Abstract: PURPOSE: To investigate the stability and influencing factors of proximal bone segment in patients with bony Class Ⅲ malocclusion with mandibular deviation after surgery, and to provide reference for the design of preoperative digital orthognathic surgery. METHODS: Seventy patients with bony Class Ⅲ malocclusion were included in this study, which included 35 patients with non-deviated jaws (group A) and 35 patients with deviated jaws(group B), all of them underwent bilateral sagittal split ramus osteotomy(BSSRO). CBCT data were obtained from the patients at 1 week preoperatively(T0), 1 week postoperatively(T1), and 1 year postoperatively (T2). ProPlan CMF 3.0 software was used for reconstruction and separation, and three-dimensional models of the upper and lower jaws were obtained. Relevant items on bilateral paracardial bone segments were measured at different periods. The spatial position movement and rotation of the plane where the connections of the proximal condyle center(CoC), coronoid point(Cor) and mandibular angle point(Go) were located were included. The factors affecting the stability of proximal bone segment after BSSRO in patients with bony Class Ⅲ malocclusion and mandibular deviation were analyzed. SPSS 27.0 software package was used for data analysis. RESULTS: There were statistically significant differences in yaw and roll positions of left and right mandibular ascending branches in group B at stage T0(P< 0.05), but no significant differences in other stages. Compared with stage T0, both proximal cardial segments moved vertically downward, tilted forward, and rotated in patients in stage T1(P< 0.05); meanwhile, in group B, skewed proximal cardial segment rolled(P< 0.05), and the changes of yaw and roll in the skewed and oblique proximal cardial segments were statistically significant(P< 0.05). In group B, the internal rotation and extroversion of the oblique side were correlated with the preoperative mandibular deviation (R-values were -0.590 and 0.573, respectively). Compared with T1, the proximal core segment in T2 group moved vertically upward, tilted backward and abducted (P< 0.05), and the proximal core segment in group B moved horizontally to the medial side as a whole(P< 0.05). ROC curve was used to obtain the preoperative truncation value of the skew of the skew proximal bone segment that caused the horizontal recurrence of the skew proximal bone segment, which was 6.31mm. For patients with skew greater than 6.31mm before surgery, the horizontal change of the skew proximal bone segment had statistical significance(P< 0.05). CONCLUSIONS: The proximal bone segment on the deviated side is more unstable after BSSRO in patients with bony Class Ⅲ malocclusion with mandibular deviation, and may be displaced horizontally and medially. If the preoperative mandibular deviation is large(more than 6.31 mm), when designing the preoperative digital orthognathic surgery plan, it is necessary to be aware of the horizontal movement of the proximal cardiac segment of the postoperative deviation to the medial side, which may aggravate or cause temporomandibular joint disorder, and the risk of postoperative recurrence.;

Key words: Bony Class Ⅲ malocclusion, Mandibular deviation, Bilateral sagittal split ramus osteotomy, Proximal bone segment, Stability