Changes of biomechanical parameters in dysplasia of the hip by total hip replacement

Aim: Dysplastic hips or dislocated hips lead to loss of muscle strength claudication, and finally to coxarthrosis. This study analytically compares the hip joint forces in normal, dysplastic hips and hips after implantation of a hip prosthesis for several positions. Method: The results of 173 total hip replacements in 153 cases of either severe congenital dysplasia or dislocation were analyzed. A 2 dimensional mathematical model of the hip was developed to evaluate the effects of surgically achievable mechanical alterations such as acetabular placement, femoral shaft-prosthetic neck angle, and neck length of the femoral prosthesis. Results: 63% of the hip prosthesis were implanted on the right, 72% on theleft. Before implantation, the femoral shaft-prosthetic neck angle in mean was 146degrees +/- 10degrees, after implantation it was 135degrees. The hip centre was displaced onin average 10 mm distally. Postoperatively hip joint force was increased 21%. Conclusion: Minimum joint contact forces occurred when the femoral shaft-prosthetic neck angles were small. The loads on the hip were lowered significantly by placing the centre of the acetabulum as far inferiorly and medially as possible. Another important finding is that displacement of the hip centre distally has a great effect on muscle performance and hip joint force.