Can you give me a quick overview of what causes ACL injuries?

Q: I am a high school athlete working on something called a "senior project." We are supposed to take a topic of interest to us and explore or investigate it thoroughly. I have picked the subject of ACL injuries in athletes because it happened to me and to several of my teammates. I'm glad I found your website to help me. Can you give me a quick overview of what causes ACL injuries to help me get organized?

A: As you have found out the hard way, anterior cruciate ligament (ACL) injuries are more common than athletes would like and can be very disabling. They can take a long time to heal and rehab. An injury forceful enough to rupture a ligament like the ACL can also cause damage to other soft tissues in the knee making recovery an even greater challenge.

For an overview, it may be helpful to recognize that there are many identified causes of ACL injuries. Most are multifactorial meaning there is more than one factor increasing the risk of ACL injury. Two broad categories for these risk factors are: intrinsic risk factors and extrinsic factors.

Anything intrinsic refers to the patient or affected individual. Intrinsic risk factors potentially contributing to ACL injuries include sex (male or female), hormones, genetic factors, and anatomy. Females are at greater risk than males for ACL injuries for a combination of reasons including anatomy and hormone shifts. Previous injury(ies) to the leg (especially the knee) is also an important risk factor.

Anatomical and neuromuscular features play a big role in ACL injuries. Variables such as knee geometry, alignment, and joint laxity are half the problem. Knee geometry refers to three specific measures of shape, depth, and angle that have been linked with ACL injuries. These include decreased intercondylar femoral notch size, decreased depth of concavity of the medial tibial plateau, and increased slope of the tibial plateaus. No doubt you will come across these in your research.

Neuromuscular risk factors include posture, landing biomechanics, ground reaction forces, core stability, trunk displacement, and active proprioceptive repositioning error. Changes in movement patterns from any one of these factors increase the strain on the ACL and seem to be contributing to the increased risk of ACL injuries. Your coach and/or athletic trainer will help define and describe these factors to you.

Extrinsic refers to something outside the body such as the type of playing surface or equipment in use. Sometimes weather and field condition contribute to increasing the risk of any injury including ACL tears or ruptures.

The benefit of research projects like the one you are working on is in being able to counsel athletes wisely and develop effective prevention strategies. Injuries like ACL ruptures can end an athlete's career -- or at least sideline the player for a season or more. There is a future risk of knee osteoarthritis that must be considered as well.

With proper training and good body mechanics, even athletes with nonmodifiable anatomic risk factors may be protected from such injuries. Knee geometry cannot be changed but balanced muscle strength, motor control, and neuromuscular coordination could make a difference.

Exercise programs aimed at these areas during different stages of growth and development may help. Further research is needed to fully understand all ACL risk factors and find appropriate prevention strategies. The role of skeletal and muscular maturation versus conditioning must be evaluated as well. If you get started with this basic information, it won't be difficult to expand and fill in the details. Good luck!

Reference: Helen C. Smith, MS, ATC, et al. Risk Factors for Anterior Cruciate Ligament Injury: A Review of the Literature - Part 1: Neuromuscular and Anatomic Risk. In Sports Health. January/February 2012. Vol. 4. No. 1. Pp. 69-78.