February 2010 Newsletter

Archived Newsletters

Anterior Cruciate Ligament (ACL) Injury Prevention
By Dr. Stephen A. Hunt

What is the ACL?
The ACL is a ligament that connects the tibia (shin bone) to the femur (thigh bone). It is located in the center of the knee and is one of the main ligaments that stabilizes the knee. While its overall function is complicated, the two main ways the ACL stabilizes the knee is by preventing the tibia from shifting too far in front of the femur and from rotating too far inward.

How does an ACL injury occur?
The ACL can be injured in one of two ways. A contact injury occurs when the knee suffers a direct impact from another object (often another player’s body). It is more common for the ACL to be injured by a noncontact mechanism. Noncontact mechanisms include jumping, landing, cutting, or pivoting,. The ACL can tear when the stress on the knee exceeds the strength of the ligament.

What happens if the ACL is injured?
Unlike some other ligament and bone injuries, complete ruptures of the ACL do not heal. The knee may not function well for an athlete because it will be unstable. This means the knee may shift out of position during physical activity. In addition to not performing well during athletics, an unstable knee is at risk for further injury such as meniscus or cartilage tears. Surgery, in the form of ligament reconstruction, is usually recommended for active patients with ACL injuries.

Can ACL injuries be prevented?
Nothing can prevent all ACL injuries, but research has found that certain youth groups are at a particularly high risk for this injury. Female athletes, for example, have a 5 to 9 times increased chance of an ACL injury in sports such as soccer and basketball versus their male counterparts. While many reasons have been provided to explain this finding (anatomic, hormonal, and neuromuscular differences between genders), there is no one isolated risk factor. Of these potential differences, the only one that can be modified through intervention is neuromuscular control.

What is neuromuscular control?
Neuromuscular control involves the complex interaction of nerves and muscles that allows for coordinated movement. There is evidence that differences exist in the patterns of muscle activation between males and females during jumping and landing. These differences may alter the forces that are absorbed by muscles, placing more force on the ACL. Additionally, weakness in the core muscles (lower back, abdominal, and hip muscles) can place excessive stress on the ACL. This would usually be experienced during the landing part of a jump. In someone with weak core muscles, there is a tendency to land flatfooted, with a straight back, and the knees in a knock-kneed position (see figure below). There is a higher tendency in young female athletes to land in this position.

Bad Landing Position	Good Landing Position

How do ACL injury prevention programs work?
ACL injury prevention programs are designed to correct deficiencies in neuromuscular control and teach proper techniques for athletic movements. While there are several programs available, they all provide instruction in proper techniques for jumping, and landing, while improving core strength. The theory behind these programs is that by correcting core weakness and retraining neuromuscular activation patterns, the athlete will experience an at risk situation for a noncontact ACL injury less frequently.

For example, athletes will retrain their body to land on the balls of their feet with their knees and hips flexed and their knees centered over their feet (see figure above). These programs should be initiated 6-8 weeks prior to the start of the athletic season, and should have some components incorporated into warm-ups during the season to maintain the gains made during preseason.

Do ACL prevention programs work?
There are several studies that demonstrate that ACL injury prevention programs reduce the risk of sustaining injuries, though a handful of studies suggest there is no benefit. ACL injury prevention programs are relatively easy to implement by trainers and coaches during the preseason (refer to the presentation online). Therefore, I think there is little harm in incorporating a formal program during preseason or having screenings performed and encouraging at risk individuals to work on these exercises. Even if there is no perceived benefit for preventing ACL injuries during a given season, the exercise program can improve the athletic performance of the athletes by improving core and plyometric (explosive) strength.

References

1. Hewett TE et al. Anterior cruciate ligament injuries in female athletes. Am J Sports Med 2006;34:490-498.
2. Mandelbaum BR et al. Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: two-year follow up. Am J Sports Med 2005;33:1003-1010.
3. Griffin LY et al. Understanding and preventing noncontact anterior cruciate injuries. Am J Sports Med 2006;34:1512-1532.