Which mechanism is characteristic of ACL injury in the material?

• A. Deceleration and acceleration motions with noncontact valgus load at or near full knee extension
• B. Direct contact to knee
• C. Repeated impact with no valgus
• D. Hyperextension of the knee with rotation

Answer: (A)

ACL injuries commonly happen during noncontact events when a person suddenly decelerates or changes direction while the knee experiences a valgus load near full extension. In this position the tibia can move forward (anteriorly) and rotate relative to the femur, placing strain on the ACL and often resulting in rupture. The extended knee also makes muscular stabilization less effective at that moment, so a pivot or landing that drives the knee into valgus is especially risky. This pattern—rapid deceleration or pivot with a valgus load on an almost straight knee—best explains how the injury occurs. Direct contact, repetitive impact without valgus, or purely hyperextension with rotation can injure the knee too, but they are not as characteristic of the classic ACL tear as the noncontact valgus mechanism during deceleration.