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Abstract
Introduction Musculoskeletal foot and ankle injuries are commonly experienced by soldiers during military training. We performed a systematic review to assess epidemiological patterns of foot and ankle injuries occurring during military training.
Methods A review of the literature was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The search, done on 14 February 2019, resulted in 1603 reports on PubMed, 565 on Embase and 3 on the Cochrane Library. After reading the remaining full-text articles, we included 91 studies.
Results Among a population of 8 092 281 soldiers from 15 countries, 788 469 (9.74%) foot and ankle injuries were recorded. Among the 49 studies that reported on length of training, there were 36 770/295 040 (18.17%) injuries recorded among women and 248 660/1 501 672 (16.56%) injuries recorded among men over a pooled mean (±SD) training period of 4.51±2.34 months. Ankle injuries were roughly 7 times more common than foot injuries, and acute injuries were roughly 24 times more common than non-acute injuries. Our findings indicated that, during a 3-month training period, soldiers have a 3.14% chance of sustaining a foot and ankle injury. The incidence of foot or ankle injury during military parachutist training was 3.1 injuries per thousand jumps.
Conclusions Our findings provide an overview of epidemiological patterns of foot and ankle injuries during military training. These data can be used to compare incidence rates of foot and ankle injuries due to acute or non-acute mechanisms during training. Cost-effective methods of preventing acute ankle injuries and non-acute foot injuries are needed to address this problem.
- foot & ankle
- epidemiology
- orthopaedic & trauma surgery
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Key messages
Ankle injuries during military training are roughly seven times more likely than foot injuries.
Acute injuries are 24 more times likely than non-acute injuries.
The overall chance of foot or ankle injury during a 3-month training period is 3.14%.
The chance of sustaining a foot or ankle injury during a 3-month training period for women was 1.9%.
The chance of sustaining a foot or ankle injury during a 3-month training period for men was 3.2%.
The incidence of foot and ankle parachute injuries was 3.1 injuries per thousand jumps.
Introduction
Military training is a demanding process that aims to prepare soldiers both physically and psychologically for their time in the military. While efforts have been made to limit the number of injuries sustained during this time, foot and ankle injuries remain common,1 with significant implications for soldiers’ health and overall military readiness. In particular, musculoskeletal injuries lead to attrition in military training and present an enormous cost to militaries each year. Effective measures to limit these injuries have the potential to significantly reduce financial cost to the military, as well as improve physical and combat preparedness by minimising interruptions to training. While attempts to limit foot and ankle injuries have been made, there is little evidence supporting a cost-effective technique or device to accomplish this task.
The mixture of high-intensity and long-duration activities that comprise military training predisposes soldiers to both acute injuries and non-acute overuse injuries. Previous studies have addressed some of the methods that can help prevent both traumatic and non-traumatic injuries. Stretching, bracing, shock-absorbing insoles, foot morphology and range of motion were all investigated in connection to foot and ankle injuries.2–5 However, due to variations in research methodology, the outcomes reported in these studies vary widely.
While numerous studies examined and reported on musculoskeletal injuries in general, analyses specific to foot and ankle injuries while in military training are limited. The aim of the current study was to provide an overview of the worldwide epidemiological patterns of foot and ankle injuries during military training. This includes the likelihood of sustaining foot and ankle injuries during land-based training and parachute training and the requisite time to recover from such injuries.
This systematic review was conducted according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. A literature search was performed in PubMed, Embase and the Cochrane library on 14 February 2019 (Figure 1). The search was not limited to any particular year of publication. Studies were identified using synonyms for ‘military’, ‘ankle’, ‘foot’ and ‘injury’. We included English studies that reported on the number of foot and ankle injuries sustained during military training and military parachute training. Exclusion criteria were
Case reports, including less than five subjects.
Systematic reviews.
Studies on animals.
Cadaver or in vitro studies.
Biomechanical reports.
Letter to editors.
Studies that included injuries not related to training.
Studies that included subjects who were not in the military.
Studies that did not include subjects with foot and ankle injuries.
Overlapping cohorts.
Cohorts including cadets and people at the US Military Academy (excluded due to physical requirements outside training).
Military training period of less than 3 months.
Flowchart of the literature search and selection of relevant articles. The last search was performed on 14 February 2019.
Data extraction and quality assessment
The following data were extracted from included articles: author, year of publication, country of cohort, study design, age, sex, number of injuries, number of subjects, injury type, number of parachute jumps, branch of training, training length and recovery period. Two reviewers independently assessed the quality of included studies by examining the number of subjects included, the per cent of follow-up for each study, the amount of missing data as reported in the study, level of evidence, detail of injury data and selection criteria for each study (Online supplemental table 1).
Supplemental material
Data analysis
The overall incidence rate of foot and ankle injuries during a 3-month military training was calculated using only studies that reported explicitly the length of training. The total number of injuries, subjects and number of training months trained were described in 49 studies. While the other 42 studies observed over the course of the minimum amount of time, they either observed multiple trainings at a base simultaneously (resulting in an unclear mix of individual training durations) or retrospectively followed up a group of soldiers over a period of multiple trainings. While these could not be kept in our analysis of incidence rate of injury, we believe they contributed to a true overview of foot and ankle injuries. Thus, these were to remain included but not used within this particular analysis. The total number of injuries was divided by the total number of subjects to generate an injuries:subject ratio, which was then divided by the average amount of time spent in training. This would give the injury/person over the course of the average length of training, which was then adjusted to reflect a 3-month time period of training. Data specific to injury type were omitted due to the nature of the studies. For example, if a study examined the incidence ankle sprains over the course of years with millions of participants, the percentage may be low; however, a study that examined stress fractures and included only those with stress fracture may create a misleading per cent of stress fractures. To avoid misleading on a particular injury, we chose to omit completely and offer a higher-level assessment of foot and ankle injuries.
Limited duty days were calculated by using all studies that reported days lost due to injury. The reported number of days lost to injury was extracted from each article, and the pooled mean and SD were calculated. The pooled incidence rate of foot and ankle injuries following parachute training was described as the total number of injuries per thousand jumps. Parachute injuries were analysed separately because of the increased risk the training presented.
Results
A literature search resulted in 1603 reports in PubMed, 565 in Embase and 3 in Cochrane. After removal of duplicates, two reviewers independently screened titles and abstracts of 1928 studies by applying the predefined eligibility criteria. Subsequently, the reviewers read 174 full-text articles to assess eligibility for data extraction. Eighty-three articles were excluded: 26 studies did not have sufficient training periods; 14 were in the setting of military academies; 25 did not specifically report foot and ankle injuries; 4 included deployed soldiers; 1 included non-military personnel; 2 were literature reviews; 10 were unable to be found; and 1 consisted of an overlapping cohort with an already included study.
In total, 91 studies describing injuries to the foot and ankle during military training were included in this systematic review. Seven of these studies reported specifically on foot and ankle injuries following parachute training.6–12 Of the 91 studies included, 5 studies were scored as level I evidence.13–17 Forty-two studies were level II evidence.1 3 4 9 11 18–54 Forty-two studies were level III,2 5–8 10 12 54–89 and 2 studies were level IV.90 91 While assessing study size, we found that 85 (93%) studies included over 120 subjects; 4 (4%) studies included between 40 and 120 subjects; and only 2 (3%) studies included fewer than 40 subjects. Sixty-one (67%) studies reported that 100% of participants completed the study; however, few studies reported any missing outcome data (75 studies). In assessing patient selection process in each study, we found in 79 (82%) studies the selection process in detail, while 12 (18%) did not offer this information. Finally, we note how each study reported the injuries—whether they describe the mechanism (acute or non-acute) and the specific location of injury (foot or ankle) in their study. Fifty (55%) studies reported clearly defined foot and ankle injuries, while 41 (45%) were not specific either in the location (grouped foot and ankle into single category) or mechanism of injury (did not distinguish acute and non-acute). Quality assessment and critical analysis data can be found in Online supplemental table 1.
Among the 91 studies from 15 countries, the total study population included 8 092 281 soldiers with a median age of 20.8 (IQR 19.8–25.2) years. There were 6 513 391 (80.49%) men and 1 096 508 (13.55%) women. Sex was not reported for 482 382 (5.96%) individuals (Table 1). Overall, 788 469 foot and ankle injuries were recorded. Forty-nine studies reported on the length of training period. The pooled mean (SD) was 4.51±2.4 months. Ankle injuries were roughly seven times more common than foot injuries, and acute injuries were roughly 24 times more common than non-acute injuries. We found that injuries to the foot were more commonly non-acute (non-acute:acute ratio=19:1), whereas injuries to the ankle were more commonly acute (acute:non-acute ratio=157:1) in nature. The most common non-acute injuries noted were stress fracture of the tarsals and metatarsals, plantar fasciitis and tenosynovitis. The most common acute injuries noted were lateral ankle sprains or ankle fractures. Six studies that grouped foot and ankle injuries together were also recorded but accounted for less than 2% of the total injury data (Table 2).
Demographics of the included studies
Acute and non-acute foot and ankle injury data (studies, n=84)
Based on 49 studies, soldiers have a 3.14% chance of sustaining a foot or ankle injury during a 3-month training period. Thirty studies that reported on the number of injuries in men and women separately were analysed to examine the difference in injury incidence between sexes. The analysis revealed that the incidence rate of foot and ankle injuries for women was 1.9% per 3 months, while that of men was 3.2% per 3 months. Injuries resulted in a mean (SD) of 36±25 limited duty days based on analysis of eight studies.
Injuries during parachute training
Among seven studies that reported on foot and ankle injuries after parachute training, the incidence of injury was found to be 3.1 injuries/thousand jumps.6–12 These data included 281 527 jumps and 872 foot or ankle injuries: 81 (9%) foot, 644 (74%) ankle and 147 (17%) foot and ankle (unspecified). Ankle injuries were eight times more common than foot injuries during parachuting training.
Discussion
Previous studies have evaluated frequencies and mechanisms of foot and ankle injuries occurring during military training.4 47 79 92 However, to our knowledge, this study represents the first systematic review which integrates data from countries all over the world to provide a comprehensive worldwide overview of foot and ankle injuries in military training. This study aimed to understand the worldwide epidemiological patterns of foot and ankle injuries during military training. We found that among a total of 8 092 281 soldiers from 15 countries, 788 469 (9.74%) foot and ankle injuries were recorded over a pooled mean (SD) training period of 4.51±2.4 months.
Acute and non-acute injuries
Many studies included in this analysis and in general specify the ankle as the most commonly injured location of the distal lower extremity.4 23 30 37 38 61 92–95 However, some studies17 29 59 60 66 67 81 91 localise the majority of these injuries to the foot, contrasting with this study’s findings that acute ankle injuries are the most common type of injuries to the distal lower extremity during military training. This seemingly contradicting finding can be explained by the fact that all but one of the articles91 citing the foot as the most commonly injured location are studies primarily focused on stress fractures or overuse injuries. The current study found that injuries to the foot were more commonly non-acute (non-acute:acute ratio=19:1), whereas injuries to the ankle were more commonly acute (acute:non-acute ratio=157:1) in nature. Due to this finding, we recommend that preventative measures for acute lower extremity injuries should be focused on protecting the ankle, while preventative measures for non-acute injuries should focus on reducing overuse stress to the foot. Some studies suggest shock-absorbing insoles, for example, have been found to reduce the incidence of lower limb stress fractures, tibial periostitis, tenosynovitis of the foot and Achilles tendinopathy, but more research is needed to examine the cost effectiveness of such a strategy.68 96 Further studies should be conducted to identify the most common mechanisms responsible for acute injuries to the ankle. This would allow for more efficient targeting of future preventative tactics during especially high-risk activities, such as parachute jumps.
Chance of injury
We found that soldiers have a 3.14% chance of sustaining a foot or ankle injury during a 3-month training period. While this represents the overall chance of sustaining a foot or ankle injury, our analysis did not take into account potential risk factors linked to an increased chance of sustaining general musculoskeletal injury, such as increased body mass index (BMI), smoking, male sex and low level of physical fitness. These factors should be considered independently in order to truly identify those soldiers that at risk of injury. Previous studies have investigated if these risk factors were independently associated with occurring musculoskeletal injuries.9 18 25 26 77 90 97 However, these studies did not determine if these factors were independently associated with musculoskeletal foot and ankle injuries specifically. Future research assessing independent risk factors and their association with foot and ankle injuries would be valuable.
Length of recovery
This study also examined the recovery time for soldiers who sustained a foot or ankle injury. We found that after such injury, the time to return to duty was, on average, 35 days. Similarly, a retrospective study by Schuh-Renner et al found that ankle injuries are one of the top causes of limited duty days and that these injuries lead to an average of 41 limited duty days per injury.98 Significant efforts should be directed towards lowering the number of limited duty days among soldiers in training. Not only do the limited duty days pose a risk of combat unpreparedness but also they present a significant financial cost to the military. Hauschild et al investigated the financial burden of knee injuries in the military, reporting an estimate of 40 million dollars spent in one calendar year in managing 12 706 knee injuries.99 On average, these injuries resulted in about 30 limited duty days. The study noted that this estimate was conservative and that the true cost is likely higher. We would expect to find similar financial costs when examining the management of foot and ankle injuries due to the comparable incidence rates and the number of resulting limited duty days. However, there are currently no studies focused on the economic costs of foot and ankle injuries in the military, again highlighting the need for research focused specifically on foot and ankle. Nevertheless, it is clear that limiting the number of acute ankle injuries would likely yield significant reductions in military spending, given their predominant contribution to the total number of limited duty days.
Parachute training
Due to the nature of parachutist training, injuries of varying severity are common and, moreover, lower extremity injuries are very common.6 7 11 Our data on military parachute training indicated that there are 3.1 injuries per 1000 jumps. Current literature found that lower extremity injuries are the most commonly associated with military parachutist training.89 Studies investigating interventions to prevent injuries, such as ankle braces or new parachute types, have also been conducted, but the benefit of such interventions remains unclear.12 100
Prevention of overuse injury
Our review included several articles assessing overuse injuries in the foot and ankle.3 4 17 24 38 49 68 77 Three studies specifically assessed prevention of overuse injuries in feet3 17 24; however, none of the studies found an intervention which produced significant reduction in injury incidence. The studies examined shock-absorbent insoles, modified basketball shoes, and shoe type and size, but all failed to reduce the number of overuse injuries during military training.3 17 24 This suggests that current standardised treatment strategies to prevent overuse injuries are not effective. Kaufman et al reported that foot structure was linked to a likelihood of suffering from an overuse injury of the foot.4 The study found that patients with pes planus or pes cavus had nearly twice the incidence of stress fractures compared with subjects with average arch height.4 Future studies should assess the effectiveness of insoles specific to these foot types in reducing overuse injuries to the foot and ankle.
Limitations
Our review has a few limitations that must be taken into consideration. Several studies categorised injury data simplified as ‘lower extremity’ without specifying the precise anatomical location of the injury.101 102 These data could not be extracted and therefore limited the total number of foot and ankle injuries described in this study. Conversely, some studies focused only on specific injury types (eg, metatarsal stress fractures), neglecting other injuries to the foot and ankle, which may have occurred during the same training period.25 43 These injuries are therefore over-represented in our data. Included studies also lacked data about population characteristics, such as smoking status, BMI and mechanism of injury. This prevented our ability to compare the impact of these factors on injury incidence. Future studies should be aimed at determining demographic risk factors for foot and ankle injuries among these subgroups. This information is necessary to understand the entire scope of injuries and to begin considering effective preventative strategies which can be targeted to the highest-risk subpopulations.
Conclusion
The current research provides a worldwide epidemiological overview of foot and ankle injuries during military training. Throughout the literature and embedded within our own analysis, it is clear that these injuries present a serious problem in military training. Acute and non-acute foot and ankle injuries continue to disrupt training and increase attrition rates. Our research shows that there has been little success in preventing these injuries but that there is the potential to reduce foot and ankle injuries if more focused approaches are taken in addressing the issue. Future studies assessing trainee risk factors, identifying high-risk training activities and comparing strategies for prevention are necessary to curb this serious problem in military training.
References
Supplementary materials
Supplementary Data
This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.
Footnotes
Contributors BPF, BL and JS were responsible for the design, data acquisition, analysis, interpretation and drafting of the manuscript. JC assisted with the design and data acquisition. GRW, CWD and DG contributed significantly to the design, drafting, editing and review of the manuscript. BPF is responsible for the submission of the article. All authors significantly contributed to the work.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data are available upon reasonable request.
Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.