Anatomy and Purpose of the ACL
The ACL attaches inside of the knee joint diagonally from the front of the tibia (shin bone) to the back of the femur (thigh bone). It runs through a space called the intercondylar notch of the femur and prevents the femur from moving posteriorly (backward) and the tibia from moving anteriorly (forward) during weight bearing activity. It also prevents knee hyperextension and works with the hamstring muscles to stabilize the knee joint. The ACL is the crucial structure for knee joint stability, especially for athletes in sports that involve running, jumping, and pivoting. The most common mechanisms of ACL injuries are the non-contact movements of planting the foot and twisting, hyperextending the knee, or decelerating from a run or a jump. An ACL tear can result in serious damage to other joint structures and take months to heal after surgical reconstruction.

Risk Factors
In a study that reviewed the data submitted to the National Collegiate Athletic Association Injury Surveillance System from 1989 – 1998, the possible causes of increased ACL injuries among women was described [2]. These researchers classified the risk factors as either extrinsic (relating to the type of sports activities, the manner in which the sport is practiced, the environmental conditions, and the equipment used to play a sport) or intrinsic (relating to the factors that are individual, physical, or psychosocial). They further explained the extrinsic causation factors to relate to body movement, muscular strength, shoe-surface interface, and skill level and the intrinsic factors to comprise of joint laxity, limb alignment, intercondylar notch dimensions, ligament size, and hormonal influences.

Extrinsic Factors
Most reported ACL injuries among women have occurred in team sports. A significantly high female ACL injury rate was reported in collegiate women's soccer and basketball programs [1]. In Norwegian team handball during the 1989-90 and 1990- 91 seasons, 1.8% of the ACL injuries were women, as opposed to 1.0% for the men [4]. A prospective study was done on team handball during the 1993-94, 1994-95, and 1995-96 seasons. There were 23 ACL injuries among women and 5 among men [5]. Results of a 30 month study compared 19 ACL ruptures for female basketball players to only 4 for males [6]. A retrospective study of 176 Norwegian patients who had participated in organized soccer showed that women had an incidence rate of 0.10 ACL injuries per 1000 game hours, significantly higher than that for men (0.057) [7]. During a single basketball season, an injury survey of girls' varsity teams at 100 class 4A and 5A high schools in Texas revealed that female athletes had a significantly higher rate of knee injuries including a 3.79 times greater risk of ACL injuries [3]. Similar results were found in the sports of women’s volleyball and rugby [8, 9]. In the majority of these cases the injuries occurred during competition when the athletes were pushing themselves harder, and nearly all the injuries occurred in non-contact situations when the women performed plant-and-cut movements.
Environmental conditions, equipment problems, and shoe/surface interface have been discussed as possible ACL injury causes, but more intriguing are the effects of muscular strength, coordination, neuromuscular activation patterns, and proprioception (position sense and balance) on ACL injuries. The quadriceps and hamstring muscles provide dynamic restraint to the knee joint by absorbing high loads generated during sports. If these muscles are not strong enough, or if they contract in the wrong sequence, the ACL could be exposed to excessive forces. In addition, the hamstrings are the primary decelerators of the knee joint and vital for slowing down the body’s momentum during running and jumping. Without the deceleration of the hamstrings, the ACL is vulnerable. One study has shown that female athletes and a control group demonstrated significantly less quadriceps and hamstring muscle strength and endurance compared to male athletes, and the female athletes took significantly longer to generate maximum hamstring muscle torque during isokinetic testing. In addition, the female athletes appeared to rely more on their quadriceps muscles in response to anterior tibial translation and the three other test groups relied more on their hamstring muscles for initial knee stabilization. [10]. This places an increased stress on the ACL and may predispose it to injury. Mechanoreceptors located in the knee tendons and ligaments are responsible for detecting joint loads and responding to muscular activity. They basically give the muscle information about where the joint is in space, and what the muscle’s next job must be to accomplish the brain’s goal. When fatigued, both male and female athletes exhibit a decline in proprioception [11], however, when compared with males, females who participated in basketball and soccer took a longer time to detect knee joint motion moving into extension. Without this proprioceptive ability, they are at risk for ACL injuries. [12].
The argument used to be that because women weren’t as skilled as men in sports, or because they haven’t played organized sports as long as men, they would be more susceptible to injuries like ACL tears. Well, as the years have passed and more women are participating in sports, have better coaching, and are starting at a young age, skill level isn’t a consideration anymore. Presently, we should see a decline in the number of ACL injuries if those injuries are due to skill, not an incline [2, 13]. Even a study on women’s and men’s soccer and basketball players at NCAA Division I, II, and III institutions failed to relate skill level to ACL injuries [14].

Intrinsic Factors
Joint laxity means the stabilizing joint ligaments allow excessive translation, or movement. This could lead to structural damage inside the joint, as well as predisposing the joint to a traumatic event. One study revealed that in 34 healthy, collegiate-level female athletes who played soccer, basketball, or both possessed significantly greater knee joint laxity values than men [12]. Interestingly, ligament laxity increased during a female semi-professional basketball players’ practice, but returned to normal 5 hours after the practice [15].
The way the female body is designed predisposes her to non-contact ACL injuries where the primary mechanism is planting and twisting. Typically women have a wider pelvis, excessive foot pronation (where the inside border of the foot is pressed closely to the ground), increased genu valgum (“knock-knees”) and recurvatum (hyperextension), and more external tibial torsion (where the shin bone is rotated towards the outside of the body) than men. These faulty alignments are similar to the ACL injury body positions where ACL injuries occur [16].
Much research has confirmed that women have smaller intercondylar notch dimensions and ligament size than men, and ACL tears are correlated with statistically smaller notches. Data gathered from 902 high school athletes, demonstrated that athletes with a smaller intercondylar notch were at significantly greater risk for sustaining non-contact ACL injuries [17]. A study of 100 men and 100 women concluded that the intercondylar notch width is narrower in women than men, and, in both men and women, the notch width is narrower in patients who sustain ACL tears compared with controls [18]. For female handball players, there was an increased risk of ACL injury associated with decreasing notch opening. In fact, the handball players with 17 mm or less anterior notch width were 6 times more susceptible to an ACL injury than compared to players with wider notch widths [19].
Perhaps the greatest area of interest, and one in need of future study, is the effect that female hormones have on ACL injuries. For a hormone to have an effect on a body tissue, a receptor must be present to accommodate that particular hormone. ACLs have been found to possess estrogen and progesterone receptors [20]. Estrogen fluctuations during a woman’s menstrual cycle may change the composition of the ligament and render it more susceptible to injury [21]. The menstrual cycle is divided into four phases: the menstrual phase, the follicular phase, the ovulatory phase, and the luteal phase. In a recent study on ACL laxity and hormones, the follicular, ovulatory, and luteal phases were found to have the peak values for estrogen and progesterone. Estrogen levels rise during the follicular phase and progesterone levels rise during the ovulatory and luteal phases. ACL laxity was shown to increase with increasing levels of circulating estrogen associated with the follicular phase, and the greatest ACL laxity was associated with the luteal phase [22]. These results did not confirm whether ACL laxity is a combined effect of estrogen and progesterone, or if it is due to another hormone. This study also did not demonstrate if the laxity directly correlated with ACL injuries. However, a study on (high-level team handball players confirmed that five of the ACL injuries occurred in the menstrual phase, 2 in the follicular phase, 1 in the early luteal phase, and 9 in the late luteal phase. These results suggest that there may be an increased risk of ACL injury during the week prior to or after the start of the menstrual period [5]. Another study confirmed that a significant statistical association was found between the stage of the menstrual cycle and the likelihood for an ACL injury. This study also showed that women taking birth control pills had a lower injury rate, which may suggest that hormone stabilization decreases the possibility of laxity [23]. From a review of the data submitted to the National Collegiate Athletic Association Injury Surveillance System from 1989 – 1998, Arendt and Dick also found that females were more likely to be injured just before or just after their menses and not midcycle [1].

Solutions
Although ACL injuries do occur more often in women than in men, and some risk factors are unavoidable, preventing injuries are the best way to decrease injuries. Using the right equipment, training properly, warming up, and stretching before activity can lessen the risk of injuries for all athletes [24]. Increasing hamstring strength and endurance cannot be underestimated in the prevention of ACL injuries. But the role of neuromuscular training seems to be the key to reducing the risk of ACL injuries for women (take this ref out or change)[13]. Many ACL reconstruction physical therapy protocols now include emphasis on challenging the neuromuscular system through proprioception, kinesthesia, dynamic joint stability, balance, and reactivity exercises [25].

Why women?
Athletic injuries on the whole are multifactorial and result from a complex interaction of risk factors. It is difficult to isolate one factor without acknowledging the possibility of others. In addition, injuries may be more sport specific than sex specific [2]. However, there is proof that women have risk factors that predispose them to ACL injuries. Armed with this knowledge, we can now focus on specific women’s protocols for prevention and rehabilitation. Part II of this article will describe in detail prevention and rehabilitation programs.

References
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