Stability after mandibular setback: mandible-only versus 2-jaw surgery.

PURPOSE

The objective of this study was to evaluate whether changes in the technique for mandibular setback surgery since the introduction of rigid internal fixation have improved postoperative stability in Class III correction with setback alone and 2-jaw surgery.

PATIENTS AND METHODS

Cephalometric (skeletal and dental) outcomes for 17 patients with mandibular setback alone were compared with outcomes in 83 patients with 2-jaw surgery for Class III correction. Demographic characteristics in the 2 groups were similar, and the mean amount of setback (-4.7 mm) was the same; however, given a mean maxillary advancement of 4.9 mm, the 2-jaw patients had a greater total Class III correction.

RESULTS

Greater than 4 mm of posterior movement of the gonion at surgery and a resulting significant change in ramus inclination were found in 8 of the mandible-only patients (47%) but only 1 of the 2-jaw patients (1%). Postoperatively, the mean changes for the 2 groups were similar, with mean forward movement of the chin (pogonion) of 2.8 mm in both groups, but the mechanism was different. In the mandible-only patients, the major reason for forward movement of the chin was recovery of ramus inclination. In the 2-jaw group, about half the change in chin position was because of forward movement of the gonion; the other half was because of small upward movement of the maxilla that allowed upward-forward rotation of the mandible. In both groups there was a significant correlation (r = 0.42, P < .0001) between postoperative change in the position of the chin and gonion.

CONCLUSIONS

Despite improvements in surgical techniques for mandibular setback since 1995, postoperative stability still leaves something to be desired, but there is better control of the ramus position when 2-jaw surgery is performed.