A unified theory for osteonal and hemi-osteonal remodeling

Glucocorticoids disrupt longitudinal advance of cortical bone basic multicellular units in the rabbit distal tibia

Glucocorticoids (GCs) are the leading cause of secondary osteoporosis. The emerging perspective, derived primarily from 2D histological study of trabecular bone, is that GC-induced bone loss arises through the uncoupling of bone formation and resorption at the level of the basic multicellular unit (BMU), which carries out bone remodeling. Here we explore the impact of GCs on cortical bone remodeling in the rabbit model. Based upon the rapid reduction of bone formation and initial elevation of resorption caused by GCs, we hypothesized that the rate of advance (longitudinal erosion rate; LER) of cortical BMUs would be increased. To test this hypothesis we divided 20 female New Zealand White rabbits into four experimental groups: ovariohysterectomy (OVH), glucocorticoid (GC), OVH + GC and SHAM controls (n = 5 animals each). Ten weeks post-surgery (OVH or sham), and two weeks after the initiation of dosing (daily subcutaneous injections of 1.5 mg/kg of methylprednisolone sodium succinate in the GC-treated groups and 1 ml of saline for the others), the right tibiae were scanned in vivo using Synchrotron Radiation (SR) in-line phase contrast micro-CT at the Canadian Light Source. After an additional 2 weeks of dosing, the rabbits were euthanized and ex vivo images were collected using desktop micro-CT. The datasets were co-registered in 3D and LER was calculated as the distance traversed by BMU cutting-cones in the 14-day interval between scans. Counter to our hypothesis, LER was greatly reduced in GC-treated rabbits. Mean LER was lower in GC (4.27 μm/d; p < 0.001) and OVH + GC (4.19 μm/d; p < 0.001), while similar in OVH (40.13 μm/d; p = 0.990), compared to SHAM (40.44 μm/d). This approximately 90 % reduction in LER with GCs was also associated with an overall disruption of BMU progression, with radial expansion of the remodeling space occurring in all directions. This unexpected outcome suggests that GCs do not simply uncouple formation and resorption within cortical BMUs and highlights the value of the time-lapsed 4D approach employed.

Trabecular and cortical bone microarchitecture using high-resolution peripheral quantitative computed tomographic imaging in diabetic peripheral neuropathy

CONTEXT

Type 2 Diabetes Mellitus (T2D) is associated with an increased risk of fragility fracture despite normal areal bone mineral density (BMD). The contribution of diabetic peripheral neuropathy (PN) to volumetric BMD (vBMD) and bone microarchitecture in T2D is not explored.

OBJECTIVE

To assess vBMD and microarchitectural properties of bone using high-resolution peripheral quantitative computed tomography (HR-pQCT) in patients of T2D with or without PN.

DESIGN

This is a cross-sectional study of patients of T2D divided into two groups [patients with T2D without PN (Group A) and T2D with PN (Group B)]. All patients underwent clinical examination, biochemical evaluation, dual-energy X-ray absorptiometry (DXA), and HR-pQCT of the radius and tibia.

RESULTS

A total of 296 patients were included in the study [Group A (n = 98), Group B (n = 198)]. HR-pQCT demonstrated a significant difference in total vBMD[mg/cm3] at tibia (291.6 ± 61.8 vs. 268.2 ± 63.0; p-0.003); cortical vBMD[mg/cm3] at tibia [912.5 (863.3, 962.4) vs. 853.8 (795.3, 913.2) p-0.000], among groups A and B respectively. Among the microarchitecture parameters, there was a significant difference in cortical porosity at the tibia (2.5% ±1.7% vs. 3%±1.7%; p-0.004), trabecular number[mm-1] at the tibia [1.080 (0.896, 1.237) vs. 1.140 (0.983, 1.286), p-0.045] and trabecular thickness[mm] at the radius [0.228 (0.217, 0.247) Vs. 0.238 (0.224, 0.253); p-0.006], among groups A and B respectively.

CONCLUSION

Despite comparable areal BMD, T2D patients with PN have diminished vBMD and deteriorated skeletal microarchitecture, compared to those without PN.

Vascular study of decellularized porcine long bones: Characterization of a tissue engineering model

INTRODUCTION

Massive bone allografts enable the reconstruction of critical bone defects in numerous conditions (e.g. tumoral, infection or trauma). Unfortunately, their biological integration remains insufficient and the reconstruction may suffer from several postoperative complications. Perfusion-decellularization emerges as a tissue engineering potential solution to enhance osseointegration. Therefore, an intrinsic vascular study of this novel tissue engineering tool becomes essential to understand its efficacy and applicability.

MATERIAL AND METHODS

32 porcine long bones (humeri and femurs) were used to assess the quality of their vascular network prior and after undergoing a perfusion-decellularization protocol. 12 paired bones were used to assess the vascular matrix prior (N = 6) and after our protocol (N = 6) by immunohistochemistry. Collagen IV, Von Willebrand factor and CD31 were targeted then quantified. The medullary macroscopic vascular network was evaluated with 12 bones: 6 were decellularized and the other 6 were, as control, not treated. All 12 underwent a contrast-agent injection through the nutrient artery prior an angio CT-scan acquisition. The images were processed and the length of medullary vessels filled with contrast agent were measured on angiographic cT images obtained in control and decellularized bones by 4 independent observers to evaluate the vascular network preservation. The microscopic cortical vascular network was evaluated on 8 bones: 4 control and 4 decellularized. After injection of gelatinous fluorochrome mixture (calcein green), non-decalcified fluoroscopic microscopy was performed in order to assess the perfusion quality of cortical vascular lacunae.

RESULTS

The continuity of the microscopic vascular network was assessed with Collagen IV immunohistochemistry (p-value = 0.805) while the decellularization quality was observed through CD31 and Von Willebrand factor immunohistochemistry (p-values <0.001). The macroscopic vascular network was severely impaired after perfusion-decellularization; nutrient arteries were still patent but the amount of medullary vascular channels measured was significantly higher in the control group compared to the decellularized group (p-value <0.001). On average, the observers show good agreement on these results, except in the decellularized group where more inter-observer discrepancies were observed. The microscopic vascular network was observed with green fluoroscopic signal in almost every canals and lacunae of the bone cortices, in three different bone locations (proximal metaphysis, diaphysis and distal metaphysis).

CONCLUSION

Despite the aggressiveness of the decellularization protocol on medullary vessels, total porcine long bones decellularized by perfusion retain an acellular cortical microvascular network. By injection through the intact nutrient arteries, this latter vascular network can still be used as a total bone infusion access for bone tissue engineering in order to enhance massive bone allografts prior implantation.

A review of mathematical modeling of bone remodeling from a systems biology perspective

Bone remodeling is an essential, delicately balanced physiological process of coordinated activity of bone cells that remove and deposit new bone tissue in the adult skeleton. Due to the complex nature of this process, many mathematical models of bone remodeling have been developed. Each of these models has unique features, but they have underlying patterns. In this review, the authors highlight the important aspects frequently found in mathematical models for bone remodeling and discuss how and why these aspects are included when considering the physiology of the bone basic multicellular unit, which is the term used for the collection of cells responsible for bone remodeling. The review also emphasizes the view of bone remodeling from a systems biology perspective. Understanding the systemic mechanisms involved in remodeling will help provide information on bone pathology associated with aging, endocrine disorders, cancers, and inflammatory conditions and enhance systems pharmacology. Furthermore, some features of the bone remodeling cycle and interactions with other organ systems that have not yet been modeled mathematically are discussed as promising future directions in the field.

Modelling the Effects of Growth and Remodelling on the Density and Structure of Cancellous Bone

A two-stage model is proposed for investigating remodelling characteristics in bone over time and distance to the growth plate. The first stage comprises a partial differential equation (PDE) for bone density as a function of time and distance from the growth plate. This stage clarifies the contributions to changes in bone density due to remodelling and growth processes and tracks the rate at which new bone emanates from the growth plate. The second stage consists of simulating the remodelling process to determine remodelling characteristics. Implementing the second stage requires the rate at which bone moves away from the growth plate computed during the first stage. The second stage is also needed to confirm that remodelling characteristics predicted by the first stage may be explained by a realistic model for remodelling and to compute activation frequency. The model is demonstrated on microCT scans of tibia of juvenile female rats in three experimental groups: sham-operated control, oestrogen deprived, and oestrogen deprived followed by treatment. Model predictions for changes in bone density and remodelling characteristics agree with the literature. In addition, the model provides new insight into the role of treatment on the density of new bone emanating from the growth plate and provides quantitative descriptions of changes in remodelling characteristics beyond what has been possible to ascertain by experimentation alone.

An in silico micro-multiphysics agent-based approach for simulating bone regeneration in a mouse femur defect model

Bone defects represent a challenging clinical problem as they can lead to non-union. In silico models are well suited to study bone regeneration under varying conditions by linking both cellular and systems scales. This paper presents an in silico micro-multiphysics agent-based (micro-MPA) model for bone regeneration following an osteotomy. The model includes vasculature, bone, and immune cells, as well as their interaction with the local environment. The model was calibrated by time-lapsed micro-computed tomography data of femoral osteotomies in C57Bl/6J mice, and the differences between predicted bone volume fractions and the longitudinal in vivo measurements were quantitatively evaluated using root mean square error (RMSE). The model performed well in simulating bone regeneration across the osteotomy gap, with no difference (5.5% RMSE, p = 0.68) between the in silico and in vivo groups for the 5-week healing period - from the inflammatory phase to the remodelling phase - in the volume spanning the osteotomy gap. Overall, the proposed micro-MPA model was able to simulate the influence of the local mechanical environment on bone regeneration, and both this environment and cytokine concentrations were found to be key factors in promoting bone regeneration. Further, the validated model matched clinical observations that larger gap sizes correlate with worse healing outcomes and ultimately simulated non-union. This model could help design and guide future experimental studies in bone repair, by identifying which are the most critical in vivo experiments to perform.

Daily administration of parathyroid hormone slows the progression of basic multicellular units in the cortical bone of the rabbit distal tibia

Basic Multicellular Units (BMUs) conduct bone remodeling, a critical process of tissue turnover which, if imbalanced, can lead to disease, including osteoporosis. Parathyroid hormone (PTH 1-34; Teriparatide) is an osteoanabolic treatment for osteoporosis; however, it elevates the rate of intra-cortical remodeling (activation frequency) leading, at least transiently, to increased porosity. The purpose of this study was to test the hypothesis that PTH not only increases the rate at which cortical BMUs are initiated but also increases their progression (Longitudinal Erosion Rate; LER). Two groups (n = 7 each) of six-month old female New Zealand white rabbits were both administered 30 μg/kg of PTH once daily for a period of two weeks to induce remodeling. Their distal right tibiae were then imaged in vivo by in-line phase contrast micro-CT at the Canadian Light Source synchrotron. Over the following two weeks the first group (PTH) received continued daily PTH while the second withdrawal group (PTHW) was administrated 0.9 % saline. At four weeks all animals were euthanized, their distal tibiae were imaged by conventional micro-CT ex vivo and histomorphometry was performed. Matching micro-CT datasets (in vivo and ex vivo) were co-registered in 3D and LER was measured from 612 BMUs. Counter to our hypothesis, mean LER was lower (p < 0.001) in the PTH group (30.19 ± 3.01 μm/day) versus the PTHW group (37.20 ± 2.77 μm/day). Despite the difference in LER, osteonal mineral apposition rate (On.MAR) did not differ between groups indicating the anabolic effect of PTH was sustained after withdrawal. The slowing of BMU progression by PTH warrants further investigation; slowed resorption combined with elevated bone formation rate, may play an important role in how PTH enhances coupling between resorption and formation within the BMU. Finally, the prolonged anabolic response following withdrawal may have utility in terms of optimizing clinical dosing regimens.

Conceptual design of compliant bone scaffolds by full-scale topology optimization

A promising new treatment for large and complex bone defects is to implant specifically designed and additively manufactured synthetic bone scaffolds. Optimizing the scaffold design can potentially improve bone in-growth and prevent under- and over-loading of the adjacent tissue. This study aims to optimize synthetic bone scaffolds over multiple-length scales using the full-scale topology optimization approach, and to assess the effectiveness of this approach as an alternative to the currently used mono- and multi-scale optimization approaches for orthopaedic applications. We present a topology optimization formulation, which is matching the scaffold's mechanical properties to the surrounding tissue in compression. The scaffold's porous structure is tuneable to achieve the desired morphological properties to enhance bone in-growth. The proposed approach is demonstrated in-silico, using PEEK, cortical bone and titanium material properties in a 2D parameter study and on 3D designs. Full-scale topology optimization indicates a design improvement of 81% compared to the multi-scale approach. Furthermore, 3D designs for PEEK and titanium are additively manufactured to test the applicability of the method. With further development, the full-scale topology optimization approach is anticipated to offer a more effective alternative for optimizing orthopaedic structures compared to the currently used multi-scale methods.

Both network architecture and micro cracks effects on lacuno-canalicular liquid flow efficiency within the context of multiphysics approach for bone remodeling

When physical forces are applied to bone, its mechanical adaptive behaviors change according to the microarchitecture configuration. This leads to changes in biological and physical thresholds in the remodeling cell population, involving sensor cells (osteocytes) interacting with each other and changes in osteocyte shape due to variation in lacunar shape. The resulting alterations in fluid flow leads to changes in the membrane electrical potential and shear stress. Eventual creation of microcracks, may lead in turn to modify cell activity. In contrast, the redundancy in the lacuno canalicular network (LCN) interconnectivity maintains partial flow. Our goal was to investigate the role of fluid flow in LCN by proposing a model of electro-mechanical energy spread through inhomogeneous microarchitectures. We focused on mechano-sensitivity to changes in load-induced flow impacted by neighboring micro cracks and quantifying its critical role in changing, velocity, shear stress and orientation of liquid mass transportation from one cell to another. To enhance the concept of intricacy LCN micro-structure to fluid flow, we provide a new combined effects factor considered as osteocytes sensor efficiency. We customized an influence function for each osteocyte, coupling: in one hand, the spatial distribution within remodeling influence areas, conducting a significant fluid spread, leading hydro-dynamic behavior and impacted further by presence of micro cracks and; in other hand, the fluid electro kinetic behavior. As an attempt to fill the limitations stated by many of the recent studies, we reveal in numerical simulation, some results which cannot be measured in vitro/in vivo studies. Numerical calculations were performed in order to evaluate, among many others, how liquid flow conditions changes between lacunas, how the orientation and the magnitude of the governing flow in LCN can regulate osteocytes efficiency. In addition to be regulated by osteocytes, a direct effects of fluid flow are also acting on osteoblast activity. In summary, this new approach considers mechano-sensitivity in relation to liquid flow dynamic and suggests additional pathway for Osseo integration via osteoblast regulation. However, this novel modeling approach may help improve the mapping and design bone scaffolds and/or selection of scaffold implantation regions.

A multimodal 3D imaging approach of pore networks in the human femur to assess age-associated vascular expansion and Lacuno-Canalicular reduction

Cellular communication in the mechanosensory osteocyte Lacuno-Canalicular Network (LCN) regulates bone tissue remodeling throughout life. Age-associated declines in LCN size and connectivity dysregulate mechanosensitivity to localized remodeling needs of aging or damaged tissue, compromising bone quality. Synchrotron radiation-based micro-Computed Tomography (SRμCT) and Confocal Laser Scanning Microscopy (CLSM) were employed to visualize LCN and vascular canal morphometry in an age series of the anterior femur (males n = 14, females n = 11, age range = 19-101, mean age = 55). Age-associated increases in vascular porosity were driven by pore coalescence, including a significant expansion in pore diameter and a significant decline in pore density. In contrast, the LCN showed significant age-associated reductions in lacunar volume fraction, mean diameter, and density, and in canalicular volume fraction and connectivity density. Lacunar density was significantly lower in females across the lifespan, exacerbating their age-associated decline. Canalicular connectivity density was also significantly lower in females but approached comparable declining male values in older age. Our data illuminate the trajectory and potential morphometric sources of age-associated bone loss. Increased vascular porosity contributes to bone fragility with aging, while an increasingly reduced and disconnected LCN undermines the mechanosensitivity required to repair and reinforce bone. Understanding why and how this degradation occurs is essential for improving the diagnosis and treatment of age-related changes in bone quality and fragility.

Mechanoregulated trabecular bone adaptation: Progress report on in silico approaches

Adaptation is the process by which bone responds to changes in loading environment and modulates its properties and spatial organization to meet the mechanical demands. Adaptation in trabecular bone is achieved through increase in bone mass and alignment of trabecular-bone morphology along the loading direction. This transformation of internal microstructure is governed by mechanical stimuli sensed by mechanosensory cells in the bone matrix. Realisation of adaptation in the form of local bone-resorption and -formation activities as a function of mechanical stimuli is still debated. In silico modelling is a useful tool for simulation of various scenarios that cannot be investigated in vivo and particularly well suited for prediction of trabecular bone adaptation. This progress report presents the recent advances in in silico modelling of mechanoregulated adaptation at the scale of trabecular bone tissue. Four well-established bone-adaptation models are reviewed in terms of their recent improvements and validation. They consider various mechanical factors: (i) strain energy density, (ii) strain and damage, (iii) stress nonuniformity and (iv) daily stress. Contradictions of these models are discussed and their ability to describe adequately a real-life mechanoregulation process in bone is compared.

Evaluation of Stresses on Implant, Bone, and Restorative Materials Caused by Different Opposing Arch Materials in Hybrid Prosthetic Restorations Using the All-on-4 Technique

The long-term success of dental implants is greatly influenced by the use of appropriate materials while applying the “All-on-4” concept in the edentulous jaw. This study aims to evaluate the stress distribution in the “All-on-4” prosthesis across different material combinations using three-dimensional finite element analysis (FEA) and to evaluate which opposing arch material has destructive effects on which prosthetic material while offering certain recommendations to clinicians accordingly. Acrylic and ceramic-based hybrid prosthesis have been modelled on a rehabilitated maxilla using the “All-on-4” protocol. Using different materials and different supports in the opposing arch (natural tooth, and implant/ceramic, and acrylic), a multi-vectorial load has been applied. To measure stresses in bone, maximum and minimum principal stress values were calculated, while Von Mises stress values were obtained for prosthetic materials. Within a single group, the use of an acrylic implant-supported prosthesis as an antagonist to a full arch implant-supported prosthesis yielded lower maximum (Pmax) and minimum (Pmin) principal stresses in cortical bone. Between different groups, maxillary prosthesis with polyetheretherketone as framework material showed the lowest stress values among other maxillary prostheses. The use of rigid materials with higher moduli of elasticity may transfer higher stresses to the peri implant bone. Thus, the use of more flexible materials such as acrylic and polyetheretherketone could result in lower stresses, especially upon atrophic bones.

Closing the osteon: Do osteocytes sense strain rate rather than fluid flow?

Osteons are cylindrical structures of bone created by matrix resorbing osteoclasts, followed by osteoblasts that deposit new bone. Osteons align with the principal loading direction and it is thought that the osteoclasts are directed by osteocytes, the mechanosensitive cells that reside inside the bone matrix. These osteocytes are presumably controlled by interstitial fluid flow, induced by the physiological loading of bones. Here I consider the stimulation of osteocytes while the osteon is closed by osteoblasts. In a conceptual finite element model, bone is considered a poro-elastic material and subjected to locomotion-induced loading conditions. It appears that the magnitude of flow is constant along the closing cone, while shear strain rate in the bone matrix diminishes linearly with the deposition of bone. This suggests that shear strain rate, rather than fluid flow, is the physical cue that controls osteocytes and bone deposition in newly formed osteons.

Load adaptation through bone remodeling: a mechanobiological model coupled with the finite element method

This work proposes a novel tissue-scale mechanobiological model of bone remodeling to study bone's adaptation to distinct loading conditions. The devised algorithm describes the mechanosensitivity of bone and its impact on bone cells' functioning through distinct signaling factors. In this study, remodeling is mechanically ruled by variations of the strain energy density (SED) of bone, which is determined by performing a linear elastostatic analysis combined with the finite element method. Depending on the SED levels and on a set of biological signaling factors ([Formula: see text] parameters), osteoclasts and osteoblasts can be mechanically triggered. To reproduce this phenomenon, this work proposes a new set of [Formula: see text] parameters. The combined response of osteoclasts and osteoblasts will then affect bone's apparent density, which is correlated with other mechanical properties of bone, through a phenomenological law. Thus, this novel model proposes a constant interplay between the mechanical and biological components of the process. The spatiotemporal simulation used to validate this new approach is a benchmark example composed by two distinct phases: (1) pre-orientation and (2) load adaptation. On both of them, bone is able to adapt its morphology according to the loading condition, achieving the required trabecular distribution to withstand the applied loads. Moreover, the equilibrium morphology reflects the orientation of the load. These preliminary results support the new approach proposed in this study.

An organic-inorganic hybrid scaffold with honeycomb-like structures enabled by one-step self-assembly-driven electrospinning

Electrospun organic/inorganic hybrid scaffolds have been appealing in tissue regeneration owing to the integrated physicochemical and biological performances. However, the conventional electrospun scaffolds with non-woven structures usually failed to enable deep cell infiltration due to the densely stacked layers among the fibers. Herein, through self-assembly-driven electrospinning, a polyhydroxybutyrate/poly(ε-caprolactone)/58S sol-gel bioactive glass (PHB/PCL/58S) hybrid scaffold with honeycomb-like structures was prepared by manipulating the solution composition and concentration during a one-step electrospinning process. The mechanisms enabling the formation of self-assembled honeycomb-like structures were investigated through comparative studies using Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) between PHB/PCL/58S and PHB/PCL/sol-gel silica systems. The obtained honeycomb-like structure was built up from nanofibers with an average diameter of 370 nm and showed a bimodal distribution of pores: large polygonal pores up to hundreds of micrometers within the honeycomb-cells and irregular pores among the nanofibers ranging around few micrometers. The cell-materials interactions were further studied by culturing MG-63 osteoblast-like cells for 7 days. Cell viability, cell morphology and cell infiltration were comparatively investigated as well. While cells merely proliferated on the surface of non-woven structures, MG-63 cells showed extensive proliferation and deep infiltration up to 100-200 μm into the honeycomb-like structure. Moreover, the cellular spatial organization was readily regulated by the honeycomb-like pattern as well. Overall, the newly obtained hybrid scaffold may integrate the enhanced osteogenicity originating from the bioactive components, and the improved cell-material interactions brought by the honeycomb-like structure, making the new scaffold a promising candidate for tissue regeneration.

Osteon: Structure, Turnover, and Regeneration

Bone is composed of dense and solid cortical bone and honeycomb-like trabecular bone. Although cortical bone provides the majority of mechanical strength for a bone, there are few studies focusing on cortical bone repair or regeneration. Osteons (the Haversian system) form structural and functional units of cortical bone. In recent years, emerging evidences have shown that the osteon structure (including osteocytes, lamellae, lacunocanalicular network, and Haversian canals) plays critical roles in bone mechanics and turnover. Therefore, reconstruction of the osteon structure is crucial for cortical bone regeneration. This article provides a systematic summary of recent advances in osteons, including the structure, function, turnover, and regenerative strategies. First, the hierarchical structure of osteons is illustrated and the critical functions of osteons in bone dynamics are introduced. Next, the modeling and remodeling processes of osteons at a cellular level and the turnover of osteons in response to mechanical loading and aging are emphasized. Furthermore, several bioengineering approaches that were recently developed to recapitulate the osteon structure are highlighted. Impact statement This review provides a comprehensive summary of recent advances in osteons, especially the roles in bone formation, remodeling, and regeneration. Besides introducing the hierarchical structure and critical functions of osteons, we elucidate the modeling and remodeling of osteons at a cellular level. Specifically, we highlight the bioengineering approaches that were recently developed to mimic the hierarchical structure of osteons. We expect that this review will provide informative insights and attract increasing attentions in orthopedic community, shedding light on cortical bone regeneration in the future.

Toward a Mathematical Modeling of Diseases' Impact on Bone Remodeling: Technical Review

A wide variety of bone diseases have hitherto been discovered, such as osteoporosis, Paget's disease, osteopetrosis, and metastatic bone disease, which are not well defined in terms of changes in biochemical and mechanobiological regulatory factors. Some of these diseases are secondary to other pathologies, including cancer, or to some clinical treatments. To better understand bone behavior and prevent its deterioration, bone biomechanics have been the subject of mathematical modeling that exponentially increased over the last years. These models are becoming increasingly complex. The current paper provides a timely and critical analysis of previously developed bone remodeling mathematical models, particularly those addressing bone diseases. Besides, mechanistic pharmacokinetic/pharmacodynamic (PK/PD) models, which englobe bone disease and its treatment's effect on bone health. Therefore, the review starts by presenting bone remodeling cycle and mathematical models describing this process, followed by introducing some bone diseases and discussing models of pathological mechanisms affecting bone, and concludes with exhibiting the available bone treatment procedures considered in the PK/PD models.

INTERFERENCES EFFECTS BETWEEN OSTEOCYTES LOCATED AT THE SAME ZONE DURING BONE REMODELING PERIOD: A THEORETICAL AND NUMERICAL STUDY

Background: Several analytical models have been developed in the past to analyze the specific role of osteocytes in the process of bone remodeling, which can be considered as the response of bone material to functional requirements. Most of them considered both the number of osteocytes and their spatial distribution in one area of influence, while others suggested in addition to include considerations of the size of the basic multi-cellular unit. Methods: Taking advantage of previous works, the standard model equation is revisited by incorporating two complementary parameters: (a) the possibility of resorption of osteocytes, apoptosis or function inhibition during remodeling process triggered by the transduction phase of osteocytes embedded within the bone matrix and; (b) the interference of influence zones for the same osteocyte. Results: Bone density evolution has been calculated starting with a medical imaging of an implanted femur. It is shown that the management of interference zone and the possibility of resorption or inhibition of osteocytes have a direct impact upon the value of the mechanical stimulus and hence on the recruitment of Bone Multicellular Units (BMUs). From a mathematical point of view, this effect has been considered by modifying mechanical stimulus of the standard model such that it is impacted by a scalar factor ranged in the interval (0.5–1). Conclusion: It is clearly demonstrated that predicted of the added bone mass amount shows that the new model is more active in low density regions where requiring rapid adaptation to the behavior of the implant, and that the standard model takes the lead in the regions with high density.

Association between diabetic neuropathy and osteoporosis in patients: a systematic review and meta-analysis

OBJECTIVES

Many studies have explored the association between neuropathy and osteoporosis in patients with diabetes mellitus. However, the results still remain inconsistent and controversial. We aimed to estimate the association between diabetic neuropathy and osteoporosis.

METHODS

Databases, including PubMed, Embase, Web of Science, the Cochrane library, Chinese Biomedical Literature Database (CBM), and Wanfang, were screened from inception to 30 March 2020. Studies were selected and data were extracted by two independent reviewers. Study characteristics and quality sections were reviewed independently. Pooled ORs and 95% CIs were calculated using random effects model when evidence of heterogeneity was present; otherwise, fixed effects model was used. Meta-regression and subgroup analyses were performed to explore the source of heterogeneity. Sensitivity analysis and publication bias were also tested.

RESULTS

A total of 11 studies with 27,585 participants were included in this analysis which indicated that there was an increased odd between diabetic neuropathy and osteoporosis (overall OR 2.20, 95% CI 1.71-2.83). In the subgroup analyses and meta-regression, diabetic neuropathy has no significant difference in osteoporosis or fracture (p = 0.532). And osteoporosis also has no significant difference in type 1 or type 2 diabetic neuropathy (p = 0.668).

CONCLUSIONS

This meta-analysis suggests that patients with diabetic neuropathy have a significantly increased chance of developing osteoporosis, even fragility fracture. The clinicians should pay more attention to the patients with diabetic neuropathy. Further studies were still needed to explore the confounding factors among studies and to elucidate the underlying biological mechanisms.

BONE ORTHOTROPIC REMODELING AS A THERMODYNAMICALLY-DRIVEN EVOLUTION

In this paper, we present and discuss a model of bone remodeling set up in the framework of the theory of generalized continuum mechanics which was first introduced by DiCarlo et al. [Sur le remodelage des tissus osseux anisotropes, Comptes Rendus Mécanique 334(11):651–661, 2006]. Bone is described as an orthotropic body experiencing remodeling as a rotation of its microstructure. Thus, the complete kinematic description of a material point is provided by its position in space and a rotation tensor describing the orientation of its microstructure. Material motion is driven by energetic considerations, namely by the application of the Clausius–Duhem inequality to the microstructured material. Within this framework of orthotropic remodeling, some key features of the remodeling equilibrium configurations are deduced in the case of homogeneous strain or stress loading conditions. First, it is shown that remodeling equilibrium configurations correspond to energy extrema. Second, stability of the remodeling equilibrium configurations is assessed in terms of the local convexity of the strain and complementary energy functionals hence recovering some classical energy theorems. Eventually, it is shown that the remodeling equilibrium configurations are not only highly dependent on the loading conditions, but also on the material properties.

Theoretical concept of cortical to cancellous bone transformation

Microstructures of cortical and cancellous bones are altered continually by load-adaptive remodeling; in addition, their cellular mechanisms are similar despite the remarkably different porosities. The cortico-cancellous transitional zone is a site of vigorous remodeling, and intracortical remodeling cavitates the inner cortex to promote its trabecularization, which is considered the main cause of bone loss because of aging. Therefore, to prevent and treat age-related cortical bone loss effectively, it is indispensable to gain an integrated understanding of the cortical to the cancellous bone transformation via remodeling. We propose a novel theoretical concept to account for the transformation of dense cortical bone to porous cancellous bone. We develop a mathematical model of cortical and cancellous bone remodeling based on the concept that bone porosity is determined by the balance between the load-bearing function of mineralized bone and the material-transporting function of bone marrow. Remodeling simulations using this mathematical model enable the reproduction of the microstructures of cortical and cancellous bones simultaneously. Furthermore, current remodeling simulations have the potential to replicate cortical-to-cancellous bone transformation based on changes in the local balance between bone formation and resorption. We anticipate that the proposed mathematical model of cortical and cancellous bone remodeling will contribute to highlighting the essential features of cortical bone loss due to trabecularization of the cortex and help predict its spatial and temporal behavior during aging.

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A novel theoretical concept to account for cortical-to-cancellous bone transformation is proposed.

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A remodeling model to reproduce cortical and cancellous bone microstructures is developed.

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The remodeling simulation replicates cortical-to-cancellous bone transformation.

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The proposed method is valuable in clinical applications such as in predicting age-related cortical bone loss.

Tumour Initiation: a Discussion on Evidence for a "Load-Trigger" Mechanism

Appropriate mechanical forces on cells are vital for normal cell behaviour and this review discusses the possibility that tumour initiation depends partly on the disruption of the normal physical architecture of the extracellular matrix (ECM) around a cell. The alterations that occur thence promote oncogene expression. Some questions, that are not answered with certainty by current consensus mechanisms of tumourigenesis, are elegantly explained by the triggering of tumours being a property of the physical characteristics of the ECM, which is operative following loading of the tumour initiation process with a relevant gene variant. Clinical observations are consistent with this alternative hypothesis which is derived from studies that have, together, accumulated an extensive variety of data incorporating biochemical, genetic and clinical findings. Thus, this review provides support for the view that the ECM may have an executive function in induction of a tumour. Overall, reported observations suggest that either restoring an ECM associated with homeostasis or targeting the related signal transduction mechanisms may possibly be utilised to modify or control the early progression of cancers. The review provides a coherent template for discussing the notion, in the context of contemporary knowledge, that tumourigenesis is an alliance of biochemistry, genetics and biophysics, in which the physical architecture of the ECM may be a fundamental component. For more definitive clarification of the concept there needs to be a phalanx of experiments conceived around direct questions that are raised by this paper.

Peripheral Neuropathy as a Component of Skeletal Disease in Diabetes

PURPOSE OF REVIEW

The goal of this review is to explore clinical associations between peripheral neuropathy and diabetic bone disease and to discuss how nerve dysfunction may contribute to dysregulation of bone metabolism, reduced bone quality, and fracture risk.

RECENT FINDINGS

Diabetic neuropathy can decrease peripheral sensation (sensory neuropathy), impair motor coordination (motor neuropathy), and increase postural hypotension (autonomic neuropathy). Together, this can impair overall balance and increase the risk for falls and fractures. In addition, the peripheral nervous system has the potential to regulate bone metabolism directly through the action of local neurotransmitters on bone cells and indirectly through neuroregulation of the skeletal vascular supply. This review critically evaluates existing evidence for diabetic peripheral neuropathy as a risk factor or direct actor on bone disease. In addition, we address therapeutic and experimental considerations to guide patient care and future research evaluating the emerging relationship between diabetic neuropathy and bone health.

3D analysis of the osteonal and interstitial tissue in human radii cortical bone

Human cortical bone has a complex hierarchical structure that is periodically remodelled throughout a lifetime. This microstructure dictates the mechanical response of the tissue under a critical load. If only some structural features, such as the different porosities observed in bone, are primarily studied, then investigations may not fully consider the osteonal systems in three-dimensions (3D). Currently, it is difficult to differentiate osteons from interstitial tissue using standard 3D characterization methods. Synchrotron radiation micro-computed tomography (SR-μCT) in the phase contrast mode is a promising method for the investigation of osteons. In the current study, SR-μCT imaging was performed on cortical bone samples harvested from eight human radii (female, 50-91 y.o.). The images were segmented to identify Haversian canals, osteocyte lacunae, micro-cracks, as well as osteons. The significant correlation between osteonal and Haversian canal volume fraction highlights the role of the canals as sites where bone remodelling is initiated. The results showed that osteocyte lacunae morphometric parameters depend on their distance to cement lines, strongly suggesting the evolution of biological activity from the beginning to the end of the remodelling process. Thus, the current study provides new data on 3D osteonal morphometric parameters and their relationships with other structural features in humans.

Effect of Chlorella Pyrenoidosa Protein Hydrolysate-Calcium Chelate on Calcium Absorption Metabolism and Gut Microbiota Composition in Low-Calcium Diet-Fed Rats

In our current investigation, we evaluated the effect of Chlorella pyrenoidosa protein hydrolysate (CPPH) and Chlorella pyrenoidosa protein hydrolysate-calcium chelate (CPPH-Ca) on calcium absorption and gut microbiota composition, as well as their in vivo regulatory mechanism in SD rats fed low-calcium diets. Potent major compounds in CPPH were characterized by HPLC-MS/MS, and the calcium-binding mechanism was investigated through ultraviolet and infrared spectroscopy. Using high-throughput next-generation 16S rRNA gene sequencing, we analyzed the composition of gut microbiota in rats. Our study showed that HCPPH-Ca increased the levels of body weight gain, serum Ca, bone activity, bone mineral density (BMD) and bone mineral content (BMC), while decreased serum alkaline phosphatase (ALP) and inhibited the morphological changes of bone. HCPPH-Ca up-regulated the gene expressions of transient receptor potential cation V5 (TRPV5), TRPV6, calcium-binding protein-D9k (CaBP-D9k) and a calcium pump (plasma membrane Ca-ATPase, PMCA1b). It also improved the abundances of Firmicutes and Lactobacillus. Bifidobacterium and Sutterella were both positively correlated with calcium absorption. Collectively, these findings illustrate the potential of HCPPH-Ca as an effective calcium supplement.

Migration and differentiation of osteoclast precursors under gradient fluid shear stress

The skeleton can adapt to mechanical loading through bone remodeling, and osteoclasts close to microdamages are believed to initiate bone resorption. However, whether local mechanical loading, such as fluid flow, regulates recruitment and differentiation of osteoclast precursors at the site of bone resorption has yet to be investigated. In the present study, finite element analysis first revealed the existence of a low-fluid shear stress (FSS) field inside microdamage. Based on a custom-made device of cone-and-plate fluid chamber, finite element analysis and particle image velocimetry measurement were performed to verify the formation of gradient FSS flow field. Furthermore, the effects of gradient FSS on the migration, aggregation, and fusion of osteoclast precursors were observed. Osteoclast precursor RAW264.7 cells migrated along a radial direction toward the region with decreased FSS during exposure to gradient FSS stimulation for 40 min, thereby deviating from the direction of actual fluid flow indicated by fluorescent particles. When calcium signaling pathway was inhibited by gadolinium and thapsigargin, cell migration toward a low-FSS region was significantly reduced. For the other cell lines MC3T3-E1, PDLF, rat mesenchymal stem cells, and Madin-Darby canine kidney epithelial cells, gradient FSS stimulation did not lead to low-FSS inclined migration. After being cultured under gradient FSS stimulation for 6 days, RAW264.7 cells showed significantly higher density and ratio of TRAP-positive multinucleated osteoclasts in the low-FSS region to those in the high-FSS region. Therefore, osteoclast precursor cells may exhibit the special ability to sense FSS gradient and tend to actively migrate toward low-FSS regions, which are regulated by calcium signaling pathway.

Bone cortical compactness in 'tree sloths' reflects convergent evolution

Bone remodeling, one of the main processes that regulate bone microstructure, consists of bone resorption followed by the deposition of secondary bone at the same location. Remodeling intensity varies among taxa, but a characteristically compact cortex is ubiquitous in the long bones of mature terrestrial mammals. A previous analysis found that cortical bone in a few 'tree sloth' (Bradypus and Choloepus) specimens is heavily remodeled and characterized by numerous immature secondary osteons, suggesting that these animals were remodeling their bones at high rate until late in their ontogeny. This study aims at testing if this remodeling is generally present in 'tree sloths', using a quantitative analysis of the humeral cortical compactness (CC) among xenarthrans. The results of the investigation of humeral diaphyseal cross-sections of 26 specimens belonging to 10 xenarthran species including specimens from both extinct and extant species indicate that in 'tree sloths' the CC is significantly lower than in the other sampled xenarthrans. No significant difference was found between the CC of the two genera of 'tree sloths'. Our results are consistent with the hypothesis that the cortical bone of 'tree sloths' in general undergoes intense and balanced remodeling that is maintained until late (possibly throughout) in their ontogeny. In the light of xenarthran phylogeny, low CC represents another convergence between the long-separated 'tree sloth' lineages. Although the exact structural and/or functional demands that are associated with this trait are hitherto unknown, several hypotheses are suggested here, including a relationship to their relatively low metabolism and to the mechanical demands imposed upon the bones by the suspensory posture and locomotion, which was independently acquired by the two genera of 'tree sloths'.

Comparison of patient-specific computational models vs. clinical follow-up, for adjacent segment disc degeneration and bone remodelling after spinal fusion

Spinal fusion is a standard surgical treatment for patients suffering from low back pain attributed to disc degeneration. However, results are somewhat variable and unpredictable. With fusion the kinematic behaviour of the spine is altered. Fusion and/or stabilizing implants carrying considerable load and prevent rotation of the fused segments. Associated with these changes, a risk for accelerated disc degeneration at the adjacent levels to fusion has been demonstrated. However, there is yet no method to predict the effect of fusion surgery on the adjacent tissue levels, i.e. bone and disc. The aim of this study was to develop a coupled and patient-specific mechanoregulated model to predict disc generation and changes in bone density after spinal fusion and to validate the results relative to patient follow-up data. To do so, a multiscale disc mechanoregulation adaptation framework was developed and coupled with a previously developed bone remodelling algorithm. This made it possible to determine extra cellular matrix changes in the intervertebral disc and bone density changes simultaneously based on changes in loading due to fusion surgery. It was shown that for 10 cases the predicted change in bone density and degeneration grade conforms reasonable well to clinical follow-up data. This approach helps us to understand the effect of surgical intervention on the adjacent tissue remodelling. Thereby, providing the first insight for a spine surgeon as to which patient could potentially be treated successfully by spinal fusion and in which patient has a high risk for adjacent tissue changes.

3D micro structural analysis of human cortical bone in paired femoral diaphysis, femoral neck and radial diaphysis

Human bone is known to adapt to its mechanical environment in a living body. Both its architecture and microstructure may differ between weight-bearing and non-weight-bearing bones. The aim of the current study was to analyze in three dimensions, the morphology of the multi-scale porosities on human cortical bone at different locations. Eight paired femoral diaphyses, femoral necks, and radial diaphyses were imaged using Synchrotron Radiation µCT with a 0.7 µm isotropic voxel size. The spatial resolution facilitates the investigation of the multiscale porosities of cortical bone, from the osteonal canals system down to the osteocyte lacunar system. Our results showed significant differences in the microstructural properties, regarding both osteonal canals and osteocytes lacunae, between the different anatomical locations. The radius presents significantly lower osteonal canal volume fraction and smaller osteonal canals than the femoral diaphysis or neck. Osteocytes lacunae observed in the radius are significantly different in shape than in the femur, and lacunar density is higher in the femoral neck. These results show that the radius, a non-weight-bearing bone, is significantly different in terms of its microstructure from a weight-bearing bone such as the femur. This implies that the cortical bone properties evaluated on the femoral diaphysis, the main location studied within the literature, cannot be generalized to other anatomical locations.

Cortical Histomorphometry of the Human Humerus During Ontogeny

Modeling and remodeling are two key determinants of human skeletal growth though little is known about the histomorphometry of cortical bone during ontogeny. In this study, we examined the density and geometric properties of primary and secondary osteons (osteon area and diameter, vascular canal area and diameter) in subperiosteal cortical bone from the human humerus (n = 84) between birth and age 18 years. Sections were removed from the anterior midshaft aspect of humeri from skeletons. Age-at-death was reconstructed using standard osteological techniques. Analyses revealed significant correlation between the histomorphometric variables and age. Higher densities of primary osteons occurred between infancy and 7 years of age but were almost completely replaced by secondary osteons after 14 years of age. The geometry of primary osteons was less clearly related to age. Secondary osteons were visible after 2 years of age and reached their greatest densities in the oldest individuals. Osteon size was positively but weakly influenced by age. Our data imply that modeling and remodeling are age-dependent processes that vary markedly from birth to adulthood in the human humerus.

Microfluidic co-culture platform for investigating osteocyte-osteoclast signalling during fluid shear stress mechanostimulation

Bone cells exist in a complex environment where they are constantly exposed to numerous dynamic biochemical and mechanical stimuli. These stimuli regulate bone cells that are involved in various bone disorders, such as osteoporosis. Knowledge of how these stimuli affect bone cells have been utilised to develop various treatments, such as pharmaceuticals, hormone therapy, and exercise. To investigate the role that bone loading has on these disorders in vitro, bone cell mechanotransduction studies are typically performed using parallel plate flow chambers (PPFC). However, these chambers do not allow for dynamic cellular interactions among different cell populations to be investigated. We present a microfluidic approach that exposes different cell populations, which are located at physiologically relevant distances within adjacent channels, to different levels of fluid shear stress, and promotes cell-cell communication between the different channels. We employed this microfluidic system to assess mechanically regulated osteocyte-osteoclast communication. Osteoclast precursors (RAW264.7 cells) responded to cytokine gradients (e.g., RANKL, OPG, PGE-2) developed by both mechanically stimulated (fOCY) and unstimulated (nOCY) osteocyte-like MLO-Y4 cells simultaneously. Specifically, we observed increased osteoclast precursor cell densities and osteoclast differentiation towards nOCY. We also used this system to show an increased mechanoresponse of osteocytes when in co-culture with osteoclasts. We envision broad applicability of the presented approach for microfluidic perfusion co-culture of multiple cell types in the presence of fluid flow stimulation, and as a tool to investigate osteocyte mechanotransduction, as well as bone metastasis extravasation. This system could also be applied to any multi-cell population cross-talk studies that are typically performed using PPFCs (e.g. endothelial cells, smooth muscle cells, and fibroblasts).

Localized tissue mineralization regulated by bone remodelling: A computational approach

Bone is a living tissue whose main mechanical function is to provide stiffness, strength and protection to the body. Both stiffness and strength depend on the mineralization of the organic matrix, which is constantly being remodelled by the coordinated action of the bone multicellular units (BMUs). Due to the dynamics of both remodelling and mineralization, each sample of bone is composed of structural units (osteons in cortical and packets in cancellous bone) created at different times, therefore presenting different levels of mineral content. In this work, a computational model is used to understand the feedback between the remodelling and the mineralization processes under different load conditions and bone porosities. This model considers that osteoclasts primarily resorb those parts of bone closer to the surface, which are younger and less mineralized than older inner ones. Under equilibrium loads, results show that bone volumes with both the highest and the lowest levels of porosity (cancellous and cortical respectively) tend to develop higher levels of mineral content compared to volumes with intermediate porosity, thus presenting higher material densities. In good agreement with recent experimental measurements, a boomerang-like pattern emerges when plotting apparent density at the tissue level versus material density at the bone material level. Overload and disuse states are studied too, resulting in a translation of the apparent-material density curve. Numerical results are discussed pointing to potential clinical applications.

Bone remodeling as a spatial evolutionary game

Bone remodeling is a complex process involving cell-cell interactions, biochemical signaling and mechanical stimuli. Early models of the biological aspects of remodeling were non-spatial and focused on the local dynamics at a fixed location in the bone. Several spatial extensions of these models have been proposed, but they generally suffer from two limitations: first, they are not amenable to analysis and are computationally expensive, and second, they neglect the role played by bone-embedded osteocytes. To address these issues, we developed a novel model of spatial remodeling based on the principles of evolutionary game theory. The analytically tractable framework describes the spatial interactions between zones of bone resorption, bone formation and quiescent bone, and explicitly accounts for regulation of remodeling by bone-embedded, mechanotransducing osteocytes. Using tools from the theory of interacting particle systems we systematically classified the different dynamic regimes of the spatial model and identified regions of parameter space that allow for global coexistence of resorption, formation and quiescence, as observed in physiological remodeling. In coexistence scenarios, three-dimensional simulations revealed the emergence of sponge-like bone clusters. Comparison between spatial and non-spatial dynamics revealed substantial differences and suggested a stabilizing role of space. Our findings emphasize the importance of accounting for spatial structure and bone-embedded osteocytes when modeling the process of bone remodeling. Thanks to the lattice-based framework, the proposed model can easily be coupled to a mechanical model of bone loading.

Governing Equations of Tissue Modelling and Remodelling: A Unified Generalised Description of Surface and Bulk Balance

Several biological tissues undergo changes in their geometry and in their bulk material properties by modelling and remodelling processes. Modelling synthesises tissue in some regions and removes tissue in others. Remodelling overwrites old tissue material properties with newly formed, immature tissue properties. As a result, tissues are made up of different “patches”, i.e., adjacent tissue regions of different ages and different material properties, within evolving boundaries. In this paper, generalised equations governing the spatio-temporal evolution of such tissues are developed within the continuum model. These equations take into account nonconservative, discontinuous surface mass balance due to creation and destruction of material at moving interfaces, and bulk balance due to tissue maturation. These equations make it possible to model patchy tissue states and their evolution without explicitly maintaining a record of when/where resorption and formation processes occurred. The time evolution of spatially averaged tissue properties is derived systematically by integration. These spatially-averaged equations cannot be written in closed form as they retain traces that tissue destruction is localised at tissue boundaries. The formalism developed in this paper is applied to bone tissues, which exhibit strong material heterogeneities due to their slow mineralisation and remodelling processes. Evolution equations are proposed in particular for osteocyte density and bone mineral density. Effective average equations for bone mineral density (BMD) and tissue mineral density (TMD) are derived using a mean-field approximation. The error made by this approximation when remodelling patchy tissue is investigated. The specific signatures of the time evolution of BMD or TMD during remodelling events are exhibited. These signatures may provide a way to detect remodelling events at lower, unseen spatial resolutions from microCT scans.

Investigating histomorphometric relationships at the human femoral midshaft in a biomechanical context

Cortical bone histomorphometry utilised in human and animal bone biology studies has demonstrated that osteon densities and their geometric properties may be in a relationship with biomechanical load application. Further research is required to investigate mutual links between bone histological variables to elucidate their usefulness in future biomechanical studies. Here, a series of correlations exploring bone biology relationships at the human midshaft femur were performed using a large sample. Mean intact, fragmentary and total osteon population densities, Haversian canal diameter and area, osteon area, as well as osteocyte lacunae density were measured along the sub-periosteal cortex in sections removed from the posterior midshaft aspect of modern human male (n = 233) and female (n = 217) femora (total n = 450). Parametric and non-parametric correlations between the histology variables were sought in the entire sample, as well as within age and sex sub-groups. Several significant positive and negative correlations explaining a large proportion of data variation were found. Haversian canal area, diameter, and osteon area were positively correlated. As the density of osteocyte lacunae increased, Haversian canals and osteons became smaller. As osteons increased in density, so did osteocyte lacunae, but Haversian canal and osteon area became smaller. Results were consistent across age and sex groups. Findings suggest that an increased rate of bone remodelling is associated with a decrease in geometrical properties of osteons. An increased density of osteocyte lacunae and osteons indicates the involvement of bone maintenance cells in remodelling potentially induced by mechanical stimuli. Future histomorphometry studies will benefit from examining multiple bone histology variables due to many mutual bone biology relationships that exist at the human midshaft femur.

Chemistry of bone remodelling preserved in extant and fossil Sirenia

Bone remodelling is a crucial biological process needed to maintain elemental homeostasis.

Effect of Ceramic Scaffold Architectural Parameters on Biological Response

Numerous studies have focused on the optimization of ceramic architectures to fulfill a variety of scaffold functional requirements and improve biological response. Conventional fabrication techniques, however, do not allow for the production of geometrically controlled, reproducible structures and often fail to allow the independent variation of individual geometric parameters. Current developments in additive manufacturing technologies suggest that 3D printing will allow a more controlled and systematic exploration of scaffold architectures. This more direct translation of design into structure requires a pipeline for design-driven optimization. A theoretical framework for systematic design and evaluation of architectural parameters on biological response is presented. Four levels of architecture are considered, namely (1) surface topography, (2) pore size and geometry, (3) porous networks, and (4) macroscopic pore arrangement, including the potential for spatially varied architectures. Studies exploring the effect of various parameters within these levels are reviewed. This framework will hopefully allow uncovering of new relationships between architecture and biological response in a more systematic way as well as inform future refinement of fabrication techniques to fulfill architectural necessities with a consideration of biological implications.

A THREE-DIMENSIONAL COMPUTER MODEL TO SIMULATE SPONGY BONE REMODELING UNDER OVERLOAD USING A SEMI-MECHANISTIC BONE REMODELING THEORY

Overload has been suggested as a contributing factor for bone loss, for instance at the bone implant interface. The objective of this study is to investigate spongy bone resorption under overload using a semi-mechanistic bone remodeling theory. Since overload can cause the accumulation of microdamage in bone, in this study, it is assumed that overload will increase the osteoclastic activity, and also will reduce the osteocyte influence distance. First, a previously proposed semi-mechanistic bone remodeling theory was extended by defining a new form for the resorption probability function, which is based on experimental evidence. Then, in order to investigate the validity of our hypothesis, a three-dimensional finite element model of spongy bone was developed. The simulation results show that, first, trabeculae adapt with the mechanical stimuli placed on them. Secondly, a sharp reduction in spongy bone density will be resulted, in agreement with experimental evidence, when bone is under overload.