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Why women have an increased risk of ACL injury

By Timothy E. Hewett, PhD

Decreased neuromuscular control of the trunk leads to valgus torques at the knee

Female athletes who participate in jumping and pivoting sports are 2 to 10 times more likely to sustain a knee ligament injury, such as an anterior cruciate ligament (ACL) injury, than male athletes participating in the same sports. The substantial increase in the number of women and girls participating in athletics, coupled with the 2- to 10-fold higher injury rate, widened the gender gap in the number of serious knee ligament injuries.

Most ACL injuries—whether in male or female athletes—occur by noncontact mechanisms, often during landing from a jump or making a lateral pivot while running. Osteoarthritis of the knee will develop in 50 percent to 100 percent of injured female athletes within one to two decades after injury.

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Several biomechanic and neuromuscular factors may be related to the increased incidence of knee injury in female athletes. Our findings—collected over the last two decades—indicate that valgus knee torques and neuromuscular control of the trunk predict ACL injury risk in female athletes with similar levels of sensitivity and specificity. These predictors may be linked, because lateral positioning of the trunk can create high valgus torques at the knee by both biomechanic and neuromuscular mechanisms.

Timothy E. Hewett, PhD

Mechanisms of injury
The mechanisms of ACL injury in female athletes include high knee valgus torques and lateral trunk motion, with most of the body’s weight shifted over the injured limb and the foot placed lateral to the body’s center of mass. An unanticipated disturbance often contributes to the injury mechanism. A strong association exists between increased lateral trunk motion and coronal plane abduction valgus torques on the knee. Neuromuscular control deficits at the hip and trunk may contribute to decreased active neuromuscular control of the lower extremity that may increase knee abduction valgus torques and strain on the knee ligaments.

Trunk motion may result in increased knee abduction by increasing the magnitude or distance of the ground reaction force vector to the knee center or reactive hip adductor torque or by both knee loading mechanisms. These associations provide a strong, evidence-based rationale for the development of effective trunk or “core-based” interventions to decrease ACL injury risk in high-risk female athletes.

The combination of the greater susceptibility and a 10-fold increase in the female sports population since the inception of Title IX has resulted in a dramatic increase in the number of ACL injuries in females. In the United States, 100,000 to 300,000 total ACL injuries occur each year. The treatment costs exceed $1 billion annually in female varsity athletics alone and do not end with the ACL reconstruction and rehabilitation. A strong association exists between ACL injury and development of posttraumatic knee osteoarthritis at a relatively young age, which also occurs with much greater incidence in females than males.

Coronal plane knee valgus torques and trunk displacement both predict ACL injury risk in female athletes. Valgus torques predicted ACL injury risk with 78 percent sensitivity and 73 percent specificity. Trunk displacement predicted risk of knee, knee ligament, and ACL injuries with high sensitivity and specificity in female, but not male, athletes. A logistic regression model that incorporated lateral trunk motion predicted ACL injury risk in female athletes with 83 percent sensitivity and 76 percent specificity, but did not predict knee or ACL injury risk in male athletes.

The mechanism of ACL injury may differ in female and male athletes, especially with respect to the dynamic positioning of the knee, because female athletes demonstrate greater valgus collapse of the lower extremity, primarily in the coronal plane. The mechanism of noncontact ACL injuries as observed on video has several common components in female athletes: high knee abduction, lateral trunk motion with the body shifted over one leg; the plantar surface of the foot fixed flat on the playing surface and displaced away from the trunk; and low knee flexion. Unanticipated disturbance in the motion of the trunk is another common component.

Deficits in neuromuscular control of the trunk may contribute to knee joint instability and injury. Landing and cutting require high levels of neuromuscular control to maintain stability and performance. Dynamic stability of the knee depends on accurate sensory input and appropriate motor responses to rapid changes in body position during cutting and landing.

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Neuromuscular training may help reduce the incidence of ACL injuries in female athletes.

The impact of neuromuscular training
Neuromuscular control of the hip, trunk, and knee is based on feedback control. The position and load of each body segment is used to modify the descending movement commands. Impaired control of the hip and trunk can increase lower extremity injury. For example, abdominal muscle fatigue contributes to hamstrings injuries. Patients with ankle sprains had a delay in onset of gluteus maximus and medius activation.

Movement biomechanics and knee joint strength can be altered in female athletes with neuromuscular training (NMT). Neuromuscular power can increase within 6 weeks of training and may reduce peak impact forces and knee abduction torques. Because baseline neuromuscular performance levels are lower in female than in male athletes, the observed changes may be greater in female athletes as well. If NMT can decrease ACL injury risk, the mechanisms underlying increased risk are likely to be neuromuscular in nature.

ACL injury risk may be reduced in trained female athletes during landing and cutting. Prospective studies show that NMT has the potential to decrease ACL injury rates in females. Neuromuscular control of the trunk and knee can be improved with NMT, which may increase coronal plane trunk and hip control in females. For example, during a drop-jump—a two-footed plyometric activity—post-training results showed that lower extremity valgus was reduced at the hip. Conversely, during a single-leg landing task, the most significant modifications may occur at the knee. Therefore, the effects of training in the coronal plane are likely to be movement task specific.

These results indicate that resultant valgus torque is controlled primarily by hip torque during a two-footed landing, but more by the change in the ground reaction force during single-leg tasks. Increased coronal plane control at both the hip and the trunk may be necessary to reduce ACL injury risk. Lower extremity coronal joint motions and torques linked to increased ACL injury risk are often correlated, indicating that control of knee valgus torques may require synergistic and antagonistic contribution from the trunk, hip, and knee. Perturbation-enhanced training may increase trunk control and decrease hip adduction and knee abduction valgus torques in females.

Female athletes tend to use greater coronal plane control rather than a sagittal plane control strategy for the lower extremity. They tend to use a “trunk dominant strategy” for single-leg control and balance during landing and cutting. For example, coronal plane excursions are greater and more rapid at the hip and knee during walking in females. The knee functions optimally as a sagittal plane hinge, not a coronal plane hinge or ball and socket joint, because the large muscles of the lower extremity that limit coronal plane trunk, hip, and knee motion or torque absorb and dissipate force most effectively and efficiently are in the sagittal, rather than the coronal, plane. Women also demonstrate less active joint stiffness than men.

Applying the findings
These findings have advanced the understanding of the mechanisms and prevention of ACL injuries in female athletes. Prophylactic intervention could prevent a significant percentage of the ACL injuries that occur each year. If injury rates for male and female athletes were equal, female athletes could continue the health benefits of sports participation and avoid the long-term complications of osteoarthritis, which occurs with a 10- to 100-fold greater incidence in ACL-injured than in uninjured athletes.

Orthopaedics is at the verge of making great inroads into translating these important research findings from the research laboratory (bench) to the sports field and court to decrease care of these injuries at the bedside.

Dr. Hewett’s work was supported in part by NIH/NIAMS R01 AR049735, R01 AR05563 and R01 AR056259 (TEH). Dr. Hewett is director of sports medicine research, professor of physiology and cell biology, orthopaedic surgery, family medicine and biomedical engineering at The Ohio State University and professor of pediatrics, orthopaedic surgery, biomedical engineering and rehabilitation sciences at the University of Cincinnati and the University of Kentucky; he can be reached at tim.hewett@cchmc.org or tim.hewett@osumc.edu

Editor’s Note: This review is a compilation of the last 20 years of Dr. Hewett’s work and the results of his latest epidemiologic-biomechanical and neuromuscular findings associated with the sex differences in anterior cruciate ligament (ACL) injury. The article also details the development of strategies for risk assessment and prevention of ACL injuries.

AAOS Now
November 2010 Issue
http://www.aaos.org/news/aaosnow/nov10/research3.asp