EP1.55 Combined version better predicts femoral head coverage and range of motion in simulated hip models

Abstract Introduction Combined version (CV), the sum of acetabular and femoral version, informs surgical decision making in various hip conditions and treatments, including femoroacetabular impingement (FAI) and arthroplasty, yet its application in open hip preservation surgery is limited. Periacetabular osteotomy (PAO) remains the mainstay treatment for hip dysplasia, but the influence of femoral version on PAO long-term outcomes is poorly understood. We hypothesized that CV better predicts femoral head coverage (FHC) and impingement-free range of motion (ROM) better than acetabular version (FV) or femoral version (FV) alone. Methods Using a validated hip modeling program (VirtualHip, Boston Children’s Hospital), we systematically generated 49 hip models in which LCEA and Tonnis angles were held constant while AV and FV angles were each varied from -10o to 50o in 10o increments, representing every permutation within this range. FHC was measured in the standing position. Impingement-free internal rotation (IR) with hip flexion to 90o and external rotation (ER) with hip extension were calculated. Linear and quadratic regression analyses assessed associations between FHC and ROM with AV, FV, and CV. Results CV demonstrated stronger associations with FHC and ROM than AV or FV alone. FHC was better represented as a quadratic function of CV (r2=0.702) than AV (r2=0.083) or FV (r2=0.459), and was maximized at CV 25-40o. IR with hip flexion increased with increasing CV, AV, and FV; CV showed the strongest linear correlation with IR (r2=0.952), outperforming FV (r2=0.595) and AV (r2=0.370). ER with hip extension decreased with increasing CV, AV, and FV, with CV again demonstrating the strongest association (r2=0.918) versus FV (r2=0.409) and AV (r2=0.303). Discussion In this parametric study with standardized LCEA and Tonnis angles (to isolate the effect of version), CV outperformed AV and FV alone in predicting FHC and ROM. FHC was optimized at CV 25-40o, and rotational ROM was strongly correlated with CV in both flexion and extension. These findings support CV as a clinically relevant surgical planning parameter. For patients undergoing PAO without femoral osteotomy, target AV should be individualized to the patient’s native FV to optimize postoperative CV, FHC, and ROM. CT-based planning or intraoperative navigation should be considered to enable precise acetabular fragment correction. Future work is focused on developing a decision tree algorithm to assist with treatment choice based on CV in addition to other morphological and patient-related factors.