Fracture resistances of zirconia, cast Ni-Cr, and fiber-glass composite posts under all-ceramic crowns in endodontically treated premolars

PURPOSE: The aim of the present study was to evaluate the fracture resistances of zirconia, cast nickel-chromium alloy (Ni-Cr), and fiber-composite post systems under all-ceramic crowns in endodontically treated mandibular first premolars. MATERIALS AND METHODS: A total of 36 extracted human mandibular premolars were selected, subjected to standard endodontic treatment, and divided into three groups (n=12) as follows: cast Ni-Cr post-and-core, one-piece custom-milled zirconia post-and-core, and prefabricated fiber-glass post with composite resin core. Each specimen had an all-ceramic crown with zirconia coping and was then loaded to failure using a universal testing machine at a cross-head speed of 0.5 mm/min, at an angle of 45 degrees to the long axis of the roots. Fracture resistance and modes of failure were analyzed. The significance of the results was assessed using analysis of variance (ANOVA) and Tukey honest significance difference (HSD) tests (α=.05). RESULTS: Fiber-glass posts with composite cores showed the highest fracture resistance values (915.70±323 N), and the zirconia post system showed the lowest resistance (435.34±220 N). The corresponding mean value for the Ni-Cr casting post and cores was reported as 780.59±270 N. The differences among the groups were statistically significant (P<.05) for the zirconia group, as tested by ANOVA and Tukey HSD tests. CONCLUSION: The fracture resistance of zirconia post-and-core systems was found to be significantly lower than those of fiberglass and cast Ni-Cr post systems. Moreover, catastrophic and non-restorable fractures were more prevalent in teeth restored by zirconia posts.

Effect of zirconia substructure design on the in-vitro fracture load of molar zirconia core crowns

In the available all-ceramic systems the zirconia core is commonly fabricated as one layer with an even thickness, but the veneering porcelain has different thicknesses in different parts of the restoration and therefore, undergoes chipping and fracture more rapidly under masticatory forces. This study aimed at comparing the fracture load of all-ceramic crowns with 2 different zirconia core designs in Cercon system and in-vitro conditions. In the present experimental study, 10 metal dies of mandibular first molar were fabricated with NNB base metal alloy using lost wax technique. Ten standard zirconia cores were then fabricated with an even thickness of 0.5 mm using Cercon CAD/CAM System. Also, 10 more were fabricated in the customized form with a 1 mm labial collar and 2 mm lingual shoulder. Porcelain was applied on all samples by an expert technician using an index. The prepared crowns were placed on their respective dies and cemented with Panavia F resin cement and under the constant pressure of 25 N. Vertical compressive force was applied to the samples by means of a stainless steel ball at a crosshead speed of 0.5 mm/min using Universal Testing Machine until failure. Data were analyzed using student t test. Our study results demonstrated the fracture load of 1852.11 +/- 587.9 N for zirconia core crowns with standard core design and 3332.63 +/- 916.38 N for custom design crowns. Statistical analyses demonstrated that the fracture load was significantly higher for customized core designs than the standard design cores[P<0.0001]. Considering the obtained results we can conclude that crowns with customized core design have a greater fracture resistance than those with standard design