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 clinical evaluation of a painful UKA begins with a history and physical examination to determine the location, time course, and inciting variables producing the pain. As with most arthroplasties about the knee (UKA, TKA, and total knee revision), complete pain resolution may take up to 12–18 months. Early weight-bearing pain after UKA may be amenable to treatment with the use of an assist device such as a cane or walker to allow the bone and soft tissues to settle over a 4- to 6-week period. Often the pain is over the anteromedial tibia and is correlated with tenderness over the pes anserine insertion. Debate continues as to whether this is soft tissue related or bony in nature and is discussed below.

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 shed important light on this subject, demonstrating that anterior knee pain resolves after medial UKA with central and medial PFJ degeneration. Further, anterior knee pain does not correlate with intraoperative or radiographic findings in patients with anteromedial osteoarthritis (OA). There are no published data to refute this approach of largely ignoring the PFJ in patients with anteromedial OA of the knee and proceeding with medial UKA.

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