Recovery of spaceflight-induced bone loss: Bone mineral density after long-duration missions as fitted with an exponential function

Abstract

The loss of bone mineral in NASA astronauts during spaceflight has been investigated throughout the more than 40 years of space travel. Consequently, it is a medical requirement at NASA Johnson Space Center (JSC) that changes in bone mass be monitored in crew members by measuring bone mineral density (BMD) with dual-energy x-ray absorptiometry (DXA) before and after flight on astronauts who serve on long-duration missions (4-6 months). We evaluated this repository of medical data to track whether there is recovery of bone mineral that was lost during spaceflight.Our analysis was supplemented by BMD data from cosmonauts (by convention, a space traveler formally employed by the Russia Aviation and Space Agency or by the previous Soviet Union) who had also flown on long-duration missions. Data from a total of 45 individual crew members -a small number of whom flew on more than one mission -were used in this analysis.Changes in BMD (between 56 different sets of pre-and postflight measurements) were plotted as a function of time (days after landing). Plotted BMD changes were fitted to an exponential mathematical function that estimated: i) BMD change on landing day (day 0) and ii) the number of days after landing when 50% of the lost bone would be recovered ("50% recovery time") in the lumbar spine, trochanter, pelvis, femoral neck and calcaneus. In sum, averaged losses of bone mineral after long-duration spaceflight ranged between 2-9% across all sites with our recovery model predicting a 50% restoration of bone loss for all sites to be within 9 months.Recovery of spaceflight-induced bone loss 3

Trabecular bone recovers from mechanical unloading primarily by restoring its mechanical function rather than its morphology

“…However, once mechanical loads are reintroduced to skeletal extremities during reambulation, recovery of bone mass is often slow and incomplete. If full recovery is accomplished, it requires several times the duration of unloading [16][17][18]. During the initial phase of reambulation, bone mass may even continue to deteriorate in humans [16,19] and rodents [20], further compromising bone's structural integrity at a time when mechanical support is critical to withstand reloading of the skeleton.…”

Effects of oriented substrates on cell morphology, the cell cycle, and the cytoskeleton in Ros 17/2.8 cells

“…Human bone loss during spaceflight is up to 2% bone mass per month with exposure to a space microgravity environment [1,2]. Bone-forming osteoblasts are vital factors in mechanosensing and mechanotransduction in vivo, and the biological responses of osteoblasts in altered gravity provide basic understanding of human bone loss in space.…”

Adaptation of the Skeletal System During Long-Duration Spaceflight

“…The derivation of a "half-life" index provided a time point (days after landing) which represented the timing of 50% restoration of BMD with Table 3 summarizes the "half-lives" and the losses at the time of landing for the skeletal sites evaluated for recovery. In spite of the large variability in the BMD measurements, and the uncertainty in half-life values (generally 3-9 months dependent upon skeletal site), the asymptotic increase in BMD over the post-flight period was clearly apparent and provided the basis for substantial recovery at ~ 4 times the half-life [33].…”

“…Recently, a novel method of analyzing areal BMD has been reported that characterizes postflight skeletal recovery [33]. BMD measurements have been accumulated over a post-flight period lasting as long as five years.…”