Review Question - QID 218994

The most common cause for early (<5 years) unicompartmental knee arthroplasty (UKA) failure is aseptic loosening. Component malpositioning, fixed bearing designs, and poor initial fixation can all lead to aseptic loosening.

Since its inception, UKA has been increasingly utilized throughout the world as a less invasive treatment for isolated compartment arthritis over conventional TKA. Proponents of UKAs cite its faster rate of recovery, improved kinematics, and better functional outcomes compared with TKA in age-matched control studies. Despite similar long-term survival rates to TKAs, UKAs are not without potential complications. In the early (<5 years) follow-up period, aseptic loosening is the most common cause for the need for revision surgery after UKA. In particular, all-polyethylene tibial tray designs, fixed-bearing implants, component malalignment, and poor initial fixation all place patients at an increased risk for aseptic loosening. If left untreated, aseptic loosening can progress to gradual subsidence requiring conversion to TKA with augments, as was seen in this patient at 2 years follow-up (Illustrations A and B).

Jennings and colleagues provided a comprehensive review article regarding unicompartmental knee arthroplasty. The authors discuss the changes to indications over time, the long-term outcomes between fixed and mobile bearing implants, and surgical techniques. They also discuss the importance of volume in determining outcomes, as high-volume UKA surgeons have significantly better long-term survival rates compared to low-volume UKA surgeons. The authors conclude that UKA is both beneficial and cost-effective when performed in appropriate patient populations.

Crawford and colleagues provided a comprehensive review regarding the management of failed medial unicompartmental knee arthroplasty. They discuss the most common reason for early (<5 years) failure is aseptic loosening whereas long-term (>5 years) failure is due to osteoarthritis progression. The authors also discuss that the functional outcomes of UKA converted to TKA may not be as good as an initial primary TKA, however, are better than those seen in revised TKAs. The authors conclude that many complications can be avoided with proper patient selection, component alignment, and surgical technique.

Figure A shows immediate postoperative AP and lateral radiographic imaging of a left medial UKA. The implant had approximately 10 degrees of varus (normal is neutral to 3 degrees of varus), indicative of malalignment. She developed aseptic loosening recognized by 6 months, however was lost to follow-up until over a year later when she presented with implant subsidence (Illustration A) requiring revision to TKA with a medial-sided augment (Illustration B).

Incorrect Answers:
Answer 1: Periprosthetic joint infections are thankfully uncommon in the setting of UKA. There are various cohort and registry studies showing a lower rate of periprosthetic joint infections in UKAs as compared to primary TKAs (<2%).
Answer 2: Progression of arthritis is the most common reason for UKA failure at mid and long- term (5-10+ years) follow-up. Progression of arthritis is unlikely to occur within the first couple of years after surgery.
Answer 4: Polyethylene wear is an infrequent cause for failure in UKA and accounts for a minority of revision cases. Like the progression of arthritis, it takes years for the polyethylene wear to become symptomatic and therefore only seen in long-term follow-up (>5 years).
Answer 5: Tibial stress fractures are important to recognize early to prevent catastrophic failure. Nondisplaced fractures can be treated nonsurgically with protected weight bearing whereas displaced fractures require either open reduction internal fixation or revision arthroplasty. They represent approximately 2% of UKA failures.