Acute ACL rupture is a common knee injury in sports. The incidence of ACL ruptures has been estimated at 1 in 3,000 people within the general population in the United States per year. Over 70% of all ACL injuries occurred in recreational and competitive sport activities. The estimated incidence of ACL injuries is over 3 per 100 athletes over the course of a season. The ACL injury rate has been reported at 3.5 per 1000 game hours in indoor soccer. Recent studies reported that approximately 175,000 primary ACL reconstruction surgeries were performed annually in the United States, and estimated annual cost for ACL reconstruction surgeries alone was over $2 billion.

Acute ACL rupture is a devastating injury that can significantly affect a patient's activity level and quality of life. Complete ACL tears can induce many chronic knee problems including knee instability, menisci and chondral surface damage, and osteoarthritis. Two thirds of the patients with complete ACL tears had chronic knee instability and secondary damage to menisci and chondral surfaces. These damages to joint structures significantly affect knee functions and often force patients to decrease their activity levels and change their life styles. After acute ACL injuries, 31% patients reported moderate to severe overall disabilities in walking activities alone, 44% patients in routine activities of daily living, 77% patients in sports activities. In addition, damages to joint structures of the knee due to an acute ACL rupture and menisci damage have been identified as inducers of knee osteoarthritis, a condition that can severely impair patients' functional activities and independence.

The majority of ACL injuries are non-contact in nature, that is, there was no physical contact between athletes when the injuries occurred. Women have an ACL injury rate two to eight times higher than that for men. Basketball and soccer are two leading sports for non-contact ACL injuries among female athletes. High school students have more ACL injuries than college students. Data on ACL reconstruction procedures performed by the candidates for the certification of the American Board of Orthopaedic Surgeons suggest that more ACL reconstruction procedures were performed for high school senior students than for any other age group. The number of ACL reconstruction procedures for high school senior aged females is three times that for college aged females while the number of ACL reconstruction procedures for high school senior aged males is 1.5 times more than the college aged males. These data indicate that high school aged females are at an even higher risk for ACL injuries than that for college aged females.

To prevent non-contact ACL injuries, especially non-contact ACL injuries in women and high school aged young adolescents, modifiable risk factors have to be identified to prevent non-contact ACL injuries. We have conducted a series of preliminary studies in an attempt to identify modifiable risk factors for non-contact ACL injuries. A study by Malinazk et al. suggest that female recreational athletes tend to have smaller knee flexion angle, greater knee valgus angle, greater quadriceps muscle activation, and lower hamstring activation in comparison to the male subjects in running, jumping, and cutting tasks. Literature suggests that small knee flexion angle, large knee valgus angle, high quadriceps activation, and low hamstring activation tend to increase the load on the ACL.

A study by Chappell et al. also suggest that women have greater peak proximal tibia anterior shear force, greater peak knee extension moment, and greater peak knee varus moment during landing of stop jump tasks in comparison to men. The increased peak proximal tibia anterior shear force in women suggests that women on average may have increased tendency to have a tibia anterior translation and strain their ACLs in comparison to men. The increased knee extension moment at the peak proximal tibia anterior shear force in women suggests that the increased peak proximal tibia anterior shear force in women is likely a result of increased quadriceps muscle contraction or decreased hamstring muscle contraction. A study by Chappell et al. further suggest that both female and male recreational athletes tend to increase the peak anterior shear force and the knee valgus moment and decrease knee flexion angle during landing in stop jump tasks when fatigued. The results of our preliminary studies indicate that lower extremity motor control in athletic tasks may be an important risk factor for non-contact ACL injuries.

Besides motor control related risk factors, we also investigated anatomical risk factors for ACL injuries. A study by Nunley et al. show that the patella-tendon tibia shaft angle is essentially a linear function of the knee flexion angle, which can be accurately determined from two local knee x-ray films with a relative large difference in knee flexion angle. The results also show that the patella-tendon tibia shaft angle has a large variation between subjects and between genders, which may significantly affect the anterior shear force applied on the tibia by the quadriceps muscle.