Fluid shifts due to microgravity and their effects on bone: a review of current knowledge

Bone loss during long-term space flight is potentially a significant problem. Bone mineral density measurements show that bone is lost at a rate of about 1% per month from the lumbar spine, and 1.5% per month from the hip. Bone density changes are not uniform throughout the skeleton, and there is accumulating evidence that the distribution of bone density changes may be related to fluid shifts observed in microgravity. The paper summarises the data relating fluid shift of bone density changes in microgravity. In addition, microgravity and ground-based experiment were carried out to investigate the effects of interstitial fluid flow on bone formation. Data from these experiments were interpreted in light of the review data relating fluid shift to bone density changes in microgravity. Both experiments assessed the effect of an external pneumatic venous tourniquet on bone distal to the tourniquet. In the first experiment, a pneumatic tourniquet was designed to be placed around one ankle of a single astronaut on a 180-day mission on the MIR space station. Ultrasound measurements of both calcanei were made during the mission, and dual energy x-ray densitometry (x-ray absorption) measurements were performed at the start and end of the mission. In the second experiment, ground-based experiments have been performed to investigate possible mechanisms for the action of such a tourniquet on bone. A venous tourniquet was designed to be placed around the hind limb of a rat, proximal to the knee, applying a continuous pressure of 30 mmHg. The tourniquet increased significantly tibial fluid weight, periosteal bone formation and intracortical bone remodelling. Some of the effects appeared to due to a nitric oxide dependant pathway. It is proposed that tissue fluid pressures have an effect on bone that is independent of mechanical load, which may explain the variation in bone mineral density observed in space flight.