Plane-to-plane analysis of mandibular misalignment in patients with facial asymmetry.

INTRODUCTION: Little is known regarding how the mandible rotates in facial asymmetry. The purpose of this study was to study mandibular misalignment with a new plane-to-plane analysis method in patients with facial asymmetry. METHODS: Optimal symmetry planes (OSPs) were generated by computing the greatest count of paired voxels on opposing sides of the computerized tomography image of the structure. The mandibular OSP was measured against the midfacial OSP for its alignment. The deviation angle formed by the 2 OSPs was broken down into a y-axis component (frontal deviation angle) and a z-axis component (horizontal deviation angle). Fifty-nine patients who sought correction for facial asymmetry were included for study. RESULTS: The new analysis method was feasible. Fifty patients (83%) had significant mandibular misalignment (deviation, ≥4° or 4 mm). The locations of the rotational axes exhibited significant variations that could explain the varied features of the asymmetry. The frontal deviation angle (mean, 3.80° ± 3.89°) was significantly larger than the horizontal deviation angle (mean, 2.77° ± 1.71°). There was no significant correlation between the horizontal deviation angle and the anterior deviation distance or the posterior deviation distance. CONCLUSIONS: Proper mandibular realignment was suggested to be the primary aim in surgical correction of most jawbone asymmetries. Because of the greatly varied rotational axes and the obscure z-axis rotation, realignment could be difficult with the traditional approach. The OSP-based analysis is advocated to guide planning.

Use of the matching optimal symmetry plane method in planning surgical correction of facial asymmetry--a preliminary report of 20 patients.

PURPOSE: A voxel-based median plane (optimal symmetry plane [OSP]) was developed to assess facial bone asymmetry. The purpose of the present study was to introduce a new method of planning surgical correction of facial asymmetry using the OSPs as guides and test its effectiveness. PATIENTS AND METHODS: A retrospective study was conducted of 20 facial asymmetry patients with a mandibular deviation of 4 mm or greater or 4° or more that required surgical correction. In the test group (n = 8), the plans for asymmetry correction were formulated using the matching OSP method, in which the OSPs of the facial bones are tracked and matched during the model surgery setup. In the control group (n = 12), traditional planning was conducted. The traditional plans were cross-checked for symmetry through tracking and revised as needed. The symmetry results of the plans were compared between the 2 groups and within the control group. The outcome measures were the deviation distances between the OSPs of the midface and mandible at the anterior or posterior mandible, the occlusal plane cant, and the angle formed by the 2 OSPs. Surgery was performed in accordance with the final plans, and the results were assessed for symmetry. RESULTS: The traditional plans left a major mandibular deviation in 5 of the 12 control subjects compared with none in the test group. The test group did significantly better than the control group. The revised plans were significantly better than the initial plans. Postoperatively, significant improvements in symmetry were observed. CONCLUSION: The new method resulted in surgical plans that brought about significantly less postoperative mandibular deviation while maintaining a reasonable occlusion.

Optical tracking-based model surgery for orthognathic surgical planning using a quantifying evaluation method.

Traditional cephalometry with a cast-mounted articulator is a useful and well-established tool for orthognathic surgery 7 planning. However, 2-D planning with dental casts cannot provide 8 comprehensive information on facial bone conditions, especially 9 with regards to symmetry. To plan and predict postsurgical facial 10 symmetry and occlusions, this paper uses an optical navigation system to track the movement of the upper and lower dental models 12 in model surgery. The corresponding movement and the new position of the jawbones are demonstrated in the computer and the 14 symmetry status can be evaluated. Surgical splints can be fabricated from the virtual models and used in surgery. The procedure 16 provides more realistic predictions, which can assist surgeons to 17 better control postsurgical facial harmony.