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, investigator s have used routine annual radiographs made,vith the patient in the supine position in order to measure penetration by the femoral head into the polye thylene liner, However, researchers have begun to question the effect of we ight-bearing on the position of the head within the acetabular cup and, con sequently the effect of weight-bearing on measurements of penetration by th e head, The purpose of the current study was to determine the effect of wei ght-bearing on the two-dimensional radiographic position of the femoral hea d within the acetabular cup. Methods: Thirty-seven patients (forty-seven hips) who had had a total hip a rthroplasty had radiographs made at one of two separate institutions. A set of anteroposterior radiographs was made for each patient: one radiograph w as made,vith the patient supine and one was made with the patient upright b earing full weight on the replaced hip, At one of the institutions, a third anteroposterior radiograph was made with the patient in the same upright p osition but not bearing weight on the replaced hip. All measurements of the two-dimensional position of the head were performed by a single observer w ith use of a previously published computerized measurement system, Results: Data from both institutions revealed that measurements of the posi tion 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,vith the patient bearing weight (r(2) > 0.93, p < 0.001 for both regressions), Examination of the differences between the measurem ents revealed no bias for one set of measurements to consistently underesti mate or overestimate the values derived with use of the other method. Moreo ver, we found a nearly perfect relationship between the measurements of the position of the head on radiographs made,vith the patient standing and bea ring weight and those on radiographs made,vith the patient standing but not bearing weight (r(2) = 0.97, p < 0.001, slope = 0.99, intercept = 0.02 mil limeter), Conclusions: On the basis of these findings, there is no evidence that radi ographs must be made,vith the patient bearing weight in order to accurately measure the position of the femoral head within the polyethylene liner.