Reliability and Failure Mode of Reduced Thickness Occlusal Overlays.

Objectives: To evaluate the flowable composite as an alternative material for use in occlusal overlay in comparison to the standard pressed ceramic material.
Methods: An anatomic model of a mandible first molar was created with an occlusal reduction of 0.5 mm. Based on the designed tooth preparation, a 0.5 mm thickness occlusal overlays were modeled and fabricated using different materials: (i) lithium disilicate pressed ceramic (IPS e.max Press, Ivoclar Vivadent) (LIDI) and (ii) flowable composite (G-Aenial Composite) (FC) (n=18). The specimens were etched according to manufacturer’s instruction, cemented with adhesive resin cement (relyXäultimate, 3M) and kept in distilled water for 7 days. For the fatigue test, overlays were subjected to single load using specifically designed profiles (n=3/group):mild, moderate and aggressive resulting in a specimen ratio distribution of 3:2:1. Mouth-motion step-stress fatigue was performed under water by sliding an indenter 0.5mm lingually down the distobuccal cusp. The use level probability Weibull curves and reliabilities for missions of cycles varying from 100,000 to 200,000 at use stress levels from 100 to 200N were calculated.
Results: The beta (β) values were 1.27(0.67-2.39) and 2.30(1.49-3.55) for LIDI and FC respectively, indicating that failure rates increased with fatigue. With use stress level of 100 N, FC demonstrated higher reliability up to 250,000 cycles, which is equivalent to one year use. At use stress level higher than 100 N (e.g., 150 N and 200 N), LIDI outperformed FC. Failure of LIDI and FC chiefly comprised small chips with the fracture origin associated with the occlusal indentation area, which was demonstrated by hackles pointing towards the restoration margins and arrest lines presenting their concave portion pointing towards the fracture origin.
Conclusions: Statistical analysis demonstrates that FC is as reliable as LIDI use up to a year (250,000 cycles) at 100 N use stress level.