Impact fracture has frequently been used to study the failure of various post/core restoration supporting cast metal crowns. An alternative clinical failure mode is by fatigue. Objectives: The advent of high modulus, high fracture toughness fibre composites (Nulite F, SDI Australia) raises the question whether these materials alone could be used for post/cores supporting crowns. This study seeks to answer this. Methods: Sixteen central incisors were decoronated and root canals opened to 1.5mm. Cast metal post/cores and cast crowns were prepared for eight teeth. Post/cores and crowns were cemented with zinc phosphate cement (Shofu Hy Bond). Root canals of the remainder were treated with 3M Single-Bond and packed with Nulite-F composite to form post and core. Metal crowns were then fabricated and cemented. Strain gauges were cemented across the tooth-crown junction on the palatal and connected to an electronic strain gauge digital read-out. Teeth were supported in resin 30 degrees to the horizontal and loaded at 37cycles/minute under 100 N force until failure or a maximum of 300,000 cycles. Results: The mean cycles to failure for crowns with cast metal post/cores was 57047 ± 19381. No crowns with composite cores failed by the "cut off" point of 300,000 cycles. A t-test gave a probability of p=0.0000. Conclusion: This technique provides a simple, but time consuming method for the study of the failure of crowns with varying modes of support, under fatigue loading.

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