Abstract: Objective：To measure the anatomical data of root surface of three-rooted PMFMs in adolescents with CBCT, to provide theoretical support and clinical guidance for the prognosis and treatment of periodontitis. Methods：175 CBCT images of three-rooted PMFMs were collected. The occurrence of teeth, the length of root, the length of root trunk, the degree of concavities of root trunk at the level of 2mm below CEJ, the width of root bifurcation and the degree of root concavities at the level of 2mm below apex of root bifurcation vault and the depth of concavities at different levels of mesial root surface were recorded and analyzed. Results：The length of mesial root、distal buccal root and distal lingual root were 13.06±1.26mm,11.85±1.30mm and 10.26±1.61mm respectively. Except for distal lingual root,the root length of male was longer than that of female(P<0.05).The length of distal root trunk (4.87±1.00) mm was the longest, and the length of buccal root trunk(2.56±0.74) mm was the shortest.The degree of root trunk concavities at the distal side was the smallest at the level of 2mm below CEJ.The width of buccal root bifurcation was the broadest at the level of 2mm below apex of root bifurcation vault , with an average width of 2.77±0.67mm.The depth and degree of concavities of the mesial root-surface were greater than those of the distal buccal one. From the level of 2mm to 6mm below apex of root bifurcation vault, the depth of the concavities of mesial root surface became shallow gradually.The above differences were statistically significant(P<0.05).Conclusions:The three-rooted PMFMs had long distal root trunk.Once the lesion extends to the root bifurcation,the treatment will be difficult and the prognosis will be poor. The mesial root was obviously depressed.When the lesion spreads here, the treatment can be entered from the buccal bifurcation.

Key words: cone beam computed tomography; three-rooted PMFMs, distal lingual root; furcation ;root trunk