Upper Airway shape changes after Bilateral Sagittal Split Osteotomies

Method:  Longitudinal cone-beam computed tomography (CBCT) scans were obtained for 22 patients that underwent BSSO surgery for the correction of skeletal Class II malocclusions. Image capture was done supine position. Longitudinal scans were acquired pre-surgery (T1), six weeks after surgery (T2) and one year follow-up after surgery (T3). Segmentation of the CBCT images delineates the anatomy of the upper airway from the cervical vertebra to the level of hard palate/posterior nasal spine superiorly and the lower border of C2 inferiorly. For shape measurements, point-based correspondent models and mean latitude axis are obtained for all the data using SPHARM-PDM correspondence. All 3D correspondent models were rigidly aligned using Procrustes before distance computation to analyze shape changes rather than positional changes. Absolute distance maps and corresponding vector maps showing shape differences were computed by measuring differences between each correspondent point. Mean latitude axis is a new imaging method to calculate minimum cross-sectional areas along the long axis of the airway independently of head position/alignment. 

Result:  Three-dimensional color maps and semi-transparency overlays allow qualitative and quantitative assessment of treatment changes in time. All subjects show lateral and anterior expansion of the airway after BSSO Advancement. Minimum cross-sectional area measured by mean latitude axis showed a statistically significant increase (120 to 173mm³) after surgery, and displays a minimal relapse (173 to 163mm³) one year after follow up.

Conclusion:  Shape analysis is a powerful tool to assess upper airway morphology. BSSO advancement produces a significant increase in pharyngeal airway space and minimum cross-sectional area. The greatest increase in airway space occurs laterally, not anteriorly.