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The lack of sufficient mechanical stimulation to the human bone, results in disuse osteoporosis. Low-intensity pulsed ultrasound (LIPUS) promotes fracture healing and the treatment of disuse osteoporosis, but its biomechanical mechanism remains unknown. Simulative research on the mechanical effects of LIPUS on disuse trabecular bone and osteoblasts have been performed. The von Mises stress of disuse trabecular bone and osteoblasts obviously increased under LIPUS irradiation. The average von Mises stress of osteoblasts were two orders of magnitude higher under the irradiation of simulant LIPUS than that without LIPUS irradiation, and the von Mises stress of osteoblasts was positively correlated with the amplitude of sound pressure excitation. The results showed that LIPUS irradiation could obviously improve the mechanical micro-environment of trabecular bone and osteoblasts to alleviate the lack of mechanical stimulation. The results of the research can reveal the biomechanical mechanism of LIPUS in the treatment of disuse osteoporosis to some extent and provide theoretical guidance for clinical treatment of disuse osteoporosis through physical methods., (Copyright © 2023 by ASME.)

Introduction: The bone lacunar-canalicular system (LCS) is an important microstructural basis for signaling and material transport in bone tissue, guaranteeing normal physiological processes in tissues. Spaceflight astronauts and elderly osteoporosis are related to its function, so it is necessary to reveal the mass transfer laws in bone microstructure under different gravity fields to provide insight for effective clinical treatment., Materials and Methods: Using the natural LCS structure of bovine tibial cortical bone as the object, the mass transfer experiments on cortical bone were conducted by using sodium fluorescein tracer through different frequency pulsating pressure provided by dynamic perfusion loading device and different high G environments provided by high-speed centrifuge to analyze the mass transfer laws under different gravity fields and different pulsating pressures., Results: The fluorescence intensity of lacunae within the osteon was lower the farther away from the Haversian canal. As the gravity field magnitude increased, the fluorescence intensity within each lacuna enhanced, and the more distant the lacunae from the Haversian canal, the greater the fluorescence intensity enhancement. High-frequency pulsating pressure simulated high-intensity exercise in humans can improve mass transfer efficiency in the LCS., Conclusion: High-intensity exercise may greatly increase solute molecules, nutrients, and signaling molecules in osteocytes and improve the activity of osteocytes. Hypergravity can enhance the transport of solute molecules, nutrients, and signaling molecules in the LCS, especially promoting mass transfer to deep layer lacunae. Conversely, mass transfer to deep layer lacunae may be inhibited under microgravity, causing bone loss and ultimately leading to osteoporosis., (© 2022. The Japanese Society Bone and Mineral Research.)

During spaceflight, astronauts experience 1-1.5% bone loss per month, especially in the lumbar spine, pelvis and lower limbs. The bone loss leads to osteoporosis and increased the risk of fracture. Current researches focus on anti-osteoporosis under microgravity mainly by inhibiting bone resorption of osteoclasts and / or increasing bone formation of osteoblasts. However, studies on the effects of mass transfer in the bone lacunar-canalicular system (LCS) on osteoporosis are lacking. Osteocytes reside in the lacunae and communicate with other osteocytes, osteoblasts and osteoclasts through the LCS in the bone matrix. Osteocytes are mainly responsible for mechanosensing and signal regulation in bone, and the LCS is the basic structure for signaling, mass transfer and mechanical stimulation. Microgravity causes deficient mass transfer in the LCS, especially in the outer layer of osteon. Osteocytes far away from the Haversian canals are inhibited or accelerated apoptosis to stimulate osteoclasts which result in bone loss. Deficient mass transfer in the LCS may be a determinant of human osteoporosis under microgravity, which will open up a new way to treat osteoporosis in space., (Copyright © 2021 The Committee on Space Research (COSPAR). Published by Elsevier B.V. All rights reserved.)

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]

Irreversible osteoporosis may occur in astronauts during long-term space flight. The flow field of tissue fluid in the lacunar-canalicular system (LCS) of osteon and the mechanical response of osteocytes to the flow field under different gravity fields were studied by numerical simulation. This study is expected to explain how the decrease in liquid transmission within microgravity can be a cause of osteoporosis in astronauts from the perspective of biomechanics using a fundamental research approach. A 3D axisymmetric fluid-solid coupling finite element model of an osteon with a two-stage pore structure (Haversian canals and lacunar-canalicular network) and osteocytes was established. The model compared the influence of differences in pulsating pressure of arterioles in Haversian canal, from 33 mmHg to 45 mmHg within a microgravity field (0 g), Earth's gravity field (1 g), and a high G gravitational fields (2-8 g). The liquid flow velocity in the LCS within a microgravity field was less than that within a normal gravitational field, and the flow velocity increased with gravitational acceleration. There was a significant liquid pressure gradient in the osteocytes within a normal and higher gravitational field compared with in microgravity. A reduction in the fluid flow velocity and fluid shear stress on osteocytes in different zones in microgravity compared with Earth's gravitational field. For these reasons, possibly causing a decrease in mechanical conduction and biochemical function, even cell death, leads to increased osteoclast activity, eventually causing the loss of a large quantity of bone. Graphical abstract A 3D axisymmetric fluid-solid coupling finite element model of an osteon with a two-stage pore structure was established. The model compared the influence of magnitudes of gravity on liquid transmission in LCS and mechanical response of osteocytes. The mean flow velocity of liquid in various layers (shallow, middle, and deep) increased linearly as acceleration due to gravity increased, and there was a significant liquid pressure gradient in osteocytes within a normal gravitational field compared with in microgravity. In microgravity environment, the osteocytes were unable to experience the pressure difference compared to that of Earth, possibly causing a decrease in mechanical conduction and biochemical function, even cell death, leading to increased osteoclast activity, eventually causing the loss of a large quantity of bone. [ABSTRACT FROM AUTHOR]