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|Anterior cruciate ligament injury|
|Classification and external resources|
Diagram of the right knee
|Anterior cruciate ligament injury|
|Classification and external resources|
Diagram of the right knee
The anterior cruciate ligament is an important, internal, stabilizer of the knee joint, restraining hyperextension. It is injured when its biomechanical limits are exceeded (over stretched), often with a hyperextension mechanism. Formerly, this occurred most often in a sports contact injury, when other structures were frequently involved. A particularly severe form of the contact injury is called the "unhappy triad" or "O'Donaghue's triad", and involves the anterior cruciate ligament, the medial collateral ligament, and the lateral meniscus. Presently, ACL injury is more commonly a non-contact injury, such as a dismount from a layup in basketball. Both forms occur more frequently in athletes than in the general population and are prevalent in alpine skiing, Association football, American football, Australian rules football, basketball, rugby, professional wrestling, martial arts, and artistic gymnastics. It is also known to be about three times more common in women than men.
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, and this can vary on a case-by-case basis. If instability is evident, particularly rotatory instability, then the menisci will get injured, sooner or later, setting the scene for progressive, degenerative, arthritis of the knee.
The combination of "pop" during a twisting movement or rapid deceleration, together with inability to continue participation, and followed by early swelling, is said to indicate a 90% probability of rupture of the anterior cruciate ligament.
ACL injuries occur when an individual stops suddenly or plants his/her foot hard into the ground (cutting). ACL failure has also been linked to heavy or stiff-legged landing; the knee rotating while landing, especially when the knee is in an unnatural position.
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. Women also have a relatively wider pelvis, requiring the femur to angle toward the knees. The role of genetics is currently speculative.
Significantly many ACL injuries occur in athletes landing flat on their heels. The latter 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.
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. 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. Additionally, females typically land with their tibia rotated internally or externally. As a result of increased knee valgus stress, ground reaction forces are greater and laterally directed.
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. 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.
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 typicall 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.
|Right knee, front, showing interior ligaments||Left knee, behind, showing interior ligaments|
ACL tears occur for two reasons: the failure load of the ligament and the mechanical load applied to it. Female ACLs will fail at relatively lower loads than males, and female pelvic anatomy also predisposes women to higher mechanical loads on the knee. The combination of these factors leads to an increased likelihood – four to six times – for females to tear their ACLs than males.
There are both proximate and ultimate causes for the increased susceptibility of women to ACL tears. Proximate, or immediate, causation is that women have wider pelvises than men. This widened pelvis creates a wider valgus knee angle: with wider hips, the femur must angle towards the knee at a wider angle. This difference in skeletal anatomy between men and women makes women more susceptible to ACL tears due to greater rotational force placed upon the knee.
Underlying this proximate cause is the ultimate cause of male and female anatomical divergence due to the influence of sex hormones. Before puberty, there is no observed difference in frequency of ACL tears between the sexes. Changes in sex hormones, specifically increased estrogen and progesterone in women, make possible many of the anatomical changes necessary for successful reproduction and childbirth. Through the influence of sex hormones, female pelvises widen during puberty. The proximate cause of increased likelihood of ACL tears in women thus stems from the ultimate cause of differences in sex hormones between males and females.
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. Sex hormones, the ultimate cause of ACL tear differences, create 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.
While these sex hormones may appear detrimental to women in terms of sports injuries, they are necessary for childbirth and thus are an intrinsic part of the evolution of the human species. Females face an evolutionary trade-off in anatomy between a body adapted for efficient bipedal movement and one adapted for successful childbirth. Trade-offs, a common theme in the history of human evolution, occur when humans evolve a change in physiology in order to reduce illness or injury (in this case, death in childbirth). This change may have deleterious effects on another aspect of human physiology: in the case of pelvic anatomy, a too-wide pelvis would not be able to be supported by gluteal muscles and would be an inefficient means of bipedal locomotion.
Applied to female pelvic width, a narrower pelvis would reduce valgus knee angle, leading, among other things, to lower rates of ACL tears and other physical benefits like a more efficient stride and running gait. Yet this narrow pelvis would constrain childbirth, possibly resulting in the death of the mother and child. Thus, throughout human history, women with wider pelvises had higher rates of survival in childbirth and passed on these wider-hipped genes to their offspring.
Pelvic width was constrained by the trade-off between locomotion and childbirth: wider pelvises offered a fitness advantage up to a certain point, where they became a liability due to decreased bipedal abilities. Pelvic width thus could not expand as wide as it might to make childbirth easier, due to the necessity of human bipedal locomotion. Humans evolved from quadruped primates who had only the physiological capabilities for inefficient, infrequent bipedal locomotion. Evolutionary adaptations like larger, more powerful gluteal muscles allowed humans to stabilize their hips and trunk during bipedal locomotion. Multiple theories exist about why bipedalism conferred a reproductive advantage. Bipedalism allowed humans to use their hands to carry food, was a more efficient form of long-distance transportation than quadrupedal locomotion, improved thermoregulation by reducing the amount of skin exposed to direct sunlight (the top of the head vs. the entire back), and permitted humans to engage in persistence hunting. East Africa was changing from a forest to a grassland when bipedalism first evolved in humans' ancestors approximately 8 million years ago, and the new behaviors it enabled them to engage in would have made them better fit to survive in this changed environment.
Thus female pelvic width is a trade-off, where childbirth is easier but not as easy as it might be in other quadruped primates; and bipedal locomotion is efficient but not as efficient as it might be with narrower hips. These sex differences in locomotion underlie differential rates of ACL injury in men and women.
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. The ACL can also be visualized using a magnetic resonance imaging scan (MRI scan).
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 MRI, which has greatly lessened the need for diagnostic arthroscopy and which has a higher accuracy than clinical examination. It may also permit visualization of other structures which may have been co-incidentally involved, such as a meniscus, or collateral ligament, or posterolateral corner of the knee joint.
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 measureable, and that applies particularly to women. 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.
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.
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.
These issues are fully explored at ACL Reconstruction,
Before undertaking ACL reconstruction, the patient must accept that rehabilitation is mandatory, not optional, is fatiguing and time-consuming, and will last at least six months, more likely twelve, before optimal function is regained. Initial rehabilitation emphasizes recovery of range of motion, the preferred modality being isometric exercise that does not place stress on the knee joint. One goal is the achievement of full knee extension after two weeks. Although a minimum of six weeks is required for bony consolidation of the graft, walking is generally permitted. Thereafter, increased strengthening along with flexibility takes place until the important twelve-week post surgical timeline, when a more aggressive regimen begins. Rehabilitation is not only recovery but protection against future injury and attention to quadriceps strength, and importantly, ratio of hamstring to quadriceps strength as previously mentioned, will be emphasized.
Rehabilitation and recovery is discussed in detail at ACL reconstruction.
Gender difference in ACL tears in relationship with physical activities have been asserted. A review of NCAA data has found relative rates of injury per1000 athlete exposures as follows:
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.
It is relatively easy to identify differences between male and female gender that might be significant and relevant. These have been hypothesized to include the influence of estrogen, the difference in angle between the junction of the femur and tibia because of the relatively wider female hip, the slope of the tibial plateau, the size of the inter-condylar and questionable differences in hamstrings and quadriceps strength, and joint position sense, and so on. Although such assertions are made with confidence from time to time, they come nowhere close to establishing a 'cause and effect' relationship. (See Koch's postulates and Bradford Hill criteria) More recently, interest has surrounded the subject of femoral–tibial compression forces in their relationship to ground reaction force. These studies have suggested that certain circumstances induce a spontaneous 'pivot-shift' mechanism whereby the tibia translates anteriorly on the femur, placing the anterior cruciate ligament at risk. The factors that encourage this seem to include a valgus position of the knee, a relatively straight leg position minimizing the influence of the ankle and hip, and a relatively flat footed position, with weight concentrated toward the heel.