ACL Tear

Mechanisms of ACL Injuries

Activities where the athlete is involved in cutting, twisting, jumping and landing subject the ACL to risk.  Surprisingly, non-contact ACL injuries are more common than contact ACL injuries.  Torn ACL Surgeons often hear injury stories where “no one hit me- my knee just buckled!”   

The ACL stabilizes the connection of the femur and the tibia.  Specifically it prevents the tibia from sliding forward relative to the femur.  There is also a rotational component of stability, namely the ACL prevents “over-twisting” of the two bones when your foot is planted on the ground.  

Two basic mechanisms have been described for ACL failure.  The first is a valgus force applied to the knee (a bending-in) while the knee is flexed and the foot is externally rotated (twisted out).

The second main mechanism is hyperextension of the knee, which causes a rupturing, or pulling apart of the ACL.  This often happens when an athlete is landing from a jump with the leg straight.  Incidentally, awkward landing is one of the chief culprits in the surprisingly high rate of ACL tears among women.  Alert therapists and doctors address this, and encouraging studies are emerging where ACL injury rates are decreasing among young women who train their “landing muscle.”

Of course there are a number of foot, knee, and hip positions that result in ACL tears, but in general, tears occur when the ACL experiences tensions and shear forces that overwhelm its mechanical integrity.

The ACL exists within the “notch” of the femur.  It originates along the side-wall of the femoral notch, and hugs against the bony notch on its way to the center of the tibial plateau.  In some individuals, this bony notch is too narrow.  This places the ACL at risk, because the bony constraints of the notch lever against the ACL during a forceful knee flexion and twisting event.  This in part explains how some athletes have torn their ACLs in otherwise harmless episodes.

If you are concerned about an ACL tear or other type of acl injury, contact Denver torn ACL surgeon, Dr. Schneider today.