The Charnley-Hastings bipolar prosthesis in femoral neck fractures - a study of dynamic motion

Fourteen patients who had a Charnley-Hastings bipolar prosthesis inserted for a fracture of the neck of the femur underwent a fluoroscopic assessment of the prosthesis while walking on a treadmill at an average of 24 months (range 8-48 months) post-insertion. This motion was compared with static weight-bearing and non-weight-bearing motion. The area of interest was the motion at the interprosthetic junction. Previous studies of this and similar prostheses had suggested that interprosthetic motion lessened with time, as the inner bearing stiffened up. However, in this study it was found that although the interprosthetic motion during non-weight-bearing abduction was in the region of 20-30% of total abduction, the interprosthetic motion during flexion and extension in gait accounted for 70-80% of motion. We suggest this can be explained by considering the biomechanical effect of sliding friction at the metal-acetabular bone junction during weight-bearing gait, as well as the impingement of the neck on the prosthetic socket during extremes of movement. The reported superior long term results of the bipolar prosthesis over its unipolar counterparts in femoral neck fracture treatment can be explained by the delayed acetabular wear due to reduced motion, and shear forces, at the prosthetic-bone interface.