Accelerated bruxism-simulating fatigue test of occlusal veneers

The aim of this in vitro study was to analyze the fatigue behavior of occlusal veneers made of three different Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) restorative materials using accelerated bruxism-simulating cyclic loading.

Thirty occlusal veneers were fabricated for natural lower molars using 3 different CAD/CAM materials (n = 10 per material): lithium disilicate (IPS e.max CAD), hybrid ceramic (Vita Enamic) and translucent zirconia (Bruxzir). The specimens were preconditioned mechanically in a chewing simulator (ART1, University of Minnesota, USA) with a maximum load of 50 N at a frequency of 4 Hz for 300,000 cycles. These loading parameters simulated the normal physiological challenges of approximately one-year duration. The specimens were then cyclically fatigued to failure by lateral loading at a frequency of 2 Hz to simulate bruxing using a custom-made device attached to a universal testing machine. A linear increase of the lateral load from 50 N to 500 N over a period of 8 h was applied to accelerate the fatigue. Failure loads were recorded and compared between the groups using One-way ANOVA followed by Tukey's test. The level of significance was set at α = 0.05. Weibull statistical analysis was performed to evaluate the reliability of the tested restorations against fatigue failure.

All samples survived the chewing simulation without any cracking or chipping, but they all failed by fatigue under cyclic lateral loading before the maximum load was reached. The load at failure was 250.7 ± 46.1 N, 274.1 ± 50.3 N and 335.3 ± 55.0 N for lithium disilicate, hybrid ceramic and zirconia, respectively. The zirconia group had a significantly higher failure load than the other two groups (p < 0.05). The hybrid ceramic samples, on the other hand, showed the highest Weibull modulus for fatigue (1.77), followed by IPS e.max CAD (1.21) and Bruxzir (1.14). For the zirconia samples, those fractured had a significantly higher mean load at failure than those debonded.

Under normal cyclic occlusal loads, using hybrid ceramics for occlusal veneers might be advantageous because of their higher reliability for fatigue. However, under heavy occlusal loads such as bruxing, zirconia veneers will provide a better fatigue performance.