Factors associated with spontaneous mesialization of impacted mandibular third molars after second molar protraction

Objectives: To investigate factors associated with spontaneous mesialization of impacted third molars after second molar protraction to close the space caused by a missing mandibular first molar (L-6) or retained deciduous mandibular second molars with a missing succedaneous premolar (L-E). Materials and Methods: Panoramic radiographs of patients treated with mandibular second molar protraction to close the space due to missing L-6 or L-E (14 males, 36 females, mean age ¼ 18.6 6 4.4 years) were analyzed before treatment (T1) and after second molar protraction (T2). Factors associated with the amount of third molar mesialization were investigated using regression analyses. Results: Mandibular second molars were protracted by 5.1 6 2.1 mm and 5.8 6 2.7 mm, measured at the crown and root furcation, respectively. After second molar protraction, third molars showed spontaneous mesialization by 4.3 6 1.6 mm and 3.8 6 2.6 mm, measured at the crown and root furcation, respectively. Nolla’s stage of the third molar at T1 (B ¼ 0.20, P ¼ .026) and second molar protraction time (B ¼ 0.04, P ¼ .042) were significantly associated with the amount of third molar mesialization. Conclusions: Greater third molar mesialization was observed when Nolla’s stage of the third molar was higher before treatment and when the second molar protraction time was longer.