Risk factors for bone stress injuries: a follow-up study of 102,515 person-years

The aim of the present study was to assess the risk factors for magnetic resonance imaging (MRI)-detected bone stress injuries in the pelvis, hip, thigh, and knee in a large cohort of Finnish conscripts during a follow-up of 102,515 person-years.

An epidemiologic prospective cohort study of 152,095 conscripts, including 2345 (1.5%) females, was conducted. Localized pain in the pelvis, hip, thigh, or knee resulted in an orthopedic surgeon's consultation and subsequent MRI examination at the Central Military Hospital, Helsinki, Finland. Risk factors were systematically collected from 1998 to 2004, including data on conscripts' physical fitness and body composition measured at the beginning of their military service.

Altogether, 319 MRI-detected bone stress injuries of the pelvis, hip, thigh, or knee were identified in our cohort; thus, the incidence was 311 (95% CI: 277-345) per 100,000 person-years. The female:male ratio varied substantially, depending on the anatomic location of the injury; it was highest for sacral injuries (female:male ratio = 51.1) and lowest for injuries of the femoral condyle (female:male ratio = 0.8). In univariate Cox regression analysis, poor muscle strength and a poor result in a 12-min run were significantly associated with bone stress injuries. In multivariable analysis, the strongest risk factors for bone stress injuries were female gender (hazard ratio 8.2; 95% CI: 4.8-14.2) and higher age (hazard ratio 2.1; 95% CI: 1.4-3.1).

Female military trainees have a highly increased risk of bone stress injuries of the pelvis and hip compared with male conscripts. Sacral stress fractures are typical bone stress injuries in female military recruits. Physicians should remember the possibility of bone stress injury, especially when examining stress-related pain symptoms of the pelvic area in physically active young adult females.