[Three dimensional finite element analysis of the influence of stress on occlusal surface of the mandibular first molar].

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.

[Effect of clinical experience on the shade matching accuracy].

PURPOSE: To evaluate the effect of clinical experience on the shade matching accuracy. METHODS: Thirty-seven dental students (group A) who had 1 to 2 years of clinical experience and 57 dental students (group B) who didn't have clinical experience took part in this experiment. Participants were divided into two groups (TTB group and TT group) and used the Toothguide Training (TT) and Toothguide Training Box (TTB) respectively. All participants received training session once a week for 3 weeks. Before training and each time after finished training all participants were asked to identify ten unlabeled tabs which had been randomly chosen from Vita 3D-master shadeguide. SPSS 16.0 software package was used for Wilcoxon signed rank test. RESULTS: Before training, the accuracy rate of group A and group B was (65.67±20.4)% and (70.17±17.57)% respectively. After training, the accuracy rate of group A and group B was (78.91±22.82)% and (80.17±17.77)% respectively. No significant difference was found between group A and B, but significant difference was found between before and after training. CONCLUSIONS: This study shows clinical experience is not found to be a significant factor to the teeth shade matching. The training can improve the accuracy of shade matching.