Intervertebral Disc Swelling Demonstrated by 3D and Water Content Magnetic Resonance Analyses after a 3-Day Dry Immersion Simulating Microgravity

Abstract

Background: Vertebral deconditioning is commonly experienced after space flight and simulation studies. Disc herniation is quadrupled after space flight. Purpose: The main hypothesis formulated by the authors is that microgravity results in intervertebral disc (IVD) swelling. Study Design: The aim of the study was to identify the morphological changes of the spine and their clinical consequences after simulated microgravity by 3-day dry immersion (DI). The experimental protocol was performed on 12 male volunteers using magnetic resonance imaging and spectroscopy before and after DI. Methods: All the experiment was financially supported by CNES (Centre national d'études spatiales i.e., French Space Agency). Results: We observed an increase in spine height of 1.5 ± 0.4 cm and a decrease in curvature, particularly for the lumbar region with a decrease of -4 ± 2.5°. We found a significant increase in IVD volume of +8 ± 9% at T₁₂-L₁ and +11 ± 9% at L₅-S₁. This phenomenon is likely associated with the increase in disc intervertebral water content (IWC), 17 ± 27%. During the 3 days in DI, 92% of the subjects developed back pain in the lumbar region below the diaphragmatic muscle. This clinical observation may be linked to the morphological changes of the spine. Conclusions: The morphological changes observed and, specifically, the disc swelling caused by increased IWC may contribute to understanding disc herniation after microgravity exposure. Our results confirmed the efficiency of the 3-day DI model to reproduce quickly the effects of microgravity on spine morphology. Our findings raise the question of the subject selection in spatial studies, especially studies about spine morphology and reconditioning programs after space flight. These results may contribute to a better understanding of the mechanisms underlying disc herniation and may serve as the basis to develop countermeasures for astronauts and to prevent IVD herniation and back pain on Earth.

Keywords: back pain; disc herniation; disc swelling; dry immersion; magnetic resonance imaging/magnetic resonance spectroscopy; microgravity; vertebral deconditioning.

Figure 1

Dry immersion condition. © CNES/MEDES/E.GRIMAULT, 2015.

Figure 2

Spine height measurement (A) between the foramen magnum (C) and the superior and posterior endplate of the first sacral vertebra (B).

Figure 3

Intervertebral disc water spectrum analysis: example with the voxel of interest in L₅-S₁, position (P) of water spectrum P_w = 4.7; integral (I) represents the water content calculated under the peak spectrum.

Figure 4

Position during dry immersion (DI) showing the discomfort or painful sensation. The most significant discomfort was located in the thoracolumbar region. The pelvis is heavy, so it had a tendency to sink; and the thorax is full of air, so it rose while in DI. Moreover, the viscera draws toward the diaphragm muscle. All these phenomena concentrated constraints in the thoracolumbar region.

Figure 5

Pain intensity per region using the visual analog scale (VAS, 0 to 10 where 10 is the worst pain) before (BDC-2), during dry immersion (DI 2), and at recovery (R+0). Paired t-test (BDC vs. DI), p-value significant (^*p = 0.019; ^**p = 0.006).

Figure 6

Spinal curvature (in degree °) changes compared to baseline [before (BDC-4) and after dry immersion (DI3)]. Lumbar curvature decreased after DI for −8 ± 6% (^***p < 0.001) (Comparison Pre vs. Post DI, T-Paired Test).

Figure 7

MRI 3D analysis of intervertebral disc volume (IVD). IVD volume increased after dry immersion (DI) in L5-S1 (+11 ± 5%^***) and in T12-L1 (+8 ± 9%^*). Comparison pre (BDC-4) vs. post DI (DI3) (Mean ± SD, T-Paired test, ^*p < 0.05; ^***p < 0.001).

Figure 8

Intervertebral disc volume 3D reconstruction after regions of interest selected on hyper-signal disc on OsiriX software crosschecking horizontal and axial planes (T1 vibe Dixon opposition phase).