Adaptation of the Proximal Femur to Skeletal Reloading After Long-Duration Spaceflight

Abstract

We studied the effect of re-exposure to Earth's gravity on the proximal femoral BMD and structure of astronauts 1 year after missions lasting 4-6 months. We observed that the readaptation of the proximal femur to Earth's gravity entailed an increase in bone size and an incomplete recovery of volumetric BMD. Bone loss is a well-known result of skeletal unloading in long-duration spaceflight, with the most severe losses occurring in the proximal femur. However, there is little information about the recovery of bone loss after mission completion and no information about effect of reloading on the structure of load-bearing bone. To address these questions, we carried out a study of the effect of re-exposure to Earth's gravity on the BMD and structure of the proximal femur 1 year after missions lasting 4-6 months. In 16 crew members of the International Space Station (ISS) making flights of 4.5-6 months, we used QCT imaging to measure the total, trabecular, and cortical volumetric BMD (vBMD) of the proximal femur. In addition to vBMD, we also quantified BMC, bone volume, femoral neck cross-sectional area (CSA), and femoral neck indices of compressive and bending strength at three time-points: preflight, postflight, and 1 year after mission. Proximal femoral bone mass was substantially recovered in the year after spaceflight, but measures of vBMD and estimated bone strength showed only partial recovery. The recovery of BMC, in the absence of a comparable increase in vBMD, was explained by increases in bone volume and CSA during the year after spaceflight. Adaptation of the proximal femur to reloading entailed an increase in bone size and an incomplete recovery of vBMD. The data indicate that recovery of skeletal density after long-duration space missions may exceed 1 year and supports the evidence in the aging literature for periosteal apposition as a compensatory response for bone loss. The extent to which this compensatory effect protects against fracture remains to be seen.