Anterior Cruciate Ligament Injuries in Female Soccer Players

Female athletes at the high school and collegiate level in the United States suffer more than 30,000 serious knee injuries each year. In recreational sports, the number of injuries is even greater. Many of these injuries are anterior cruiciate ligament (ACL) tears. Why women are 2 to 8 times more likely than men to sustain ACL tears, especially in sports like soccer that involve deceleration, twisting, cutting, and jumping, continues to be a challenging question. Differences in the anatomy, knee alignment, hormonal profile, muscle strength, and conditioning of men and women are just some of the possible reasons for the different rates of injury.

Anatomic differences
The knee joint comprises the cartilage-covered surfaces of three bones: the femur (thigh bone), the tibia (the shin bone), and the patella (knee cap). Four main ligaments help to stabilize the knee: the medial (inner side) and lateral (outer side) collateral ligaments resist side-to-side motion, and the anterior (front) and posterior (back) cruciate ligaments resist forward and backward motion, respectively (Fig. 1). The ACL lies inside the knee joint in the intercondylar notch, which is a depressed area at the end of the femur. This ligament provides most of the support that prevents the tibia from slipping too far forward and rotating too far inward under the femur. Women have a narrower notch than men, therefore, the space for the ACL's movement is more limited (Fig. 2). Within this restricted space, the thigh bone can easily pinch the ACL as the knee bends and extends, especially with the pivoting and cutting motions required in sports like soccer. Pinching of the ACL in the knee joint can lead to rupture of the ligament.

Alignment of the knee
As the femur meets the tibia, it creates an angle called the quadriceps, or Q angle. The width of the pelvis determines the size of the Q angle. Women have a wider pelvis than men; therefore, the Q angle is greater in women than in men (Fig. 3). A large Q angle can concentrate increasing force on the ACL each time the knee twists, increasing the risk of an ACL tear. A twisting injury in a man's knee may only stretch his ACL; however, because of the greater Q angle, the same type of twisting injury in a woman's knee may cause complete rupture of the ACL.

Hormonal profile
The possible role of hormones in predisposing female athletes to injury of the ACL has recently been an area of active investigation. In 1996, estrogen and progesterone (female sex hormones) receptor sites were found in female ACL cells, suggesting that hormones may play a role in ACL structure. Several researchers have since determined that female sex hormones can influence the composition and mechanical properties of the ACL, as well as the flexibility of muscles and tendons surrounding the knee. This flexibility helps prevent many injuries because it enables certain joints and muscles to absorb more impact before being damaged. However, this looseness does not necessarily prevent ACL injuries in a woman's knee. If the other ligaments and muscles around the knee are so loose that they cannot absorb the stresses placed on them, then even normal loads or forces may be transferred directly to the ACL, making it prone to rupture. In this sense, the ACL not only has to maintain stability about the knee, but it also must make up for the instability in a generally loose knee.

Muscle strength and conditioning
When women and men compete in the same sport, at the same high level of competition, they have nearly equal twisting and loading forces placed across their knee joints. However, women have less muscle strength in proportion to bone size than men. Muscles that help hold the knee in place are stronger in men than in women. Therefore, women's knees rely less on the muscles and more on the ACL for stability. Once again, the ACL may have to work overtime, making it more prone to injury.

Another point to take into consideration is the conditioning aspect of training. Traditionally, male athletes participate in twisting sports, like soccer, at an earlier age than women. This helps them develop muscle coordination and reflexes that can protect the knee once they reach the high school and collegiate competitive level. These knee reflexes allow strong muscles to control the knee, thereby maintaining its stability. Most female athletes do not participate in the same sports until a later age. Therefore, their muscle strength and coordination, as well as their reflexes, may not be as fully developed when they reach higher levels of competition.

For women, the ACL must provide most of the stability in the knee making it prone to injury. As women begin participating in sports at an earlier age and as they continue conditioning and strengthening the muscles around the knee, it is hoped that the rate of ACL tears in the female athlete will decrease.

Stephanie L. Silberberg, MD
Fort Walton Beach, Florida

Further Reading:
Griffin LY. Noncontact ACL Injuries: risk factors and prevention strategies. Journal of the American Academy of Orthopaedic Surgeons. 2000;8:141-150.
Evans NA. The Natural History and Tailored Treatment of ACL Injury. Physician and Sportsmedicine. 2001;29(9):19-34.