Age-Related Risk of Conversion to Total Knee Arthroplasty After Mobile-Bearing Unicompartmental Knee Arthroplasty: A Survival and Competing Risk Analysis.

Abstract

Mobile-bearing unicompartmental knee arthroplasty (UKA) is a well-established treatment for isolated compartment osteoarthritis. However, long-term outcomes may vary with the age of the patient at arthroplasty. This study evaluated the association between age and the risk of conversion to total knee arthroplasty (TKA) and the ultimate outcome for patients undergoing conversion from UKA to TKA. A consecutive series of 468 patients who underwent mobile-bearing UKA between December 2006 and June 2023 were prospectively followed. Patients were stratified by age: ≤ 60 years (n = 180) and > 60 years (n = 288). The mean follow-up was 10.7 years (range, two to 18). Survivorship free from TKA conversion was analyzed using competing risk methods, with death as a competing event. Multivariate Cox regressions identified independent predictors of conversion. Outcomes following conversion to TKA were also evaluated. The overall rate of conversion to TKA was 7.5% for the entire cohort. Competing risk analysis revealed cumulative incidence of TKA conversion at two, five, 10, and 15 years was 4.4, 5.6, 10.2, and 15.0% in the younger group, compared to 1.0, 3.2, 5.1, and 6.0% in the older age group (P = 0.009). Multivariate analyses identified younger age (hazard ratio (HR) 2.2; 95% confidence interval (CI), 1.1 to 4.6; P = 0.029) and operative time (HR 1.01; P = 0.017) as independent predictors of conversion. Of the 21 patients < 60 years who underwent conversion from UKA to TKA, none required subsequent revision during an average of eight years of subsequent follow-up. Patients aged ≤ 60 years had an annual failure rate near 1%, which was higher than patients > 60 years at the time of UKA. Despite a higher conversion rate seen in young patients, these patients had favorable outcomes at final follow-up, further supporting mobile-bearing UKA as a highly viable bone-preserving option in this patient population with appropriate counseling.