Contralateral and Ipsilateral Second Injuries of the Anterior Cruciate Ligament After Primary Reconstruction in Sport-Active Patients: A Retrospective Cohort Study

Introduction: After anterior cruciate ligament (ACL) reconstruction, a second injury may affect either the graft or the native contralateral ACL.In this exploratory secondary analysis, we aimed to describe the cumulative incidence of contralateral ACL rupture and ipsilateral re-rupture after primary reconstruction in sport-active patients and to report these events according to surgical technique: single-bundle (SB) or double-bundle plus lateral reinforcement (DB + LR).Materials and methods: This was a secondary analysis of a retrospective, single-center cohort of 56 patients aged 16-45 years who underwent ACL reconstruction using hamstring autografts between 2016 and 2021 (SB, n = 31; DB + LR, n = 25).Second ACL injuries documented in the center's medical records through June 2023 were analyzed (median time from surgery to data extraction, 3.3 years).Contralateral ACL rupture was not a predefined outcome of the parent study.Results: The mean age was 25.6 ± 8.3 years; 67.9% were male, and 48.2% played rugby.At clinical discharge, 51 of 53 examined knees (96.2%) were clinically stable, and 50 of 56 patients (89.3%) had a documented return to pre-injury activity.Twelve patients (21.4%; 95% CI: 11.6-34.4)had a recorded second ACL injury: seven contralateral ACL ruptures (12.5%; 95% CI: 5.2-24.1)and five ipsilateral re-ruptures (8.9%; 95% CI: 3.0-19.6),with no significant difference between the two (exact McNemar test, p = 0.77).Contralateral ACL rupture occurred in 9.7% of SB and 16.0% of DB + LR patients (p = 0.69); this comparison was underpowered and confounded by sport.Conclusions: Most patients achieved a clinically stable knee and had a documented return to pre-injury activity.However, about one in five had a recorded second ACL injury, and contralateral ruptures were at least as frequent as ipsilateral ruptures.The contralateral knee deserves attention during follow-up and rehabilitation; prospective studies with complete follow-up are needed to confirm these rates.