Objective: The purpose of this study was to evaluate the influence of preparation design on the structural integrity of resin-bonded ceramic crowns. The hypothesis to be tested was that a reduction in the occlusal thickness of the restoration would not significantly affect the fracture strength of the crown.

Method: Three groups of upper premolar teeth (n=9) were mounted in resin blocks and prepared for a ceramic crown with a 1.0mm thick shoulder, rounded line angles and an occlusal reduction of 0.5mm (Group 1), 1.0mm (Group 2) and 2.0mm (Group 3). IPS Empress® crowns were constructed for each group, etched, silanated and luted using a composite luting resin (Variolink®). The specimens were subjected to a compressive load test using a 4mm diameter stainless steel bar, placed across the occlusal midline of the teeth. The fracture strength and mode of fracture were recorded for each group. Data were analysed using on-way analysis of variance.

Result: A mean fracture strength of 1134± 272 N, 1260±561 N and 1489±N were recorded for Groups 1, 2 and 3 respectively. Statistical analysis showed no significant difference in the fracture strength of the three groups, although there were differences in the mode of failure.

Conclusion: The fracture strength of resin-bonded ceramic crowns restorations was unaffected by the amount of occlusal reduction, supporting the original hypothesis. Thus, this preliminary study indicates that excessive removal of sound tooth structure to create space for ceramic bulk may be contraindicated.

® Ivoclar-Vivadent, Schaan, Lichtenstein

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