Predicting wear and blood metal ion levels in metal‐on‐metal hip resurfacing

Abstract

Suboptimal component position and design are thought to lead to edge wear and raised blood metal ion levels in metal-onmetal hip resurfacing (MOM-HR). These factors are thought to influence the "contact patch to rim distance" (CPRD), and calculation of this distance may improve prediction of wear and blood metal ion levels. We measured blood cobalt and chromium ion levels and the wear rates of the bearing surfaces in 165 MOM-HR retrieval cases. We then determined the contribution and effect sizes of cup inclination and version angles, component size and design, and CPRD (calculated from case-specific data) on blood metal ion levels and component wear rates. Acetabular orientation explained between 16.3% and 28.5% of the variation in wear rates and metal ion levels, whereas component size and design explained between 7.3% and 21.8% of the variability. In comparison, CPRD explained up to 67.7% of the variability, significantly greater than any other variable (all p < 0.0001). CPRD is a good predictor of wear and improves our understanding of wear performance and the mechanisms leading to edge loading. Keywords: metal-on-metal; hip resurfacing; retrievals; wear; metal ions High revision rates have been widely reported for metal-on-metal hip resurfacing (MOM-HR), 1 and subsequently the use of MOM has become a major public health concern subject to widespread coverage in both the scientific and lay press. High revision rates have been partly attributed to the release of metal debris from the bearing surfaces. High wear, occurring as a result of edge loading, has been associated with devastating inflammatory soft tissue reactions. 2-4These patients have poorer outcomes after revision surgery. 5Previous studies reported a significant association between cup inclination angle and both wear rates and blood metal ion levels.6-8 However, the correlations with cup inclination are only weakly positive and therefore can not alone explain all the variation in wear rates and ion levels. 6,8 Indeed, edge loading in MOM-HR is likely attributable not only to component position, but also component design 9,10 and patient activity patterns. 11The Articular Surface Replacement (ASR; DePuy Orthopaedics) is widely recognized as having the poorest clinical and wear performance of the contemporary MOM-HR designs. 1,12,13 Design features such as the reduced "arc of cover" (the angle subtended by the articular surface of the cup) and head-cup clearance are thought to increase the likelihood of edge loading and high wear.12,13 An overview of the design variations among manufacturers is shown in Table 1.Many of the variables that affect wear, both surgical and design, likely do so by moving the contact area between the femoral and acetabular bearing surfaces closer to the cup rim. This distance has been described in the literature , and in this study we have termed it the 'contact patch to rim distance' (CPRD). As shown by the two-dimensional schematic (Fig. 1), a reduction in this distance increase the likelihood of edge loading a...

Influence of implant design on blood metal ion concentrations in metal-on-metal total hip replacement patients

“…Linear regression and t tests were performed to investigate relationships between the base 10 logarithm of blood metal ion concentrations and the different patient, surgeon and implant covariates. Similar to previous studies [25,34,35], the logarithm was necessary to transform the asymmetric blood metal ion concentration distributions so that the residuals in the regression models were more normally distributed. Adjusted R 2 was also calculated to give the percentage of variance in the logarithm blood metal ion concentrations explained by the models.…”

Patient Specific Component Alignment in Total Hip Arthroplasty

“…The simulation calculates the magnitude and direction of the hip joint reaction force throughout the two activities and determines the path of the contact patch [13,14] as it traces across the articulating surface, Fig 3. These contact patch paths are presented in a polar plot that represents the bearing surface in two dimensions viewed perpendicular to the face of the cup. The polar plots are generated for nine different cup orientations, defined in angles of radiographic inclination and anteversion [15], Fig 4.…”

Management of metal-on-metal hip implant patients: Who, when and how to revise?

“…Following revision of a MOM implant to an alternative bearing, blood ion levels reduce but do not normalise in the postoperative period [19,20] . Component design and positioning has been shown to be associated with increased wear and as a result raised metal ion levels [21][22][23][24][25] . Blood cobalt and chromium ion levels in patients with unexplained painful MOM hips are double those of well-functioning MOM hips [13] .…”