Recently, women's participation in collegiate sports has increased dramatically with a 9% rise from 1989-1992 as doc-umented by the National Collegiate Athletic Association (NCAA). Prior to the introduction of Title IX in 1972, which allowed equal opportunities for female athletes at the intercollegiate level, there was no noted gender difference with regard to sports injuries. Subsequent to this, certain sports such as soccer, basketball, volleyball, field hockey, gymnastics, softball and handball noted a predominance of gender specific injuries. Although the ankle is the most commonly injured joint during sporting activities, knee injuries are often the most debilitating. Anterior cruciate ligament (ACL) injuries have been reported to be 2-8 times more common in women (1,2) . A 5-year study by the NCAA revealed that women were 2.4 times more likely to tear their ACL than men in soccer, and 4.1 times more likely in basketball. This high rate of ACL injuries has become a near epidemic problem to players, teams and the sports medicine community.

Numerous intrinsic and extrinsic factors have been investigated to ascertain the cause of the higher rate of ACL injuries among women athletes. Extrinsic sources include athlete conditioning, level of motor skill, muscular strength, shoe/floor interface, coordination, and body mechanics. Intrinsic causes include anatomical and physiologic factors such as ligament size, intracondylar notch size, lower extremity alignment, hormonal effects, and joint laxity.

Dr. Arnold is Sports Medicine Fellow at Massachusetts 
General Hospital
Dr. Boland is Chief of Sports Medicine at Harvard University
and Assistant Professor of Orthopaedic Surgery, Harvard
Medical School

Please address correspondence to:

Arthur Boland, MD
Sports Medicine Service
Massachusetts General Hospital
Boston, MA 02114

It is believed that the majority of ACL injuries are the result of a non-contact mechanism such as landing from a jump, cutting, or sudden deceleration (3) . Due to the non-contact nature of ACL injuries, extrinsic factors such as muscular weakness and fitness level have been investigated as possible causes. Women were found to have a lower hamstring-to-quadriceps strength ratio than men (5) . Huston and Wojyts noted that women athletes took longer than men did to generate maximum hamstring torque during isokinetic testing (9) . Whether this contributes to the higher rate of ACL injuries among women is unclear. Grace et al., found no difference between strength ratio and knee injuries in high school foot-ball player's (4) . Hewett et al. noted a difference between ham-string and quadriceps strength in female athletes before training (5) . They also noted that men have flexor moments about the knee that are three times that of women during jumping (5) . Furthermore, ACL injuries were reduced by decreasing the adduction/abduction moment arm and through improving hamstring to quadriceps ratio by a jump training program (23) .

The intrinsic causes of ACL injuries have also been studied extensively. The issue of intracondylar notch size as a pre-disposing factor has been an area of particular interest. Souryal and Freeman noted a positive correlation between notch stenosis and ACL tears (6) . Laprade et al. noted a positive correlation between a narrow notch and ACL tears but did not find a difference in notch width between men and women (7) . It has been suggested that the formation of a stenotic notch con-tributing to an ACL is due to increased impingement of the ACL on the lateral femoral condyle during a forceful external rotation of the tibia on the femur (7) . It has also been proposed that a narrow notch may contain a smaller ACL (7) . However, whereas notch size may be related to ACL rupture, it does not appear to be found preferentially in women.

Several studies have investigated joint laxity as a possible factor related to the higher incidence of ACL injuries in women (8,15) . Haycock and Gillette suggested that the higher ACL injury rate in women may be due to their increased flexibility and ligamentous laxity (8) . Women have been shown to be significantly more flexible and have increased joint laxity compared with men (9,15) . Although some authors have documented a relationship between joint laxity and ACL injury (10) , others have been unable to demonstrate this relationship (11) .

Much attention has also focused on the higher quadriceps angle (Q angle) in women as a potential cause of higher ACL rupture rates. The Q angle is typically 10 degrees in men, and 15 degrees in women. A larger Q angle in women may predis-pose the knee to greater biomechanical stresses, thus poten-tially increasing the likelihood of an ACL rupture (8) . However, as with other theories, no conclusive evidence is available to support this hypothesis.

Hormone levels should be considered as another potential contributory factor. Women possess a unique hormonal cycle, during which time the hormonal levels change dramatically over a 28-day period. The follicular phase (day 1-9) is charac-terized by low levels of estrogen and progesterone. A mid-cycle rise in estrogen precedes ovulation (day 10-14). In the final stage, the luteal phase (day 15-28), progesterone levels rise, and relaxin levels increase halfway through the phase. Estrogen and relaxin are known to affect tissue away from the reproductive system. These hormones can affect the growth and development of bone, muscle, and connective tissue (12) . Liu et al. reported the presence of estrogen and progesterone receptors in the ACL (18) . Similarly, Harris et al., likewise found relaxin receptors in the ACL (19) . These studies suggest that variations in hormone levels may have a direct effect on the predisposition to ACL rupture.

Estrogen is known to affect muscle function and soft tis-sue strength. It has been reported to decrease collagen synthesis in rat periodontal tissue, (20) . When estrogen is given to oophorectomized rats, the result is a reduction in the force to ligament failure (21) . Liu et al. investigated the effects of 17-beta-estradiol on rabbit ACL fibroblast proliferation and collagen synthesis in vitro, noting decreased fibroblast proliferation and collagen synthesis with increased estradiol exposure (12) . He concluded that cumulative or sudden fluctuations in estrogen concentration, as seen in the menstrual cycle, may induce changes in ACL fibroblasts resulting in structural and compositional changes that could weaken the ACL making them more susceptible to injury (12) .

Relaxin is an insulin-like hormone whose primary function is to cause collagen remodeling in pregnant women. These changes result in lengthening of the intrapubic ligament, softening of the birth canal, inhibition of uterine contraction, and mammary gland stimulation. Relaxin has been proposed to affect connective tissues outside of the reproductive system (17) In non-pregnant females, relaxin is produced by the corpus luteum, with a peak rise 6-9 days after the luteal surge. The precise effects of relaxin on soft tissue physiology are currently unclear. Relaxin has been proposed to have a role in collagen degradation as well as inhibiting collagen synthesis in both the pregnant and non-pregnant female (17) . Various authors have noted that serum relaxin levels may have cumu-lative effects on soft tissues such as the ACL, causing increased joint laxity (16) .

The joint laxity seen in pregnancy is present in joints other than the pelvis (16) . Calguneri suggested that prolonged exposure to relaxin, as seen in pregnancy, may have cumulative effects on ligaments causing increased laxity with prolonged exposure (16) . Blecher and Richmond reported a case of a woman with an ACL reconstruction resulting in a stable knee post-operatively, who subsequently experienced a transient increase in translation at 9 weeks and 1 year postpartum, returning to normal at 16 months 13 . Conceivably, relaxin could cause increased knee laxity in certain women with higher serum concentrations, and subsequently could put them at higher risk for ACL rupture. Arnold et al., failed to find a statistically significant rela-tionship between weekly knee joint laxity as determined by KT-1000 and weekly serum relaxin levels. They noted that knee laxity and serum relaxin levels were higher in the ACL injured women, although this was not statistically significant (15) .

Various authors have reported decreased athletic performance, and increased risk of injury during the premenstrual or menstrual phases (14,22,24) . Wojyts et al. noted more ACL tears dur-ing the ovulation phase of the cycle in non-contact injuries (24) . They also noted that a majority of women in their study (20 of 28) felt their athletic performance was worse during the pre-menstrual period (24) . Moller-Nielson and Hammer studied women soccer players and noted an increased injury rate during the pre-menstrual and menstrual period (14) . They also found that women with pre-menstrual and menstrual discomfort were at higher risk of injury during these phases (14) . Hormonal fluctuations may also affect the neuromuscular system. Posthuma et al. noted a decrease in fine motor skills during the premenstrual cycle (22) .

ACL injuries are more common in women than in men who participate in certain sports such as soccer, basketball, vol-leyball, field hockey, gymnastics, and softball (1,2) . The reason for these differences remains unclear. As participation by women in sports and level of competition continue to increase, these trends are likely to continue. Additional research is needed to identify the etiologic factors that predispose women athletes to an increased rate of ACL injury.