Abstract: Objective To investigate the influence of ferrule and crown-to-root ratio on fracture resistance of mandibular first premolars treated with prefabricated fiber post and core restoration. Methods Forty sound extracted mandibular first premolars with single root canal in each tooth, were cut 2.0 mm coronal to the buccal cemento-enamel junction for residual root models. The roots were divided into five groups (n=8) at random. Simulated surgical crown lengthening procedures were done in the cervical region of the roots to provide ferrule lengths of 0.0 mm, 1.0 mm, 2.0 mm, 3.0 mm and 4.0 mm for groups A0 to A4 respectively. The corresponding crown-to-root ratios of each group were 0.62, 0.75, 0.91, 1.10 and 1.33 respectively. A glass fiber post-and-core combined with a cast Co-Cr alloy crown was used for the restoration of each root. Each specimen was embedded to a height 2.0 mm below the crown margin from the apical surface in an acrylic resin block and was loaded in a universal testing machine at a 135° angle to its long axis. The fracture loads and fracture patterns of the specimen were recorded and analyzed by SPSS software. Results Fracture loads of A0 to A4 groups were (0.54±0.09), (1.03±0.11), (1.06±0.17), (0.85±0.11), (0.57±0.10) kN. When a 1.0-2.0 mm ferrule was prepared in the cervical region of residual root and the crown-to-root ratio was less than 1, the fracture resistance of teeth increased significantly compared with the group without ferrule (P＜0.05). When the height of ferrule was more than 3.0 mm and the crown-to-root ratio was greater than 1, the fracture resistance of teeth decreased significantly (P＜0.05). Conclusion Different ferrule heights and crown-to-root ratios can significantly affect the fracture resistance of residual teeth. To ensure a better fracture resistance of mandibular first premolars, a 1.0-2.0 mm ferrule should be prepared in the cervical root and the crown-to-root ratio of teeth after restoration should be controlled within 1.

Key words: ferrule, crown-to-root ratio, residual root, fracture resistance, fiber post and core