Understanding Chronic Ankle Instability: Facts and Figures

Acute ankle sprains are among the most common musculoskeletal injuries, with a particularly high incidence in physically active populations. Additionally, the recurrence rate of acute ankle sprains is high, which is associated with the development of chronic ankle instability (CAI). Understanding the epidemiology of these injuries is crucial for improving patients' musculoskeletal health and reducing the burden of ankle sprains and their associated sequelae.

In 2016, the International Ankle Consortium published a consensus statement and evidence review on the prevalence, impact, and long-term prognosis of lateral ankle sprains, providing an evidence base for the occurrence of lateral ankle sprains, chronic ankle instability, and the associated direct and indirect costs, establishing specific goals for future research. This study aims to provide a focused review of the existing literature on the epidemiology of acute ankle sprains, including lateral, medial, and high/syndesmotic ligament types, with a focus on incidence studies from the United States. The study also offers a brief overview of CAI, post-traumatic osteoarthritis, and injury prevention to help understand the epidemiology and current state of science regarding ankle sprains and ankle instability in sports medicine.

Acute ankle sprains are characterized by the stretching or tearing of ankle ligaments. Sprains of the lateral ligament complex are the most common type of ankle sprain; meta-analyses indicate an incidence rate of 0.93/1000 athlete exposures (AE), where 1 AE is defined as one athlete participating in one game or practice. In contrast, reported incidence rates for acute medial and high/syndesmotic ankle sprains are lower, approximately 0.06 and 0.38/1000 AE, respectively. In fact, more than three-quarters of acute ankle sprains are lateral ankle sprains, with about 73% involving the anterior talofibular ligament. The remaining approximately 25% of acute ankle sprains are medial (deltoid ligament) or high ankle/syndesmotic (anterior inferior tibiofibular ligament or posterior inferior tibiofibular ligament injuries). This review focuses on the various categories of acute ankle sprains, including lateral, medial, and high/syndesmotic ligament sprains.

In the United States, there are approximately 2 million acute ankle sprains annually. Emergency department visit data indicate an incidence rate of 2-7/1000 person-years for acute ankle sprains; however, this likely significantly underestimates the actual incidence because many injured individuals may not present to the emergency department or seek medical care at all. In fact, one estimate suggests that the true incidence in the general population in the Netherlands is 5.5 times higher than that provided by emergency department data. Correspondingly, the incidence rate of acute ankle sprains in general population surveys (including those treated and untreated in the emergency department) ranges from 19.0 to 26.6/1000 person-years, while the incidence rate based on emergency department data in the same population is 2.1 to 3.2/1000 person-years. Furthermore, half of all ankle sprains treated in the emergency department in the U.S. are not related to sports, indicating that these injuries can affect a wide range of individuals, not just those participating in organized sports or high-level physical activities.

A meta-analysis of 181 prospective epidemiological studies on ankle sprains in different populations and data sources indicates that overall, the incidence rate of acute ankle sprains is higher in females (13.6/1000 exposures) than in males (6.9/1000 exposures). The incidence rate also appears to decline with age, with an estimated incidence of 2.85/1000 exposures in children, decreasing to 1.94/1000 exposures in adolescents, and 0.72/1000 exposures in adults. However, the peak incidence for males and females may differ; one study indicates that the highest incidence is in females aged 10-14 years, while the lowest incidence is in males aged 15-19 years. Current understanding of the incidence of acute ankle sprains in the general population remains limited due to the scarcity of data. Most existing incidence rates include only the most severe injuries requiring treatment in the emergency department. As mentioned earlier, most acute ankle sprains in the general population may not be treated in the emergency department, and many injured individuals may not seek care at all. To better understand the distribution of these injuries in the general population, alternative data sources, including administrative data sources, general population surveys, and primary care data, should be utilized to gain a broader understanding of the true incidence and prevalence of ankle sprains.

In U.S. collegiate athletes, acute ankle sprains are the most common injury, accounting for 15% of all injuries in this population. From 1988-1989 to 2003-2004, the incidence rate of acute ankle sprains in 15 National Collegiate Athletic Association sports ranged from 0.75 to 0.89 sprains/1000 AE. In contrast, the incidence rate of concussions was between 0.15 and 0.41/1000 AE, and the incidence rate of anterior cruciate ligament injuries was between 0.11 and 0.17/1000 AE in the same population during the same period. The incidence rate of acute ankle sprains also varies greatly by sport, as is the case with other sports-related injuries. Sports characterized by running, cutting, and jumping, such as basketball, soccer, football, and volleyball, typically have the highest incidence rates of acute ankle sprains. For example, the reported incidence rates for men's spring football (1.34/1000 AE), men's basketball (1.30/1000 AE), and women's soccer (1.30/1000 AE) are much higher than for men's and women's ice hockey (0.23 and 0.14/1000 AE, respectively) and men's baseball (0.23/1000 AE).

Acute ankle sprains are common injuries at various levels of competition. In U.S. high school sports, acute ankle sprains account for approximately 15% to 17% of all reported injuries, similar to the prevalence at the collegiate level. Notably, the incidence rate of acute ankle sprains in this population decreased from about 4.6 per 10,000 AE in 2005-2006 to about 2.5 per 10,000 AE in 2010-2011. Researchers studying sports-related injuries reported to the Athletic Training Practice-Based Research Network, an interscholastic athletic training electronic medical record system including U.S. high school athletes, also identified acute ankle sprains as one of the most common injuries, with the highest incidence rates reported among basketball and volleyball players.

At the highest levels of competition, acute ankle sprains were one of the most common injuries during the 2004 Olympic Games. The incidence of acute ankle sprains is particularly high among elite basketball players. The incidence rate of acute ankle sprains in National Basketball Association athletes is about 3.2-3.5/1000 player-games. In contrast, the next most commonly suffered specific injury in National Basketball Association players is lumbar sprain/strain, with an incidence rate of 1.1/1000 player-games. Similarly, although the overall injury incidence rate is lower among professional volleyball players compared to other elite athletic populations, ankle sprains are the most common specific diagnosis, accounting for 19.8% of all injuries. Interestingly, in high-level rugby competition, acute ankle sprains are also common injuries affecting players; however, these athletes appear to sustain a higher proportion of high/syndesmotic sprains compared to other populations. A National Football League team reported 36 high/syndesmotic sprains and 53 lateral ankle sprains over 15 seasons. Therefore, about 40% of ankle sprains in this population were high/syndesmotic sprains, compared to about 6% of ankle sprains in other populations.

Beyond athletic populations, acute ankle sprains also frequently occur among those engaged in vigorous physical activities, such as military personnel. The incidence rate among U.S. active-duty military personnel is 58.3/1000 person-years. Thus, compared to the incidence rate of 2 to 7 ankle sprains per 1000 person-years in the general population based on emergency department data, the incidence rate in military personnel is significantly higher, even considering the potential underestimation in the general population. For a recent systematic review, investigators compiled 173 studies of athletic populations and 8 studies of military populations published before July 2012 and identified an incidence rate of 11.55 ankle sprains/1000 exposures (95% confidence interval [CI] 11.54, 11.55) and an overall period prevalence of 11.88% (95% CI 0.56, 13.19). Compared to data from the general population, these rates illustrate a strong positive correlation between the prevalence of ankle sprains and the level of physical activity.

Data from high-quality injury monitoring and research projects, such as the National Collegiate Athletic Association Injury Surveillance Program, the High School Reporting Information Online system, and the Athletic Training Practice-Based Research Network, provide insight into the consequences of ankle sprains among athletes, but assessment across various levels of participation and a broader distribution of sports is needed. In particular, evidence from the "extremes" of sports participation levels, such as youth and professional sports, is lacking. Innovative data collection methods will allow for prospective epidemiological studies of youth and recreational sports populations.

A significant proportion of the high incidence rate of acute ankle sprains is attributed to the frequency of subsequent injuries following an initial ankle sprain. Authors of a systematic review noted that a considerable portion of all acute ankle sprains are recurrent. For instance, recurrent ankle sprains in volleyball, football, basketball, and soccer occur in 46%, 43%, 28%, and 19% of cases, respectively. Other researchers have identified the proportion of recurrent ankle sprains to range from 12% to 47%. Particularly, a history of lateral ankle sprain is described as one of the strongest risk factors for future lateral ankle sprains. In a prospective cohort study involving 9,811 military academy students, individuals with a history of ankle sprains were at approximately 3.5 times higher risk of sustaining another sprain, even after adjusting for age, participation in injury prevention programs, frequency of high school sports participation, and running index. This effect was similar for both males (adjusted risk ratio [RR] 3.40, 95% confidence interval [CI] 2.72–4.26) and females (adjusted RR 3.53, 95% CI 2.79–4.48), except for those without a history of lower limb injuries other than ankle sprains.

As previously stated, not only is there a high recurrence rate of ankle sprains, but athletes also tend to have a significant number of prior ankle injuries. Among recreational and elite basketball players in Australia, 73% reported a history of ankle injuries, with an average of 3.5 previous injuries (standard deviation 2.7). A survey of elite, competitive, and recreational athletes in China indicated that 73.5% reported experiencing at least two episodes of same-side ankle sprains in June 2019. Moreover, 22% of all patients had previously suffered five or more sprains. Among freshmen military cadets in the United States, 15.5% described experiencing an ankle sprain within the six months prior to entering the academy. Thus, both acute initial sprains and recurrent injuries play significant roles in the overall incidence of these injuries in populations, and those designing future injury prevention interventions should consider the risk of subsequent injuries following the initial ankle sprain.

While the literature has examined the prevalence of recurrent injuries, there is a lack of assessment of time-dependent measurements of ankle sprain history as a predictor of ankle sprains. Further investigation into this topic will clarify the connection between prior injury characteristics and the risk of subsequent injuries. These findings can also provide information for future injury prevention measures aimed at reducing the impact of these injuries. Additionally, more research is needed to further elucidate the relationship between multiple injuries and adverse long-term outcomes such as chronic ankle instability (CAI) and post-traumatic osteoarthritis.

Associated with the high rate of recurrent injuries following acute lateral ankle sprains is the development of chronic ankle instability (CAI), characterized by laxity and mechanical instability that disrupts activities. CAI can result from multiple injuries to the same structures or other mechanisms following the initial ankle sprain, ultimately leading to dysfunction of the lateral ankle ligament complex. It is important to recognize the overlap between the rate of recurrent injuries and the prevalence of CAI. A recent review of the incidence of acute ankle sprains indicated that up to 70% of individuals may develop acute ankle instability shortly after the initial injury. Researchers found in a prospective cohort study that the prevalence of CAI was 40% one year after the initial lateral ankle sprain. However, this estimate may involve a continuous unified entity based on reported symptoms and duration of disability, which vary depending on the population studied. Not surprisingly, individuals engaged in running and jumping activities appear to have higher rates of CAI, as well as the incidence of acute ankle sprains. However, contrary to previous findings regarding the incidence rate of acute ankle sprains, the literature suggests that individuals engaged in dance and gymnastics may have higher rates of CAI than other athletic populations. This finding emphasizes the differences between acute ankle sprains and acute ankle instability, which includes a broader spectrum of diseases affecting ankle stability.

When assessing the prevalence of CAI among high school and college athletes using ankle instability tools like the Ankle Instability Instrument and the Cumberland Ankle Instability Tool, 23.4% of athletes reported CAI. It was reported that female athletes (32%) had nearly twice the frequency of chronic ankle instability compared to male athletes (17%). Interestingly, the prevalence appears to decrease with higher levels of participation, with rates of 19% among college athletes and 31% among high school athletes. It is challenging to determine whether these variations represent true differences in the risk of CAI among different athletic populations or represent a selection bias, where healthier athletes are more likely to continue higher levels of participation. Also noteworthy is that in previous studies, nearly two-thirds of athletes reporting CAI did not indicate previous ankle sprains in their assessment surveys. This highlights the ambiguity in defining CAI and the potential for a broader range of ankle disorders that may lead to CAI. It is reported that a significant number of people experience inadequate ankle stability and associated disabilities, underscoring the importance of further research on this topic. Screening tools to assess this condition need evaluation and improvement based on the work of the authors who identified predictive factors for CAI development following the initial lateral ankle sprain. Future researchers should also focus on identifying mechanisms for the development of chronic ankle instability to reduce the risk.

Perhaps the most concerning long-term consequence of ankle sprain injuries is the development of post-traumatic osteoarthritis (PTOA). A recent review by authors identified lateral ankle sprains as accounting for 13% to 22% of all cases of osteoarthritis involving the ankle joint and 80% of cases of post-traumatic osteoarthritis. Other known causes of post-traumatic ankle osteoarthritis include fractures and osteochondral injuries. In cases of post-traumatic ankle osteoarthritis following lateral ankle sprains, half occur following a single acute injury, while the other half result from recurrent sprains or CAI. Individuals with post-traumatic ankle osteoarthritis may be younger than those with primary osteoarthritis, with an average onset age in the 10th decade of life, including patients in their twenties. Although this review provides a key overview of the link between ankle sprains and post-traumatic osteoarthritis, more research is needed, as most literature focuses on patients with post-traumatic osteoarthritis. Future researchers should prospectively evaluate the development and progression of this condition, including both injured and uninjured individuals. Additionally, the causes of post-traumatic osteoarthritis are not well understood and require continued efforts in this area.

Injury prevention interventions focusing on musculoskeletal strengthening, balance, proprioception, and biomechanical improvements are known to have benefits in preventing lower limb musculoskeletal injuries, alleviating pain, and restoring functional impairments. Specifically, a randomized controlled trial conducted among elite male basketball players compared the FIFA injury prevention guidelines with a control group, which consisted of a warm-up program involving stretching, strengthening, and balance exercises tailored to the sport of interest. The incidence of all injuries was lower (0.95 vs. 2.16 per 1000 AEs), especially lower limb injuries (0.68 vs. 1.4; P .022) under the FIFA prevention guidelines intervention. Proprioception and balance training programs are effective in reducing ankle sprains. In a prospective analysis of a six-year balance training program, acute ankle sprains were reduced by 81%. A randomized controlled trial showed that a multi-exercise program reduced the probability of ankle sprains by nearly 65% compared to the control group. This evidence suggests that proprioception exercise programs are beneficial in reducing the risk of acute ankle sprains.

However, it is important to note that studies evaluating the effectiveness of these interventions often have limited scope, despite their potential to yield results. Primarily, authors focused on ensuring strong internal validity (i.e., experimental control of confounding factors to ensure causal pathways between interventions and reduced injury rates); however, external validity (i.e., generalizability of conclusions from the sample to the entire population) may thus be compromised. Meanwhile, these interventions are not widely implemented. In many cases, there is little investigation into the actual adherence to intervention schemes and their impact on anticipated outcomes. Therefore, ongoing evaluation of interventions is needed to assess the effectiveness of their adoption, implementation, and maintenance. It is important to reiterate that evaluations of injury prevention should consider not only interventions aimed at reducing injury rates and severity but also comprehensive prevention strategies.

van Mechelen et al.'s prevention sequence proposes a cyclical, four-step approach to assessing injuries: (1) identifying incidence and severity, (2) determining causes of injury, (3) introducing preventive measures, and (4) evaluating the effectiveness of proposed preventive measures. Given the widespread distribution of acute ankle sprains in the general population, future researchers should assess injury prevention plans adapted and implemented in the general population from a public health perspective.

Due to the high prevalence of recurrent ankle sprains, CAI, and their association with post-traumatic osteoarthritis, it is necessary to develop and evaluate secondary and tertiary injury prevention interventions to reduce the incidence of recurrent ankle sprains and CAI. Rehabilitation following initial ankle sprains as a secondary injury prevention plan is aimed at preventing subsequent injuries, but there is relatively little data on the actual prevention of recurrent injuries. Tertiary prevention measures to reduce the likelihood of developing post-traumatic osteoarthritis are also urgently needed to mitigate the long-term adverse consequences of these injuries, and the first step in advancing in this area is to study the continuous pathway from initial ankle sprain to post-traumatic osteoarthritis.

Despite the many health benefits of physical activity, ankle sprains are a common injury, often resulting in recurrent injuries, chronic ankle instability, and post-traumatic osteoarthritis. Although the literature has shown that interventions for primary prevention are effective in reducing injury rates, there is limited data on the effectiveness of these interventions and the generalizability of the findings. Therefore, ongoing evaluation of these interventions is needed to assess their impact on reducing the prevalence of ankle sprains.

Besides the high incidence rate of acute, initial ankle sprains, the concern of re-injury is also significant. Individuals with a history of acute ankle sprains are approximately 3.5 times more likely to suffer another ankle sprain compared to those without such a history. This strong association between previous ankle sprains and increased risk of future ankle sprains is evidenced by findings that 12% to 47% of ankle sprains are recurrent. Associated with the increased risk of re-injury is the development of chronic ankle instability (CAI), which involves chronic functional impairment of the lateral ligament complex. Up to 70% of individuals with acute lateral ankle sprains may develop chronic ankle instability, disrupting normal activities and leading to disability. Moreover, the link between acute ankle sprains, CAI, and post-traumatic osteoarthritis is concerning. Future injury prevention measures must continue to consider the subsequent injury risks following initial ankle sprains, CAI development, and post-traumatic osteoarthritis to mitigate the burden of these injuries across populations. It is important to note the limitations over time in describing and comparing the epidemiology of acute ankle sprains and CAI. Given ongoing advancements in injury diagnosis, treatment, and prevention strategies, the identification and definition of these conditions also evolve over time. Specifically, defining and quantifying cases of chronic ankle instability in the literature presents a unique challenge due to the diversity of synonyms and characteristics used to describe this phenomenon. Subsequently, when drawing inferences based on recent advances, consideration must be given to the time periods and populations included in the reference studies. Continued investigation into the epidemiology of acute ankle sprains and the development of adverse long-term outcomes such as CAI and post-traumatic osteoarthritis is needed to fill gaps in the literature and deepen understanding of this public health issue. Additionally, if successful prevention observed in specific populations is to be translated to a broader population, the adoption, evaluation, implementation, and maintenance of primary, secondary, and tertiary injury prevention interventions are crucial.