Computational Modelling of the Mechanics of Trabecular Bone and Marrow Using Fluid Structure Interaction Techniques

Scaffold geometries designed to promote bone ingrowth by enhancing mechanobiological stimulation and biotransportation - A multiobjective optimisation approach

In a tissue-engineered bone scaffold implant, the process of neo-tissue ingrowth and remodelling into hard lamellar bone occurs slowly; it typically requires a period of several months to a year (or more) to complete. This research considers the design optimisation of a scaffold's unit cell geometry for the purpose of accelerating the rate at which neo-tissue forms in the porous network of the scaffold (ingrowth), and hence, reduce the length of time to complete the bone ingrowth process. In this study, the basic structure of the scaffold is the Schwarz Primitive (P) surface unit cell, selected for its compelling biomechanical and permeability characteristics. The geometry of the scaffold is varied using two parameters (namely iso-value, k, and spatial period, a) within the surface equation defining the Schwarz P-surface unit cell. In total, sixteen different unit cell geometries are considered here with the porosity ranging from 50% to 82%. The design objectives for the scaffold are to (i) enhance mechanobiological stimulus conditions conducive to bone apposition and (ii) enhance permeability to improve the transport of nutrients/oxygen and metabolities to and from the sites of neo-tissue formation throughout the porous scaffold. The independent design variables (k and a) of the periodic unit cell geometry are optimised to best satisfy the design objectives of appositional mechanobiological stimulus and biotransporting permeability. First, a reconstructed sheep mandible computed tomographic (CT)-based finite element (FE) analysis model is used to relate the strain energy density and mechanobiological stimulus to the design variables. Next, a computational fluid dynamics (CFD) model of a 5 × 5 × 5 unit cell scaffold is developed to relate the distributions of pressure and fluid velocity to the design variables. Then, surrogate modelling is undertaken in which bivariate cubic polynomial response surfaces are fitted to the FE and CFD analysis output data to form mathematical functions of each objective with respect to the two design variables. Finally, a multiobjective optimisation algorithm is invoked to determine the best trade-off between the competing design objectives of mechanobiological stimulus and biofluidic permeability. The novel design of the scaffold structure is anticipated to provide a better biomechanical and biotransport environment for tissue regeneration.

Fluid-solid coupling numerical simulation of the effects of different doses of verapamil on cancellous bone in type 2 diabetic rats

BACKGROUND

The purpose of this study was to investigate the effects of four different doses of verapamil on the mechanical behaviors of solid and the characteristics of fluid flow in cancellous bone of distal femur of type 2 diabetes rats under dynamic external load.

METHODS

Based on the micro-CT images, the finite element models of cancellous bones and fluids at distal femurs of rats in control group, diabetes group, treatment groups VER 4, VER 12, VER 24, and VER 48 (verapamil doses of 4, 12, 24, and 48 mg/kg/day, respectively) were constructed. A sinusoidal time-varying displacement load with an amplitude of 0.8 μm and a period of 1s was applied to the upper surface of the solid region. Then, fluid-solid coupling numerical simulation method was used to analyze the magnitudes and distributions of von Mises stress, flow velocity, and fluid shear stress of cancellous bone models in each group.

RESULTS

The results for mean values of von Mises stress, flow velocity and FSS (t = 0.25s) were as follows: their values in control group were lower than those in diabetes group; the three parameters varied with the dose of verapamil; in the four treatment groups, the values of VER 48 group were the lowest, they were the closest to control group, and they were smaller than diabetes group. Among the four treatment groups, VER 48 group had the highest proportion of the nodes with FSS = 1-3 Pa on the surface of cancellous bone, and more areas in VER 48 group were subjected to fluid shear stress of 1-3 Pa for more than half of the time.

CONCLUSION

It could be seen that among the four treatment groups, osteoblasts on the cancellous bone surface in the highest dose group (VER 48 group) were more easily activated by mechanical loading, and the treatment effect was the best. This study might help in understanding the mechanism of verapamil's effect on the bone of type 2 diabetes mellitus, and provide theoretical guidance for the selection of verapamil dose in the clinical treatment of type 2 diabetes mellitus.

Computational simulation of applying mechanical vibration to mesenchymal stem cell for mechanical modulation toward bone tissue engineering

Evaluation of cell response to mechanical stimuli at in vitro conditions is known as one of the important issues for modulating cell behavior. Mechanical stimuli, including mechanical vibration and oscillatory fluid flow, act as important biophysical signals for the mechanical modulation of stem cells. In the present study, mesenchymal stem cell (MSC) consists of cytoplasm, nucleus, actin, and microtubule. Also, integrin and primary cilium were considered as mechanoreceptors. In this study, the combined effect of vibration and oscillatory fluid flow on the cell and its components were investigated using numerical modeling. The results of the FEM and FSI model showed that the cell response (stress and strain values) at the frequency of 30 H z mechanical vibration has the highest value. The achieved results on shear stress caused by the fluid flow on the cell showed that the cell experiences shear stress in the range of 0 . 1 - 10 Pa . Mechanoreceptors that bind separately to the cell surface, can be highly stimulated by hydrodynamic pressure and, therefore, can play a role in the mechanical modulation of MSCs at in vitro conditions. The results of this research can be effective in future studies to optimize the conditions of mechanical stimuli applied to the cell culture medium and to determine the mechanisms involved in mechanotransduction.

Evaluating the cell migration potential of TiO2 nanorods incorporated in a Ti6Al4V scaffold: A multiscale approach

Improvement of cell migration by the nano-topographical modification of implant surface can directly or indirectly accelerate wound healing and osseointegration between bone and implant. Therefore, modification of the implant surface was done with TiO2 nanorod (NR) arrays to develop a more osseointegration-friendly implant in this study. Modulating the migration of a cell, adhered to a scaffold, by the variations of NR diameter, density and tip diameter in vitro is the primary objective of the study. The fluid structure interaction method was used, followed by the submodelling technique in this multiscale analysis. After completing a simulation over a global model, fluid structure interaction data was applied to the sub-scaffold finite element model to predict the mechanical response over cells at the cell-substrate interface. Special focus was given to strain energy density at the cell interface as a response parameter due to its direct correlation with the migration of an adherent cell. The results showed a huge rise in strain energy density after the addition of NRs on the scaffold surface. It also highlighted that variation in NR density plays a more effective role than the variation in NR diameter to control cell migration over a substrate. However, the effect of NR diameter becomes insignificant when the NR tip was considered. The findings of this study could be used to determine the best nanostructure parameters for better osseointegration.

Osteoclastogenesis Requires Primary Cilia Disassembly and Can Be Inhibited by Promoting Primary Cilia Formation Pharmacologically

The primary cilium is a solitary, sensory organelle with many roles in bone development, maintenance, and function. In the osteogenic cell lineage, including skeletal stem cells, osteoblasts, and osteocytes, the primary cilium plays a vital role in the regulation of bone formation, and this has made it a promising pharmaceutical target to maintain bone health. While the role of the primary cilium in the osteogenic cell lineage has been increasingly characterized, little is known about the potential impact of targeting the cilium in relation to osteoclasts, a hematopoietic cell responsible for bone resorption. The objective of this study was to determine whether osteoclasts have a primary cilium and to investigate whether or not the primary cilium of macrophages, osteoclast precursors, serves a functional role in osteoclast formation. Using immunocytochemistry, we showed the macrophages have a primary cilium, while osteoclasts lack this organelle. Furthermore, we increased macrophage primary cilia incidence and length using fenoldopam mesylate and found that cells undergoing such treatment showed a significant decrease in the expression of osteoclast markers tartrate-resistant acid phosphatase, cathepsin K, and c-Fos, as well as decreased osteoclast formation. This work is the first to show that macrophage primary cilia resorption may be a necessary step for osteoclast differentiation. Since primary cilia and preosteoclasts are responsive to fluid flow, we applied fluid flow at magnitudes present in the bone marrow to differentiating cells and found that osteoclastic gene expression by macrophages was not affected by fluid flow mechanical stimulation, suggesting that the role of the primary cilium in osteoclastogenesis is not a mechanosensory one. The primary cilium has been suggested to play a role in bone formation, and our findings indicate that it may also present a means to regulate bone resorption, presenting a dual benefit of developing ciliary-targeted pharmaceuticals for bone disease.

The role of bone marrow on the mechanical properties of trabecular bone: a systematic review

Background

Accurate evaluation of the mechanical properties of trabecular bone is important, in which the internal bone marrow plays an important role. The aim of this systematic review is to investigate the roles of bone marrow on the mechanical properties of trabecular bone to better support clinical work and laboratory research.

Methods

A systematic review of the literature published up to June 2022 regarding the role of bone marrow on the mechanical properties of trabecular bone was performed, using PubMed and Web of Science databases. The journal language was limited to English. A total of 431 articles were selected from PubMed (n = 186), Web of Science (n = 244) databases, and other sources (n = 1).

Results

After checking, 38 articles were finally included in this study. Among them, 27 articles discussed the subject regarding the hydraulic stiffening of trabecular bone due to the presence of bone marrow. Nine of them investigated the effects of bone marrow on compression tests with different settings, i.e., in vitro experiments under unconfined and confined conditions, and computer model simulations. Relatively few controlled studies reported the influence of bone marrow on the shear properties of trabecular bone.

Conclusion

Bone marrow plays a non-neglectable role in the mechanical properties of trabecular bone, its contribution varies depending on the different loading types and test settings. To obtain the mechanical properties of trabecular bone comprehensively and accurately, the solid matrix (trabeculae) and fluid-like component (bone marrow) should be considered in parallel rather than tested separately.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12938-022-01051-1.

Changes in interstitial fluid flow, mass transport and the bone cell response in microgravity and normogravity

In recent years, our scientific interest in spaceflight has grown exponentially and resulted in a thriving area of research, with hundreds of astronauts spending months of their time in space. A recent shift toward pursuing territories farther afield, aiming at near-Earth asteroids, the Moon, and Mars combined with the anticipated availability of commercial flights to space in the near future, warrants continued understanding of the human physiological processes and response mechanisms when in this extreme environment. Acute skeletal loss, more severe than any bone loss seen on Earth, has significant implications for deep space exploration, and it remains elusive as to why there is such a magnitude of difference between bone loss on Earth and loss in microgravity. The removal of gravity eliminates a critical primary mechano-stimulus, and when combined with exposure to both galactic and solar cosmic radiation, healthy human tissue function can be negatively affected. An additional effect found in microgravity, and one with limited insight, involves changes in dynamic fluid flow. Fluids provide the most fundamental way to transport chemical and biochemical elements within our bodies and apply an essential mechano-stimulus to cells. Furthermore, the cell cytoplasm is not a simple liquid, and fluid transport phenomena together with viscoelastic deformation of the cytoskeleton play key roles in cell function. In microgravity, flow behavior changes drastically, and the impact on cells within the porous system of bone and the influence of an expanding level of adiposity are not well understood. This review explores the role of interstitial fluid motion and solute transport in porous bone under two different conditions: normogravity and microgravity.

Integration of mechanics and biology in computer simulation of bone remodeling

Bone remodeling is a complex physiological process that spans across multiple spatial and temporal scales and is regulated by both mechanical and hormonal cues. An imbalance between bone resorption and bone formation in the process of bone remodeling may lead to various bone pathologies. One powerful and non-invasive approach to gain new insights into mechano-adaptive bone remodeling is computer modeling and simulation. Recent findings in bone physiology and advances in computer modeling have provided a unique opportunity to study the integration of mechanics and biology in bone remodeling. Our objective in this review is to critically appraise recent advances and developments and discuss future research opportunities in computational bone remodeling approaches that enable integration of mechanics and cellular and molecular pathways. Based on the critical appraisal of the relevant recent published literature, we conclude that multiscale in silico integration of personalized bone mechanics and mechanobiology combined with data science and analytics techniques offer the potential to deepen our knowledge of bone remodeling and provide ample opportunities for future research.

Fluid-structure interaction (FSI) modeling of bone marrow through trabecular bone structure under compression

The present study has sought to investigate the fluid characteristic and mechanical properties of trabecular bone using fluid-structure interaction (FSI) approach under different trabecular bone orientations. This method imposed on trabecular bone structure at both longitudinal and transverse orientations to identify effects on shear stress, permeability, stiffness and stress regarded to the trabeculae. Sixteen FSI models were performed on different range trabecular cubes of 27 mm³ with eight models developed for each longitudinal and transverse direction. Results show that there was a moderate correlation between permeability and porosity, and surface area in the longitudinal and transverse orientations. For the longitudinal orientation, the permeability values varied between 3.66 × 10^-8 and 1.9 × 10^-7 and the sheer stress values varied between 0.05 and 1.8 Pa, whilst for the transverse orientation, the permeability values varied between 5.95 × 10^-10 and 1.78 × 10^-8 and the shear stress values varied between 0.04 and 3.1 Pa. Here, transverse orientation limits the fluid flow from passing through the trabeculae due to high shear stress disturbance generated within the trabecular bone region. Compared to physiological loading direction (longitudinal orientation), permeability is higher within the range known to trigger a response in bone cells. Additionally, shear stresses also increase with bone surface area. This study suggests the shear stress within bone marrow in real trabecular architecture could provide the mechanical signal to marrow cells that leads to bone anabolism and can depend on trabecular orientation.

Biomimetic Aspects of Oral and Dentofacial Regeneration

Biomimetic materials for hard and soft tissues have advanced in the fields of tissue engineering and regenerative medicine in dentistry. To examine these recent advances, we searched Medline (OVID) with the key terms “biomimetics”, “biomaterials”, and “biomimicry” combined with MeSH terms for “dentistry” and limited the date of publication between 2010–2020. Over 500 articles were obtained under clinical trials, randomized clinical trials, metanalysis, and systematic reviews developed in the past 10 years in three major areas of dentistry: restorative, orofacial surgery, and periodontics. Clinical studies and systematic reviews along with hand-searched preclinical studies as potential therapies have been included. They support the proof-of-concept that novel treatments are in the pipeline towards ground-breaking clinical therapies for orofacial bone regeneration, tooth regeneration, repair of the oral mucosa, periodontal tissue engineering, and dental implants. Biomimicry enhances the clinical outcomes and calls for an interdisciplinary approach integrating medicine, bioengineering, biotechnology, and computational sciences to advance the current research to clinics. We conclude that dentistry has come a long way apropos of regenerative medicine; still, there are vast avenues to endeavour, seeking inspiration from other facets in biomedical research.

Pumpless microfluidic device with open top cell culture under oscillatory shear stress

Here we developed a 96-well plate-based pumpless microfluidic device to mimic bidirectional oscillatory shear stress experienced by osteoblasts at the endosteal niche located at the interface between bone and bone marrow. The culture device was designed to be high-throughput with 32 open top culture chambers for convenient cell seeding and staining. Mathematical modeling was used to simulate the control of oscillatory shear stress with the peak stress in the range of 0.3 to 50 mPa. Osteoblasts, cultured under oscillatory shear stress, were found to be highly viable and significantly aligned along the direction of flow. The modeling and experimental results demonstrate for the first time that cells can be cultured under controllable oscillatory shear stress in the open top culture chamber and pumpless configurations.

Enhancement of cartilage extracellular matrix synthesis in Poly(PCL-TMC)urethane scaffolds: a study of oriented dynamic flow in bioreactor

The development of new technologies to produce three-dimensional and biocompatible scaffolds associated with high-end cell culture techniques have shown to be promising for the regeneration of tissues and organs. Some biomedical devices, as meniscus prosthesis, require high flexibility and tenacity and such features are found in polyurethanes which represent a promising alternative. The Poly(PCL-TMC)urethane here presented, combines the mechanical properties of PCL with the elasticity attributed by TMC and presents great potential as a cellular carrier in cartilage repair. Scanning electron microscopy showed the presence of interconnected pores in the three-dimensional structure of the material. The scaffolds were submitted to proliferation and cell differentiation assays by culturing mesenchymal stem cells in bioreactor. The tests were performed in dynamic flow mode at the rate of 0.4 mL/min. Laser scanning confocal microscopy analysis showed that the flow rate promoted cell growth and cartilage ECM synthesis of aggrecan and type II collagen within the Poly(PCL-TMC)urethane scaffolds. This study demonstrated the applicability of the polymer as a cellular carrier in tissue engineering, as well as the ECM was incremented only when under oriented flow rate stimuli. Therefore, our results may also provide data on how oriented flow rate in dynamic bioreactors culture can influence cell activity towards cartilage ECM synthesis even when specific molecular stimuli are not present. This work addresses new perspectives for future clinical applications in cartilage tissue engineering when the molecular factors resources could be scarce for assorted reasons.

A Novel 3D Osteoblast and Osteocyte Model Revealing Changes in Mineralization and Pro-osteoclastogenic Paracrine Signaling During Estrogen Deficiency

Recent in vitro studies have revealed that the mechanobiological responses of osteoblasts and osteocytes are fundamentally impaired during estrogen deficiency. However, these two-dimensional (2D) cell culture studies do not account for in vivo biophysical cues. Thus, the objectives of this study are to (1) develop a three-dimensional (3D) osteoblast and osteocyte model integrated into a bioreactor and (2) apply this model to investigate whether estrogen deficiency leads to changes in osteoblast to osteocyte transition, mechanosensation, mineralization, and paracrine signaling associated with bone resorption by osteoclasts. MC3T3-E1s were expanded in media supplemented with estrogen (17β-estradiol). These cells were encapsulated in gelatin-mtgase before culture in (1) continued estrogen (E) or (2) no further estrogen supplementation. Constructs were placed in gas permeable and water impermeable cell culture bags and maintained at 5% CO2 and 37°C. These bags were either mechanically stimulated in a custom hydrostatic pressure (HP) bioreactor or maintained under static conditions (control). We report that osteocyte differentiation, characterized by the presence of dendrites and staining for osteocyte marker dentin matrix acidic phosphoprotein 1 (DMP1), was significantly greater under estrogen withdrawal (EW) compared to under continuous estrogen treatment (day 21). Mineralization [bone sialoprotein (BSP), osteopontin (OPN), alkaline phosphatase (ALP), calcium] and gene expression associated with paracrine signaling for osteoclastogenesis [receptor activator of nuclear factor kappa-β ligand (RANKL)/osteoprotegerin OPG ratio] were significantly increased in estrogen deficient and mechanically stimulated cells. Interestingly, BSP and DMP-1 were also increased at day 1 and day 21, respectively, which play a role in regulation of biomineralization. Furthermore, the increase in pro-osteoclastogenic signaling may be explained by altered mechanoresponsiveness of osteoblasts or osteocytes during EW. These findings highlight the impact of estrogen deficiency on bone cell function and provide a novel in vitro model to investigate the mechanisms underpinning changes in bone cells after estrogen deficiency.

Mechanical Loading Promotes the Expansion of Primitive Osteoprogenitors and Organizes Matrix and Vascular Morphology in Long Bone Defects

Elucidating the effects of mechanical stimulation on bone repair is crucial for optimization of the healing process. Specifically, the regulatory role that mechanical loading exerts on the osteogenic stem cell pool and vascular morphology during healing is incompletely understood. Because dynamic loading has been shown to enhance osteogenesis and repair, we hypothesized that loading induces the expansion of the osteoprogenitor cell population within a healing bone defect, leading to an increased presence of osteogenic cells. We further hypothesized that loading during the repair process regulates vascular and collagen matrix morphology and spatial interactions between vessels and osteogenic cells. To address these hypotheses, we used a mechanobiological bone repair model, which produces a consistent and reproducible intramembranous repair response confined in time and space. Bilateral tibial defects were created in adult C57BL/6 mice, which were subjected to axial compressive dynamic loading either during the early cellular invasion phase on postsurgical days (PSDs) 2 to 5 or during the matrix deposition phase on PSD 5 to 8. Confocal and two-photon microscopy was used to generate high-resolution three-dimensional (3D) renderings of longitudinal thick sections of the defect on PSD 10. Endomucin (EMCN)-positive vessels, Paired related homeobox 1 (Prrx1+) stem cell antigen-1 positive (Sca-1+) primitive osteoprogenitors (OPCs), and osterix positive (Osx+) preosteoblasts were visualized and quantified using deep tissue immunohistochemistry. New bone matrix was visualized with second harmonic generation autofluorescence of collagen fibers. We found that mechanical loading during the matrix deposition phase (PSD 5 to 8) increased vessel volume and number, and aligned vessels and collagen fibers to the load-bearing direction of bone. Furthermore, loading led to a significant increase in the proliferation and number of Prrx1+ Sca-1+ primitive OPCs, but not Osx+ preosteoblasts within the defect. Together, these data illustrate the adaptation of both collagen matrix and vascular morphology to better withstand mechanical load during bone repair, and that the mechanoresponsive cell population consists of the primitive osteogenic progenitors. © 2019 American Society for Bone and Mineral Research.

A simple rocker-induced mechanical stimulus upregulates mineralization by human osteoprogenitor cells in fibrous scaffolds

Biodegradable electrospun polycaprolactone scaffolds can be used to support bone-forming cells and could fill a thin bony defect, such as in cleft palate. Oscillatory fluid flow has been shown to stimulate bone production in human progenitor cells in monolayer culture. The aim of this study was to examine whether bone matrix production by primary human mesenchymal stem cells from bone marrow or jaw periosteal tissue could be stimulated using oscillatory fluid flow supplied by a standard see-saw rocker. This was investigated for cells in two-dimensional culture and within electrospun polycaprolactone scaffolds. From day 4 of culture onwards, samples were rocked at 45 cycles/min for 1 h/day, 5 days/week (rocking group). Cell viability, calcium deposition, collagen production, alkaline phosphatase activity and vascular endothelial growth factor secretion were evaluated to assess the ability of the cells to undergo bone differentiation and induce vascularisation. Both cell types produced more mineralized tissue when subjected to rocking and supplemented with dexamethasone. Mesenchymal progenitors and primary human mesenchymal stem cells from bone marrow in three-dimensional scaffolds upregulated mineral deposition after rocking culture as assessed by micro-computed tomography and alizarin red staining. Interestingly, vascular endothelial growth factor secretion, which has previously been shown to be mechanically sensitive, was not altered by rocking in this system and was inhibited by dexamethasone. Rocker culture may be a cost effective, simple pretreatment for bone tissue engineering for small defects such as cleft palate.

Infill Optimization for Additive Manufacturing-Approaching Bone-Like Porous Structures

Porous structures such as trabecular bone are widely seen in nature. These structures are lightweight and exhibit strong mechanical properties. In this paper, we present a method to generate bone-like porous structures as lightweight infill for additive manufacturing. Our method builds upon and extends voxel-wise topology optimization. In particular, for the purpose of generating sparse yet stable structures distributed in the interior of a given shape, we propose upper bounds on the localized material volume in the proximity of each voxel in the design domain. We then aggregate the local per-voxel constraints by their p-norm into an equivalent global constraint, in order to facilitate an efficient optimization process. Implemented on a high-resolution topology optimization framework, our results demonstrate mechanically optimized, detailed porous structures which mimic those found in nature. We further show variants of the optimized structures subject to different design specifications, and we analyze the optimality and robustness of the obtained structures.

Osteoblast-derived paracrine factors regulate angiogenesis in response to mechanical stimulation

Angiogenesis is a process by which new blood vessels emerge from existing vessels through endothelial cell sprouting, migration, proliferation, and tubule formation. Angiogenesis during skeletal growth, homeostasis and repair is a complex and incompletely understood process. As the skeleton adapts to mechanical loading, we hypothesized that mechanical stimulation regulates "osteo-angio" crosstalk in the context of angiogenesis. We showed that conditioned media (CM) from osteoblasts exposed to fluid shear stress enhanced endothelial cell proliferation and migration, but not tubule formation, relative to CM from static cultures. Endothelial cell sprouting was studied using a dual-channel collagen gel-based microfluidic device that mimics vessel geometry. Static CM enhanced endothelial cell sprouting frequency, whereas loaded CM significantly enhanced both frequency and length. Both sprouting frequency and length were significantly enhanced in response to factors released from osteoblasts exposed to fluid shear stress in an adjacent channel. Osteoblasts released angiogenic factors, of which osteopontin, PDGF-AA, IGBP-2, MCP-1, and Pentraxin-3 were upregulated in response to mechanical loading. These data suggest that in vivo mechanical forces regulate angiogenesis in bone by modulating "osteo-angio" crosstalk through release of paracrine factors, which we term "osteokines".

A simulation study on marrow fat effect on biomechanics of vertebra bone

Trabecular bone and bone marrow are main components of cancellous bone. Most mechanical studies for bone mainly focus on hard tissues, while if bone marrow contributes to bone biomechanics is not clear yet. This study was proposed to investigate marrow fat effect on trabecular bone biomechanics by simulation. Finite element (FE) bone models were established based on quantitative CT images at L3 lumbar spine, from which trabecular structures with and without marrow fat were investigated respectively. Auni-static compressive test was applied on the proposed models until to the appearance of fracture. Simulation results showed that trabecular models filled with marrow fat had about 3%-9% less maximum stress in volume than models with only trabeculae. However, its average stress in volume was about 9%-56% larger than those with only trabeculae. The strain energy density of the bone model with marrow fat showed a more uniformed distribution. As a conclusion, marrow fat has contributions to the bone mechanics. It can balance the stress distribution of the bone tissue, which may reduce bone deformation under a compressive loading. The mixture of trabecular structure and marrow fat would be against higher compress load before the failure point.

Altered mechanical environment of bone cells in an animal model of short- and long-term osteoporosis

Alterations in bone tissue composition during osteoporosis likely disrupt the mechanical environment of bone cells and may thereby initiate a mechanobiological response. It has proved challenging to characterize the mechanical environment of bone cells in vivo, and the mechanical environment of osteoporotic bone cells is not known. The objective of this research is to characterize the local mechanical environment of osteocytes and osteoblasts from healthy and osteoporotic bone in a rat model of osteoporosis. Using a custom-designed micromechanical loading device, we apply strains representative of a range of physical activity (up to 3000 με) to fluorescently stained femur samples from normal and ovariectomized rats. Confocal imaging was simultaneously performed, and digital image correlation techniques were applied to characterize cellular strains. In healthy bone tissue, osteocytes experience higher maximum strains (31,028 ± 4213 με) than osteoblasts (24,921 ± 3,832 με), whereas a larger proportion of the osteoblast experiences strains >10,000 με. Most interestingly, we show that osteoporotic bone cells experience similar or higher maximum strains than healthy bone cells after short durations of estrogen deficiency (5 weeks), and exceeded the osteogenic strain threshold (10,000 με) in a similar or significantly larger proportion of the cell (osteoblast, 12.68% vs. 13.68%; osteocyte, 15.74% vs. 5.37%). However, in long-term estrogen deficiency (34 weeks), there was no significant difference between bone cells in healthy and osteoporotic bone. These results suggest that the mechanical environment of bone cells is altered during early-stage osteoporosis, and that mechanobiological responses act to restore the mechanical environment of the bone tissue after it has been perturbed by ovariectomy.

The in situ mechanics of trabecular bone marrow: the potential for mechanobiological response

Bone adapts to habitual loading through mechanobiological signaling. Osteocytes are the primary mechanical sensors in bone, upregulating osteogenic factors and downregulating osteoinhibitors, and recruiting osteoclasts to resorb bone in response to microdamage accumulation. However, most of the cell populations of the bone marrow niche,which are intimately involved with bone remodeling as the source of bone osteoblast and osteoclast progenitors, are also mechanosensitive. We hypothesized that the deformation of trabecular bone would impart mechanical stress within the entrapped bone marrow consistent with mechanostimulation of the constituent cells. Detailed fluid-structure interaction models of porcine femoral trabecular bone and bone marrow were created using tetrahedral finite element meshes. The marrow was allowed to flow freely within the bone pores, while the bone was compressed to 2000 or 3000 microstrain at the apparent level.Marrow properties were parametrically varied from a constant 400 mPas to a power law rule exceeding 85 Pas. Deformation generated almost no shear stress or pressure in the marrow for the low viscosity fluid, but exceeded 5 Pa when the higher viscosity models were used. The shear stress was higher when the strain rate increased and in higher volume fraction bone. The results demonstrate that cells within the trabecular bone marrow could be mechanically stimulated by bone deformation, depending on deformation rate, bone porosity, and bone marrow properties. Since the marrow contains many mechanosensitive cells, changes in the stimulatory levels may explain the alterations in bone marrow morphology with aging and disease, which may in turn affect the trabecular bone mechanobiology and adaptation.

Pressure and shear stress in trabecular bone marrow during whole bone loading

Skeletal adaptation to mechanical loading is controlled by mechanobiological signaling. Osteocytes are highly responsive to applied strains, and are the key mechanosensory cells in bone. However, many cells residing in the marrow also respond to mechanical cues such as hydrostatic pressure and shear stress, and hence could play a role in skeletal adaptation. Trabecular bone encapsulates marrow, forming a poroelastic solid. According to the mechanical theory, deformation of the pores induces motion in the fluid-like marrow, resulting in pressure and velocity gradients. The latter results in shear stress acting between the components of the marrow. To characterize the mechanical environment of trabecular bone marrow in situ, pore pressure within the trabecular compartment of whole porcine femurs was measured with miniature pressure transducers during stress-relaxation and cyclic loading. Pressure gradients ranging from 0.013 to 0.46 kPa/mm were measured during loading. This range was consistent with calculated pressure gradients from continuum scale poroelastic models with the same permeability. Micro-scale computational fluid dynamics models created from computed tomography images were used to calculate the micromechanical stress in the marrow using the measured pressure differentials as boundary conditions. The volume averaged shear stress in the marrow ranged from 1.67 to 24.55 Pa during cyclic loading, which exceeds the mechanostimulatory threshold for mesenchymal lineage cells. Thus, the loading of bone through activities of daily living may be an essential component of bone marrow health and mechanobiology. Additional studies of cell-level interactions during loading in healthy and disease conditions will provide further incite into marrow mechanobiology.

SIMULATION ON TISSUE DIFFERENTIATIONS FOR DIFFERENT ARCHITECTURE DESIGNS IN BONE TISSUE ENGINEERING SCAFFOLD BASED ON CELLULAR STRUCTURE MODEL

In bone tissue engineering, mechanical stimuli are among the key factors affecting cell proliferation and differentiation. This study aimed to investigate the effects of different inlet fluid velocities and axial strains on the differentiation of bone marrow mesenchymal stem cells (BMSCs) on the surface of scaffolds with different morphologies. Five three-dimensional bone scaffold architectures with 65% porosity were designed using typical cellular structural models of trabecular bone. Apparent compressive strains between 0% and 5% were applied to simulate an unconfined compression test. Strain distributions were analyzed on the wall surface of the solid model. The interstitial fluid flow at inlet velocities ranging between 0.01 mm/s and 1 mm/s was applied to interconnected pores, simulating a steady state flow in the scaffold. The shear stress distributions on the surface of the scaffolds were calculated. The differentiation of BMSCs on the surface of the scaffolds with different morphologies was predicted according to mechanoregulation theory. This study shows that different levels of mechanical stimuli can be generated as a result of different scaffold morphologies under compressive loading and fluid flow to satisfy the mechanical requirements for different bone defect sites.

Biomimetic approaches in bone tissue engineering: Integrating biological and physicomechanical strategies

The development of responsive biomaterials capable of demonstrating modulated function in response to dynamic physiological and mechanical changes in vivo remains an important challenge in bone tissue engineering. To achieve long-term repair and good clinical outcomes, biologically responsive approaches that focus on repair and reconstitution of tissue structure and function through drug release, receptor recognition, environmental responsiveness and tuned biodegradability are required. Traditional orthopedic materials lack biomimicry, and mismatches in tissue morphology, or chemical and mechanical properties ultimately accelerate device failure. Multiple stimuli have been proposed as principal contributors or mediators of cell activity and bone tissue formation, including physical (substrate topography, stiffness, shear stress and electrical forces) and biochemical factors (growth factors, genes or proteins). However, optimal solutions to bone regeneration remain elusive. This review will focus on biological and physicomechanical considerations currently being explored in bone tissue engineering.

Primary cilia exist in a small fraction of cells in trabecular bone and marrow

Primary cilia are potent mechanical and chemical sensory organelles in cells of bone lineage in tissue culture. Cell culture experiments suggest that primary cilia sense fluid flow and this stimulus is translated through biochemical signaling into an osteogenic response in bone cells. Moreover, in vivo, primary cilia knockout in bone cells attenuates bone formation in response to loading. However, understanding the role of the primary cilium in bone mechanotransduction requires knowledge of its incidence and location in vivo. We used immunohistochemistry to quantify the number of cells with primary cilia within the trabecular bone tissue and the enclosed marrow of ovine cervical vertebrae. Primary cilia were identified in osteocytes, bone lining cells, and in cells within the marrow, but were present in only a small fraction of cells. Approximately 4% of osteocytes and 4.6% of bone lining cells expressed primary cilia. Within the marrow space, only approximately 1% of cells presented primary cilia. The low incidence of primary cilia may indicate that cilia either function as mechanosensors in a selected number of cells, function in concert with other mechanosensing mechanisms, or that the role of primary cilia in mechanosensing is secondary to its role in chemosensing or cellular attachment.

Multiscale Modeling of Trabecular Bone Marrow: Understanding the Micromechanical Environment of Mesenchymal Stem Cells During Osteoporosis

Mechanical loading directs the differentiation of mesenchymal stem cells (MSCs) in vitro and it has been hypothesized that the mechanical environment plays a role in directing the cellular fate of MSCs in vivo. However, the complex multicellular composition of trabecular bone marrow means that the precise nature of mechanical stimulation that MSCs experience in their native environment is not fully understood. In this study, we developed a multiscale model that discretely represents the cellular constituents of trabecular bone marrow and applied this model to characterize mechanical stimulation of MCSs in vivo. We predicted that cell-level strains in certain locations of the trabecular marrow microenvironment were greater in magnitude (maximum ε12 = ∼24,000 με) than levels that have been found to result in osteogenic differentiation of MSCs in vitro (>8000 με), which may indicate that the native mechanical environment of MSCs could direct cellular fate in vivo. The results also showed that cell–cell adhesions could play an important role in mediating mechanical stimulation within the MSC population in vivo. The model was applied to investigate how changes that occur during osteoporosis affected mechanical stimulation in the cellular microenvironment of trabecular bone marrow. Specifically, a reduced bone volume (BV) resulted in an overall increase in bone deformation, leading to greater cell-level mechanical stimulation in trabecular bone marrow (maximum ε12 = ∼48,000 με). An increased marrow adipocyte content resulted in slightly lower levels of stimulation within the adjacent cell population due to a shielding effect caused by the more compliant behavior of adipocytes (maximum ε12 = ∼41,000 με). Despite this reduction, stimulation levels in trabecular bone marrow during osteoporosis remained much higher than those predicted to occur under healthy conditions. It was found that compensatory mechanobiological responses that occur during osteoporosis, such as increased trabecular stiffness and axial alignment of trabeculae, would be effective in returning MSC stimulation in trabecular marrow to normal levels. These results have provided novel insight into the mechanical stimulation of the trabecular marrow MSC population in both healthy and osteoporotic bone, and could inform the design three-dimensional (3D) in vitro bioreactor strategies techniques, which seek to emulate physiological conditions.

Mechanical stimulation of bone marrow in situ induces bone formation in trabecular explants

Low magnitude high frequency (LMHF) loading has been shown to have an anabolic effect on trabecular bone in vivo. However, the precise mechanical signal imposed on the bone marrow cells by LMHF loading, which induces a cellular response, remains unclear. This study investigates the influence of LMHF loading, applied using a custom designed bioreactor, on bone adaptation in an explanted trabecular bone model, which isolated the bone and marrow. Bone adaptation was investigated by performing micro CT scans pre and post experimental LMHF loading, using image registration techniques. Computational fluids dynamic models were generated using the pre-experiment scans to characterise the mechanical stimuli imposed by the loading regime prior to adaptation. Results here demonstrate a significant increase in bone formation in the LMHF loaded group compared to static controls and media flow groups. The calculated shear stress in the marrow was between 0.575 and 0.7 Pa, which is within the range of stimuli known to induce osteogenesis by bone marrow mesenchymal stem cells in vitro. Interestingly, a correlation was found between the bone formation balance (bone formation/resorption), trabecular number, trabecular spacing, mineral resorption rate, bone resorption rate and mean shear stresses. The results of this study suggest that the magnitude of the shear stresses generated due to LMHF loading in the explanted bone cores has a contributory role in the formation of trabecular bone and improvement in bone architecture parameters.

Rheological behavior of fresh bone marrow and the effects of storage

The progression of several diseases, such as osteoporosis and diabetes, are associated with changes in marrow composition and physiology. As these diseases are affected by aging and activity, the biomechanical properties and mechanobiology of marrow may play a role in their progression. Bone marrow is comprised primarily of cells, and provides a niche for several mechanosensitive cell lineages. The mechanical signals imparted to the cells depend on their interaction with one another, the extracellular matrix, and the intercellular fluid. At a macroscopic scale, these interactions manifest as viscosity in marrow. Marrow viscosity has been measured in human and bovine bone. However, a large range of storage, retrieval, and measurement techniques has resulted in inconsistent data. To provide physiologically relevant data, marrow samples from young adult pigs were harvested and tested within less than 8h of slaughter. The viscosity was over 100Pas at a shear rate of 1s(-1), and decreased with shear rate according to a power law. However, the marrow did not exhibit a measurable yield stress as some complex fluids do. The viscosity of samples that had been frozen and thawed prior to testing was lower by an order of magnitude. The difference in properties was associated with a loss of integrity of the marrow adipocyte membranes. Previous reports of bone marrow viscosity have shown inconsistent results, which may be due to different storage and handling prior to testing. The higher viscosity compared to previous reports would impact poroelastic models of bone, and suggests that the stress on marrow cells during whole bone loading may be higher than previously believed.

Multiscale fluid-structure interaction modelling to determine the mechanical stimulation of bone cells in a tissue engineered scaffold

Recent studies have shown that mechanical stimulation, by means of flow perfusion and mechanical compression (or stretching), enhances osteogenic differentiation of mesenchymal stem cells and bone cells within biomaterial scaffolds in vitro. However, the precise mechanisms by which such stimulation enhances bone regeneration is not yet fully understood. Previous computational studies have sought to characterise the mechanical stimulation on cells within biomaterial scaffolds using either computational fluid dynamics or finite element (FE) approaches. However, the physical environment within a scaffold under perfusion is extremely complex and requires a multiscale and multiphysics approach to study the mechanical stimulation of cells. In this study, we seek to determine the mechanical stimulation of osteoblasts seeded in a biomaterial scaffold under flow perfusion and mechanical compression using multiscale modelling by two-way fluid-structure interaction and FE approaches. The mechanical stimulation, in terms of wall shear stress (WSS) and strain in osteoblasts, is quantified at different locations within the scaffold for cells of different attachment morphologies (attached, bridged). The results show that 75.4 % of scaffold surface has a WSS of 0.1-10 mPa, which indicates the likelihood of bone cell differentiation at these locations. For attached and bridged osteoblasts, the maximum strains are 397 and 177,200 με, respectively. Additionally, the results from mechanical compression show that attached cells are more stimulated (maximum strain = 22,600 με) than bridged cells (maximum strain = 10.000 με)Such information is important for understanding the biological response of osteoblasts under in vitro stimulation. Finally, a combination of perfusion and compression of a tissue engineering scaffold is suggested for osteogenic differentiation.