Investigation of three-rooted deciduous mandibular second molars in a Chinese population using cone-beam computed tomography

Background To investigate the anatomic features of three-rooted deciduous mandibular second molars (DMSMs) in Chinese children by using cone-beam computed tomography (CBCT). Methods A total of 247 CBCT scans of Chinese children were selected and retrospectively analyzed. The occurrence, gender and side predilection of three-rooted DMSMs were examined. The pattern of concurrence of bilateral three-rooted DMSMs, and concurrence of three-rooted DMSM and three-rooted permanent mandibular first molar (PMFM) was analyzed by the concurrence rate and Spearman's rank correlation test. The geometric parameters of the disto-buccal (DB) and disto-lingual (DL) roots, including the vertical root length, level and angle of distal root furcation, angle of root curvature (by Schneider technique) and the spreading angle, were measured and compared to the three-rooted PMFMs (n = 42) from 100 randomly selected adult subjects. Results The occurrence of three-rooted DMSMs was 24.0% (54/225) calculated by individual, and 18.6% (88/472) by tooth. A significant right-side predilection was detected (23.0% vs 14.2%, p < 0.05), while gender predilection was not detected (p > 0.05). The bilateral concurrence rate was 49.0%, and Spearman's correlation test indicated a significant relationship between the antimetric teeth (rho = 0.609, p < 0.01); whereas a weak but significant co-relationship was detected between the three-rooted DMSM and three-rooted PMFM (right side: concurrence rate = 31.6%, rho = 0.325, p < 0.01; left side: concurrence rate = 23.0%, rho = 0.260, p < 0.01). The length of DL roots in the DMSMs was 7.4 +/- 1.5 mm, and the curvature angle was 16.4 +/- 11.3 degrees, which was significantly (both p < 0.01) lower than that of the three-rooted PMFMs (root length = 11.0 +/- 1.3 mm; degrees of curvature = 34.2 +/- 16.1 degrees), whereas the spreading angle of the DL root in DMSMs (34.6 +/- 8.4 degrees) was significantly (p < 0.01) greater than in the PMFMs (26.8 +/- 6.5 degrees). Conclusions Three-rooted DMSMs have a high occurrence rate in the Chinese children with a right-side predilection, and they have a weak but statistically significant correlation with three-rooted PMFMs. The DL roots of DMSMs are shorter, less curved, and spreading more widely as compared with those in the three-rooted PMFMs.