We have studied the kinematics of the knee in the sagittal plane, using a four-bar linkage as model, and assuming that a "neutral fibre" in each ligament remains isometric throughout flexion. We devised a computer program to calculate the distance separating any pair of points, one on each bone, for various cruciate attachments at various angles of flexion. The parameters for the linkage in four cadaveric knees were obtained by marking the centre of attachment of the cruciate ligaments with tacks and taking lateral radiographs. The movements of the bones were then calculated, in the computer model, for various attachments of "replacement" ligament fibres, the distance between the attachment sites being plotted against the angle of flexion. It was then possible to define zones around the isometric attachment points within which changes in length would be predictable. Our results show that the position of the femoral sites of attachment of both anterior and posterior cruciate replacement was more critical than that of the tibial attachments.

Thirty-two neglected congenital dislocations of the hip in twenty-two children over the age of six years were treated by traction, open reduction and Chiari osteotomy. In five hips, where prolonged traction failed to bring the femoral heads into the vicinity of the acetabulum, a shortening subtrochanteric osteotomy of the femur was performed. The overall results were good and this regime of treatment is recommended.