Time to First Injury After Knee Surgery in U.S. Army Soldiers: A Survival Analysis.

BACKGROUND: Knee injuries, including those requiring surgical intervention, are a leading cause of disability in U.S. Army soldiers. Previous studies investigating the risk of injury after surgery have primarily focused on civilian populations and lack specific information on the timing of new injuries. OBJECTIVE: The purpose of this study was to identify the windows of peak injury risk after knee surgery in active-duty soldiers. We hypothesized that injury risk would peak around return to duty and remain elevated beyond completion of post-operative rehabilitation. METHODS: U.S. Army administrative and medical data were extracted from the Soldier Performance, Health, and Readiness database. An accelerated failure time model adjusted for demographic-, surgical-, and military-specific variables was used to identify post-operative injury risk by procedure type. RESULTS: Of the 7595 soldiers who had knee surgery from 2017 to 2020, 71% (5357) sustained a new musculoskeletal injury within 4 years after surgery. Soldiers with a prior non-knee injury were estimated to sustain re-injury 11% sooner than those without a history of injury, regardless of surgical procedure. Peak hazard for injury occurred around 2.5 months after surgery when physical activity restrictions were lifted. Furthermore, soldiers were more likely to sustain an injury after reaching 14.3 months post-surgery. CONCLUSIONS: These results indicate that both at the end of rehabilitation and 1 year post-surgery may represent critical windows for targeted injury-prevention strategies; however, future studies are needed to confirm these opportunities. CLINICAL RELEVANCE: These findings suggest potential implications for guiding provider care and informing future policy in military post-operative management, with the goal of improving long-term outcomes.