Gearing Effect in Clockwise Rotational Orthognathic Surgery.

BACKGROUND: The standard procedure for managing skeletal class III malocclusion is maxillary advancement with mandibular setback. Occlusal plane-altering orthognathic surgery, such as jaw rotation, is useful as well. Although clockwise jaw rotation is a common procedure, its mechanism has not been well investigated. With this study, the authors aim to introduce the gearing effect to correct class III malocclusion in Asians by maxillary posterior impaction using clockwise rotation without advancing the maxilla. METHODS: Patients with class III correction with clockwise rotation of the maxillomandibular complex without maxillary advancement were included; those who underwent genioplasty were excluded. Various facial skeletal cephalometric landmarks were measured using artificial intelligence-based cephalometric analysis software. The gearing effect was determined by dividing the lower anterior facial height (LAFH) in relatively short and long groups compared with those in the anterior nasal spine to posterior nasal spine length. RESULTS: In a total of 29 patients, the amount of menton setback between group 1 ( n = 15; short LAFH) and group 2 ( n = 14; long LAFH) was 1.67 +/- 0.66 and 2.74 +/- 0.99 mm per 1 mm of posterior nasal spine impaction, respectively ( P = 0.002), and 1.58 (interquartile range, 0.78) and 1.95 (interquartile range, 1.05) mm per 1 degree of clockwise rotation of the palatal angle, respectively ( P = 0.007). The convexity of the A point was improved without any significant change in the sella-nasion-A point angle before and after surgery. CONCLUSIONS: This article addresses the scientific evidence regarding the impact of clockwise rotational orthognathic surgery based on the gearing effect. The mandibular setback turned out to be more effective in patients with a long LAFH. CLINICAL QUESTION/LEVEL OF EVIDENCE: Risk, II.