ACL Injury Part 1: Mechanism, Function, and Risk Factors

When I was in high school my brother and I played on the varsity basketball team. He was amazing and we were from a small town; this meant that he played basically every minute of every game. When he was in his final year of school I remember we were playing in a tournament. It was the last quarter, my brother drove to the hoop, went for the easy lay up AND.... dropped like a ton of bricks. Later he told me he heard a pop and his knee gave way. That, my friends, was an complete ACL Rupture.

Mechanism of Injury

Up to 80% of all ACL injuries occur without contact - meaning that you don't have to get hit to get injured. Most injuries occur when landing  or quickly stopping with the knee fully extended (think after a lay-up or stoping after a 100m dash) or when quickly changing direction.

In injuries that are sustained because of contact to the knee is nearly always due to valgus stress - such as getting tackle or struck on the outside of the knee. Contact injuries like this are typically accompanied by damage to the medial collateral ligament (MCL) and the medial meniscus; collectively known as the unhappy traid.

Function of the ACL

The ACL is the most important ligament in the knee because it provides the most stability due to its orientation and thickness. The anterior cruciate ligament (ACL) is responsible for limiting anterior translation of the tibia on the femur, especially during knee extension. What this means is that it stops the shin from moving too far forward on the thigh. It also limits rotational movement at the knee joint along with the posterior cruciate ligament (PCL).

Who Is At Risk Of Injury

Well, anyone can get injured,but typically younger athletes in sports like soccer and basketball have the highest injury rate, in older people skiing has a large rate of injury. Here are common factors that may increase your risk of having an ACL injury.

  1. Athletes are at a much higher risk than the average person for an ACL injury due to the extreme forces about the knee in activities such as jumping, kicking, and quick changes in direction, with soccer having one of the highest ACL injury rates of any sport.

  2. Women athletes are 2-4 times more likely to experience an ACL Injury in their career than their male counterparts.
  3. Tying in to the point above, ovulation has been proposed to put a woman at an increased risk of ACL injury because of the excess of estrogen and relaxin hormones, making ligaments less tensile, and therefore more susceptible to injury.

  4.  Increased knee laxity. This means that people with more flexible knees are at higher risk of sustaining and ACL injury.

  5. People with a narrow femoral notch

  6. A higher than average BMI has also been suggested, but only in women - a high BMI doesn't seem to affect a male's risk of ACL injury.

  7. Muscles imbalances. People who have much stronger quadriceps than hamstrings are at an increased risk because the hamstrings work as a protagonist to knee extension, prevent excessive tibial translation. Weak hamstrings mean that the ACL takes on more strain to keep the shin from moving too far forward on the thigh.

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