A 76-year-old woman who underwent bilateral metal-on-metal total hip arthroplasty fell 3 years after this procedure and subsequently incurred continuous pain in her buttock. Plain radiographs showed no fracture and no loosening of the hip prosthesis. Magnetic resonance imaging revealed an abnormal, large, thick-walled mass with heterogeneous signal intensity at the right buttock. The prerevision diagnosis was adverse reaction to metal debris. The mass was surgically resected, and the metal femoral head was replaced by a dual-mobility prosthesis. The intraoperative and histological analyses indicated an expanding hematoma. Cobalt ion concentrations of whole blood and effusion around the hematoma-1.9 µg/L and 1.3 µg/L, respectively-were not indicative of adverse reaction to metal debris. Transcatheter arterial embolization was performed 2 days postoperatively. The hematoma was reduced and was not present after 9 months. The diagnosis of a periprosthetic soft tissue mass after metal-on-metal total hip arthroplasty should be carefully reached with magnetic resonance imaging and assessment of blood metal ion concentrations. Expanding hematoma should be considered a potential diagnosis if metal ion concentrations are not increasing and magnetic resonance imaging shows a periprosthetic mass with a heterogeneous lesion. Embolization is useful for the management of an expanding hematoma. [Orthopedics. 2017; 40(6):e1103-e1106.].
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