A Cephalometric Analysis of Skeletal and Soft tissue Changes in 48 Patients with Mandibular Prognathism Before and After Mandibular Setback Surgery
Date Issued
2008
Date
2008
Author(s)
Lin, Hung-Ying
Abstract
Purposehe purpose of this retrospective study was to compare post-operative changes and skeletal stability between bilateral saggital split ramus osteotomy (BSSO) and intraoral vertical ramus osteotomy (IVRO) used for mandibular setback. Factors contributing to skeletal instability were also identified. atients and Methodshe study included 48 patients with mandibular prognathism, who underwent mandibular setback surgery at the Department of Oral and Maxillo-facial Surgery, National Taiwan University Hospital (NTUH) from January 2001 to December 2006. Twenty-five of them recieved with rigid internal fixation, the remaining 23 underwent IVRO with intermaxillary fixation. Lateral cephalometric radiographs were taken within 1 month before surgery (T1), immediately after the operation (within 1 week, T2), and at the time of completion of post-op orthodontic treatment (T3). The T1,T2, and T3 radiographs were traced and superimposed with “best-fit” technique and the data were digitalized to analyze the changes at T2-T1 and T3-T2. Mann-Whitney test was used to verify the differences in post-operative changes and long-term skeletal stability between the two groups. And the linear regression model was established to find the contributing factors.esultshe mean amount of horizontal setback at pogonion was 6.55 mm in BSSO group and 9.68 mm in IVRO group. The mean amount of vertical movement at menton was 0.99 mm upward in BSSO group and 0.12 mm downward in IVRO group. Long-term observation (T3-T2) showed that the chin moved upward (1.10 mm) and forward (1.48 mm) after BSSO, and moved downward (0.32 mm) and backward (1.19 mm) after IVRO. Significant differences were noted between the two procedures in horizontal skeletal instability at pogonion (T3-T2, p=.002), and vertical skeletal instability at menton (T3-T2, p=.004). n BSSO group, the horizontal soft/hard tissue ratio was 1.06 at B point, 0.88 at pogonion, 1.04 at menton. And the vertical soft/hard tissue ratio was 0.97 at B point, 1.00 at pogonion, 0.57 at menton. In IVRO group, the horizontal soft/hard tissue ratio was 1.03 at B point, 0.88 at pogonion, 1.01 at menton. And the vertical soft/hard tissue ratio was 0.91 at B point, 1.00 at pogonion, 2.07 at menton. No significant difference was found in the soft/hard tissue ratio.n BSSO group, the significant predictor for the horizontal skeletal instability was age and rotation of condylar axis. For vertical skeletal instability, it was significantly correlated to change of mandibular plane angle.n IVRO group, the significant predictors for horizontal skeletal instability were gender, openbite, and jaw deviation. And vertical skeletal instability was significantly correlated to age, amount of horizontal setback, grade of openbite, and follow-up period.iscussionhe patterns of post-operative instability are significantly different between BSSO and IVRO. The key factors contributing to this difference was considered be the change in condylar axis and the application of RIF or IMF. In BSSO, the tooth-bearing segment was rigidly fixed with the bilateral condylar segments. Therefore, the chin moves upward and forward as the condylar axis rotates counter-clockwise post-operatively. But in IVRO, osseous remodeling takes place at the osteotomy site. The chin may not move while the condylar axis rotates counter-clockwise post-operatively. Moreover, the application of IMF will lead to an increase of lower facial height. Thus, the chin moves downward and backward after the surgery.
Subjects
mandibular prognathism
setback
BSSO
IVRO
cephalometric analysis
skeletal instability
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