Anterior cruciate ligament (ACL) injury is a common sports injury which can have severe negative consequences. Neuromuscular factors that increase risk, such as knee landing kinematics, may be ameliorated through training. Effective ACL injury prevention programs exist, although the ideal program is yet to be determined. It is recommended that athletes engaged in high-risk sports participate in an ACL injury prevention program to reduce the risk of sustaining this injury.
Key points
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Anterior cruciate ligament injuries are common in athletes.
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Anterior cruciate ligament injuries can have long-term consequences for the affected athlete.
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Widespread implementation of anterior cruciate ligament injury prevention programs has not occurred.
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This article reviews some strategies for prevention of anterior cruciate ligament injuries based on current research.
Introduction
Anterior cruciate ligament (ACL) injury is a common and severe sports injury. The female athlete is at particular risk for this type of injury. ACL injuries have immediate and long-term consequences for affected athletes. Despite much medical literature concerning these injuries, widespread implementation of effective injury prevention programs has not occurred. The aim of this article is to review ACL injury epidemiology, including risk factors, mechanisms for injury, and strategies for ACL prevention based on current research.
Introduction
Anterior cruciate ligament (ACL) injury is a common and severe sports injury. The female athlete is at particular risk for this type of injury. ACL injuries have immediate and long-term consequences for affected athletes. Despite much medical literature concerning these injuries, widespread implementation of effective injury prevention programs has not occurred. The aim of this article is to review ACL injury epidemiology, including risk factors, mechanisms for injury, and strategies for ACL prevention based on current research.
Epidemiology
ACL injury is a common sports injury that often occurs during adolescence and young adulthood. It often occurs from a noncontact knee injury, but can occur from direct trauma, such as a blow to the knee. It is estimated that year-round high-level female soccer and basketball participants have approximately a 5% risk of having an ACL tear. Defining 1 athletic exposure (AE) as 1 athlete participating in 1 game or practice, studies at the high school level suggest football has the highest rate of knee injury at 6.29 per 10,000 AE. High school girls’ soccer and girls’ gymnastics are also huge contributors to knee injuries with 4.53 per 10,000 AE in girls’ soccer and 4.23 per 10,000 AE in girls’ gymnastics. When we examine the specific risk of ACL injury for girls compared with boys in sex-comparable sports at the high school level, we find that girls are generally at a profoundly higher risk of ACL injury with a relative risk (RR) of 2.38. This statement remains valid for softball versus baseball (RR, 4.99), basketball (RR, 4.54), and soccer (RR, 2.33) Joseph and colleagues, in a recent epidemiologic multisport comparison of high school athletics at 100 representative US high schools, found that 74.9% of ACL injuries occurred during competition versus practice. They found the highest injury rates per 100,000 AE in girl’s soccer (12.2) followed by football (11.1) and girl’s basketball. Interestingly, boy’s basketball had a relatively low risk at 2.3.
Studies of collegiate athletes note that men’s spring football and women’s gymnastics have equal ACL injury rates per 1000 AE (0.33). The authors also note that 3 of the 4 collegiate sports with the highest ACL injury rate included women’s gymnastics (0.33), women’s soccer (0.28), and women’s basketball (0.23).
Given the above epidemiology and that ACL injuries frequently require surgery, the expense of these injuries becomes a significant public health concern. A 2013 study of 2 prospective orthopedic cohorts estimated the costs for surgical repair and rehabilitative conservative care of ACL injuries. The Multicenter Orthopedics Outcomes Network cohort included 988 primary ACL tears followed up for a minimum of 6 years. The KANNON (knee anterior cruciate ligament, nonsurgical vs surgical treatment) cohort studied 121 patients for a minimum of 2 years. The societal costs of each type of treatment of this injury are significant. The lifetime cost estimate for ACL surgical repair was $38,121 compared with $88,538 for rehabilitative conservative care. The author’s acknowledge that the long-term economic cost predictions were based on lower levels of evidence than level I. The authors additionally note that ACL tears may cause profound long-term issues for patients regardless of treatment approach, highlighting the importance of primary prevention.
Although the above comments concern the long-term issues that arise after ACL injuries, there are immediate immense impacts of ACL injuries on health care expenditures. This serious injury necessitates physician evaluation, radiography, and rehabilitation in addition to potential surgical intervention and postoperative care; these represent a public health emergency. Given the huge health, economic, and societal tolls of ACL injury, it becomes imperative for physicians to better understand how to prevent this catastrophic injury. Third-party payer data on ACL injury costs are difficult to obtain. It is estimated, however, that up to 250,000 ACL injuries occur yearly in the United States, at a cost of more than $2 billion per year. Given the above startling and troubling statistics, it is critical that physicians have a thorough understanding of the causes of ACL injuries and the ways in which ACL injuries can be prevented.
It is difficult to prevent acute knee injuries secondary to direct knee trauma because of the large contact forces on the joint encountered with direct knee impacts. In the United States, football participation is the leading cause of sports trauma overall and a common cause of injury to the ACL. However, it is surprising that most sports-related ACL injuries are actually secondary to noncontact-related forces on the knee joint. Contact causes fewer ACL tears than cutting maneuvers or speed decelerations.
Immediate morbidity and loss of function in the athlete are significant public health concerns. Larger public health concerns are the long-term sequelae of sports-related knee injuries. Of young adults with knee injuries, 13.9% will have knee osteoarthritis by age 65 compared with 6.0% in those without histories of knee injuries. A study of female soccer players with a prior history of ACL injury found arthritic radiographic changes such as joint space narrowing or osteophytes in 82% of those examined radiographically 12 years after injury. Fifty-one percent of women fulfilled the radiographic requirements for radiographic knee osteoarthritis in the cohort with a median age of 31 years (range, 26 to 40 years old.) The cutoff for radiographic osteoarthritis in this study approximated grade 2 on the Kellgren/Lawrence scale. Seventy-five percent of respondents in this study noted symptoms affecting their knee-related quality of life. These symptoms resulted in lifestyle modifications in 50% of respondents.
A study of male soccer players noted similar outcomes. Male soccer players with a history of ACL injuries were radiographically assessed 14 years after ACL injury. This radiographic examination documented abnormalities in 78% of evaluated knees with 41% of the injured knees showing findings of Kellgren/Lawrence grade 2 or higher. Surgical versus conservative treatment of these injured knees did not affect the radiographic outcome. Noninjured knees by contrast showed advanced changes in only 4% of knees. Study participants also responded to a questionnaire to further assess patient-relevant outcomes 14 years after ACL injury. Eighty percent of the study participants reported reduced activity level. Of these, 69% noted the knee injury as the cause of reduced activity. Thirty percent experienced severe changes in lifestyle.
In addition to the physical toll of these injuries, the emotional functioning of otherwise healthy young people may be negatively affected. The long-term impact of the depression experienced by some of those who have an ACL injury remains to be determined.
Anatomy
The cruciate ligaments are integral to knee joint stability ( Fig. 1 ). The knee is described as having a screw-home mechanism where the tibia rotates as the knee flexes and extends through the tibiofemoral articulation. As the knee straightens, the tibia externally rotates. During this motion, contact points of the joint actually migrate anteriorly with knee extension. Knee ligaments become more taut with extension. The ACL is described as the primary structure preventing anterior tibial translation. ACL strain injury occurs primarily through shear forces with additional contributions to injury from coronal and axial plane stress. The ACL is actually 2 discrete anatomic bundles ( Fig. 2 ). The anteromedial and posterolateral bundles begin from the posteromedial portion of the lateral femoral condyle and insert between and slightly anteriorly to the tibial intercondylar eminence. The bundles’ names are descriptive of their relationship at this tibial insertion. There are 2 bundles that comprise the ACL, which spiral and increase in tension with tibial internal rotation. The bundles also have variable tension based on knee flexion angle and varus/valgus and rotational stress. Because of this, the ACL is at particular risk with sidestepping or cutting maneuvers in knee flexion angles of 0° to 40°.
The majority of the blood supply to the ACL is through the middle genicular artery, which derives from the popliteal artery. There are additional blood flow contributions from the inferomedial and inferolateral genicular arteries, which traverse the anterior fat pad. Tears of the ACL commonly result in permanent damage to the blood supply of the ligament, which impairs innate healing potential.
Injury Risk Factors
Our knowledge of the various risk factors for a tear of the ACL and our ability to predict which athletes are at particularly high risk for this injury continue to evolve. Both extrinsic factors and intrinsic factors impact ACL injury risk. It remains unsettled, however, as to how all risk factors interplay and coalesce in each individual athlete to result in ACL injury.
Several external and internal risk factors appear to predispose some athletes to ACL injury risk. Playing American style football exposes the athlete’s knee to large direct contact forces. Tackling or being tackled leads to most ACL injuries in football players. From 1987 to 2000, ACL reconstruction was the third most common orthopedic procedure incurred by National Football League prospects and the most common medical condition noted for players who received a medical fail with regard to their projected ability to play successfully in the National Football League.
External ACL injury risk factors such as shoe type worn and type of playing surface have been evaluated. For example, football cleat design is found to have an impact on ACL injury risk. A 3-year prospective study of high school football players found that wearing a shoe with an increased torsional resistance was associated with a significantly higher risk of ACL injury. In this study, the edge design of longer irregular cleats at the shoe sole periphery with smaller pointed cleats positioned interiorly was a higher-risk design than shoe designs with less torsional resistance.
Playing surface coefficient of friction may also affect ACL injury risk. In a Norwegian handball study, the higher friction floor type increased the ACL injury risk for female handball players by a factor of 2.35. Their male counterparts did not accrue any statistically significant additional risk secondary to change in floor type. A 3-year prospective study of game-related football injuries in college athletes evaluated the impact of FieldTurf, a polyethylene fiber blend with ground rubber infill versus natural grass turf. No significant difference in knee injuries was noted on FieldTurf versus natural grass.
Intrinsic factors also play a role. There is a known familial predisposition to ACL tears. In a study by Flynn and colleagues, 23.4% of individuals with an ACL tear were noted to have a first-degree relative with such an ACL injury compared with 11.7% of a control group of matched individuals without a history of an ACL tear. Posthumus and colleagues noted that variants in matrix metalloproteinase genes on chromosome 11q22 were associated with increased risk of ACL tears.
Anatomic indices such as femoral intercondylar notch width, notch width index, ( Fig. 3 ) and femoral intercondylar notch morphology are assessed as independent risk factors for ACL injury. Ireland and colleagues evaluated the radiographs of 108 individuals (55 women, 53 men) with a history of ACL injury and compared the radiographic findings with those of 186 cases (94 women, 92 men) with an intact ACL. Small intercondylar notches were noted in those with ACL injury with a mean notch of 18.9±4.0 mm in those with ACL injury compared with 20.7±3.9 mm in those whose ACLs were intact. A smaller notch width index (ration of intercondylar notch width to width of distal femur) may also potentially increase ACL injury risk.
The slope of the posterior tibia may also have an impact on ACL injury risk ( Fig. 4 ). The posterior tibial slope is the posterior inclination of the tibial plateau. The tibial slope is associated with an anterior position of the tibia. In one study concerning those with chronic ACL rupture, each 10° increase in posterior tibial slope led to increased anterior laxity with a 6-mm increase in anterior tibial translation when the individual was in monopodal stance at 20° of flexion.
The potential effect of hormones and oral contraceptives on ACL rupture risk also remains a matter for continued investigation. In a study by Bell and colleagues, 30 women were studied to assess the effect of oral contraceptives on hamstring function in physically active female study participants. Oral contraceptive use did not alter muscle neuromechanics. Wild and colleagues evaluated the role of the female adolescent growth spurt with its large estrogen increases as a contributor to knee laxity and the risk of ACL injury. All participants were premenarchal 10- to 13-year-old girls to exclude cyclic hormone variation. Participants were biomechanically assessed up to 4 times during the 12-month period of the adolescent growth spurt. Interestingly, estradiol concentrations remained constant. Although quadriceps strength increased during the study, hamstring strength did not. The authors theorize that strength imbalance might place the ACL at injury risk. Passive knee joint laxity of study participants also increased during the testing period. It has also been observed that the peak velocity for lower limb growth precedes the peak height velocity. This growth rate discrepancy has an unclear biomechanical impact on lower extremity vulnerability. Hormonal influences remain an area of intense interest. A recent symposium, ACL Research Retreat VI, highlighted the complexity of studying the effects of hormonal influences on mechanical properties of the ACL and ACL injury vulnerability.
The injury predisposition in women may also be enhanced by properties of the ACL itself. The female ACL has an 8.3% lower tensile load to failure compared with the male ACL in one cadaveric study. The female ACL also had a 22.49% lower modulus of elasticity. An additional cadaveric study found that the female ACL is shorter than the male ACL, with ACL mass correlating with height in women but not in men. A retrospective cohort of subjects referred for knee magnetic resonance imaging noted that ACL volume correlated with height differences rather than gender. The maximal ACL area is noted to be smaller in women than men when standardized for body weight. A prospective study evaluating the effect of generalized joint laxity noted that passive knee hyperextension and asymmetry of knee anterior-posterior laxity enhanced ACL injury risk.
Research suggests that neuromuscular activation in a gender-specific pattern likely contributes to ACL injury risk. The proposed differences include female athletes landing differently from jumps than male athletes. Findings include landing from jumps with a relatively straight knees. A study of adolescent soccer players noted that female athletes exhibited decreased hip and knee flexion in jump tests. Women have a greater vertical ground reaction force than is found in male athletes while performing jump tests. Knee valgus moments were significantly increased compared with those in male athletes for single and bilateral leg landings. A retrospective analysis of 1718 athletes with ACL injury noted the dynamic alignment at time of injury in 1603 subjects (781 men, 822 women).The alignment of knee-in and toe-out was the most common, noted in 793 of 1603 with no significant difference in male and female subjects.
The trunk also contributes to the ground reaction forces on the ACL. Relatively greater hip adduction, decreased hip abductor strength, and diminished trunk control can contribute to knee injury risk. Proprioceptive deficiency has been correlated with knee injury in women but not in men. Impaired core proprioception in women was highly predictive of knee injury risk in this prospective study. Another prospective study of 277 college athletes found that decreased trunk control was strongly predictive of knee injury in female athletes.
The morphology of the knee joint articulation surface is also smaller in the female athlete. magnetic resonance imaging of the joint surface of 18 healthy subjects found interesting differences in male and female joint anatomy. Compared with those in men, women’s medial tibias had 32.9% less surface area and women’s lateral tibias had 33.4% less lateral tibial surface area and 21% less femoral joint surface area.
The quadriceps angle or Q angle is greater in women than in men ( Fig. 5 ). This measure of the femoral to tibial angle is an additional factor that may increase load on the ACL through increased lateral quadriceps pull. Increased knee joint laxity has also been proposed to increase the risk of ACL injury in the athlete. As previously noted, the ACL is the principal restraint to anterior tibial translation. Anterior knee laxity in non–weight bearing is a test of ACL integrity. Shultz and colleagues found that increased anterior knee laxity was in fact associated with increased anterior tibial translation, as the knee accepted weight.
