Cortical and Trabecular Bone Mineral Loss From the Spine and Hip in Long-Duration Spaceflight

Abstract

We measured cortical and trabecular bone loss using QCT of the spine and hip in 14 crewmembers making 4-to 6-month flights on the International Space Station. There was no compartmentspecific loss of bone in the spine. Cortical bone mineral loss in the hip occurred primarily by endocortical thinning. Introduction:In an earlier study, areal BMD (aBMD) measurements by DXA showed that cosmonauts making flights of 4-to 12-month duration on the Soviet/Russian MIR spacecraft lost bone at an average rate of 1%/month from the spine and 1.5%/month from the hip. However, because DXA measurements represent the sum of the cortical and trabecular compartments, there is no direct information on how these bone envelopes are affected by spaceflight. Materials and Methods: To address this, we performed a study of crewmembers (13 males and 1 female; age range, 40 -55 years) on long-duration missions (4 -6 months) on the International Space Station (ISS). We used DXA to obtain aBMD of the hip and spine and volumetric QCT (vQCT) to assess integral, cortical, and trabecular volumetric BMD (vBMD) in the hip and spine. In the heel, DXA was used to measure aBMD, and quantitative ultrasound (QUS) was used to measure speed of sound (SOS) and broadband ultrasound attenuation (BUA). Results and Conclusions: aBMD was lost at rates of 0.9%/month at the spine (p Ͻ 0.001) and 1.4 -1.5%/month at the hip (p Ͻ 0.001). Spinal integral vBMD was lost at a rate of 0.9%/month (p Ͻ 0.001), and trabecular vBMD was lost at 0.7%/month (p Ͻ 0.05). In contrast to earlier reports, these changes were generalized across the vertebrae and not focused in the posterior elements. In the hip, integral, cortical, and trabecular vBMD was lost at rates of 1.2-1.5%/month (p Ͻ 0.0001), 0.4 -0.5%/month (p Ͻ 0.01), and 2.2-2.7%/month (p Ͻ 0.001), respectively. The cortical bone loss in the hip occurred primarily by cortical thinning. Calcaneal aBMD measurements by DXA showed smaller mean losses (0.4%/month) than hip or spine measurements, with SOS and BUA showing no change. In summary, our results show that ISS crewmembers, on average, experience substantial loss of both trabecular and cortical bone in the hip and somewhat smaller losses in the spine. These results do not support the use of calcaneal aBMD or QUS measurements as surrogate measures to estimate changes in the central skeleton.

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

“…Studies using calcium kinetics, site-specific bone densitometry and bone turnover markers document a net loss of bone mineral in the gravitationally-unloaded skeleton of crew members who had flown either on Skylab (28, 56 and 84 days) or on long-duration missions (>4 months) aboard the Russian Mir spacecraft and the International Space Station (ISS) [2][3][4][5][6][7][8][9][10][11][12].…”

“…Since the Skylab missions of the 1970's, measurements of bone mineral and bone mineral density had been used to evaluate the effects of spaceflight on the skeleton [4,5,8,9]. More recently, QCT scans of long-duration crew members was used to evaluate changes in mineral density and in hip structure after spaceflight [10].…”

Lower body negative pressure treadmill exercise as a countermeasure for bed rest-induced bone loss in female identical twins

“…Data from Skylab and Mir missions show that astronauts continue to lose bone despite participation in vigorous in-flight exercise protocols [11,26,35,41]. The inflight exercise program on early International Space Station (ISS) flights was also not successful at mitigating weightlessness-induced bone loss [25], although exercise hardware availability in those missions was inconsistent.…”

“…Mir crew members using treadmill exercise with bungee-cord restraints had maximum mechanical loads of only 60-70% of body weight [44]. A similar loading configuration has been used on the ISS with a resistive exercise device, and in ISS crew members losses of bone mineral density were similar to those on Mir [25]. It is generally accepted that higher-impact exercise is more effective at maintaining bone than lower-impact exercise [14,39,46].…”

Osteocytes and Homeostasis of Remote Organs

“…They are mechanosensors of bone and contribute to bone homeostasis by converting mechanical stress to biologic signals; therefore, the major players in bone turnover, osteoblasts and osteoclasts, are regulated at the local bone surface (6, 7). For instance, reduced mechanical stress on the bones of astronauts or bedridden patients leads to rapid progression of osteoporosis and impaired immunity (29)(30)(31)(32). Microgravity is known to induce local osteoporosis on the hindlimbs in the mouse tail suspension system.…”