Gigantic popliteal synovial cyst caused by wear particles after total knee arthroplasty

We present a case exhibiting persistent joint effusion and formation of a gigantic popliteal synovial cyst 8 years after total knee joint arthroplasty. Assessment using flow cytometry revealed that both joint and cyst fluid contained abundant macrophage-phagocytosing wear particles. This finding indicates that wear particles participated in formation of the cyst through the communication between the joint cavity and cyst. Intraoperatively, prominent villous synovial proliferation was observed in both the joint and cyst, and delamination failure of the polyethylene insert was identified. Because no evidence of prosthetic loosening was found, only polyethylene insert revision and synovectomy were undertaken, resulting in a successful outcome. This case suggests that synovial cyst formation in popliteal lesions might represent a sign of wear-particle generation after total knee joint arthroplasty.