Concern of Intervertebral Disc Damage Upon and Immediately After Re-Exposure to Gravity (Inactive)

There is an increased incidence of back pain expressed by crewmembers in space. Additionally, herniated Intervertebral Discs (IVD) have been diagnosed in returning Skylab and Shuttle astronauts on landing day, and after varying periods of time in the postflight period. Such injuries in astronauts, however, may be related to their careers as aviators (either high performance jet pilots and/or helicopter pilots). However, the evidence of IVD injuries raises the concern that astronauts are at increased risk during loading scenarios experienced during or after exploration missions (for example, re-entry to a gravitational field, activities on planetary surfaces).

To date, flight data related to potential back injuries have focused upon spine elongation and the well-established effects of mechanical unloading on intervertebral discs (IVDs). IVDs are the articulating connective tissue between vertebral bodies of the spinal column where the IVD acts as a shock absorber to the mechanical loads experienced in the axial direction. The connective tissue of joints is devoid of vasculature so exchanges between nutrients and waste products are accomplished by the influx and efflux of fluid. In general, the diurnal fluctuations in IVD volume of the spine are induced as the individual transitions between sleep (supine) and ambulation (upright), although the spine is subjected to a variety of mechanical forces with daily activities in 1 G. However, during prolonged bed rest or spaceflight, the absence of axial and muscular loading to the spine causes the IVDs to swell with increased fluid intake. Consequently, the changes in IVD volume were considered major factors for the elongation of the spine, the increase in height and the loss of lordotic curvature. IVD changes may also account for the occurrence of back pain, although the exact cause for the latter is not well defined. Tissue analyses of animals, mechanically unloaded in space and ground-based models, reveal changes in IVD biochemical composition. Spaceflight-induced changes to IVDs may predispose the IVD to injury during reloading. No flight studies have investigated whether a specific exercise regimen, performed during real or simulated weightlessness, can effectively impart axial loads to the human spine to mimic the diurnal changes in IVD volume experienced on Earth. Restoration of IVD volume, after spaceflight and bed rest, has been observed with return to upright position in a 1-G environment, but the recovery time course has not been systematically assessed. Likewise, IVD biochemical and biomechanical properties, before and after spaceflight, have not been investigated.

In brief, extended exposures to microgravity are associated with increased reports of back pain during flight and may be related to the occurrence of IVD herniations in astronauts after flight. The etiology for these observations may be multi-factorial given the number of documented physiological risk factors induced in space, which include muscle atrophy, tissue degeneration, bone fracture and accelerated bone loss. Thus, further investigations are needed to generate more evidence to better define the risk.