What are the mechanisms of ACL injury?

A simple diagram illustrates the function of the ACL. The ACL opposes anterior translation of the tibia (advancement). During sporting activity, such as jumping or landing a jump, the body’s most powerful muscle, the quadriceps, sometimes contracts violently. The forces are transmitted to the tibia via the patellar tendon.

This resulting force can be broken down into a compressive force and an anterior translational force. The ACL opposes this force. It is then put under tension. Tensed, the ACL is liable to rupture when rotated and/or twisted. It’s like an over-stretched guitar string which, when strummed, breaks with an audible noise. This mechanism explains the majority of non-contact ACL ruptures: landing a jump with a bad landing, shooting into empty space… This often leads to a so-called isolated ACL rupture.

Sometimes the mechanism is different, and the contact is sustained. In rugby, for example, during a tackle, the opponent falls on the side of the knee. Under pressure, the knee deforms, yawning, with rupture of one or more collateral ligaments. Then the energy runs out, dissipating in the rupture of the ACL, sometimes even in other ligaments.