CHAPTER 26 The Painful Medial Unicompartmental Knee Arthroplasty
Introduction
Long-term survivorship of unicompartmental knee arthroplasty (UKA) has increased with contemporary prosthetic designs and improved patient selection and has been reported to be between 96% and 98% at 10–13 years of follow-up.1–4 Failure leading to revision in UKA has been ascribed to progression of arthritis in retained compartments, polyethylene wear, patient selection, implant malpositioning, loosening, fracture, and persistent pain.1–5 This chapter describes the evaluation and management of a painful UKA. A differential diagnosis is outlined in Box 26–1. Clinical and radiographic evaluations should seek to understand the diagnoses that may account for pain with well-positioned and stable implants. In addition, the assumption that the painful UKA will be solved with a conversion to a total knee arthroplasty (TKA) is discussed.
Often the evaluation of a painful UKA may lead the surgeon biased against medial UKA to believe that a TKA should have been indicated as the original arthroplasty. The reality, however, is that not all TKAs are pain free,6 with a series by Price et al. reporting pain at midterm follow-up at 41%. In addition, patient satisfaction following TKA may not be as high as assumed by many surgeons (many of whom do not perform UKAs), as Bourne et al. reported a 19% patient dissatisfaction rate in a large cohort of TKAs.7 Finally, the decision to revise a painful UKA to a TKA may have a lower threshold than a painful TKA and must be carefully considered. This “threshold for revision” may underscore higher revision rates in national registry data. A UKA may also have been performed prematurely without full-thickness cartilage loss and result in incomplete pain relief following the arthroplasty.8
Clinical Evaluation
The “appropriate” indications for medial UKA have long been debated.9–12 Some have suggested a combination of patient factors and examination findings that may limit the number of appropriate candidates to 4–6% of varus knees.9–12 Others have followed more physiologic criteria as documented by “anteromedial osteoarthritis” with intact collateral and cruciate ligaments, which may increase the percentage of varus knees that are appropriate candidates for medial UKA to as high as 30%. The published long-term data support the latter approach.4 The status of the patellofemoral joint (PFJ) as a contraindication to UKA is misunderstood, and long-term data by Beard et al.13,14
