The influence of veneering porcelain thickness of all-ceramic and metal ceramic crowns on failure resistance after cyclic loading.

STATEMENT OF PROBLEM In some clinical situations, the length of either a prepared tooth or an implant abutment is shorter than ideal, and the thickness of a porcelain crown must be increased. Thickness of the coping and the veneering porcelain should be considered to prevent mechanical failure of the crown. PURPOSE The purpose of this study was to investigate the influence of veneering porcelain thickness for all-ceramic and metal ceramic crowns on failure resistance after cyclic loading. MATERIAL AND METHODS All-ceramic and metal ceramic crowns (n=20) were fabricated on an implant abutment (RN Solid Abutment) for the study. Two different framework designs with 2 different incisal thicknesses of veneering porcelain (2 mm and 4 mm) were used for each all-ceramic and metal ceramic crown system, resulting in 4 experimental groups (n=10) with identically shaped crowns. The all-ceramic crown consisted of alumina (Procera AllCeram) frameworks and veneering porcelain (Cerabien), while metal ceramic crowns were made of high noble metal (Leo) frameworks and veneering porcelain (IPS Classic). All crowns were cemented on the corresponding abutments using a resin cement (Panavia 21). They were subjected to 1000 cycles of thermal cycling (5 degrees C and 55 degrees C; 5-second dwell time). The crowns were tested with a custom-designed cyclic loading apparatus which delivered simultaneous unidirectional cyclic loading at 135 degrees, vertically, at an rpm of 250, with a load of 49 N. Each specimen was loaded for 1.2 x 106 cycles or until it failed. The specimens were thoroughly evaluated for cracks and/or bulk fracture with an optical stereomicroscope (x10) and assigned a score of success, survival, or failure. The specimens without bulk fracture after cyclic loading were loaded along the long axis of the tooth, on the incisal edge, in a universal testing machine at a crosshead speed of 1.5 mm/min, until fracture. Fisher's exact test was used to compare the success and survival rate between the 2 different materials (alpha=.05). Two-way ANOVA was used to analyze data in terms of material, porcelain thickness, and interaction effect. Also, a 2-sample t test was performed to compare between 2 thicknesses within the same material (alpha=.05). RESULTS According to the Fisher's exact test, the all-ceramic group showed significantly higher success (P=.003) and survival rates (P=.001) than the metal ceramic group. For the failure load, the 2-way ANOVA showed significant effects for material (P<.001) and porcelain thickness (P=.004), but not a significant interaction effect (P=.198). For the metal ceramic groups, crowns with a 2-mm porcelain thickness showed a significantly greater failure load than crowns with a 4-mm porcelain thickness (P=.004). However, all-ceramic groups did not show a significant difference between the 2 different thicknesses of veneering porcelain (P=.198). CONCLUSIONS The all-ceramic crowns showed significantly higher success and survival rates after cyclic loading, but lower failure loads than metal ceramic crowns. The thickness of the veneering porcelain affected the failure load of the metal ceramic crowns, but not that of the all-ceramic crowns.