Radiological difficulty in identifying unicompartmental knee replacement dislocation.

Unicondylar knee replacement is a relatively common elective orthopedic procedure but is not often seen in the Emergency Department setting. Familiarity with normal clinical and radiological appearances is difficult to gain. Dislocation of the mobile bearing component "spacer" is a known complication of unicondylar knee replacements, and these patients will initially present to the accident and Emergency Department. In this setting, an accurate and prompt diagnosis is necessary to appropriately manage the patient's condition. There is normally a radiological challenge in identifying dislocated mobile bearings on plain radiographs. These patients may need to have further imaging, such as a computer tomographic scan to identify the dislocated mobile bearing.

Introduction

Partial knee replacements “unicondylar arthroplasties” are a relatively commonly performed orthopedic elective procedure. The Oxford unicompartmental knee is comprised of 2 metallic components, separated by a mobile polyethylene bearing, which acts as a mobile spacer between the metallic femoral and tibial components which are normally fixed to the bone with bone cement. The aim of these mobile bearings is to reduce polyethylene wear and to improve the congruity between the articulating surfaces [1].

Case

We present a case of a 47-year-old man who is from Barnsley and was in Aberdeen for work. He was performing an activity as part of his normal employment and suffered a relatively innocuous valgus and external rotation injury to his right knee. He had previously had an uncomplicated right Oxford unicompartmental knee replacement performed in January 2013 in Chesterfield. He was otherwise fit and well, with no allergies and no preceding problems with his knee after his operation and before his presentation to our department.

At time of presentation to our Emergency Department, he was unable to weight bear and fully extend his right knee with range of movement of 20-50 degrees. Distal neurovascular examination was normal. A hard object was palpable on the subcutaneous medial aspect of his knee joint, and radiographs performed revealed a dislocated spacer which could be difficult to spot on plain radiographs (Figs. 1 and 2). This was evident from the abnormal position of the wire within the spacer as otherwise the polyethylene spacer is radiolucent. Early evidence of patellofemoral and lateral compartment osteoarthritis was incidentally noted. The original operation note was transferred from Chesterfield which showed he had large 4-mm meniscal bearing initially inserted.

Fig. 1

Anteroposterior radiograph of dislocated mobile bearing (white arrow).

Fig. 2

Lateral radiograph of dislocated mobile bearing (white arrow).

The following day he underwent operative intervention in the form of removal of dislocated spacer and washout with insertion of a new 3-mm meniscal bearing as perioperatively, it was thought the 4-mm spacer may have “overstuffed” the compartment (Figs. 3 and 4). There were no signs of infection clinically. He was discharged to Barnsley a few days later.

Fig. 3

Postoperative anteroposterior radiograph with well-placed mobile bearing.

Fig. 4

Postoperative lateral radiograph with well-placed mobile bearing.

Discussion

Dislocation of the mobile bearing is a recognized but rare complication of the Oxford unicompartmental knee when used to replace the medial compartment, occurring in less than 0.6% of patients. The risk of dislocation increases to 10% when used to replace the lateral compartment. Dislocation commonly occurs through a significant twisting injury, resulting in immediate severe localized pain with or without loss in knee motion [2]. Dislocated bearing inserts are usually found on the anterior, medial, or lateral side of the intracapsular area. However, posterior dislocation of a polyethylene bearing to the extra-articular area after unicompartmental knee arthroplasty has also been reported [3]. There have been several cases of missed dislocated mobile bearing from unicondylar knee replacement on initial radiographic examination [2], [4]. Recognition of this diagnosis on plain radiographs and prompt referral to the Orthopaedic Department will greatly improve the chances of a successful outcome and decrease the chances of a long-term negative outcome with regards to the life and performance of the unicondylar arthroplasty.