Differences in the posterior space available for molar distal movement according to the presence and position of the third molar: A cone-beam computed tomography study

Objective: To assess differences in the available posterior space for molar distal movement according to the presence and three-dimensional position of third molars. Methods: This study included 46 and 59 patients for the maxilla and mandible, respectively. The patients were assigned to third molar and no-third molar groups. Retromolar distances were measured at different vertical root levels of the second molar. The positions and axial angulations of the third molars were also examined. An independent t test or Mann–Whitney U test was performed to compare the retromolar distance between groups. The Pearson correlation coefficient was calculated to investigate the relationship between retromolar distance and age or third molar variables. Results: The retromolar distance was shortest at the 2 mm apical level in the maxilla and at the 10 mm apical level in the mandible. The maxillary retromolar distance of the distobuccal root was significantly longer at all apical levels in the third molar group than in the no-third molar group (P < 0.01), but was not significantly different at the 10 mm apical level in the mandible (P = 0.969). The number of buccally positioned third molars in the mandible was significantly higher than that in the maxilla (P < 0.001). The shortest retromolar distance in both jaws was not significantly associated with age, the third molar’s position, or eruption status. Conclusions: The available posterior space with the third molar present was significantly larger than that without the third molar in the maxilla, but was not significantly different in the mandibular posterior space.