Objective: Fiber-reinforced composite restorations have become popular due to their excellent esthetics; however, little has been reported on the effect of preparation design on the marginal accuracy of these restorations. The aim of this study was to evaluate the effect of variations in tooth preparation design and die material on the marginal accuracy of fiber-reinforced composite crowns. Methods: Sixty standardized FibreKor crowns were made on stone and epoxy resin dies (n = 30 each) duplicated from three metal master dies representing complete crown tooth preparations with a total convergence of 5^o. Each die group represented a different finish line design (n = 10 each). Group A (narrow chamfer finish line), Group B (wide chamfer finish line), and Group C (shoulder finish line). Marginal discrepancy was evaluated by measuring the distances between each of four pairs of indentations on the crowns and on the dies with a toolmaker's microscope. Results: Analysis of seating measurements with parametric analysis of variance and Scheffé multiple comparison test disclosed a statistically significant difference for tooth preparation design (P <0.05). However, the type of die material was not significantly different. The least mean marginal opening value was for FibreKor crowns made on epoxy resin dies with a light chamfer finish line (57 mm), but the difference was not statistically significantly different from the same group of crowns made on the stone die (66 mm). Restorations made on the stone die with a shoulder finish line had a greater mean marginal opening value (95 mm), however, they were not statistically significantly different from the same group of crowns made on epoxy resin dies (81 mm). Conclusion: The marginal accuracy of fiber-reinforced composite crowns was affected by the tooth preparation design.

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