Activity and functional readiness, not age, are the critical factors for second anterior cruciate ligament injury - the Delaware-Oslo ACL cohort study.

High rates of second ACL injury in young athletes may be driven by a mismatch between RTS rates and functional readiness to RTS. Passing RTS criteria was independently associated with a lower second ACL rate. Allowing more time prior to RTS, and improving rehabilitation and RTS support, may reduce second ACL injury rates in young athletes with ACL reconstruction.