Most people who enjoy playing or watching sports are familiar with the devastating progression of athletics-related ACL injuries: the initial pain, the invasive surgery, the lengthy rehabilitation process, and the prolonged absence from competition. ACL injuries are prevalent in professional, collegiate, and youth athletics. The immediate aftermath of an ACL tear is painful and expensive to manage, and the injury places individuals at risk for future long-term knee complications (1). This is in addition to the fact that ACL injuries invariably bring athletic careers to a screeching halt (8). The success of all athletes, from a star NFL running back to a budding middle school soccer star, is therefore dependent on the athletes’ ability to minimize the risk of ACL injury. So which athletes are at the highest risk for ACL injury? And what preventative strategies can be used to protect these athletes?

The number one risk factor for ACL injuries is gender. It has been well-established in the scientific literature that female athletes are at a greater risk for ACL injuries compared to their male counterparts, with some studies predicting as much as a 6-fold increased risk of ACL injury for females (2). There are a myriad of suspected reasons as to why the female body tends to sustain ACL injuries at a greater rate than the male body, however many of these theories boil down to females engaging in high-risk movements with greater force and less flexibility in the knee (3). This is a dangerous combination that may underlie the overrepresentation of females in the ACL injury population. A valgus rotation of the knee (when the knees collapse inward compared to the hips and ankles), is more likely to occur in females, potentially due to anatomical differences between females and males, especially in the pelvis (2). A great deal of attention has been given to the role of estrogen in female susceptibility to ACL injuries. A study using female soccer players supposedly recorded a greater number of ACL injuries in athletes in the ovulating stage of their menstrual cycle when estrogen levels are highest (4). However subsequent studies examining the effect of estrogen on muscle properties have generated mixed results, with some studies suggesting that increased estrogen levels alter ligament properties, and others not identifying an effect (5, 6). Regardless of the precise underlying mechanism, it is clear that being a female athlete raises the risk of sustaining an athletics-related ACL injury.

A second important factor that determines whether or not an athlete is at risk for an ACL injury is whether the sport and position require the athlete to engage in high-risk movement patterns that frequently result in ACL injury. At first thought, one might assume that athletes who engage in high-contact, collision-heavy sports such as football and wrestling would be the most at risk for sustaining an ACL injury. However, the vast majority of ACL injuries are non-contact related. Basketball and soccer are frequently cited as having some of the highest rates of ACL injuries (7). Even in sports where heavy contact between players is normal, ACL injuries tend to result from non-contact-related mechanisms. In the NFL, 72.5% of ACL injuries are not associated with direct contact to the injured knee (8). However, it is also important to note that even when ACL injury does not result from direct contact to the knee, injuries often arise from contact between players that alters their movement patterns and places them at risk for injury (3). The two movements that place an athlete at greatest risk for an ACL injury are deceleration after landing and cutting maneuvers (9). Both of these movements often lead to full extension of the knee and/or valgus knee collapse that can result in ACL injury (9). When picturing a basketball player landing after a layup, or a soccer player weaving around a defender, it is easy to understand why these sports boast such high rates of ACL injury. Athletes who play sports or positions that require these movement patterns are therefore at a higher risk of sustaining an ACL injury.

A final factor that must be considered when assessing an athlete’s risk of ACL injury is their level of competition. There are mixed opinions as to whether competing at a more elite or a more recreational level increases an athlete’s risk for ACL injury (7). The peak age for ACL reconstruction surgery is 17 years old, indicating that youth athletes may be among the highest at risk for ACL injuries (10). This is a particularly telling statistic given that older, elite athletes engage in more hours of risky athletic activity, yet still have fewer ACL injuries (7). Furthermore, there are several risks associated with youth athletics that may explain the high rates of youth athletics-related ACL injuries. Younger athletes are still developing their musculoskeletal system and coordination, placing them at risk for injuries that may arise from a lack of control over their relatively immature bodies (7). These issues are exacerbated by the current trend for youth athletes to specialize in a single sport early in their careers. Premature athletic specialization places youth athletes at risk for overuse injuries, a major problem considering that their bodies are still developing (7). Finally high school athletics, as opposed to collegiate and professional leagues, include a wide range of skill levels from future pro athletes to novices. The wide disparity in skill level can make these athletes unreliable and increases the likelihood of injury from unpredictable movements (7). This combination of factors highlights the dangers of youth sports for ACL health and indicates that preventative ACL injury programs should include and perhaps even focus on the youth demographic.

Given the high rates of ACL injury in athletes, considerable effort has gone into developing training programs designed to prevent ACL tears. However for the most part, these programs fail to prevent injury because it is difficult for athletes to translate the movements they learn in training to the fast-paced field of competition (11). When athletes participate in ACL prevention programs, they learn to avoid high-risk movements by consciously thinking about them in drills: a thought process that is far too extensive for an athlete to use when they are reacting to sudden plays during competition (11). To address this issue, Esurgi is in the process of developing a real-time, wearable device that allows athletes to receive instantaneous feedback when they engage in high-risk movements with the goal of training athletes to avoid high-risk movements automatically. Do you think that the Esurgi biofeedback device would be useful in preventing ACL injuries in athletes?

Citations

1. Beynnon, B. D., Vacek, P. M., Newell, M. K., Tourville, T. W., Smith, H. C., Shultz, S. J., Slauterbeck, J. R., & Johnson, R. J. (2014). The Effects of Level of Competition, Sport, and Sex on the Incidence of First-Time Noncontact Anterior Cruciate Ligament Injury. The American Journal of Sports Medicine, 42(8), 1806-1812. doi:10.1177/0363546514540862

2. Hewett, T. E., Myer, G. D., & Ford, K. R. (2006). Anterior Cruciate Ligament Injuries in Female Athletes. The American Journal of Sports Medicine, 34(2), 299-311. doi:10.1177/0363546505284183

3. Hewett, T. E., Shultz, S. J., & Griffin, L. Y. (2007). Understanding and preventing noncontact ACL injuries. Champaign, IL: Human Kinetics.

4. Wojtys, E. M., Huston, L. J., Lindenfeld, T. N., Hewett, T. E., & Greenfield, M. L. (1998). Association Between the Menstrual Cycle and Anterior Cruciate Ligament Injuries in Female Athletes. The American Journal of Sports Medicine, 26(5), 614-619. doi:10.1177/03635465980260050301

5. Samuel, C. S., Butkus, A., Coghlan, J. P., & Bateman, J. F. (1996). The effect of relaxin on collagen metabolism in the nonpregnant rat pubic symphysis: The influence of estrogen and progesterone in regulating relaxin activity. Endocrinology, 137(9), 3884-3890. doi:10.1210/endo.137.9.8756561

6. Strickland, S. M., Belknap, T. W., Turner, S. A., Wright, T. M., & Hannafin, J. A. (2003). Lack of Hormonal Influences on Mechanical Properties of Sheep Knee Ligaments. The American Journal of Sports Medicine, 31(2), 210-215. doi:10.1177/03635465030310020901

7. Gornitzky, A. L., Lott, A., Yellin, J. L., Fabricant, P. D., & Ganley, T. J. (2016). Sport-Specific Yearly Risk and Incidence of Anterior Cruciate Ligament Tears in High School Athletes: A Systematic Review and Meta-Analysis. Pediatrics, 137(Supplement 3). doi:10.1542/peds.137.supplement_3.561a

8. Johnston, J. T., Mandelbaum, B. R., Schub, D., Rodeo, S. A., Matava, M. J., Silvers-Granelli, H. J., Cole, B. J., ElAttrache, N. S., McAdams, T. R., & Brophy, R. H. (2018). Video Analysis of Anterior Cruciate Ligament Tears in Professional American Football Athletes. The American Journal of Sports Medicine, 46(4), 862-868. doi:10.1177/0363546518756328

9. Chaudhari, A. M., & Andriacchi, T. P. (2006). The mechanical consequences of dynamic frontal plane limb alignment for non-contact ACL injury. Journal of Biomechanics, 39(2), 330-338. doi:10.1016/j.jbiomech.2004.11.013

10. Dodwell, E. R., Lamont, L. E., Green, D. W., Pan, T. J., Marx, R. G., & Lyman, S. (2014). 20 Years of Pediatric Anterior Cruciate Ligament Reconstruction in New York State. The American Journal of Sports Medicine, 42(3), 675-680. doi:10.1177/036354651351841211. Benjaminse, A., & Otten, E. (2011). ACL injury prevention, more effective with a different way of motor learning? Knee Surgery, Sports Traumatology, Arthroscopy,19(4), 622-627. doi:10.1007/s00167-010-1313-z

11. Benjaminse, A., & Otten, E. (2011). ACL injury prevention, more effective with a different way of motor learning? Knee Surgery, Sports Traumatology, Arthroscopy, 19(4), 622-627. doi:10.1007/s00167-010-1313-z