RESUMO
Background: Stress fracture of army recruits usually occur during basic military training. The symptoms of stress fracture are local tenderness and inability to run. Currently, no methods of risk identification are available in spite of many stress fractures. If bone mineral density (BMD), measured by calcaneal quantitative ultrasound (QUS), is related to stress fractures, QUS may be a tool for identifying army recruits at risk.Objectives: To evaluate the relationship between bone mineral density and stress fractures, and to study the incidence of stress fracture and risk factors among Thai army recruits during basic military training.Material and methods: One-thousand two-hundred sixty-three new army recruits were enrolled from different 10 battalions of Thai army in Bangkok. Before the 10-week basic military training, their heels were measured by calcaneal QUS and a risk factor questionnaire was administered. During training, their musculoskeletal injuries were monitored, especially stress fracture. Radiographic examination or a bone scintigraphy was performed in suspect of stress fracture. At the end of the training, they filled-out another questionnaire.Results: The cumulative incidence of stress fracture was 6.57% (95%CI: 5.27, 8.08) and the incidence rate was 1.22 per 1,000 person-days (95%CI: 0.97, 1.51). The Cox proportional hazards model showed that the BMD, measured by calcaneal QUS, had a significant relationship to stress fracture. The lowest quartile (Q₁) of speed of sound (SOS) was significantly related to stress fracture (Hazard ratio (HR) = 3.42; 95%CI: 1.74, 6.75; p-value \< 0.001), history of fracture (HR=2.20; 95%CI: 1.15, 4.21; p-value=0.017), and heavy smoking (HR=2.08; 95%CI: 1.23, 3.50; p-value=0.006). The area under the ROC curve of SOS was 61.1% (95%CI: 54.70, 67.39).Conclusion: The bone mineral density, measured by calcaneal QUS, was significantly related to stress fracture. In the lowest group of SOS measurement, heavy smokers, and recruits with a history of fracture had high risk of stress fracture. The incidence of stress fracture might be some what reduced by the application of adapted basic military training for high-risk recruits.