Four-rooted permanent maxillary first and second molars in a northwestern Chinese population.

Objectives: To detect the incidence and anatomic features of 4-rooted permanent maxillary molars in a northwestern Chinese population by using cone-beam computed tomography (CBCT).

Design: A total of 725 Chinese patients with well-developed maxillary permanent first and/or second molars were examined by using CBCT. The number of roots and canals, shape, location, and division level of the roots were evaluated. Four-rooted maxillary first and second molars were classified according to modified Versiani's classification. The root length and cross-sectional diameters of 4-rooted molars were measured by the software Galileos Implant 1.7 (SICAT GmbH & Co. KG, Bonn, Germany). One way analysis of variance followed by a Newman-Keuls post hoc test was used to compare the groups in relation to the root length and diameters (P = 0.05).

Results: Among 1365 maxillary first molars, only one tooth had four separated roots, whereas in second molars, 12 out of 1226 teeth (0.98%) exhibited an extra root; the incidences were 1.28% (7/549) for males and 0.74% (5/677) for females. All 4-rooted molars observed were unilateral. Out of a total of thirteen 4-rooted maxillary molars, six exhibited the type I root form, four exhibited the type II root form and one exhibited the type III root form; a type IV category (two molars) was added to Versiani's classification, which described maxillary molars with three buccal roots. A transitional root form between the typical 3-rooted form and double palatal roots was observed in six cases of 3-rooted molars. Measurements of root size showed that the diameter of the distopalatal roots was significantly larger than the extra mesiopalatal roots (P < 0.01).

Conclusion: The prevalence of the 4-rooted maxillary first and second molars in the Chinese population is low. They usually occurred unilaterally, and the shape, location and size of the four roots may be variable.