Radiographic measures of hip dysplasia correlate only moderately with computationally derived contact mechanics

Hip dysplasia is currently diagnosed with two-dimensional radiographic measurements that may not adequately describe the aetiology of pain and dysfunction associated with dysplastic deformity and future osteoarthritis progression. This study aimed to assess how accurately two-dimensional radiographic measurements represent the three-dimensional mechanical joint environment. Spearman ρ correlation was used to determine associations between radiographic measurements of hip dysplasia made on pre-operative and 6-month post-operative standing AP pelvis radiographs, and hip contact stress calculated based on pre- and post-operative pelvis CT scans for a series of 100 patients (125 dysplastic hips) treated with periacetabular osteotomy using discrete element analysis. Only moderate correlations were observed between radiographic measurements and contact mechanics. Pre-operative lateral centre edge angle of Wiberg had the highest, though moderate, correlation with mean cumulative contact stress-time exposure (ρ = -0.5112, P < .0001). Tönnis angle (ρ = 0.4898, P < .0001) and extrusion index (ρ = 0.4579, P < .0001) also had moderate correlations with pre-operative mean cumulative contact stress-time exposure. Only weak correlations were found post-operatively and when comparing change in radiographic measurements to computational contact mechanics metrics before and after surgery. These findings indicate the mechanical environment of the hip joint is more complex than simple radiographic measurements that quantify coverage of the femoral head by the acetabulum. Evaluation of three-dimensional contact mechanics in dysplastic hips should be used in combination with two-dimensional radiographic evaluation for guiding clinical care for patients with hip dysplasia.