The anterior cruciate ligament, or ACL, is a widely-referenced ligament, especially in sports-related knee injuries. The knee joint is comprised of three bones – the femur (thigh bone), tibia (shin bone) and patella (kneecap). These bones are connected via non-contractile tissues (tissues that do not produce force, but act to restrict movement) called ligaments. The ligaments that connect the femur and tibia include the ACL, the posterior cruciate ligament (PCL), the medial collateral ligament (MCL) and lateral collateral ligament (LCL). The knee cap also has ligaments that connect it to the tibia (patellar ligament) and to the femur (medial patellofemoral ligament). In addition to these ligaments, the muscles surrounding the knee joint assist in providing stability.
The ACL is made up of two bundles. The posteriolateral bundle of the ACL resists internal rotation of the tibia (shin bone), while the anteriomedial bundle resists anterior translation of the tibia (shin) on the femur (thigh bone). Each of these bundles originates on the femur, fans slightly and rotates, then connects slightly forward on the tibia. As the knee moves from an flexed position to an extended position, the fibers of the two bundles are stressed differently to allow for proper biomechanical movement of the knee. In sports, this is crucial to getting in and out of positions requiring the extremes of knee flexion and extension, as well as stabilization when the leg is planted on the ground.
The requirements placed on the ACL with fast-paced and multidirectional activities, such as running, cutting, and jumping, are aided by muscles that surround the lower extremity joints specifically the hamstrings and hip external rotators. When these muscles do not produce enough force to maintain good joint mechanics, the ACL can quickly become stressed and often times causes it to fail. Think of a string connecting the flat end of two cylinders. If you move one cylinder past the other too quickly, the string will likely snap. That is similar to how the ACL ruptures. The fibers of the ACL are made up of contractile tissues, and do not have motor innervations, the ACL itself cannot be trained to withstand additional force.
In most cases, an ACL injury can be categorized as a contact injury or a non-contact injury. An ACL rupture due to a contact injury are difficult to prevent, and usually occurs when another player contacts the knee, abruptly forcing it inward. A non-contact injury can sometimes be prevented, however is thought to account for up to 70% of all ACL injuries. The two most frequent positions that can cause an ACL to rupture are hyperextension of the knee while the foot is planted, and when changing directions quickly while allowing the knee to drop inward while the hips and foot are moving in opposite directions of the knee. To watch a video that further explains the mechanisms of a non-contact ACL tear, press play below.
Because prevalence of these injuries can be reduced through intervention, several tools have been developed to screen athletes for predisposing factors. To learn more about these screening tools, stay tuned for Part 2 in the ACL series!
