Association between surgically treated knee injury and knee arthroplasty: an explorative study based on Finnish nationwide register-based data

BACKGROUND AND PURPOSE: Knee injuries are established risk factors for knee osteoarthritis (OA). We aimed to investigate whether surgically treated knee injuries are associated with an increased incidence of end‑stage knee OA necessitating knee arthroplasty (KA). METHODS: Using nationwide Finnish registers, we identified all individuals aged 15-99 years who underwent surgery for knee injuries between 1986 and 2024. Injuries were categorized as knee fracture surgeries or non‑fracture knee injury surgeries. The date of the first identified injury surgery served as the index point from which person years under the exposure of interest were accrued. The control population comprised all Finnish residents without a recorded knee injury diagnosis and surgical treatment. Diagnoses and procedures were retrieved from the Care Register for Health Care (CRHC); KAs were identified from the Finnish Arthroplasty Register (FAR) and CRHC. Incidence rates of KA were calculated for both injury categories and the control population, stratified by age, sex, and year. Incidence comparisons between exposure and control groups were performed using age‑, sex‑, and year-standardized standardized incidence ratios (SIRs). RESULTS: In the fracture surgery cohort, there were 2,163 KAs with over 489,000 person‑years of follow‑up. In the non‑fracture injury surgery cohort there were 13,570 KAs with over 3.01 million person‑years of follow‑up. In the control population there were 205,440 KAs with over 167 million person‑years of follow‑up. Relative to the corresponding general population, the incidence of KA was increased in both injury categories, non-fracture injury surgery SIR 3.74 (95% confidence interval [CI] 3.68-3.80) and fracture surgery SIR 2.08 (CI 1.99-2.17). The greatest excess was among the youngest group (15-54 years): fracture surgery SIR 8.12 (CI 7.40-8.89); non‑fracture injury surgery SIR 6.05 (CI 5.84-6.27). CONCLUSION: Surgically treated knee injuries were associated with an increased incidence of KA, most notably in those aged 15-54 years. In older age groups, the additional effect of the injury surgery appears smaller, likely because age itself markedly increases the baseline incidence of KA.