Altered architecture and cell populations affect bone marrow mechanobiology in the osteoporotic human femur

Navigating the biophysical landscape: how physical cues steer the journey of bone metastatic tumor cells

Many tumors prefer to metastasize to bone, but the underlying mechanisms remain elusive. The human skeletal system has unique physical properties, that are distinct from other organs, which play a key role in directing the behavior of tumor cells within bone. Understanding the physical journey of tumor cells within bone is crucial. In this review we discuss bone metastasis in the context of how physical cues in the bone vasculature and bone marrow niche regulate the fate of tumor cells. Our objective is to inspire innovative diagnostic and therapeutic approaches for bone metastasis from a mechanobiological perspective.

Mechanobiological optimization of scaffolds for bone tissue engineering

Synthetic bone graft scaffolds aim to generate new bone tissue and alleviate the limitations of autografts and allografts. To meet that aim, it is essential to have a design approach able to generate scaffold architectures that will promote bone formation. Here, we present a topology-varying design optimization method, the "mixed-topology" approach, that generates new designs from a set of starting structures. This approach was used with objective functions focusing on improving the scaffold's local mechanical microenvironments to mechanobiologically promote bone formation within the scaffold and constraints to ensure manufacturability and achieve desired macroscale properties. The results demonstrate that this approach can successfully generate scaffold designs with improved microenvironments, taking into account different combinations of relevant stimuli and constraints.

Prediction of Bone Remodeling in Rat Caudal Vertebrae Based on Fluid-Solid Coupling Simulation

Some previous researches have demonstrated that appropriate mechanical stimulation can enhance bone formation. However, most studies have employed the strain energy density (SED) method for predicting bone remodeling, with only a few considering the potential impact of wall fluid shear stress (FSS) on this process. To bridge this gap, the current study compared the prediction of bone formation and resorption via SED and wall FSS by using fluid-solid coupling numerical simulation. Specifically, 8-week-old female Sprague-Dawley rats were subjected to stretching of the eighth caudal vertebra using a custom-made device. Based on micro-computed tomography images, a three-dimensional model integrating fluid-solid coupling was created to represent compact bone, cancellous bone, and bone marrow. The animals were grouped into control, 1 Hz, and 10 Hz categories, wherein a tensile displacement load of 1000 με was applied to the loading end. The results revealed that SED values tended to increase with elevated porosity, whereas wall FSS values decreased it. Notably, wall FSS demonstrated the higher predictive accuracy for cancellous bone resorption than SED. These findings support the notion that fluid flow within cancellous bone spaces can significantly impact bone resorption. Therefore, the findings of this study contribute to a more comprehensive understanding of the role of wall FSS in bone remodeling, providing a theoretical support for the dynamic evolution of bone structures under mechanical stimulation.

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.

Fluid-solid coupling numerical simulation of entire rat caudal vertebrae under dynamic loading

Trabeculae bone undergoes directional growth along the applied force under physiological loading. The growth of bone structure relies on the coordinated interplay among osteocytes, osteoblasts, and osteoclasts. Under normal circumstances, bone remodeling maintains a state of equilibrium. Excessive bone formation can lead to osteosclerosis, while excessive bone resorption can result in osteoporosis and osteonecrosis. The investigation of the structural characteristics of trabeculae and the mechanotransduction between bone cells plays a vital role in the treatment of bone-related diseases. In this study, a fluid-solid coupling model of the entire vertebral bone was established based on micro-CT images obtained from rat tail vertebrae subjected to tensile loading experiments. The flow characteristics of bone marrow and the mechanical response of osteocytes in different regions under physiological loading were investigated. The results revealed a U-shaped distribution of wall fluid shear stress (FSS) along the longitudinal axis in trabecular bone, with higher FSS regions exhibiting greater mechanical stimulation on osteocytes. These findings elucidate a positive correlation between the mechanical microenvironment among osteocytes, osteoblasts, and osteoclasts, providing potential strategies for the prevention and treatment of bone diseases.

Numerical simulations of fluid flow in trabecular-lacunar cavities under cyclic loading

BACKGROUND

Under external loading, the fluid shear stress (FSS) in the porous structures of bones, such as trabecular or lacunar-canalicular cavity, can influence the biological response of bone cells. However, few studies have considered both cavities. The present study investigated the characteristics of fluid flow at different scales in cancellous bone in rat femurs, as well as the effects of osteoporosis and loading frequency.

METHODS

Sprague Dawley rats (3 months old) were divided into normal and osteoporotic groups. A multiscale 3D fluid-solid coupling finite element model considering trabecular system and lacunar-canalicular system was established. Cyclic displacement loadings with frequencies of 1, 2, and 4 Hz were applied.

FINDINGS

Results showed that the wall FSS around the adhesion complexes of osteocyte on the canaliculi was higher than that on the osteocyte body. Under the same loading conditions, the wall FSS of the osteoporotic group was smaller than that of the normal group. The fluid velocity and FSS in trabecular pores exhibited a linear relationship with loading frequency. Similarly, the FSS around osteocytes also showed the loading frequency-dependent phenomenon.

INTERPRETATION

The high cadence in movement can effectively increase the FSS level on osteocytes for osteoporotic bone, i.e., expand the space within the bone with physiological load. This study might help in understanding the process of bone remodeling under cyclic loading and provide the fundamental data for the development of strategies for osteoporosis treatment.

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.

Effects of mechanobiological signaling in bone marrow on skeletal health

Bone marrow is a cellular tissue that forms within the pore space and hollow diaphysis of bones. As a tissue, its primary function is to support the hematopoietic progenitor cells that maintain the populations of both erythroid and myeloid lineage cells in the bone marrow, making it an essential element of normal mammalian physiology. However, bone's primary function is load bearing, and deformations induced by external forces are transmitted to the encapsulated marrow. Understanding the effects of these mechanical inputs on marrow function and adaptation requires knowledge of the material behavior of the marrow at multiple scales, the loads that are applied, and the mechanobiology of the cells. This paper reviews the current state of knowledge of each of these factors. Characterization of the marrow mechanical environment and its role in skeletal health and other marrow functions remains incomplete, but research on the topic is increasing, driven by interest in skeletal adaptation and the mechanobiology of cancer metastasis.

Shear Stress in Bone Marrow has a Dose Dependent Effect on cFos Gene Expression in In Situ Culture

Introduction

Mechanical stimulation of bone is necessary to maintain its mass and architecture. Osteocytes within the mineralized matrix are sensors of mechanical deformation of the hard tissue, and communicate with cells in the marrow to regulate bone remodeling. However, marrow cells are also subjected to mechanical stress during whole bone loading, and may contribute to mechanically regulated bone physiology. Previous results from our laboratory suggest that mechanotransduction in marrow cells is sufficient to cause bone formation in the absence of osteocyte signaling. In this study, we investigated whether bone formation and altered marrow cell gene expression response to stimulation was dependent on the shear stress imparted on the marrow by our loading regime.

Methods

Porcine trabecular bone explants were cultured in an in situ bioreactor for 5 or 28 days with stimulation twice daily. Gene expression and bone formation were quantified and compared to unstimulated controls. Correlation was used to assess the dependence on shear stress imparted by the loading regime calculated using computational fluid dynamics models.

Results

Vibratory stimulation resulted in a higher trabecular bone formation rate (p = 0.01) and a greater increase in bone volume fraction (p = 0.02) in comparison to control explants. Marrow cell expression of cFos increased with the calculated marrow shear stress in a dose-dependent manner (p = 0.002).

Conclusions

The results suggest that the shear stress due to interactions between marrow cells induces a mechanobiological response. Identification of marrow cell mechanotransduction pathways is essential to understand healthy and pathological bone adaptation and remodeling.

Trabecular resorption patterns of cement-bone interlock regions in total knee replacements

With in vivo service, there is loss of mechanical interlock between trabeculae and PMMA cement in total knee replacements. The mechanisms responsible for the loss of interlock are not known, but loss of interlock results in weaker cement-bone interfaces. The goal of this study was to determine the pattern of resorption of interdigitated bone using a series of 20 postmortem retrieved knee replacements with a wide range of time in service (3-22 years). MicroCT scans were obtained of a segment of the cement-bone interface below the tibial tray for each implant. Image processing methods were used to determine interface morphology and to identify supporting, interdigitated, resorbed, and isolated bone as a function of axial position. Overall, the amount of remaining interdigitated bone decreased with time in service (p = 0.0114). The distance from the cement border (at the extent of cement penetration into the bone bed) to 50% of the interdigitated volume decreased with time in service (p = 0.039). Isolated bone, when present, was located deep in the cement layer. Overall, resorption appears to start at the cement border and progresses into the cement layer. Initiation of trabecular resorption near the cement border may be a consequence of proximity to osteoclastic cells in the adjacent marrow space.

CLINICAL SIGNIFICANCE

Aseptic loosening of joint replacements remains an important clinical problem. This work explores the process and pattern of trabecular bone resorption responsible for loss of interface fixation. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2773-2780, 2017.