ACL Function

How the ACL Works

The ACL has surprisingly complex anatomy. It is not simply a single ligament. It has two primary bundles: anteromedial and posterolateral, so named based on their tibial insertions. One clever way of remembering the twisting shape of the ACL is by crossing your fingers. Do this with both hands: cross your middle finger over your index finger. The anteromedial bundle is like your middle finger, crossing over your index finger (which is like the posterolateral bundle).

One can imagine then, that the anteromedial bundle gets tightened when the knee flexes. Simulate this action by again crossing your fingers…and then bending both fingers. Your middle finger is placed under tension in this action. When your knee is fully extended (made all the way straight), it is the posterolateral bundle that is tensioned. Theoretically, there is a collection of fibers within the ACL that is at the same tension or length through a complete range of flexion and extension. These fibers are considered the “isometric” fibers. That is to say, these fibers are “the same length” through an entire range of motion. Finding these fibers is like finding a unicorn or Bigfoot- they might not actually exist. But, conceptually re-creating isometricity is the key to performing ACL reconstruction. See more about isometric placement of the graft under ACL TREATMENT OPTIONS.

The ACL is composed of connective tissue that is very strong. Strength testing (biomechanical testing) of the ACL has been performed in many studies over the last 25 years. There is general consensus that the average ultimate tensile load required to tear an ACL is approximately 2,160 Newtons (almost 500 pounds!). The stiffness (defined as the resistance of an elastic body to deformation by an applied force) of your native ACL is 242 N/mm (or 54 pounds per millimeter). Stated another way, your native, healthy ACL moves one millimeter per 54 pounds applied in an even manner. The cross section of your native ACL is 44 mm2.