Bone density and pore size distribution were measured by mercury porosimetry in the left humerus, while humerus mechanical properties were evaluated with a standard torsion test machine. Bone formation, mineralization, and resorption rates were determined by quantitative histological techniques using the left tibia, while osteoblastic and osteoclastic cell populations were determined from the right. Length measurements were made with calipers, and correlation, regression, and covariance analyses were made by means of computer programs based on standard statistical methods.

Results:

Space flight had little effect on the bone porosity parameters measured, while the flight and synchronous animals (compared to vivarium controls) did show a significant decrease in bone density immediately postflight. The most striking effects were those on bone formation; all parameters investigated in the flight animals immediately after flight were significantly decreased from both vivarium and synchronous controls. An arrest line was found at both the endosteum and the periosteum of flight animals suggesting that a complete cessation of bone growth occurred during the flight. By 25 days postflight, flight animals showed a significant increase in formation, suggesting that a rebound in bone formation had occurred following flight.