Polyethylene acetabular wear in hip prostheses: computer-simulated quantification of error caused by changes in pelvic orientation and direction of wear.

Background And Purpose: Polyethylene is commonly employed for bearings in acetabular cups used in hip replacements. Assessment of in vivo wear is important for evaluation and monitoring of wear in individual patients, as well as in different implant designs. Polyethylene wear is quantified by comparisons of radiographic measurements made on sequential pelvic radiographs. Variations in pelvic orientation and variation in direction of wear may cause underestimation of polyethylene wear measurements. The purpose of this study was to quantify these effects on 2-dimensional measurements of polyethylene wear.

Methods: A computer program designed to simulate radiographs was employed to generate virtual radiographs of a virtual pelvis with a total hip replacement. Effects caused by variation in pelvic spatial orientation and variations in wear direction on wear measurements were analyzed separately. A Monte Carlo computational algorithm was employed to describe the combined effects of these two factors.

Results: Variation in pelvic orientation induced a mean underestimation of wear of 0.4% (0-2.6). Variation in direction of wear introduced a mean underestimation of 8.5% (0-42). A mean underestimation of wear of 9% (0-99) was found when varying both pelvic orientation and direction of wear simultaneously.

Interpretation: Errors caused by variations in pelvic orientation and wear direction are likely to be small compared to other sources of error when performing polyethylene wear measurements in acetabular components.