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Anterior cruciate ligament injury
Classification and external resources
File:Knee diagram.svg
Diagram of the right knee
ICD-10 S83.5
ICD-9 844.2
eMedicine pmr/3

An injury to the anterior cruciate ligament can be a debilitating musculoskeletal injury to the knee, seen most often in athletes. Non-contact tears and ruptures are the most common causes of ACL injury.


The anterior cruciate ligament (ACL) is a vital ligament for proper movement. ACL injury more commonly causes knee instability than injury to other knee ligaments.[1] Injuries of the ACL range from mild such as small tears to severe when the ligament is completely torn. There are many ways the ACL can be torn; the most prevalent is when the knee is bent too much toward the back and when it goes too far to the side (this has been proven by a said individual that was hit in the side by an on coming car). Tears in the anterior cruciate ligament often take place when the knee receives direct impact[clarification needed] while the leg is in a stable position, for example a standing football player is tackled sideways when his feet are firmly planted. Torn ACL’s are most often related to high impact sports or when the knee is forced to make sharp changes in movement and during abrupt stops from high speed. These types of injuries are prevalent in Alpine skiing,[2] American football, basketball, high jump, rugby, martial arts and soccer. Research has shown that women involved in sports are more likely to have ACL injuries than males. ACL tears can also happen among older individuals by slips and falls and they are seen mostly in people over forty due to wear and tear of the ligaments. An ACL tear can be determined by an individual if a popping sound is heard after impact, swelling after a couple of hours, severe pain when bending the knee, and when the knee buckles or locks during movement or gives way while standing still with weight on the affected knee.


Mountcastle et al. performed a study on gender difference in ACL tears in relationship with physical activities.[3] The researchers performed an epidemiology study on young athletic populations. Preceding studies have signified that women that participate in the same physical activities as men are more at risk for ACL injuries. The authors hypothesize that the frequency rate for males and females in the athletic and college aged population is the same. The procedures for the study was college graduation classes from 1994-2003 at a major institutions. The players who received a whole tear were examined for apparatus of injury and the type of sport they played when the injury occurred. The authors calculated the accident rate, opinion of danger, gender incidents, class year, and the accident rate differentiating men and women. There was 353 ACL injuries in 10 classes during the span of the study. The researchers calculated a 4 year accident proportion of 3.24 per 100 students for men, and 3.51 for women. Overall, the ACL injury rate not including male only sports was substantial greater in women with an incidence ratio of 1.51 (pg 5). Women are more likely to get injured at gymnastics course with an incidence ratio of 5.67, with an indoor obstacle course test is 3.72, and 2.42 incidence ratio on basketball. The authors concluded that there is slim gender difference in gender ACL tear. On the other hand, there were significant gender differences ACL injury rates when particular specific sports and physical activities were compared. Also, when male only sports were detached from the whole rate evaluation.

A notable finding is that women are three times more likely to have an ACL injury than men. The reason is because of the variation of hormone levels. Also, ligament strength of the ACL handles more force in men than in women. Most importantly, there is substantial difference neuromuscular coordination and control in landing, women have less hip and knee flexion. Athletic trainers and team physicians advise female athletes to adapt an ACL conditioning program.


Symptoms of an ACL injury include hearing a sudden popping sound, swelling, and instability of the knee (i.e., a "wobbly" feeling). Pain is also a major symptom in an ACL injury and can range from moderate to severe.[4] Continued athletic activity on a knee with an ACL injury can have devastating consequences, resulting in massive cartilage damage, leading to an increased risk of developing osteoarthritis later in life.


ACL injuries occur when an athlete rapidly decelerates, followed by a sharp or sudden change in direction (cutting). ACL failure has been linked to heavy or stiff-legged landing; as well as twisting or turning the knee while landing, especially when the knee is in the valgus (knock-knee) position.

Women in sports such as football (soccer), basketball, tennis and volleyball are significantly more prone to ACL injuries than men. The discrepancy has been attributed to differences between the sexes in anatomy, general muscular strength, reaction time of muscle contraction and coordination, and training techniques. A recent study suggests hormone-induced changes in muscle tension associated with menstrual cycles may also be an important factor.[5] Women have a relatively wider pelvis, requiring the femur to angle toward the knees.[6] Recent research also suggests that there may be a gene variant that increases the risk of injury [7]

The majority of 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 lateral 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.


File:VKB-Riss MRT T1 PDW sag.jpg

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

The pivot-shift test, anterior drawer test and the Lachman test are used during the clinical examination of suspected ACL injury. The ACL can also be visualized using a magnetic resonance imaging scan (MRI scan).

An ACL tear can be determined by the an individual if a popping sound is heard after impact, swelling after a couple of hours, severe pain when bending the knee, and when the knee buckles or locks during movement.

Though clinical examination in experienced hands is highly accurate, the diagnosis is usually confirmed by MRI, which has greatly lessened the need for diagnostic arthroscopy. MRI has a higher accuracy than clinical examination in detecting ACL tears when multiple ligaments are torn. This is of particular benefit if there is a coexisting posterolateral corner injury. Addressing the posterolateral corner injury at the time of ACL reconstruction will prevent premature graft failure.

Anterior drawer test

The anterior drawer test for anterior cruciate ligament laxity is one of many medical tests used to determine the integrity of the anterior cruciate ligament.[8] It can be used to help diagnose sprain and tears.

The test is performed as follows: the patient is positioned lying supine with the hip flexed to 45° and the knee to 90°. The examiner positions themselves by sitting on the examination table in front of the involved knee and grasping the tibia just below the joint line of the knee. The thumbs are placed along the joint line on either side of the patellar tendon. The index fingers are used to palpate the hamstring tendons to ensure that they are relaxed; the hamstring muscle group must be relaxed to ensure a proper test. The tibia is then drawn forward anteriorly. An increased amount of anterior tibial translation compared with the opposite limb or lack of a firm end-point indicates either a sprain of the anteromedial bundle of the ACL or a complete tear of the ACL. This test should be performed along with other ACL-specific tests to help obtain a proper diagnosis.

Lachman test

File:ACLI 17.jpg

Lachman test

  • The knee is flexed at 30 degrees
  • Examiner pulls on the tibia to assess the amount of anterior motion of the tibia in comparison to the femur
  • An ACL-deficient knee will demonstrate increased forward translation of the tibia at the conclusion of the movement

The Lachman test is an orthopedic test used for examining the anterior cruciate ligament (ACL) in the knee for patients where there is a suspicion of a torn ACL.[9] The Lachman test is recognized by most authorities as the most reliable and sensitive clinical test for the determination of anterior cruciate ligament integrity, superior to the anterior drawer test commonly used in the past. To do this, lay the patient supine on an examination table. Put the patient's knee in about 20-30 degrees flexion, also according to Bates' Guide to Physical Examination the leg should be externally rotated. The examiner should place one hand behind the tibia and the other on the patient's thigh. It is important that the examiner's thumb be on the tibial tuberosity. On pulling anteriorly on the tibia, an intact ACL should prevent forward translational movement of the tibia on the femur ("firm endpoint").

Anterior translation of the tibia associated with a soft or a mushy endpoint indicates a positive test. More than about 2 mm of anterior translation compared to the uninvolved knee suggests a torn ACL ("soft endpoint"), as does 10 mm of total anterior translation. An instrument called a "KT-1000" can be used to determine the magnitude of movement in mm.

This test can be done in an on-the-field evaluation in an acute injury setting, or in a clinical setting when a patient presents with knee pain. In either situation, ruling out fracture is important in the evaluation process. Also when evaluating the integrity of the ACL, it is important to test the integrity of the MCL, because this is a common ligament torn in an ACL injury as well.[10] This test is named after orthopaedic surgeon, John Lachman.

Pivot shift test

  • Person lies on one side of the body
  • Knee is extended and internally rotated
  • Doctor applies stress to lateral side of the knee, while the knee is being flexed
  • A positive test indicates a crash felt at 30 degrees flexion.


Research has shown that the incidence of non-contact ACL injury can be reduced anywhere from 20% to 80% by engaging in regular neuromuscular training that is designed to enhance proprioception, balance, proper movement patterns and muscle strength.[11]

A National Institutes of Health funded study is underway with the objective of identifying unique movement patterns that predispose female athletes to ACL injuries and evaluate and improve injury prevention programs. The study is overseen by Dr. Christopher Powers at the University of Southern California's Division of Biokinesiology. An initial phase of the project evaluated the Prevent Injury and Enhance Performance (PEP) program developed by the Santa Monica Orthopaedic and Sport Medicine Research Foundation. During the final stage of the study, the Competitive Athlete Training Zone ("CATZ") in Pasadena, CA the ACL injury prevention training program is being enhanced and continually improved by CATZ founders Jim Liston and Kevin Wentz. Information on the PEP program, and the latest developments at CATZ can be monitored at the project website.[12][13]


The ACL primarily serves to stabilize the knee in an extended position and when surrounding muscles are relaxed; so if the muscles are strong, many people can function without it. Fluids will also build the muscle.

The term for non-surgical treatment for ACL rupture is "conservative management", and it often includes physical therapy and using a knee brace. Lack of an ACL increases the risk of other knee injuries such as a torn meniscus, so sports with cutting and twisting motions are strongly discouraged. For patients who frequently participate in such sports, surgery is often indicated.


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 it, ice it every fifteen to twenty minutes, produce 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.


If the tear is severe, surgery may be necessary because the ACL can not heal independently because there is a lack of blood supply going to this ligament. Surgery is usually required among athletes because the ACL is needed in order to perform sharp movements safely and with stability. The surgery of the ACL is usually done several weeks after the injury in order to allow the swelling and inflammation to go down. During surgery the ACL is not repaired instead, it is reconstructed using other ligaments in the body. There are three different types of ACL surgery. Patella tendon-bone auto graft and hamstring auto graft are the most common and preferred and tend to produce the best results. For the Patella tendon-bone auto graft, the central 1/3 of the patella tendon is removed along with a piece of bone at the attachment sites on the kneecap and tibia. The advantages of using this method is that the patella tendon and ACL are relatively the same length and it uses a bone to bone attachment which most surgeons agree is much stronger than other healing methods. Disadvantages of this method is common anterior knee pain due to the removal of bone from the kneecap. For the hamstring auto graft, two tendons are taken from the hamstring muscles and wrapped together forming the new ACL. Advantages of this method are less pain associated with post surgery healing than that of the patella tendon-bone graft due to the fact no bone was removed and the incision is small. Disadvantages of this method is that it takes longer to heal since there is no bone to bone healing and the tendon to bone takes awhile to become rigid.[14]After the surgery, rehabilitation is required in order to strengthen the surrounding muscles and stabilize the joint.

There are two main options for ACL graft selection, allograft and autograft. Autografts are the patients' own tissues, and options include the hamstring tendons or middle third of the patella tendon. Allograft is cadaveric tissue sourced from a tissue bank. Each method has its own advantages and disadvantages; hamstring and middle third of patella tendon having similar outcomes. Patellar grafts are often incorrectly cited as being stronger, but the site of the harvest is often extremely painful for weeks after surgery and some patients develop chronic patellar tendinitis. Replacement via a posthumous donor involves a slightly higher risk of infection. Additionally, donor grafts eliminate tendon harvesting which, due to improved arthroscopic methods, is responsible for most post-operative pain.

The surgery is typically undertaken arthroscopically, with tunnels drilled into the femur and tibia at approximately the original ACL attachments. The graft is then placed into position and held in place. There are a variety of fixation devices available, particularly for hamstring tendon fixation. These include screws, buttons and post fixation devices. The graft typically attaches to the bone within six to eight weeks[citation needed]. The original collagen tissue in the graft acts as a scaffold and new collagen tissue is laid down in the graft with time. Hence the graft takes over six months to reach maximal strength.[citation needed]

After surgery, the knee joint loses flexibility, and the muscles around the knee and in the thigh tend to atrophy. All treatment options require extensive physical therapy to regain muscle strength around the knee and restore range of motion (ROM). For some patients, the lengthy rehabilitation period may be more difficult to deal with than the actual surgery. In general, a rehabilitation period of six months to a year is required to regain pre-surgery strength and use.[citation needed] This is very dependent on the rehabilitation assignment provided by the surgeon as well as the person who is receiving the surgery. External bracing is recommended for athletes in contact and collision sports for a period of time after reconstruction. It is important however to realize that this type of prevention is given by a 'surgeon to surgeon' basis; all surgeons will prescribe a brace and crutches for post surgery recovery total usage time is one month. After surgery no sports for 6 to 7 months. Whether the ACL deficient knee is reconstructed or not, the patient is susceptible to early onset of chronic degenerative joint disease.


The rehabilitation process is a very important part of the surgery. There is a long and rigorous process involved in getting back to one hundred percent. The doctor will start the patient on the rehabilitation program, which is broken down into phases:

Phase 1: This step is called the early rehabilitation phase. This is basically the things that were covered in short term, things to reduce pain and swelling while gaining movement.

Phase 2: This phase covers weeks 3 and 4. At this point the pain should be subsiding and the patient will be ready to try more things that their knee isn’t willing perform. That is why there is a lot of emphasis put on joint protection during this step. The patient will be able to start doing exercises such as mini wall sits and riding stationary bikes. The aim of this is to be able to bend the knee 100 degrees.

Phase 3: This phase is known as the controlled ambulation phase and it covers weeks 4 to 6. At this point the patient will be doing the same exercises from phase 2 plus some more challenging ones. The patient will try to get their knee to bend 130 degrees during this stage. The aim during this period is to focus heavily on improving balance.

Phase 4: This is the moderate protection phase and it covers weeks 6 to 8. In this period the patient will try to obtain full range of motion as well as increase resistance for the workouts.

Phase 5: This is the light activity phase and it covers weeks 8 to 10. This period will place particular emphasis on strengthening exercises with increased concentration on balance and mobility.

Phase 6: This is the return to activity phase and it lasts from week 10 until the target activity level is reached. At this point the patient will be able to start jogging and performing moderately intense agility drills. Somewhere between month 3 and month 6 the surgeon will probably request that the patient perform physical tests so s/he can monitor the activity level. When the doctor feels comfortable with the progress of the patient, s/he will clear that person to resume a fully active lifestyle.[15]


ACL injury prevention should be taken sincerely. The best way to prevent an ACL injury is to implement and add warm up drills like jumping and balancing. These drills will induce increase neuromuscular control and conditioning. In turn, muscular reactions will improve thus decreasing the risk of an ACL injury. A warm up program of at least 15 minutes 2-3 times per week is essential in order to prevent an ACL injury. Identifying the causes of the ACL and how painful they are the best way to avoid or escaped a painful experience it is to stretch the ligament before a physical activity. The leg muscles like the quadriceps and hamstrings have to be made stronger.

  • Backward running to warm up the hip extensors and hamstrings
  • One of the fundamental ways to avoid an ACL injury is to not wear shoes that have cleats in contact sports.
  • When a person has already suffered an ACL injury, but wants to return to competitive sports, the best way to prevent another injury is to strengthen the quadriceps and hamstrings.
  • Another way is to change mechanics like pivoting, cutting excessively because it puts extra stress on the knee.
  • Overall, sports like football, soccer, basketball, and other contact sports the risk is always high.
  • The best way is to wear a knee brace.


Stretching the quadriceps and hamstrings before an event will also prevent ACL injury because it promotes flexibility, decrease firmness, and increase performance. The muscle stretching has to be done in reps.

  • Calf Stretch 1-2 minute of stretching the lower leg muscles. Ankle circles will stretch the gastrocnemius.
  • Quadriceps- 2–3 minutes of seated butterfly 3 reps of 20 seconds
  • Hamstrings-1 minute of wall sits 2 reps of 30 seconds
  • Inner thigh stretch- 1 minute of knee to chest
  • Hip flexors-2 reps of at least 20 seconds of lunges

See also

Additional images


  1. Levy DB, Dickey-White HI. Knee Injury, Soft Tissue, eMedicine, Dec 15, 2009
  2. Dr Langran. "Alpine Ski Injuries". Retrieved 5 December 2010.
  3. Mountcastle, Sally; et al. "Gender Differences in Anterior Cruciate Ligament Injury Vary With Activity. The American Journal of Sports Medicine. 35.10 (2007)".
  5. Catalyst: Weak at the Knees - ABC TV Science
  6. McLean SG, Huang X, van den Bogert AJ (2005). "Association between lower extremity posture at contact and peak knee valgus moment during sidestepping: implications for ACL injury". Clin Biomech (Bristol, Avon) 20 (8): 863–70. doi:10.1016/j.clinbiomech.2005.05.007. PMID 16005555.
  7. A Genetic Link for ACL Injuries, Science of Soccer Online
  8. "Anterior Drawer Test - Wheeless' Textbook of Orthopaedics".
  9. "Lachman Test - Wheeless' Textbook of Orthopaedics".
  10. Starkey's Evaluation of Orthopedic and Athletic Injuries
  11. Patrick, Dick (2003-06-24). "Specific exercises may be key to preventing ACL injuries". USA Today.
  14. Cluett, Jonathan. "ACL Reconstruction." The New York Times Company, 07 Sept 2010. Web. 7 Dec 2010. <

External links

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