Long-term risk of knee replacement after ACL reconstruction using the contralateral knee as an internal control: a National Hospital Episode Statistics database study of 135 881 patients.

OBJECTIVE: Estimate the population-specific relative risk of knee arthroplasty following anterior cruciate ligament (ACL) reconstruction (ACLr) in patients with a history of ACLr in only one knee. METHODS: A population-based cohort of patients undergoing their first ACLr, with no previous or subsequent ACLr to their contralateral knee was extracted from National Hospital Episode Statistics data, England, UK (1997/1998-2022/2023). Mortality-adjusted Kaplan-Meier survival analysis (survival defined as not undergoing knee replacement) of the index knee undergoing ACLr versus the non-intervened knee. Subgroup analysis was done examining sex, age, index of multiple deprivation, ethnicity and whether ACLr was performed in isolation or with concurrent meniscal surgery. A comparison of the arthroplasty rate in both knees versus general population data was also performed. RESULTS: 135 881 ACLr patients were included. At 20 years post-ACLr, the rate of ipsilateral knee arthroplasty was 2.44% (95% CI 2.18% to 2.73%) versus the contralateral knee (0.88%, 95% CI 0.72% to 1.06%). The HR for knee arthroplasty versus the contralateral knee was 3.00 (95% CI 2.56 to 3.51) in isolated ACLr cases and 3.50 (95% CI 2.55 to 4.81) in cases with simultaneous meniscal surgery. In males, the HR was 3.27 (95% CI 2.72 to 3.92) versus the contralateral knee, and in females, the HR was 2.85 (95% CI 2.28 to 3.57) versus the contralateral knee. The increased HR of arthroplasty in the ACLr knee remained present across the different strata of index of multiple deprivation, ethnicity and age. The rate of arthroplasty was slightly higher in the contralateral (non-ACLr) knee when compared with the background population risk of knee arthroplasty. CONCLUSIONS: This is the first study to estimate the population-specific relative risk of knee replacement following an ACL injury and reconstruction by using the contralateral knees of the same patients as the control. The relative risk is approximately three times that of the contralateral, uninjured knee. Our findings may better inform patients when discussing their long-term prognosis after ACL injury.