Spaceflight alters bone mechanics and modeling drifts in growing rats

Background: Alterations in bone metabolism may be a particularly serious co nsequence of spaceflight and a major obstacle to long-term space exploratio n. The effects of spaceflight on bone mechanics are unclear. This study exa mined the effects of spaceflight on bone mechanics in a growing rat model d uring a 17-d mission aboard the space shuttle (STS-78). Methods: There were 18 rats that were divided into 3 experimental groups: flight rats (n = 6), ground-based control rats housed in an animal enclosure module (AEM, n = 6 ), and ground-based control rats housed in standard vivarium caging (n = 6) . At the conclusion of the mission, rat femurs were tested in three-point b ending followed by static and dynamic bone histomorphometry. Results: Maxim um stress was unaffected by spaceflight, but flexural rigidity was signific antly decreased in flight animals. Much of the decrease appeared to be the result of decreases in tissue properties (elastic modulus) rather than stru ctural changes within the bone. No significant differences in cortical bone mass or geometry were observed. In contrast, endocortical resorption was s ignificantly decreased in flight rats accompanied by a nonsignificant decre ase in periosteal bone formation, suggesting alterations in bone modeling d rifts during spaceflight. For nearly all measured indices, ground-based AEM rats displayed values intermediate to flight and ground-based vivarium rat s. Conclusions: Spaceflight can impair tissue properties in femoral cortica l bone during growth without significant decreases in bone mass or geometry .