Medial Unicompartmental Arthroplasty of the Knee

Jason M. Jennings, MD, DPT; Lindsay T. Kleeman-Forsthuber, MD; Michael P. Bolognesi, MD


J Am Acad Orthop Surg. 2019;27(5):166-176. 

In This Article

Understanding Failure in Unicompartmental Knee Arthroplasty

Caseload and Experience

Analysis of the National Joint Registry showed that optimal results were obtained by surgeons performing UKAs in 40% to 60% of their patients, with poorest results in those performing UKA <5% of the time.[32] Survival rates for high-volume surgeons were 96% at 5 years compared with 90% in low-volume surgeons.[32] Performing UKA through a minimally invasive surgery (MIS) approach adds another level of complexity, but with the potential benefit of earlier recovery. Some centers have experienced high failure rates with MIS, with revision rates as high as 11% at 2-year follow-up.[37] Reasons for revision included retained cement, contralateral compartment chondral pathology or meniscal defects, and wound complications. Hamilton et al[37] reviewed their series of 445 MIS UKAs and found minimal improvement in reoperation surgery and revision rates over a 2-year period, suggesting absence of a learning curve. This finding highlights that even at high-volume institutions, complication rates for MIS UKA are notable and perhaps longer follow-up and higher volume are needed to detect improvement in the surgical technique.

Mechanisms of Failure

The primary mechanisms for UKA failure have remained consistent since early clinical reports in the 1980s. Goodfellow et al[12] cited aseptic loosening as the primary reason for revision (incidence 6.6%) in their series of 103 patients with an MB implant at mean follow-up of 3 years. A recent systematic review found that the most common reasons for UKA failure were aseptic loosening (36%), progression of osteoarthritis (20%), unexplained pain (11%), instability (6%), infection (5%), and polyethylene wear (4%).[25] The majority of early failures (<5 years) were from aseptic loosening (25%), osteoarthritis progression (20%), and bearing dislocation (17%), whereas midterm and later revisions were performed primarily for osteoarthritis progression (38 to 40%), aseptic loosening (29%), and polyethylene wear (10%).[25] Dyrhovden et al[38] reviewed mechanisms for UKA failure in the Norwegian database between early and later periods (1992 to 2004 and 2005 to 2015) with a decrease in failures from aseptic loosening, polyethylene wear, and periprosthetic fractures from early to later time points but more revisions for osteoarthritis progression. Unlike TKA that experienced an improvement in the 10-year survival between time periods (91% to 94%), UKA survival remained stable (80% to 81%).[38] These findings suggest that despite advances in UKA implant technology and technique, osteoarthritis progression is a consistent mode of UKA failure. Unexplained pain is another major factor contributing to the discrepancy in revision rates between UKA and TKA, because it accounts for 1.6% to 11% of UKA revision.[20,25] The threshold to revise a painful UKA is much lower than for TKA, because UKA can be converted to a TKA, while revision of a TKA tends to be a more extensive surgery.[39]