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

Abstract

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.

Correction of radiographic measurements of acetabular cup wear for variations in pelvis orientation

“…A true wear penetration of 0.4 mm, for instance, would be reduced by approximately 0.02 mm (same settings as above), which is below the accuracy of the best radiographic wear measurement systems. 26 In their theoretical study of a standard cup, Foss et al 8 also concluded that wear measurement errors caused by changes in pelvis orientation were likely to be small compared to other error sources. Notably, though, they did not determine wear from two, sequential displacement vectors (reference and follow-up) using the duo-radiographic technique.…”

“…Using a computer generated ‘virtual radiographic laboratory’, Foss et al 8 studied the effects of variations in pelvis orientation on the virtual wear measurement of an acetabular cup and concluded that measurement errors were likely to be small. Most modern measurement systems use the duo-radiographic wear measurement technique 9,10 in which wear is determined by comparing the two-dimensional (2D) displacement vector (usually between the cup and head centres) on a follow-up radiograph to that on a reference (e.g.…”

Analysis of polyethylene wear in plain radiographs

“…The accuracy and precision of clinical PE wear estimates depend on several variables, including patient factors (Schmalzried and Huk 2004), radiographic quality (Sychterz et al 2001), assumptions of linear wear patterns (Yamaguchi et al 1999), hip angulations (Collier et al 2003, Foss et al 2008), the wear analysis method used (Bragdon et al 2006a), intraobserver variance (Engh, Jr. et al 2002), and manufacturing tolerances of acetabular components (Hui et al 2003). Plain AP radiographs used for wear analysis are not calibrated (position coordinates), and in retrospective studies radiographs are often not obtained according to a standardized protocol.…”

“…Plain AP radiographs used for wear analysis are not calibrated (position coordinates), and in retrospective studies radiographs are often not obtained according to a standardized protocol. The clinical positioning of patients with the risk of slight changes in hip angulations between radiographic follow-ups has been shown experimentally to influence wear results (Collier et al 2003, Foss et al 2008). The greater the change in angulations between follow-ups, the larger the magnitude of wear measured (Collier et al 2003).…”

“…Previous studies have shown great variation in wear measurements for specific components, which may in part be caused by intraobserver variance, component and patient factors (Orishimo et al 2003), pelvic orientation (Collier et al 2003, Foss et al 2008), and the radiographic quality (Sychterz et al 2001). Furthermore, it is unlikely that different methods used to measure PE wear will agree exactly by giving identical results for all individuals (Bland and Altman 1986)—as has also been demonstrated in comparative studies (Collier et al 2003, Hui et al 2003, Bragdon et al 2006a).…”