Bone loss during simulated weightlessness: a biomechanical and mineralization study in the rat model

Abstract

Background: Astronauts exposed to weightlessness for extended periods experience significant decreases in bone mineral density. The clinical implications of this demineralization are not entirely clear, and the biomechanics involved are not completely understood.

Hypothesis: Local (rather than global) measurements of geometry and calcium concentration effectively predict femur strength in adult rats exposed to a hind-limb suspension model of weightlessness.

Methods: Female Fischer rats (6-mo-old) were divided into groups of control and hind-limb-suspended animals. Animals were sacrificed after 2 or 4 wk of hind-limb suspension, and both femurs removed from each animal. The 3-point bending strength and total bone mineralization were determined for one femur from each animal, and the mid-shaft cross-sectional geometrical properties and distribution of calcium were determined for the contralateral femur.

Results: Although suspension led to significant decreases in total bone mineralization, the concentration of calcium at the anterior periosteal surface was unaffected. Total bone percent mineralization was not well correlated with structural properties, but bone geometrical properties (particularly cross-sectional moment of inertia and length) correlated strongly with ultimate bending strength (r2 = 0.81). Differences in bone geometry due to suspension were consistent with a distribution of bone material closer to the axis of the femur.

Conclusions: Structural properties of bone are predicted well by bone geometry and poorly by total bone percent mineralization. Decreased bone mechanical strength in this model of weightlessness is primarily due to a distribution of bone material nearer the axis of the bone.