X-ray transparency interpositions after reduction of traumatic dislocations of the hip in children.

Traumatic dislocations of the hip are much less frequent in children than in adults. If some complications are well known (avascular necrosis of the femoral head, recurrent dislocation), the lateralization of the head by x-ray transparency interposition is less appreciated. We report 4 cases of x-ray transparency interposition in 15 reductions of incomplete traumatic dislocation of the hip. These 4 dislocations were posterior in non-pathological hips, three on the left side and one on the right. Our reductions have been easy and the hips remained stable. However, the post-reduction roentgenogram shows the same unusual enlarged joint space corresponding to the lateralization of the femoral head. Four arthrotomies were performed, tracing a cartilaginous fragment free or fixed to the articular capsule into the femoral joint. We have found a concentric reduction of the hip in 4 cases and enlargement from 1 to 3 mm of the joint space of the hip. After several years, articular function of the four hips is normal, nevertheless, they have shown a coxa valga with a femoral elongation to 10 mm, a coxa magna with an articular space wider than 1 mm, 2 very good concentric reductions of the hip. The lateralization of the head after reduction had to be detected as soon as the first radiographic control because it can cause immediate instability or arthrosis of the hip. Because of the cartilaginous or capsular composition and the size of the interposition, an arthrotomy is performed in front of a radiological enlarging of the space joint, or a loss of parallelism of osteochondral acetabulum bone. Computerized tomography and MRI can reveal the nature, the place of the included fragment, as well as the surgical approach. An arthrogram is not absolutely indispensible. Surgical exploration should avoid degenerative arthrosis after a lateralisation left untreated.