Abstract:

Objective： To compare the fracture resistance of roots of mandibular premolar with diffe-rent apical preparation diameters. Methods: Sixty single-rooted single canal permanent mandibular premolar teeth extracted newly for orthodontic reason without immatureness, fracture or cracks were selected, with a curvature less than 10°, and internal length: short diameter of less than 2 at a level 5 mm from the apex. All the teeth were decoronated, leaving roots 13 mm in length. The initial apical file size for the teeth was ≤15#. The roots were assigned to 6 groups based on weights with random block design. Group A:  blank control group, no instrumentation was performed. Groups B-F: the master apical file (MAF) was 40#, 45#, 50#, 55# and 60#, respectively. In the five experimental groups the roots were instrumented using hand files with stepback technique at 1 mm increments, resulting in a taper of 0.05. The irrigant used was distilled water. After mounted in acrylic resin, all the teeth were subject to vertical loading using an Instron testing machine until fractured. The occurrence of fractures was detected when the applied load suddenly decreased. The fracture load values and fracture modes were recorded. One-way ANOVA and post-hoc Tukey test were used to determine the difference of fracture load values between the groups (P＜0.05). Chi-square tests were used to compare the modes of root fracture. Results: Five experimental groups exhibited lower fracture load values than that of control group ［(1 444±155) N］. The mean fracture load values for roots instrumented to an apical diameter of 50# ［(1 027±128) N］, 55# ［(994±150) N］ and 60# ［(983±166) N］ were significantly lower than that of control group and 40# group ［(1 339±131) N］ and 45# ［(1 287±144) N］ (P<0.05). Buccal-lingual fracture, mesio-distal fracture and compound fracture occurred 55%, 13% and 32%, respectively. No difference of fracture mode was detected in the six groups. Conclusion: The fracture resistance reduced significantly when the roots were instrumented to an apical diameter of 50# or larger.

Key words: Root canal preparation, Fracture resistance, Bicuspid