A 3-Dimensional Approach for Analysis in Orthognathic Surgery-Using Free Software for Voxel-Based Alignment and Semiautomatic Measurement

Purpose: In orthognathic surgery, the repeatability of 3-dimensional (3D) measurements is limited by the need for manual reidentification of reference points, which can incorporate errors greater than 1 mm for every 4 repeated measurements. This report describes a semiautomatic approach to decrease the manual reidentification error. This study evaluated the repeatability of surgical outcome measurements using the semiautomatic approach. Furthermore, a step-by-step guide is provided to enable researchers and clinicians to perform the 3D analysis by themselves. Materials and Methods: Evaluating surgical outcome consists of 2 parts. First, the scans are aligned at the anterior cranial base. Second, a semiautomatic approach is used to place 3 dental reference points at exactly the same sites of the pre-and postoperative maxilla. Because the maxilla is repositioned during surgery but otherwise unaltered, the reference points should be identical if the pre-and postoperative scans are aligned at the maxilla. Therefore, the authors propose the insertion of reference points on the preoperative scan and then repositioning a copy of the preoperative reference points relative to the postoperative scan. To align the reference points on the postoperative scan, the hard palate is used as a mutual maxillary reference structure. A reproducibility test was performed in 10 participants by analyzing the difference between repeated measurements. Results: Repeated linear measurements differed by less than 0.1 mm along all 3 axes (standard deviations, < 0.1 mm). The 2 largest differences between repeated measurements were 0.33 mm along the superoinferior axis and 0.29 along the anteroposterior axis. Repeated rotational measurements differed by less than 0.1 degrees around all 3 axes (standard deviations, <= 0.1 degrees). Conclusion: The semiautomatic approach showed excellent linear and angular repeatability. The algorithm can be implemented in the clinical evaluation of orthognathic surgical outcome and postoperative relapse. (C) 2017 American Association of Oral and Maxillofacial Surgeons