Abstract
Metal-on-metal (MOM) retrieval studies have demonstrated that CoCr bearings used in total hip arthroplasty (THA) and resurfacing (RSA) featured stripe wear damage on heads, likely created by rim impact with CoCr cups.1-3 Such subluxation damage may release quantities of large CoCr particles that would provoke aggressive 3rd-body wear. With RSA, the natural femoral neck reduces the head-neck ratio but avoids risk of metal-to-metal impingement (Fig. 1).4 For this study, twelve retrieved RSA were compared to 12 THA (Table 1), evaluating, (i) patterns of habitual wear, (ii) stripe-wear damage and (iii) 3rd-body abrasive scratches. Considering RSA have head/neck ratios much lower than large-diameter THA, any impingement damage should be uniquely positioned on the heads.
Twelve RSA and THA retrievals were selected with respect to similar diameter range and vendors with follow-up ranging typically 1–6 years (Table 1). Patterns of habitual wear were mapped to determine position in vivo. Stripe damage was mapped at three sites: polar, equatorial and basal. Wear patterns were examined using SEM and white light interferometry (WLI). Graphical models characterized the complex geometry of the natural femoral neck in coronal and sagittal planes and provided RSA head-neck ratios.4
Normal area patterns of habitual wear were similar on RSA and THA bearings. The wear patterns showing cup rim-breakout proved larger for RSA cups than THA. Polar stripes presented in juxtaposition to the polar axis in both RSA and THA (Fig. 1). As anticipated, basal stripes on RSA occurred at steeper cup-impingement angles (CIA) than THA. The micro-topography of stripe damage was similar on both RSA and THA heads. Some scratches were illustrative of 3rd-body wear featuring raised lips, punctuated terminuses, and crater-like depressions (Fig. 2).
Neck narrowing observed following RSA procedures may be a consequence of impingement and subluxation due to the small head-neck ratios. However, lacking a metal femoral neck, such RSA impingement would not result in metal debris being released. Nevertheless it has been suggested that cup-to-head impingement produced large CoCr particles and also cup “edge wear” as the head orbits the cup rim.4 Our study showed that impingement had occurred as evidenced by the polar stripes and 3rd-body wear by large hard particles as evidenced by the wide scratches with raised lips. We can therefore agree with the prior study, that 2-body and 3rd-body wear mechanisms were present in both RSA and THA retrievals.
