Implant related hypersensitivity is an infrequent complication after total knee replacement. It remains a relatively unpredictable and poorly understood cause of failure of an implant.

We present a report of five patients who presented with persistent hypertrophic synovitis after total knee replacement using a cobalt chrome component. Extensive preoperative and intraoperative attempts ruled out infection as a cause of symptoms. The knees had good ligamentous balance and were well aligned and fixed.

The clinical condition improved after revision to a zirconium femoral and titanium metal backed tibial components. Intraoperative histopathology revealed thickened synovium with a predominantly monocellular (lymphocytic or histiocytic) response.

Where infection has been excluded as a cause of persistent pain and swelling, consideration should be given to metal allergy as a cause of failure in primary knee replacement surgery.