From pocket to profile: Scale-free pelvic sex estimation from 3D photogrammetry

BACKGROUND: Biological sex estimation is a cornerstone of the forensic biological profile, and the adult pelvis remains the most informative skeletal region due to pronounced sexual dimorphism. At the same time, three-dimensional documentation of skeletal evidence is rapidly expanding beyond computed tomography and laboratory scanners toward inexpensive, accessible approaches such as photogrammetry-with mobile phone cameras. While the reliability of photogrammetry for osteological documentation has been supported, most established pelvic sexing approaches still assume measurements in absolute units or rely on expert scoring of morphological traits. METHODS: We propose and validate a scale-free workflow for sex estimation from unscaled 3D pelvic models. The final approach is based on within-specimen log-ratios of seven collected pelvic measurements rather than on absolute units. Model development used a pooled reference dataset of 2,321 known-sex os coxae from documented dry-bone and CT-derived sources, and external implementation testing targeted unscaled photogrammetric pelvic models acquired under realistic conditions (n = 103). RESULTS: In the Coimbra photogrammetric subset, the final scale-free model achieved 97.1% binary accuracy, 97.1% balanced accuracy, and an area under curve of 0.997. Under conservative strict thresholds, 90.3% of cases received a definitive classification, all of which were correct, while the remaining 9.7% were retained as indeterminate. CONCLUSIONS: Scale-free pelvic sex estimation supports the forensic use of unscaled photogrammetric 3D bone models in situations where reliable external calibration is unavailable. The method does not replace standard calibrated osteometry, but it provides a practical and defensible alternative for low-cost digital documentation, remote analysis, and triage-oriented forensic workflows.