Mechanical loading attenuates bone loss due to immobilization and calcium deficiency

Our purpose was to determine the effects of a mechanical loading interventi on on mass, geometry, and strength of rat cortical bone during a period of disuse concurrent with calcium deficiency (CD). Adult female rats were assi gned to unilateral hindlimb immobilization, immobilized-loaded, or control (standard chow 1.85% calcium) treatments. Both immobilized groups were fed a CD rat chow (0.01% calcium) to induce high bone turnover. Three times wee kly, immobilized-loaded rats were subjected to 36 cycles of 4-point bending of the immobilized lower leg. After 6 wk, the immobilized rats exhibited d ecreased tibial shaft bone mineral density (-12%), ultimate load (-19%), an d stiffness (-20%; tested in S-point bending to failure) vs. control rats. Loading prevented this decline in bone density and attenuated decreases in ultimate load and stiffness. Elastic modulus was unaffected by disuse or lo ading. Bone cross-sectional area in the immobilized-loaded rats was equival ent to that of control animals, even though endocortical resorption continu ed unabated. On the medial periosteum, percent mineralizing surface doubled vs. that in immobilized rats. This loading regimen stimulated periosteal m ineralization and maintained bone mineral density, thereby attenuating the loss :in bone strength incurred with disuse and concurrent calcium deficien cy.