Anterior cruciate ligament injury

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Anterior cruciate ligament injury
Knee diagram.svg
Diagram of the right knee
Specialty orthopedics
Symptoms a "pop" with pain, swelling of knee
Causes noncontact athletic injury or direct blow
Risk factors athletes, female gender
Diagnostic method physical exam, MRI
Prevention neuromuscular training,[1] core strengthening[2]
Treatment physical therapy, surgery
Frequency > 200,000 per year in U.S.

Anterior cruciate ligament injury occurs when the biomechanical limits of the ligament are exceeded (over-stretched). The anterior cruciate ligament (ACL) is an important internal stabilizer of the knee joint. The primary function of the ACL is to prevent hyperextension; its secondary function is to restrain tibial rotation and varus/valgus stress.[3]

ACL injury is most commonly a non-contact injury involving a sudden stop or twisting movement, such as a dismount from a layup in basketball.[4] ACL injury occurs more frequently in athletes than in the general population and is prevalent in alpine skiing, association football, American football, Australian rules football, basketball, rugby, professional wrestling, martial arts, and gymnastics.[5] It is also known to be about three times more common in women than men.[5]

The consequences of the injury depend on how much the stability of the knee is affected, and the extent to which other structures have been involved. If instability is evident, particularly rotatory instability, then the menisci may get injured; this may lead to progressive, degenerative, arthritis of the knee.

Signs and symptoms[edit]

An individual may feel or hear a "pop" in their knee during a twisting movement[6] or rapid deceleration, followed by an inability to continue participation in the sport and early swelling from hemarthrosis. This combination is said to indicate a 90% probability of rupture of the anterior cruciate ligament.[7] An individual may experience instability in the knee once they resume walking and other activities, and they may feel their knee is "giving out".


ACL Tear.

ACL injury is most commonly a non-contact injury that occurs when an individual stops suddenly or plants his or her foot hard into the ground (cutting). ACL injury also has been linked to heavy or stiff-legged landing; the knee rotating while landing, especially when the knee is in an unnatural position.

Significantly, many ACL injuries occur in athletes landing flat on their heels. This movement directs the forces directly up the tibia into the knee, while the straight-knee position places the anterior femoral condyle on the back-slanted portion of the tibia. The resultant forward slide of the tibia relative to the femur is restrained primarily by the now-vulnerable ACL.

ACL injuries also can be caused by direct contact or trauma, such as in a motor vehicle collision or from a tackle in football. A severe form of ACL injury caused by direct contact is called the "unhappy triad," also known as the "terrible triad," or "O'Donaghue's triad." The "unhappy triad" involves injury of the anterior cruciate ligament, the medial collateral ligament, and the medial meniscus.[8]

Risk factors[edit]

Sex-related differences[edit]

Women in sports such as association football, basketball, and tennis are significantly more prone to ACL injuries than men. The discrepancy has been attributed to gender differences in anatomy, general muscular strength, reaction time of muscle contraction and coordination, and training techniques.

Gender differences in ACL injury rates become evident when specific sports are compared.[9] A review of 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.[10]

According to recent studies, female athletes are two to eight times more likely to strain their anterior cruciate ligament (ACL) in sports that involve cutting and jumping as compared to men who play the same particular sports (soccer, basketball, and volleyball).[11] Differences between males and females identified as potential causes are the active muscular protection of the knee joint, the greater Q angle putting more medial torque on the knee joint, relative ligament laxity caused by differences in hormonal activity from estrogen and relaxin, intercondylar notch dimensions, and muscular strength.[11][12]

Hormonal and anatomic differences[edit]

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.[11] 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.[13] 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[edit]

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 ACL tears.[14]

In addition, the quadriceps femoris muscle is an antagonist to the ACL. According to a study done 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".[15]

Ligament dominance[edit]

The increased risk of anterior cruciate ligament injury among female athletes is best predicted by the motion and loading of the knee during performance situations.[16] The ligament dominance theory suggests that females typically perform athletic movements with greater knee valgus angles. A greater amount of stress is placed on the ACL in these situations because there is high activation of the quadriceps muscles despite limited knee flexion, limited hip flexion, greater hip adduction, and a large knee adductor moment.[17][18] Additionally, females typically land with their tibia rotated internally or externally.[19] As a result of increased knee valgus stress, ground reaction forces are greater and laterally directed.[20]

Quadriceps dominance[edit]

Ligament dominance is observed when there is excessive movement in the frontal plane to accommodate limited movement in the sagittal plane. This is caused by weakness in the hamstring muscles or reliance on the strength of the quadriceps muscles.[18] This quadriceps dominance theory identifies when the hamstring muscles are notably weaker than the quadriceps muscles. As a result, knee stability in performance situations depends on the quadriceps due to a discrepancy in the pattern in recruiting quadriceps and hamstring muscles.[21]

Trunk and leg dominance[edit]

Other theories used to explain the increased risk of ACL injury among female athletes include the trunk dominance and leg dominance theories. 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. Leg dominance suggests that females exhibit greater kinematic leg asymmetry in knee valgus angles, hip abduction, and ankle abduction in performance situations.[17]

Gray347.png Gray348.png
Right knee, front, showing interior ligaments Left knee, behind, showing interior ligaments


Manual tests[edit]

The pivot-shift test, anterior drawer test, and Lachman test are used during the clinical examination of suspected ACL injury. The Lachman test is recognized by most authorities as the most reliable and sensitive test, and usually superior to the anterior drawer test.[22]

An ACL tear can present with a popping sound heard after impact, swelling after a couple of hours, severe pain when bending the knee, and buckling or locking of the knee during movement.

Though clinical examination in experienced hands can be accurate, the diagnosis is usually confirmed by using an arthrometer or MRI, which have greatly lessened the need for diagnostic arthroscopy and which have a higher accuracy than clinical examination. It may also permit visualization of other structures which may have been coincidentally involved, such as a meniscus, or collateral ligament, or posterolateral corner of the knee joint.


Laximetry is a reliable technique for diagnosing a torn anterior cruciate ligament.[23]

MRI scan[edit]

Anterior cruciate ligament tear seen on MRI. T1 left, right PDW.

The MRI is perhaps the most used technique for diagnosing the state of the Anterior Cruciate Ligament but it not always the most reliable. In some cases the Anterior Cruciate Ligament can indeed not be seen because of the blood surrounding it.


Interest in reducing non-contact ACL injury has been intense and the observed, increased, liability of the female gender in some sports has added to this. The International Olympic Committee, after a comprehensive review of preventive strategies, has stated that injury prevention programs have an effect on reducing injuries that is measurable, and that applies particularly to women.[24] Further, paying attention to the balance of strength between hamstrings and quadriceps will help prevent the anterior cruciate ligament from being overpowered by over-emphasized quadriceps strength. It is also stressed that landing forces should be reduced together with emphasizing proper landing technique. It has been previously reported that landing on the heel, rather than forefoot with progressive transfer of weight to the heel, is potentially injurious to the ACL because of the hyperextension forces created. The closer the knee is to full extension, the more likely this is to occur.[25]

Accordingly, it is generally recommended that injury prevention programs stress these principles.


The term for non-surgical treatment for ACL rupture is "conservative management", and it often includes physical therapy and using a knee brace. Instability associated with ACL deficiency increases the risk of other knee injuries such as a torn meniscus, so sports with cutting and twisting motions are problematic and surgery is often recommended in those circumstances.

Patients who have suffered an ACL injury should be evaluated for other injuries that often occur in combination with an ACL tear and include cartilage/meniscus injuries, bone bruises, PCL tears, posterolateral injuries and collateral ligament injuries.


A torn ACL is less likely to restrict the movement of the knee. When tears to the ACL are not repaired it can sometimes cause damage to the cartilage inside the knee because with the torn ACL the tibia and femur bone are more likely to rub against each other. Immediately after the tear of the ACL, the person should rest the knee, ice it every 15 to 20 minutes, provide compression on the knee, and then elevate above the heart; this process helps decrease the swelling and reduce the pain. The form of treatment is determined based on the severity of the tear on the ligament. Small tears in the ACL may just require several months of rehab in order to strengthen the surrounding muscles, the hamstring and the quadriceps, so that these muscles can compensate for the torn ligament. Falls associated with knee instability may require the use of a specific brace to stabilize the knee. Women are more likely to experience falls associated with the knee giving way. Sudden falls can be associated with further complications such as fractures and head injury.


Main article: ACL reconstruction

If surgery is decided upon, either because obvious instability interferes with activities of daily living, or because the knee is subject to repeated, severe, provocative maneuvers, such as the case of the competitive athlete involved in cutting and rapid deceleration etc., then several issues need to be decided upon.

  • Timing. Immediate repair is usually avoided and initial swelling and inflammatory reaction allowed to subside.
  • Choice of graft material, autograft or allograft.
  • Choice of anterior cruciate ligament augmentation, patellar tendon or hamstring tendon.[26]

These issues are fully explored in ACL Reconstruction.


There are around 200,000 ACL tears each year in the United States, with over 100,000 ACL reconstruction surgeries per year. Over 95% of ACL reconstructions are performed in the outpatient setting. The most common procedures performed during ACL reconstruction are partial meniscectomy and chondroplasty.[27]

Special populations[edit]

Young athletes[edit]

High school athletes are at increased risk for ACL tears when compared to non-athletes. This risk increases with certain types of sports. Among high school girls, the sport with the highest risk of ACL tear is soccer, followed by basketball and lacrosse. The highest risk sport for boys was basketball, followed by lacrosse and soccer.[28] Children and young athletes may benefit from early surgical reconstruction after ACL injury. Young athletes who have early surgical reconstruction of their torn ACL are more likely to return to their previous level of athletic ability when compared to those who underwent delayed surgery or nonoperative treatment. They are also less likely to experience instability in their knee if the undergo early surgery.[29]

See also[edit]


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  2. ^ Sugimoto D, Myer GD, Foss KD, Hewett TE. "Specific exercise effects of preventive neuromuscular training intervention on anterior cruciate ligament injury risk reduction in young females: meta-analysis and subgroup analysis.". Br J Sports Med. 49: 282–9. doi:10.1136/bjsports-2014-093461. PMID 25452612. 
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