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Different gravity fields have important effects on the structural morphology of bone. The fluid flow caused by loadings in the bone lacunar-canalicular system (LCS), converts mechanical signals into biological signals and regulates bone reconstruction by affecting effector cells, which ensures the efficient transport of signaling molecules, nutrients, and waste products. In this study, the fluid flow and mass transfer effects of bone lacunar-canalicular system at multi-scale were firstly investigated, and a three-dimensional axisymmetric fluid-solid coupled finite element model of the LCS within three continuous osteocytes was established. The changes in fluid pressure field, flow velocity field, and fluid shear force variation on the surface of osteocytes within the LCS were studied comparatively under different gravitational fields (0 G, 1 G, 5 G), frequencies (1 Hz, 1.5 Hz, 2 Hz) and forms of cyclic compressive loading. The results showed that different frequencies represented different exercise intensities, suggesting that high-intensity exercise may accelerate the fluid flow rate within the LCS and enhance osteocytes activity. Hypergravity enhanced the transport of solute molecules, nutrients, and signaling molecules within the LCS. Conversely, the mass transfer in the LCS may be inhibited under microgravity, which may cause bone loss and eventually lead to the onset of osteoporosis. This investigation provides theoretical guidance for rehabilitative training against osteoporosis. [ABSTRACT FROM AUTHOR]

The bone lacunar-canalicular system (LCS) is a unique complex 3D microscopic tubular network structure within the osteon that contains interstitial fluid flow to ensure the efficient transport of signaling molecules, nutrients, and wastes to guarantee the normal physiological activities of bone tissue. The mass transfer laws in the LCS under microgravity and hypergravity are still unclear. In this paper, a multi-scale 3D osteon model was established to mimic the cortical osteon, and a finite element method was used to numerically analyze the mass transfer in the LCS under hypergravity, normal gravity and microgravity and combined with high-intensity exercise conditions. It was shown that hypergravity promoted mass transfer in the LCS to the deep lacunae, and the number of particles in lacunae increased more significantly from normal gravity to hypergravity the further away from the Haversian canal. The microgravity environment inhibited particles transport in the LCS to deep lacunae. Under normal gravity and microgravity, the number of particles in lacunae increased greatly when doing high-intensity exercise compared to stationary standing. This paper presents the first simulation of mass transfer within the LCS with different gravity fields combined with high-intensity exercise using the finite element method. The research suggested that hypergravity can greatly promote mass transfer in the LCS to deep lacunae, and microgravity strongly inhibited this mass transfer; high-intensity exercise increased the mass transfer rate in the LCS. This study provided a new strategy to combat and treat microgravity-induced osteoporosis. [ABSTRACT FROM AUTHOR]

A three-dimensional model for osteocyte was established to research the mechanisms of mechano-transduction and amplification of primary cilium and osteocyte process in every gravity field. The results showed that significant stress concentration was observed in the area of physical connection between TES and the osteocyte process, where the fluid shear stress (FSS) was around two orders of magnitude higher than that in other areas. Due to the significant amplification effect of the TES structure on mechanical stimulation, making osteocyte process the "optimal mechanical receptor". In microgravity, the mechanical signal conduction ability of the osteocyte decreased significantly.. At the micro-nano scale, a 3D model for single bone lacunae-osteocyte system is established. The stress amplification mechanism of the transverse element is verified. Compared with the primary cilium, osteocyte process is the 'optimal mechanical receptor'. In microgravity, the mechanical signal conduction ability of osteocyte system decreased. [ABSTRACT FROM AUTHOR]