Comparing Femur Cross‐sectional Morphology in Young Outbred Mice Following Daily Exposure to Muscle‐, Impact‐, or Combined Loading of the Hind Limb Skeleton

Abstract

Limb bones are exposed to a multitude of intrinsic and extrinsic loading forces which they must resist in order to prevent fracture and maintain function. Bone is a plastic tissue which can adapt to encountered loads by increasing cortical thickness in areas of high strain and resorbing cortex in areas of low strain, thereby optimizing strength, weight, and metabolic requirements. While exercises such as running and jumping are known to induce changes in the cross‐sectional dimensions of limb bones, the relative contributions of muscular contractions and extrinsic ground reaction forces (GRF) in altering bone morphology are not fully understood. We compared femur gross and cross‐sectional morphology in mice exposed to four loading environments that differ in the source of forces imposed on the skeleton: swimming (muscle‐dominated), impact loading via falling (GRF‐dominated), running (combined muscle and GRF characteristic of natural locomotion), and sedentary controls. Four‐week‐old male Hsd:ICR mice were randomly assigned to treatment groups and completed loading exercises daily for 20 days. Mice were injected twice with fluorochrome labels during the final week of treatment to allow measurement of mineral apposition rates and regionalization. Undecalcified femoral mid‐diaphyseal sections were photographed using light and fluorescent microscopy and analyzed for cross‐sectional properties and mineral apposition patterns. We found that the falling impact group had the greatest distal femoral width, but the swim group had the greatest width at the mid‐diaphysis. The jump impact group demonstrated significantly greater cross‐sectional area and moments of inertia than controls, indicating increased resistance to torsion and bending compared to other loading regimes. Examination of mineral apposition patterns showed that mineral apposition was greatest posterolaterally in all groups, with additional medial or posterior cortical drift characterizing each loading treatment. These results indicate regional sensitivity of the growth and morphology of the femur to muscle‐ and GRF‐derived loads and suggest that impact loading induces the growth of stronger bones than other forms of exercise. Future analyses will use 3D geometric morphometrics to examine overall shape differences in femur, tibia, and pelvis following loading treatment.Support or Funding InformationUniversity of Missouri School of Medicine