 A new study suggests that early ACL reconstruction surgery delays secondary injury to the knee meniscus compared with delayed surgery or non-surgical treatment. The findings are published in the American Journal of Sports Medicine. The anterior cruciate ligament, or ACL, is an important stabilizing structure in the knee. Injury to the ACL places additional stress on neighboring knee structures, such as the meniscus, increasing the risk of secondary injury. However, it's unclear whether the type and timing of ACL reconstruction affect the risk of subsequent meniscal injury. To find out, researchers recently used machine learning to compare the risk and timing of secondary meniscal injury among patients whose ACL injuries were treated non-surgically with early reconstruction surgery or with delayed reconstruction surgery. There was no significant difference in the overall occurrence of secondary meniscal injury in the patients who were treated with versus without surgery. However, the patients who underwent early ACL surgery went longer without developing meniscal injury than those who underwent delayed surgery or no surgery at all. Random survival forest models outperformed the traditional Kaplan-Meyer method in predicting the risk of secondary meniscal tear and the duration without meniscal injury. Risk factors for secondary meniscal injury were also identified. In the surgical cohort, an increased time to surgery, a faster return to sport, a younger age at the time of injury, and participation in sports involving high impact or rotational landings were significant risk factors. In contrast, in the non-surgical cohort, the significant factors were a faster return to sport, greater pain at the time of injury, hypermobility, and participation in non-contact sports. Notably, this study was retrospective and there were some differences in baseline characteristics between groups. For example, the surgical cohort had a greater incidence of simultaneous meniscal injury at the time of ACL injury than the non-surgical group. In addition, the non-surgical cohort had a greater proportion of partial ACL tears. Therefore, the initial injuries may have been more severe in the surgical cohort. Furthermore, there were differences in sports involvement and follow-up times among the groups. And surgical techniques for ACL reconstruction changed considerably during the study period. Overall, the findings show that performing ACL reconstruction surgery soon after injury may help delay a secondary meniscal injury. They also identify machine learning models with good performance that might be useful for clinical decision making regarding the treatment of ACL injuries.