There is a growing demand for total hip arthroplasty (THA). Pseudotumor formation is increasingly reported as a common complication in patients receiving metal-on-metal (MoM) hip implants. It is recognized that a pseudotumor can generally be formed through adverse local tissue reactions secondary to metal corrosion at the head-neck junction and excessive metal ion release. Nevertheless, we experienced a case of a pseudotumor in a patient with a metal-on-polyethylene (MoP) implant without metal corrosion.

 A 55-year-old man underwent a primary right THA for the treatment of idiopathic necrosis of the femoral head using MoP bearings. He felt discomfort after seven and a half years postoperatively, and swelling with fluctuation was noted in his right hip. Although lateralization of the femoral head was found on a plain radiograph, there was no periprosthetic osteolysis. A computed tomography scan revealed the presence of a large soft-tissue mass anterior to the right hip. Serological abnormalities and increased serum metal ion levels were not identified. A histological analysis identified a mixture of large cells and plasma cells and the formation of a granuloma surrounding debris formed from wear of the polyethylene component. Although the pseudotumor was surgically removed, it relapsed in the same region after three years. A revision THA was performed to exchange the metal head, shell, and polyethylene liner, and the postoperative course has been uneventful so far.

 There were no obvious signs of metal corrosion or excessive metal wear in the retrieved implants, but wear of the conventional polyethylene liner was observed. Our findings suggest that in this case, the pseudotumor was more likely to be associated with debris from the wear of the conventional polyethylene liner rather than wear and ion release from the metal components.