The midlumbar myelomeningocele hip: mechanism of dislocation and treatment.

We define the "bowstring" force of the iliopsoas, the force applied to the femoral head as the tendon angles across the hip joint, which we believe is an important cause of dislocation of the hip in patients with a midlumbar myelomeningocele. An operative procedure consisting of iliopsoas recession and suture of its tendon to the anterolateral hip joint capsule has been developed and used in 10 dislocated and 9 subluxated hips. The procedure was performed on patients with an average age of 6.6 months; the average follow-up was 45 months. Arthrography demonstrated the subluxation pattern of Leveuf in 17 hips. Surgical observations included: (a) flattening of the femoral head beneath the iliopsoas, (b) "bowstringing" of the iliopsoas across the hip joint, (c) posterolateral displacement of the femoral head with hip extension, (d) apparent increase in leg length after release of the iliopsoas, and (e) decrease of hip flexion contracture after iliopsoas release. Following iliopsoas recession alone, 11 hips were stable; 7 had subluxation; 1 was dislocatable. Secondary varus derotation osteotomy for valgus and anteversion was performed on five hips with subluxation; each was stable at review. A combined varus derotation osteotomy and Chiari osteotomy was performed on the one dislocatable hip that remained dislocatable. Therefore, at review 16 of 19 hips were stable, two have subluxation, and one was dislocatable. Early surgical treatment to prevent secondary adaptive changes in the hip is recommended. Additional surgery to correct retained valgus and anteversion is frequently necessary to achieve stability.