To observe and analyze the root canal anatomy of mandibular first premolar, second premolar and first permanent molar in Yunnan province by using cone-beam computed tomography (CBCT) , and to provide theoretical basis and reference for clinical root canal therapy.

400 patients were selected by stratified sampling based on the age (100 cases aged 20-29, 100 cases aged 30-39, 100 cases aged 40-49, and 100 cases aged 50-59) from January 2017 to January 2018 at the Department of radiology of Yunnan stomatological hospital. CBCT data of 324 patients met the standard, including 166 males and 158 females, from which 82 cases aged 20-29, 82 cases aged 30-39, 82 cases aged 40-49, and 78 cases aged 50-59. The number, length and curvature of root canals of mandibular first premolar, second premolar and first permanent molar was analyzed. The data of root canal length, the distance from the dental cusp of mandibular first permanent molar to the root bifurcation and the distance from the apical foramen to the mandibular cannal of different age were analysed by single factor analysis of variance. The data of the distance from the dental cusp of mandibular first permanent molar to the root bifurcation and the distance from the apical foramen to the mandibular cannal of different gender were analysed by independent sample t test.

CBCT images of 324 patients showed that the mandibular premolars were basically single root canal (647/648) , and only one case of right mandibular first premolar with 2 root canals. The curvature of mandibular premolars were mainly level Ⅰ (5°, 10°) and level Ⅱ (10°, 25°) . The length of root canal of mandibular first permanent molars were mostly in the normal range (15 ~ 25 mm) . The curvature of mandibular first permanent molars were mainly level Ⅱ (10°, 25°) . The distance from the dental cusp of mandibular first molar to the root bifurcation became shorter and shorter with age (F_left = 11.16, P_left<0.001; F_right = 11.51, P_right<0.001) , there was no statistically significant difference between male and female (t_left = 1.31, P_left = 0.19; t_right = 0.51, P_right = 0.61) . The distance from the apical foramen of mandibular first molar to the mandibular canal increased with age (F_left = 7.03, P_left<0.001; F_right = 12.25, P_right<0.001) . There was no statistically significant difference between male and female (t_left = -0.64, P_left = 0.52; t_right = -0.11, P_right = 0.91) .

The root canal variation of mandibular first premolar were less complex than others, the root canal anatomy of mandibular first permanent molar were complex. Compared with other regions, the root canal anatomical system of the first permanent molar and premolars of the mandibular in Yunnan is different, but there is no difference in aging. CBCT can provide an evidence for root canal therapy.