Evaluation of antero-posterior instability of the hip using modified Lequesne's false profile view

Accurate evaluation of hip instability is critical for the diagnosis and successful treatment of developmental dysplasia of the hip (DDH). However, dynamic evaluation of hip instability is not well established. This study aimed to use the lateral view from a radiograph to evaluate dynamic antero-posterior hip instability in patients with DDH. Seventy-four patients (92 hips) with DDH (DDH group) and 46 patients (59 hips) without hip pain and DDH (Control group) were examined. A false profile view (FPV) according to Lequesne was taken at standard and 90° flexion with the hip of interest defined as functional FPV; the translation of the center of the femoral head (CFH) obtained from the functional FPV was measured. As a validation test, we measured the anterior translation of the CFH using ultrasonography (US). There was a significant difference between the two groups in the translation of the CFH ( p < 0.01). The degree of CFH translation depended on the severity of DDH (lateral center edge angle, r = −0.56, p < 0.01; vertical center anterior margin angle, r = −0.57, p< 0.01) and lateralization of the femoral head (head lateralization index, r = 0.54, p < 0.01). There was a significant correlation between functional FPV and US measurements ( r = 0.71, p < 0.01). The present study confirmed that antero-posterior hip instability in DDH patients can be detected using functional FPV. Our novel measurement, as a new method for assessing hip instability, may be useful for evaluating hip dynamic instability in diagnosing the etiology, and determining and evaluating the treatment for DDH at lower cost and improved accessibility.