Objectives: To assess the fracture strength and failure mode of fiber reinforced composite (FRC) in cusp-replacing restorations. Methods: Forty-five extracted sound upper premolars of similar dimensions were selected. Identical MODB cavities were prepared and the palatal cusps were reduced in height in order to create cuspal coverage. The premolars were randomly divided into 3 groups of 15 premolars each. All specimens were restored with resin composite (Clearfil Photo Posterior, Kuraray). Before applying the composite in Group A two layers of resin impregnated woven FRC (everNet, Stick Tech Ltd.) were applied to the cavity surface and in Group B one layer of unidirectional FRC (everStick, Stick Tech Ltd.) was used. In Group C no fibers were used (control). The specimens were subjected to thermocycling (6000 x 55-5ºC) and subsequent static load tests by applying vertical load to the central fossa of the tooth with a cylinder. For each tooth the load until fracture was registered. Failure mode was categorized as tooth fracture above or below CEJ. Results: Weibull analysis revealed a characteristic strength and Weibull modulus (m) at 2367.1N for Group A (m=7.6), 2454.1N for Group B (m=5.3) and 2182.9N for Group C (m=10.7). Fracture strengths were not significantly different (ANOVA, p>0.05). Teeth restored with FRC showed less fractures extending below the cemento-enamel junction (38% and 23% for Groups A and B respectively) compared to specimen without FRC (86%) (chi-square, p<0.05). Conclusion: The results suggest that incorporation of FRC does not increase the static load bearing capacity of premolars with cusp-replacing restorations. However, fiber reinforcement has a beneficial effect on the failure mode and thereby on the restorability in case of fracture. Woven fibers gave more consistent results than unidirectional fibers.

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