Two hundred and ninety-two patients under the age of 50 years, presenting with mechanical hip pain, were included in a prospective multicenter study. In 241 cases, imaging assessment included AP standing pelvic X-ray and Lequesne's false profile (LFP) and/or lateral neck (Ducroquet, Dunn or variant) hip X-ray. Cross-sectional arthroscan and/or arthro-MRI images were available in 81 cases. Exploration looked for acetabular and femoral head/neck dysplasia liable to induce cam or pincer anterior femoroacetabular impingement (AFAI), respectively. Labral and chondral lesions arise secondarily to hip osteoarthritis (HOA) and/or AFAI. Two-thirds of patients showed HOA. Only 6% showed a strictly normal aspect on imaging. More than half (52%) of cases had cam AFAI, half of these involving an osteophytic neck, associated in more than 90% of cases with large multifocal bone spurs of the head, neck and acetabula. These cases were considered ambiguous, due to the uncertainty as to the congenital nature of the cervico-cephalic dysmorphy; if they are excluded, only 23% of the series involved cam AFAI. Crossover sign on AP standing pelvic X-ray is the best assessment criterion for acetabular retroversion, the most frequent form of acetabular dysplasia underlying pincer AFAI, and should be explored for. Secondary neck lesions were visible only on lateral neck view in 42% of cases: this view should be included in standard radiologic work-up in under-50 year-olds. The alpha angle can be measured on this type of lateral view and on axial arthroscan and arthro-MR images; more than half of the cases in which it was pathological involved an osteophytic neck and thus a pseudo-cam effect.
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