The effect of weight-bearing on the radiographic measurement of the position of the femoral head after total hip arthroplasty.

BACKGROUND: In most radiographic studies of polyethylene wear, investigators have used routine annual radiographs made with the patient in the supine position in order to measure penetration by the femoral head into the polyethylene liner. However, researchers have begun to question the effect of weight-bearing on the position of the head within the acetabular cup and, consequently, the effect of weight-bearing on measurements of penetration by the head. The purpose of the current study was to determine the effect of weight-bearing on the two-dimensional radiographic position of the femoral head within the acetabular cup. METHODS: Thirty-seven patients (forty-seven hips) who had had a total hip arthroplasty had radiographs made at one of two separate institutions. A set of anteroposterior radiographs was made for each patient: one radiograph was made with the patient supine and one was made with the patient upright bearing full weight on the replaced hip. At one of the institutions, a third anteroposterior radiograph was made with the patient in the same upright position but not bearing weight on the replaced hip. All measurements of the two-dimensional position of the head were performed by a single observer with use of a previously published computerized measurement system. RESULTS: Data from both institutions revealed that measurements of the position of the head on radiographs made with the patient supine were strongly and significantly correlated with measurements of the position of the head on radiographs made with the patient bearing weight (r2>0.93, p<0.001 for both regressions). Examination of the differences between the measurements revealed no bias for one set of measurements to consistently underestimate or overestimate the values derived with use of the other method. Moreover, we found a nearly perfect relationship between the measurements of the position of the head on radiographs made with the patient standing and bearing weight and those on radiographs made with the patient standing but not bearing weight (r2 = 0.97, p<0.001, slope = 0.99, intercept = 0.02 millimeter). CONCLUSIONS: On the basis of these findings, there is no evidence that radiographs must be made with the patient bearing weight in order to accurately measure the position of the femoral head within the polyethylene liner.