Anthropometric Analysis of Frontal and Orbital Dimorphism: Quantifying Population Averages and Variability in Cis-Gender Men and Women

SDCC 

Purpose: Surgical contouring of the frontal bone and orbital rims are foundational components of facial feminization surgery, which can have a transformative impact on gender identity and quality of life in patients. There is minimal evidence with which an individual patient's anatomy can be objectively placed in the context of natural morphologic variations within a population. As a result, identifying surgical candidates and defining the optimal morphologic endpoints of intervention remain subjective decisions. Borrowing from the field of biological anthropology, this study uses three-dimensional geometric morphometrics to: (1) compare skeletal morphology of the frontal and orbital regions between cis-gender men and women, and (2) measure population averages and variability within genders.
Methods: We retrospectively reviewed adults who underwent computerized tomography (CT) imaging of the cranium prior to orthognathic surgery. Patients with a history of craniofacial trauma, genetic syndromes, metabolic growth disorders, or exogenous hormone exposure were excluded. A standardized set of 501 digital landmarks was used to capture surface morphometrics of the forehead, supraorbital ridges, and orbital rims. All landmarked crania were aligned using Generalized Procrustes Analysis to assess for differences in shape without the influence of scale, and principal component analysis evaluated the contribution of gender to the overall variance in coordinates. Using Robust Rank Aggregation, a ranked list of important landmarks most predictive of gender was generated after integrating the relative and absolute differences in male and female mean values, linear discriminant analysis weights and magnitudes, and random forest feature importance scores.
Results: A total of 154 patients were analyzed, including 70 (45%) males and 84 (55%) females who underwent imaging at a mean age of 23.6±7.7 and 26.0±11.1 years, respectively. A ranked list of 119 important features with significant aggregate rank scores (p<0.05) demonstrated that the highest degree of inter-gender variability concentrated around the following landmarks: (1) the glabella and adjacent points between the brows, (2) the orbitale and adjacent points along the lateral and infraorbital rim, and (3) the supraorbital notch and adjacent points along the medial supraorbital rim. Landmarks along the lateral infraorbital demonstrated the greatest absolute differences between male and female average values, with females displaying a 2.23 mm greater projection in the anteroposterior dimension. The degree of morphologic variability within genders was similar in the male and female populations.
Conclusions: This study applies anthropometric methods of shape analysis to create a normative anatomical map with which skeletal dimorphisms can be quantitatively studied at the population level. An analysis of frontal and orbital dimorphism found the greatest degree of inter-gender variability surrounding the glabella, lateral infraorbital rim, and medial supraorbital rim, which may help inform preoperative evaluation and planning for feminizing procedures of the upper facial third.

Gender Affirmation

PSTM 2024