In 3 patients who developed acute arthritis of the knee 3 months to 5 years after insertion of a total condylar prosthesis, it was possible to formally exclude an infectious cause. A number of cytological and histological arguments confirm that these were cases of foreign body arthritis (polyethylene debris and metal dust). These cases of arthritis responded to local steroid treatment combined with a Yttrium synoviorthesis, in the absence of any antibiotic treatment. The synoviorthesis was performed in order to eliminate the real foreign body granuloma which develops and which can contribute to the loosening of these prostheses.