根尖弯向近中的下颌阻生第三磨牙拔牙断根率高的原因及预防

Analysis of the High Rate of Root-Fracture in the Extraction of Teeth for Lower Impacted Third Molars with Radicis Dentis Apices of Mesial Twist

对５０颗根尖弯向近中的下颌阻生第三磨牙断根率高的原因和预防进行了分析。断根原因：（１）牙挺旋转用力方向中含有较大的远中方向的力量；牙根尖近中的牙槽骨面阻力很大，两方面的力量使牙根沿着根尖转弯处、牙槽骨的反作用力方向折断。（２）牙片根尖的畸形显示不清，导致错误的牙挺用力位置和用力方向。预防措施：（１）术前认真分析Ｘ线片阻生牙的阻力所在，特别是根尖有无近中弯曲，选择去除各方面阻力的最佳方案。（２）去除近中弯曲的根尖阻力：牙挺的近中舌侧方向的旋转用力与楔入力量配合并且去除向远中方向的用力。

In this study, reasons and precautions of the high rate of root-fractures in the extraction of teeth for 50 lower impacted third molars with radicis dentis apices of mesial twist (RDAMT) were analyzed by mechanics. The reasons were: (1) There were much distal strength in the direction whirled strength of elevetor and strong arrested strength in mesial surface of the alveolus. These two kinds of strength made the roots broken at the root-twisting points, in the contra-stregnth direction of distal surface of the alveolus. (2) The images of detormed roots were not clear on the X-ray films, so the position and strength direction of the elevator that we got from it were wrong. The precautions were: (1) Resistance of the extraction should beanalysedbymeansofobservingtheX-rayfilms carefully and theRDAMT espesially before the oprations. (2) Remove the resistance of RDAMT: the whirled strength of the elevator was only in the mesiolingual direction, and wedge direction stregnth were at work in common.