Anterior cruciate ligament (ACL) injury prevention

INTRODUCTION — 

Anterior cruciate ligament (ACL) injuries continue to be a common, debilitating injury among competitive and recreational athletes. Nearly a quarter of a million ACL injuries occur in the United States and Canada alone each year [1]. The acute and chronic morbidity associated with ACL injuries is substantial, as are related costs. A substantial number of athletes are unable to regain their preinjury level of performance, or even resume their primary sport at all.

While ACL injuries can occur by a variety of mechanisms, low-energy, noncontact injuries are most common, accounting for approximately 70 percent of ACL tears [2]. Such noncontact injuries may be amenable to prevention, and substantial research has been performed to elucidate the biomechanics of noncontact ACL tears and the most effective interventions to prevent them.

The risk factors for noncontact ACL tears and strategies for preventing such injuries are reviewed here. The diagnosis and treatment of ACL tears are discussed separately. (See "Anterior cruciate ligament injury".)

BIOMECHANICS OF ACL AND NONCONTACT INJURY — 

The ACL spans the knee joint from the medial aspect of the lateral femoral condyle to the anterior tibial spine. The ligament functions as a limit to anterior translation and rotation of the tibia. A complex yet repeatable series of motions is seen in noncontact ACL injuries. Several biomechanical models have described the process of an ACL tear, which include the following factors [3]:

●Hip internal rotation and adduction

●Dynamic knee valgus

●Tibial rotation and anterior translation

●Ankle eversion

The anatomy of the ACL and knee, and the mechanism of ACL injury, are reviewed in greater detail separately. (See "Anterior cruciate ligament injury", section on 'Anatomy and function' and "Physical examination of the knee", section on 'Anatomy' and "Physical examination of the knee", section on 'Biomechanics'.)

RISK FACTORS FOR NONCONTACT ACL INJURY — 

While the mechanism of noncontact ACL injury is common and consistent across disparate populations, certain groups show increased susceptibility due to a range of established and potential risk factors.

The single best predictive factor for noncontact ACL injury is a history of prior ACL injury. The risk of ACL "rerupture" to the same side after surgical reconstruction ranges from 6 to 25 percent, while the risk of contralateral ACL rupture after reconstruction is nearly the same, ranging from 2 to 20.5 percent [4].

Increased risk among female athletes — The female athlete is the largest at-risk population for noncontact ACL injuries. Overall, female athletes are significantly more likely to sustain an ACL tear than their male counterparts participating in the same sport or activity [5]. A 2016 systematic review and meta-analysis reported an incidence of 0.08 in female athletes and 0.05 in male athletes per 1000 exposures [6]. Proposed explanations for this disparity include a combination of physical and biomechanical attributes, which are discussed below [7,8].

Age — Young age is a risk factor for ACL injury, with the peak incidence occurring at 16 to 18 years, and this rate is rising [9,10]. These phenomena are thought to be attributable in part to earlier participation in organized sports and higher rates of youth sport specialization than seen in previous generations.

The relationship between age and sex in determining injury risk remains to be clarified. In a retrospective database review of 129 children younger than 12 years, boys sustained a significantly higher rate of ACL injury than girls (85 versus 44; male: female, 1.93:1) [11].

Anatomic and hormonal risk factors — Differences between male and female bony anatomy and knee alignment serve as a starting point for the study of sex disparities in noncontact ACL injuries. The Q angle is made by drawing a line from the anterior superior iliac spine to the patella and a second line from the patella to the tibial tubercle (figure 1). The relatively wide pelvis and short femur of females creates a larger Q angle. Although some researchers claim an association exists between a larger Q angle and increased risk of ACL tear, no convincing evidence exists to substantiate this claim [12,13].

Another anatomic difference is the decreased width of the intercondylar notch of the distal femur in females, which some research associates with ACL tear [14-16]. However, others refute this association [17], and the potential role of notch width remains debatable. Regardless, notch width, like the Q angle, cannot be modified.

Serum estrogen and relaxin exert effects upon the strength and flexibility of soft tissues, including ligaments, and may influence neuromuscular function, although these effects and their impact on ACL injury remain unclear and controversial [12,18-21]. Studies differ in their conclusions about the relationship between ACL injury risk and fluctuations in estrogen during the menstrual cycle [22-25]. Oral contraceptives modulate hormonal effects on soft tissue, and several observational studies suggest that they may reduce the risk of ACL rupture [12,26,27], but other study results refute this concept [28]. The use of oral contraceptives to decrease the risk of ACL injuries remains controversial and requires further study.

Neuromuscular and biomechanical risk factors — In studies comparing female and male athletes and their susceptibility to ACL injury, researchers have noted differences in relative muscle strength and the biomechanics of deceleration, pivoting, and landing. These factors appear to play a major role in the increased risk of ACL injury among female athletes, and when present in males, increase their risk as well [12,29-31]. Important factors include:

●Hamstring weakness and under-recruitment (ie, quadriceps dominance) [32-36].

●Valgus angulation of knee during landing or change of direction [29,30,37-40].

●Hip external rotator weakness and under-recruitment [41-43].

●Torso (ie, core) muscle weakness [44].

●Quadriceps dominance – Quadriceps dominance refers to the muscle group used preferentially to control deceleration. Several biomechanical studies have found that in female athletes, the quadriceps group generally contracts first during deceleration, while in males, the hamstring group generally contracts first [7,32-34]. More specifically, the posterior chain (hamstrings and gluteus muscles) is critical to counteracting the effect of the quadriceps on anterior tibial translation, especially at small knee flexion angles (<45 degrees). Analyses of bone bruise location patterns after ACL injury indicate that tibial anterior translation relative to the femur is a primary injury mechanism in the majority of such injuries, supporting the important role of the posterior chain [45]. For prevention purposes, increasing posterior chain strength and recruitment during dynamic activities is critical to reducing the stress placed on the ACL from sagittal plane loads.

Studies also suggest that females generally have weaker hamstrings and greater strength imbalances between the two muscle groups, and such imbalances increase knee instability [35,36,46]. These findings suggest an important role in ACL injury prevention for exercises that correct relative muscle weakness and imbalance.

Sex differences in muscle group recruitment extend beyond deceleration. Coactivation of the hamstrings and quadriceps protects the knee by improving control and stability. Female athletes who demonstrate quadriceps dominance also exhibit weak or under-recruited hamstrings, limiting the potential for such coactivation [12].

●Dynamic knee angulation – Valgus angulation (medial knee collapse) and limited knee flexion during jump landings and rapid changes in direction (ie, cutting) increase the loads placed on the ACL, and thereby the risk of injury.

During jump landings, a decreased ratio of medial to lateral quadriceps recruitment, along with greater firing of the lateral hamstrings, leads to decreased control of forces exerted in the coronal plane of the knee. This imbalance produces increased valgus angulation at the knee (ie, knee bends inward or medially). A valgus moment during landing or cutting substantially increases the stress placed on the ACL [12].

Limited knee flexion also increases the stress placed on the ACL during jump landings and cutting maneuvers [12,47]. A reduced knee flexion angle increases vertical ground reaction forces and can contribute to increased anterior shear forces exerted on the tibia, both of which increase loads sustained by the ACL.

Several biomechanical studies, including some using video analysis, have found that female athletes are more likely to place their knees in positions of increased valgus angulation when changing direction during sport [29,30,37-39].

The role of sex differences in knee flexion angles during landing and their role in ACL injury are debated. One research group has reported that female athletes land with significantly greater knee flexion than male athletes [48]. However, another research group found no significant difference in flexion angles between female athletes who went on to tear their ACL and their uninjured peers [37].

●Hip musculature weakness – Imbalances in hip muscle activation and control contribute to sex disparities in ACL injury rates. Hip muscle control and mobility play an important role in determining the forces exerted on the knee during sport. One research team reported significantly lower gluteal muscle activity (as determined by surface electromyogram [EMG]) in females compared with males during landing [41]. Another group reported greater hip internal rotation and maximum angular displacement during jump landing in American collegiate female athletes (n = 13) compared with their male counterparts [42]. Combined, the decreased gluteal muscle activation and increased hip internal rotation increase the strain placed on the ACL [12]. Other evidence suggests a clinically significant association between ACL injury and restrictions in hip internal and external motion, as well as radiographic evidence of femoroacetabular impingement [49].

●Torso musculature weakness – Studies of torso (ie, core) strength and proprioception suggest that deficits in these areas increase ACL injury risk in females. In a prospective observational study of 277 American collegiate athletes, females (n = 140) who exhibited impaired core stability and proprioception sustained higher rates of ligament and meniscal knee injuries (11 total) over the subsequent three years [44]. No such increased risk was noted among male athletes.

The importance of relative muscle weakness and poor biomechanics as risk factors for ACL injury is further supported by studies of dancers, whose training involves holding positions that develop strength in the knee and hip stabilizers and torso, and perfecting jumping and landing technique. Female dancers sustain ACL injuries at much lower rates than their field sport counterparts [50]. Laboratory studies of high-level dancers and team sport athletes performing a 30 cm single-leg drop-landing have noted that female dancers and male athletes (dancers and team sport participants) land in a similar fashion, with little or no knee valgus and greater hip and trunk stability, whereas female team athletes demonstrate significantly greater knee valgus and less hip and trunk stability [51,52].

Environment and other extrinsic factors — Fatigue and a number of extrinsic factors, some partially modifiable, may predispose athletes to ACL injury. Factors that have been studied include playing surface (eg, synthetic versus grass), environmental conditions (eg, dry versus wet), footwear, physical fatigue (eg, early versus late period of a game; match played early or late in a season), and legal or illegal play by opponents [53]. It is worth emphasizing the multifactorial nature of many of these risk factors and the dearth of controlled studies. Given the wide variety of playing surfaces, environmental conditions, and shoe wear in which sports are played, as well as the limited and heterogeneous published evidence, it is difficult to generalize about interventions to reduce the impact of potential extrinsic risk factors for ACL injury.

●Fatigue and mechanical perturbation – Neuromuscular fatigue diminishes an athlete's ability to control, absorb, and transfer force, thereby placing greater stress on the ACL and potentially increasing the risk of injury [54-56]. However, a systematic review of 37 studies of the effect of fatigue on lower limb mechanics during landing or cutting did not identify consistent changes in movement that increase the risk for ACL injuries [55]. The authors noted that methods varied among studies, and further research into the role of fatigue is warranted. Another systematic review did not find an association between ACL injury and play later in a season or during a game, when fatigue would likely be more prevalent, but noted that many studies did not provide information about timing [57].

Mechanical perturbation during play is a risk factor. As an example, while 88 percent of ACL injuries occur without direct knee contact, injuries sustained during indirect contact (eg, getting bumped when landing while rebounding a basketball or kicking a soccer ball) occur as often as truly noncontact injuries [58]. Several of the ACL injury prevention programs discussed below address the phenomenon of being knocked off balance during a sporting activity.

●Environmental conditions, field surface, and footwear – The roles of ambient environmental conditions, field surface, and footwear in ACL injury are the subject of a number of studies, several but not all of which have concluded that risk increases as traction improves, whether due to shoe type or playing surface (eg, synthetic versus wood gym floor, wet versus dry outdoor playing surface) [12,39,53,59-62]. As an example, a systematic review of three prospective studies involving 4972 male American football players found a strong relationship between higher rotational traction and increased risk of lower extremity injury, including ACL tears [60]. A survey of noncontact injuries among professional American football players found that fewer than 5 percent occurred on a wet playing surface, when traction was diminished [63].

One team of researchers has examined the effect of cold and wet conditions on knee injuries sustained during Australian rules and American football [59,64]. They found that high water evaporation in the month before an Australian football match and low rainfall in the year before (ie, dry playing conditions) were associated with a statistically significantly higher incidence of ACL injuries. With American football, they found that cold weather was associated with lower rates of ankle and knee injuries, including ACL tears, whether the competition took place on grass or turf.

The effect of playing surface per se, regardless of other environmental conditions, is unclear. A systematic review of studies investigating the effect of playing surface on knee injury rates, while noting large variability among studies (eg, sports included, level of play, definition of injury, competition versus practice, methods of inquiry), reported that the evidence suggests there is no significant difference in injury risk between grass and synthetic turf [65].

A study of knee injuries using data from the United States' National Collegiate Athletic Association (NCAA) Injury Surveillance System for American football reported that ACL injury rates at all levels of competition (ie, Divisions I, II, and III) were higher during competition than practice [66]. The study found no difference in injury rates at the most elite level (Division I) when matches were played on artificial turf compared with grass, but it noted higher rates of ACL tears among players in lower divisions (II and III) playing on artificial turf (rate ratio 1.63, 95% CI 1.10-2.45).

The potential risk for an ACL injury posed by playing on natural grass versus turf may differ depending upon the sport. As an example, a systematic review (including 10 high-quality studies) looking at ACL injury in soccer and American football cohorts found an increased rate of ACL injury on synthetic surfaces (both earlier generation "AstroTurf" and contemporary field turf) for American football players but no similar increased risk among soccer players [53].

Regarding footwear, research is limited and consists primarily of small pilot studies or biomechanical studies of the shoe-surface interaction, rather than the incidence of injury [67,68]. Overall, the results of available studies are mixed, and there is no consensus concerning the role of shoe-surface interaction in ACL injury [54]. Pending the results of large, prospective studies, the authors believe that cleated shoes can be worn on both natural and field turf surfaces [67,68].

DETERMINING INDIVIDUAL RISK — 

While it is possible to assess the cumulative risk of ACL injury in select populations (eg, female athletes, adolescents, participants in particular sports), all individuals within such groups do not carry the same risk. There may be instances when it is prudent to evaluate specific individuals for increased risk of noncontact ACL injuries compared with their age-, sex-, or sports-matched peers, but this is an area of controversy [69].

Advanced three-dimensional video motion analysis has been used to identify the biomechanical risk factors outlined in previous sections [37,70,71]. However, such analysis is costly and typically unavailable to the practicing community clinician.

In the absence of such video analysis, several simple examination maneuvers can be used to assess an athlete's control of knee motion. These include the single-leg squat and vertical drop jump tests. While useful assessment tools, when performed by experienced clinicians, these examination maneuvers cannot predict ACL injury risk directly. Studies of such screening tests show considerable overlap between the results obtained from athletes who go on to sustain an ACL injury and those who do not, and so no clear cutoff value can be established [69,72].

Though the issue of screening is controversial, some researchers and clinicians agree on the utility of assessments (eg, tuck jump assessment, landing error scoring system [LESS] [73,74]) that can be integrated into a training program to help target deficits. High-risk athletes (eg, female soccer players) may benefit from more detailed assessment to guide training for the reduction of ACL injury risk factors. In addition, athletes who demonstrate the highest risk biomechanics are more responsive to injury prevention training programs and demonstrate the greatest improvements in biomechanics [75].

The focus of this topic is the use of ACL injury prevention programs without prescreening to determine those athletes at greatest risk. Rather, we support the use of such prevention programs for all participants in high-risk sports. These include basketball, football (soccer), American football, lacrosse, rugby, and netball [6,76]. (See 'Participation and maintenance' below.)

GENERAL ACL INJURY PREVENTION PROGRAMS

Available programs — Several quality prevention programs have been developed to reduce the risk of noncontact ACL injury. Examples of programs aimed at decreasing injuries in the lower extremity in general and the ACL in particular include those developed by the Fédération Internationale de Football Association (FIFA, the world governing body of soccer) and the International Olympic Committee (IOC):

●The FIFA 11+ (form 1A-C), a program developed by the FIFA Medical Assessment and Research Centre [77,78] (videos from FIFA demonstrating the program are provided here; videos from New Zealand Football can be found here)

●The IOC "Get Set" app

High-quality evidence supports the effectiveness of neuromuscular training (NMT) programs for preventing ACL injuries. (See 'Evidence for ACL injury prevention programs' below.)

The interested reader may want to investigate specific, evidence-based NMT programs available in the sports medicine literature that are aimed at lower extremity and ACL injury prevention. Several of these programs offer freely available resources on the internet. These include:

●The Prevent injury and Enhance Performance (PEP) program (PEP practical guide and PEP program exercises)

●The HarmoKnee program (program details can be found in the following references [79,80])

●The RIIP Reps program and app (program access can be found in the following reference) [81]

Core components including neuromuscular training — A common theme of many ACL injury prevention programs is a focus on NMT. NMT refers to exercises that are distinct from stretching, resistance training, or balance training [82]. NMT focuses on movements that enhance unconscious motor responses by stimulating both the afferent signals (sensory) and central neuromuscular control. NMT that includes exercise progressions involving unanticipated reactions and perturbations of movement aims to improve central neuromuscular control and dynamic joint stability, and to optimize movement patterns [83-87]. NMT is central to all evidence-based ACL injury prevention programs.

After reviewing over 166 meta-analyses, systematic reviews, and cohort or randomized controlled studies, the Osteoarthritis Action Alliance established six core components that should be included in any ACL injury prevention training program [88]. Additional systematic reviews support these findings. Core components include:

●Lower extremity and core strengthening exercises [89]

●Plyometrics [90]

●Balance training [91]

●Optional stretching or agility exercises

●Targeted feedback to athletes regarding proper technique (eg, landing, pivoting)

●Sufficient volume of exercises and frequency of training sessions (at least twice per week) [92]

●Minimal to no additional equipment requirements

A 2019 systematic review and meta-analysis found that the effectiveness of ACL injury prevention programs was inversely related to the volume of balance training exercises [89], while overall effectiveness of an intervention improved with increased time spent on landing stabilization and lower body strength exercises during each session.

A 2020 systematic review and meta-analysis limited to seven cluster randomized trials and one randomized trial involving over 10,000 athletes identified significant heterogeneity in the specific exercises included in prevention programs, but found that the majority incorporated the following components [93]:

●Lower extremity and core strengthening exercises

●Plyometrics

●Agility exercises with feedback on proper landing technique

Systematic reviews have sought to determine whether a particular program may offer better protection against ACL injury when compared with other programs; these studies have not demonstrated the clear superiority of any one program [1]. The effectiveness of a well-designed program may not vary by sport, but athletes with more risk factors and more problematic movement patterns are most likely to benefit from participation in such a program [75].

ACL injury risk is reduced if NMT is performed at least two times per week [94]. In addition, studies that included programs with a greater number of exercises that progressed in difficulty and intensity within the NMT reported the most benefit.

Neurocognitive training — Emerging research supports the benefit of augmenting traditional NMT programs with neurocognitive training to further reduce injury risk. Neurocognitive training incorporates exercises that challenge the brain and nervous system, and includes such techniques as visual motor training and dual task scenarios (ie, simultaneously performing motor and cognitive tasks) [95]. These exercises are intended to improve reaction times to visual cues and increase the range of motor responses, thereby improving resilience, adaptability, and coordination. While neurocognitive training is promising, we caution the reader interested in implementing injury prevention strategies to focus first on traditional NMT. NMT has a strong evidence base, an established record of effectiveness, and most programs require no extra equipment or resources.

EVIDENCE FOR ACL INJURY PREVENTION PROGRAMS — 

There is considerable evidence supporting the efficacy of a variety of programs for the prevention of ACL injury, so much so that the Fédération Internationale de Football Association (FIFA) and the International Olympic Committee (IOC), two of the larger and more influential sport organizations in the world, recommend regular implementation of these programs in sport. The evidence for these interventions is especially strong for adolescent female athletes [94].

Individual trials have focused on particular elements of prevention. In one such trial, high-level intercollegiate women's soccer (football) teams in the United States were randomly assigned to participate three times per week before practice in a neuromuscular training (NMT) program designed to reduce the rate of noncontact ACL injuries or to engage in their standard team warm-up [96]. Athletes participating in the prevention program (n = 583) sustained only two noncontact ACL injuries over the course of one season, while those following their team's standard warm-up (n = 852) sustained 10.

The program used in this study (known as PEP for "Prevent injury and Enhance Performance") requires approximately 10 to 15 minutes to perform and consists of a warm-up followed by several strength, agility, plyometric, and flexibility exercises [97]. The goal is to increase lower extremity and core muscle strength and to improve neuromuscular function such that athletes avoid positions that increase their susceptibility to ACL injury. A similar trial performed in adolescent female soccer players reported comparable findings [98]. Other smaller studies involving athletes in a number of high-risk sports (eg, basketball, soccer) have demonstrated decreased rates of ACL tear among athletes who participate in similar well-designed prevention programs [96,97,99-104].

Multiple, high-quality meta-analyses provide still stronger support for injury prevention interventions targeting intrinsic risk factors. (See 'Risk factors for noncontact ACL injury' above.)

●A 2018 "meta-analysis of meta-analyses" assessed all meta-analyses evaluating the effectiveness of ACL injury prevention training programs [105]. This study reported a 50 percent reduction in the overall risk for ACL injuries, including contact and noncontact mechanisms in male and female athletes, for those participating in such programs (odds ratio [OR] 0.5 [95% CI 0.41-0.59]). In addition, the study found a remarkable 67 percent reduction in noncontact injuries among female athletes (OR 0.33 [95% CI 0.27–0.41]).

●A 2020 systematic review and meta-analysis limited to high-quality trials (one randomized trial and seven cluster randomized trials) corroborates the significance and effectiveness of prevention programs in all athletes, reporting an ACL injury reduction rate of 53 percent (incidence ratio 0.47; 95% CI 0.30-0.73) [93].

Additional important details can be found by analyzing individual meta-analyses. A meta-analysis limited to prospective studies found that the overall risk of ACL injury was reduced in female athletes who participated in NMT programs [106]. A total of 29 ACL injuries occurred among program participants compared with 100 injuries among nonparticipant athletes (OR 0.40; 95% CI 0.26-0.61). The authors noted the following:

●All four programs that incorporated high-intensity jumping plyometric exercises reduced injury rates.

●All three programs that included biomechanical analysis and provided direct feedback to the athletes about proper position and movement reduced injury rates.

●Programs that incorporated strength training reduced injury rates, although strength training alone did not.

●Balance training alone is unlikely to reduce injury rates, although it may enhance other prevention techniques.

●Athletes must participate in prevention training at least two times per week for a minimum of six consecutive weeks to accrue any benefit.

In addition, programs that include strengthening exercises for the core musculature (muscles of the hips, pelvis, and lower torso) produce statistically significant reductions in ACL injury rates, according to a subsequent meta-analysis of 14 prospective controlled trials [107]. This meta-analysis also found benefit from lower extremity strengthening exercises (particularly Nordic hamstring lowers) and prevention programs that incorporated multiple types of exercises (eg, core strength, extremity strength, plyometrics).

Another meta-analysis of eight prospective studies did not comment on the best methods for ACL injury prevention but reported a significant reduction in injury rates for athletes who participate regularly in well-designed neuromuscular prevention programs (pooled risk ratio [RR] 0.38, 95% CI 0.20-0.72) [108]. The reduction in injury risk was statistically significant for female athletes (RR 0.48, 95% CI 0.26-0.89). Of note, the results of negative studies included in the review suggest that neither plyometric training (20 minutes, twice weekly) nor balance training (15 minutes, three times weekly), when performed alone, reduce the risk of ACL injury. The limited effectiveness of balance training alone is supported by other studies [107,109].

Data supporting ACL injury prevention are strongest for young female athletes [89,110]. A 2010 meta-analysis found that NMT prevention programs are especially effective in female athletes younger than 18 years [110]. It noted that programs were most effective when athletes performed them during the preseason and in-season. According to this review, females in their mid-teens had greater reductions in their rate of ACL injury (OR 0.28; 95% CI 0.18-0.42) than females in their late teens (OR 0.48; 95% CI 0.21-1.07) or early adulthood (OR 1.01; 95% CI 0.62-1.64) [94]. These results suggest that it is important to implement prevention programs before female athletes develop poor mechanics during movements associated with an increased risk of ACL injury (eg, suddenly changing direction while running, landing from a jump). The authors of that study advocate early use of NMT, and we concur.

Hip strength and stability, in particular external rotator strength, appear to be important facets of ACL injury prevention in young female athletes. In a 12-year, prospective cohort study involving several hundred female collegiate basketball players, a "hip-focused" injury prevention program reduced the relative risk of ACL injury by 38 percent, from 16 tears during the initial four-year observation period to nine tears during the subsequent eight-year implementation period (95% CI 0.17-0.87) [111]. Some of the exercises included in the study program are similar to some found in the FIFA 11+ program (form 1A and form 1B and form 1C).

Although we advocate ACL injury prevention for male athletes, studies of prevention programs in this population are less conclusive. A 2014 systematic review of studies of ACL injury prevention programs for male athletes identified only two studies whose outcome was a reduction in ACL tears (five studies assessed risk factor reduction) [112]. These two studies of intervention programs for soccer athletes reached opposite conclusions, but each had major limitations [99,113]. In one study, the intervention was limited to proprioception training only, and the other involved only 41 participants. A 2015 systematic review reported a statistically significant benefit for intervention (summary incidence rate ratio 0.49 [95% CI 0.29, 0.85]) but concluded that the majority of the 16 studies included in the analysis focused exclusively on prevention in female athletes, and therefore the "…results should be generalized cautiously to male athletes" [114].

EXTRINSIC SUPPORTS — 

Knee bracing does not prevent ACL tears, and one observational study suggests that such bracing may increase morbidity [115-118]. A small laboratory study suggests medially posted orthotics may reduce the risk of valgus knee angulation, and thereby ACL injury, but further research is needed before this intervention can be recommended [119].

COST SAVINGS — 

Injury prevention programs may involve some cost, but most ACL injury prevention programs can be implemented with little investment aside from time and supervision. Conversely, numerous studies describe the cost savings of implementing such programs in both the direct savings from avoiding ACL reconstruction surgeries and indirect future health care costs [120-125].

It is estimated that the lifetime cost to society of an ACL reconstruction is approximately USD $38,121 [123]. Implementation of an ACL injury prevention program in high-risk 12- to 25-year-olds was estimated to prevent 842 cases of knee osteoarthritis per 100,000 interventions and up to 584 total knee replacements. Overall, this averts nearly USD $700 of direct health care costs per person per lifetime [122]. In 2010, Sweden introduced a neuromuscular training program (the Swedish Knee Control Program) nationally using downloadable apps, among other supports [126]. While the program has yet to be fully implemented, it is estimated that the annual costs will be less than the cost of treating one ACL injury.

New Zealand determined that the cost benefits of a limited rollout of the Fédération Internationale de Football Association (FIFA) 11+ amounted to over eight dollars saved for every dollar invested. Consequently, it has begun a nationwide initiative mandating that all teams participating in high-risk sports (eg, rugby, soccer, netball) adopt ACL injury prevention [127,128]. The Fit4Football website is an excellent resource the country has developed as part of their prevention programming. This website provides detailed instructions for interested parties (coaches, athletes, and parents or other caregivers) who want to adopt the FIFA 11+ and FIFA 11+ Kids interventions to prevent ACL injury. Child athletes are an important population to target. (See 'At what age should athletes begin?' below.)

OVERCOMING BARRIERS TO IMPLEMENTATION AND COMPLIANCE WITH PREVENTION PROGRAMS — 

The adoption of injury prevention practices is often not straightforward. Obstacles to the adoption of such practices are not unique to ACL injury prevention and hold true for efforts ranging from the adoption of bicycle helmets to the prevention of concussions in contact sports. In addition to the recommendations provided below, some of which are summarized in the following table (table 1), detailed guidance for stakeholders about overcoming barriers to implementing an injury prevention program can be found in the National ACL Injury Coalition's Field Guide.

Common barriers to the implementation of an injury prevention program include the following [129]:

●Cost.

●Equipment.

●Time required.

●"Buy-in" from involved parties (especially coaches, but also players, parents, athletic directors, governing bodies).

●Coach(es) and trainer(s) confidence and self-efficacy [130].

●Supervision (ie, ensuring the program is performed correctly).

●Cost and equipment – Cost and equipment do not present barriers to many ACL injury prevention programs, including the Fédération Internationale de Football Association (FIFA) 11+ and the authors' approach described below, as no additional cost or equipment is required.

●Time required – Time may be a challenge, but if advocates emphasize using the program as a warm-up, this potential obstacle should be surmountable.

A meta-analysis of female athletes investigated the impact of session training time on injury risk reduction [92]. It divided interventions into tertiles by time per session (low = <15 minutes; moderate = 15 to 30 minutes; high = >30 minutes). It found a significant benefit for all time lengths, with the low, moderate, and high tertiles achieving 44, 54, and 68 percent risk reduction, respectively (ie, the "high" time group avoided 68 percent of expected ACL injuries).

●Coach buy-in – Typically, buy-in from involved parties is the chief barrier to overcome. Buy-in is typically less of a problem in a setting where a clinician is responsible in some measure for the health of a team, for instance, when serving as a team physician. However, considerable evidence suggests that the coach is the most important person to convince of the importance of adopting an ACL injury prevention program [131]. Understanding coaches' perceptions of injury prevention programs and providing education directed first towards coaches is an important first step in implementing a neuromuscular training (NMT)-based ACL injury prevention program with any team [132].

Prevention programs are often most appealing to a coach when their perceptions are acknowledged, and the educational material is delivered in a "propose" rather than "impose" manner [133]. It is ideal if the coach can adopt the injury prevention program, or possibly the movement and mechanics portion of the program, as the team warm-up. Furthermore, it is important that the coach maintain fidelity to the prescribed program. According to one published survey of high school basketball and soccer coaches, only 9 percent reported having their athletes perform the prescribed injury prevention program precisely as written [134].

●Supervision – Proper supervision and feedback to ensure that exercises are performed properly is a common thread among effective NMT protocols [82,94,135]. Qualified instruction is integral to ensuring that young athletes are performing exercises properly and not reinforcing poor movement patterns that increase the risk of injury. The professionals who supervise young athletes should be skilled in recognizing proper technique and be able to provide constructive feedback during the learning process, especially for exercises in which improper technique increases injury risk (eg, landing from a jump) [94].

The prescribed exercises, sets, and repetitions for an effective program should be attainable for each athlete but may need some modification. The initial volume (ie, number of sets and repetitions) should be low to allow for the development of proper exercise technique. Volume (or intensity/resistance, when applicable) should be increased gradually after the athlete can properly perform the exercise at the initially prescribed volume and intensity.

PARTICIPATION AND MAINTENANCE

Who should participate? — We recommend that individuals or teams participate regularly in an ACL injury prevention program if any of the following conditions exist:

●Athlete has sustained a prior ACL injury.

●Athlete or team participates in a high-risk sport (eg, football [soccer], basketball).

●Athlete or team is female and involved in any sport or activity involving frequent landing, cutting, or direction changes and decelerations. As an example, this recommendation would apply to swimmers or snowboarders whose land-based training involves plyometric exercises.

The program should be performed during both the preseason and in-season and should be continued for the duration of an individual's participation in any high-risk sport (eg, basketball, soccer, lacrosse, football, rugby, netball). Many injuries occur early during a season, and thus, beginning a prevention program during the preseason is important.

At what age should athletes begin? — For the reasons provided here, early adolescence is likely the optimal time to integrate neuromuscular training (NMT)-based injury prevention programs into sport. The rate of sports-related ACL injuries increases during adolescence, with a peak incidence during the mid to late teens [136-139]. Furthermore, biomechanical and epidemiologic data indicate that the window of opportunity for optimizing reductions in ACL injury risk is widest during adolescence, before neuromuscular deficits and abnormal mechanics that increase risk develop [1,94,140-144]. Additional evidence suggests that even younger athletes may benefit, as preadolescent children may be at the optimal age to master fundamental motor skills [145,146]. (See 'Risk factors for noncontact ACL injury' above.)

While there is no minimum age for participation in an ACL injury prevention program, all participants must be able to follow coaching instructions and pay proper attention to the feedback and instructions given during exercise implementation [84,147]. ACL injury prevention programs can potentially start as early as middle school (fifth grade [ie, 10 to 11 years]).

High-quality evidence that informs the practice guidelines issued by the American Academy of Sports Physical Therapy and similar bodies notes that the greatest impact on reducing medical morbidity and costs (from ACL injuries and subsequent osteoarthritis) is achieved by encouraging ongoing performance of ACL injury prevention programs in athletes ages 12 to 25 who engage in high-risk sport [148].

The importance of rigorous compliance — Injury prevention programs require regular participation and close compliance to achieve their greatest benefit. This has been demonstrated in multiple studies [91,92,131,149]:

●According to a cluster-randomized trial involving 31 female soccer teams (ages 13 to 18) using the FIFA 11+ prevention program, players on teams categorized as "high adherence" demonstrated a 57 percent lower injury risk (incidence rate ratio 0.43, 95% CI 0.19-1.) compared with players on "low adherence" teams [131].

●In a prospective cohort study involving over 4000 female soccer players (ages 12 to 17), players in the high-compliance tertile had an 88 percent reduction in ACL injury rates (rate ratio [RR] 0.12, 95% CI 0.01-0.85), whereas players in the low-compliance tertile had a 23 percent reduction (RR 0.77, 95% CI 0.27-2.21), an injury rate similar to controls who did not participate in any formal prevention program [149].

●A meta-analysis of studies of female athletes participating in a prevention program found that participants with low (<33.3 percent) and moderate (33.3 to 66.6 percent) compliance rates had a 4.9 and 3.1 times greater relative risk of ACL injury compared with those in the high-compliance (>66.6 percent) tertile [92]. Compliance rate was defined as the number of training sessions completed divided by the total number of sessions offered.

OUR PROGRAM RECOMMENDATIONS — 

Many well-designed programs for ACL injury prevention are freely available on the internet. One well-studied example of such a program is the FIFA 11+ (form 1A-C) [78] (the program can be accessed here, with videos provided here). (See 'General ACL injury prevention programs' above.)

The program guidance we offer below uses the principles elucidated in this topic and was developed after careful analysis of the available literature. It is designed to emphasize the key components needed to optimize ACL injury prevention [129,150]. Coaches, athletes, parents or other caregivers, and clinicians can use the following checklist (table 2) to assess the quality of their current ACL injury prevention programming.

We recommend programs that include the exercise types described below.

●Landing stabilization – Proper landing technique is important for ACL protection. Exercises to develop this ability include those below.

Examples of two-leg exercises: Vertical drop jump and hold, long jump and hold (picture 1), jump over obstacle and hold.

Examples of single-leg exercises: Long jump and hold (picture 2), vertical drop jump and hold, jump over obstacle and hold.

●Lower body strength – Strong torso stabilizers, hip stabilizers (extensors), and knee flexors are needed to support the ACL when landing, cutting, and performing other high-intensity maneuvers during sport.

Useful exercises include: Nordic hamstring lowers (picture 3 and picture 4 and picture 5), lunges (movie 1 and movie 2), single-leg squat (picture 6), calf (heel) raises (picture 7), planks (picture 8)

●Plyometrics – Plyometric exercises develop strength and power in the lower extremities. Exercises should progress from lower intensity and less complex to higher intensity and more complex.

Lower intensity exercises include: 90-degree hops, multidirectional hopping on two legs, scissor jumps, depth jump exercise progressions.

Higher intensity exercises include: Tuck jumps, jumps with 180-degree rotation, 180-degree hops (picture 9), multidirectional hopping on single-leg, scissor jumps holding weights, bounding (picture 10).

A selection of each type of the exercises listed above should be performed at least twice per week as warm-ups prior to sport activity and should be performed during both preseason and in-season. Fifteen to 30 minutes should be dedicated to their performance, with longer sessions demonstrating greater benefit.

As stated above, prevention programs are important for male and female athletes in high-risk sports, but with an emphasis on female athletes because of their higher risk. It may be used as early as fifth grade (10 to 11 years old), or as soon as the athlete is ready to take instructive feedback. (See 'At what age should athletes begin?' above.)

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anterior cruciate ligament injury".)

SUMMARY AND RECOMMENDATIONS

●Anatomy and biomechanics – The anterior cruciate ligament (ACL) spans the knee joint from the medial aspect of the lateral femoral condyle to the anterior tibial spine. The ligament functions as a limit to anterior translation and rotation of the tibia. Noncontact ACL injuries often involve complex but repeatable motions, including dynamic knee valgus, hip internal rotation and adduction, tibial rotation and anterior translation, and ankle eversion. (See "Anterior cruciate ligament injury", section on 'Anatomy and function' and "Physical examination of the knee", section on 'Anatomy' and "Physical examination of the knee", section on 'Biomechanics'.)

●Risk factors – Several anatomic and hormonal factors may contribute to ACL injury, but these cannot be modified. Most notably, females are at significantly higher risk than males participating in the same sport. A number of extrinsic factors, ranging from fatigue to playing surface to footwear, may predispose athletes to ACL injury. Risk appears to increase as traction improves, whether due to shoe type or playing surface. Neuromuscular and biomechanical factors that contribute to ACL injury risk, which are modifiable with proper training, include (see 'Risk factors for noncontact ACL injury' above):

•Hamstring weakness and under-recruitment (ie, quadriceps dominance)

•Valgus angulation of knee during landing or change of direction

•Hip external rotator weakness and under-recruitment

•Core (torso) muscle weakness

●Prevention programs – A number of prevention programs have been developed that reduce the risk of noncontact ACL injury (table 2). Examples and links to such programs are provided in the text. Perhaps the best studied of the programs demonstrated to reduce injury rates is the Fédération Internationale de Football Association (FIFA) 11+ (form 1A-C). New Zealand Football has produced a video series demonstrating the FIFA 11+ program that can be accessed here, while videos produced by FIFA can be found here. (See 'General ACL injury prevention programs' above and 'Evidence for ACL injury prevention programs' above.)

Methods for overcoming common barriers to implementing an injury prevention program are described in the text; some are summarized in the following table (table 1). More detailed guidance about overcoming such barriers can be found in the National ACL Injury Coalition's Field Guide.

●Criteria for effective prevention programs – A common theme of many ACL injury prevention programs is a focus on neuromuscular training. A successful program will include the following exercises and elements (see 'General ACL injury prevention programs' above and 'Our program recommendations' above):

•Lower extremity and core strengthening exercises

•Low- and high-intensity plyometric exercises

•Landing stabilization exercises

•Continual feedback to athletes regarding proper movement technique (eg, landing, pivoting)

•Sufficient performance time (at least twice per week) and rigorous compliance

•Minimal to no additional equipment requirements

●Starting age and indications for participation – ACL injury prevention programs can potentially start as early as middle school (fifth grade [ie, 10 to 11 years]). In any case, prevention should begin while the athlete is young, specifically under 18 years. We recommend individuals or teams participate regularly in an ACL injury prevention program if any of the following conditions exist (see 'Participation and maintenance' above):

•Athlete has sustained a prior ACL injury.

•Athlete or team participates in a high-risk sport (eg, football [soccer], basketball).

•Athlete or team is female and involved in any sport or activity involving frequent landing, cutting, or direction changes and decelerations.