Stress fractures: Lessons from military research

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Lower extremity stress fractures are not limited to members of the armed forces, but have been studied extensively in military populations. That body of evidence has important implications for stress fracture prevention and management in runners, other athletes, and even nonathletes.

By Baris K. Gun, DO; Andrew C. McCoy, DPM; Kevin C. Wang, BS; and Brian R. Waterman, MD

In 1855, the Prussian military physician Breithaupt described a condition that he called “march foot” in soldiers who participated in long road marches or periods of sustained training.¹ This was later identified as the metatarsal stress fracture, which continues to plague us today, constituting an estimated 10% of all sports medicine injuries in athletic populations.² Although rarely occurring in other locations, such as ribs or the axial spine; 99% of stress fractures occur in the lower extremities.²

Stress fractures are frequently reported to affect distance runners and military recruits, but these injuries can affect any athletes, including those who participate in ballet, volleyball, rowing, basketball, and gymnastics. Although stress fractures in an athletic population occur more commonly in the athletic population compared with the .5% incidence rate reported in the general population, the exact incidence rate in athletes has not been well defined.³

However, stress fractures in military recruits—another population with high levels of physical demand—have been studied extensively. In the largest study to date, examining an at-risk US military population of 5,580,875 service members between 2009 and 2012, participants experienced 5.69 stress fractures of the lower extremity per 1000 person-years.⁴ This risk is especially heightened with sudden increases in physical demand, such as military boot camp. In a 1999 study of 1296 randomly selected male Marine recruits, 4% were found to have incurred a stress fracture during basic training.⁵

The economic cost to the military is profound, with some soldiers undergoing prolonged treatment requiring a medical separation board, during which their status must come under review for possible separation (discharge) from active military service. Individuals who sustain stress fractures in basic combat training are removed from training and placed on rehab for an average of 62 days.⁶ In fact, the single most powerful medical predictor of discharge during Marine basic training is a stress fracture, with a four-fold higher rate of discharge in soldiers who suffer from a stress fracture, compared with those without this injury.⁷ Given the analogous demands required of many military recruits during basic training, including repetitive, high-impact activity, this finding is likely broadly applicable to military trainees in the US Army and Marines, two branches of the military service with higher levels of physicality than other branches, though the stress fracture-related rate of discharge may not be as high in the US Air Force or Navy.

These injuries aren’t restricted to elite athletes or military recruits. They also can be seen in anyone who increases high-impact training, such as running on hard surfaces or too rapidly, and have been reported following orthopedic procedures, including bunion­ectomy, metatarsophalangeal joint arthroplasty, and extensive rearfoot-ankle fusion.⁸ Early identification and surgical intervention is key for “high-risk” fracture sites, which include the medial malleolus, talus, navicular bone, base of fifth metatarsal, and hallux sesamoids. These high-risk fractures are less likely to heal with rest and activity de-escalation alone.⁹

Stress fractures are caused by an overuse injury in otherwise healthy bones. Repetitive mechanical loading causes increased hydrostatic pressure.¹⁰ Bone, being a dynamic tissue in accordance with Wolff’s law, remodels in response to physiologic stress. However, osteoclastic resorption reaches its peak at three weeks, sooner than osteoblastic bone formation, which can take roughly three months to form normal lamellar layers.^11,12 This increase of bone resorption relative to bone formation is associated with partial or incomplete bone rupture, which can later be diagnosed by the “dreaded black line” on radiograph, magnetic resonance imaging (MRI), or bone scan.¹³

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Nonmodifiable risk factors

Intrinsic risk factors for the development of stress fractures have been studied extensively.  Studies focusing on military populations have identified female gender, initial entry to service, increasing chronological age, white race, enlisted rank, Army or Marines branch of service, increased bone turnover, anatomic malalignment, and decreased tissue or bone vascularity as risk factors.^4,13  Specific to the foot and ankle, intrinsic factors include a high longitudinal arch, leg-length discrepancy, and excessive forefoot varus.⁹

In a 2013 US Military Academy study, the cumulative four-year incidence of stress fracture was 5.7% for male cadets and 19.1% for female cadets.¹⁴ Among all active duty service members, women are more than three times as likely to suffer a lower extremity stress fracture and more than seven times more likely to suffer a femoral neck stress fracture than men.⁴

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There are many reasons why women may be at greater risk of lower extremity stress fracture than men. Anatomically, female bone geometry, with a smaller and narrower tibia than in men, and sex-specific differences in bone microarchitecture, with lower density and greater trabecular volume in women than in men, contribute to an increased risk for these injuries.¹⁵ It does appear, however, that taller and heavier men in military service have a higher risk of stress fracture than taller and heavier women, which may be attributed to the greater bending strain experienced in long bones of men relative to women of comparable height and weight percentiles.^16,17 Military studies also have shown female service members have lower average physical fitness than their male counterparts, which is also associated with increased injury rates.^18,19

Extremes of chronological age are also associated with an increased risk of stress fractures. Teenaged service members are more than three times as likely to have a lower extremity stress fracture compared with those aged 20 to 24 years, and those older than 40 years are nearly six and a half times as likely.⁴

This bimodal distribution has multiple likely explanations. Young recruits may be more likely to suffer from stress fractures given their recent introduction to the intense, high-impact demands of boot camp that can be in stark contrast to a less-active, premilitary lifestyle. As service members age, the risk of fractures may decrease as they become more accustomed to the physical rigors of military life. Additionally, bone mass peaks in young adulthood (around age 25 years for men and a slightly younger age in women).²⁰ After this peak, a slowly increasing rate of bone loss, primarily trabecular, begins, which may explain the increased rate of stress fractures in older service members.¹⁶

White individuals in the military are one and half times more likely to have a stress fracture than their African American counterparts.^4,21 This may be due to several factors, including bone mineral density and inherent bone geometry, which contribute to greater mechanical strength in African Americans.^16,22-24

The physical demands placed on junior enlisted military members, compared with more senior-ranking and officer service members, as well as the physicality of the Army and Marines branches of service, relative to comparable populations in the Navy and Air Force, may account for increases in stress fracture incidence.⁴

Modifiable risk factors

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To curtail the burden of stress fractures, a large portion of the medical and public health community has focused on modifiable risk factors. A plethora of modifiable risk factors, including general physical fitness, muscle endurance, bone density, training regimen, dietary profile, footwear, training surface, and steroid, tobacco, or alcohol abuse, have been established for all individuals experiencing high-impact activity, and these findings have been applied to the military population.^10,13,25 

Although many modifiable risk factors are multifactorial, a sedentary period followed by a sudden increase in activity appears to be one of the strongest contributors to the development of stress fractures. Nowadays, many recruits have a sedentary lifestyle or a relatively low level of physical fitness prior to initiating basic training, which can put them at risk for stress fracture. The majority of stress fractures in the 2013 Cosman et al study¹⁴ of West Point cadets occurred in the first three months of the four-year study. Cadets who exercised less than seven hours per week in the year prior to matriculation were at double the risk of developing a stress fracture compared with those who worked out more than an hour a day.¹⁴ One reason for this is that lower levels of fitness result in greater relative effort, using a greater proportion of maximum strength, which results in higher perceived effort, more rapid fatigue, and consequently poorer form and mechanics—all of which can contribute to injury.¹⁶

People with extremely high or low body mass index (BMI) are at heightened risk for stress fractures, although interestingly, in the military, underweight recruits appear to have the greatest risk for tibia and fibula stress fractures, whereas in the civilian population increased risk is associated with obesity.^26-28 Many orthopedic injuries are associated with a similar bimodal BMI injury distribution, as increased weight causes increased loads through a patient’s musculoskeletal system, and lower than normal BMI is associated with poor bone density.²⁹ It has been demonstrated that overweight football players are 19 times more likely to suffer a noncontact ankle sprain than those with a normal BMI due to the increased instantaneous joint loads and angular momentum;³⁰ these can also contribute to overuse injuries over time. Repetitive motion injuries, including stress fractures, are often associated with activities like ruck marching, where the same standard-weight sack represents a larger percentage increase of total cyclical load in a small or underweight person than in a heavier person. Additionally, entrance to military service requirements screen out individuals with very high BMIs.³¹

Deficiencies in calcium and vitamin D levels have been investigated and increase risk for the development of stress fractures.³² Supplementation is an effective treatment for these risk factors. Lappe et al demonstrated that an addition of 2000 mg of calcium and 800 IU of vitamin D supplementation per day in female military recruits led to a 20% reduction in stress fracture incidence.²⁹

Long-term alcohol use and high-volume consumption have been associated with low bone mass in general, and a higher incidence of stress fractures in women.^33,34 Smoking (currently or in the past) has been associated with a negative impact on bone health and an increased incidence of stress fractures; higher numbers of packs per day and more years of smoking can further increase the relative risk of stress fractures.^33-35

Prevention

Adjusted training schedules and individualized risk stratification can successfully reduce stress fracture risk.^22,36 Intensity of training should gradually be increased over a number of weeks, with high-strain sport-specific activities beginning approximately six weeks after graduated training.¹¹ Early trials of physical readiness training programs and individualized injury-prevention programs have shown promise, and these programs may eventually be employed for higher-risk service members.^37,38 Although the Air Force issues soldiers running shoes for physical training and is the service branch with the lowest incidence of stress fractures, there is limited evidence that shock-absorbing insoles prevent stress fractures.¹³ Activity-specific athletic shoes should be in good condition and replaced every 200 to 300 miles.¹¹

The relationship between stress fractures and BMI in the military population may come from a number of causative factors, and these should all be considered in an effort to reduce the BMI-specific risk for stress fractures. Lower-BMI service members may demonstrate decreased lean muscle mass or nutritional deficiencies, two problems that can lead to decreased bone mass or increased risk for bone injury.²⁸ Higher-BMI individuals subject their skeletons to higher biomechanical loading during high-impact exercise and should be encouraged to lose excess weight to reduce the risk of stress fractures. Thus, BMI may dictate the physical fitness goals of military service members, with those with lower BMI focusing on increasing strength and lean body mass with resistance training and adequate nutrition, and those with high BMI focusing on reducing caloric intake and body fat percentage.

Calcium and vitamin D supplementation have been widely promoted for prevention of fragility fractures, especially when a nutritional deficiency exists.²⁹ Further studies have shown that serum vitamin D levels greater than 40 ng/mL are associated with a lower risk of stress fractures and are recommended for stress fracture prophylaxis.^39,40 Oral contraceptive pills, taken by women with hypo­estrogenic amenorrhea, have not proven in longitudinal prospective studies to increase bone mineral density or to decrease stress fracture risk.⁴¹

A thorough assessment of lifestyle factors should be included in any stress fracture prevention physical exam. Examinations of patient exercise routines and frequency, as well as alcohol and tobacco consumption, should be an integrated part of stress fracture prevention.

Conclusion

Lower extremity stress fractures are classically associated with the military and elite athletes; however, they can affect anyone, especially those who experience a rapid increase in repetitive loading exercises. Early diagnosis, through clinical suspicion and advanced imaging, are critical to reducing the morbidity associated with stress fractures.

Individual risk factors are broken down into intrinsic and modifiable. Established intrinsic risk factors include female gender, increasing chronological age, white race, positions in the military with increased physical demands, and anatomical and pathologic disease processes. Modifiable risk factors include the aforementioned increases in training regimen, general physical fitness, dietary profile, and steroid, tobacco, or alcohol abuse. Adjusted training schedules and individualized risk stratification are key to reducing the burden of these injuries.

Baris K. Gun, DO, is an army physician stationed at Ft. Hood, TX, with a special interest in orthopedic surgery and sports medicine, especially as it relates to an active military population. Andrew C. McCoy, DPM, is a podiatric medicine and surgery resident at Jackson Memorial Hospital in Miami, FL. Kevin C. Wang, BS, is a fourth-year medical student at Northwestern University’s Feinberg School of Medicine in Chicago. Brian R. Waterman, MD, is an orthopedic surgeon specializing in sports medicine and shoulder and elbow care at Wake Forest University in Winston-Salem, NC.

Author’s note: The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the US military or the Department of Defense.

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