An unusual cause of locking.

This case report describes the discovery of a plastic foreign body in a patient's knee joint 21 years from the initial injury. The patient was thoroughly investigated radiologically with plain X-rays both immediately after the initial injury and years later in orthopaedic outpatients. He also subsequently received an MRI. All these were reported as normal. The patient's symptoms persisted and an arthroscopy was performed. At this procedure the presence of the foreign body was confirmed and the offending plastic was safely removed. The patient's symptoms of intermittent locking resolved. This experience stresses this importance of clinical suspicion despite apparently normal radiology.