Category: Ankle, Trauma
Keywords: Distal Tibia, Bone Stress Injuries, Bone Mineral Density
Introduction/Purpose: Repetitive loading during basic combat training (BCT) results in tibial bone adaptation. Men show increases in cortical thickness, trabecular thickness, trabecular volumetric bone density, and cortical bone density during 8-10 weeks of U.S. Army BCT, yet ~5-7% of men sustain bone stress injuries (BSIs) during training. BSIs are a leading cause of BCT attrition.
Sex steroid hormones regulate bone metabolism. While testosterone supports bone formation, work in older men suggests estradiol is a stronger determinant of skeletal health.8 Whether this is true in younger men remains to be studied. We tested whether cumulative estradiol, testosterone, and free testosterone exposure during BCT is associated with favorable changes in distal tibial microarchitecture.
Methods: We collected high resolution peripheral quantitative computed tomography (HR-pQCT) scans at the distal tibia (4% of tibial length) in 67 men at the beginning and end of BCT. We measured serial serum hormones by LCM/MS/MS at weeks 0, 2, 4, 6, 8 and 10. Exposures were log2-means of estradiol, total testosterone, and free testosterone across all timepoints. Outcomes included total bone mineral density (Tt.BMD), cortical BMD (Ct. BMD), Ct area (Ct.Ar), trabecular BMD (Tb.BMD), Tb Thickness (Tb.Th), and Tb spacing (Tb.Sp). We fit ANCOVA models with robust standard errors, coefficients for hormones represent change per exposure doubling. False-discovery rate control (Benjamini–Hochberg) was applied across outcomes.
Results: Participants were 17-28 (mean 20.9 ± 3.7) years old with normal BMI (mean 24.9 ± 3.9 kg/m2). Racial and ethnic diversity was represented: 38% identified as White, 23% as Black, and 39% as Other racial categories. Per-doubling estradiol was associated with higher distal tibial Tt.BMD (β=+2.6 mgHA/cm³, 95% CI 0.2–5.0, p=0.03), greater Ct.Ar (β=+1.13 mm2, 95% CI 0.17–2.10, p=0.02), and greater Tb.vBMD (β=+2.2 mgHA/cm³, 95% CI 0.1-4.3, p=0.04) at the end of BCT, adjusted for baseline bone parameter, age, and BMI. Other microarchitectural indices were null. Adding total or free testosterone did not attenuate estradiol effects, and testosterone measures were not independently associated with bone adaptation.
Conclusion: In male trainees, greater estradiol exposure was associated with greater gains in measures of distal tibial size, density and microarchitecture independent of testosterone. These findings suggest estradiol is a key regulator of bone adaptation in young adult men. Given the modest sample and multiple outcomes, larger studies should confirm these associations and test links to tibial bone stress injury risk.
