Investigation of bone changes in microgravity during long and short duration space flight: comparison of techniques

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Abstract
Loss of bone mass is a continuing problem in long‐term space flight. Although counter‐measure programmes have been developed, effective assessment of these programmes is hampered by a lack of monitoring techniques that can be used in‐flight. Three techniques were used to evaluate changes in bone during two missions of 180 and 20 days to the MIR space station, involving three subjects. Dual energy X‐ray absorptiometry (DXA) was used before and after flight to measure whole body and regional bone mineral density (BMD). Ultrasonic measurements of velocity (SOS) and broadband attenuation (BUA) of the calcaneus were measured during the 180 day mission and before and after the 20 day mission. Phase velocity of flexural waves in the tibia was also measured on the same days as the ultrasonic measurements of the calcaneus. DXA measurements demonstrated significant variation between different sites in the body for changes in BMD, with the greatest changes occurring in the lumbar spine and proximal femur. There was a trend for increasing phase velocity in the tibia during the 180 mission, but this was not significant. BUA and SOS measurements of the calcaneus showed consistent but divergent patterns of changes during the mission. Although in‐flight measurements of bone using ultrasound or phase velocity may provide information on the kinetics of bone loss in space flight, the heterogeneity of response in the skeleton means that it is difficult to predict overall bone loss from measurements at one particular site.