Abstract: PURPOSE: To compare the posterior space of the maxillary molar with osseous dislocation and investigate the retromolar space available for molar distalization in patients with maxillary prognathism. METHODS: Cone-beam CT (CBCT) data of 136 posterior maxillary segments in 32 patients with skeletal Class I malocclusion and 36 patients with skeletal Class Ⅱ malocclusion were analyzed. The functional tip of the maxillary first and second molars was connected as the measuring reference line, the reference plane for measurement parallel to the reference line, which passed the furcation of the mandibular second molar root, was set as the “0 mm” plane. Four additional planes, which were parallel to the 0 mm-plane and located at 2, 4, 6 mm apical to the 0 mm-plane, cemento-enamel junction (CEJ) and root tip, were named the"2 mm, 4 mm, 6 mm, CEJ and TIP", in order to measure the distance from the maxillary second molar to maxillary tubercle bone cortex in the edge. SPSS 23.0 software package was used to analyze the data. RESULTS: At all levels of measurement, the retromolar space of the values in male and female patients was not significantly different, ClassⅡ malocclusion were significantly greater than those in ClassⅠmalocclusion, with the minimum values at the CEJ level, (4.18±1.40) and (5.12±2.16) mm, respectively. As the measured level moved up, the measured clearance value increased and reached the maximum at the root tip, which was (8.64±2.41) and (10.02±2.27) mm, respectively. CONCLUSIONS: Compared with ClassⅠmalocclusion, patients with ClassⅡmalocclusion have greater retromolar space for maxillary molar distalization along the posterior line of occlusion. More attention should be paid to the anatomical limit of maxillary second molar at the CEJ level along the bite line in the distal direction when maxillary molar is pushed backward.

Key words: Retromolar space, Molar distalization, Maxillary tubercle, Maxillary prognathism