Fracture load of zirconia crowns according to the thickness and marginal design of coping

Abstract

Statement of problem: The opaque color of zirconia copings may compromise esthetics, especially in the facial cervical area. A collarless zirconia coping can be an alternative. However, the strength of zirconia crowns with collarless copings is unknown.

Purpose: The purpose of this study was to compare the fracture load of zirconia crowns according to coping thickness and facial collar design.

Material and methods: Forty-eight zirconia crowns were fabricated and divided into 4 groups: standard coping group (0.5 mm coping thickness, 0.2 mm facial collar height); collarless coping group (0.5 mm coping thickness, no facial collar); modified thicker coping group (0.7 mm coping thickness, 0.2 mm facial collar height); and thicker coping group (0.7 mm coping thickness including collar height). The fracture load for the crowns was measured in a universal testing machine. One-way ANOVA and the Tukey HSD test for post hoc analysis were used for statistical analysis (α=.05). Ten additional zirconia crowns were fabricated to test the effect of cyclic loading on fracture resistance between the standard coping group and the collarless coping group, and data were analyzed with the Mann-Whitney U test.

Results: The fracture load in the collarless coping group (2329.1 N±948.3) was not significantly different from that of the other groups. The thicker coping group (3179.3 N±1137.7) resulted in significantly greater fracture load than the standard (2126.9 N±576.9) and modified thicker coping groups (2112.7 N±623.9) (F=4.193; P=.011). The fracture load after cyclic loading did not differ significantly between the standard coping group and the collarless coping group.

Conclusions: Within the limitations of this in vitro study, the thicker coping group had the highest fracture strength, but the collarless coping group using a 0.5 mm thickness coping was not significantly different from the standard coping group.