Oct. 1, 2004
R. Alexander Creighton, M.D.
Soccer Team Physician
University of North Carolina
Injury to the anterior cruciate ligament (ACL) is a common problem in the athletic population. In the United States in any given year, it is estimated that roughly 1 of every 3000 people will suffer an ACL injury, with nearly 70% of these occurring during a sporting activity. The mechanism of injury usually involves twisting of the knee during deceleration. Landing awkwardly, landing with the knee straight after jumping, or changing directions suddenly while running put the ACL at risk for injury. Most ACL injuries are "non-contact" and do not involve a collision with another player. The incidence of ACL injuries has been reported to be greater in women than men. Data from soccer, basketball, and volleyball suggest that women are 3 to 7 times more likely to tear their ACL than men. On average women are five years younger then men when they rupture their ACL. Possible explanations for the gender difference in the rate of ACL injuries are intrinsic factors such as differences in anatomy, hormones, generalized laxity and neuromuscular function or extrinsic factors such as coaching, training, and conditioning techniques.
The anatomic differences that may predispose women to an ACL injury include a wider pelvis, increased genu valgum ("knock knee"), increased external tibial torsion ("out-toeing"), a narrower intercondylar notch, and a smaller ACL. These anatomic differences all place the women's ACL at a mechanical disadvantage compared to men.
In general, women have greater ligamentous laxity than men. There may be a hormonal basis for this difference. There are receptors for the female hormones estrogen and progesterone in the ACL. These hormones influence the cells on the surface of the ACL, the cells that make up the fibers of the ACL and the blood vessels of the ACL. Some studies have shown that an increase in estrogen concentration may decrease the strength of the ligament. Corresponding with this increase of estrogen, some authors have found a higher incidence of ACL tears during the ovulatory phase (days 5 through 12) of the menstrual cycle.
Some of the most interesting, and possibly most helpful findings have been looking at the neuromuscular differences between women and men. Some female athletes have a tendency to fire their quadriceps first before their hamstrings in response to the lower leg moving forward called anterior tibial translation. The problem with this is that the hamstring muscles decrease anterior tibial force, like a brake in a car, which is protective to the ACL. In contrast, the quadriceps pull the tibia forward, thus placing additional stress on the ACL.
Understanding these intrinsic differences and understanding the biomechanics of ACL injuries may influence how we coach, train, and condition our female athletes. For example, evaluation of jumping activities in motion laboratories has found differences between women and men. Women land with lower knee flexion angles than men. When landing from jumping, women on average have their knees approximately 10 degrees straighter than their male counterparts. This is true in both recreational and highly skilled athletes. Landing with a higher knee flexion angle is protective the ACL. It is also known that coaching can influence landing posture.
Fatigue also influences our bodies' protective mechanisms. The endurance of the quadriceps and hamstrings is less for female athletes even when normalized for weight, compared to male athletes. A fatigued athlete begins to stand up straighter, getting out of their athletic position and lowering their knee flexion angle, and putting their ACL at greater risk.
Age may play a role as it relates to level of competition. Female soccer athletes who are less than 16 years of age, who move up to compete at a higher level, are more likely to injure their ACL. Why this occurs is not clear.
The increased rate of ACL tears among female athletes is clearly established. The cause of this is not attributed to just one factor, but is multifactorial. Training regimens and coaching may modify some of these predisposing factors. The effect of jump training programs on high school volleyball players improved their muscle coordination. Even a six week training program may be enough to alter knee injuries in a high school population. Untrained female athletes had a 3.6 times higher incidence of knee injury than trained female athletes and a 4.8 times higher incidence of knee injury than male athletes.
There are ACL prevention programs being established across the country. These programs focus on dynamic motion, postural position, neuromuscular control, and education. If these prevention programs are to be successful, they should be started prior to maturation. Prior to puberty, males and females are very similar in their body habitus and neuromuscular control. It makes sense that these prevention programs would be most successful during the maturation process. Various programs incorporate plyometric exercises, core stability, improving hamstring-quadriceps strength ratio, and understanding advantageous biomechanical positioning. It is essential to the athlete to have good coaching and technique perfection. The success of these programs has yet to be determined, but their influence could be significant for the female athlete in prevention of ACL injuries.