Timing of stress fracture in soldiers during the first 6 career months: a retrospective cohort study

Abstract

Context: Stress fractures (SF) are injuries that can result from beginning new or higher volume physical training regimens. The pattern of clinical presentation of SF over time after individuals start a new or more demanding physical training regimen is not well defined in medical literature.

Objective: Report trends in the clinical presentation of stress fractures over the first six month of soldiers' time in the service.

Design: Retrospective Cohort study Setting: This study was conducted using medical encounter and personnel data from U.S. Army soldiers during the first 6 months of their career.

Participants: U.S. Army soldiers beginning their careers from 2005-2014 (N=701,027).

Data collection and analysis: Weekly SF numbers and incidence were calculated overall, as well as by sex, over the first 6 months of military service.

Results: SF diagnoses (N=14,155) increased steeply in weeks 3 and 4, with a peak in the overall incidence of SF diagnoses occurring during weeks 5-8. Although clinical incidence of stress fracture generally decreased beyond 8 weeks, incident lower extremity stress fractures continued to present for over 20 weeks. The hazard ratio (HR) for SF among women compared to men was 4.14 (95% CI = [4.01, 4.27]).

Conclusions: Across the 6-month study period, women showed over 4 times greater hazard for stress fracture. The results also suggest that health care providers should be particularly vigilant for stress fractures within 3 weeks after the beginning of a new or higher intensity exercise regimen. The incidence of SF may continue to climb for several weeks. Even as stress fracture incidence declines, it should be noted that these injuries may also continue to appear clinically even several months after a change in activity or training.

Keywords: military; overuse injury; stress fracture; tactical athlete.

Figures

Figure 1

Weekly numbers and incidence of stress fractures (per 1000 soldiers) over the first 26 weeks (6 months) of service in the Army. A, Represents the number of stress fractures per week of service; B, represents the weekly incidence. Numbers and incidences are depicted for all soldiers (solid line) and separately for male (dashed line) and female (dotted line) soldiers.

Figure 2

Survival curve for incident lower extremity stress fractures over the 6-month study period. The hazard ratio for lower extremity stress fractures in women versus men was 4.14, with 95% CI = 4.01, 4.27.