Effect of microgravity on mechanical loadings in lumbar spine at various postures: a numerical study

Abstract

The aim of this study was to quantitatively analyze the mechanical change of spinal segments (disc, muscle, and ligament) at various postures under microgravity using a full-body musculoskeletal modeling approach. Specifically, in the lumbar spine, the vertebra were modeled as rigid bodies, the intervertebral discs were modeled as 6-degree-of-freedom joints with linear force-deformation relationships, the disc swelling pressure was deformation dependent, the ligaments were modeled as piecewise linear elastic materials, the muscle strength was dependent on its functional cross-sectional area. The neutral posture and the "fetal tuck" posture in microgravity (short as "Neutral 0G" and "Fetal Tuck 0G", in our simulation, the G constant was set to 0 for simulating microgravity), and for comparison, the relaxed standing posture in 1G and 0G gravity (short as "Neutral 1G" and "Standing 0G") were simulated. Compared to values at Neutral 1G, the mechanical response in the lower spine changed significantly at Neutral 0G. For example, the compressive forces on lumbar discs decreased 62-70%, the muscle forces decreased 55.7-92.9%, while disc water content increased 7.0-10.2%, disc height increased 2.1-3.0%, disc volume increased 6.4-9.3%, and ligament forces increased 59.5-271.3% at Neutral 0G. The fetal tuck 0G reversed these changes at Neutral 0G back toward values at Neutral 1G, with magnitudes much larger than those at Neutral 1G. Our results suggest that microgravity has significant influences on spinal biomechanics, alteration of which may increase the risks of disc herniation and degeneration, muscle atrophy, and/or ligament failure.

Fig. 1. The postures simulated in 1G gravity and microgravity.

a Relaxed standing posture in 1G gravity (Neutral 1G); this posture was also used in microgravity for comparison (Standing 0G), b relaxed floating posture in microgravity (Neutral 0G), and c fetal tuck posture in microgravity (Fetal Tuck 0G).

Fig. 2. Comparison of mechanical forces on lumbar discs among various postures in 1G gravity and microgravity.

a Compressive force and b shear force among Neutral 1G, Standing 0G, Neutral 0G, and Fetal Tuck 0G.

Fig. 3. Comparison of disc morphology change among various postures in 1G gravity and microgravity.

a Disc height, b cross-sectional area, and c disc volume change among Neutral 1G, Standing 0G, Neutral 0G, and Fetal Tuck 0G.

Fig. 4. Comparison of water content in lumbar discs among various postures in 1G gravity and microgravity.

Water content in a NP and b AF among Neutral 1G, Standing 0G, Neutral 0G, and Fetal Tuck 0G. NP nucleus pulposus, AF annulus fibrosus.

Fig. 5. Comparison of muscle forces and ligament forces in lumbar spine among various postures in 1G gravity and microgravity.

a Muscle forces and b ligament forces in lumbar spine among Neutral 1G, Standing 0G, Neutral 0G, and Fetal Tuck 0G. MF multifidus, ES erector spinae, PM psoas major, QL quadratus lumborum, OE obliquus externus, OI obliquus internus, SR semispinalis, TMF thoracic multifidus, RA rectus abdominis, Tra transversus abdominis, ALL anterior longitudinal ligament, PLL posterior longitudinal ligament, IS interspinous, SS supraspinous, FL flavum, IT intertransverse.