Take-Home Points
- Stress injuries, specifically of the lower extremity, are very common in new military trainees.
- Stress injury can range from benign periosteal reaction to displaced fracture.
- Stress injury should be treated on a case-by-case basis, depending on the severity of injury, the location of the injury, and the likelihood of healing with nonoperative management.
- Modifiable risk factors such as nutritional status, training regiment, and even footwear should be investigated to determine potential causes of injury.
- Prevention is a crucial part of the treatment of these injuries, and early intervention such as careful pre-enrollment physicals and vitamin supplementation can be essential in lowering injury rates.
Bone stress injuries, which are common in military recruits, present in weight-bearing (WB) areas as indolent pain caused by repetitive stress and microtrauma. They were first reported in the metatarsals of Prussian soldiers in 1855.1 Today, stress injuries are increasingly common. One study estimated they account for 10% of patients seen by sports medicine practitioners.2 This injury most commonly affects military members, endurance athletes, and dancers.3-5 Specifically, the incidence of stress fractures in military members has been reported to range from 0.8% to 6.9% for men and from 3.4% to 21.0% for women.4 Because of repetitive vigorous lower extremity loading, stress fractures typically occur in the pelvis, femoral neck, tibial shaft, and metatarsals. Delayed diagnosis and the subsequent duration of treatment required for adequate healing can result in significant morbidity. In a 2009 to 2012 study of US military members, Waterman and colleagues6 found an incidence rate of 5.69 stress fractures per 1000 person-years. Fractures most frequently involved the tibia/fibula (2.26/1000), followed by the metatarsals (0.92/1000) and the femoral neck (0.49/1000).6 In addition, these injuries were most commonly encountered in new recruits, who were less accustomed to the high-volume, high-intensity training required during basic training.4,7 Enlisted junior service members have been reported to account for 77.5% of all stress fractures.6 Age under 20 years or over 40 years and white race have also been found to be risk factors for stress injury.6
The pathogenesis of stress injury is controversial. Stanitski and colleagues8 theorized that multiple submaximal mechanical insults create cumulative stress greater than bone capacity, eventually leading to fracture. Johnson9 conducted a biopsy study and postulated that an accelerated remodeling phase was responsible, whereas Friedenberg10 argued that stress injuries are a form of reduced healing, not an attempt to increase healing, caused by the absence of callous formation in the disease process.
Various other nonmodifiable and modifiable risk factors predispose military service members to stress injury. Nonmodifiable risk factors include sex, bone geometry, limb alignment, race, age, and anatomy. Lower extremity movement biomechanics resulting from dynamic limb alignment during activity may be important. Cameron and colleagues11 examined 1843 patients and found that those with knees in >5° of valgus or >5° of external rotation had higher injury rates. Although variables such as sex and limb alignment cannot be changed, proper identification of modifiable risk factors can assist with injury prevention, and nonmodifiable risk factors can help clinicians and researchers target injury prevention interventions to patients at highest risk.
Metabolic, hormonal, and nutritional status is crucial to overall bone health. Multiple studies have found that low body mass index (BMI) is a significant risk factor for stress fracture.7,12,13 Although low BMI is a concern, patients with abnormally high BMI may also be at increased risk for bone stress injury. In a recently released consensus statement on relative energy deficiency in sport (RED-S), the International Olympic Committee addressed the complex interplay of impairments in physiologic function—including metabolic rate, menstrual function, bone health, immunity, protein synthesis, and cardiovascular health—caused by relative energy deficiency.14 The committee stated that the cause of this syndrome is energy deficiency relative to the balance between dietary energy intake and energy expenditure required for health and activities of daily living, growth, and sporting activities. This finding reveals that conditions such as stress injury often may represent a much broader systemic deficit that may be influenced by a patient’s overall physiologic imbalance.
Diagnosis
History and Physical Examination
The onset of stress reaction typically is insidious, with the classic presentation being a new military recruit who is experiencing a sudden increase in pain during physical activity.15 Pain typically is initially present only during activity, and is relieved with rest, but with disease progression this evolves to pain at rest. It is crucial that the physician elicit the patient’s history of training and physical activity. Hsu and colleagues7 reported increased prevalence of overweight civilian recruits, indicating an increase in the number of new recruits having limited experience with the repetitive physical activity encountered in basic training. Stress injury should be suspected in the setting of worsening, indolent lower extremity pain that has been present for several days, especially in the higher-risk patient populations mentioned. Diet should be assessed, with specific attention given to the intake of fruits, vegetables, and foods high in vitamin D and calcium and, most important, the energy balance between intake and output.16 Special attention should also be given to female patients, who may experience the female athlete triad, a spectrum of low energy availability, menstrual dysfunction, and impaired bone turnover (high amount of resorption relative to formation). A key part of the RED-S consensus statement14 alerted healthcare providers that metabolic derangements do not solely affect female patients. These types of patients sustain a major insult to the homeostatic balance of the hormones that sustain adequate bone health. Beck and colleagues17 found that women with disrupted menstrual cycles are 2 to 4 times more likely to sustain a stress fracture than women without disrupted menstrual cycles, making this abnormality an important part of the history.
Examination should begin with careful evaluation of limb alignment and specific attention given to varus or valgus alignment of the knees.11 The feet should also be inspected, as pes planus or cavus foot may increase the risk of stress fracture.18 Identification of the area of maximal tenderness is important. The area in question may also be erythematous or warm secondary to the inflammatory response associated with attempted fracture healing. In chronic fractures in superficial areas such as the metatarsals, callus may be palpable. Although there are few specific tests for stress injury, pain may be reproducible with deep palpation and WB.
If a femoral fracture is suspected, the fulcrum test can be performed by applying downward pressure on the patient’s knee while levering the thigh over the examiner’s opposite arm or thigh (Figure 1).19 Patients with sacral stress fractures may have pain when standing or hopping on the affected side (positive flamingo test).20Laboratory Testing
When a pathology is thought to have a nutritional or metabolic cause, particularly in a low-weight or underweight patient, a laboratory workup should be obtained. Specific laboratory tests that all patients should undergo are 25-hydroxyvitamin D3, complete blood cell count, and basic chemistry panel, including calcium and thyroid-stimulating hormone levels. Although not necessary for diagnosis, phosphate, parathyroid hormone, albumin, and prealbumin should also be considered. Females should undergo testing of follicle stimulating hormone, luteinizing hormone, estradiol, and testosterone and have a urine pregnancy test. In patients with signs of excessive cortisone, a dexamethasone suppression test can be administered.21 In males, low testosterone is a documented risk factor for stress injury.22
Imaging
Given their low cost and availability, plain radiographs typically are used for initial examination of a suspected stress injury. However, they often lack sensitivity, particularly in the early stages of stress fracture development (Figure 2).
Although a fracture line or callus formation is present occasionally, findings may be subtler. Images should be inspected for blunting of cortical bone and periosteal reaction, which should be correlated with the site of maximal tenderness.11 When there is high clinical suspicion based on history and physical examination, but radiographs are negative, magnetic resonance imaging (MRI) or bone scan can be useful.23 MRI is the most accurate imaging modality, with sensitivity ranging from 86% to 100% and specificity as high as 100%.2,24,25 On MRI, stress fractures typically are seen as bright areas of increased edema. Arendt and Griffiths24 proposed an MRI-based grading system for stress fractures, with grades 1 and 2 representing low-grade injuries, and 3 and 4 representing high grade. Computed tomography (CT) also has a role in diagnosis and may be better than MRI in imaging stress fractures in the pelvis and sacrum.2 In a study involving tibial stress fractures, Gaeta and colleagues26 found MRI was 88% sensitive and 100% specific and had a positive predictive value of 100%, and CT was 42% sensitive and 100% specific and had a positive predictive value of 100%. They concluded MRI was superior to CT in the diagnosis of tibial stress fractures.Management
Management of bone stress injury depends on many factors, including symptom duration, fracture location and severity, and risk of progression or nonunion (Table).13
Patients thought to have an underlying metabolic or nutritional derangement should be treated accordingly. Injuries with a low risk of nonunion or further displacement typically can be managed with a period of modified physical activity or reduced or non-WB (NWB) ambulation; higher risk injuries may require operative intervention.5