Study compares unicompartmental knee arthroplasty and total knee arthroplasty

Reviewed by Kate Anderton, B.Sc. (Editor)Nov 25 2019

Unicompartmental (or "partial") knee arthroplasty (UKA) is an alternative to total knee arthroplasty (TKA) for some patients with knee osteoarthritis. When UKA fails, conversion to TKA has been thought to be a reliable solution. However, TKA converted from UKA had inferior outcomes compared with TKA performed as the primary procedure reports a study in the November 20, 2019 issue of The Journal of Bone & Joint Surgery. The journal is published in the Lippincott portfolio in partnership with Wolters Kluwer.

The risk of revision surgery in three times higher for TKA converted from UKA compared with primary TKA, according to the new research by Anders El-Galaly, MD, and colleagues of Aalborg University Hospital, Denmark.

Higher Revision Rate after UKA Is Converted to TKA

In UKA, just one of three compartments of the knee is replaced with a prosthesis. Because UKA is a less extensive procedure that preserves most of the normal knee anatomy, it offers faster recovery than the more common TKA. An advantage of UKA is that it can be readily converted to TKA, if needed.

With use of Danish national registry data from 1997 to 2017, the researchers compared the outcomes of 1,012 TKAs converted from UKA 73,819 primary TKAs, and 2,572 revision TKAs. The study focused on implant survival rate – implant failure requiring removal and replacement – across groups.

Patients who underwent TKA converted from UKA were younger (average 66 years old) compared with those who underwent primary or revision TKA (average 70 years), and also had less severe knee-joint degeneration (according to Charnley class). For patients who underwent conversion of UKA to TKA, long-term outcomes were similar to those who underwent revision TKA. Both of these groups had an estimated 15-year implant survival rate of 78 percent.

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

Implant type did not affect outcomes for patients who underwent conversion of UKA to TKA. As in previous studies, the main reasons for UKA conversion were implant loosening, unexplained pain, and progression of arthritis.

Partial knee replacement is an increasingly popular option for patients with knee osteoarthritis who have limited knee-joint damage. Although UKA doesn't last as long as TKA, the ability to convert to TKA if needed has been thought to offset that disadvantage, and it is sometimes assumed that the outcomes of UKA to TKA conversion are similar to those of primary TKA.

The new study questions that assumption, suggesting that the risk of revision following conversion of UKA to TKA is similar to that of TKA revisions - indicating 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. The authors conclude:

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.