EP1.36 Preventing Femoral Head Collapse in Perthes Disease Early MRI-Guided Risk Stratification and Arthrodiastasis During the Fragmentation Phase

Abstract Introduction The fragmentation phase of Perthes disease is the period of greatest vulnerability to the femoral epiphysis but also the key window for intervention. Radiographic prognostic systems are widely used, but definitive classification often cannot be assigned until collapse has occurred. We evaluated a treatment pathway combining subtraction MRI (SMRI) for early risk stratification, standardized hip arthrodiastasis, and longitudinal geometric radiographic analysis to determine whether progressive deformation could be prevented. Methods We performed a retrospective longitudinal cohort study of 23 unilateral Perthes hips treated with a standardized arthrodiastasis-based joint preservation protocol. Pre-operative SMRI was used to predict lateral pillar severity and support treatment selection before definitive radiographic collapse classification. Serial calibrated anteroposterior pelvic radiographs were analysed using contour-based geometric methods implemented in custom Python software. Continuous rather than categorical or threshold measures were used to quantify femoral epiphyseal structural integrity, sphericity, and containment. Longitudinal change from baseline to early and final follow-up was assessed using mixed-effects modelling. Associations between containment and sphericity were further examined using correlation and multivariable regression. Results Mean age at baseline was 7.17 ± 2.33 years, and 87% of hips were predicted lateral pillar C. Following treatment initiation, femoral epiphyseal structural integrity did not show progressive decline. Total epiphyseal area under the contour increased by +57.8 mm2 at early follow-up and +386.8 mm2 at final follow-up relative to baseline. In contrast, the course of femoral head sphericity was variable, with different hips showing different patterns of change over time. Increasing extrusion was strongly associated with worsening overall and lateral sphericity (ρ = 0.81 and 0.79, respectively). On multivariable analysis, worsening extrusion remained the dominant independent predictor of sphericity deterioration after adjustment for epiphyseal height change. Conclusion In this Perthes cohort, an SMRI-guided arthrodiastasis pathway was associated with preservation of femoral head structural height during the fragmentation phase, suggesting protection against progressive collapse at the cohort level. However, preservation of height alone did not ensure spherical remodelling, as worsening containment was closely linked to loss of femoral head shape. These findings support earlier identification of high-risk hips and indicate that containment and shape, rather than collapse alone, should be central targets in Perthes hip preservation.