TP9.4 Epiphyseal tubercle morphology is similar in SCFE and cam-type FAI: evidence for a shared morphological precursor?

Abstract Introduction Cam-type FAI and SCFE both exhibit alteration in proximal femoral epiphyseal morphology. Prior reports independently associate cam deformity and SCFE to smaller epiphyseal tubercles, yet a comparison of epiphyseal tubercle morphology between cam FAI and SCFE has not been reported, and no direct link between the two conditions have been established. We hypothesized that epiphyseal tubercle morphologies in SCFE and cam FAI are similar and pathologically smaller than in normal hips. Methods A retrospective analysis was performed on 54 patients with cam-type FAI and 56 patients with SCFE with CTs including pelvis and femur. 80 asymptomatic hips without radiographic pathology served as controls. A custom-developed and validated program (VirtualHip, Boston Children’s Hospital) was used to automatically segment bones, identify landmarks, define anatomical coordinate system based on ISB recommendations, and measure anatomical features. Primary epiphyseal outcomes were tubercle height, width, and length. We fit an ANCOVA (ordinary least squares) with Group (Control, FAI, SCFE) as the categorical factor, and age and sex as covariates. Pairwise comparisons were evaluated using Dunn tests, with significance levels corrected for multiple testing using the Bonferroni-Holm procedure. Results After adjusting for age and sex, both SCFE and cam-type FAI groups differed from controls for epiphyseal tubercle height and length, whereas no difference was found in tubercle width. Tubercle height and width were similar between SCFE and cam-type FAI. For tubercle height, adjusted contrasts showed smaller values in FAI and SCFE versus controls (FAI-control -4.21, 95%CI -6.39,-2.02, p<0.001; SCFE-control -3.69, 95%CI -5.46,-1.93, p<0.001). Tubercle length was reduced in both pathology groups relative to controls (FAI-control -20.35, 95%CI -24.19,-16.52, p<0.001; SCFE-control -13.39, 95%CI -16.49,-10.29, p<0.001). Tubercle width did was not different. Adjusted mean differences in epiphyseal height and width were not significantly different between FAI and SCFE. Discussion Epiphyseal tubercle morphology is similar in SCFE and cam-type FAI relative to normal hips, supporting the theory of a shared epiphyseal morphologic substrate. The diminutive nature of the epiphyseal tubercle may predispose the hip to increased physeal stress and micro-instability. A delicate balance between biomechanical loading and peri-physeal remodeling may bias towards SCFE when remodeling is insufficient, or toward cam morphology when remodeling sufficiently counters load. Further studies are necessary to better understand the developmental etiologies of SCFE and cam-type FAI. Significance/Conclusion Our data supports a common epiphyseal morphology in SCFE and cam-type FAI, which may help elucidate the etiologies of these conditions and motivate longitudinal studies in at-risk adolescents.