An anterior cruciate ligament injury occurs when the anterior cruciate ligament (ACL) is either stretched, partially torn, or completely torn. The most common injury is a complete tear. Symptoms include pain, an audible cracking sound during injury, instability of the knee, and joint swelling. Swelling generally appears within a couple of hours. In approximately 50% of cases, other structures of the knee such as surrounding ligaments, cartilage, or meniscus are damaged.
The underlying mechanism often involves a rapid change in direction, sudden stop, landing after a jump, or direct contact to the knee. It is more common in athletes, particularly those who participate in alpine skiing, football (soccer), netball, American football, or basketball. Diagnosis is typically made by physical examination and is sometimes supported and confirmed by magnetic resonance imaging (MRI). Physical examination will often show tenderness around the knee joint, reduced range of motion of the knee, and increased looseness of the joint.
Prevention is by neuromuscular training and core strengthening. Treatment recommendations depend on desired level of activity. In those with low levels of future activity, nonsurgical management including bracing and physiotherapy may be sufficient. In those with high activity levels, surgical repair via arthroscopic anterior cruciate ligament reconstruction is often recommended. This involves replacement with a tendon taken from another area of the body or from a cadaver. Following surgery rehabilitation involves slowly expanding the range of motion of the joint, and strengthening the muscles around the knee. Surgery, if recommended, is generally not performed until the initial inflammation from the injury has resolved. It should also be taken into precaution to build up as much strength in the muscle that the tendon is being taken from to reduce risk of injury.
About 200,000 people are affected per year in the United States. In some sports, women have a higher risk of ACL injury, while in others, both sexes are equally affected. While adults with a complete tear have a higher rate of later knee osteoarthritis, treatment strategy does not appear to change this risk. ACL tears can also occur in some animals, including dogs.
Signs and symptoms When an individual has an ACL injury, they are likely to hear a "pop" in their knee followed by pain and swelling. Specifcally, the knee joint fills with blood. They may also experience a specific type of instability in the knee called a "pivot shift" once they resume walking and other activities. Pivot shift occurs because in a knee without an ACL, the tibia slides forward on the lateral side. The reduction that occurs with knee flexion is the pivot shift.
The blood in the joint makes knee movement painful and the capsule thickens and contracts, reducing range of motion (stiffness). The pain, effusion, and stiffness resolve with time and exercises, but the pivot shift remains.
Causes
Tearing occurs when the tibia moves too far forwards or the femur moves too far backwards. Causes may include: Changing direction rapidly (also known as "cutting") Landing from a jump awkwardly Coming to a sudden stop when running A direct contact or collision to the knee (e.g. during a football tackle or a motor vehicle collision) These movements cause the tibia to shift away from the femur rapidly, placing strain on the knee joint and potentially leading to rupture of the ACL. About 80% of ACL injuries occur without direct trauma. Risk factors include female anatomy, specific sports, poor conditioning, fatigue, and playing on a turf field.
Injuries to the ACL are common, 250,000 ACL injuries occur on an annual basis. This corresponds to a 1 in 3,000 chance of an individual sustaining an ACL injury. Ligaments in the ACL or meniscus are usually torn with an external force being applied to the knee joint. The ACL can be torn without an external force being applied
Female predominance Female athletes are two to eight times more likely to rupture their ACL in sports that involve cutting and jumping as compared to men who play the same particular sports. NCAA data has found relative rates of injury per 1000 athlete exposures as follows: Men's basketball 0.07, women's basketball 0.23 Men's lacrosse 0.12, women's lacrosse 0.17 Men's football 0.09, women's football 0.28 The highest rate of ACL injury in women occurred in gymnastics, with a rate of injury per 1000 athlete exposures of 0.33. Of the four sports with the highest ACL injury rates, three were women's – gymnastics, basketball and soccer.
Differences between males and females identified as potential causes are the active muscular protection of the knee joint, differences in leg/pelvis alignment, and relative ligament laxity caused by differences in hormonal activity from estrogen and relaxin. Limited and low-quality evidence suggests that birth control pills may decrease the risk of ACL injury.
Dominance theories
Some studies have suggested that there are four neuromuscular imbalances that predispose women to higher incidence of ACL injury. Female athletes are more likely to jump and land with their knees relatively straight and collapsing in towards each other, while most of their bodyweight falls on a single foot and their upper body tilts to one side. Several theories have been described to further explain these imbalances. These include the ligament dominance, quadriceps dominance, leg dominance, and trunk dominance theories.
The ligament dominance theory suggests that when females athletes land after a jump, their muscles do not sufficiently absorb the impact of the ground. As a result, the ligaments of the knee must absorb the force, leading to a higher risk of injury. Quadriceps dominance refers to a tendency of female athletes to preferentially use the quadriceps muscles to stabilize the knee joint. Given that the quadriceps muscles work to pull the tibia forward, an overpowering contraction of the quadriceps can place strain on the ACL, increasing risk of injury.
Leg dominance describes the observation that women tend to place more weight on one leg than another. Finally, trunk dominance suggests that males typically exhibit greater control of the trunk in performance situations as evidenced by greater activation of the internal oblique muscle. Female athletes are more likely to land with their upper body tilted to one side and more weight on one leg than the other, therefore placing greater rotational force on their knees.
Governments and healthcare professionals acknowledge the high incidence of ACL injuries and have dedicated significant research efforts to prevention and rehabilitation. Studies have demonstrated the effectiveness of diverse training methods, such as balance, plyometric, resistance, and technique training, in reducing ACL injury risk among adolescent females. However, evidence supporting this approach for adult sport-active populations, both male and female, is limited. Two underdeveloped areas are the specificity of exercises used in interventions and the consideration of athletes' experiences, including adherence and motivation. Therefore, there is a need for injury prevention researchers to optimize training content and delivery methods to better translate research findings for diverse sport populations of varying ages and genders.
Hormonal and anatomic differences Before puberty, there is no observed difference in frequency of ACL tears between the sexes. Changes in sex hormone levels, specifically elevated levels of estrogen and relaxin in females during the menstrual cycle, have been hypothesized as causing predisposition of ACL ruptures. This is because they may increase joint laxity and extensibility of the soft tissues surrounding the knee joint. Ongoing research has observed a greater occurrence of ACL injuries in females during ovulation and fewer injuries during the follicular and luteal phases of the menstrual cycle.
Study results have shown that female collegiate athletes with concentration levels of relaxin that are greater than 6.0 pg/mL are at four times higher risk of an ACL tear than those with lower concentrations.
Relaxin is increased when estrogen levels are increased during the female menstrual cycle. Estrogen is at its peak just before ovulation, which makes it fall into the follicular phase of the menstrual cycle. These hormonal rises often fall within a 2-3 day period of an increase in knee laxity. These times of increased estrogen concentration are subsequently related to times of reduced tendon strength and stability. Additionally, the use of hormonal medication such as birth control may fluctuate the window and level of laxity which should be monitored. The NHI, found that the use of hormonal oral contraceptives reduced the risk of tearing by 68%. Moreover, women are more likely to face an ACL tear or injury when they are experiencing elevated levels of estrogen or progesterone. During these times, females don't necessarily need to limit their activity levels out of fear or precaution but it is beneficial if they participate in the proper warm-ups or strengthening exercises to limit potential risks.
Additionally, female pelvises widen during puberty through the influence of sex hormones. This wider pelvis requires the femur to angle toward the knees. This angle towards the knee is referred to as the Q angle. The average Q angle for men is 14 degrees and the average for women is 17 degrees. Steps can be taken to reduce this Q angle, such as using orthotics. The relatively wider female hip and widened Q angle may lead to an increased likelihood of ACL tears in women.
ACL, muscular stiffness, and strength During puberty, sex hormones also affect the remodeled shape of soft tissues throughout the body. The tissue remodeling results in female ACLs that are smaller and will fail (i.e. tear) at lower loading forces, and differences in ligament and muscular stiffness between men and women. Women's knees are less stiff than men's during muscle activation. Force applied to a less stiff knee is more likely to result in tears.
In addition, the quadriceps femoris muscle is an antagonist to the ACL. According to a study undertaken on female athletes at the University of Michigan, 31% of female athletes recruited the quadriceps femoris muscle first as compared to 17% in males. Because of the elevated contraction of the quadriceps femoris muscle during physical activity, an increased strain is placed onto the ACL due to the "tibial translation anteriorly".
Pathophysiology The knee joint is formed by three bones: the femur (thighbone), the tibia (shinbone), and the patella (kneecap). These bones are held together by ligaments, which are strong bands of tissue that keep the joint stable while an individual is walking, running, jumping, etc. There are two types of ligaments in the knee: the collateral ligaments and the cruciate ligaments.
The collateral ligaments include the medial collateral ligament (along the inside of the knee) and the lateral or fibular collateral ligament (along the outside of the knee). These two ligaments function to limit sideways movement of the knee.
The cruciate ligaments form an "X" inside the knee joint with the anterior cruciate ligament running from the front of the tibia to the back of the femur, and the posterior cruciate ligament running from the back of the tibia to the front of the femur. The anterior cruciate ligament prevents the tibia from sliding out in front of the femur and provides rotational stability.
There are also two C-shaped structures made of cartilage called the medial meniscus and lateral meniscus that sit on top of the tibia in the knee joint and serve as cushion for the bones. Right knee, front, showing interior ligaments Left knee, behind, showing interior ligaments
Diagnosis Manual tests