anterior cruciate ligament injury anterior cruciate ligament injury 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. anterior cruciate ligament injurydiagram of the right kneespecialtyorthopedicssymptomsaudible "crack" with pain, knee instability, swelling of kneecausesnon-contact injury, contact injuryrisk factorsathletes, femalesdiagnostic methodphysical exam, mripreventionneuromuscular training, core strengtheningtreatmentbraces, physical therapy, surgeryfrequencyc. 200,000 per year (us) 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 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. about 200,000 people are affected per year in the united states. in some sports, females 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 knee osteoarthritis, treatment strategy does not appear to change this risk. acl tears can also occur in some animals, such as dogs. the most common knee injury in dogs is acl or ccl rupture; in fact, till proven otherwise, unexpected hind limb lameness is usually due to a ruptured cruciate. 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. they may also experience instability in the knee once they resume walking and other activities, as the ligament can no longer stabilize the knee joint and keep the tibia from sliding forward. reduced range of motion of the knee and tenderness along the joint line are also common signs of an acute acl injury. the pain and swelling may resolve on its own; however, the knee will remain unstable and returning to sport without treatment may result in further damage to the knee. causes acl tear 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. female predominance female athletes are two to eight times more likely to strain 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. birth control pills also appear to decrease the risk of acl injury. dominance theories femur with q angle: the angle formed by a line drawn from the anterior superior iliac spine through the center of the patella and a line drawn from the center of the patella to the center of the tibial tubercle 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. 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. 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 acl tears. 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". 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 most acl injuries can be diagnosed by examining the knee and comparing it to the other, non-injured knee. when a doctor suspects acl injury in a person who reports a popping sound in the knee followed by swelling, pain, and instability of the knee joint, they can perform several tests to evaluate the damage to the knee. these tests include the pivot-shift test, anterior drawer test, and lachman test. the pivot-shift test involves flexing the knee while holding onto the ankle and slightly rotating the tibia inwards. in the anterior drawer test, the examiner flexes the knees to 90 degrees, sits on the person's feet, and gently pulls the tibia towards themself. the lachman test is performed by placing one hand on the person's thigh and the other on the tibia and pulling the tibia forward. these tests are meant to test whether the acl is intact and therefore able to limit the forward motion of the tibia. the lachman test is recognized by most authorities as the most reliable and sensitive of the three. medical imaging anterior cruciate ligament tear seen on mri. t1 left, right pdw. though clinical examination in experienced hands can be accurate, the diagnosis is usually confirmed by magnetic resonance imaging, which provides images of the soft tissues like ligaments and cartilage around the knee. it may also permit visualization of other structures which may have been coincidentally involved, such as the menisci or collateral ligaments. an x-ray may be performed in addition to evaluate whether one of the bones in the knee joint was broken during the injury. mri is perhaps the most used technique for diagnosing the state of the acl, but it is not always the most reliable technique as the acl can be obscured by blood that fills the joint after an injury. arthrometers/laximeters another form of evaluation that may be used in case physical examination and mri are inconclusive is laximetry testing (i.e. arthrometry and stress imaging), which involve applying a force to the leg and quantifying the resulting displacement of the knee. these medical devices basically replicate manual tests but offer objective assessments. the gnrb arthrometer, for example, is a knee arthrometer that is considered more effective than the lachman test. classification an injury to a ligament is called a sprain. the american academy of orthopedic surgeons defines acl injury in terms of severity and classifies them as grade 1, 2, or 3 sprains. grade 1 sprains occur when the ligament is stretched slightly but the stability of the knee joint is not affected. grade 2 sprains occur when the ligament is stretched to the point that it becomes loose; this is also referred to as a partial tear. grade 3 sprains