Tissue Reactions to Wear Products around Metal-On-Metal Hip Replacements

The causes of failure of metal-metal arthroplasties due to hypersensitivity reactions are not well understood. Unlike osteolysis that is induced by wear particles, hypersensitivity does not appear to depend on the amount of wear debris in the tissue. Rather, it is evident that hypersensitivity can be triggered by certain characteristics of metallic wear debris, even when present in minute amounts. Recently, solid corrosion products in periprosthetic tissues have been associated with hypersensitivity reactions that cause pain and implant revision (Huber et al, Acta Biomat 5:172, 2009). However, it is unclear whether it is the presence of solid corrosion products per se that initiates a hypersensitivity response, or if the amount and/or composition of these products are responsible. This uncertainty is, in part, because corrosion products have not been examined in tissues from control cases, that is, metal-metal joints that were revised for causes other than hypersensitivity.

The aim of this study was to identify the nature and amount of particulate debris and corrosion products and the associated histological reactions that are characteristic of hips in patients with hypersensitivity. Samples of periprosthetic tissue from ten cases revised for metal sensitivity and twenty from cases revised for mechanical loosening or femoral neck fracture (resurfacings) were analyzed by light microscopy and scanning electron microscopic spectroscopy.

Solid corrosion products of the type described to be associated with metal hypersensitivity were not found in the cases examined in this study. Particulate debris was minimal in cases with clinical features of metal allergy and the wear of those components was typically within the expected range. Mechanical failures had more particulate debris. Non-particulate, metallic debris consistent with corrosion products were only seen where total hip taper corrosion was identified. Further work is required to better characterize the nature of corrosion products around metal-on-metal hip replacements.