Excessive Implant Wear Reaction Mimicking Malignant Tumors: A Rare Orthopedic Case Report.

A 75-year-old patient was transferred to the oncology department due to the discovery of a large pelvic tumor compressing the femoral neurovascular bundle suspected to be of malignant origin. Further investigation revealed a rare complication related to a 27-year-old total hip arthroplasty (THA). The final diagnosis was a severe adverse local tissue reaction (ALTR) resulting from excessive implant wear-first from a metal-on-metal (MoM) bearing and later exacerbated by a revision to a metal-on-polyethylene (MoP) articulation. The clinical course was further complicated by periprosthetic joint infection (PJI). The patient underwent extensive tumor-like mass resection followed by two-stage revision arthroplasty. Despite these interventions, infection persisted, ultimately necessitating joint resection. This case highlights the rare but serious convergence of dreaded orthopedic complications (ALTR and PJI). It underscores the diagnostic challenge posed by wear-induced pseudotumors, which are rare even among arthroplasty specialists and are often unfamiliar to oncologists. This case illustrates the importance of early orthopedic evaluation, maintaining a high index of suspicion in atypical presentations, and invites further discussion about the interplay between ALTRs and infection risk in arthroplasty patients.