Abstract: PURPOSE: Use of three-dimensional finite element to analyze stress distribution and transient displacement of mandibular first molar both at the occlusal surface level, to investigate the biomechanical basis of vertical root fracture occurring in the mesial root. METHODS: Mimics finite element software was used to build the horizontal and tilt abrasion of the mandibular first permanent molar. Each status was then divided into 4 kinds of wearing degree and 8 models were then established including sliced 1, sliced 2, sliced 3, sliced 4 and miter 5°,miter10°,miter15°,miter20° respectively. 200 N of bite force was loaded on the occlusal surface using MSC. Marc nonlinear finite element analysis software was used to investigate the distribution of stress and transient displacement. RESULTS: When distal tipping angle of the abrasion plane of mandibular first molar increased from 5° to 15°, the angle between instant displacement contours and tooth long axis become smaller, the mesial root suffered from the biggest stress concentration and thus the possibility of longitudinal root fracture increased. CONCLUSIONS: When the abrasion plane of mandibular first molar tilted distally, the possibility of the mesial root longitudinal fracture increased.

Key words: Mandibular first molar, Mesial canal, Attrition, Three-dimensional finite element, Equivalent stress, Wink movement