Head orientation method affects symmetry assessment.

Head orientation is a key component of orthognathic surgical planning, and sets the reference frame on which clinical assessment and surgical planning are performed. Hard tissue-based (HT) and soft tissue-based (ST) head orientation methods are commonly used, but their impact on symmetry is unclear. The purpose of this study was to analyze the impact of head orientation technique on symmetry assessment. Patients who underwent orthognathic surgery at a single center over 13 months were studied retrospectively. Preoperative computed tomography were oriented using HT and ST methods, and the roll and mediolateral positions of the maxillary dental midline (U1) and pogonion (Pog) were compared. In fifty-seven eligible subjects, the median roll difference was 0.64° (range 0-2.85°; P = 0.003), and the median absolute mediolateral position difference was 0.9 mm at U1 and 1.2 mm at Pog (both P = 0.003). The reliability of the both methods was excellent (intra-class correlation coefficient > 0.974; both P < 0.001). Use of HT and ST head orientation methods resulted in differing symmetry assessments, which could result in clinically significant deviations. Clinicians must balance the advantages and disadvantages of the HT and ST methods to properly apply each to their surgical workup protocols.