Epidemiology, Injury Mechanism, and Etiology of ACL Injuries

With an estimated annual incidence of 120,000 in the United States, anterior cruciate ligament (ACL) injuries are commonly encountered in athletes of all skill levels. The anterior cruciate ligament is an important stabilizer of the knee, primarily resisting anterior translation and internal rotation of the tibia in relation to the femur. When the knee experiences forces that exceed the tensile strength of the ACL, a rupture of the ligament occurs that can be associated with concomitant injuries to the meniscus, cartilage, and other ligaments. The majority of ACL tears are noncontact injuries occurring during landing, pivoting, or cutting. The typical mechanism is an abrupt axial force applied to a slightly flexed knee causing a dynamic valgus moment with internal tibial rotation. The etiology of ACL tears is multifactorial, with varying contributions from variations in landing mechanics, anatomy of the ACL and intercondylar notch, hormonal effect on ligamentous laxity, as well as neuromuscular control and strength of the core muscles, hip abductors, and hamstrings. This chapter will explore the epidemiology, mechanism of injury, and underlying etiology of this devastating injury.