Location of the femoral head in developmental dysplasia of the hip: three-dimensional evaluation by means of magnetic resonance image.

By using magnetic resonance imaging, we analyzed the position of the femoral head in 21 hips of 21 infants with developmental dislocation. The femoral head changed its location according to the hip position. In type A dislocation, the head was anterior to the acetabulum when the hip was extended, and it was posterior when the hip was flexed. There was wide contact area between the femoral head and the acetabulum in any hip position. In type B, the head was on the rim of the acetabulum when the hip was extended. When the hip was flexed, the head was on the posterior rim of the acetabulum, and there was no contact between the femoral head and acetabular cartilage. Some heads slipped into the socket with a click phenomenon when the hip was flexed/abducted. In type C dislocation, the head was on the edge of the acetabulum when the hip was extended. When the hip was flexed, the head moved posteriorly, and it was completely out of the socket.