The ACL has become one of the best-known acronym in sport. The ACL is the expression employed to describe the anterior cruciate ligament, one of the important connective tissues in the knee joint. The ACL is often associated with serious knee injury. With the rise to prominence of female athletes in numerous sports in recent years (with the passage and implementation of Title IX), the corresponding increase in the incidence of ACL injuries among female athletes has been a subject of both considerable concern on the part of sports organizations, as well as the subject of intensive sports science research.
The knee is a hinge joint, created by the junction of the femur (thigh bone) and the tibia and the fibula (lower leg bones). There are six distinct sets of ligaments in the joint that provide various types of connectivity between the bones in the joint. The ligaments of the knee generally assist in maintaining the stability of the joint when it is subjected to the extension and flexion forces required for running and jumping, movements that are powered by the combined action of the quadriceps (thigh muscles) and hamstrings.
The most important ligament with respect to knee function is the anterior cruciate ligament, or ACL, which is positioned in the center of the knee, providing a connection between the femur and the tibia. The passage within the knee through which the ACL connects these two bones is the intercondylar notch. The two chief stabilizing functions of the ACL are the prevention of the tibia from excessive forward movement, as well as the limitation on the degree of rotation permitted to the tibia when the lower leg is extended. When the ACL is damaged, the athlete will not be able to move dynamically, as the affected knee is unstable and the joint cannot support the forces of movement to any significant degree.
While ACL injuries are relatively common as a result of a blow delivered into the knee from either the lateral side (outside) of the joint, or a direct frontal blow, the majority of all athletic ACL injuries occur in non-contact situations. Non-contact causation is especially prevalent in the ACL injuries sustained by female athletes. Numerous studies conducted by both sports science researchers and major governing bodies of women's sports such as the National Collegiate Athletic Association (NCAA) have determined that the ACL injury rate among female athletes is between three and six times higher than for comparable male athletes.
While female athletes may sustain an ACL injury in a wide variety of sports, basketball and soccer are the two female sports with the greatest incidence of these occurrences. The non-contact scenarios that commonly produce injury are usually ones involving a degree of sudden or explosive leg movement on the part of the athlete. Athletic movements such as a sudden deceleration that is accompanied by a pivot by the athlete, or a forward stride where the athlete plants the leg, intending to cut in the opposite direction, are two such actions that carry a greater risk of ACL injury.
Jumping actions have also been identified as creating a similar risk. When a player stops suddenly to jump, as in basketball or volleyball, performs a straight knee landing from a jump, or when the landing creates a hyperextension of the knee joint (the joint is straightened beyond its normal alignment by greater than 10%), each of these mechanisms places significant additional stresses on the joint.
A number of explanations for the greater risk of ACL injury among female athletes have been presented through the course of a considerable number of research studies conducted throughout the world. The research suggests that the female ACL injury rate is attributable to the combination of one or more of the following factors: the width of the female pelvis relative to the length of the femur, known as the Q line; an imbalance in the relative strength of the quadriceps and hamstrings; naturally greater joint laxity among women than men; and the use of ankle braces.
The female anatomy tends to create a lower leg structure where the femur descends from the hip joint toward the knee at an inward angle, departing from the perpendicular orientation of the upright body to the ground. This Q line will vary from person to person given the individual build of each person. This structure tends to make a female athlete more knock-kneed than a male. In such structures, the forces that are directed into the knee on sudden movement can overpower the ACL, as the Q line creates an inherently greater degree of instability in the joint. This anatomical difference is magnified by the tendency among female athletes to possess a greater degree of joint laxity, which contributes to the ability of the knee to withstand force.
The female anatomy also provides a smaller intercondylar notch and a smaller ACL tissue structure than that found in males. The structural differences are also exaggerated by the strength imbalance commonly observed in female athletes' quadriceps and hamstring muscles and their connecting tendons. The ideal relationship between the relative strength in these two muscle groups is generally regarded to be a 3:2 ratio in favor of the quadriceps; when one of these structures is able to overpower the other, the knee may not be able to withstand the stress, leading to ACL injury.
In sports such as basketball and soccer, female athletes are often encouraged to wear ankle braces to protect the ankle against sprains. Many commercially available ankle braces worn by young female athletes are stiff, tending to radiate the forces of running and jumping upwards into the knee, forces that would otherwise be dissipated at the ankle.
The repair of an ACL injury will depend on the extent of damage observed within the joint. In a serious occurrence, other connective tissues in the joint may also be damaged, such as the meniscus (the cartilage that provides cushion and absorption of force in the space between the tibia and femur). When such damage is determined, surgery will be required.
Different surgical techniques are employed depending on the nature of the ACL tear. When the tear is partial, arthroscopic surgery is an option, using techniques that limit the extent of the incisions and the disruption to the surrounding structures of the knee. In serious ACL tears, when the tissue is completely torn apart, some surgeons favor techniques that employ grafts from other living tissues or from artificial products, each of which is connected to the existing ACL remnants. Given the nature of the injury, any fragments of tissue and bone are removed during this surgery.
The recovery rate from ACL surgery for female (and male) athletes is dependent to a large degree on the quality of the rehabilitation undertaken by the individual. When the athlete avoids the temptation to rush the recovery process and not undertake movements that are unsafe for the injured knee, a typical recovery time from a torn ACL is approximately nine to 12 months, with a complete recovery expected in over 80% of such cases.