Abstract: Objective To study the relationship between bone mass of space missing mandibular first molar and bone loss of the mandibular second molars moved into the space, and to provide reference for risk prediction and prognosis in clinical treatment. Methods Fifty orthodontic patients with mandibular first molar loss were included in the study. Patients were treated with bracket, and the space was closed by mesially moving molars with the aid of implant anchorage. Cone-beam CT was taken and bone mass was measured before and after treatment. SPSS22.0 statistical software was used to analyze the data. Results The mean bone height loss of mandibular second molars at seven out of eight sites was less than 1 mm. Moreover, 34.5% of all sites had bone height increase, and 90% molars had sites with bone height increase. The height and width absorption of alveolar bone in space missing mandibular first molar had no significant correlation with the bone height loss of the mandibular second molar moved into space. Conclusion Closing the space missing mandibular first molar by orthodontic mesial movement of adjacent molars will not cause severe bone loss, and even the increase of bone height may be obtained at some sites. Risk of treatment cannot be predicted through the absorption of alveolar bone in the tooth missing area, and incremental bone surgery is unnecessary before treatment.

Key words: mandibular first molar, orthodontic space closure, bone mass of tooth missing space, molar bone loss