Biomechanics and Risk Factors for ACL Tears

ACL—anterior cruciate ligament. Major stabilizing ligament connecting the tibia (shin bone) to the femur (thigh bone) and prevents forward motion of the tibia on the femur. It also has a small role in controlling rotation of these two bones.

• 75% of all ACL injuries in women are non-contact.
• Women in soccer are 6 times more likely and women in basketball are 8 times more likely than their male counterparts to injure their ACL.
• 20,000 high school and 10,000 college girls will sustain a season ending knee injury this year.

Non-contact mechanisms of ACL injuries:

• Planting and cutting--athlete who is running and decelerates quickly, with or without pivoting, causing a rotational force on the knee.
• Straight knee or awkward landing from a jump--athlete who jumps and lands with their knee locked out in full extension causing a large compression force.
• One step stop landing with knee inwardly rotated or in hyperextension--the ACL is stretched beyond its limits.

Contact mechanisms of ACL injuries:

• Direct blow--athlete who is hit from the posterior aspect of the knee causing the tibia to move anteriorly, or from the lateral side causing a rotational force onto the tibia.

Contributing Factors

The anatomical differences between men and women may play a role in injury incidence.

Intrinsic factors: Intrinsic factors are related to the anatomy, biomechanics and other physiologic factors

• Intercondylar notch--This is a groove in the bone where the ACL travels through as it attaches to the base of the femur. The size and shape of the notch tends to be smaller in women, possibly shearing on the ACL .
• Pelvis size--women have a wider pelvis, which makes the thigh bones come down at a greater angle as it sits on the tibia. This results in a greater "Q" angle, and subsequently causes the knee to bend inward,. The wider the angle, the more pressure put through the medial aspect of the knee.
• Hormones--changes in estrogen levels may effect the strength of the ligaments. Research suggests that injuries may be more common at days 9-14 of a women's cycle as the estrogen peaks relaxes ligaments.
• Ligament laxity--excessive joint motion can predispose a woman to injury.
• Other specific factors--less muscular thigh development (hamstring to quadriceps ratio), increased flexibility/hyperextension of the knees, increased pronation or inward rotation of the ankle.

Extrinsic factors: This involves other influences other than mechanical disadvantages

• Level of experience--less exposure to the activity makes the more complex movements more difficult
• Muscle strength and coordination--athlete may be over or under developed in certain areas (quadriceps and medial hamstring muscles in particular).
• Level of skill--playing with athletes of the same age, but different skill level.

Important considerations

• Avoid standing, walking or running with the knees in hyperextension.
• Keep your knee in alignment with your second toe whenever able (i.e. going up and down stairs, getting up from sitting down).
• Maintain strong trunk and core musculature—back, hips and abdominals—as they are the source from which everything moves from.
• Talk with your doctor or ask someone at a shoe store to tell you if you pronate excessively. If so, make sure to wear good shoes and probably arch supports to hold your foot in a more stable position.
• Balance training on an unstable surface anytime it is available is helpful. Keep a skateboard around the house or have a pillow handy that you can fold over a few times and balance on.
• Keep the hamstrings strong as well as the quadriceps muscles. Men's hamstrings are typically about 60-70 percent as strong as their quads while women's are about 45 to 55 percent as strong.
• Focus on your posture from the standpoint of keeping your lower abdominal muscles lightly drawn in and the lower shoulder blade muscles lightly engaged. Breathing principles need to account for this posture.