Structural Changes of the Anterior Cruciate Ligament (ACL), Evaluated by MRI, and Their Relation to Clinical Outcomes, Within 2 Years After ACL Injury. Results From the NACOX Cohort Study

<b>OBJECTIVE:</b> The aim was to (1) describe structural changes of the anterior cruciate ligament (ACL), evaluated by magnetic resonance imaging (MRI), within 6 weeks until 24 months after ACL injury, and (2) investigate the relationship between ACL structural continuity and patient-reported and clinically assessed outcomes. <b>DESIGN:</b> Longitudinal pragmatic cohort study. <b>METHODS:</b> We included 129 patients, aged 15 to 40 years, with acute ACL injury from the NACOX (natural corollaries and recovery after an acute ACL injury) cohort. Patients were treated according to usual clinical practice, which comprised supervised rehabilitation before considering ACL reconstruction. At baseline and at 3, 6, 12, and 24 months follow-up, 3-Tesla 3-dimensional proton-density fat-saturated MRI scans, and patient-reported and objectively measured outcomes were assessed. The ACL Continuity, Thickness, and Shape (ACTS) scoring system was used for MRI assessment. <b>RESULTS:</b> At 24 months, 60 (47%) participants had received ACL reconstruction. At the last available MRI, 55 (48%) patients had overall ACL structure in continuity and 49 (43%) had ACL fibers in continuity. The overall ACL structure and fiber continuity on ACTS improved by at least 1 step in 31% to 81% of the patients at the different follow-ups. Ligament structure in continuity was associated with fewer giving-way episodes (11% versus 50%, <i>P</i> = .033) and reduced knee laxity at 12 months (side-to-side difference ≥ 3 mm: 53% versus 88%, <i>P</i> = .026) and 24 months (52% versus 100%, <i>P</i> = .013). Results for patient-reported outcomes were inconclusive due to wide confidence intervals. <b>CONCLUSION:</b> The ACL structure improved during the first 24 months after ACL injury when managed without ACL reconstruction. Continuity of ligament structure was associated with fewer giving-way episodes and lower knee laxity. <i>J Orthop Sports Phys Ther 2026;56(3):209-219. Epub 4 February 2026. doi:10.2519/jospt.2026.13397</i>.