1906 Disruption of the ACL is among the most frequent major musculoskeletal injuries affecting physically active men and women. Each year in the United States, more than 100 000 new cases of ACL injury are reported. 56 An estimated 50 000 to 75 000 ACL reconstructions are performed annu- ally to prevent instability episodes and potentially improve the natural history of the injury. 25,30 However, the effect of ACL reconstruction on subsequent knee reinjury is not known. Although many comparative studies have been done regarding different methods of ACL reconstruc- tion, 3,7,9,22,38,39,45,47,48 few studies have compared ACL recon- struction with conservative treatment in adults. 16,18,23,55 The Effect of Anterior Cruciate Ligament Reconstruction on the Risk of Knee Reinjury Warren R. Dunn,* † MD, MPH, Stephen Lyman, † PhD, Andrew E. Lincoln, ‡§ ScD, MS, Paul J. Amoroso, ll MD, MPH, Thomas Wickiewicz,* MD, and Robert G. Marx,* †¶ MD, MSc, FRCSC From the *Sports Medicine and Shoulder Service, Hospital for Special Surgery, New York, New York, † Foster Center for Clinical Outcome Research, Hospital for Special Surgery, New York, New York, ‡ War-Related Illness and Injury Study Center, Veterans Affairs Medical Center, Department of Veterans Affairs, Washington, DC, § Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, and ll US Army Research Institute of Environmental Medicine, Natick, Massachusetts Background: Although there is evidence that very active, young patients are better served with anterior cruciate ligament recon- struction, there is a lack of objective data demonstrating that future knee injury is prevented by these procedures. Hypothesis: Anterior cruciate ligament reconstruction protects against reinjury of the knee that would require reoperation. Study Design: Retrospective cohort study. Methods: A cohort of 6576 active-duty army personnel who had been hospitalized for anterior cruciate ligament injury from 1990 to 1996 were identified. Using the Total Army Injury and Health Outcomes Database, the authors followed these individuals for up to 9 years and collected clinical, demographic, and occupational data. These data were evaluated with bivariate and multi- variable analyses to determine the effect of anterior cruciate ligament reconstruction on the rate of knee reinjury that required operation. Results: Of the 6576 study subjects, 3795 subjects (58%) underwent anterior cruciate ligament reconstruction and 2781 (42%) did not. The rate of reoperation was significantly lower among the anterior cruciate ligament reconstruction group (4.90/100 person- years) compared with those treated conservatively (13.86/100 person-years; P < .0001). Proportional hazard regression analy- ses adjusted for age, race, sex, marital status, education, and physical activity level confirmed that anterior cruciate ligament reconstruction was protective against meniscal and cartilage reinjury (P < .0001). Secondary medial meniscal injury was more common than secondary lateral meniscal injury (P < .003). Younger age was the strongest predictor of failure of conservative management leading to late anterior cruciate ligament reconstruction (P < .0001). Conclusions: Anterior cruciate ligament reconstruction protected against reoperation in this young, active population; younger subjects were more likely to require late anterior cruciate ligament reconstruction. Clinical Relevance: Strong consideration should be given to anterior cruciate ligament reconstruction after anterior cruciate lig- ament injury in young, active individuals. Keywords: adult; anterior cruciate ligament (ACL); injury; reinjury; reoperation; military; outcome study ¶ Address correspondence to Robert G. Marx, MD, MSc, FRCSC, Hospital for Special Surgery, 535 East 70th St, New York, NY 10021 (e- mail: marxr@hss.edu). The views expressed in this article are those of the authors and do not reflect the official policy or position of the Department of the Army, Department of Defense, Department of Veterans Affairs, or the US gov- ernment. The analyses conducted for this article adhere to the policies for protection of human subjects as prescribed in Army Regulation 70- 25 and with the provisions of 45 CFR 46. No potential conflict of interest declared. See Acknowledgment for funding information. The American Journal of Sports Medicine, Vol. 32, No. 8 DOI: 10.1177/0363546504265006 © 2004 American Orthopaedic Society for Sports Medicine Winner of the 2003 O’Donoghue Award