Adaptation of the Skeletal System During Long-Duration Spaceflight

Abstract

This review will highlight evidence from crew members flown on space missions >90 days to suggest that the adaptations of the skeletal system to mechanical unloading may predispose crew members to an accelerated onset of osteoporosis after return to Earth. By definition, osteoporosis is a skeletal disorder − characterized by low bone mineral density and structural deterioration − that reduces the ability of bones to resist fracture under the loading of normal daily activities. "Involutional" or agerelated osteoporosis is readily recognized as a syndrome afflicting the elderly population because of the insipid and asymptomatic nature of bone loss that does not typically manifest as fractures until after age 1 https://ntrs.nasa.gov/search.jsp?R=20080014098 2018-05-10T03:42:55+00:00Z ~60. It is not the thesis of this review to suggest that spaceflight-induced bone loss is similar to bone loss induced by metabolic bone disease; rather this review draws parallels between the rapid and earlier loss in females that occurs with menopause and the rapid bone loss in middle-aged crew members that occurs with spaceflight unloading and how the cumulative effects of spaceflight and ageing could be detrimental, particularly if skeletal effects are totally or partially irreversible.In brief, this report will provide detailed evidence that long-duration crew members, exposed to the weightlessness of space for the typical long-duration (4-6 months) mission on Mir or the International Space Station --1. Display bone resorption that is aggressive, that targets normally weight-bearing skeletal sites, that is uncoupled to bone formation and that results in areal BMD deficits that can range between 6-20% of preflight BMD; 2. Display compartment-specific declines in volumetric BMD in the proximal femur (a skeletal site of clinical interest) that significantly reduces its compressive and bending strength and which may account for the loss in hip bone strength (i.e., force to failure); 3. Recover BMD over a post-flight time period that exceeds spaceflight exposure but for which the restoration of whole bone strength remains an open issue and may involve structural alteration; and 4. Display risk factors for bone loss --such as the negative calcium balance and down-regulated calcium-regulating hormones in response to bone atrophy --that can be compounded by the constraints of conducting mission operations (inability to provide essential nutrients and vitamins).The full characterization of the skeletal response to mechanical unloading in space is not complete. In particular, countermeasures used to date have been inadequate and it is not yet known 2 whether more appropriate countermeasures can prevent the changes in bone that have been found in previous flights, Knowledge gaps related to the effects of prolonged (> 6months) space exposure and to partial gravity environments are substantial, and longitudinal measurements on crew members after spaceflight are required to assess the full impact on skeletal recovery.

Space, Gravity and the Physiology of Aging: Parallel or Convergent Disciplines? A Mini-Review

“…Computer tomography used to measure bone mineral density (BMD) in 4 cosmonauts who spent up to 7 months on the Russian space station Mir showed all had lost BMD mainly from the posterior vertebra, and dual X-ray photon absorptiometry showed loss in the spine, femoral neck, trochanter and pelvis of about 1-1.6% per month and 0.3-0.4% per month in the legs and the whole body, but no loss at all in the forearm in 1-and 6-month missions [37] .…”

“…A 4-year study [36][37][38] of the long-term effects of weightlessness in the microgravity of space on the bones of the ISS crew showed that on average they lost 11% (range 0-24%) [Schneider pers. commun.]…”

“…ISS crew members lost both trabecular (inner spongy layer) and cortical (outer layer) bone in the hip, whereas bone loss with aging usually occurs mostly at the inner bone layers [36][37][38] . There appeared to be a gradient of mineral loss beginning at the lumbar spine and increasing in the hip, reinforcing the role of gravity in this pattern in the astronauts.…”

Bone Quality and Quantity are Mediated by Mechanical Stimuli

“…In situations of disuse, bone loss will occur quickly because the body no longer needs to metabolically support such a large structure for load bearing ability [22]. A common example is astronauts, who often experience bone loss due to microgravity while in space [23][24][25][26]. In a study of long-duration flights (average duration approximately 6 months), almost all long-duration astronauts experienced at least a 3% bone loss in at least one skeletal site, while 43% showed at least a 10% bone loss in at least one skeletal site [27].…”

Postflight reconditioning for European Astronauts – A case report of recovery after six months in space

“…The effects of prolonged exposure to microgravity (µG) on the human body have been well studied [1][2][3][4] and include reductions of muscle volume and strength, bone mass and aerobic capacity [1] as the human body adapts to its new environment. Reductions of muscle strength and stability are associated with lower physical performance capacity and have a number of health implications [1,5].…”