Objective:To compare the fracture resistance of roots of mandibular premolar with different 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.Oneway 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.%目的:观察不同根尖预备直径对下前磨牙牙根抗折强度的影响.方法:收集因正畸新鲜拔除的牙根发育完成、无牙根折裂的单根下前磨牙,单根管、根管弯曲度小于10°、距根尖5 mm处根管颊舌径与近远中径之比小于2,初锉≤15#.离体牙截冠后,保留13 mm牙根,按重量以随机区组分组法分为6组(n=10),根管预备至不同主锉号数:A组,空白对照组,不进行根管预备;B组,40#主锉组;C组,45#主锉组;D组,50#主锉组;E组,55#主锉组;F组,60#主锉组.预备采用不锈钢手用K锉,以步退法进行预备,步退4号,每次步退1 mm,预备锥度为0.05,预备过程中冲洗液采用蒸馏水.将牙根用义齿基托树脂包埋,制作试件,使用万能试验机垂直加载,直至牙根折裂,记录抗压载荷(N)和折裂的类型.用单因素方差分析法、Tukey检验比较各实验组牙根抗压载荷,卡方检验比较不同实验组的牙根折裂类型.结果:根管预备后,5个预备实验组平均抗压载荷均明显低于未经根管预备的空白对照组,其中50#主锉组[(1 027±128)N]、55#主锉组[(994±150) N]、60#主锉组[(983±166)N]的平均抗压载荷降低明显,与空白对照组[(1 444±155)N]、40#主锉组[(1 339±131)N]、45#主锉组[(1 287 ±144)N]相比差异有统计学意义(P＜0.05).试件折裂类型分析显示各实验组无明显差别,发生于颊舌向折裂的最高(55％),近远中向折裂率为13％,复合折裂率为32％.结论:根尖预备大于50#后,下前磨牙牙根抗折强度明显下降.
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