Participation in high school athletics has increased for decades (23) and will likely continue. These activities benefit young athletes by improving physical fitness and increasing academic achievement and school performance (22). As participation increases, sports-related injuries, which can impose significant burdens on the athlete, their families, and the medical system, may also increase (15). Among the most common sports-related injuries are knee injuries (5).
Knee injuries pose serious health burdens to athletes of all ages in nearly every sport (4,6,18,29). They account for 15.2% of all high school sports injuries (13), often requiring expensive surgical treatment (8,24,25) and prolonged time lost from school and sports participation (15,26). One international study reports youth athlete knee injury rates, but these may not represent U.S. high school injury patterns because of country-specific differences in sport availability and participation (5). Previous studies describing U.S. high school sports-related knee injuries reported high rates of knee injuries, high proportions of knee injuries compared with other injuries, increased risk of reinjury, and high numbers of knee injuries requiring surgical treatment, but these were limited in geographic region, described injury patterns in general but not in detail, or did not report injury patterns across a large number of sports (1,13,14,20,24,26). Multiple studies have investigated knee injury patterns by sex (2,7,9–12,19), frequently reporting that knee injury rates are higher in female athletes, but few studies have directly compared knee injury rates and patterns in both sex-comparable and sex-specific sports.
To our knowledge, no prior study has described U.S. high school sports-related knee injury epidemiology over multiple academic years in a large national sample of athletes participating in a large number of sports. This study used a nationwide injury surveillance system to describe rates and patterns of knee injuries across 20 high school sports. Such information is needed to drive the development and evaluation of injury prevention programs (28).
MATERIALS AND METHODS
The objective of this study was to describe the epidemiology of U.S. high school sports-related knee injuries across multiple years. Specifically, the study aims were 1) to describe rates of knee injury by sport, 2) to describe patterns of knee injury by specific injured structures, and 3) to describe risk factors and outcomes associated with specific knee injuries. Rates and patterns of knee injuries with national estimates were calculated for nine sports (football, boys’ and girls’ soccer, girls’ volleyball, boys’ and girls’ basketball, wrestling, baseball, and softball) for the 2005/2006–2010/2011 academic years. In addition, rates and patterns of knee injuries were calculated from a convenience sample of nine sports (girls’ field hockey, girls’ gymnastics, boys’ ice hockey, boys’ and girls’ lacrosse, boys’ and girls’ swimming and diving, and boys’ and girls’ track and field) for the 2008/2009–2010/2011 seasons and for boys’ volleyball and cheerleading for the 2009/2010–2010/2011 seasons.
Data were collected using the National High School Sports-Related Injury Surveillance System, High School RIO™ (Reporting Information Online), an Internet-based sports injury surveillance system (3). In brief, high schools with one or more National Athletic Trainers’ Association–affiliated certified athletic trainers (AT) with valid email addresses were invited to participate. Interested high schools were categorized into eight strata based on school population (enrollment ≤ 1000 or > 1000) and U.S. Census geographic region (27). For the nine sports originally included in the High School RIO™ study (football, boys’ and girls’ soccer, girls’ volleyball, boys’ and girls’ basketball, wrestling, baseball, and softball), 100 high schools were randomly selected to participate (12 or 13 from each of the eight strata). If a school dropped out of the study, a replacement from the same stratum was randomly selected to maintain the 100-school study population.
For the additional 11 sports added since 2008/2009 (girls’ field hockey, girls’ gymnastics, boys’ ice hockey, boys’ and girls’ lacrosse, boys’ and girls’ track and field, boys’ and girls’ swimming and diving, boys’ volleyball, and cheerleading), not enough schools volunteered to report for all sports to fill the eight strata. Thus, rather than assembling a nationally representative sample, exposure and injury data for these sports were collected from a convenience sample of U.S. high schools with an AT. Although all participating schools were asked to report for at least 10 sports, with an objective of having 100 schools reporting for each sport, schools with some of the less nationally popular sports (i.e., ice hockey, lacrosse) were enrolled even if they could not report for all sports. If an AT from a convenience sample school also reported information for athletes in one of the original nine sports, these data were included in the overall convenience sample data but were not included in weighted national estimates.
Definition of injury and exposure.
Athlete exposure (AE) was defined as one athlete participating in one practice or competition. A reportable injury was one which 1) occurred as a result of participation in an organized practice, competition, or performance (cheerleading only); 2) required medical attention by an AT or physician; and 3) resulted in restriction of the athlete’s participation for one or more days. AT logged onto the High School RIO™ Web site weekly throughout the academic year to report injury incidence and AE information. For each injury, ATs completed detailed injury reports on the injured athlete (age, height, weight, etc.), the injury (site, diagnosis, severity, etc.), and the injury event (activity, mechanism, etc.). Throughout the study, reporters were able to view previously submitted information and update reports as needed.
Data were analyzed using the Statistical Package for the Social Sciences software, version 19.0 (SPSS Inc., Chicago, IL), calculating rates and rate comparisons using unweighted case counts for the convenience sample of all 20 sports. Weighted analyses were used to calculate national estimates from data reported by the 100 randomly selected high schools for the nine original sports. Weighting factors are based on the inverse probability of selection into the study based on school size and location.
Injury rates were calculated as the number of knee injuries per 10,000 AE. Injury rate ratios (RR) and injury proportion ratios (IPR) were calculated with P values and 95% confidence intervals (CI). CI not including 1.00 and P values < 0.05 were considered statistically significant. An RR or IPR > 1.00 suggests a risk association, whereas an RR or IPR <1.00 suggests a protective association. An example RR calculation is as follows:
An example IPR calculation is as follows:
This study was approved by the institutional review board at Nationwide Children’s Hospital, Columbus, Ohio. No informed consent was required because only de-identified data were collected in this purely observational epidemiologic study.
From 2005/2006 to 2010/2011, 25,700 injuries occurred during 11,268,426 AE in the nine sports included in the original study sample for an overall rate of 22.81 injuries per 10,000 AE. Of these injuries, 3864 (15.1%) were knee injuries, primarily ligament sprains (48.2% of all knee injuries), contusions (14.9%), and meniscal injuries (9.3%). Using our weighted analyses, we estimate 1,234,291 knee injuries occurred during the 2005/2006–2010/2011 academic years nationally in these nine sports. To more broadly describe knee injuries in a larger number of U.S. high school sports, we will discuss from this point forward only data from all 20 sports included in the combined convenience and original samples.
During the 2005/2006–2010/2011 academic years, 5116 knee injuries were reported from the combined original and convenience samples during 17,172,376 AE, or 2.98 knee injuries per 10,000 AE (Table 1). The highest injury rates were in football (6.29), girls’ soccer (4.53), and girls’ gymnastics (4.23). The lowest rate was in boys’ swimming and diving (0.15).
Knee injuries were more common in competition than practice overall (RR = 3.53, 95% CI = 3.34–3.73) and for every sport except boys’ and girls’ swimming and diving, boys’ and girls’ track and field, and cheerleading (Table 1). The highest injury rates in competition were in football (21.08 competition knee injuries per 10,000 competition AE), girls’ soccer (10.84), and girls’ gymnastics (9.36).
Overall, boys sustained 70.8% of all knee injuries, with football accounting for 62.0% of boys’ knee injuries and 43.9% of all knee injuries. Knee injury rates in football were significantly higher than in girls’ soccer—the sport with the second highest rate (RR = 1.39, 95% CI = 1.26–1.53), all other boys’ sports combined (RR = 3.24, 95% CI = 3.03–3.46), and all girls’ sports combined (RR = 2.75, 95% CI = 2.57–2.93). In sex-comparable sports (soccer, volleyball, basketball, baseball/softball, lacrosse, swimming and diving, and track and field), knee injury rates were significantly higher for girls than boys (RR = 1.52, 95% CI = 1.39–1.65). Girls had higher rates than boys in soccer (RR = 1.71, 95% CI = 1.50–1.95), basketball (RR = 1.89, 95% CI = 1.62–2.20), baseball/softball (RR = 1.68, 95% CI = 1.28–2.21), and track and field (RR = 1.49, 95% CI = 1.08–2.06).
Specific injury patterns.
Beginning with the 2007/2008 academic year, specific structures involved in knee injuries were reported. The most commonly involved structure was the medial collateral ligament (MCL; involved in 36.1% of knee injuries, 0.80 injuries per 10,000 AE), followed by the patella/patellar tendon (29.5%, 0.65), anterior cruciate ligament (ACL; 25.4%, 0.56), meniscus (23.0%, 0.51), lateral collateral ligament (7.9%, 0.17), and posterior cruciate ligament (2.4%, 0.05). The most common injury patterns were isolated patella/patellar tendon involvement (26.3%), MCL (25.1%), ACL (12.2%), meniscus (10.7%), lateral collateral ligament (5.1%), and ACL + meniscus (4.7%). Most isolated patella/patellar tendon injuries were dislocation/subluxations (32.7% of patella/patellar tendon injuries), contusions (24.2%), and tendonitis (16.5%).
Football and girls’ soccer had the highest rate of ACL injury (1.17 per 10,000 AE each) followed by girls’ gymnastics (1.14) and girls’ basketball (1.07) (Table 2). Football was the only sport with a meniscal injury rate of more than 1 per 10,000 AE and had the highest rate of MCL injury (2.42 per 10,000 AE). The overall rate of ACL injury between girls and boys was similar (RR = 1.01, 95% CI = 0.87–1.17) but in sex-comparable sports girls had higher rates (RR = 2.38, 95% CI = 1.91–2.95). Girls had higher rates in softball versus baseball (RR = 4.99, 95% CI = 1.86–13.36), basketball (RR = 4.54, 95% CI = 2.99–6.89), and soccer (RR = 2.33, 95% CI = 1.67–3.26).
The proportion of knee injuries treated with surgery overall was 21.2% and was highest in cheerleading (40.0%), girls’ lacrosse (39.3%), and girls’ basketball (33.8%) (Fig. 1). Knee injuries were significantly more likely to be treated with surgery than all other injuries (IPR 5.52, 95% CI = 5.10–5.97). Most knee surgeries were for ligament strains (65.4%) and meniscal injuries (22.0%); 54.0% of all cartilage (meniscal) injuries and 29.6% of ligament strains were treated with surgery. Knee injury diagnosis often involved advanced diagnostic modalities including MRI, accounting for 54.0% of MRI scans for all injuries.
Girls’ knee injuries were more often treated with surgery than boys overall (IPR 1.37, 95% CI = 1.22–1.52) and in sex-comparable sports (IPR 1.30, 95% CI = 1.11–1.53). Girls were more likely to undergo surgery than boys in soccer (IPR 1.36, 95% CI = 1.04–1.77) and basketball (IPR 1.34, 95% CI = 1.04–1.72). Surgery was delayed to allow continued sports participation in 10.3% of meniscal injuries and 6.8% of ACL injuries. Delayed surgical treatment of meniscal injuries occurred most frequently in gymnastics (50.0%), boys’ ice hockey (28.6%), girls’ field hockey (22.2%), and boys’ wrestling (21.1%). Delayed surgery for ACL injuries occurred most frequently in girls’ field hockey (20.0%), boys’ basketball (15.0%), boys’ wrestling (14.3%), boys’ soccer (12.8%), and boys’ lacrosse (12.0%).
The most common outcomes associated with knee injuries were loss of participation from 1 to 6 d (35.2% of all knee injuries) or 1 to 3 wk (26.9%), although these were primarily patella and/or patellar tendon injuries. ACL sprains often resulted in restriction from participation for the duration of the season (47.9% of all ACL injuries).
Factors associated with knee injury.
Athletes were wearing knee braces at the time injury occurred in 6.5% of all knee injuries. Athletes were wearing braces in 30.4% of recurrent knee injuries but only 3.4% of new knee injuries. Of the 194 knee injuries that occurred while athletes were wearing braces, 88 (45.4%) were wearing a rigid frame, 77 (39.7%) a neoprene sleeve, and 29 (14.9%) were not specified.
The mechanisms of injury varied by sport but most often involved contact with another person (50.3% of all knee injuries) (Table 3). Contact with another person was the most common mechanism of knee injury for boys’ football (69.4%), boys’ soccer (52.9%), girls’ soccer (49.8%), boys’ wrestling (50.9%), and boys’ ice hockey (49.0%). Noncontact mechanisms were most common in girls’ volleyball (42.9%), boys’ basketball (35.3%), girls’ basketball (35.7%), girls’ gymnastics (57.7%), and girls’ lacrosse (51.9%).
Participation in high school athletics in the United States continues to increase (23) with an expectant increase in the number of injuries. Knee injuries in particular can impose serious physical and economic burdens on athletes, such as ACL reconstructive surgery, which can cost more than $5000 (21). Our study, the largest, most nationally representative epidemiological study of knee injuries among U.S. high school athletes to our knowledge, demonstrates the importance of knee injuries and the need to develop more effective injury prevention programs to reduce the occurrence of sports-related knee injuries.
Consistent with previous high school and collegiate athletic injury studies (1,6,13), knee injuries accounted for 15% of all high school sports injuries. Our results also supported previous reports showing higher knee injury rates in competition compared with practice (6,13,14). Equally important, a high number of injuries were treated with surgery (8,24,25). Consistent with a prior study (24), we found that girls were more likely than boys to undergo surgery for knee injuries in soccer and basketball, but we did not find significant sex differences specifically for ACL or meniscus injuries. Potential explanations for sex differences in treatment may include injury severity, physician treatment preferences, or athlete’s personal choice. Future research should investigate sex-based differences in surgical treatment decisions.
Girls had higher knee injury rates than boys in each sex-comparable sport except lacrosse, which was consistent with many prior studies (5,8,18). Collegiate men’s and women’s lacrosse ACL injury rates also do not differ significantly by sex (18). Boys’ and girls’ lacrosse have dramatically different required protective equipment and rules regarding player–player contact, but the mechanics of player–ground contact (running, pivoting, and stopping) are similar. Thus, we expected to find knee injury rates in lacrosse to follow a similar pattern to that of basketball and soccer where girls’ rates were higher than boys’ rates, but this was not the case. Investigators have suggested possible explanations and interventions aimed at reducing knee injury rates among female athletes (2,7,9,11,12), and potential screening tests have been suggested to evaluate female athletes at risk for ACL injury (12). Evidence for effective physical preventive measures (e.g., functional knee bracing) is inconclusive, but focused training and therapies have been shown to be effective at reducing injury rates (10,11,16,17).
Knee injury rates and patterns differed by sport with the highest rates among those with significant player–player contact, pivoting, jumping, and landing. Despite noteworthy sex-based differences in knee injury patterns, knee injury rates were highest among football players. Meniscal injury rates were higher, and MCL injury rates were more than twice as high as in any other sport. The burden of knee injuries among football players is even greater considering that 1,109,836 U.S. high school students participated in football during the 2010/2011 academic year—more than any other sport including boys’ and girls’ basketball (984,777 combined) and boys’ and girls’ outdoor track and field (1,054,567 combined) (23). Although knee injuries in girls’ sports have received considerable attention in recent years, the highest burden of knee injuries is among football players. As bracing has limited abilities to prevent ACL injuries (16), screening measures and therapies addressing neuromuscular and biochemical risk factors in female athletes (10) may be adapted to male athletes to reduce ACL and other knee injuries among football players.
Limitations of our study should be noted. We limited our sample to high schools with an AT, which restricted the potential schools that could be included. However, this ensured that a medically trained professional documented injury, thus increasing the quality and consistency of our data. In addition, because AE were unit based rather than time based, we were unable to report participation/exposure rates by minute or hour of practice and competition. However, this limitation was necessary to reduce reporter burden. In addition, data from two samples concurrently enrolled in High School RIO™, the original randomly selected nationally representative sample and the convenience sample, were combined in this report. Although only the data from the randomly selected sample is nationally representative of all U.S. high schools with regard to geographical distribution and school size, including the convenience sample data allowed a direct comparison of rates and patterns of injury across a larger number of sports because the reporting methodology for the two samples is identical. Although combining the original and convenience samples may limit the generalizability of our findings, this limitation was necessary to enable the comparison across such a large number of sports. Thus, although the convenience sample may not be nationally representative, it remains the largest national sample from which such high school sports-related injuries have been captured to date. Finally, the convenience sample of high schools did not allow us to provide national estimates for knee injury patterns in all 20 sports. Despite these limitations, this study remains one of the largest nationwide epidemiologic studies describing knee injuries among U.S. high school athletes.
We believe that participation in high school sports will continue to increase in the United States. Knee injuries are a potentially devastating problem among U.S. high school athletes and impose substantial time and financial burdens on athletes’ families and the healthcare system. In most major sex-comparable sports, girls sustained knee injuries at higher rates than boys. Sex-specific lacrosse injuries were not significantly different, which differs from the pattern of soccer and basketball and warrants further investigation. Football players account for nearly half of all knee injuries and sustain them at rates higher than those in any other sport. Injury prevention strategies should continue to be a high priority given the findings herein and the need to reduce the incidence and severity of knee injuries among U.S. high school athletes.
The content of this report was funded in part by the Centers for Disease Control and Prevention (grant nos. R49/CE000674-01 and R49/CE001172-01) and the National Center for Research Resources (award no. KL2 RR025754). The content of this report is solely the responsibility of the authors and does not necessarily represent the official views of the Centers for Disease Control and Prevention, the National Center for Research Resources, the National Institutes of Health, or the American College of Sports Medicine. The authors also acknowledge the generous research funding contributions of the NFHS, NOCSAE, DonJoy Orthotics, and EyeBlack.
The authors declare no conflict of interest.
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