Determinants of Upper Lip Form: An Analysis Using Geometric Morphometrics

Abstract

Orthodontics and orthognathic surgery can alter an individual's facial form, and it is the clinician's goal to predictably control changes in the facial soft tissues to achieve treatment goals. Within the facial profile, controlling projection of certain structures, such as the chin, is predictable, as the soft-tissue response to hard-tissue changes in such areas is predictable. The upper lip, on the other hand, presents a different challenge. While multiple studies have shown relationships between advancement of the upper incisor and advancement of the soft-tissue lip, unlike the chin, the anterior position of the upper lip is only a small component of upper-lip profile esthetics. Given the structure of the upper lip, the angulation plays a prominent role in upper-lip profile esthetics besides just its anteroposterior projection. No direct or indirect relationships have been established between the maxillary incisal orientation and upper-lip angulation. The clinical significance in understanding the upper lip to hard-tissue relationship is to predictably control the upper-lip angulation in orthodontics and/or orthognathic surgery in patients with dentofacial deformities. In the absence of reliable data, planning often becomes overly subjective or arbitrary, which could lead to poor esthetic outcomes. To the author's knowledge, there are no studies that analyze the relationship between the hard tissues and the upper-lip angulation. This is presumably the case because of the difficulty in comparing the contours of the hard and soft tissues in these non-linear structures. Any “parallelism” cannot be reliably measured in a reliable or quantitative way using conventional cephalometric or other linear techniques.A retrospective cross-sectional study was designed to analyze cone beam computerized tomography (CBCT) scans from 104 patients, using geometric morphometrics. The contour of the hard tissue was evaluated by taking a point below anterior nasal spine (ANS) at the point of greatest convexity between ANS and maxillary alveolus, as well as the most anterior point of the maxillary alveolar bone between the central incisors. Once these points were identified, 3 semi-landmarks were taken at equidistant points between these in order to best trace the curvature of this area. For the soft-tissue analysis, subnasale and labrale superioris were identified and, once again, 3 semi-landmarks were taken at equidistant points between these landmarks to map the upper-lip curvature.The data were then compiled, giving a total of 1040 data points, 520 hard-tissue and 520 soft-tissue points. While the results of the principal components analysis and partial least squares are pending, the initial data show a positive correlation between contour of the alveolar bone and teeth and the upper-lip position and form.Based on these findings, it appears that the upper-lip contour in the dentofacial deformity patient can be predictably controlled by manipulating the position of the maxillary alveolus and incisor position via orthodontic and surgical means.