Gluteus minimus-induced femoral head deformation in dysplasia of the hip.

Lateral notching of the femoral head is considered pathognomonic for spastic subluxation of the hip. Less frequently, flattening is seen with extrusion of the femoral head in nonspastic hip dysplasia. The aim of this study was to throw light on its underlying pathomechanism. On the radiographs of 297 hips with developmental dysplasia, lateral flattening of the femoral head was seen in 18 hips (6%), but notching was present in only 1. Of 7 dysplasias due to cerebral palsy, 6 showed lateral notching. The gluteus minimus was felt to be responsible for the lateral femoral head changes as the muscle counteracts lateral migration of the femoral head. Intraoperative dissection of 3 hips supported this view. 1 hip with developmental dysplasia and lateral notching was subjected to a periacetabular osteotomy. At surgery, the tendon of the gluteus minimus was found to fit tightly into the notch. Of 2 hips with spastic dysplasia, 1 presented with and the other without lateral notching. In the hip with lateral notching, the gluteus minimus had a normal appearance and it lay in the defect of the femoral head. In the hip without notching, the gluteus minimus was atrophied with signs of fatty degeneration. We therefore believe that lateral notching is a sign of hypertonicity of the gluteus minimus muscle.