Not All Cams Are Equal: MRI-Based Statistical Shape Modeling Reveals High Variability in Cam Shape in a Large FAI...

Evidence indicates that alpha angle (AA) is inadequate for describing cam morphology, contributing to poor patient selection and clinical outcomes for cam-type femoroacetabular impingement syndrome (FAI). While techniques such as statistical shape modeling (SSM) have advanced knowledge of 3D femoral morphology, many studies utilize computed tomography, which incurs ionizing radiation, or small sample sizes, limiting clinical implementation and generalization. Here, we conducted SSM using MRI-derived 3D femur models from a large cohort of FAI patients to explore: (1) What proximal femur features drive shape variability? (2) What variability exists within the area of the cam deformity? (3) Are shape modes correlated to AA, age, sex, and BMI? 103 FAI patients were enrolled (µ ± σ): Age (29.9 ± 12.1 Years), BMI (24.6 ± 4.8), AA (60.3° ± 9.8), Female/Male (75/28). Shape modes were determined through principal component analysis. Relationships between shape modes and AA, age, and BMI were evaluated through linear regression. Sex differences were analyzed using linear discriminant analysis. Thirteen modes explained 91.0% of femoral shape variance, with the first six modes explaining 82.2%, four of which were associated with the cam deformity region. No relationship was found between any mode versus AA, BMI, or age. Sex-based shape differences were primarily captured by the first mode. CLINICAL SIGNIFICANCE: The proximal femur in cam-type FAI patients demonstrated substantial regional shape variability, with cam-related morphology distributed across multiple independent modes rather than a single uniform deformity pattern. Sex was a major driver of proximal femoral shape variation. These findings may inform morphology-based classification, outcome analysis, and patient-specific surgical planning.