There have been many studies conducted over the last couple of years confronting the issue of ACL injury and the female athlete. The data indicates that there has been a dramatic increase in the number of injuries of this nature. Female soccer players, gymnasts, and basketball players are at the greatest risk for an ACL injury (NCAA, 1994-95). One study indicates that female basketball players are 5-8 times more likely to sustain an ACL injury than their male counterparts (Malone et al, 1993). In a study that was conducted by Ireland et al (1997), the results were that in soccer players, the female to male ratio of ACL tears was 2:4 overall; for non-contact ACL tears, there was a 3:4 ratio. Basketball also showed a 4:1 female to male ratio in overall ACL tears and a 5: 3 ratio in non-contact injuries.

Many of these studies have focused on variables that are thought to be indicative of the increase in prevalence of ACL tears. There are many differences that may contribute to the increase in ACL tears in female athletes. Females tend to have a wider pelvis, increased femoral anteversion, less muscular development, a less developed VMO, increased flexibility, increased genu recurvatum, a more narrow intercondylar notch, more external tibial torsion, and more forefoot pronation (Ireland et al., 1997). Females also tend to be more ligament dominant, relying more on their ligaments for support. Males tend to be more muscle dominant, relying on muscle mass as a major support for their joints. There is also a relationship between having a small intercondylar notch and sustaining an ACL tear (Souryal et al, 1993).

Looking at these gender differences, we can target the areas that appear to offer the most potential for injury reduction in females. These are core stability to maintain proper pelvic alignment in a neutral range, strengthening of the gluteus medius and external rotators to control femoral adduction and internal rotation, and strengthening of the quadriceps, hamstrings, and ankles. Balance and proprioceptive training is also essential, because neuromuscular control is as important to lower extremity function as strength itself (Ireland et al, 1997).

Another controversy that centers on the female athlete is over the effect that estrogen has on the development and maintenance of the ACL. Female sex hormones exhibit a widespread effect on the growth and development of bone, muscle, and connective tissue (Lebrun, 1994). Although a number of animal studies have documented the effects of estrogen on collagen metabolism in various tissues, ligaments have not been the subjects of investigation until relatively recently. Several studies in vivo have shown that the quantity of collagen in certain tissues is influenced by estrogen. The presence of estrogen receptors in the human anterior cruciate ligament has been established (Liu et al, 1997). It is believed that alterations in anterior cruciate ligament cellular metabolism caused by estrogen fluctuations may change the composition of the ligament, rendering it more susceptible to injury (Liu et al, 1997).

Fluctuations in serum estrogen concentration during the menstrual cycle may induce changes in the metabolism, amount, and type of collagen in the ACL (Liu et al, 1997). The net effect of these cellular events may be the altered structure and composition of the ligament and a potential higher risk for ACL injury. Collagen, which is produced by fibroblasts, performs the load-bearing function of the ACL (Smith et al, 1993). Changes in the metabolism of these fibroblasts, by local stimuli with growth factors, mechanical strain, or hormones, have an influence on the quantity, type, and stability of the collagen in the ACL (Liu et al, 1995).

Physiologic levels of estriadol, as found in the human menstrual cycle, have a significant dose-dependent effect on the fibroblasts of the ACL. Both fibroblast proliferation and rate of collagen synthesis are significantly reduced with increasing estriadol concentrations. It is for this reason that it is theorized that cumulative or sudden changes in serum estrogen concentrations, such as those occurring during the menstrual cycle or from exogenous estrogen present in oral contraceptives, may induce changes in the metabolism of ACL fibroblasts. The resulting structural and compositional changes could result in reduced strength of the ACL, predisposing female athletes to ligament injury.

Further studies are needed to clarify the role of estrogen on ACL fibroblast metabolism in the human knee.