Comparison of predicted and measured contact pressures in normal and dysplastic hips

Hip dysplasia, a congenital and developmental deformity characterized by malorientation and a reduction of contact area between the femur and acetabulum, is the most common cause of osteoarthritis of the hip. According to current estimates, dysplasia accounts for nearly 76% of all cases of osteoarthritis, and many who are affected require a total hip replacement before the age of 50. It is theorized that in the poorly oriented and deformed pelvis, a reduction in contact area leads to an increase in contact pressure during normal activities. Currently, clinicians attempt to reposition the joint, assuming that improving the position of the existing contact surface will lead to decreased pressures. It is also assumed that improving certain geometric parameters correlates indirectly with decreased contact pressures. Neither these simple estimates nor other non-invasive models have ever been shown to be related to contact pressure. The purpose of this study was to evaluate a computerized method of predicting hip joint contact pressures, which applies known hip joint reaction forces to the three-dimensional surface of the hip joint. To this end, cadaveric and plastic pelvic models were developed to test whether the computer model could predict the magnitude and location of maximum pressure. Mechanical testing revealed that the computer model could be used to predict pressure in cadaveric pelves at prescribed locations (r2 = 0.64). The computerized model could also be used to predict the magnitude and location of maximum pressure in a series of plastic models where the load vector and the degree of dysplasia were parametrically varied (r2 = 0.7). These findings suggest that the computer model may be useful in identifying patients who will fail osteotomy or whether they can be used to select the best osteotomy for each patient.