The ACL, or anterior cruciate ligament, extends from the lateral femoral condyle to the anterior medial aspect of the tibia. The bands of the ACL act as a biomechanical restraint for rotation and limits anterior translation of the tibia on the femur. It assists with knee stability during running, jumping, cutting or pivoting.
There are both structural features and dynamic biomechanics that have been shown to increase risk of injury to the ACL. The structural features are only modifiable via surgery. These include intercondylar notch size, narrow intercondylar notch, tibial slope and the integrity of the menisci. The dynamic and modifiable biomechanics that stress the ACL are hyperextension, excessive valgus and excessive abduction. Poor muscular control results in improper knee alignment and increased translation or rotary shear forces (1).
It is estimated that 250,000-400,000 ACL injuries occur in the United States each year and roughly half of them undergo ACL reconstruction (2,3). Despite being studied extensively and having many methods of repair, up to 80% of individuals undergoing reconstruction develop knee osteoarthritis (4). Individuals undergoing reconstructions have a 20-30 percent risk of injuring the ACL of the same knee or the opposite knee (5). Athletes suffering a knee injury prior to a division 1 career have been shown to have an eightfold risk of suffering an additional knee injury and spend 50% more time on the disabled list (6). Additionally, it can be very difficult for athletes psychologically to recover from ACL injuries.
Because of the nature of ACL injuries, time lost and financial burden, there have been attempts to make programs for ACL prevention. This started more than two decades ago with the training programs attempting to influence neuromuscular system with programs that include plyometrics, strengthening, technique and balance exercises (7). Many took interest in tailoring these programs and many studies were done with mainly female basketball, volleyball, soccer and handball athletes. This was likely due to the increased risk of ACL injury for females at about a 3 to 1 ratio when compared to males (8).
There is a growing body of evidence with ACL prevention programs and most of the studies have shown positive results in regards to overall prevention. There is a lack of high quality studies despite more than 36 intervention programs described since 1995. A recent presentation at the American Academy of Orthopedic Surgeons showed that only programs were shown to have a significant impact on improving neuromuscular patterns and significantly reducing non-contact ACL injuries (9). Factors involved in the failure of other programs included lack of compliance, unsupervised sessions, lack of increasing challenge in training exercises, failure to include plyometrics and lack of comprehensive strength training for the lower extremity, core and trunk. It is worth noting that other programs for general lower extremity injuries and general injury prevention exist, such as the FIFA 11+ program and are widely used in the soccer athlete population. The focus will remain mainly on the original and high quality studies on specifically ACL prevention that have been performed.
Carrafa et al. (10) had one of the earliest programs in 1996 and examined a balance board program in 600 male semi-professional Italian soccer players. This program was done thirty days before the season, three days a week and took around twenty minutes to complete. There were a total of 70 ACL injuries in the control group and 10 ACL injuries in the intervention group. This study did not have an overall incidence per 1,000 occurrences and also was not a multi-faceted program. However, the results were encouraging and sparked interest for further programs and studies.
Hewett and colleagues (11) studied a neuromuscular training program in soccer, volleyball and basketball female athletes. The first study was a six week program with 366 females in the intervention with a male and female control group. The program was supervised and included plyometrics, weight training, flexibility training and emphasis on proper landing patterns. This as a more comprehensive program that lasted 60-90 minutes and was done 3 times a week. Overall the female control group sustained a 3.6 times higher rate of total knee injuries compared with the female intervention group and 4.8 times higher rate of knee injuries compared with the male control group, with both being statistically significant. This program was modified and updated and is now a program called Sportsmetrics. There are now 1,280 sites with Sportsmetrics and 2,200 instructors certified throughout North America for this program (12).
Another program was shown to have a significant improvement occurred in 2005 by Mandelbaum and colleagues (13). This program was named the PEP (Prevent Injury and Enhance Performance Program) and had around 2,000 control female soccer club players and 1,000 in the intervention group. This was a 20 minute regimen performed 2-3 times a week during 12 weeks of the season. An educational video on safe and unsafe landing patterns was shown and athletes participated in team workouts for strengthening, plyometrics, soccer-specific drills that replaced the team’s warm-up. There were 2 ACL injuries in the intervention group and 32 in the control group with an 88% overall reduction in ACL injury rate during the first year. The second year yielded similar results with 4 ACL injuries in the intervention group compared with 35 in the control group yielding a 74% reduction in ACL injuries. The same group of researchers tried to implement the program with division I women’s soccer athletes with more than 583 in the intervention group and 852 in the control group. The overall ACL rate was 1.7 times less in the control group than the intervention group and the noncontact ACL rate was 3.3 times less. These results, however, were not significant (14). This program is available through the Santa Monica Sports Medicine Foundation and videos can also be found online.
La Bella et al. (2011) (15) studied a neuromuscular warm-up program implementation and its effects on high school female athletes in Chicago public schools. There was a 2-hour training session prior to the season and a 20 minute warm-up was implemented that included plyometrics, balance, progressive strengthening, and agility exercises. Coaches also received a DVD and laminated card summarizing the exercises. At the season’s end, there was a 56% reduction in total non-contact lower extremity injuries in the intervention group compared to the control. The intervention group also had lower rates of ankle sprains, knee sprains, and gradual onset of lower extremity injuries. The study looked at “ACL sprains” as one of their injury types with 6 injuries in the control group and 2 in the intervention group. This change was found to not to be statistically significant, however. This program has been modified and is now known as KIPP or Knee Injury prevention program and training is offered at Lurie Children’s institute for Sports Medicine.
There have been roughly twenty prospective randomized studies performed with a heavy focus on female athletes and many of them involve soccer, basketball, volleyball and handball. Many studies are plagued by small incidence numbers or lack of power. There have been many recent meta-analysis studies performed over the past decade, mainly on these trials. There was also a recent meta-analysis of all of the meta-analyses that concluded there was an overall 50% reduction in the risk of all ACL injuries in all athletes and a 67% reduction in non-contact injuries in females (16). The data shown for male prevention of ACL injuries is positive, but there was insufficient evidence to come to a conclusion.
No evidence suggests that a single optimal preventive training program exists. Instead, general guidelines should be considered when developing or implementing an injury prevention training program. The National Athletic Training Association provided an excellent review in regards to current evidence in regards to specifics such as implementation and timing of programs (17). It is generally accepted that programs should begin at or close to puberty and have multiple components including plyometrics, neuromuscular training, and strengthening. One recent meta-analysis showed an increased reduction of injury rates in middle school and high school athletes when compared to collegiate or professional athletes. It also recommended incorporation of lower body strength exercises (Nordic hamstring curls, lunges, heel-calf raises) with a specific focus on landing stabilization (18). Most studies or programs have been 20-30 minutes, multiple times per week and in the preseason and throughout the season. Compliance has been shown to be important (19). Programs should be supervised by an informed individual (athletic trainer, coach, physiotherapist, etc.) that is able to identify problematic patterns (Petushek). Feedback has also been shown to be important in a program, whether it is internal or external focus or cues, with external cues shown to be more effective (20).
Summary. In summary, anterior cruciate ligament injuries occur in both the general and athletic population and can lead to immediate and long-term sequelae. There is a psychological and financial burden associated with these injuries in the short term, as well as an increased long term risk of osteoarthritis and re-injury. Many ACL programs have been proposed and studied over the past 25 years and most have shown an overall decrease in ACL incidence, which is why an ACL prevention program is generally recommended for at risk athletes. Only two programs have been shown to have a significant improvement of neuromuscular patterns and a significant reduction of non-contact ACL injuries, both of which are available to the public. While there is not a current optimal program and more research is needed, many guidelines exist on specifics when implementing an ACL prevention program.
1.) Nessler T, Denney L, Sampley J. ACL Injury Prevention: What Does Research Tell Us?. Curr Rev Musculoskelet Med. ;10(3):281–288. doi:10.1007/s12178-017-9416-5
2.) Sanders TL, Maradit Kremers H, Bryan AJ, Larson DR, Dahm DL, Levy BA, et al. Incidence of anterior cruciate ligament tears and reconstruction: a 21-year population-based study. Am J Sports Med 2016;44:1502–7.
3.) Singh, Neeraj. “International Epidemiology of Anterior Cruciate Ligament Injuries Mini Review.” (2018).
4.) Holm I, Oiestad B, Risberg M, et al. No difference in prevalence of osteoarthritis or function after open versus endoscopic technique for anterior cruciate ligament reconstruction: 12-year follow-up report of randomized controlled trial. Am J Sports Med. 2012;40:2492–2498. doi: 10.1177/0363546512458766..
5.) Flynn RK, Pedersen CL, Birmingham TB, et al. The familial predisposition toward tearing the anterior cruciate ligament: a case control study. Am J Sports Med. 2005;33:23–28.
6.) Rugg C, Wang D, Sulzicki P, et al. Effects of prior knee surgery on subsequent injury, imaging and surgery in NCAA collegiate athletes. Am J Sport Med. 2014;42:959–964
7.) Grindstaff TL, Hammill RR, Tuzson AE, et al. 2006. Neuromuscular control training programs and noncontact anterior cruciate ligament injury rates in female athletes: a numbers-needed-to-treat analysis. J Athl Train 41: 450–456.
8.) Prodromos CC, Han Y, Rogowski J, Joyce B, Shi K. A meta analysis of the incidence of anterior cruciate ligament tears as a function of gender, sport, and a knee injury-reduction regimen. Arthroscopy 2007;23:1320–5.
9.) Noyes, Frank MD, Sue Barber-Westin, BS, Stephanie Smith, MS. Which ACL Prevention Programs Are Effective in Decreasing Injury Rates and Improving Neuromuscular Control in Female Athletes?” 2018 Annual Meeting of the American Academy of Orthopedic Surgeons. Morial Convention Center, New Orleans, LA.
10.) Caraffa A, Cerulli G, Projetti M, Aisa G, Rizzo A. Prevention of anterior cruciate ligament injuries in soccer: A prospective controlled study of proprioceptive training. Knee Surg Sports Traumatol Arthrosc. 1996;4(1):19–21
11.) Hewett TE, Lindenfeld TN, Riccobene JV, Noyes FR. The effect of neuromuscular training on the incidence of knee injury in female athletes. A prospective study. Am J Sports Med. 1999;27(6): 699–706
12.) “AAOS Annual Meeting 2018: ACL Prevention Exhibit” (2018). Retreived from http://noyeskneeinstitute.com/aaos-annual-meeting-2018-acl-prevention-exhibit/
13.) Mandelbaum BR, Silvers HJ, Watanabe DS, et al. Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: 2-year follow-up. Am J Sports Med. 2005;33(7):1003–1010
14.) Gilchrist J, Mandelbaum BR, Melancon H, et al. A randomized controlled trial to prevent noncontact anterior cruciate ligament injury in female collegiate soccer players. Am J Sports Med. 2008;36:1476–83
15.) LaBella CR, Huxford MR, Grissom J, et al. Effect of neuromuscular warm-up on injuries in female soccer and basketball athletes in urban public high schools: cluster randomized controlled trial. Arch Pediatr Adolesc Med. 2011;165:1033–4
16.) Webster KE, Hewett TE. Meta-analysis of meta-analyses of anterior cruciate ligament injury reduction training programs. J Orthop Res. 2018;36(10):2696-2708.
17.) Padua DA, DiStefano LJ, Hewett TE, Garrett WE, Marshall SW, Golden GM, et al. National athletic trainers’ association position statement: prevention of anterior cruciate ligament injury. J Athl Train. 2018;53:5–19.
18.) Petushek EJ, Sugimoto D, Stoolmiller M, Smith G, Myer GD. Evidence-Based Best-Practice Guidelines for Preventing Anterior Cruciate Ligament Injuries in Young Female Athletes: A Systematic Review and Meta-analysis. The American Journal of Sports Medicine 2018:363546518782460
19.) Sugimoto D, Myer GD, Bush HM, et al. Compliance with neuromuscular training and anterior cruciate ligament injury risk reduction in female athletes: a meta-analysis. J Athl Train. 2012;47(6):714–723. doi: 10.4085/1062-6050-47.6.10
20.) Benjaminse A, Welling W, Otten B, Gokeler A. Novel methods of instruction in ACL injury prevention programs, a systematic review. Phys Ther Sport. 2015;16:176-186.