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

Background: Astronauts exposed to weightlessness for extended periods exper ience significant decreases in bone mineral density. The clinical implicati ons of this demineralization are not entirely clear, and the biomechanics i nvolved are not completely understood. Hypothesis: Local (rather than globa l) measurements of geometry and calcium concentration effectively predict f emur strength in adult rats exposed to a hind-limb suspension model of weig htlessness. Methods: Female Fischer rats (6-mo-old) were divided into group s 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 anim al, The 3-point bending strength and total bone mineralization were determi ned. for one femur from each animal, and the mid-shaft cross-sectional geom etrical properties and distribution of calcium were determined for the cont ralateral 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 no t well correlated with structural properties, but bone geometrical properti es (particularly cross-sectional moment of inertia and length), correlated strongly with ultimate bending strength (r(2) = 0.81). Differences in bone geometry due to suspension were consistent with a distribution of bone mate rial 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 weightl essness is primarily due to a distribution of bone material nearer the axis of the bone.