Shea RickettsLos Angeles Sparks forward and WNBA Rookie of the Year contender Cameron Brink was sidelined mid-season by a torn anterior cruciate ligament (ACL). Adding more insult to the injury, Brink missed competing in the 2024 Paris Olympics with the United States Basketball 3×3 Women’s National Team. Brink is not alone in missing the Paris games due to injury. Several highly competitive sportswomen missed the Paris Olympics due to ACL tears sustained earlier in the year. Women of all skill levels disproportionately experience this injury compared to men, and studies consistently report that women are two to eight times more likely to suffer an ACL tear than men. For female athletes, especially basketball and soccer athletes, it’s not a question of “if” an ACL tear will happen, but “when”.
The ACL is a ligament inside the knee that stabilizes the leg during movement. ACL tears often occur following a rapid, sometimes awkward change in lateral movements. ACL injuries come with multidimensional losses. From individual athletes’ high healthcare costs to an increased risk for degenerative joint diseases (such as early onset post-traumatic osteoarthritis) to an athletic organization’s instantaneous roster shakeup, ACL injuries impact every aspect of sports. Team USA’s 3v3 Women’s Basketball team unequivocally felt the loss of Brink. Brink underwent reconstructive ACL surgery, while Team USA brought home a Bronze medal from the Paris Games. A year prior that same team, with Brink at the helm, captured Gold in Vienna at the 2023 3×3 Wold Cup. The most devastating loss an athlete faces with an ACL tear is the reality that this injury can rob her of her competitive career. Following ACL reconstructive surgery, the most common treatment strategy for a torn ACL, only 55% of athletes return to competitive sports at pre-injury participation levels.
As of 2024, the ACL injury incidence rate is rising among women. Despite the well known disproportionate rate of injury between men and women, sports medicine research has yet to determine what causes this difference. Female athletes are significantly underrepresented in sports medicine related research. Out of over 5000+ publications on exercise science and sports medicine, only 6% of studies have focused exclusively on female athletes in contrast to the 31% that exclusively focus on male athletes. These sex differences in athletic studies are unsurprising given that women are disproportionately understudied in medical research
Women have historically and systematically been excluded from clinical trials and neglected in medical research. While Congress passed the National Institute of Health’s inclusion policy into Federal Law in 1994 and mandated that women and minorities be included in government funded research, several organizations have been lax at adopting policies to include women in studies. Only this year has the American Medical Association finally established policy efforts to increase equity in medical research by including women and sexual and gender minority populations.
The systemic health inequity and lack of inclusion for women trickles down in every facet of biomedical research. Male bias is seen across multiple aspects of basic science and pre-clinical science from subjects used to animal models tested. Often only male mice are used in pre-clinical research, which is attributed to the flawed argument that hormone fluctuations in female mice could skew the data. In fact, studies with male mice outnumber female mice at six to one. This ideology has resulted in an incomplete understanding of how men and women experience disease and respond to treatment.
When it comes to ACL tears, the medical community attributes the difference between male and female athletes to three factors: physiology, biomechanical structure, and hormones. Physiologically, female knee joints are more lax with thinner tissue than male knee joints. Additionally, women are more susceptible to quad dominance, or using the quadricep muscle more than the hamstrings and glutes during exercise. Quad dominance is thought to be the most common cause for an ACL tear in women. With no relation to a woman’s physical strength, biomechanical structure and female anatomy differs from men. Compared to men, women often present with wider hips and a tapering in of the leg. This bone structure difference changes how the thigh bone, tibia and femur synergistically function in women compared to men. The inward tapering of the leg places more rotational force on the knee in women, magnifying the risk for injury with quick changes of movement. These musculoskeletal differences can be targeted with prevention exercises and can be addressed in biomechanical rehabilitation following surgery.
The third difference attributed to ACL tears in women, hormones, is arguably the most frustrating because hormonal differences pertaining to ACL injuries have yet to be fully addressed. There is no consensus agreement among the scientific community on the exact role hormones play in the increased incidence of ACL tears in female athletes. Even though it’s widely accepted that hormones have cyclic effects, there is a large gap in knowledge about how hormone levels influence injuries, from injury prevention to repair post-injury. Epidemiological evidence suggests there is an association between hormonal fluctuations and ACL injuries. Studies have demonstrated 70% of ACL injuries occur when athletes have high estrogen levels, a period known as the follicular phase in the menstrual cycle. Other studies have demonstrated relaxin, which peaks in expression shortly before estrogen peaks, may influence susceptibility of an ACL tear in women. Relaxin receptors, components that relaxin can bind to, are more highly expressed in female ACLs compared to male ACLs. Further, higher relaxin levels can reduce ligament integrity and consequently increase risk for injury. So far (or however), research into hormone levels and ACL injury is simply correlative, lacking both magnitude and mechanism of hormonal impact. If an underlying cause of ACL injury susceptibility for women is hormonal fluctuations between estrogen and relaxin, female athletes deserve a comprehensive and thorough scientific investigation into that hypothesis.
Sports medicine research needs to understand the scope by which hormones impact ACL injury in women. Sex, while not relevant for performance outcomes, does have differences in ACL injury rates and addressing why that difference exists is long overdue. The next step in the evolution of women’s sports must include improving the quality and quantity of research on the health, recovery, and performance of female athletes.
Peer Editor: Quinn Eberhard