Implanting a knee prosthesis in patients aged over 85 years is not without risk. Unicompartmental knee arthroplasty (UKA) is reputed to show lower morbidity and mortality than total knee arthroplasty (TKA). Elderly patients with isolated unicompartmental osteoarthritis without specific contraindications are thus good candidates for this procedure. The present retrospective case-control study compared complications rates in elderly patients receiving UKA versus TKA. The study hypothesis was that UKA incurs fewer complications, with equivalent functional results. Five hundred and forty-four UKAs were performed between 1987 and 2015; all patients aged 85 years or over (n=30) were included, and age-matched at a rate of 1 UKA for 3 TKAs in our database of 4,216 TKA procedures. Mean age was 87.5 years (range, 85-92 years). The main endpoint was complications rate; secondary endpoints comprised satisfaction, IKS function and knee scores, and implant survival. Mean follow-up was 32 months (range, 12-118 months) for UKA and 34 months (range, 12-100 months) for TKA. The medical complications rate was significantly lower with UKA (6.7% versus 25.6%; P=0.02), with no early mortality. IKS scores were identical between UKA and TKA groups at last follow-up: knee, 93.8 (95% CI [89.7-98.0]) versus 89.5 (95% CI [85.6-93.4]), P=0.06; function, 63.8 (95% CI [53.1-74.5]) versus 67.0 (95% CI [61.3-72.7]), P=0.62. Satisfaction rates were likewise identical: 96% and 97%, respectively (P=0.77); and implant survivorship was identical (P=0.54). Early medical complications were fewer after UKA than TKA in a population aged ≥85 years, with comparable clinical outcome. These results argue in favor of UKA in elderly patients with comorbidities. III, case-control study. Copyright © 2018 Elsevier Masson SAS. All rights reserved.