Anterior cruciate ligament (ACL) injuries do not disproportionately affect female athletes, occur more often from player-to-player contact, and far more frequently in competition than practice than previously believed, finds a surprising and important new study.  ^[1]   

"ACL injuries are traditionally regarded as a female athlete issue, yet this study found no significant gender difference in injury rates when all nine sports were considered," says Dawn Comstock, Associate Professor, Colorado School of Public Health, and Principal Investigator for the High School RIO, ^TM the internet-based data collection tool researchers at Nationwide Children's Hospital in Columbus, Ohio and the University of Colorado at Denver used in the study.

Only when sex-comparable sports (soccer, basketball, and baseball or softball) were considered were girls found to have a significantly higher ACL injury rate than boys, with girls 2 times more likely to suffer an ACL injury playing soccer than any other sport, and 4 times more likely to sustain such an injury playing either soccer or basketball than volleyball or softball. 

Football has most injuries 

Not surprisingly, football, the high school sport with the greatest number of participants by a wide margin, accounted for the largest number of ACL injuries.

What came as a surprise was that football had the highest competition-related ACL injury rate of the nine sports studied, with boys 4 times more likely to sustain an ACL injury playing football than any other boys' sport. 

Although ACL injuries accounted for just twenty percent of all knee injuries - a decrease from the 50% or more reported in prior studies researchers found "encouraging" - they emphasized that "ACL injuries still represent an important burden to high school athletes, as they often require surgical repair followed by extensive long-term rehabilitation; they can mean the end of an athlete's competitive career, and they have been linked with negative long-term outcomes, including chronic pain and osteoarthritis." 

"Rates, however, are only one way to evaluate burden," noted Comstock. "Incidence should also be considered." "Given the large number of US high school football players relative to the number of girls' basketball and soccer players, in terms of the numbers of patients treated, sports medicine clinicians are more likely to treat a male high school athlete with an ACL injury than a female high school athlete with an ACL injury."

Prevention efforts needed 

"Although effective injury-prevention programs may need to be sex-specific, given the large number of male athletes sustaining ACL injuries, efforts to create effective programs should not be solely targeted to female athletes ... [but] should be focused on those athletes at highest risk of ACL injury," the study recommends.

The study authors urge coaches and ATs in sports with high rates of ACL injury to take special care to teach sport specific skills (e.g. planting and changing direction, jumping and landing); address neuromuscular strength, and work on coordination of stabilizing muscles about the knee joint through stretching, plyometrics and strength training drills.

"Future studies should focus on the modifiable sport-specific risk factors for ACL injuries," adds Comstock. "This will help drive the development of effective, evidence-based and targeted ACL injury prevention programs."

Until such programs are developed, the authors concluded that the incidence of injury should be expected to rise along with the increasing number of young athletes participating in sports. 

First of its kind    

The study is the first of its kind to examine ACL injuries by gender and across multiple sports, and the first large epidemiological study to focus on knee injuries among high school athletes in more than a decade. 

All high schools with one or more National Athletic Trainers' Association-affiliated athletic trainers (ATs) were invited to participate in the National High School Sports-Related Injury Surveillance Study using the High School RIO ^TM program. A random sample was used to select schools from each of eight geographic regions to achieve a nationally representative sample of 100 schools. ATs reported practice and competition data for nine sports during the 2007/2008-2011/2012 academic years, five boys' (baseball, basketball, football, soccer and wrestling) and four girls' (basketball, soccer, softball and volleyball) sports.

Analyzing the data, the authors found that: 

• ACL injuries accounted for 20.5% of all knee injuries and 3.0% of all injuries sustained by high school athletes in the nine sports studied. 
• During the study, ATs reported 617 ACL injuries occurring in 9,452,180 athletic exposures or AEs, defined as 1 athlete participating in 1 practice or competition, for an overall rate of 6.5 ACL injuries per 100,000 AEs.
• Nationally, an estimated 124,626 ACL injuries in boys and 91,002 ACL injuries in girls were suffered by athletes playing the 9 sports studied.
• While girls and boys had similar rates of ACL injury overall, in sex-comparable sports (soccer, basketball, and baseball or softball): 
  • girls sustained more injuries than boys (209 versus 75, respectively);
  • girls had a higher injury rate than boys (8.9 versus 2.6 per 100,000 AEs respectively)
  • a higher proportion of ACL injuries sustained by girls than boys were of the non-contact variety, but the difference was not statistically significant;
  • a higher percentage of ACL injuries suffered by boys' soccer players were due to player-to-surface contact compared by girls (21.% and 7.5%, respectively).
• The highest rates of ACL injuries per 100,000 AEs were in:
  • girls' soccer (12.2);
  • football (11.1); and
  • girls' basketball (10.3)
• The lowest ACL injury rates were seen in boys' basketball (2.3) and baseball (0.7)
•  71.2% of boys' ACL injuries were sustained playing football, followed by soccer (17.2%), basketball (5.5%), wrestling (4.1%) and baseball (2.0%)
• Certain sports placed athletes at higher risk for ACL injuries than others:
  • Boys were 4 times as likely to sustain an ACL injury while playing football as any other sport
  • Girls were 2 times as likely to sustain an ACL injury playing soccer than any other girls' sport
  • Girls were 4 times more likely to sustain an ACL injury playing either soccer or basketball compared to volleyball or softball. 
• Girls most often injured their ACLs playing soccer (53.2%), followed by basketball (26.5%), softball (11.4%), and volleyball (8.;8%);
• Player-to-player contact was the most common injury mechanism (42.8%), followed by no contact (37.9%), player-to-surface contact (13.6%), player-playing apparatus contact (2.4%), and other (i.e. ball, base, goalpost etc.)(3.3%), but, as noted above, the injury mechanism varied by sport and gender;
• Athletes are 7 times more likely to sustain an ACL injury in competition rather than practice, significantly higher than the 2.7 to 3 times reported in previous studies.  
• Of all ACL injuries:
  • 87.6% were new
  • 2.1% were recurrences from an injury sustained in the same academic year but from which the athlete had returned to play
  • 7.9% were recurrences from an injury sustained ina previous academic year but from which an athlete had returned to play; and
  • 2.4% were other/unknown
• 76.6% of the ACL injuries resulted in surgery, but the percentage varied by sport, ranging from 96.3% in boys' basketball to 61.1% in wrestling.  
• Of those ACL injuries in which the outcome was reported:
  • 46.4% resulted in medical disqualification for the season
  • 15.4% required a 3-week or longer recovery time 
  • Of the 12.3% requiring less than a 3-week recovery time, 83.2% did not result in surgery, while 11.7% resulted in surgery which was postponed to enable the athlete to continue to play. 
• ACL injury was assessed primarily by an AT in conjunction with a general physician or orthopaedic physican (or both), using a variety of diagnostic methods (evaluation, X-ray, MRI, CT scan, surgery)
  • Those assessed by a physician were more likely to require surgery (80.5%) than those assessed by the AT alone (25.8%)
  • Patients with ACL injuries assessed by the AT alone were more likely to return to play within 3 weeks (52.3%) compared to those assessed by a physician (8.9%)

Risk factors still unknown

Efforts to design effective target prevention programs have been hampered by a number of factors, the researchers said.  While some  ACL injury prevention programs have shown "great promise" (particularly in reducing the incidence of non-contact ACL injuries in female athletes), an understanding of their effectiveness, say researchers, "is still limited."  In addition, they say, "we still lack a complete understanding of the modifiable risk factors for ACL injury."

"That said, coaches and ATs in sports with high rates of ACL injury should take special care to teach sport specific skills (e.g. planting and changing direction, jumping and landing) and address potential deficits in the neuromuscular strength and coordination of the stabilizing muscles about the knee joint through stretching, plyometrics [1] and strength training drills." 

"Plyometric exercises mimic many of the same motions an athlete will complete on the field," says Keith Cronin, PT, OCS, CSCS, a physical therapist in St. Louis and a MomsTEAM expert. "I like plyometric exercises as a means to evaluate joint positioning during the movement to determine if an athlete is at risk of hurting a joint, such as the knee."

"An athlete who can quickly change directions and respond to changing movements is not only a better athlete, but is less likely to sustain an injury. If mechanics are good, the athlete should do then sometimes (maybe 1-2 days a week as part of conditioning), but not overemphasize them, as too much heavy loading of the joints can cause damage. The purpose of sports training is to improve performance on the field. If the athlete grinds their body down training with plyometrics, performance is decreased and risk of injury increases."   

In addition to prevention, the study recommends the use of screening tools to try to identify athletes at increased risk of ACL injury, such as through use of the tuck-jump exercise [2] developed by researchers at Cincinnati Children's Hospital, which can identify lower extremity technical flaws during plyometric activity that may indicate a higher risk of ACL injury.

Player-to-player contact

One area identified by the study as an example of a potentially modifiable risk factor is player-player contact, which accounted for 42.8% of ACL injuries among the high school athletes, a rate significantly higher than the 23.9% to 30% found in previous studies.

While the reason for the increased percentage of player-player contact injuries was unclear, the study authors suggested that it could have been due to the fact that the earlier studies focused on collegiate athletes, sex-comparable sports only, or single sports; that collegiate athletes generally have more body control and are more skilled at their sports; that the pace and style of high school level of play may leave athletes at higher risk of contact of a contact injury; that, compared to prior studies, the data in the current study was skewed by the high frequency of football ACL injuries, where 6 out of 10 injuries were caused by contact almost 20% higher than the average across the 9 sports), or that athletes predisposed to ACL injury may be self-selected or medically selected out of sports before college.

Although saying that further research was needed to fully understand contact-related ACL injuries, the study suggests that player-player contact as a risk factor might be reduced via rule changes, enforcement of existing rules, increased emphasis on sportsmanship, and teaching proper sport-specific skills without affecting the underlying purpose or culture of individual sports. 

Substantial societal burden  

Anterior cruciate ligament (ACL) injuries are among the most devastating injuries a young athlete can sustain, given the frequent need for surgery and extensive long-term rehabilitation^[2,3] as well as the potential for long-term health problems, putting a patient at risk for early osteoarthritis [3] and chronic pain 10 to 20 years after the injury.^[4,5]  

More than 7 million students participate in high school sports today. Although the benefits of sports for both boys [4] and girls  [5]are well documented, injuries can exact economic, emotional, physical and social tolls. The knee joint is the second most commonly injured joint after the ankle [6] ^[6] and the leading cause of sport-related surgeries, accounting for 60% of high school sport-related surguries.^[7]  ACL injuries account for 50 percent or more of all knee injuries^[2] and an estimated $1 billion is spent annually on ACL reconstruction in this country.^[8]

While one recent study ^[9] found no clear evidence that ACL reconstruction improves the long-term symptomatic outcome, "raising the question of how many ACL injuries could be successfully treated by more conservative rehabilitation programs rather than surgery," writes Comstock, the issue is being hotly debated. A new study by researchers at KNG Health Consulting in Rockville, Maryland ^[10]  reported that, while in the short- and intermediate term, ACL reconstruction was only slightly less costly and more effective compared with rehabilitation, over the long term ACL reconstruction not only resulted in incremental cost savings of $50,417, but quality of life gains and reduced societal costs once indirect cost factors, such as work status and earnings are considered.

The bottom line, concludes the study, is that a "certain endemic level of ACL injury will always be associated with sports, yet an increased committment should be made to reduce the incidence and severity of ACL injuries sustained by high school athletes."  Until evidence-based, targeted ACL injury prevention efforts are developed, the incidence of ACL injuries should be expected to continue to rise with the increasing number of youths participating in sports." 

1.  Joseph AM, Collins CL, Henke NM, Yard EE, Fields SK, Comstock RD. A Multisport Epidemiologic Comparison of Anterior Cruciate Ligament Injuries in HIgh School Athletics.  [7] J Athl Tr. 2013;48. DOI:10.485/1062-6050-48.6.03 (epub November 11, 2013).

2. Risberg MA, Lewek M, Snyder-Mackler L. A systematic review of evidence for anterior cruciate ligament rehabilitation: how much and what type? Phys Ther Sport. 2004;5(3):125-145.  

3. Cimino F, Volk BS, Setter D. Anterior cruciate ligament injury: diagnosis, management, and prevention.  Am Fam Physician. 2010;82(8);917-922. 

4. Lohmander LS, Englund PM, Dahl LL, Roos EM. The long-term consequence of anterior cruciate ligament and meniscus injuries: osteoarthritis.  Am J Sports Med. 2007;35(10):1756-1769.

5. Fleming BC, Hulstyn MJ, Oksendahl HL, Fadale PD. Ligament injury, reconstruction and osteoarthritis. Curr Opin Orthop. 2005;16(5):354-362.

6. Swenson D, Collins C, Fields S, Comstock R. Epidemiology of US High School Sports-Related Ligamentous Ankle Injuries, 2005/06-2010-11. Clin J Sport Med 2013;23(3):190-196.  

7. Ingram JG, Fields SK, Yard EE, Comstock RD. Epidemiology of knee injuries among boys and girls in US high school athletics.  Am J Sports Med. 2008;36(6):1116-1122. 

8. Streich NA, Zimmerman D, Bode G, Schmitt H. Reconstructive versus non-reconstructive treatment of anterior cruciate ligament insufficiency: a retrospective matched-pair long-term follow-up.  Int Orthop. 2011;35(4):607-613. 

9. Centers for Disease Control and Prevention A. GA. Preventing Injuries in Sports, Recreation, and Exercise. Atlanta, GA 2006. 

10. Mather RC, et al. Societal and Economic Impact of Anterior Cruciate Ligament Tears.  J Bone Joint Surg Am. 2013;95:1751-9.  

Posted November 14, 2013 

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