The relationship of wear volume with histological features in failed hip resurfacing arthroplasty: An insight on failures associated with pseudotumours

INTRODUCTION: Pseudotumours around metal-on-metal hip resurfacing arthroplasties (MoMHRA) have caused increased concern. Although pseudotumours are associated with increased prosthesis wear, a few studies have suggested that they are not always associated with highly worn prostheses. This has led to the suggestion that the extensive necrosis and tissue destruction seen results not from the cytotoxic effect of the large number of metal particles on macrophages, which have phagocytosed these particles, but from a hypersensitivity response to one or more metal wear particle components. In keeping with this hypothesis, a prominent perivascular lymphoid infiltrate, commonly termed ALVAL, is frequently seen. Whether pseudotumours are due to wear particle-induced cytotoxicity, hypersensitivity or both remains to be determined.
OBJECTIVES: The aim of this study was to correlate histopathological changes in periprosthetic tissues with clinical findings and the amount of implant-derived metal wear.
METHODS: We analysed morphological changes in periprosthetic soft tissues of 56 failed MoMHRAs, all of which had bearing surface wear measurements. The majority of revisions were in female hips and the mean age at primary MoMHRA was 56 years. The mean implant survival was 4.7 years. The majority of retrieved implants were BHR and Conserve Plus. The commonest cause of revision was the presence of a symptomatic pseudotumour (n=45). Other failures included fracture, impingement, unexplained pain, heterotopic ossification and cup loosening. The extent of tissue necrosis and the nature of the inflammatory cell infiltrate, including ALVAL, was evaluated semiquantitatively. Furthermore, tissues were graded using the ALVAL-score described by Campbell et al. Bearing surface wear was determined for all patients using RedLux. Prostheses were considered to be highly worn if total linear wear rate (TLWR) was 4μm/yr. All assessors were blinded to mode of failure and other measurements.
RESULTS: Substantial necrosis and a heavy macrophage infiltrate were noted in most MoMHRA periprosthetic tissues, including all pseudotumours, many of which contained a significant ALVAL infiltrate. Most pseudotumours were associated with highly worn prostheses (80%). The extent of necrosis and macrophage infiltration correlated with the amount of prosthesis wear. Wear weakly correlated with the amount of ALVAL measured quantitatively and with higher ALVAL scores as per Campbell. All pseudotumours with low wear had a strong ALVAL response.
CONCLUSION: The majority of pseudotumours are associated with increased wear. This increased wear is associated with necrosis and a heavy non-specific foreign body macrophage response coupled with a variable specific immune response (ALVAL). A minority of pseudotumours are associated with low wear and a significant immune response. We hypothesise that the amount of wear required to trigger an ALVAL response varies per individual; as the wear volume increases, the chance of exceeding the individual’s threshold and triggering a response also increases. Surgeons should aim to minimise wear as this would lead to a reduction in pseudotumour incidence. However, even under optimal conditions some pseudotumours can still occur due to an exacerbated immune response.