Infected pseudotumor following metal-on-metal total hip arthroplasty: a case report with diagnostic challenges and implications for surveillance guidelines

Background: Adverse reaction to metal debris (ARMD) includes metallosis, aseptic lymphocytic vasculitis-associated lesion (ALVAL), and pseudotumors. ARMD is rare and has many clinical presentations that can be confusing. It is the cause of 0.6% of total hip arthroplasty (THA) failures, considering all types of bearing couples, and the most frequent cause of metal-on-metal (MoM) THA failures. The main reasons for revisions related to ARMD are ALVAL and pseudotumor. Patients with MoM THA are at higher risk of developing periprosthetic joint infection (PJI). Those two complications are confusing. Case Description: We report a case of an undiagnosed specific complication in a patient with a MoM THA who presented in 2023 with an acute PJI. Imaging revealed an infected pelvic cyst. After reviewing previous scans, we concluded that the cyst was a metal-induced pseudotumor that had been present since 2019 but had been neglected. The pseudotumor was compressing the iliac vein, which explains a history of deep venous thrombosis (DVT) and lymphedema, previously and incorrectly attributed to prostatic adenocarcinoma. The patient underwent a two-stage revision; unfortunately, he died 32 days later. Conclusions: MoM THA require careful and regulated follow-up. We must pay closer attention to this specific and misleading complication, so we summarized monitoring guideline to make it clearer.