ACL Anatomy – The ACL extends from the lateral femoral condyle to the anterior medial aspect of the tibia. This ligament contributes to knee stability via passive restraint, specifically through prevention of the tibia from anteriorly translating on the femur and through limiting rotational, varus, and valgus stresses at the knee joint. In addition to this passive restraint, a combination of active muscular contraction with precise neuromuscular timing further assists with stability at the knee during functional movements (running, jumping, cutting, pivoting, etc.). Therefore, any alteration in the biomechanics or muscular control of the knee increases the risk of ACL injury.
Structural Risk Factors to ACL Tear – There are two main structural features of the knee that increase risk of injury to the ACL, which include intercondylar notch size of the femur and integrity of the menisci.
- Intercondylar notch size – If the intercondylar notch is too narrow, there is compromised space for the ACL during rotational movement. This will put added stress on the ligament and predispose an individual to ACL injury risk. This is especially seen in female athletes.
- Integrity of menisci – The integrity of the menisci can also contribute to ACL injury risk by altering the movement of the femoral condyles on the tibia. Increased femoral condyle movement will add stress to the ACL during cutting or jumping movements. With intact menisci, the femoral condyle motion is limited, allowing for decreased stress on the ACL. A tear of the menisci decreases the amount of control of femoral condyle motion during functional movements, increasing the risk of injury to the ACL.
However, these two structural risk factors are only modifiable through surgery and prevention programs are not able to target these two mechanisms.
4 Categories of Modifiable Risk Factors – ACL prevention programs are targeted at the dynamic and modifiable biomechanics of the knee. There are 4 main categories of athlete deficiencies that cause great stress on the ACL, but can be addressed through training.
- Ligament Dominance – The first type of deficiency is called ligament dominance. If an athlete is ligament dominant, they are not properly recruiting their knee musculature during single-leg landing, pivoting, or deceleration for stability. Therefore, the motion of the knee joint is directed by the ground reaction force, which results in a high knee valgus motion. Since the knee stabilizing musculature is not counteracting this force, the athlete must relay on the passive ligaments for knee stability. This causes high stress to the ACL and can lead to severe injury. Addressing musculature strength and timing will reduce the amount of stress put on the ACL in these individuals.
- Quadriceps Dominance – The second type of deficiency is termed quadriceps dominance. If an athlete is quadriceps dominant, they tend to activate their knee extensors preferentially over their knee flexors to control knee stability during landing, deceleration, or pivoting. This over-reliance on the quadriceps muscle leads to imbalances in strength and coordination between the knee extensors and knee flexors. Since the ACL is under the most stress during knee internal rotation near extension, the ACL is at greater risk for injury in these individuals. Addressing this strength and timing discrepancy will reduce the risk of injury in these athletes.
- Leg Dominance – The third type of deficiency is termed leg dominance. An athlete that has leg-to-leg imbalances in muscle recruitment, strength, and stability has an increased risk of ACL injury. These leg-to-leg imbalances are common due to sport-specific requirements of one leg versus the other leg. Addressing limb symmetry will put the athlete in a safer environment during sport participation.
- Trunk Dominance – The fourth and final type of deficiency is called trunk dominance. During deceleration, landing, or pivoting, an athlete’s trunk motion is often excessive due to inadequate core muscle contraction patterns. The excessive trunk motions, especially in the frontal and coronal planes, add an even higher ground reaction force, increasing the knee abduction torque. Stabilizing the trunk during these movements will help to decrease added stress to the ACL and reduce the risk of injury.