A Soft- Tissue Injury Diagnosis in Knee Replacement Patients cannot be made without a Plain Radiograph

INTRODUCTION: The design of some knee replacements includes the use of a mobile polyethylene bearing which can potentially dislocate away from the main bearing surfaces. This is particularly the case in unicompartmental or partial knee replacements (UKRs), and the most widely used UKR currently has this feature. Bearing dislocations are typically very painful and disabling, but not always, as in the case we present. CASE REPORT: We report the case of a patient presenting at a 1 -year follow- up following a mobile bearing UKR. At this consultation, the patient reported a smooth initial recovery but with on-going pain and swelling in the posterior aspect of his knee for 3 months, diagnosed as a soft- tissue strain. RESULTS: Radiographs at 1 year follow-up appointment showed that the polyethylene insert was dislocated posteriorly towards the popliteal fossa with metal components in direct contact. At revision surgery, this was confirmed as a chronic dislocation with severe metallosis requiring revision to a total knee replacement. CONCLUSION: We report a case of a posterior dislocation of a mobile polyethylene bearing of a partial / unicompartmental knee replacement UKR with delayed definitive treatment due to mis-diagnosis at the time of onset of symptoms and signs. The key lesson is that plain radiographs should be obtained promptly for any local issue in knee replacement patients.