Knee arthroplasty in patients under 50 years demonstrates durable survivorship despite substantially higher lifetime revision risk than in older patients

PURPOSE: Data on survivorship and epidemiology in patients aged <50 years undergoing primary knee arthroplasty (KA) are limited. This study used nationwide registry data to evaluate epidemiology, implant survivorship, and revision risk in patients aged 16-49 years. METHODS: This retrospective analysis of prospectively collected data from the Scottish Arthroplasty Project evaluated patients aged 16-49 years undergoing primary KA between 2000 and 2019. A date-matched cohort aged ≥50 years served as comparison. Data were analysed using SPSS and R. Outcomes included revision rate, time to revision, and mortality. Competing risk analysis assessed cumulative incidence, and lifetime revision risk was estimated using parametric survival modelling. RESULTS: 3069 patients (58.6% female; mean age 44, range 16-49) were included, with median follow-up of 8 years (range 1-21). Nonosteoarthritis indications were more common in patients aged <35 years. KA incidence increased significantly over time in patients aged <50 (IRR 1.05/year, p < 0.001). Overall, 8.3% underwent revision, with 21-year cumulative incidence of 15.5%. Patients aged ≥50 years had lower revision risk (HR 0.4, p < 0.001). Estimated lifetime revision risk was 31.3% (95% CI 24.6-38.4), with wide confidence intervals in younger subgroups. Mortality at 21 years was 18.2%, higher in nonosteoarthritis indications (HR 0.5 for OA, p < 0.001). CONCLUSION: KA in patients under 50 years demonstrates acceptable long-term survivorship but a higher revision risk compared to older populations, with an estimated lifetime revision risk of approximately 30%. Subgroup findings should be interpreted cautiously due to small sample sizes and key confounders, particularly implant type and underlying diagnosis. These results support KA in selected younger patients while highlighting the importance of counselling regarding long-term revision risk and the need for more granular future research. LEVEL OF EVIDENCE: Level III.