The effect of joint loading on acetabular wear measurement in total hip arthroplasty.

All radiographic calculations of acetabular wear assume concentric reduction of the prosthetic articulation. To date, no studies have shown that the femoral head is fully reduced on standard radiographs, and we have seen cases on early postoperative radiographs in which this assumption is not met. Using our computerized radiographic technique, 78 paired anteroposterior pelvic radiographs in 46 patients at a mean of 14 months after surgery (range, 1-92 months) were evaluated with and without joint loading. Displacement with loading was analyzed against time since surgery, a surrogate for acetabular wear. Regression analysis found a statistically significant increase in femoral head displacement after loading with longer duration of follow-up, but the rate of this increase was small (0.027 mm/y). This difference affects calculated wear values by <15%. We conclude that in a low-wear cohort, joint loading does not affect radiographic calculations of acetabular polyethylene wear in a clinically important way.