Strength, reliability, and mode of fracture of bilayered porcelain/core ceramics.

PURPOSE: The aim of this investigation was to compare the biaxial flexural strength, its reliability, and the mode of fracture of bilayered disks made of two core materials (In-Ceram Alumina and In-Ceram Zirconia), both veneered with conventional feldspathic porcelain (Vita Alpha). MATERIALS AND METHODS: One hundred forty specimens (monolithic and bilayered) of In-Ceram Alumina, In-Ceram Zirconia, and Vita Alpha were made and tested with the biaxial flexural test. Finite element analysis was used to estimate the maximum tensile stress at fracture. Data were analyzed with one-way ANOVA, Tukey HSD, and Weibull distribution. SEM was used to identify the initial crack and characterize the fracture mode. RESULTS: All specimens with the core material on the bottom surface were statistically significantly stronger and more reliable than those with the porcelain on the bottom surface. Among them, In-Ceram Zirconia was stronger than In-Ceram Alumina. There was no statistically significant difference among groups when the porcelain underwent tension. Two different modes of fracture were observed in the bilayered samples according to which material was on the bottom surface. CONCLUSION: The material that underwent tensile stress dictated the strength, reliability, and fracture mode of the specimens. The design of the restorations and the actual distribution of the tensile stresses must be taken into account; otherwise, the significant contribution of stronger and tougher core materials to the performance of all-ceramic restorations may be offset by the weaker veneering porcelain.

Strength, reliability and mode of fracture of bilayered porcelain/zirconia (Y-TZP) dental ceramics.

The aim of this study was to investigate the biaxial flexural strength, reliability and the mode of fracture of bilayered porcelain/zirconia (Y-TZP) disks. For this purpose, 80 specimens were made from conventional dental porcelain and Y-TZP core ceramic, and equally divided into four groups as follows: monolithic specimens of porcelain; monolithic specimens of core material; bilayered specimens with the porcelain on top (facing the loading piston during testing); bilayered specimens with core material on top. The maximum load at fracture was calculated with a biaxial flexural test and finite element analysis was used to estimate the maximum tensile stress at fracture. Results were analyzed with one-way ANOVA, Tukey HSD. The reliability of strength was analyzed with the Weibull distribution. SEM was used to identify the initial crack and characterize the fracture mode. Monolithic core specimens and bilayered sample with the core material on the bottom were statistically significantly stronger than monolithic porcelain disks and bilayered samples with the porcelain on the bottom. The study, which was conducted with sample configurations that reproduce the clinical situation of crowns and fixed partial dentures, indicates that the material which lies on the bottom surface dictates the strength, reliability and fracture mode of the specimens. The contribution of strong and tough core materials to the performance of all-ceramics restorations may be offset by the weaker veneering porcelain if the actual distribution of the tensile stresses within the restoration is not taken into consideration.

Influence of core thickness on a restored crown of a first premolar using finite element analysis.

PURPOSE: This study examined the influence of ceramic coping thickness on the maximum stresses that arise in a first premolar all-ceramic crown. MATERIALS AND METHODS: Axisymmetric finite element models with different In-Ceram Alumina coping thicknesses (0.3, 0.6, and 0.9 mm) were examined. Models with and without resin lute were constructed. To all models, an identical axial load of 600 N was applied vertically downward, over an area around the crown's fissure. RESULTS: The resulting peak tensile maximum principal stresses in each part of the crown existed below the fracture strengths of the respective materials making up the crown. This was true for all variations of core thickness, with and without resin lute. The peak tensile stresses in the coping, porcelain, and dentin decreased for an increase in core thickness. This was most evident in the porcelain and coping. CONCLUSION: The thickness of the ceramic core has a significant influence on the resulting stresses in the coping, porcelain, and dentin of this axially loaded crown.

Influence of margin design and taper abutment angle on a restored crown of a first premolar using finite element analysis.

PURPOSE: The purpose of this study was to determine the influence of margin design and taper abutment angle on the stresses developed in all-ceramic first premolar crowns. MATERIALS AND METHODS: Four margin designs and three taper abutment angles were independently incorporated into models examined by finite element analysis. A 600-N force was applied vertically downward. RESULTS: The taper abutment angle had a significant influence on the greatest peak tensile maximum principal stresses (sigma11) in the coping (16.8% change in stress for an 8-degree variation in taper angle). The margin design had significant influence on the highest peak tensile sigma11 in the dentin (60% difference in stress between designs) and lesser significance in the cement (30%). All calculated values of the highest peak tensile sigma11 were considerably lower than the fracture strengths of the respective materials in which the stresses resided. CONCLUSION: A smaller taper abutment angle and a larger chamfer radius (equivalent to the modified light chamfer) are recommended to reduce the magnitude of the greatest peak tensile sigma11 based on the finite element modeling conducted.

Influence of cement on a restored crown of a first premolar using finite element analysis.

PURPOSE: The purpose of this study was to examine the influence of the elastic modulus of cement and luting thickness on the resulting stresses in an axially loaded crown cemented onto a first premolar. A comparison of these stresses was also made with the strength of the constituent materials making up the crown. MATERIALS AND METHODS: Examination of the stresses on a restored crown was conducted using finite element analysis. Eight different axisymmetric models containing combinations of In-Ceram or gold coping, using adhesive resin or zinc phosphate cement as the luting agent, with thicknesses of either 0.05 or 0.1 mm were analyzed. RESULTS: The peak tensile principal stresses in the porcelain remained below its material fracture strength. The same was true for the peak stress in the adhesive resin compared to its fracture and chemical bond strength. This was not the case for zinc phosphate. The influence of the luting agent's elastic modulus on the stresses in the crown was minor, and the influence of luting thickness was even less. CONCLUSION: The role of the luting agent was primarily one that effectively transferred the resulting stresses between the relatively stiff coping and underlying dentin. There was no evidence of the luting agent itself playing a significant role in resisting deflection from the applied force.

Finite element analysis studies of an all-ceramic crown on a first premolar.

PURPOSE: This study examined the effects of using four different core-ceramic materials on the stresses that developed in a single all-ceramic first premolar crown. MATERIALS AND METHODS: This was done by analyzing models constructed in an axisymmetric fashion with finite element analyses. The model had a 600-N vertical load applied uniformly over a circular area on top of the crown. Particular emphasis was placed on the tensile stresses that developed. RESULTS: For this particular type of model and loading configuration, radial tensile stresses were predominant in magnitude. Peak hoop and axial tensile stresses were approximately 80% and 2% of this value, respectively. The peak radial and hoop tensile stresses were located on the inside of the coping and scaled with the elastic modulus of the coping. For the axial component, the peak was located in the veneer ceramic on its outermost perimeter. CONCLUSION: Under normal loading, a near-uniform tensile stress field developed within the coping, directly beneath the contact area. The magnitude of this stress for realistic bite forces using current design recommendations was significantly lower than the fracture strength of the four ceramic materials investigated. The stresses developed within the porcelain were in all instances well below typical strength values.