Female Susceptibility to Anterior Cruciate Ligament Tears

Christine Ogden

Writer’s comment: When my English 104F class was assigned to write a literature review, I was quick to decide that my topic would be the occurrence of ACL tears in women. After playing soccer for years, I finally experienced a ruptured ACL in high school and have been interested in the subject ever since. Recently, I have become increasingly aware that the incidence of females who tear their ACL is much higher than that of men. By sifting through many medical journals, I found that female susceptibility to ACL tears was currently being investigated. Statistical information based on experiments was useful in deciding a trend, but more importantly, I learned that recent experiments help to plan future preventative programs for women. Writing this review has given me the opportunity to research a topic that genuinely interests me, but perhaps more importantly, provided me with the skills to condense large amounts of information into a readable composition that can inform a diverse audience.
—Christine Ogden

Instructor’s comment: In this literature review assignment, students must survey, organize and summarize the current scientific or medical studies on a particular topic. Ideally, the writer will sum up the state of scientific knowledge on her chosen subject, indicating which research questions have been answered and which ones remain unsolved. The substantial challenges of the literature review primarily involve topic selection and information organization. Christine made a wise move in selecting a topic close to her heart. Her exhaustive literature searches yielded just the right amount of information, which Christine organized into logical subcategories that explain why women are more susceptible than men to ACL tears and what a coach or athlete might do to prevent such injuries. Christine’s efforts have resulted in a piece that coaches of female sports teams can actually use to educate themselves on strategies for preventing these debilitating injuries.
—Anne Fleischmann, English Department, Sierra College

         Women involved in physical activity are eight times more likely to rupture their anterior cruciate ligament (ACL) than men who are involved in the same physical activity (Lephart 2002). With this number on the rise, researchers are currently investigating some contributing factors to ACL injury in females. Targeted factors in females include anatomical positioning of hips and lower extremities, neurologic recruitment of muscle groups, reflex speed, and hormone influence on laxity within the knee joint. Although anatomical features and neurological responses are difficult to alter, recent exploration has demonstrated that prevention programs focused on muscle development and proprioception skills might decrease the risk of an ACL tear in some women (Huston 2000). This review will expand on differences in anatomical structure, neurological processes, physiological mechanisms and hormonal fluctuation in females, and will further detail prevention methods currently being tested to decrease the occurrence of ACL ruptures in women.

Anatomical Differences
         While men and women exhibit the same physical features in their lower extremities, the size and positioning of these features differ. One difference between males and females is the width of the femoral notch. On average, men have an intercondylar notch slightly larger than that of females (Shelbourne 1998). The larger notch has been speculated to facilitate movement and prevent impingement on the ACL (Huston 2000). Lund-Hanssen et al. concluded from a study performed on female handball players that women having a femoral notch width less than 17mm were six times more likely to tear their ACL than men (1994). A narrow femoral notch can inhibit range of motion and pinch the ACL during flexion (Huston 2000). Another anatomical feature that contributes to the likelihood of ACL ruptures in women is the way the knee is aligned with the hip (Huston 2000). When compared to women’s, the narrow hips of males allow the knee joints to align at a fairly minimal quadriceps femoris angle, or Q angle, with their hips. In contrast, the Q angle in women is much larger due to increased hip width. While increased hip width in females allows for ease in pregnancy and childbirth, a large Q angle causes a tremendous amount of force to be exerted on the medial aspect of the knee joint. When a large load is forced upon the knee during jumps or pivots, the ACL is strained and may not be able to prevent tibial slipping (McAlindon 2002). Unregulated tibial shiftage can lead to rupture of the ACL.

Neuromuscular and Physiological Factors
         Both reflex control and proprioception play a large role in coordination of muscle firing and balance. Without quick response to force, the knee joint can often experience ligament ruptures. When men and women were both tested to judge the ability of their knees to handle a landing following a jump, reports concluded that women exhibited more force and greater extension during landing (Chappell 2002). These findings led to the idea that women might have slower motor reflexes than their male counterparts (Kaplan 2002). On a similar note, men and women reportedly recruit muscles differently (Huston 2000). While most men successfully used their hamstrings to land a jump, those women who used more of their quadriceps to land a jump experienced injury to the knee (Huston 2000 and Lephart 2002). Such studies support the hypothesis that men and women differ in both neurological and physiological reactions to physical disturbances.
         Because studies show that there may be a difference in the reaction times and motor unit recruitment, researchers began to run tests to see if individuals who have trained to increase reaction time would tear their ACL as often as an untrained individuals. T.E. Hewett (1999) conducted a test that divided individuals into three groups: “male athletes,” “female athletes,” and “untrained females.” These three groups were then monitored over the course of their athletic seasons. Only 2 out of 434 males exhibited serious knee injuries with only one injury from a noncontact situation, while 2 of 366 trained female athletes exhibited serious knee injuries, neither of which resulted from noncontact situations. In the final group of untrained female participants 10 of 463 subjects exhibited serious knee injuries, eight of which were noncontact injuries, supporting the idea that neuromuscular reactions of untrained female athletes are slower than those of trained female and male athletes (Hewett 1999).
         Researchers are investigating a related aspect of neurology with regards to muscle firing pattern. Scott Lephart et al. (2002) have been observing motor reflex arcs in women and have speculated that women may have a longer electromechanical delay than men. When a person senses a disruption, she immediately communicates to the brain via alpha motor neurons to produce both spinal reflexes and muscle spindle triggering. Such messages usually take a short amount of time to reach the target tissue, but recent evaluations suggest that males and females exhibit slight differences in reaction times (Lephart 2002). The electromechanical delay can reduce muscle reaction and therefore allow ligaments such as the ACL to be stretched for a longer period of time than they normally would. Shorter electromechanical delay can actually bring the surrounding muscles to flexion, in turn reducing the load forced on the ACL (Lephart 2002). Slow motor reflex and quadriceps overuse leaves untrained females participating in physical activity at an extremely high risk for ACL injury.

Hormone Fluctuation
         Females experience large fluxes of hormone levels including estrogen, progesterone, and sometimes relaxin. Because of such great fluctuations in the hormones of females, researchers have linked rate of ACL injury with levels of estrogen (Huston 2002). Unfortunately, administering high doses of hormones to human patients poses great health risks, so most studies are conducted on animals. In a study performed by Slauterbeck (1999) female rabbits were subjected to different levels of estrogen to test for laxity in the knee joint. The ACL load failure rate was measured to be higher in those rabbits that received the estrogen supplements. This supported the hypothesis that females have increased laxity in their knee joints due to increased estrogen levels. The study further concluded that estrogen might play a factor in weakening the strength of the ACL in women (Slauterbeck 1999). In addition to estrogen, relaxin, though present in minute amounts, may also be a contributing factor to ACL tears in females (Liu 1996). This hormone, usually present in high amounts during pregnancy, has been detected in trace amounts in ovulating females (Lui 1996). Relaxin serves to create laxity in the hip joints during childbirth, but even small amounts found in knee joints might validate the theory that ACL ruptures in females are linked to joint laxity (Lui 1996). Further studies must be conducted to confirm such hypotheses.
         Additional research by Stephen H. Lui et al., revealed that there were hormone receptors for estrogen and progesterone on the ACL, suggesting that both hormones might influence the function of the ACL (1996). When researchers tested the precursor to estrogen, estradiol, they found that the rate of collagenous material decreased when excess estradiol was introduced into the system. Many hormone studies relate female menstrual cycles to occurrence of ACL injury (Huston 2000). While not completely supported, the hypothesis stems from the idea that estrogen and progesterone levels are high during ovulation, therefore creating a greater risk of ACL injury (Huston 2000). T.E. Hewett suggests oral contraceptives as a way to regulate hormone levels in females and further suggests that the regulation of the hormones can serve to decrease the occurrence of ACL tear (2000). Researchers suspect that if ACL injury is related to hormone flux, the occurrence of injury in females will decrease through the introduction of a consistent level of estrogen and progesterone.

         Females do have options to decrease their chance of severe knee injury. Because altering physical structure is impossible, Huston et al. suggests internal rotation exercises, which include medial hamstring exercises (2000). Such exercises allow the muscles surrounding the knee to support the medial load exerted from a wide Q angle (2000). While anatomical structure cannot be drastically changed, neurological and physiological responses most definitely can be (2000). Huston et al. (2000) along with Hewitt et al. (2000) suggest that neuromuscular strength and proprioception drills, such as balance board exercises, might stabilize the knee in strenuous situations. In the study performed by Hewett et al., where trained and untrained female athletes were observed during their athletic seasons, the researchers concluded that the women who had some sort of previous muscle training exhibited a much smaller number of ACL tears than those who were completely untrained (1999). Other suggestions from Hewett et al. explain that plyometric training and safer landing techniques can significantly reduce injury or prepare the surrounding muscles of the knee for tibial shiftage (1999). Preparation exercises allow the brain to adjust to distinct movements such as cutting, pivoting, or jumping, hence creating quicker physiological responses.

         The high rate of ACL tears in females has spurred a large number of researchers and physicians to look into the mechanism of injury in great detail. Until recently, the ACL injury to women was treated in the same manner as male patients. However, details about female joint position, condyle widths, neurological patterns, and pelvic and/or knee laxity have given researchers the chance to formulate methods of possible prevention specific to females. With an increasing number of women participating in physical activity, especially in sports, prevention programs are beginning to flourish. Most programs focus on increasing proprioceptive skills and reaction time. Programs can be selected based on level of athleticism. Those involved in competitive sports should have a more rigorous workout to prepare for greater load weight. Cone and agility drills combined with jump-landing techniques are exercises that have the greatest effect of reducing injury (Hewett 1999). Since females are at such high risk for ACL rupture, athletic trainers, physical therapists and physicians should encourage their female patients to embrace a strengthening and timing program that will decrease the effects of factors contributing to ligamentous injury to the knee.


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