Abstract
Performance of metal-on-metal (MOM) bearings was of great interest until recently. Major concerns emerged over high incidence of MOM-wear failures and initially there appeared greater risks with MOM total hip arthroplasty (THA) designs compared to resurfacing arthroplasty (RSA). Impingement of the metal neck against the THA cup was likely the differentiating risk. There is a major difference between RSA and THA in (i) size of femoral necks and (ii) risk of THA metal necks impinging on metal cups. For example, a 46mm THA with 12.5mm neck, a 3.68 head:neck (H/N) ratio, provides a suitably large range-of-motion (ROM). In contrast, an RSA patient with retained 31mm size of natural neck would only have H/N = 1.48, indicating even less ROM than a Charnley THA. However, the enigma is that RSA patients have as good or better ROM in majority of clinical studies. We studied this apparent RSA vs THA dilemma by examining MOM retrievals for signs of adverse impingement. We previously described CoCr stripe wear in failed THA bearings, notably alignment of polar and basal wear stripes coincident with the rim profiles of the cups (Clarke 2013). Our governing hypothesis was that RSA patients had to routinely sublux their hips to get ROM comparable to THA. Our THA impingement studies showed polar stripes within 15o of the polar axis in large heads. For the various RSA diameters, we calculated that wear stripes angled 40o from the femoral axis could indicate impingement with no subluxation, whereas smaller angles would indicate routine subluxation of RSA femoral-shell from cup. We compared explanted RSA (N=15) and THA (N=15) bearings representing three vendors (42–54mm diameters). Wear maps and head-stripes were ink-marked for visualization, photography, and analysis. Wear areas were calculated using spherical equations and wear-stripe angles measured by computer graphics.
The results showed that RSA femoral shells had wear areas circular in shape with areas varying 1,085- 3,121mm2. These averaged 14% larger than in matched THA heads but statistically significant difference was not proven. Polar stripes were readily identifiable on femoral components, 75% for RSA cases and 100% for THA. These contained identical linear scratches and all were sited within 30o of neck axis, confirming our hypothesis that RSA patients had to sublux their hips to achieve same motion as THA. Examination of cup wear areas revealed all showed ‘edge-loading’, but RSA cups had a significantly greater degree.
Retrieval studies are limited by uncontrolled case sources, varied brands, and small numbers. In this study, we were able to match RSA and THA cases by vendor and diameter. The RSA retrievals revealed polar stripes identical to THA by site, topography and inclination to femoral-neck axis. This confirmed our starting hypothesis and explained the large clinical ROM available in RSA patients. The larger wear areas on RSA femoral shells, although not statistically significant, and the larger ‘edge loading’ sites in RSA cups appeared as further support for routine subluxation of femoral-shells during hip impingement.
