Injury to the anterior cruciate ligament (ACL) is one of the most common knee ligament injuries evaluated by sports medicine practitioners. Approximately 250,000 ACL tears are estimated and 100,000 reconstructions are performed annually in the United States (15). Lesions are more common in young and active individuals, especially girls, who participate in sports such as soccer and basketball, which involve jumping, pivoting, and change of direction (3). The estimated long-term mean lifetime cost to society is high, over $38,121 after reconstructive surgery and $88,538 for those managed with rehabilitation (26). ACL injury is associated with potential long-term complications, including chronic knee instability, meniscus tears, cartilage injury, and development of osteoarthritis (OA). Approximately 50% of ACL-injured patients will have evidence of OA associated with pain and functional impairment within 10–20 years after the original injury regardless of surgical or conservative treatment (50). In addition, after ACL reconstruction, less than 50% of patients will return to sports within 1 year, less than 65% will return within 2 years, 24% will change sports, and 11% will cease sports activity (14,18). ACL injuries have become a public health concern, particularly with the number of young individuals involved in competitive sports, the increasing incidence of tears in pediatric patients, and the long-term consequences of the injury. This has led to a special interest in the identification of risk factors for the injury and prevention strategies that would lead to a reduction in its incidence. In this article, we review some of the available scientific evidence related to the role of modifiable and nonmodifiable risk factors in the susceptibility to ACL tears and the effectiveness of injury prevention programs in athletes who participate in high-risk sports.
Anatomy and Biomechanics
ACL is a collagenous structure that originates on the medial wall of the lateral femoral condyle and crosses anteromedially to its insertion on the anterior aspect of the tibial articular surface. It is an intraarticular but extracapsular structure with limited capacity of healing. Primarily it functions as a static stabilizer of the knee by resisting hyperextension, anterior tibial translation, and rotatory movements. Secondarily it resists varus and valgus movements in all degrees of flexion. It consists of two functional bundles: the anteromedial (AM) and the posterolateral (PL) based on their tibial insertion sites. Forces transmitted through it vary with knee-joint position. Li et al. (24) showed that ACL forces are highest in the last 30° of extension and hyperextension and under several passive conditions such as anterior tibial translation, internal rotation, and valgus stress. In addition, Gabriel et al. (12) found that forces transmitted through each individual bundle were greater at 60° and 90° of flexion for the AM bundle, and at full extension for the PL bundle. The blood supply to the ACL is provided primarily by the middle genicular artery, a branch of the popliteal artery. The innervation comes from the posterior articular nerve, a branch of the tibial nerve at the popliteal fossa, which supplies mechanoreceptors and nociceptors, which may play a major role in knee proprioception.
Mechanisms of Injury
A noncontact mechanism of ACL injury occurs in 70%–80% of cases. These injuries most often occur on landing from a jump, while cutting or with a sudden deceleration (3). One of the most commonly described mechanisms in girls involves landing with an extended hip and knee, the knee is in valgus, internal rotation of the tibia, and a pronated foot, the so called “position of no return”(3) (Fig. 1). Furthermore Hewett and Myer (20) proposed that the mechanism of noncontact ACL injuries in girls also includes poor trunk control, lateral trunk motion with the body shifted over the weight-bearing leg associated with high knee abduction moment, and medial knee collapse as essential components. On the other hand, Hame et al. (16) described a different noncontact mechanism in alpine skiers that involves internal tibial rotation with a fully extended knee or a flexed knee beyond 90°. Meanwhile contact (traumatic) injuries are frequently associated with a forceful valgus stress and concomitant injury to the medial meniscus and medial collateral ligament (3).
Multiple risk factors have been associated with ACL injury. These factors can be classified as intrinsic or extrinsic, as well as modifiable or nonmodifiable. Intrinsic nonmodifiable factors include gender, anatomic variations, history of previous ACL injury, and genetic predisposition, whereas modifiable intrinsic factors include body mass index (BMI), hormonal status at the time of sports participation, neuromuscular deficits, and biomechanical abnormalities. On the other hand, extrinsic factors, which are modifiable, include playing environment, equipment, level of competition, and type of sport (Table 1).
Anatomic risk factors for ACL injuries have been studied widely. Femoral notch width has been an area of considerable debate. The majority of the studies have found a relationship between notch width or notch width index (the ratio of the widths of the femoral notch and condyle) and risk of experiencing ACL injury (44). Athletes with a narrow notch are at significantly greater risk for sustaining a noncontact ACL injury regardless of gender or notch shape (7,43). In addition, an association of decreased ACL cross-sectional area and narrow femoral notch with increased injury risk has been proposed (6,22). Other possible anatomic risk factors include posterior-inferior directed tibial slope and shallow medial tibial plateau depth (43). Ligamentous laxity also has been associated with increase in anterior cruciate injury risk. According to Myer et al. (27), this is especially true in female athletes who participate in high-risk sports such as basketball and soccer. In terms of malalignment on static evaluation, increase in subtalar pronation, excessive navicular drop, and knee recurvatum also was found to be associated with ACL injury (25,47). A modifiable anatomic risk factor associated with ACL injuries is a high BMI (46). Moreover, Evans et al. described that higher BMI in combination with narrow notch width may predispose young athletes to noncontact ACL injury (10).
Neuromuscular and biomechanical
Neuromuscular and biomechanical factors also have received considerable attention since they are modifiable. They include differences in landing, pivoting, and cutting biomechanics between injured and noninjured subjects. It has been observed that injured participants exhibit increased knee abduction and intersegmental abduction moment as well as greater ground reaction force (21). It also has been shown that girls land from a jump and perform cutting and pivoting maneuvers with less knee and hip flexion, increased knee valgus, internal rotation of the hip, external rotation of the tibia, and high quadriceps muscle activity in relation to the hamstrings muscle. In addition, girls exhibit leg dominance and imbalance in leg strength, flexibility, and control (15). Muscle fatigue further exacerbates poor biomechanics that increase the risk of injury, especially on the latter part of sports activity (4). Core proprioception and stability also have been related to ACL tears. Poor repositioning of the trunk and lateral trunk displacement were predictors of knee injuries in girls, but not in boys (48,49).
Gender-specific and hormonal
Girls have a higher risk for noncontact ACL injuries when compared with boys on similar sports that involve cutting, deceleration, and/or jumping (15). Research has identified estrogen and progesterone receptors in the ACL, which may suggest a hormonal influence in ACL sprains (38). Multiple studies have evaluated the relationship between menstrual cycle and the risk of injury. While some studies have shown that most ACL injuries in girls appear to occur in the early and late follicular preovulatory phase, other studies have shown an increased incidence during the luteal (postovulatory) phase and during menstruation (2,15,28,39, and 42). At this moment, there is no clear answer to the role of hormonal status and sports participation in ACL injury.
The type of footwear, particularly shoes that provide higher torsional resistance with the ground, such as those with longer and higher number of cleats is associated with a significantly higher ACL injury rate (23). The type of playing surface also appears to have a role in injury risk, especially those that cause a higher shoe-surface friction. In indoor sports (e.g., handball), there appears to be a higher risk of injury for women who play on artificial (synthetic) floors when compared to wooden floors (31). Meanwhile in outdoor sports, playing on grass appears to be less risky than artificial turf; furthermore rye grass appears to offer protection compared to Bermuda grass (32). Weather conditions affect the mechanical interface between the shoes and playing surface. Studies have shown that ACL injuries are less common during low-water evaporation and high-rainfall season in Australian football and during cold weather in American football (33,34).
Other risk factors
There are other intrinsic, nonmodifiable factors that have been associated with an increased risk for ACL injury. Previous injury poses a higher risk of injury in the ipsilateral as well as in the contralateral knee (24). Genetic predisposition also has been studied. Flynn et al. (11) found that participants with an ACL tear were twice as likely to have a relative (first, second, or third degree) with an ACL tear. Particularly in girls, several genes involved with codification for types I, V, and XII collagen (COL1A1, COL5A1, and COL12A1), and matrix metalloproteinase (MMP10, MMP1, MMP3, and MMP12) have been associated with ACL rupture risk (36,37). Additional modifiable extrinsic factors include level of competition and type of sport. Injuries occur more frequently during games than practice (28). Sports such as football, soccer, basketball, volleyball, handball, lacrosse, gymnastics, and alpine skiing pose a higher risk of injury for athletes (38). A recently proposed risk factor that needs more research is the association between decreased neurocognitive performance that includes reaction time, processing speed, and visual as well as verbal memory with noncontact ACL injury (45).
Risk for recurrent ACL injuries
Studies have shown that graft ruptures and contralateral limb injury range from 6% to 32% (18,24). The risk of ACL graft rupture is 15% greater than primary ACL tear (35). Risk factors include persistent neuromuscular deficits, poor trunk control, and high levels of postsurgical activity (18). Special attention should be given to the younger patients with ACL injuries particularly those who undergo reconstruction with allograft due to a higher incidence of rerupture (17,24).
Reduction of noncontact ACL injury is the primary aim of most preventive strategies. A large number of published articles have evaluated ACL prevention programs, but only a few are randomized controlled trials (30). The majority of the programs focus on modification of intrinsic risk factors, mainly neuromuscular and biomechanical deficits. However no standardized intervention program has been established, but different training programs have been proposed.
Research has shown that a multicomponent program is needed to reduce the risk and incidence of noncontact ACL injuries (1). Most of the programs include one or more of the following: strengthening, stretching, aerobic conditioning, plyometrics, proprioceptive, and balance training as well as education and feedback regarding body mechanics and proper landing pattern (Table 2). A preseason injury prevention program combined with an in-season maintenance program is preferred (1). They should be started at least 6 wk prior to the season, followed by a maintenance program that can replace the traditional warm-up.
Every ACL prevention program should incorporate education and feedback regarding proper landing and cutting technique. Landing should be soft on the forefoot with trunk, hip, and knee flexion while avoiding knee valgus. Landing on both feet should be encouraged. A partner athlete or coach can give feedback on the correct technique.
Self-feedback with use of videos has been shown to reduce ACL injuries in experienced skiing professionals by helping to correct technical errors and reinforce good technique (9).
Incorporating strength training to a multicomponent program has been shown to reduce knee injuries (19). The hamstring muscle is an important target for strength training since it prevents anterior tibial translation and functions as an ACL agonist (1). Eccentric exercise such as the “Russian hamstring curl” has been shown to be effective in increasing hamstrings to quadriceps ratio (8). Gluteus maximus and medius strength is crucial to reduce femoral rotation and knee valgus during landing, cutting, or changing direction (48). Strengthening of the gluteus maximus can be achieved with single-limb squat or single-leg deadlifts, while side planks or side-lying hip abduction can be implemented for the gluteus medius.
Interventions incorporating core stability training, including proprioceptive exercise and correction of lateral angular displacement of the trunk, also should be an essential part of a prevention program (48,49). Core stability and proprioceptive training can include planks, bridges, and single-leg squats. The exercise regimen can progress from firm to unstable surfaces followed by adding perturbation (Fig. 2). Caraffa et al. (5) found a significant risk reduction in ACL injuries in soccer players after incorporating proprioception training using a wobble board during warm-up before practice.
Multiple programs integrate plyometric exercise in their regimen. They have been shown to decrease landing forces, decrease hip adduction and abduction moment, increase lower extremity power, and decrease the incidence of serious injuries (19). Plyometrics also can address differences in leg strength and leg dominance (19). Programs should include single-leg weight-bearing activities and incorporate high-intensity agility drills on multiple planes including sports-specific drills.
It is known that prevention programs can have a quantifiable reduction in the risk of ACL injury. Sadoghi et al. (40) performed a systematic review of the literature concluding substantial benefit of ACL injury prevention programs with a risk reduction of 52% in female and 85% in male athletes. A recent meta-analysis also demonstrated the efficacy of neuromuscular training and educational intervention, which appear to reduce the incidence of ACL injuries by approximately 50% (13). Furthermore it has been shown that specific prevention programs such as Sportsmetrics™ (Cincinnati, Ohio) and the Prevent Injury and Enhance Performance Program™ (Santa Monica, California) not only reduce injury risk but also can improve performance (30).
In addition to implementing prevention programs that focus on intrinsic risk factors, athletes who are at risk should be educated also on modifiable extrinsic risk factors. Those that participate in indoor sports should be encouraged to practice and compete on wooden floors. Meanwhile those that participate in outdoor sports should practice on natural grass and avoid hot and dry weather. Shoes with a longer and higher number of cleats should be discouraged. An area of concern is the use of prophylactic knee bracing for prevention of ACL injuries. Caution should be used when prescribing knee brace strictly for prevention of ACL injury since the literature remains equivocal and inconclusive on its effectiveness (29).
Although a preventive program for ACL injury may be implemented universally with athletes who participate in high-risk sports, identification of individuals who possess modifiable risk factors may allow for a more effective prevention strategy. Intrinsic risk factors such as female gender, high-risk sport participation, previous history, and/or family history of ACL injury can be documented with the medical history. During the physical examination, trunk, hip, and thigh muscle weakness; strength imbalance; localized or generalized ligamentous laxity; and poor balance can be identified. Extremity malalignment, which includes increased subtalar pronation, excessive navicular drop, and knee recurvatum, can be assessed on static evaluation. Dynamic evaluation may include observation of single-leg squat, step down from a bench, and vertical drop jump in an attempt to identify excessive knee valgus and landing abnormalities. Clinical screening tools such as the Landing Error Scoring System have been shown to identify subjects with potential high-risk landing mechanics; however it is unknown if they can predict ACL injuries (41). Knee x-rays (tunnel and lateral views) can be used to measure intercondylar notch width and tibial slope. Magnetic resonance imaging can be ordered to evaluate the ACL volume as well as the notch size and tibial slope. In addition, isokinetic quadriceps-hamstring strength test can be used to quantify muscle imbalance objectively. In the patient with family history of ACL injury, genetic screening may be something to consider in the near future as testing becomes more available. More information is needed to determine whether the incorporation of neurocognitive and formal balance testing may provide additional benefit to identify individuals at risk.
Injury to the ACL is common and affects young individuals, particularly girls, who are active in sports that involve jumping, pivoting, as well as change of direction. Even with appropriate diagnosis, surgical management, and aggressive rehabilitation of ACL tears, a significant number of individuals will not return to their previous level of function and will develop knee osteoarthritis. Modifiable risk factors associated with this injury include weak trunk and hip muscles, poor conditioning, abnormal landing mechanics, type of sport, and playing surface. Preventive programs with multiple components including strength, plyometric and balance exercises, as well as education and feedback on appropriate landing techniques have been shown to be effective in reducing the risk of ACL injury in both boys and girls if implemented prior to sports participation and continued as a maintenance program through the competitive season.
The authors declare no conflicts of interest and do not have any financial disclosures.
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