Background: The standard procedure for the management of 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. Therefore, in this study, we aim to introduce the gearing effect to correct class III malocclusion in Asians by maxillary posterior impaction via clockwise rotation without advancing the maxilla.
Methods: Patients with class III correction with clockwise rotation of the maxillomandibular complex without maxillary advancement were included, while those with genioplasty were excluded. Various facial skeletal cephalometric landmarks were measured through 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 to those in the ANS‒PNS 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 PNS impaction, respectively (p=0.002), and 1.58(interquartile range(IQR) 0.78) and 1.95(IQR 1.05) mm per 1° clockwise rotation of palatal angle, respectively (p=0.007). The convexity of the A point was improved without any significant change in the SNA angle before and after surgery.
Conclusions: This article addressed the scientific evidence in impact of clockwise rotational orthognathic surgery based on the gearing effect. Considering the gearing effect, the mandibular setback turned out to be more effective in patients with a long LAFH.
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