Hormonal Contraceptive Use and Stress Fracture Risk during Entry-Level Military Training

Stress fractures (SFx) are a musculoskeletal injury of urgent concern in military SMs, with an incidence rate of 5.7 per 1000 person-years. Female SMs are at even greater risk, with an incidence rate 3x that of males. SFx are particularly common among military recruits undergoing entry-level training; for instance, Knapik et al. reported an incidence rate of 19.3 and 79.9 in per 1000 male and female recruits, respectively, during U.S. Army basic training.  Depending on the site and severity of SFx, rehabilitation can last an average of 27 days and up to 12-21 weeks, requiring a reduction or cessation of training to support recovery from the injury and time to regain fitness, or medical discharge from service. The economic cost of SFx in the military is also staggering. The U.S. Air Force, alone, reported lower extremity SFx during training cost over $4.8 million per year. As such, identifying modifiable risk factors for SFx is key to developing targeted mitigation strategies.

Numerous investigations have sought to characterize SFx risk in athletes and SM, identifying factors such as older age, lower body weight, white race, taller height, caloric and micronutrient deficiencies, low bone mineral density (BMD), lower fitness levels, and more as associated with increased SFx risk.However, one risk factor that is specific to women and remains to be fully understood is the effect of hormonal contraceptive use. Despite demonstrated safety and efficacy of hormonal contraceptives for reproductive health, the effects of all types of hormonal contraception on MSK health and SFx risk remain to be elucidated. Exogenous estrogen and progesterone, the active components of hormonal contraception, can alter bone turnover and, as such, may influence BMD, an important component of bone strength related to SFx risk. Oral contraceptive use (i.e. “the pill”) has been associated with increased, decreased, or no change in BMD in a wide range of female populations (athletes, non-athletes, SM), with variability in effects likely attributable to factors such as age of use, duration of use, formulation/dose, and menstrual function prior to initiation of contraception. However, some evidence in female athletes suggests oral contraceptive use may protect against development of SFx. Alternatively, depot-medroxyprogesterone acetate (DMPA, i.e., “Depot”), the intramuscular injectable contraception, is definitively detrimental for bone health. DMPA use is associated with BMD declines up to 7.5% after two years of use and a study in the mid-1990s demonstrated DMPA use was associated with increased risk of stress fracture in non-Hispanic U.S. Army recruits during eight-week basic training. A recent investigation of British Army recruits studied over 12-week basic military training demonstrated that DMPA use was associated with significantly greater bone turnover compared to oral contraceptive users and non-users, though SFx outcomes were not evaluated. Despite these findings, recent changes to contraception education in the Marine Corps emphasizes DMPA as an attractive contraceptive option since it is long-acting, requiring only semi-monthly administration, and provides good contraceptive efficacy. In addition to DMPA, the U.S. Military Health System offers a wide range of hormonal contraceptive options to SM, including short-acting methods (oral contraceptive pill, transdermal patch, vaginal ring) and long-active reversible contraceptive methods (LARCs; intrauterine device (IUD), subdermal implant, intramuscular injection). However, there have been no studies, to date, comparing the impact of each type of hormonal contraception on SFx risk in military SM, a significant gap in the literature.

This study will make innovative use of existing data from the large-scale Musculoskeletal Outcome Readiness Evaluation (MORE) project housed within the CHAMP at the USU. The dataset comprises data on over three million SM from multiple DoD/MHS resources, including the Defense Manpower Data Center, Military Health System Data Repository, Periodic Health Assessment, and others dating back to 2011. Specifically, training records, prescription information, and ICD-9/10 codes will be queried for hormonal contraception prescription and stress fracture diagnosis information in female SM during entry-level training in all Service branches. Additional demographic (e.g., age, race) and biographic (e.g., height, weight) information relevant to MSK health will also be requested. Making use of the existing dataset is a cost-effective strategy to answer novel questions about a military-wide issue on a large-scale (all female SM during entry-level training), increasing the translatability of the findings to all Service branches.