The longevity of the fixation of implants in a formerly dysplastic hip is compromised by several risk factors:
Young age. Severity of the dislocation. Previous surgery. Hip arthroplasty after a previous intertrochanteric osteotomy is technically more demanding but not necessarily associated with a higher rate of complications. Distortion of the acetabulum. Fixation of the socket in a dysplastic hip joint acetabulum (one of the main aims of a THR) is compromised both by using a small implant and an insufficient containment of the socket in the bony acetabulum. Small cups (small implant/bone contact area, thin polyethylene wall). Small cups are especially used in cases where the implant must be positioned higher up in the iliac bone. High hip center and lateral placement of the cup. A high hip center is not to be considered as a risk factor as long as there is no simultaneous lateralisation of the cup. Upward displacement of the center of rotation must be compensated for by changing muscle length and the arms of the abductors with a longer neck in order to preserve muscle power. The acetabular component, i.e., the center of rotation of the hip articulation should be positioned as medially as possible. Insufficient containment of the acetabular socket. As a rule, the positioning of the socket into the original acetabulum creates normal mechanics of the hip and provides the best bone stock for fixation of the cup, especially in complete dislocations. However, placement of the cup into the original acetabulum of a subluxated femoral head in an angle that is not too vertical leaves a supero-lateral void. Enlargement, i.e., reinforcement of the roof of the acetabulum with screws and bone cement has not proven to be adequate. Acetabuloplasty, i.e., grafting with an autologous cortico-cancellous graft taken from the resected femoral head or using an acetabular reinforcement ring (ARR) is indicated if 20 and more degrees of the weight-bearing surface of the cup would otherwise remain uncovered. Massive cortico-cancellous bone grafts. The use of bulky autologous or homologous cortico-cancellous grafts which would be loaded over 50% or more of the weight-bearing surface of the cup is not recommended. Excessive anteversion, narrow medullary cavity, and capsular contractures on the femoral side. The most typical deformity of the proximal end of the femur in hip dysplasia is an excessive anteversion angle of the neck of the femur. Anteversion angles of 45 degrees and more are corrected by a derotational osteotomy of the femur. To avoid overlength of the leg by positioning the cup into the original acetabulum, a subtrochanteric shortening osteotomy may be indicated. Preoperative planning is mandatory. Procedure, choice of method, and availability of appropriate equipment and endoprosthetic implants must be ensured. Computerised tomography with 3-D reconstruction is recommended for more complex anatomical situations.
