Post fatigue fracture resistance of lithium disilicate and zirconia crowns: vertical versus horizontal preparations (an in vitro study).

Abstract

To assess and compare the post-fatigue fracture resistance of two types of ceramic crowns, zirconia and lithium-disilicate, when used with two margin designs (vertical and horizontal). Forty sound human premolar teeth collected from the orthodontic department were divided randomly into two main groups according to the type of ceramic material (n = 20): Group A: Lithium-disilicate and Group B: Monolithic zirconia. Each group was further subdivided according to the margin design into two subgroups (n = 10): feather-edge (1) and shoulder (2) preparation designs. After digital scanning and designing, the crowns were bonded with self-adhesive resin cement. Each assembly underwent cyclic loading (1,000,000 cycles; 100 N; 1 Hz) in water and was then subjected to axial compressive loading till fracture. Failure modes were analyzed through fractographic analysis under SEM. The highest mean of fracture resistance was recorded for subgroup B1 (Zirconia with vertical preparation: 3129 N), followed by subgroup B2 (Zirconia with horizontal preparation: 2616 N). while lithium-disilicate crowns showed lower fracture values (Group A1: 1880 N, Group A2: 1512 N). One-way ANOVA test revealed statistically significant differences in fracture resistance between the two crown materials (p < 0.05). However, there was no statistically significant difference between the vertical and horizontal preparation designs within the same material group (p > 0.05). Both zirconia and lithium-disilicate crowns demonstrated fracture strengths exceeding the level of biting forces in the premolar area, supporting the use of conservative feather-edge preparation for lithium-disilicate and zirconia crowns in posterior regions.