Rationale: Cases of pseudotumor complicated after THA with metal-on-metal articulation as a bearing surface have been reported. Most of the pseudotumors occurred near or inside the hip joint, and usually have similar features with the infectious condition. We herein report a case of pseudotumor with huge size in the pelvic cavity mimicking features of iliacus muscle abscess. Patients concerns: A 70-year-old female was referred to our emergency department due to huge mass in her right pelvic area on abdominal CT. She complained of mild febrile sensation, generalized weakness, and pain in her right leg for several months. The patient underwent metal-on-metal bearing THA 11 years ago for treatment of right-sided hip pain due to avascular necrosis. Diagnosis: Percutaneous catheter drainage was performed for both therapeutic and diagnostic reasons. No microbes were cultured from the drained fluid, and signs of chronic inflammation were shown on pathology slide samples. The patient showed positive allergic reactions to cobalt and captan on allergic patch test, and serum cobalt levels was increased as 46.78 μg/L (normal value, 0.11–0.64 μg/L). Biopsy samples of the cyst were consistent with the histological findings of pseudotumor reported by Willert et al. Intervention: For the treatment of infection, primarily, the patient was surgically treated by irrigation, debridement, and bone bead insertion. Then, after 4 weeks of antibiotic treatment, revision surgery changing the metal-on-metal articulation into poly-on-metal articulation was performed. Outcomes: CT scan at 1 year after surgery showed no findings of cyst inside the pelvic cavity. Although the patient complained of right sided hip pain (VAS score 2), she was able to perform activities of daily living. Lessons: In patients who underwent metal-on-metal THA, pseudotumor mimicked the feature of periprosthetic infection may occur not only inside the hip joint but also in the pelvic cavity.

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