Case presentation : An 81-year-old man presented with a painful and enlarging mass at the popliteal fossa and calf of his right knee.He had had a total right knee replacement seven years previously.Plain radiographs showed narrowing of the medial compartment.Magnetic resonance imaging showed a cystic lesion at the postero-medial aspect of the knee join mimicking popliteal cyst or soft tissue sarcoma.Fine needle aspiration was non diagnostic. A core-needle biopsy showed metallosis Intraoperative findings revealed massive metallosis related to extensive polyethylene wear,delamination and deformation.Revision knee and patella arthroplasty was carried out after a thorough debridement of the knee joint.