The effect of a 5-day space flight on the immature rat spine.

Background Context: Spaceflight has many reported effects upon the musculoskeletal system structure and function. This study was designed to determine the effect of a 5-day flight on the rat spine.

Methods: In September 1991, 8 neonatal rats were flown aboard the Space Shuttle Columbia flight STS-48 during a 5-day mission. Upon return to earth, the spines were dissected, frozen and shipped to our laboratory. Matched ground-based rats were used as controls. The spines were radiographed and then slowly thawed. Individual vertebrae were subjected to compressive biomechanical testing using an Instron tester (Instron Corp, Canton, MA, USA) and then processed for determination of calcium and phosphorus content. The intervertebral discs were placed in physiological saline and the stress-relaxation characteristics measured. The discs were then lyophilized and assayed for collagen and proteoglycan content. Disc height on radiographs was measured by image analysis.

Results: After space flight, the heights of the discs were found to be 150 to 200 microns greater, although the values were not statistically significant. There was no difference in the resiliency of the thoracic discs as determined by stress-relaxation. However, in the lumbar discs, space flight increased the resiliency (p<.01). There was no difference in water content. In both the thoracic and lumbar discs there was a 3.3-fold increase in hydroxyproline-proteoglycan ratio after space flight. However, because of the small sample size, these values were not statistically significant. In the vertebrae, there was no difference in calcium-phosphate ratio or compressive strength.

Conclusions: These data suggest that even after a short 5-day flight, the spine begins to undergo biomechanical and biochemical changes. In addition, the weightless environment in space may provide a good model to study the effects of immobilization on earth.