A man, aged 55, presented with a spontaneous haemarthrosis of his right knee, 1 year after an unicompartmental knee replacement. This case showed an atraumatic rupture of the metal marker wire of an all-polyethylene inlay tibial implant which caused a destruction of the polyethylene surface and a disseminated synovitis of the right knee. This is a rare but destructive complication following an unicompartmental knee arthroplasty.