont was prepared for the mandibular left sextant with implant analogs embedded at the first premolar and first molar positions with a missing second premolar. Three-unit provisional fixed dental prosthesis (FDP) was designed utilizing the 3Shape tooth library and forty-five uniform specimens were fabricated with different materials: self-cured poly(methyl methacrylate) (PMMA) (N = 15), milled PMMA CAD-CAM blocks (N = 15) and 3D-printed resin (N = 15). All specimens were tested using an Instron machine at a crosshead speed of 0.5 mm/min by an axial load on the occlusal surface of the second premolar pontic site. Statistical analysis was completed with Shapiro-Wilk, ANOVA and Tukey post-hoc tests. Mean fracture force was 300.61 N, 294.64 N and 408.49 N for self-cured PMMA, milled PMMA and 3D-printed resin, respectively. Mean force at FDP fracture of 3D-printed resin was significantly greater than the mean fracture force of either self-cured (p = 0.016, 95% CI [17.86, 197.91]) or milled (p = 0.010, 95% CI [23.83, 203.88]) PMMA.

A Novel Design Modification to Improve Flexural Strength of Zirconia Framework: A Comparative Experimental In Vitro Study

Aim:

Zirconia-based restoration is successfully replacing metal ceramic restorations in posterior areas. Although higher mechanical properties of zirconia, their use in compromised situation is questionable. Hence, there is a need to modify the design which to strengthen the framework. The aim of this in vitro study was to evaluate the influence of lingual collar design on the flexural strength of CAD/CAM-fabricated posterior three-unit zirconia framework.

Materials and Methods:

A mandibular metallic stainless steel master mold is designed for a three-unit fixed partial denture framework. All CAD-milled 20 samples are divided into two groups based on the design. Group A––with collar (10 samples) and Group B––without collar (10 samples), tested using universal testing machine to calculate the mean fracture load and flexural strength.

Statistical Analysis Used:

Descriptive statistics and independent sample t test were used to find the difference between the groups, and simple linear regression was used to find the relationship between load and displacement between the groups.

Results:

The result of the mean flexural strength for Group A was 11328.06 ± 3770MPa and for Group B was 7633.95 ± 3196MPa; the mean fracture strength observed for Group A was 1274.04 ± 424MPa and for Group B was 858.80 ± 359MPa. A statistically significant difference was observed in flexural strength between Groups A and B (P < 0.05).

Conclusion:

Zirconia framework with connector dimension of 7 mm² with lingual collar design can be successfully incorporated in compromised situation where an ideal connector dimension of 9 mm² cannot be placed.

Retentive Force of Glass-Ceramic Soldered Customized Zirconia Abutment Copings with Prefabricated Titanium Bases

Two-piece abutments consisting of customized zirconia abutment copings and prefabricated titanium bases are popular due to their biological and esthetic advantages. Glass-ceramic solder (GS) is an alternative biocompatible connective agent. This in vitro study evaluated the retentive force of GS in comparison to classical resin composite cements (RC) after artificial aging and autoclaving. Ninety specimens consisting of prefabricated titanium bases and zirconia abutment copings were fabricated. The two parts of each specimen were fixed either by RC (n = 30) or GS with a luting space of either 30 µm (n = 30) or 100 µm (n = 30). Ten specimens of each group underwent autoclaving before artificial aging (water storage, thermocycling). Twenty specimens (including the 10 autoclaved specimens) of each group were exposed to a mechanical load. The retentive force between the zirconia and titanium in all specimens was determined. A fractographic analysis was performed to analyze the fracture surfaces of the GS specimens. The RC- and GS-connected two-piece abutments showed no relevant differences, independent of the luting space. RC appears to be more vulnerable to the thermal and mechanical loads than GS. Thus, GS may be an appropriate alternative to RC for two-piece abutments, especially for patients with enhanced biocompatibility requirements.

Effect of Access Cavities and Canal Enlargement on Biomechanics of Endodontically Treated Teeth: A Finite Element Analysis

INTRODUCTION

The purpose of this study was to investigate the influence of access cavities and tapers of canal preparations on fracture resistance of endodontically treated first molars by finite element method and Weibull analysis.

METHODS

On the basis of the micro-computed tomography data of maxillary first molar, the models of endodontically treated teeth with conservative endodontic cavity, traditional endodontic cavity, and 4 tapers of canal preparations (0.02, 0.04, 0.06, and 0.08) were created. Four static loads (800 N in total) were applied vertically to the contact points. The stress distributions of maximum principal stress were recorded and analyzed. Weibull analysis was performed to analyze the failure risk in enamel and dentin.

RESULTS

The stress distributions of maximum principal stress on occlusal surfaces were similar. In cervical region, the tensile stress was mainly concentrated on mesiobuccal root and root furcation. The finite element analysis and Weibull analysis showed that conservative endodontic cavity significantly reduced the maximum principal stress in cervical region and the failure probability, compared with traditional endodontic cavity. No significant difference was detected among tapers of prepared canals.

CONCLUSIONS

Preserving coronal dentin by using conservative endodontic cavity significantly reduced the concentration of tensile stress and the failure probability of dentin, although the maximum principal stress and failure probability were less affected by taper of canal preparation.

Debonding/crack initiation and flexural strengths of bilayered zirconia core and veneering ceramic composites

The aim of this study was to evaluate the effects of the kinds of veneering ceramics and veneering methods on the debonding/crack initiation and 3-point flexural strengths in bilayered zirconia core and veneering ceramic composites. Zirconia block was used as core material, and Cerabien ZR and Lava Ceram for the layering technique and IPS e.max ZirPress and Amber LiSi-POZ for the heat pressing technique were used as veneering materials. Both debonding/crack initiation and 3-point flexural strengths of bilayered zirconia core and veneering ceramic composites as well as the bi-axial flexural strengths of veneering materials, were higher when using heat pressing technique than layering technique. It was identified that not only bonding strength between zirconia core and veneering materials but also the intrinsic strength of veneering ceramic should be high to prevent chipping of veneering material.

The Effect of Thermocycling and Surface Treatments on the Surface Roughness and Microhardness of Three Heat-Pressed Ceramics Systems

Dental ceramic restorations are widely used in restorative dentistry. However, these restorations can be affected once cemented in the oral cavity by several factors. How can conventional surface treatments, such as glazing and mechanical polishing, diminish the effects of aging? The purpose of this in vitro study was to evaluate the effect of thermocycling and conventional surface treatments on the surface roughness and microhardness of three types of glass-ceramics by using a profilometer, scanning electron microscopy (SEM), atomic force microscopy (AFM), and a microhardness tester. Three types of ceramic systems (zirconia reinforced lithium silicate glass-ceramic, lithium disilicate glass-ceramic, and feldspathic glass-ceramic) (n = 48) were prepared. The samples were subjected to thermocycling for 10,000 cycles. Surface roughness was evaluated numerically using a profilometer and visually by using SEM and AFM. Microhardness was performed using a microhardness tester. The data were interpreted using the ANOVA test, and the results were correlated using Pearson’s correlation formula (r). Significant differences were found before and after thermocycling for the Ra (p < 0.01) and Rz (p < 0.05) parameters. As well, differences between glazed and polished surfaces were significant before and after thermocycling for surface roughness and microhardness (p < 0.05). A correlation was made between average surface roughness and microhardness (r = −460) and for the maximum surface roughness and microhardness (r = −606). Aging increases the roughness and decreases in time the microhardness. The tested ceramic systems behaved differently to the aging and surface treatments. Surface treatments had a significant impact on the microhardness and surface characteristics. The glazed groups were reported with higher surface roughness and lower microhardness when compared to the polished groups before and after thermocycling. The measuring roughness techniques determine the scale-dependent values for the Ra (Sa) and Rz (Sq) parameters. Thermocycling almost doubled the surface roughness for all the tested samples. Microhardness decreased only for the Celtra glazed samples. Nano-roughness increased the values for Vita and slightly for Emax. Thermocycling had little effect on Emax ceramic and a more significant impact on Celtra Press ceramic.

Estimation of stress distribution and risk of failure for maxillary premolar restored by occlusal veneer with different CAD/CAM materials and preparation designs

OBJECTIVES

To compare stress distribution and failure probability in maxillary premolars restored by simple occlusal veneer (SOV) and buccal-occlusal veneer (BOV) with 3 different CAD/CAM materials.

MATERIALS AND METHODS

A maxillary premolar was digitized by a micro-CT scanner. Three-dimensional dynamic scan data were transformed, and finite element models of 2 different models (SOV and BOV restored teeth) were designed. Three different CAD/CAM materials, including lithium disilicate glass ceramic (LD) IPS e.max CAD, polymer-infiltrated ceramic-network (PICN) Vita Enamic, and resin nano-ceramic (RNC) Lava Ultimate, were designated to both veneers. Maximum principle stresses were determined by applying a 300-N axial load to the occlusal surface. Weibull analyses were performed to calculate the failure probability of the models.

RESULTS

LD-restored teeth showed the highest stress in the veneer, lowest stress in substrate teeth, and lowest failure probability for the overall system; RNC-restored teeth showed the lowest stress in the veneer, highest stress in substrate teeth, and highest failure probability. No significant differences were found in the cement layer among the different models. No significant differences of stress and failure probability existed between SOV and BOV preparations.

CONCLUSIONS

CAD/CAM composite resin occlusal veneers bear lower maximum stress than ceramic veneers. Teeth restored by composite veneers are more prone to failure than those restored by ceramic veneers. Additional reduction of the buccal surface did not increase the stress on the occlusal veneer under axial load.

CLINICAL RELEVANCE

Both occlusal veneers could be used under physiological masticatory force. CAD/CAM glass ceramic was safer than composite resins.

Repair bond strength of composite resin to zirconia restorations after different thermal cycles

PURPOSE

This in vitro study investigated the repair bond strength of the zirconia ceramic after different aging conditions.

MATERIALS AND METHODS

In order to imitate the failure modes of veneered zirconia restorations, veneer ceramic, zirconia, and veneer ceramic-zirconia specimens were prepared and were divided into 4 subgroups as: control (37℃ distilled water for 24 hours ) and 3000, 6000, 12000 thermal cycling groups (n=15). Then, specimens were bonded to composite resin using a porcelain repair kit according to the manufacturer recommendation. The repair bond strength (RBS) test was performed using a universal testing machine (0.5 mm/min). Failure types were analyzed under a stereomicroscope. Two-way ANOVA and Bonferroni test were used for statistical analysis.

RESULTS

The RBS values of zirconia specimens were statistically significant and higher than veneer ceramic and veneer ceramic-zirconia specimens in control, 3000 and 6000 thermal cycling groups (P<.05). When 12000 thermal cycles were applied, the highest value was found in zirconia specimens but there was no statistically significant difference between veneer ceramic and veneer ceramic-zirconia specimens (P>.05). Veneer ceramic specimens exhibited cohesive failure types, zirconia specimens exhibited adhesive failure types, and veneer ceramic-zirconia specimens exhibited predominately mixed failure types.

CONCLUSION

Thermal cycling can adversely affect RBS of composite resin bınded to level of fractured zirconia ceramics.

Evaluation of fracture strength for single crowns made of the different types of lithium disilicate glass-ceramics

Lithium disilicate glass-ceramics with high mechanical strength are being widely used as ingots for heat-pressing technique and blocks for CAD/CAM processing in clinical dentistry as aesthetic prosthetic materials. The purpose of this study was to evaluate the fracture strength of single crowns made of the different types of lithium disilicate glass-ceramics. Single crowns for mandibular second premolar with thickness of 1.5 mm were manufactured. IPS e.max Press and Amber Press crowns were produced by heat-pressing, and IPS e.max CAD and Rosetta SM crowns was produced by milling. Amber Lisi-POZ crown was produced by heat-pressing on the zirconia frame. Fracture strength test was performed at 10 degrees of inclination toward the load after bonding crown on metal abutment using dual-curing resin cement. Statistical analysis of fracture strength was conducted through Weibull statistics (n = 15 per group). The mean fracture strength (2087.4 N) of Amber Lisi-POZ group produced by heat-pressing on the zirconia frame was significantly higher than that (1479.8 N) of Rosetta SM group produced by milling. Weibull coefficients for IPS e.max CAD and Rosetta SM groups were, respectively, 14.44 and 9.39, and those for IPS e.max Press, Amber Press, and Amber Lisi-POZ groups produced by heat-pressing were in the range between 4.72 and 5.16. In conclusion, the fracture strength of Amber Lisi-POZ crown with zirconia framework was the highest, and the buccal cusps fractured from the central groove of the all crowns. Weibull modulus of crowns produced by milling was higher than those of crowns produced by heat-pressing.

Influence of Radius of Curvature at Gingival Embrasure in Connector Area on Stress Distribution of Three-Unit Posterior Full-Contour Monolithic Zirconia Fixed Partial Denture on Various Amounts of Load Application: A Finite Element Study

Objectives

To test the hypothesis that radius of curvature at gingival embrasure in connector area significantly affects the fracture resistance of full-contour monolithic zirconia three-unit posterior Fixed partial denture (FPD) on various amounts of load application.

Materials and Methods

In this study, two types of three-dimensional finite element models of a three-unit posterior full-contour monolithic zirconia FPD with two gingival embrasure radii (r_GE I, 0.45 mm and r_GE II, 0.25 mm) were constructed. The components modeled through finite element modeling were subjected to 400, 600, and 800 N vertical loads at the central fossa of the pontic, and further analysis was carried out.

Results

All the results were displayed by post-processor finite element analysis software (ANSYS). The study revealed that with increase in the amount of load application as well as decrease in the gingival embrasure radii, stress concentration values were increasing gradually for both the full-contour monolithic zirconia FPD.

Conclusion

The fracture resistance of the zirconia posterior FPD was significantly affected by the gingival embrasure radii and the mode of load application. When there is a clinical situation of heavier occlusal forces, the fracture resistance can be increased by designing greater gingival embrasure radii in the connector region.

In vitro investigation of human enamel wear by dental porcelain

BACKGROUND

Ceramic crown that fires the layering porcelain on zirconia corping is starting to be used due to aesthetic requirements. However, many ceramics are used, but wear characteristics are often unknown.

OBJECTIVE

This study was performed to investigate the wear of human enamel by porcelains, and to determine the influence of porcelain hardness, surface roughness and crystal of porcelain on the enamel wear.

METHODS

Enamel specimens were prepared from extracted human molar teeth. Porcelain specimens were prepared from Deguceram Gold, Vita Omega 900, and Cercon Ceram Kiss. Enamel wear volume was calculated and the porcelain wear depth, surface roughness and Vickers hardness was measured. Moreover, the observation of a crystal by scanning electron microscope and the crystal by X-ray diffraction were identified.

RESULTS

Omega 900 showed the smallest enamel wear volume and porcelain wear depth. The Vickers hardnesses of Omega 900 and Cercon Ceram Kiss were almost identical. Leucite was detected from Deguceram Gold and Omega 900, and Silica and Silicon Nitride were detected from Cercon Ceram Kiss.

CONCLUSIONS

The enamel wear is influenced not by the hardness of the porcelain but by the surface roughness, the size and shape of the crystal.

Influence of veneer pore defects on fracture behavior of bilayered lithium disilicate glass-ceramic crowns

OBJECTIVE

To identify the conditions under which fabrication pore defects within veneering porcelain in bilayered lithium disilicate glass-ceramic (LDG) crowns will influence and jeopardize the mechanical integrity of the structure.

METHODS

Thirty standardized molar crowns (IPS e.max Press) were fabricated and microCT scanned to 3D-analyze the size, morphology and distribution of pores in veneering porcelain, followed by in vitro fracture test and SEM fractographic observation. Finite element analysis (FEA) of the microCT reconstructed models was used to evaluate the stress state.

RESULTS

The volumes of pores in samples ranged from 3241μm³ to 1.29×10⁹μm³ with the equivalent radius between 10μm to 680μm. Deviation of sphericity of pores ranged from 0.10 to 0.81 and the average of 99.97% pores was near 0.63. For the smaller pores their distribution tended to be uniform, while the larger pores were irregular with elongated ellipsoidal form and located at or near the veneer-core interface. During wedge loading blunt contact fracture testing 21 crowns failed from the fissure on the occlusal surface, of which 16 failed from surface or near surface pores, 2 from the midpoint of the oblique ridge, and 7 from larger interfacial pores. FEA analysis indicated that defects were detrimental to veneer integrity only in regions of tensile stress and where the pore radius associated with crack initiation ranged from 30 to 50μm. Pore morphology appeared to have only a minor effect on fracture.

SIGNIFICANCE

Within the limitation of the microCT resolution and FEA, it suggests that pores radius large than 30-50μm and located in the tensile stress area like grooves and fissures on the occlusal surface or near surface as well as cervical margins of veneering porcelain will jeopardize the bilayered structure and mechanical integrity of LDG.

Effect of Connector Design on Fracture Resistance in Zirconia-based Fixed Partial Dentures for Upper Anterior Region

The purpose of this study was to investigate the influence of the cross-sectional form and area of the connector on fracture resistance in three-unit zirconia fixed partial denture (FPD) frameworks for the upper anterior region. Sixty FPD framework specimens were fabricated using the CAD/CAM system. The cross-sectional form (Type I, II, or III) and area (9.0, 7.0, 5.0, or 3.0 mm(2)) of the connectors differed. The specimens were fixed to a jig capable of applying a load axially to the abutment teeth at an angle of 135 degrees. Each specimen was subjected to fracture load measurements using a universal testing machine and cross-sectional microscopic examination. Fracture load fell significantly with a decrease in cross-sectional area (p <0.01). In terms of cross-sectional form, an isosceles triangle with a gingival base yielded the highest fracture load. These results suggest that the connector of a three-unit zirconia-based FPD framework for the upper anterior region should be triangular, have a gingival base, sufficient height in the loading direction, and a cross-sectional area of >5.0 mm(2).

Effect of Lithium Disilicate Reinforced Liner Treatment on Bond and Fracture Strengths of Bilayered Zirconia All-Ceramic Crown

This study was performed to evaluate the effect of a lithium-disilicate spray-liner application on both the bond strength between zirconia cores and heat-pressed lithium-disilicate glass-ceramic veneers, and the fracture strength of all-ceramic zirconia crowns. A lithium-disilicate reinforced liner was applied on the surface of a zirconia core and lithium-disilicate glass-ceramic was veneered on zirconia through heat press forming. Microtensile and crown fracture tests were conducted in order to evaluate, respectively, the bonding strength between the zirconia cores and heat pressed lithium-disilicate glass-ceramic veneers, and the fracture strength of bilayered zirconia all-ceramic crowns. The role of lithium-disilicate spray-liner at the interface between zirconia and lithium-disilicate glass-ceramic veneers was investigated through surface and cross-sectional analyses. We confirmed that both the mean bonding strength between the zirconia ceramics and lithium-disilicate glass-ceramic veneers and the fracture strength of the liner-treated groups were significantly higher than those of the untreated groups, which resulted, on the one hand, from the chemical bonding at the interface of the zirconia and lithium-disilicate liner, and, on the other, from the existence of a microgap in the group not treated with liner.

Residual stress of porcelain-fused to zirconia 3-unit fixed dental prostheses measured by nanoindentation

OBJECTIVE

To evaluate the residual stress (nanoindentation based on hardness) of fatigued porcelain-fused to zirconia 3-unit fixed dental prostheses (FDP) with different framework designs.

METHODS

Twenty maxillary 3-unit FDP replacing second-premolar (pontic) were fabricated with conventional framework-design (even-thickness of 0.5mm and 9mm² connector area) and modified framework-design (thickness of 0.5mm presenting lingual collar connected to proximal struts and 12mm² connector area). Connector marginal ridges were loaded and the fractured and suspended FDPs were divided (n=3/each) into: (1) Fractured zirconia even-thickness (ZrEvenF); (2) Suspended zirconia even-thickness (ZrEvenS); (3) Fractured zirconia with modified framework (ZrModF); (4) Suspended zirconia with modified framework (ZrModS); (5) Non-fatigued FDP with conventional framework design (Control). The FDPs were nanoindented at 0.03mm (Region of Interest (ROI) 1), 0.35mm (ROI 2) and 1.05mm (ROI 3) distances from porcelain veneer outer surface with peak load 4000μN. The Linear Mixed Analysis of Variance (ANOVA) Model on ranks and Least Significant Difference Test on ranks (95%) were used.

RESULTS

Highest rank hardness values were found for Control group and ZrModS, whereas the lowest values were found in ZrModF. Statistical differences (p=0.000) were found among all groups except for comparison between ZrModS and Control group (p=0.371). Hardness between ROIs were statistically significant different (p<0.001) where ROI 1 presented the lowest values.

SIGNIFICANCE

Framework-design modification did not influence the residual stress of porcelain-fused to zirconia fatigued 3-unit FDP. Whereas fractured FDPs showed the highest residual stress compared to suspended and control FDPs. Residual stress increased as nanoindented away from framework.

Effect of Endocrown Restorations with Different CAD/CAM Materials: 3D Finite Element and Weibull Analyses

The aim of this study was to evaluate the effects of two endocrown designs and computer aided design/manufacturing (CAD/CAM) materials on stress distribution and failure probability of restorations applied to severely damaged endodontically treated maxillary first premolar tooth (MFP). Two types of designs without and with 3 mm intraradicular extensions, endocrown (E) and modified endocrown (ME), were modeled on a 3D Finite element (FE) model of the MFP. Vitablocks Mark II (VMII), Vita Enamic (VE), and Lava Ultimate (LU) CAD/CAM materials were used for each type of design. von Mises and maximum principle values were evaluated and the Weibull function was incorporated with FE analysis to calculate the long term failure probability. Regarding the stresses that occurred in enamel, for each group of material, ME restoration design transmitted less stress than endocrown. During normal occlusal function, the overall failure probability was minimum for ME with VMII. ME restoration design with VE was the best restorative option for premolar teeth with extensive loss of coronal structure under high occlusal loads. Therefore, ME design could be a favorable treatment option for MFPs with missing palatal cusp. Among the CAD/CAM materials tested, VMII and VE were found to be more tooth-friendly than LU.

Bonding measurement -Strength and fracture mechanics approaches

This study investigated the effect of cross-sectional areas on interfacial fracture toughness and bond strength of bilayered dental ceramics. Zirconia core ceramics were veneered and cut to produce specimens with three different cross-sectional areas. Additionally, monolithic specimens of glass veneer were also prepared. The specimens were tested in tension until fracture at the interface and reported as bond strength. Fracture surfaces were observed, and the apparent interfacial toughness was determined from critical crack size and failure stress. The results showed that cross-sectional area had no effect on the interfacial toughness whereas such factor had a significant effect on interfacial bond strength. The study revealed that cross-sectional area had no effect on the interfacial toughness, but had a significant effect on interfacial bond strength. The interfacial toughness may be a more reliable indicator for interfacial bond quality than interfacial bond strength.

Three-dimensional finite element analysis of zirconia all-ceramic cantilevered fixed partial dentures with different framework designs

The purpose of this study were: to perform stress analyses using three-dimensional finite element analysis methods; to analyze the mechanical stress of different framework designs; and to investigate framework designs that will provide for the long-term stability of both cantilevered fixed partial dentures (FPDs) and abutment teeth. An analysis model was prepared for three units of cantilevered FPDs that assume a missing mandibular first molar. Four types of framework design (Design 1, basic type; Design 2, framework width expanded buccolingually by 2 mm; Design 3, framework height expanded by 0.5 mm to the occlusal surface side from the end abutment to the connector area; and Design 4, a combination of Designs 2 and 3) were created. Two types of framework material (yttrium-oxide partially stabilized zirconia and a high precious noble metal gold alloy) and two types of abutment material (dentin and brass) were used. In the framework designs, Design 1 exhibited the highest maximum principal stress value for both zirconia and gold alloy. In the abutment tooth, Design 3 exhibited the highest maximum principal stress value for all abutment teeth. In the present study, Design 4 (the design with expanded framework height and framework width) could contribute to preventing the concentration of stress and protecting abutment teeth.

Interfacial modulus mapping of layered dental ceramics using nanoindentation

PURPOSE

The aim of this study was to test the modulus of elasticity (E) across the interfaces of yttria stabilized zirconia (YTZP) / veneer multilayers using nanoindentation.

MATERIALS AND METHODS

YTZP core material (KaVo-Everest, Germany) specimens were either coated with a liner (IPS e.max ZirLiner, Ivoclar-Vivadent) (Type-1) or left as-sintered (Type-2) and subsequently veneered with a pressable glass-ceramic (IPS e.max ZirPress, Ivoclar-Vivadent). A 5 µm (nominal tip diameter) spherical indenter was used with a UMIS CSIRO 2000 (ASI, Canberra, Australia) nanoindenter system to test E across the exposed and polished interfaces of both specimen types. The multiple point load – partial unload method was used for E determination. All materials used were characterized using Scanning Electron Microscopy (SEM) and X – ray powder diffraction (XRD). E mappings of the areas tested were produced from the nanoindentation data.

RESULTS

A significantly (P<.05) lower E value between Type-1 and Type-2 specimens at a distance of 40 µm in the veneer material was associated with the liner. XRD and SEM characterization of the zirconia sample showed a fine grained bulk tetragonal phase. IPS e-max ZirPress and IPS e-max ZirLiner materials were characterized as amorphous.

CONCLUSION

The liner between the YTZP core and the heat pressed veneer may act as a weak link in this dental multilayer due to its significantly (P<.05) lower E. The present study has shown nanoindentation using spherical indentation and the multiple point load - partial unload method to be reliable predictors of E and useful evaluation tools for layered dental ceramic interfaces.

Effect of core ceramic grinding on fracture behaviour of bilayered zirconia veneering ceramic systems under two loading schemes

OBJECTIVE

The aim of this in vitro study was to evaluate the effect of core ceramic grinding on the fracture behaviour of bilayered zirconia under two loading schemes.

METHODS

Interfacial surfaces of sandblasted zirconia disks (A) were ground with 80 (B), 120 (C) and 220 (D) grit diamond discs, respectively. Surface roughness and topographic analysis were performed using a confocal scanning laser microscope (CSLM) and a scanning electron microscopy (SEM). Relative monoclinic content was evaluated using X-ray diffraction analysis (XRD) then reevaluated after simulated veneer firing. Biaxial fracture strength (σ) and Weibull modulus (m) were calculated either with core in compression (subgroup Ac-Dc) or in tension (subgroup At-Dt). Facture surfaces were examined by SEM and energy dispersive X-ray spectroscopy (EDS). Maximum tensile stress at fracture was estimated by finite element analysis. Statistical data analysis was performed using Kruskal-Wallis and one-way ANOVA at a significance level of 0.05.

RESULTS

As grit size of the diamond disc increased, zirconia surface roughness decreased (p<0.001). Thermal veneering treatment reversed the transformation of monoclinic phase observed after initial grinding. No difference in initial (p=0.519 for subgroups Ac-Dc) and final fracture strength (p=0.699 for subgroups Ac-Dc; p=0.328 for subgroups At-Dt) was found among the four groups for both loading schemes. While coarse grinding slightly increased final fracture strength reliability (m) for subgroups Ac-Dc. Two different modes of fracture were observed according to which material was on the bottom surface. Components of the liner porcelain remained on the zirconia surface after fracture for all groups.

SIGNIFICANCE

Technician grinding changed surface topography of zirconia ceramic material, but was not detrimental to the bilayered system strength after veneer application. Coarse grinding slightly improved the fracture strength reliability of the bilayered system tested with core in compression. It is recommended that veneering porcelain be applied directly after routine lab grinding of zirconia ceramic, and its application on rough zirconia cores may be preferred to enhance bond strength.

Effect of connector design on fracture resistance in all-ceramic fixed partial dentures for mandibular incisor region

Yttrium tetragonal zirconia polycrystal frameworks were prepared for all-ceramic fixed partial dentures (FPDs) for the mandibular incisor region. The effects of the cross-sectional area and morphology of the connector on its strength were evaluated by fracture tests. Nine types of zirconia framework for a 3-unit FPD for a defect of 1 mandibular central incisor were prepared, each differing in cross-sectional area and morphology. Fracture tests were performed by loading until fracture using a universal testing machine. Fracture load was determined and fracture site examined. Significant differences were observed in fracture load according to the morphology and cross-sectional area of the connector (p<0.05, p<0.01, p<0.001). Fracture load differed significantly among all groups according to cross-sectional area, and was also greater when the shape of the connector formed an isosceles triangle widest at the base and the connector had the same height and width. These values still far exceeded 311 N, however, which is the average occlusal force in the incisor region. The results of this study suggest that connector design affects fracture load.

Topological design of all-ceramic dental bridges for enhancing fracture resistance

Layered all-ceramic systems have been increasingly adopted in major dental prostheses. However, ceramics are inherently brittle, and they often subject to premature failure under high occlusion forces especially in the posterior region. This study aimed to develop mechanically sound novel topological designs for all-ceramic dental bridges by minimizing the fracture incidence under given loading conditions. A bi-directional evolutionary structural optimization (BESO) technique is implemented within the extended finite element method (XFEM) framework. Extended finite element method allows modeling crack initiation and propagation inside all-ceramic restoration systems. Following this, BESO searches the optimum distribution of two different ceramic materials, namely porcelain and zirconia, for minimizing fracture incidence. A performance index, as per a ratio of peak tensile stress to material strength, is used as a design objective. In this study, the novel XFEM based BESO topology optimization significantly improved structural strength by minimizing performance index for suppressing fracture incidence in the structures. As expected, the fracture resistance and factor of safety of fixed partial dentures structure increased upon redistributing zirconia and porcelain in the optimal topological configuration. Dental CAD/CAM systems and the emerging 3D printing technology were commercially available to facilitate implementation of such a computational design, exhibiting considerable potential for clinical application in the future. Copyright © 2015 John Wiley & Sons, Ltd.

Fracture toughness of chairside CAD/CAM materials - Alternative loading approach for compact tension test

OBJECTIVE

This in-vitro study determined plane-strain fracture toughness (KIC) of five different chairside CAD/CAM materials used for crown fabrication, following alternative innovative loading approach of compact tension test specimens.

METHODS

Rectangular-shaped specimens were cut from CAD/CAM blocks (n=10): Vita Mark II (Vident) (VMII); Lava-Ultimate (3M/ESPE) (LU); Vita Enamic (Vident) (VE); IPS e.max CAD (Ivoclar Vivadent); crystallized and un-crystallized (E-max and E-max-U, respectively); and Celtra Duo (Dentsply) fired and unfired (CD and CD-U, respectively). Specimens were notched with thin diamond disk prior to testing. Instead of applying tensile loading through drilled holes, a specially-made wedge-shaped steel loading-bar was used to apply compressive load at the notch area in Instron universal testing machine. The bar engaged the top ¼ of the notch before compressive load was applied at a cross-head speed of 0.5mm/min. Fracture load was recorded and KIC calculated. Data was statistically-analyzed with one-way ANOVA at 95% confidence level and Tukey's tests.

RESULTS

Means and SDs of KIC in MPam(1/2) for VMII, LU, VE, E-max, E-max-U, CD and CD-U were: 0.73 (0.13), 0.85 (0.21), 1.02 (0.19), 1.88 (0.62), 0.81 (0.25), 2.65 (0.32) and 1.01 (0.15), respectively. ANOVA revealed significant difference among the groups (p<0.001). CD and E-max had significantly highest mean KIC values.

SIGNIFICANCE

Mean KIC values of the tested materials varied considerably, however, none of them reached mean KIC of dentin (3.08MPam(1/2)) previously reported. For E-max and CD, specimens firing significantly increased mean KIC. The modified test arrangement was found to be easy to follow and simplified specimen preparation process.

Influence of core design, production technique, and material selection on fracture behavior of yttria-stabilized tetragonal zirconia polycrystal fixed dental prostheses produced using different multilayer techniques: split-file, over-pressing, and manually built-up veneers

AIM

To investigate and compare the fracture strength and fracture mode in eleven groups of currently, the most commonly used multilayer three-unit all-ceramic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) fixed dental prostheses (FDPs) with respect to the choice of core material, veneering material area, manufacturing technique, design of connectors, and radii of curvature of FDP cores.

MATERIALS AND METHODS

A total of 110 three-unit Y-TZP FDP cores with one intermediate pontic were made. The FDP cores in groups 1-7 were made with a split-file design, veneered with manually built-up porcelain, computer-aided design-on veneers, and over-pressed veneers. Groups 8-11 consisted of FDPs with a state-of-the-art design, veneered with manually built-up porcelain. All the FDP cores were subjected to simulated aging and finally loaded to fracture.

RESULTS

There was a significant difference (P<0.05) between the core designs, but not between the different types of Y-TZP materials. The split-file designs with VITABLOCS(®) (1,806±165 N) and e.max(®) ZirPress (1,854±115 N) and the state-of-the-art design with VITA VM(®) 9 (1,849±150 N) demonstrated the highest mean fracture values.

CONCLUSION

The shape of a split-file designed all-ceramic reconstruction calls for a different dimension protocol, compared to traditionally shaped ones, as the split-file design leads to sharp approximal indentations acting as fractural impressions, thus decreasing the overall strength. The design of a framework is a crucial factor for the load bearing capacity of an all-ceramic FDP. The state-of-the-art design is preferable since the split-file designed cores call for a cross-sectional connector area at least 42% larger, to have the same load bearing capacity as the state-of-the-art designed cores. All veneering materials and techniques tested in the study, split-file, over-press, built-up porcelains, and glass-ceramics are, with a great safety margin, sufficient for clinical use both anteriorly and posteriorly. Analysis of the fracture pattern shows differences between the milled veneers and over-pressed or built-up veneers, where the milled ones show numerically more veneer cracks and the other groups only show complete connector fractures.

Simplified method for determining fracture toughness of two dental ceramics

This study compared the fracture toughness values (KIC), which were derived from simplified techniques: the indentation fracture (IF), the indentation strength (IS), and fractographic approach to that from a standard testing using surface cracks in flexure (SCF). Forty bar specimens, twenty IPS Empress(®) Esthetic and twenty IPS e.max(®)Ceram were prepared. Ten specimens in each material were tested by IF technique, IS technique and fractographic approach, and additional 10 specimens were tested by the SCF technique. This study showed that the mean KIC derived from fractographic approach were not significantly different from that of the SCF in both materials (p>0.05) whereas the mean KIC from indentation techniques rarely agreed with those of the standard technique. The KIC determination is sensitive to the methods used that affect accuracy. Consequently, test selection should be based on a sound understanding and inherent limitations of each technique.

Biaxial flexural strength of bilayered zirconia using various veneering ceramics

PURPOSE

The aim of this study was to evaluate the biaxial flexural strength (BFS) of one zirconia-based ceramic used with various veneering ceramics.

MATERIALS AND METHODS

Zirconia core material (Katana) and five veneering ceramics (Cerabien ZR; CZR, Lava Ceram; LV, Cercon Ceram Kiss; CC, IPS e.max Ceram; EM and VITA VM9; VT) were selected. Using the powder/liquid layering technique, bilayered disk specimens (diameter: 12.50 mm, thickness: 1.50 mm) were prepared to follow ISO standard 6872:2008 into five groups according to veneering ceramics as follows; Katana zirconia veneering with CZR (K/CZR), Katana zirconia veneering with LV (K/LV), Katana zirconia veneering with CC (K/CC), Katana zirconia veneering with EM (K/EM) and Katana zirconia veneering with VT (K/VT). After 20,000 thermocycling, load tests were conducted using a universal testing machine (Instron). The BFS were calculated and analyzed with one-way ANOVA and Tukey HSD (α=0.05). The Weibull analysis was performed for reliability of strength. The mode of fracture and fractured surface were observed by SEM.

RESULTS

It showed that K/CC had significantly the highest BFS, followed by K/LV. BFS of K/CZR, K/EM and K/VT were not significantly different from each other, but were significantly lower than the other two groups. Weibull distribution reported the same trend of reliability as the BFS results.

CONCLUSION

From the result of this study, the BFS of the bilayered zirconia/veneer composite did not only depend on the Young's modulus value of the materials. Further studies regarding interfacial strength and sintering factors are necessary to achieve the optimal strength.

Fracture of porcelain-veneered gold-alloy and zirconia molar crowns using a modified test set-up

Objective: The main aim of this study was to compare fracture load and fracture mode of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and metal-ceramic (MC) molar crowns using a modified test set-up to produce fractures similar to those seen in vivo, i.e. fractures of the veneering material rather than complete fractures.

Materials and methods: 13 high-noble-alloy MC and 13 Y-TZP molar crowns veneered with porcelain were manufactured. The crowns were artificially aged before final load to fracture. Load was applied using a 7 mm diameter steel ball exerting force on the cusps with stresses directed toward the core-veneer interface. Fracture surface analysis was performed using light- and scanning electron microscopy.

Results: The test design produced fractures of the veneering material rather than complete fractures. MC crowns withstood significantly (p > 0.001) higher loads (mean 2155 N) than Y-TZP (mean 1505 N) crowns, yet both endure loads sufficient for predictable clinical use. Fracture mode differed between MC and Y-TZP. MC crowns exhibited fractures involving the core-veneer interface but without core exposure. One Y-TZP crown suffered a complete fracture, all others except one displayed fractures of the veneering material involving the core-veneer interface with core exposure.

Conclusions: The test set-up produces fractures similar to those found in vivo and may be useful to evaluate the core-veneer interface of different material systems, both metals and ceramics. The study confirms suggestions from previous studies of a weaker core-veneer bond for Y-TZP compared to MC crowns.

Numerical simulation of the fracture process in ceramic FPD frameworks caused by oblique loading

Using a newly developed three-dimensional (3D) numerical modeling code, an analysis was performed of the fracture behavior in a three-unit ceramic-based fixed partial denture (FPD) framework subjected to oblique loading. All the materials in the study were treated heterogeneously; Weibull׳s distribution law was applied to the description of the heterogeneity. The Mohr-Coulomb failure criterion with tensile strength cut-off was utilized in judging whether the material was in an elastic or failed state. The simulated loading area was placed either on the buccal or the lingual cusp of a premolar-shaped pontic with the loading direction at 30°, 45°, 60°, 75° or 90° angles to the occlusal surface. The stress distribution, fracture initiation and propagation in the framework during the loading and fracture process were analyzed. This numerical simulation allowed the cause of the framework fracture to be identified as tensile stress failure. The decisive fracture was initiated in the gingival embrasure of the pontic, regardless of whether the buccal or lingual cusp of the pontic was loaded. The stress distribution and fracture propagation process of the framework could be followed step by step from beginning to end. The bearing capacity and the rigidity of the framework vary with the loading position and direction. The framework loaded with 90° towards the occlusal surface has the highest bearing capacity and the greatest rigidity. The framework loaded with 30° towards the occlusal surface has the least rigidity indicating that oblique loading has a major impact on the fracture of ceramic frameworks.

Influence of peak oral temperatures on veneer-core interface stress state

Objective: There is a growing interest for the use of Y-TZP zirconia as core material in veneered all-ceramic prostheses. The objective of this study was to evaluate the influence of CET on the stress distribution of a porcelain layered to zirconia core single crowns by finite elements analysis.

Material and methods: CET of eight different porcelains was considered during the analysis.

Results: Results of this study indicated that the mismatch in CET between the veneering porcelain and the Y-TZP zirconia core has to be minimum (0.5–1 μm/mK) so as to decrease the growing of residual stresses which could bring chipping.

Conclusions: The stress state due to temperature variation should be carefully taken into consideration while studying the effect of mechanical load on zirconia core crown by FEA. The interfacial stress state can be increased by temperature variation up to 20% with respect to the relative failure parameter (interface strength in this case). This means that stress due to mechanical load combined to temperature variation-induced stress can lead porcelain veneer–zirconia core interfaces to failure.

Influence of different adherend materials and combinations on in vitro shear bond strength

The influence of different adherend and substrate materials on shear bond strength (SBS) test was estimated. Ceramic plates (IPS e.max press) were cut, polished, abraded, and applied with two resin cements (Panavia F/Biscem). The SBS values of 30 groups were measured. The groups consisted of five combinations of adherend and substrate materials for each adhesive system and three different bonded areas (2, 4, and 6 mm diameter) for each combination. The failure modes were examined using a stereomicroscope. Groups with ceramic adherends showed higher SBS values in both adhesive systems and all three bonded areas. Small bonded areas are associated with significantly high SBS values. Groups with similar bonded areas and high SBS values showed more mixed or cohesive failures. Groups with small bonded areas and high SBS values had more interfacial failures. Adherend and substrate material significantly influenced the in vitro SBS value.