A 54 year old man presented to the emergency department with an acutely locked left knee. He had had a unicondylar knee replacement done on this knee 18 months earlier. Plain x rays were taken (fig 1A) which confirmed the dislocation of the meniscal bearing. An attempt to reduce the knee under sedation in the emergency department was unsuccessful and an orthopaedic consultation was requested. An attempted manipulation under anaesthetic to reduce the dislocated bearing also failed. An open reduction was required. A miniarthrotomy was performed using the previous incision. The surface of the dislocated bearing was found to have been damaged (fig 1B) and therefore a new bearing was inserted.

• Download figure
• Open in new tab
• Download powerpoint

Figure 1

 (A) Plain x ray demonstrating the dislocated meniscal bearing (white arrows). (B) The meniscal bearing after removal, note the damaged edge (black arrow).

DISCUSSION

To our knowledge this is the first report of a dislocated bearing of a unicompartmental knee replacement presenting as an acutely locked knee. Thousands of knee replacement operations are carried out each year in the UK. Of these, up to 5% are suitable for unicompartmental knee arthroplasty. The vast majority of patients remain satisfied with their unicompartmental knee replacement at long term follow up.¹ The commonest long term complication is wear of the polyethylene liner.²

It is already recognised that most locked knees are caused by a mechanical block to full extension. Most are caused by torn menisci, 10% are caused by cruciate ligament ruptures, and loose bodies are an occasional finding.³ It is difficult to identify those patients who will require surgical intervention for their locked knee. Many are able to either unlock their knee spontaneously or are able to do so after adequate analgesia has been provided and they have been encouraged to flex and extend the knee. Arthroscopic assessment of the joint is obviously required for those knees that remain locked. Repeated attempts to unlock the knee by manipulation risks damaging the components of a joint replacement. This was clearly demonstrated in our case. It is proposed that these patients should be referred to orthopaedics early on, with a low threshold for open reduction of the dislocated meniscal bearing.