April 2011

Sprain in the Forecast: Epidemiology and risk factors for ankle sprain

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Analysis of the epidemiology of ankle sprain has revealed modifiable and non-modifiable risk factors. Understanding these will allow practitioners to help athletes minimize their risk of acute injury and chronic sequelae.

By CPT Brian R. Waterman, MD; Joseph R. Langston, BS; Kenneth L. Cameron, PhD, ATC; LTC Philip J. Belmont, Jr, MD; and LTC Brett D. Owens, MD

Ankle sprain is one of the most common injuries, if not the most common, among athletic populations, and can result in significant rates of recurrence, time lost to injury, and further long term sequelae. The epidemiology of ankle sprain is complex, and attempts at clarification have yielded several potential modifiable and non-modifiable risk factors.  Non-modifiable risk factors associated with ankle sprain include gender, age, height, and previous ankle sprain, whereas modifiable risk factors include weight, BMI, postural stability, and exposure to sport. With a broader understanding of these contributing risk factors, practitioners may be able to intervene and educate athletes to mitigate risk of ankle sprain injury.

Introduction

Ankle sprain represents one of the most common injuries sustained during sporting activities, accounting for 10% to 30% of single sport injuries.1,2 Injuries in the general population have also steadily increased for the last several decades,2-4 resulting in nearly 1.2 million ankle sprain related health care visits per year.5 With incidence rates ranging from 5 to 58.4 per 1000 person-years,6-9 and reported associated costs of up to $3.8 billion encompassing both treatment and rehabilitation,5,10 ankle sprain presents a significant problem for athletes and sports medicine practitioners alike. In addition to its financial costs, ankle sprain can result in significant sequelae, including time lost to injury and long-term disability in up to 50% of cases.11 Furthermore, repetitive ankle sprain and chronic ankle instability can result in osteochondral lesions of the talus, peroneal tendon pathology, and anterolateral ankle impingement.

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To better understand and anticipate the epidemiology of this injury, several authors have sought to identify risk factors associated with lateral ankle sprain. While risk factors have traditionally been classified as intrinsic or extrinsic,12,13 injury epidemiologists have shown increased interest in modifiable and non-modifiable risk factors associated with injury.6,14 (see table) Non-modifiable risk factors can be useful in identifying high-risk populations for injury prevention, while modifiable risk factors such as body mass index, proprioception or postural stability, and the absence of external restraints to inversion (e.g., prophylactic bracing) can be the targets for intervention. 14 Within the context of this framework, several risk factors for ankle sprain will be discussed.

Non-modifiable risk factors

Gender. With the increasing popularity and involvement of female athletes in sports, gender-related studies of musculoskeletal injury have become important in determining potential health disparities. The underpinnings of these gender-based discrepancies are likely multifactorial, although several postulated associations including inherent hormonal differences, lower extremity anatomy, limb alignment, ligamentous laxity, neuromuscular control, and the extent and type of athletic exposure have been discussed, particularly with regard to anterior cruciate ligament rupture.14,15

Gender studies on ankle sprain incidence, however, have yielded mixed results. A study of military cadets showed incidence rates (IR) of ankle sprain as 96.4 and 52.7 per 1000 person-years for women and men, respectively [incidence rate ratio (IRR) of 1.83 (95% CI, 1.52-2.20)], while no differences were detected between the male and female intercollegiate athletes.6 In a separate study of collegiate athletes, Beynnon et al showed IRs of 1.6 and 2.2 per 1000 person-days for men and women, respectively, although the difference was not statistically significant.16 Hosea et al subsequently found that while female athletes had a 25% greater risk of sustaining a Grade I ankle sprain compared with their male counterparts in both high school and intercollegiate basketball, there was no significant difference in the risk for the more severe ankle sprains.17

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Further studies in the general population of the United States show no overall differences by gender, although male individuals between the ages of 15 and 24 and female individuals over the age of 30 had higher rates of ankle sprain than their opposite-sex counterparts.7 A population-based study of active duty military personnel revealed that female servicemembers experienced an incidence rate that was 21% higher than that of male servicemembers.14

Based on the available literature, gender appears to be associated with risk of ankle sprain, although additional factors such as exposure to at-risk activity and level of activity may directly influence this complex relationship.

Age. Younger age, particularly when associated with increased exposure to at-risk activity, is also associated with the risk of ankle sprain. Studies of the general population of the United States6 and Denmark8 yielded a mean and median patient age of 26.2 and 24.4, respectively. A recent population-based study within an active-duty military population reported the highest incidence rates for ankle sprain in the group of those under 20 years old for both male and female subjects, and rates generally declined with increasing age.14 These studies suggest that peak incidence rates for ankle sprain occur during the second decade of life, with male and female peak incidence rates occurring between the ages of 15 to 19 and 10 to 14,7,9 respectively.

The mechanism of injury for ankle sprain also varies by age, with a greater preponderance of injuries occurring during recreational or competitive sports in young, active populations.6 Patients under 25 years of age were more likely to sustain ankle sprains while engaging in athletics and physical activity, while patients over 50 were more likely to incur ankle sprain in their own homes or during activities of daily living.8

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Previous ankle sprain. Ankle sprain produces damage to ligaments that maintain the stability of the ankle joint, thereby creating potential functional limitations. Some of this damage, such as that resulting in proprioceptive or neuromuscular impairment, is modifiable through exercise. However, the initial inflammatory response following injury can also lead to scar tissue formation, which is more likely than normal tissues to fail due to a 60% reduction in energy absorbing capacity.18

Functional instability and increased risk of re-injury may still occur after primary ankle sprain in athletes and military recruits undergoing basic training, even if the primary sprain is on the less severe end of the injury spectrum.19-21 A recent study on track and field athletes and rates of reinjury within 24 months showed that athletes with a grade I or II lateral ankle sprain were at higher risk of re-injury (14% and 29% respectively) than high-grade acute lateral ankle sprains (5.6%).22 However, this may also be related to inadequate rehabilitation of less severe injuries and earlier perceived healing despite persistent proprioceptive impairment, which ultimately increases further risk of recurrence.

Modifiable risk factors

Weight and body mass index (BMI). With increasing weight and body mass index (BMI), an increasing mass moment of inertia acts about the ankle, potentially increasing the risk of ankle sprain. In a study on high school football players, Tyler et al23 has shown the incidence of ankle sprain was significantly increased in patients with BMI categorized as above normal or overweight when compared to those players with a normal BMI. Waterman et al6 reported similar findings in military cadets with ankle sprains who had higher mean weight and BMI than their uninjured counterparts. Interestingly, in the same study, no statistically significant differences in height, weight, or BMI were observed between the injured and uninjured female cadets, but this may have been due to the limited female representation within the studied cohort.

Despite the evidence that weight and BMI are associated with an increased risk for ankle sprain, other studies have failed to demonstrate that these anthropometric measures are independent risk factors for ankle sprain.16,24  Certain athletic cohorts or player positions with elevated BMI may be at a greater predisposition for ankle sprain; further research is required to discern these subtle differences.

Neuromuscular control/postural stability. Proprioception and broader neuromuscular control were first proposed as a risk factor for ankle sprain by Freeman et al25 in 1965. Subsequent studies have extensively and rigorously evaluated proprioceptive deficits after primary ankle sprain and described their resultant effects on strength, postural balance, and ankle stability, particularly in athletic populations.26-28 Furthermore, McGuine et al 29 demonstrated that high school basketball athletes who subsequently sustained ankle sprains had significantly greater measures of pre-injury postural sway when measured with stabilometry than their uninjured counterparts, indicating an underlying neuromuscular predisposition. Other studies have reported similar results with clinical assessments of postural stability.30

However, while gross morphologic changes and disrupted afferent nervous networks have been noted with ankle sprain and resultant postural instability,31 its causal link with chronic ankle instability is less clear.32,33 Under these circumstances, muscular fatigue or diminished strength may potentiate neuromuscular impairment and contribute to subsequent ankle instability.34

Sport. Studies on the risk of ankle sprain associated with various sports have shown that participation in certain athletic activities increase the likelihood of ankle sprain, particularly those that involve frequent running, cutting, and jumping movements. Analysis of the National Electronic Injury Surveillance System (NEISS) for all ankle sprain injuries presenting to emergencydepartments over a five-year time period revealed that 49.3% of ankle sprains were caused by participation in sports, and basketball (41.1%), football (9.3%), and soccer(7.9%) accounted for more than half of all ankle sprains during athleticactivity.7

A more expansive systematic review of ankle sprain epidemiology revealed that incidence rates varied depending on the unit of measurement.4 When evaluating for incidence per 1000 person-hours, rugby had the highest incidence (4.20), followed by soccer (2.52). Conversely, when considering incidence more accurately in terms of athletic exposure, lacrosse had the highest incidence rate (2.56) of sprains per 1000 person-exposures, followed by basketball (1.90). Similarly, Waterman et al6 found that basketball (men’s, 1.67; women’s, 1.14), men’s rugby (1.53), and men’s lacrosse (1.34) were among the highest incidences per 1000 person-years among intercollegiate athletes.

Level of competition. As a broad category, level of competition has also been identified as a potential risk factor for ankle sprain. Traditionally, level of competition has been synonymously used to describe both intensity of competition (i.e. practice vs. game) and level of skill (e.g., recreational, intercollegiate, and professional). However, both of these components represent distinct variables that should be separately considered and evaluated.

When considering intensity of competition, there is a positive correlation between higher level of play and increased risk of ankle sprain, with approximately 55% to 66% of injuries occurring during games when compared to training sessions.35-38 This is likely attributable to the increased risk-taking activity and pace of play.

When skill level is considered separately, however, there is less of a consensus in the literature. Our previous work has revealed that ankle sprain incidence in intercollegiate athletes was seven times that of intramural athletes in terms of injuries per 1000 person-years.6 However, when more specifically controlling for the extent of athlete-exposures, no significant differences between intramural and intercollegiate athletes were noted. Other prior studies evaluating skill level are conflicting, with one report of an increased risk in higher-level, intercollegiate athletes17 and another demonstrating no significant difference.39 Conversely, two additional studies have revealed an increased risk of sports injuries in lower-skill soccer athletes than their higher- skill cohorts.36,40

Several more specific, predetermining factors may more predictably explain the different rates of ankle sprain. These include higher cumulative numbers of athlete-exposures, greater match exposure,35 low training-to-match ratio,41 and limited warm-up or stretch period.41-43

Interventions for prevention

Reasonable measures may be designed and implemented to mitigate modifiable risk factors and reduce the risk of ankle sprain. Several interventions have demonstrated success in achieving these goals without significant effects on quality of life or athletic performance. By increasing passive restraints to ankle inversion and enhancing postural stability, prophylactic bracing in high-risk athletes can be effective in reducing the risk of primary and recurrent ankle sprain by up to 50%.24,44,45 In one prospective randomized trial, Sitler et al24 showed a threefold increased risk for ankle sprain among unbraced basketball players when compared to braced athletes over a two-year time period at the United States Military Academy.

Furthermore, neuromuscular training programs have also shown merit in reducing the risk of ankle sprain. In a meta-analysis, McKeon et al33 confirmed that prophylactic and targeted balance control training resulted in a 20% to 60% relative risk reduction for sustaining lateral ankle sprain, particularly in those individuals with prior history of ankle sprain. With more consistent screening of high-risk athletes and better instrumented measures for diagnosis, prophylactic interventions may gain more widespread popularity and effectively reduce the incidence of ankle sprain.

Conclusion

Ankle sprain is a common injury affecting both competitive and recreational athletes. With the significant socioeconomic costs, time lost to injury, and long-term disability associated with ankle sprain, prevention is an imperative goal for physicians.  To this end, identification and understanding of the underlying risk factors for injury may facilitate the establishment of risk-reduction strategies for targeted athletic populations.

CPT Brian R. Waterman, MD, is a senior resident in the orthopaedic surgery program at William Beaumont Army Medical Center and Texas Tech University Health Sciences Center in El Paso, TX. Joseph R. Langston, BS, is a third year medical student at Texas Tech University Health Sciences Center in Lubbock, TX. Kenneth L. Cameron, PhD, ATC, is the Director of
Orthopaedic Research at the John A. Feagin Sports Medicine Fellowship, Keller Army
Hospital in West Point, NY. LTC Philip J. Belmont, Jr., MD, is the Residency Director of the orthopaedic surgery program at William Beaumont Army Medical Center and Texas Tech University Health Sciences Center in El Paso. LTC Brett D. Owens, MD, is an associate professor with the Uniformed Services University of Health Sciences, John A. Feagin, Jr. Sports Medicine Fellowship at Keller Army Hospital in West Point, NY.

Disclaimer: The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or reflecting the views of the Department of Defense or the U.S. government. The authors are employees of the U.S. government.

 

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