Periprosthetic Tissue Metal Content but Not Serum Metal Content Predicts the Type of Tissue Response in Failed Small-Diameter Metal-on-Metal Total Hip Arthroplasties

Background: Tissue responses to periprosthetic metal wear debris are complex and poorly understood. There are two predominant tissue responses: a nonspecific macrophage-mediated granulomatous response and lymphocyte-dominated response, which has immunological memory and is mediated by T cells. Delayed hypersensitivity-type responses may accelerate aseptic loosening of arthroplasty implants. We hypothesized that the metal content of periprosthetic tissue but not of serum would be predictive of the type of tissue response to metal wear debris. Methods: We examined twenty-eight total hip arthroplasty implant retrievals from twenty-seven patients who had undergone revision arthroplasty at one institution. Indications for revision were pain and/or osteolysis; one patient had recurrent dislocations. Tissue samples were analyzed microscopically and the metal (Co, Cr, and Ni) content was determined. Explanted prosthetic components were examined for linear wear. Intraoperatively, periprosthetic metallosis was observed in twelve cases and formation of a bursa (pseudotumor) was observed in thirteen. The acetabular cup was loose in eleven cases, the femoral stem was loose in five, and both components were loose in five. Results: The metal (Co, Cr, and Ni) content of the periprosthetic tissue ranged from 1.4 to 4604.0 mu g/g. Histologically, macrophages containing metal particles as well as diffuse and perivascular lymphocytic infiltration were observed. Fibrin exudation was also visible. Tissues that displayed a predominantly lymphocytic response had a mean metal content of 222.2 +/- 52.9 mu g/g, whereas those that displayed a macrophage-dominated response had a metal content of 3.0 +/- 0.9 mu g/g; this difference was significant (p = 0.001). The mean serum metal content did not differ significantly between the two subgroups (60.7 +/- 13.4 compared with 43.7 +/- 3.8 mu g/L, p = 0.105). Conclusions: An association between periprosthetic tissue metal content and hypersensitivity appears likely but needs to be validated with larger-scale retrieval studies.