DI-5-Cuffs: Lumbar Intervertebral Disc Proteoglycan and Water Content Changes in Humans after Five Days of Dry Immersion to Simulate Microgravity

Abstract

Most astronauts experience back pain after spaceflight, primarily located in the lumbar region. Intervertebral disc herniations have been observed after real and simulated microgravity. Spinal deconditioning after exposure to microgravity has been described, but the underlying mechanisms are not well understood. The dry immersion (DI) model of microgravity was used with eighteen male volunteers. Half of the participants wore thigh cuffs as a potential countermeasure. The spinal changes and intervertebral disc (IVD) content changes were investigated using magnetic resonance imaging (MRI) analyses with T1-T2 mapping sequences. IVD water content was estimated by the apparent diffusion coefficient (ADC), with proteoglycan content measured using MRI T1-mapping sequences centered in the nucleus pulposus. The use of thigh cuffs had no effect on any of the spinal variables measured. There was significant spinal lengthening for all of the subjects. The ADC and IVD proteoglycan content both increased significantly with DI (7.34 ± 2.23% and 10.09 ± 1.39%, respectively; mean ± standard deviation), p < 0.05). The ADC changes suggest dynamic and rapid water diffusion inside IVDs, linked to gravitational unloading. Further investigation is needed to determine whether similar changes occur in the cervical IVDs. A better understanding of the mechanisms involved in spinal deconditioning with spaceflight would assist in the development of alternative countermeasures to prevent IVD herniation.

Keywords: ADC diffusion; T1-T2 mapping; back pain; intradiscal proteins; space physiology; spine; vertebral deconditioning.

Figure 1

Magnetic resonance imaging (MRI) T2 sagittal sequence centered in the median plane. Spine height (distance occipital bone C₀ to first sacral vertebra S₁ as shown in yellow line) increased for each of subject, with a mean change from 59.81 to 61.06 cm (95% confidence interval (CI) of difference 1.428 to 1.065 cm; * p < 0.05).

Figure 2

Magnetic resonance imaging (MRI) T2 sagittal sequence centered in the median plane. Lumbar lordosis (vertebral endplates L₁-S₁ in red line) decreased pre- vs post (t-test *** p < 0.0001; 95% CI from 39.50 to 48.45° in pre to 37.05 to 46.80° in post) with no effect of the countermeasure (p < 0.1906).

Figure 3

Apparent diffusion coefficient (ADC; value in ms ×10⁻³ mm²/s) per intervertebral lumbar disc level (** p < 0.001; *** p < 0.0001). The ADC increased significantly after dry immersion, with an average increase of 7.34 ± 2.23% (p < 0.05).

Figure 4

Apparent diffusion coefficient (ADC) in lumbar intervertebral discs (IVD) from the MRI sagittal view (region of interest in red circle inside IVD L3-L4). All of the lumbar levels significantly increased in water content after dry immersion (* p < 0.05; ** p < 0.01; *** p < 0.001). There were no statistical differences between the Cuffs and Control groups (p = 0.92).

Figure 5

T1 mapping images pre vs. post are presented with an OleaSphere filter with the region of interest (ROI) inside the nucleus pulposus (NP). Protein content in the NP significantly increased after DI (* p < 0.05 and ** p < 0.01).

Figure 6

Back pain intensity (0 to 10 on a visual analog scale). Data collection before (B-1) to recovery (R0) every day for five days of dry immersion (D₁ to D₅) in the morning (m) and in the evening (ev). Two-way analysis of variance (ANOVA) revealed a significant global effect of DI (p < 0.0001), and no effect of countermeasure (p = 0.34).