Abstract: Objective To explore the influence on retention force and fracture resistance of single root tooth with diagonal fracture when casting post was locally enlarged. Methods 105 mandibular first premolars newly extracted were gathered, with the decoronation from 2 mm above buccal cemento-enamel junction (CEJ) to 2 mm under lingual CEJ prepared for the root canal therapy. 60 teeth were randomly assigned to 4 groups. In three experiment groups, the tips of casting posts were enlarged by 0.1 mm, 0.2 mm, 0.3 mm respectively; in the control group, the posts were made by convention. Then all these posts were knocked into the corresponding teeth with luting agent, fractures of the teeth were observed with stereomicroscope, and the tensile tests were made to those teeth with no fractures. The remaining 45 teeth were assigned to 3 groups, and conventional casting posts, casting posts with 0.1 mm enlarged tip, and fiber posts were made respectively for fracture resistance tests. Then breaking-resistant tests were done. Results No fracture was found in the control group and the 0.1 mm enlarged group, while in the 0.2 mm, 0.3 mm enlarged groups, dental fractures were observed. There was statistical difference concerning retention force (P=0.000), but no difference concerning fracture resistance (P=0.134) between the control group and the 0.1 mm enlarged group. There was statistical difference concerning fracture resistance between the two casting posts groups and the fiber posts group (P=0.000). Conclusion The retention force of single root tooth with diagonal fracture can be enhanced with the casting posts when the tips were enlarged by 0.1 mm, with no risk of dental fracture.

Key words: Casting post, Enlarging, Retentive strength, Breaking-resistant strength