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Prosthetic joint infection
1%
15/1717
ACL rupture
5%
90/1717
Bearing dislocation
86%
1470/1717
Progression of lateral compartment arthritis
4%
66/1717
Lateral meniscal tear
3%
60/1717
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Bearing dislocation is a complication occasionally encountered with mobile bearing prosthetics as seen here. The mobile bearing polyethylene has a marker in it, which can help identify a dislocation. Bearing dislocation is a complication unique to mobile bearing medial UKA. This is most commonly caused by surgeon technical errors such as flexion/extension gap mismatch, medial collateral ligament (MCL) damage, impingement by remnant cement or osteophytes, or component misalignment. To assist in the identification of this complication, many mobile bearing polyethylene contain a marker that allows them to be visualized radiographically. Prompt surgical treatment is often required as many patients are very symptomatic. Treatment options consist of closed reduction, open exploration with bearing exchange, conversion to fixed UKA or conversion to TKA. Kim et al review redislocation after bearing exchange for patients who previously had bearing dislocation following mobile bearing UKA. They report a high incidence of redislocation occurring in 50% of their cohort (n=9). They conclude that bearing exchange as a sole treatment should be carefully considered in selected patients with only correctable causes of bearing dislocation. Campi et al review their experience with the oxford mobile bearing UKA. They report on 1200 consecutive patients who had an overall cumulative survival of 91% at 10 year follow up. The most common cause of revision was progression of lateral compartment disease. They conclude that mobile bearing UKA can provide excellent results if proper surgical technique and patient selection are performed. Figure A are the AP and lateral radiographs of a medial mobile bearing UKA with polyethylene dislocation. The radiographic marker in the polyethylene can easily be identified in the midline posteriorly. Incorrect Answers: Answer 1: PJI would not be common in the setting of normal inflammatory markers. Answer 2: While ACL rupture is a reason for failure following UKA, this patients ligamentous exam is stable and there are other radiographic reasons for his symptoms. Answer 4: AP radiographs do not show significant degenerative changes of the lateral compartment that would lead to this patient's symptoms. Answer 5: While lateral meniscal tears may occur, this patients symptoms are due to the dislocated polyethylene.
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