A study of how microgravity affects the biomechanical properties of caudal and lumbar mouse discs showed that caudal discs that had been exposed to microgravity for 15 days experienced biomechanical diminishment, while their lumbar discs were relatively unaffected. The caudal discs lost height, were more likely to creep, and demonstrated less nuclear swelling strain dependence than the discs used in the control group.
The mice involved in the study were part of NASA’s Biospeciman Sharing Program and were exposed to 15 days of flight on the space shuttle prior to being sacrificed for analysis. Each mouse was radiographed prior to being dissected and having the soft tissues removed. The mice were mounted onto custom mechanical instruments that measured the rate of compressive creep and tissue deformation under a constant load. After five cycles of compressive creep loading and measuring, a displacement controlled stress relaxation test was used to measure the amount of nuclear swelling pressure, then the researchers calculated the change displacement and load response to determine the swelling pressure. All results were then analyzed using the Levonberg-Marquardt algorithm, and creep parameters were analyzed with a non-parametric test. The results of the control (non-flight) and space-flight group were compared for all parameters to determine any statistical differences or similarities.
There was a statistically-significant difference in the caudal discs of the control and spaceflight specimen groups, but there was little difference in the lumbar discs of the two groups. The caudal discs of the spaceflight group were 32 percent shorter and crept 2.5 times more than those in the non-flight subjects. Strain-dependence of the spaceflight group’s swelling pressure was 70 percent lower than that of the non-flight group. This could help to explain the back-pain complaints of astronauts returning to Earth after long spaceflights.
The results of this study indicate that the effects of microgravity on the biomechanical properties of intervertebral caudal discs is significant in murine samples. Specifically, there is a diminishing effect in that microgravity appears to lessen disc height, increase the rate of creep, and lower strain-dependence swelling pressure in caudal discs. Though there were no statistically-significant differences between the spaceflight and non-flight study groups at the lumbar disc levels, it is possible that this is because mice are quadrupedal and thus may not have the excess swelling capacity that humans require to counter the daily loading of an upright posture. It is also possible that mice counter loading demands by transferring loads to their tails or adapting their movements during usual and zero-gravity conditions. In any event, the study findings indicate that constraining spinal movements to reduce nucleus swelling during the return to normal gravity conditions could help to lessen the negative effects of microgravity on astronauts on their return to Earth after a lengthy space flight.