A comparison of failure load for zirconia-ceremics restorations with different zirconia/veneer thickness and cooling rate

Abstract

Objectives: The purpose of the current study was to evaluate the influence of different zirconia coping thicknesses and cooling rates on the failure load of zirconia crowns. Methods: Forty identical abutment models were milled out of polymethylmethacrylate, and zirconia copings of two thicknesses (0.5 mm or 1.5 mm; n = 20 each) were fabricated using a dental computer-aided design and computer-aided manufacturing system. Zirconia crowns were completed by veneering feldspathic ceramics under different cooling rates (conventional or slow, n = 20 for each cooling rate), resulting four zirconia crown groups (n = 10 per group). Each crown was cemented on the abutment and 300,000 cycles of a 50-N load was applied on the crowns in conjunction with 1263 thermocyclings. After fatigue loading, a static load was applied on each crown until failure using a universal testing machine. The mean failure loads were statistically evaluated with one-way and two-way analysis of variance tests (p = 0.05). Results: No cohesive or adhesive failure was observed after fatigue loading. The greatest mean failure load occurred in zirconia crowns that had 1.5-mm thick coping and had undergone slow cooling (p < 0.001). Furthermore, six of 10 crowns with the 1.5-mm thick coping in the slow cooling group showed coping fractures. However, no coping fractures occurred in the other groups. Conclusions: Coping thickness and the cooling rate had a significant influence on the mean failure loads of the zirconia crowns. Under conventional cooling conditions, the mean failure load was not influenced by the coping thickness; however, under slow cooling conditions, the mean failure load was significantly influenced by the coping thickness. Clinical significance: A thicker coping design or slow cooling after the final firing of the veneer ceramic would be beneficial in reducing the incidence of chipping failure in zirconia crowns. A thicker coping design with slow cooling is therefore recommended to minimize chipping failures.