All articles published in the Journal of Special Operations Medicine are protected by United States copyright law and may not be reproduced, distributed, transmitted, displayed, or otherwise published without the prior written permission of Breakaway Media, LLC. Contact [email protected]. An Ongoing Series Stress Fractures Etiology, Epidemiology, Diagnosis, Treatment, and Prevention Joseph J. Knapik, ScD; Katy Reynolds, MD; Kyle L. Hoedebecke, MD ABSTRACT Stress fractures are part of a continuum of changes in KEYWORDS: stress fracture; risk factors; diagnosis; treatment healthy bones in response to repeated mechanical defor- mation from physical activity. If the activity produces excessive repetitive stress, osteoclastic processes in the Introduction bone may proceed at a faster pace than osteoblastic processes, thus weakening the bone and augmenting Stress fractures are part of a continuum of changes in the susceptibility to stress fractures. Overall stress fracture bone in response to mechanical stress. The continuum incidence is about three cases per 1,000 in active duty spans events from normal bone remodeling to a frank Servicemembers, but it is much higher among Army ba- fracture. Figure 1 is a summary of terms used to describe sic trainees: 19 per 1,000 for men and 80 per 1,000 for this spectrum, obtained from a review of various terms women. Well-documented risk factors include female used in the literature1–4 and clinical experience. The term sex, white ethnicity, older age, taller stature, lower aero- “stress reactions” has been used to identify conditions bic fitness, prior physical inactivity, greater amounts of where early changes occur in the bone and the Soldier current physical training, thinner bones, cigarette smok- experiences pain symptoms, but there is no evidence of ing, and inadequate intake of vitamin D and/or calcium. bone lesions on imaging. Stress fractures usually refer to Individuals with stress fractures present with focal ten- conditions where there are distinct partial or incomplete derness and local pain that is aggravated by physical fracture lines on imaging. The term “bone stress injury” activity and reduced by rest. A sudden increase in the has been used to refer to the portion of the continuum volume of physical activity along with other risk factors (Figure 1) encompassing both stress reactions and stress is often reported. Simple clinical tests can assist in diag- fractures. nosis, but more definitive imaging tests will eventually need to be conducted if a stress fracture is suspected. Figure 1 Spectrum of bone changes in response to repetitive stress. Plain radiographs are recommended as the initial im- aging test, but magnetic resonance imaging has higher Bone Stress Injury sensitivity and is more likely to detect the injury sooner. Normal Bone Stress Stress Frank Treatment involves first determining if the stress frac- Remodeling Reaction Fracture Fracture ture is of higher or lower risk; these are distinguished by anatomical location and whether the bone is loaded in Increasing Physical Damage tension (high risk) or compression (lower risk). Lower- risk stress fractures can be initially treated by reduc- ing loading on the injured bone through a reduction in Bone stress injuries are typically associated with ha- activity or by substituting other activities. Higher-risk bitual mechanical deformation of the bone. The injury stress fractures should be referred to an orthopedist. In- generally occurs in individuals with otherwise healthy vestigated prevention strategies include modifications to bones, often in association with unaccustomed but re- physical training programs, use of shock absorbing in- petitive physical activity.5–8 This type of fracture is often soles, vitamin D and calcium supplementation, modifi- referred to as a “fatigue fracture” and is to be distin- cations of military equipment, and leadership education guished from “insufficiency fractures,” which are par- with injury surveillance. tial or incomplete bone ruptures due to loading on bone 120 All articles published in the Journal of Special Operations Medicine are protected by United States copyright law and may not be reproduced, distributed, transmitted, displayed, or otherwise published without the prior written permission of Breakaway Media, LLC. Contact [email protected]. that has become osteopenic with aging or disease (e.g., hypervitaminosis A, sarcoma). Insufficiency fractures may occur spontaneously in association with normal loading or minimal trauma that would not result in a fracture in healthy bones.9–14 Soldiers involved in heavy physical activity may be at risk for stress fractures, especially if the activity involves sudden increase in frequency, duration, or intensity. Re- Figure 2 Structure of a covery times can be extended, negatively affecting both long bone. Soldier and unit effectiveness for a considerable time.15–18 This article covers the etiology, epidemiology, diagnosis, treatment, and prevention of fatigue fractures, which will hereafter be referred to as stress fractures. Etiology Bone will bend when subjected to impact forces induced Obtained from https://commons.wikimedia.org/wiki/File:Structure_of by activities like running or foot marching with loads. _a_Long_Bone.png They will return to their original configuration when the impact forces are reduced or removed. The repetitive 2007 reported incidences of 19 cases per 1,000 Service- bending results in an increase in cyclic hydrostatic pres- members (SMs) for men and 80 cases per 1,000 SMs sures that are sensed by osteocytes within the bone ma- for women.28 The incidence among active duty military trix. Mechanical pressures stimulate osteocyte-directed personnel from 2003 to 2012 was 2.7 cases per 1,000 transcription of osteoclasts, which begin the bone remod- SMs.29 During the Central Burma campaign in the Sec- eling process. If the repetitive mechanical stress is applied ond World War, 80 stress fracture cases were reported gradually over time (i.e., progressive overload), normal in one 7,000-man infantry unit (1.1% incidence) during bone remodeling occurs. That is, osteoclastic processes, a long road march.33,34 Early in the study of this injury, which resorb bone, approximately balance osteoblastic stress fractures were termed “march fracture” because processes, which form new bone. However, if the repeti- Soldiers would often present with this injury after road tive stress is applied in a relatively short time, as with a marching35–38; however, cases emerged in which the on- long foot march with a heavy load for which the Soldier set was insidious and the injury could not be linked to has not sufficiently trained, osteoclastic processes may a single event.35,37,39 Breithaupt is credited with the first proceed at a faster pace than osteoblastic processes. This description of the condition in Prussian Army Soldiers.37 imbalance produces a vulnerable period when the bone is During the Second World War, cases and case series be- weakened and susceptible to rupture.5,19,20 gan to appear in the literature in both British35,39 and American33 36–38,40 military training. Stress fractures appear more likely to occur where bone remodeling is slower. Areas of slow remodeling include We obtained data on ambulatory (outpatient) stress frac- compact (cortical) bone, the long bone diaphysis (Figure tures directly from the Defense Medical Epidemiology 2), and bodies of cuboid-shaped bones like the vertebrae, Database (http://www.health.mil/Military-Health-Topics tarsals, and carpals. In spongy (cancellous) bone where /Health-Readiness/Armed-Forces-Health-Surveillance- more active remodeling occurs, there is a lower likeli- Branch/Data-Management-and-Technical-Support/ hood of stress fractures. Cancellous bone is contained Defense-Medical-Epidemiology-Database) for the Army, in the metaphysis and epiphysis of both long and square Navy, Marine Corps, and Air Force. The data were visits bones (Figure 2). Cancellous bone has a metabolic turn- to medical care providers with International Classification over rate eight times faster than cortical bone.3,4,12,21 of Diseases, Ninth Revision codes of 733.93 to 733.98 (the codes for stress fractures at various anatomical loca- tions), diagnosed from 2006 through 2014. The overall Epidemiology rate was 17.3 visits per 1,000 SMs. Data by year, sex, age, and military service are shown in Figure 3. Rates tended Incidence to decrease in the period examined (Figure 3A). Military Stress fractures are a commonly encountered injury in Servicewomen had 3.6 times the stress fracture rate of athletics, among military recruits22–29 and among trained military Servicemen (Figure 3B) and rates decreased with military personnel.29–32 A study that examined the entire age (Figure 3C). The Marine Corps and Army had the population of US Army basic trainees from 1997 to highest rates and the Navy had the lowest (Figure 3D). Stress Fractures 121 All articles published in the Journal of Special Operations Medicine are protected by United States copyright law and may not be reproduced, distributed, transmitted, displayed, or otherwise published without the prior written permission of Breakaway Media, LLC. Contact [email protected]. Figure 3 Rates and factors associated with stress fractures bone section modulus and a lower bone strength index in US military personnel. (A) Rates from 2006 to 2015. (ratio of section modulus to bone length) than men.63 (B) Association with sex. (C) Association with age. Female bones are thinner and narrower, which provides (D) Association with military service. AF, Air Force; MC, less bone strength.63,64 With regard to anatomical dif- Marine Corps. ferences, women have wider pelves,80 which result in a varus tilt of the hip and a larger bicondylar angle,81 which places greater stresses on the hips and lateral as- pects of the knee and lower leg during physical activ- ity. Both men and women with wider pelves have higher stress fracture incidence, suggesting that a wider pelvis alone (regardless of sex) increases injury risk.63 The association between age and stress fractures may be related to changes in bone with aging.