Relationships Between Bone Density, Stiffness, And Failure Load At The Distal Tibia: The ARMI Study

PURPOSE: In animals, adaptive bone formation (ABF) at the diaphysis in response to mechanical loading has been associated with disproportionate increases in strength compared to non-loaded controls. Men and women undergoing US Army Basic Combat Training (BCT) have consistently shown ABF at the distal tibia, however, the relationship between ABF and changes in stiffness (S) and estimated failure load (F.ult) are not well understood. The purpose of this analysis was to determine how distal tibial S and F.ult change during BCT and how these changes are related to changes in bone mineral density (BMD) and volume. METHODS: 576 trainees (193 females, 385 males) completed high resolution peripheral quantitative computed tomography (HR-pQCT) scans at 4% of the distal tibia at weeks 1 and 9 of BCT for assessment of BMD and volume. Micro-finite element analysis of HR-pQCT scans estimated S and F.ult. Total group and sex-stratified linear regression models compared percent (%) change in S and F.ult with % change in total (Tt.), trabecular (Tb.), and cortical (Ct.) volumetric BMD and Tb. bone volume fraction (Tb.BV/TV). Models were adjusted for age, race, height, weight, tobacco use, and prior physical activity. The total group model was also adjusted for sex. RESULTS: Trainees were young (mean ± SD, 21.4 ± 3.8 years) and had a mean BMI of 24.8 ± 3.2 kg/m2. On average, S and F.ult increased during BCT by 2.88% and 2.74% (p < 0.01 for both), respectively. In the total group, a 1% increase in Tt.BMD, Tb.BMD, and Tb.BV/TV was associated with a 1.31%-1.79% increase in S and a 1.16%-1.58% increase in F.ult (p < 0.01 for all). Meanwhile, a 1% increase in Ct.BMD was associated with a 3.56% increase in S and a 3.25% increase in F.ult (p < 0.01 for both). Sex stratified analyses yielded similar results such that a 1% increase in Ct.BMD was associated with the greatest increases in S and F.ult in males (3.51% and 3.22%, respectively) and females (3.57% and 3.22%, respectively) compared to other bone parameters. CONCLUSIONS: As expected, S and F.ult increased significantly during BCT. Additionally, the greatest mechanically meaningful changes are associated with improvements in Ct.BMD, despite the thin cortex at the distal tibia. Exploring interventions to improve Ct.BMD has the potential to provide the greatest benefit to mechanical strength in the distal tibia. Supported by: The opinions or assertions contained herein are the private views of the author(s) and are not to be construed as official or as reflecting the views of the Army, the Department of Defense, or the U.S. Government.