Impact of gravity on fluid flow and solute transport in the bone lacunar-canalicular system: a multiscale numerical simulation study

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.