UKAs Converted to TKAs Are More Likely to Be Revised than Primary TKAs

    Unicompartmental knee arthroplasty (UKA) can be a good alternative to total knee arthroplasty (TKA) in patients with osteoarthritis (OA) in only 1 compartment of the knee. However, UKAs generally have a higher failure rate than TKAs, typically from progression of OA, aseptic loosening, and bearing dislocation.

    When a UKA fails, conversion to TKA has been thought to be a reliable solution. A study from Denmark, however, shows that outcomes for a TKA converted from a failed UKA are inferior to outcomes of TKA performed as the primary procedure, with a much higher rate of revision.

    The study by Anders El-Galaly, MD, and colleagues from Aalborg University Hospital in Aalborg, Denmark, was published The Journal of Bone & Joint Surgery.

    Using data from the Danish national registry, the researchers compared outcomes of 1012 TKAs converted from UKA, 73,819 primary TKAs, and 2572 revision TKAs performed between 1997 and 2017. The study focused on implant survival rate – implant failure requiring removal and replacement – across groups.

    Patients who underwent TKA converted from UKA were younger than patients who underwent primary or revision TKA (average age 66 vs 70, respectively), and also had less-severe knee joint degeneration according to Charnley class. Long-term outcomes were similar among patients who underwent conversion of UKA to TKA and patients who underwent revision TKA. Both groups had an estimated 15-year implant survival rate of 78%.

    In contrast, the 15-year implant survival rate for primary TKA patients was 94% percent. After adjustment for other factors, the risk of revision was 3 times higher for patients who underwent UKA-to-TKA conversion than for those who underwent primary TKA.

    Implant type did not affect outcomes for patients who underwent conversion of UKA to TKA.

    UKA is an increasingly popular option for patients only limited knee joint damage from OA. The ability to convert an UKA to TKA if needed has been thought to offset UKA’s disadvantage of shorter longevity compared with primary TKA. It has also been assumed that outcomes of UKA-to-TKA conversion are similar to those of primary TKA.

    The new study questions this assumption, suggesting that the risk of revision following conversion of UKA to TKA is similar to that of TKA revision, which may indicate a similar surgical complexity.

    Dr. El-Galaly and colleagues note that their analysis of Danish registry data supports the results of similar studies from Norway and Sweden. “On the basis of this study, we believe that careful consideration is necessary before using medial UKA as treatment for knee osteoarthritis, as a potential conversion to a TKA decreased implant survival when compared with that following primary TKA,” they conclude.


    El-Galaly A, Kappel A, Nielsen PT, Jensen SL. Revision risk for total knee arthroplasty converted from medial unicompartmental knee arthroplasty: comparison with primary and revision arthroplasties, based on mid-term results from the Danish Knee Arthroplasty Registry. J Bone Joint Surg Am. 2019 Nov 20;101(22):1999-2006. doi: 10.2106/JBJS.18.01468.