Prolonged microgravity degenerates biomechanical properties of the murine caudal intervertebral discs

INTRODUCTION: Prolonged exposure to microgravity has adverse effects on the human intervertebral disc (IVD), causing low back pain during spaceflight and an increased incidence of post-spaceflight herniated nucleus pulposus (HNP). Cervical HNP incidence among an astronaut sample population was nearly 5-fold higher compared to a control population, while incidence of lumbar HNP was not signifcantly different. Theoretically, microgravity increases spinal injury risk by removing diurnal loading that expels IVD fluid that has been osmotically imbibed during bedrest. Discs freely swell throughout the entire flight, leading to increased disc height and spinal stiffness among astronauts. We propose that degenerative effects of microgravity will vary among spinal regions due to level-dependent differences in range of motion (ROM). Specifically, imposing larger ROM on microgravity stiffened spinal segments could cause accelerated degeneration and increased HNP risk immediately post-spaceflight. As part of the Biospecimen Sharing Program with NASA, we acquired murine lumbar spines and tails from STS-131, a 15-day shuttle mission. We hypothesize the caudal IVD will exhibit a greater difference in biomechanical properties between the spaceflight and ground control mice due to the greater ROM at caudal segments compared to lumbar segments.