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Pseudotumours are well recognised as a complication of metal-on-metal hip arthroplasties and are thought to develop on the basis of an innate and adaptive immune response to cobalt-chrome (Co-Cr) wear particles. We report a case of a large pseudotumour that developed following a knee endoprosthetic replacement (EPR) undertaken for Ewing sarcoma. The lesion contained necrotic and degenerate connective tissue in which there were numerous scattered metal wear-containing macrophages, eosinophil polymorphs, lymphocytes, plasma cells and aseptic lymphocyte-dominated vascular-associated lesion-like lymphoid aggregates. Metal ion levels were elevated. No evidence of infection or tumour was noted and it was concluded that the lesion was most likely an inflammatory pseudotumour developing on the basis of an innate and adaptive immune response to components of Co-Cr metal wear derived from the knee EPR. [ABSTRACT FROM AUTHOR]

The imaging findings of periprosthetic soft tissue lesions (pseudotumours) have been typically defined in the context of newer second-generation metal-on-metal hip arthroplasty. More recently, similar findings have been described in the setting of non-metal-on-metal prostheses. Although uncommon, wear and corrosion between the metal surfaces at the head-neck ('trunnionosis') and neck-stem interfaces are the potential culprits. With modular junctions containing at least one cobalt chromium component frequently present in hip arthroplasty prostheses, the incidence of this mode of adverse wear may be higher than previously thought (irrespective of the specific bearing couple used). In the present report, we described a case of a severe adverse local tissue reaction secondary to suspected corrosion at the head-neck taper in a metal-on-polyethylene total hip arthroplasty and reviewed the literature. Knowledge of this topical entity should help radiologists facilitate early diagnosis and ensure early management of this potentially serious complication. [ABSTRACT FROM AUTHOR]