Anterior cruciate ligament injury
Anterior cruciate ligament injury is 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, a popping 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 injury|
|Diagram of the right knee|
|Symptoms||A "pop" with pain, knee instability, swelling of knee|
|Causes||Non-contact injury, contact injury|
|Risk factors||Athletes, females|
|Diagnostic method||Physical exam, MRI|
|Prevention||Neuromuscular training, core strengthening|
|Treatment||Braces, physical therapy, surgery|
|Frequency||c. 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), 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. Many people with a complete tear who do not receive surgery are unable to play sports, and may develop osteoarthritis.
Signs and symptomsEdit
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.
ACL injury is most likely to occur in the following situations:
- 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 (i.e. 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.
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 appear to decrease the risk.
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 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 differencesEdit
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.
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 strengthEdit
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".
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.
|Right knee, front, showing interior ligaments||Left knee, behind, showing interior ligaments|
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 him or herself. 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.
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.
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.
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 occur when the ligament is completely torn into two pieces, and the knee joint is no longer stable. This is the most common type of ACL injury.
Around half of ACL injuries occur in conjunction with injury to other structures in the knee, including the other ligaments, menisci, or cartilage on the surface of the bones. A specific pattern of injury called the “unhappy triad” (also known as the "terrible triad," or "O'Donoghue's triad") involves injury to the ACL, MCL, and medial meniscus, and occurs when a lateral force is applied to the knee while the foot is fixed on the ground.
Interest in reducing non-contact ACL injury has been intense. The International Olympic Committee, after a comprehensive review of preventive strategies, has stated that injury prevention programs have a measurable effect on reducing injuries. These programs are especially important in female athletes who bear higher incidence of ACL injury than male athletes, and also in children and adolescents who are at high risk for a second ACL tear.
Researchers have found that female athletes often land with the knees relatively straight and collapsing inwards towards each other, with most of their bodyweight on a single foot and their upper body tilting to one side; these four factors put excessive strain on the ligaments on the knee and thus increase the likelihood of ACL tear. There is evidence that engaging in neuromuscular training (NMT) to counter those factors and emphasize proper landing technique can reduce risk of ACL injury, particularly in young female athletes.
Nonsurgical treatment for ACL rupture (also referred to as "conservative management") is suggested for individuals who are not highly active or engage in sports that cutting and twisting motions, and often includes physical therapy and the use of a hinged knee brace. In athletes or individuals with additional knee injuries such as damage to the cartilage, menisci or other ligaments, surgery is usually advised.
A torn ACL will not heal without surgery (i.e. the torn pieces will not come back together to form a functional ligament). However, if the knee remains stable enough to allow for walking and the individual does not plan to participate in high level of activity, doctors will recommend bracing and physical therapy rather than surgery.
Rest, icing, compression, and elevation may be recommended. A brace may be used to protect the knee form instability, and crutches may be used to prevent weight-bearing while the knee is healing. As swelling goes down, physical therapy may help restore function to the knee and strengthen the surrounding muscles (hamstring and quads) so that the muscles can compensate for the torn ligament and stabilize the knee.
ACL reconstruction surgery involves replacing the torn ACL with a “graft,” which is a tendon taken from another source. Grafts can be taken from the patellar tendon, hamstring tendon, quadriceps tendon from either the patient ("autograft") or a cadaver ("allograft"). The graft serves as scaffolding upon which new ligament tissue will grow.
The surgery is done with an arthroscope or tiny camera inserted inside the knee, with additional small incisions made around the knee to insert surgical instruments. This method is less invasive and is proven to result in less pain from surgery, less time in the hospital, and quicker recovery times than “open” surgery (in which a long incision is made down the front of the knee and the joint is opened and exposed).
The timing of ACL reconstruction has been controversial, with some studies showing worse outcomes when surgery is done immediately after injury, and others showing no difference in outcomes when surgery is done immediately compared to when surgery is delayed. The American Academy of Orthopedic Surgeons has stated that there is moderate evidence to support the guideline that ACL reconstruction should occur within five months of injury in order to improve patient function and protect the knee from further injury; however, additional studies need to be done to determine the best time for surgery and to better understand the effect of timing on clinical outcomes.
The goals of rehabilitation following an ACL injury are to regain knee strength and motion. If an individual with an ACL injury undergoes surgery, the rehabilitation process will first focus on slowly increasing the range of motion of the joint, then on strengthening the surrounding muscles to protect the new ligament and stabilize the knee. Finally, functional training specific to the activities required for certain sports is begun. It may take six or more months before an athlete can return to sport after surgery, as it is vital to regain a sense of balance and control over the knee in order to prevent a second injury.
The prognosis of ACL injury is generally good, with many people regaining function of the injured leg within months. ACL injury used to be a career-ending injury for competitive athletes; however, in recent years ACL reconstruction surgery followed by physical therapy has allowed many athletes to return to their pre-injury level of performance.
Long term complications of ACL injury include early onset arthritis of the knee and/or re-tearing the ligament. Factors that increase risk of arthritis include severity of the initial injury, injury to other structures in the knee, and level of activity following treatment. Not repairing tears to the ACL 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.
Unfortunately, young female athletes have a significant risk of re-tearing a repaired ACL, or tearing the ACL on the other knee after their recovery. This risk has been recorded as being nearly 1 out of every 4 young athletes. Therefore, athletes should be screened for any neuromuscular deficit (i.e. weakness greater in one leg than another, or incorrect landing form) before returning to sport.
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.
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 in the US, the sport with the highest risk of ACL tear is soccer, followed by basketball and lacrosse. The highest risk sport for high school boys in the US was basketball, followed by lacrosse and soccer.
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 they undergo early surgery.
- "Anterior Cruciate Ligament (ACL) Injuries-OrthoInfo - AAOS". orthoinfo.aaos.org. March 2014. Archived from the original on 5 July 2017. Retrieved 30 June 2017.
- "ACL Injury: Does It Require Surgery?-OrthoInfo - AAOS". orthoinfo.aaos.org. September 2009. Archived from the original on 22 June 2017. Retrieved 30 June 2017.
- Hewett TE, Ford KR, Myer GD (March 2006). "Anterior cruciate ligament injuries in female athletes: Part 2, a meta-analysis of neuromuscular interventions aimed at injury prevention". The American Journal of Sports Medicine. 34 (3): 490–8. doi:10.1177/0363546505282619. PMID 16382007.
- Sugimoto D, Myer GD, Foss KD, Hewett TE (March 2015). "Specific exercise effects of preventive neuromuscular training intervention on anterior cruciate ligament injury risk reduction in young females: meta-analysis and subgroup analysis". British Journal of Sports Medicine. 49 (5): 282–9. doi:10.1136/bjsports-2014-093461. PMID 25452612.
- Prodromos CC, Han Y, Rogowski J, Joyce B, Shi K (December 2007). "A meta-analysis of the incidence of anterior cruciate ligament tears as a function of gender, sport, and a knee injury-reduction regimen". Arthroscopy. 23 (12): 1320–25. doi:10.1016/j.arthro.2007.07.003. PMID 18063176.
- "ACL Injury: Does It Require Surgery? - OrthoInfo - AAOS". Retrieved 2018-03-22.
- Montalvo, Alicia M.; Schneider, Daniel K.; Yut, Laura; Webster, Kate E.; Beynnon, Bruce; Kocher, Mininder S.; Myer, Gregory D. (2018-03-07). ""What's my risk of sustaining an ACL injury while playing sports?" A systematic review with meta-analysis". Br J Sports Med: bjsports–2016–096274. doi:10.1136/bjsports-2016-096274. ISSN 0306-3674. PMID 29514822. Archived from the original on 2018-03-22. Retrieved 2018-03-22.
- "ACL injury - Symptoms and causes". Mayo Clinic. Retrieved 2018-03-22.
- Faryniarz, Deborah A.; et al. (2006). "Quantitation of Estrogen Receptors and Relaxin Binding in Human Anterior Cruciate Ligament Fibroblasts". In Vitro Cellular & Developmental Biology – Animal. 42 (7): 176–181. doi:10.1290/0512089.1. JSTOR 4295693. PMID 16948498.
- Hootman JM, Dick R, Agel J (April–June 2007). "Epidemiology of collegiate injuries for 15 sports: summary and recommendations for injury prevention initiatives". Journal of Athletic Training. 42 (2): 311–9. PMC 1941297. PMID 17710181.
- Wojtys EM, Huston LJ, Schock HJ, Boylan JP, Ashton-Miller JA (May 2003). "Gender differences in muscular protection of the knee in torsion in size-matched athletes". The Journal of Bone and Joint Surgery. American Volume. 85-A (5): 782–9. PMID 12728025.
- Samuelson, Kathleen; Balk, Ethan M.; Sevetson, Erika L.; Fleming, Braden C. (November–December 2017). "Limited Evidence Suggests a Protective Association Between Oral Contraceptive Pill Use and Anterior Cruciate Ligament Injuries in Females: A Systematic Review". Sports Health. 9 (6): 498–510. doi:10.1177/1941738117734164. ISSN 1941-0921. PMC 5665118. PMID 29016234.
- Hewett TE, Myer GD, Ford KR, Heidt RS, Colosimo AJ, McLean SG, van den Bogert AJ, Paterno MV, Succop P (April 2005). "Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study". The American Journal of Sports Medicine. 33 (4): 492–501. doi:10.1177/0363546504269591. PMID 15722287. Archived from the original on 2018-04-22. Retrieved 2019-01-18.CS1 maint: Uses authors parameter (link)
- Hewett TE, Ford KR, Hoogenboom BJ, Myer GD (December 2010). "Understanding and preventing acl injuries: current biomechanical and epidemiologic considerations - update 2010". North American Journal of Sports Physical Therapy. 5 (4): 234–51. PMC 3096145. PMID 21655382.
- Pappas E, Carpes FP (January 2012). "Lower extremity kinematic asymmetry in male and female athletes performing jump-landing tasks". Journal of Science and Medicine in Sport. 15 (1): 87–92. doi:10.1016/j.jsams.2011.07.008. PMID 21925949.
- Emery, Carolyn; Roy, Thierry-Olivier; Whittaker, Jackie; Nettel-Aguirre, Alberto; van Mechelen, Willem (2015-04-01). "Neuromuscular training injury prevention strategies in youth sport: A systematic review and meta-analysis". British Journal of Sports Medicine. 49 (13): 865–870. doi:10.1136/bjsports-2015-094639. PMID 26084526.
- McLean SG, Huang X, van den Bogert AJ (October 2005). "Association between lower extremity posture at contact and peak knee valgus moment during sidestepping: implications for ACL injury". Clinical Biomechanics. 20 (8): 863–70. doi:10.1016/j.clinbiomech.2005.05.007. PMID 16005555.
- Slauterbeck JR, Hickox JR, Beynnon B, Hardy DM (October 2006). "Anterior cruciate ligament biology and its relationship to injury forces". The Orthopedic Clinics of North America. 37 (4): 585–91. doi:10.1016/j.ocl.2006.09.001. PMID 17141016.
- Biondino CR (November 1999). "Anterior cruciate ligament injuries in female athletes". Connecticut Medicine. 63 (11): 657–60. PMID 10589146.
- "Pivot Shift Test - Orthopedic Examination of the Knee - PHYSICAL THERAPY WEB". PHYSICAL THERAPY WEB. Retrieved 2018-03-22.
- "Anterior Drawer Test - Orthopedic Examination of the Knee - PHYSICAL THERAPY WEB". PHYSICAL THERAPY WEB. Retrieved 2018-03-22.
- "Lachman Test". Physiopedia. Retrieved 2018-03-22.
- van Eck CF, van den Bekerom MP, Fu FH, Poolman RW, Kerkhoffs GM (August 2013). "Methods to diagnose acute anterior cruciate ligament rupture: a meta-analysis of physical examinations with and without anaesthesia". Knee Surgery, Sports Traumatology, Arthroscopy. 21 (8): 1895–903. doi:10.1007/s00167-012-2250-9. PMID 23085822.
- "MRI for Anterior Cruciate Ligament Injury: Overview, Anatomy, Mechanism of Injury". 2017-10-27. Archived from the original on 2018-03-22. Retrieved 2018-03-22.
- Rohman, Eric M.; Macalena, Jeffrey A. (2016-06-01). "Anterior cruciate ligament assessment using arthrometry and stress imaging". Current Reviews in Musculoskeletal Medicine. 9 (2): 130–138. doi:10.1007/s12178-016-9331-1. ISSN 1935-973X. PMC 4896874. PMID 26984335.
- O'Donoghue DH (October 1950). "Surgical treatment of fresh injuries to the major ligaments of the knee". The Journal of Bone and Joint Surgery. American Volume. 32 A (4): 721–38. PMID 14784482.
- Ardern, Clare L.; Ekås, Guri Ranum; Grindem, Hege; Moksnes, Håvard; Anderson, Allen F.; Chotel, Franck; Cohen, Moises; Forssblad, Magnus; Ganley, Theodore J. (2018-04-01). "2018 International Olympic Committee consensus statement on prevention, diagnosis and management of paediatric anterior cruciate ligament (ACL) injuries". Br J Sports Med. 52 (7): 422–438. doi:10.1136/bjsports-2018-099060. ISSN 0306-3674. PMC 5867447. PMID 29478021. Archived from the original on 2018-03-22. Retrieved 2018-03-21.
- Lang PJ, Sugimoto D, Micheli LJ (2017-06-12). "Prevention, treatment, and rehabilitation of anterior cruciate ligament injuries in children". Open Access Journal of Sports Medicine. 8: 133–141. doi:10.2147/OAJSM.S133940. PMC 5476725. PMID 28652828.
- Dekker, Travis J.; Godin, Jonathan A.; Dale, Kevin M.; Garrett, William E.; Taylor, Dean C.; Riboh, Jonathan C. (2017-06-07). "Return to Sport After Pediatric Anterior Cruciate Ligament Reconstruction and Its Effect on Subsequent Anterior Cruciate Ligament Injury". The Journal of Bone and Joint Surgery. 99 (11): 897–904. doi:10.2106/jbjs.16.00758. ISSN 0021-9355. PMID 28590374.
- Boden BP, Sheehan FT, Torg JS, Hewett TE (September 2010). "Noncontact anterior cruciate ligament injuries: mechanisms and risk factors". The Journal of the American Academy of Orthopaedic Surgeons. 18 (9): 520–7. doi:10.5435/00124635-201009000-00003. PMC 3625971. PMID 20810933.
- Myer GD, Sugimoto D, Thomas S, Hewett TE (January 2013). "The influence of age on the effectiveness of neuromuscular training to reduce anterior cruciate ligament injury in female athletes: a meta-analysis". The American Journal of Sports Medicine. 41 (1): 203–15. doi:10.1177/0363546512460637. PMC 4160039. PMID 23048042.
- "Common Sports Injuries and Their Prevention". Fitness and Exercise Sourcebook. Omnigraphics. 2011. p. 463. Retrieved 19 May 2019.
- Jarvela, Timo (February 2008). "Double-bundle anterior cruciate ligament reconstruction using hamstring autografts and bioabsorbable interference screw fixation: prospective, randomized, clinical study with 2-year results". The American Journal of Sports Medicine. 36 (2): 290–297. doi:10.1177/0363546507308360. PMID 17940145.
- Bottoni, Craig R. (April 2008). "Postoperative range of motion following anterior cruciate ligament reconstruction using autograft hamstrings: a prospective, randomized clinical trial of early versus delayed reconstructions". American Journal of Sports Medicine. 36 (4): 656–662. doi:10.1177/0363546507312164. PMID 18212347.
- "OrthoGuidelines". Retrieved 23 March 2018.
- "Anterior Cruciate Ligament Injury". Retrieved 23 March 2018.
- Paterno, MV (October 2010). "Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport". American Journal of Sports Medicine. 10 (10): 1968–1978. doi:10.1177/0363546510376053. PMC 4920967. PMID 20702858.
- Mall NA, Chalmers PN, Moric M, Tanaka MJ, Cole BJ, Bach BR, Paletta GA (October 2014). "Incidence and trends of anterior cruciate ligament reconstruction in the United States". The American Journal of Sports Medicine. 42 (10): 2363–70. doi:10.1177/0363546514542796. PMID 25086064.
- Gornitzky AL, Lott A, Yellin JL, Fabricant PD, Lawrence JT, Ganley TJ (October 2016). "Sport-Specific Yearly Risk and Incidence of Anterior Cruciate Ligament Tears in High School Athletes: A Systematic Review and Meta-analysis". The American Journal of Sports Medicine. 44 (10): 2716–2723. doi:10.1177/0363546515617742. PMID 26657853.
- Ramski DE, Kanj WW, Franklin CC, Baldwin KD, Ganley TJ (November 2014). "Anterior cruciate ligament tears in children and adolescents: a meta-analysis of nonoperative versus operative treatment". The American Journal of Sports Medicine. 42 (11): 2769–76. doi:10.1177/0363546513510889. PMID 24305648.