Objective To investigate the development of third molar embryo and establish the normal value of posterior arch length of adolescence in Chengdu.Methods The samples consisted of 40 males and 41 females with Class 1 dentitions, normal second molar occlusion, no history of orthodontic treatment, and good facial balance. Cephalometry and statistic analysis were conducted for all the subjects.Results The normal value of posterior arch length of maxilla in male was(16 52±2 35)mm and in female was(16 42±2 55)mm The normal length of mandible was(13 00±2 44)mm in male and(12 43±2 18)mm in female But statistic analysis showed no gender difference. Most of the third molar embryo had been formed and their tooth crowns had been calcified, the occurrence ratio was 84 0 % in maxilla and 85 2 % in mandible. Most of them located in the middle and inclined mesially.Conclusion The data obtained in this study will be valuable for our clinical practice and be helpful for the study concerning dento-facial growth and development.
