A Quantitative Analysis of Sex-Related Differences of the Lower Facial Third.

BACKGROUND: Understanding sexual dimorphism of the lower facial third is critical to planning facial gender confirmation surgery (FGCS). PURPOSE: The purpose of this study was to evaluate differences in the angles and proportions of the lower facial third between sexes and to provide objective measurements of these anatomical relationships. STUDY DESIGN, SETTING, SAMPLE: A retrospective cross-sectional study was conducted at a tertiary academic site. Computed tomography (CT) scans from adult patients with anatomically normal maxillofacial structures were included. Patients with a history of craniofacial pathology, facial fractures, or prior FGCS were excluded. PREDICTOR VARIABLE: Sex, coded as male or female. MAIN OUTCOME VARIABLES: The primary outcome variables of three-dimensional cephalometric measures including linear distances (tragus-tragus, gonial-gonial, and genial width), the ratio of genial to mandibular width, and angular measurements (genial, gonial, and geniolabial angles). To manage multiple comparisons, the Benjamini-Hochberg procedure was applied to control the false discovery rate at 5%. COVARIATES: Covariates included age, race, and body mass index. ANALYSES: Analytic statistics were calculated to assess sex differences utilizing the Wilcoxon rank-sum test. A multivariable linear regression evaluated the independent effect of sex while adjusting for covariates. Statistical significance was set at P value < .05. RESULTS: The sample was composed of 50 patients with average the age of 50.2 (SD = 16.3) years, and 25 (50%) were male. Males had statistically significantly wider faces than females as measured by tragus to tragus (males = 159.5 (8.0) mm; females = 148.2 (6.2) mm; P < .001) and gonial angle to angle (males = 97.2 (6.1) mm; females = 89.5 (6.2) mm; P < .001). The gonial (males = 125.0° (7.0°); females = 129.8° (5.6°); P = .01) and genial angle (males = 136.8° (7.3°); females = 148.0° (6.0°); P < .001) were more acute in males. CONCLUSIONS AND RELEVANCE: Males exhibit statistically significantly wider jaws and more acute mandibular angles than females. These objective measurements of lower facial third sexual dimorphism can inform three-dimensional virtual surgical planning to improve patient outcomes and satisfaction.