The importance of murine cortical bone microstructure for microcrack initiation and propagation

Osteoporosis and Covid-19: Detected similarities in bone lacunar-level alterations via combined AI and advanced synchrotron testing

While advanced imaging strategies have improved the diagnosis of bone-related pathologies, early signs of bone alterations remain difficult to detect. The Covid-19 pandemic has brought attention to the need for a better understanding of bone micro-scale toughening and weakening phenomena. This study used an artificial intelligence-based tool to automatically investigate and validate four clinical hypotheses by examining osteocyte lacunae on a large scale with synchrotron image-guided failure assessment. The findings indicate that trabecular bone features exhibit intrinsic variability related to external loading, micro-scale bone characteristics affect fracture initiation and propagation, osteoporosis signs can be detected at the micro-scale through changes in osteocyte lacunar features, and Covid-19 worsens micro-scale porosities in a statistically significant manner similar to the osteoporotic condition. Incorporating these findings with existing clinical and diagnostic tools could prevent micro-scale damages from progressing into critical fractures.

The synergy of synchrotron imaging and convolutional neural networks towards the detection of human micro-scale bone architecture and damage

The growing health and economic burden of bone fractures, their intricate multiscale features and the existing knowledge gaps in the comprehension of micro-scale bone damage occurrence make fracture diagnosis a challenging issue. In this scenario, deep-learning and artificial intelligence embody the new frontier of healthcare system, by overcoming the subjectivity of clinicians in the analysis of medical images. However, the preliminary attempts in exploiting the power of machine learning algorithms such as neural networks are still limited to bone macro-scale, while there is an evident lack in their application to smaller scales, where damage starts nucleating. Currently, speculations at the micro-scale are only feasible with the aid of high-resolution imaging techniques, that are particularly time consuming in terms of output images analysis. In this context, this works aims at combining the visualization of the micro-crack propagation mechanism with the promising application of convolutional neural networks. The implemented artificial intelligence tool is based for the first time on a large number of human synchrotron images coming from healthy and osteoporotic femoral heads tested under micro-compression. The designed convolutional neural networks are able to automatically detect lacunae and micro-cracks at different compression levels with high accuracy levels; indeed, with the baseline setup, networks achieve more than 0.99 level of accuracy for both cracks and lacunae, and more than 0.87 of the meanIoU adopted as validation metric. This approach is particularly encouraging for the development of powerful recognition system to comprehend bone micro-damage initiation and propagation, paving the way to the application of machine learning studies to bone micromechanics. This could be additionally crucial for future patient specific micro-scale observations to be related to the clinical practice.

Characterizing the Mechanical Behavior of Bone and Bone Surrogates in Compression Using pQCT

Many axial and appendicular skeleton bones are subjected to repetitive loading during daily activities. Until recently, the structural analysis of fractures has been limited to 2D sections, and the dynamic assessment of fracture progression has not been possible. The structural failure was analyzed using step-wise micro-compression combined with time-lapsed micro-computed tomographic imaging. The structural failure was investigated in four different sample materials (two different bone surrogates, lumbar vertebral bodies from bovine and red deer). The samples were loaded in different force steps based on uniaxial compression tests. The micro-tomography images were used to create three-dimensional models from which various parameters were calculated that provide information about the structure and density of the samples. By superimposing two 3D images and calculating the different surfaces, it was possible to precisely analyze which trabeculae failed in which area and under which load. According to the current state of the art, bone mineral density is usually used as a value for bone quality, but the question can be raised as to whether other values such as trabecular structure, damage accumulation, and bone mineralization can predict structural competence better than bone mineral density alone.

Induction of somatopause in adult mice compromises bone morphology and exacerbates bone loss during aging

Somatopause refers to the gradual declines in growth hormone (GH) and insulin‐like growth factor‐1 throughout aging. To define how induced somatopause affects skeletal integrity, we used an inducible GH receptor knockout (iGHRKO) mouse model. Somatopause, induced globally at 6 months of age, resulted in significantly more slender bones in both male and female iGHRKO mice. In males, induced somatopause was associated with progressive expansion of the marrow cavity leading to significant thinning of the cortices, which compromised bone strength. We report progressive declines in osteocyte lacunar number, and increases in lacunar volume, in iGHRKO males, and reductions in lacunar number accompanied by ~20% loss of overall canalicular connectivity in iGHRKO females by 30 months of age. Induced somatopause did not affect mineral/matrix ratio assessed by Raman microspectroscopy. We found significant increases in bone marrow adiposity and high levels of sclerostin, a negative regulator of bone formation in iGHRKO mice. Surprisingly, however, despite compromised bone morphology, osteocyte senescence was reduced in the iGHRKO mice. In this study, we avoided the confounded effects of constitutive deficiency in the GH/IGF‐1 axis on the skeleton during growth, and specifically dissected its effects on the aging skeleton. We show here, for the first time, that induced somatopause compromises bone morphology and the bone marrow environment.

Analysis of type I osteoporosis animal models using synchrotron radiation

Preclinical experiments to analyze the trabecular space of spongy bones using small animals are required for the evaluation and treatment of patients with osteoporosis (OP). We performed ovariectomy to create OP models. A total of four mice were used. Ovariectomized group (OVX, n = 2) in which both ovaries were resected at random, and the sham operated group (SHAM, n = 2) performed surgery without resecting the ovaries. We propose a study that enables OP analysis by analyzing tibia microstructures of OVX and SHAM using synchrotron radiation (SR). SR imaging is a technology capable of irradiating an extremely small object in the order of several tens of nanometers using a nondestructive method at the microscopic level. Unlike previous imaging diagnoses (staining, micro-CT [Computed Tomography]) it was possible to preserve the real shape and analyze bone microstructures in real-time and analyze and evaluate spongy bones to secure data and increase the reliability of OP analysis. We were able to confirm the possibility of OP diagnosis through experimental animals for spongy bone damage related to bone mineral density. Therefore, we aimed to provide a rehabilitation and medicine therapy intervention method through basic research on the evaluation of OP diagnosis through human-based segmentation of challenging spongy bones while supplementing the limitations of existing imaging methods. RESEARCH HIGHLIGHTS: We present an analysis of osteoporosis through spongy bone using phase-contrast X-ray source. Unlike existing methods, it is possible to analyze the internal microstructure of the tibia with this method. This is an objective mechanism for OP and a basis for rehabilitation.

Anatomical variation in intracortical canal network microarchitecture and its influence on bone fracture risk

Intracortical canals are a major contributor to cortical bone porosity and influence its mechanical response. Canal networks act as stress concentrators and the magnitude of which depends on the size and spatial distribution of canals. In the present study, we investigated site-dependent variation in intracortical canal network morphological indices and their effect on the mechanical response of bone. For this, mid-diaphysis of rat tibia bones were scanned using high-resolution micro-CT and morphological indices were measured for four main anatomical sites-anterior, posterior, medial and lateral. Further, a micro-finite element (μFE) model was developed to quantify the stress concentration regions in different cortices. The fracture risk was assessed using an effective strain approach. Results show that canal porosity, canal orientation and canal length are site-dependent whereas canal diameter and canal number density are independent of the site. The lateral cortex has significantly higher porosity compared to the posterior cortex (p < 0.05). The orientation of canals is found significantly different between endosteal and periosteal regions for anterior and medial quadrants. Canals are inclined at higher angles with bone axis in the endosteal region as compare to the periosteal region. The μ-FE results show that the regions with higher effective strain are concentrated around the canals. Further, failed element volume per unit bone volume is found highest for medial cortex whereas lowest for posterior cortex. The higher failed volume is associated with more radial canals in the medial cortex as compare to other cortices. The linear regression analysis shows that the volume of overstrained elements strongly depends on canal orientation (R² = 0.73, p < 0.0001) and canal porosity (R² = 0.61, p < 0.0001). The findings from this study suggest that along with vascular canal porosity, canal orientation and canal diameter can further improve the bone fracture risk assessment.

A Review on Multiscale Bone Damage: From the Clinical to the Research Perspective

The investigation of bone damage processes is a crucial point to understand the mechanisms of age-related bone fractures. In order to reduce their impact, early diagnosis is key. The intricate architecture of bone and the complexity of multiscale damage processes make fracture prediction an ambitious goal. This review, supported by a detailed analysis of bone damage physical principles, aims at presenting a critical overview of how multiscale imaging techniques could be used to implement reliable and validated numerical tools for the study and prediction of bone fractures. While macro- and meso-scale imaging find applications in clinical practice, micro- and nano-scale imaging are commonly used only for research purposes, with the objective to extract fragility indexes. Those images are used as a source for multiscale computational damage models. As an example, micro-computed tomography (micro-CT) images in combination with micro-finite element models could shed some light on the comprehension of the interaction between micro-cracks and micro-scale bone features. As future insights, the actual state of technology suggests that these models could be a potential substitute for invasive clinical practice for the prediction of age-related bone fractures. However, the translation to clinical practice requires experimental validation, which is still in progress.

Heterogeneity of the osteocyte lacuno-canalicular network architecture and material characteristics across different tissue types in healing bone

Various tissue types, including fibrous connective tissue, bone marrow, cartilage, woven and lamellar bone, coexist in healing bone. Similar to most bone tissue type, healing bone contains a lacuno-canalicular network (LCN) housing osteocytes. These cells are known to orchestrate bone remodeling in healthy bone by sensing mechanical strains and translating them into biochemical signals. The structure of the LCN is hypothesized to influence mineralization processes. Hence, the aim of the present study was to visualize and match spatial variations in the LCN topology with mineral characteristics, within and at the interfaces of the different tissue types that comprise healing bone. We applied a correlative multi-method approach to visualize the LCN architecture and quantify mineral particle size and orientation within healing femoral bone in a mouse osteotomy model (26 weeks old C57BL/6 mice). This approach revealed structural differences across several length scales during endochondral ossification within the following regions: calcified cartilage, bony callus, cortical bone and a transition zone between the cortical and callus region analyzed 21 days after the osteotomy. In this transition zone, we observed a continuous convergence of mineral characteristics and osteocyte lacunae shape as well as discontinuities in the lacunae volume and LCN connectivity. The bony callus exhibits a 34% higher lacunae number density and 40% larger lacunar volume compared to cortical bone. The presented correlations between LCN architecture and mineral characteristics improves our understanding of how bone develops during healing and may indicate a contribution of osteocytes to bone (re)modeling.

Mechanical and thermal damage in cortical bone drilling in vivo

Recently, the failure rate of fracture fixation to fractured bone has increased. Mechanical and thermal damage to the bone, which influences the contact area and cell growth between the bone and the screw, is the primary reason for fixation failure. However, research has mainly focused on force and temperature in bone drilling. In this study, the characteristics of hole edges, microcracks, empty lacunae, and osteon necrosis were investigated as viewed in the transverse and longitudinal sections after drilling. Drilling force and temperature were also recorded for comparing the relationship with mechanical and thermal damage. Experiments were conducted in vivo using five different drill geometries under the same drilling parameters. Characteristics of the hole wall were detected using computed tomography. Microcracks and necrosis were analyzed using the pathological sectioning method. The maximum microcrack was approximately 3000 and 1400 μm in the transverse section and longitudinal section, respectively, which were much larger than those observed in previous studies. Empty lacuna and osteon necrosis, starting from the Haversian canal, were also found. The drill bit geometry, chisel edge, flute number, edges, and steps had a strong effect on bone damage, particularly the chisel edge. The standard and classic surgical drill caused the greatest surface damage and necrosis of the five drill bit geometries studied. The microstructural features including osteons and matrix played an important role in numbers and length of microcracks and necrosis. More microcracks were generated in the transverse direction, while a greater length of the empty lacuna was generated in the longitudinal direction under the same drilling parameters. Microcracks mainly propagated in a straight manner in and parallel to the interstitial bone matrix and cement line. Drilling forces were not directly correlated with bone damage; thus, hole performance should be considered to evaluate the superiority and inferiority of drill bits rather than the drill force alone.

A network of trans-cortical capillaries as mainstay for blood circulation in long bones

Closed circulatory systems (CCS) underlie the function of vertebrate organs, but in long bones their structure is unclear, although they constitute the exit route for bone marrow (BM) leukocytes. To understand neutrophil emigration from BM, we studied the vascular system of murine long bones. Here we show that hundreds of capillaries originate in BM, cross murine cortical bone perpendicularly along the shaft and connect to the periosteal circulation. Structures similar to these trans-cortical-vessels (TCVs) also exist in human limb bones. TCVs express arterial or venous markers and transport neutrophils. Furthermore, over 80% arterial and 59% venous blood passes through TCVs. Genetic and drug-mediated modulation of osteoclast count and activity leads to substantial changes in TCV numbers. In a murine model of chronic arthritic bone inflammation, new TCVs develop within weeks. Our data indicate that TCVs are a central component of the CCS in long bones and may represent an important route for immune cell export from the BM.

Examining the influence of PTH(1-34) on tissue strength and composition

The lacunar-canaliculi system is a network of channels that is created and maintained by osteocytes as they are embedded throughout cortical bone. As osteocytes modify their lacuna space, the local tissue composition and tissue strength are subject to change. Although continual exposure to parathyroid hormone (PTH) can induce adaptation at the lacunar wall, the impact of intermittent PTH treatment on perilacunar adaptation remains unclear. Therefore, the primary objective of this study was to establish how intermittent PTH(1-34) treatment influences perilacunar adaptation with respect to changes in tissue composition. We hypothesized that local changes in tissue composition following PTH(1-34) are associated with corresponding gains in tissue strength and resistance to microdamage at the whole bone level. Adult male C57BL/6J mice were treated daily with PTH(1-34) or vehicle for 3 weeks. In response to PTH(1-34), Raman spectroscopy revealed a significant decrease in the carbonate-to-phosphate ratio and crystallinity across the entire tissue, while the mineral-to-matrix ratio demonstrated a significant decrease in just the perilacunar region. The shift in perilacunar composition largely explained the corresponding increase in tissue strength, while the degree of new tissue added at the endosteum and periosteum did not produce any significant changes in cortical area or moment of inertia that would explain the increase in tissue strength. Furthermore, fatigue testing revealed a greater resistance to crack formation within the existing tissue following PTH(1-34) treatment. As a result, the shift in perilacunar composition presents a unique mechanism by which PTH(1-34) produces localized differences in tissue quality that allow more energy to be dissipated under loading, thereby increasing tissue strength and resistance to microdamage. In addition, our findings demonstrate the potential for PTH(1-34) to amplify osteocytes' mechanotransduction by producing a more compliant tissue. Overall, the present study demonstrates that changes in tissue composition localized at the lacuna wall contribute to the strength and fatigue resistance of cortical bone gained in response to intermittent PTH(1-34) treatment.

Aging, Osteocytes, and Mechanotransduction

PURPOSE OF REVIEW

The bone is able to adapt its structure to mechanical signals via the bone remodeling process governed by mechanosensitive osteocytes. With aging, an imbalance in bone remodeling results in osteoporosis. In this review, we hypothesized that changes in lacunar morphology underlie the decreased bone mechanoresponsiveness to mechanical loading with aging.

RECENT FINDINGS

Several studies have reported considerable variations in the shape of osteocytes and their lacunae with aging. Since osteocytes can sense matrix strain directly via their cell bodies, the variations in osteocyte morphology may cause changes in osteocyte mechanosensitivity. As a consequence, the load-adaptive response of osteocytes may change with aging, even when mechanical loading would remain unchanged. Though extensive quantitative data is lacking, evidence exists that the osteocyte lacunae are becoming smaller and more spherical with aging. Future dedicated studies might reveal whether these changes would affect osteocyte mechanosensation and the subsequent biological response, and whether this is (one of) the pathways involved in age-related bone loss.

Effect of combined treatment with zoledronic acid and parathyroid hormone on mouse bone callus structure and composition

In recent years, great interest in combined treatment of parathyroid hormone (PTH) with anti-resorptive therapy has emerged. PTH has been suggested to aid bridging of atrophic fractures and improve strength in closed fracture models. Bisphosphonate treatments typically result in a larger woven bone callus that is slower to remodel. The combination of both drugs has been demonstrated to be effective for the treatment of osteoporotic bone loss in many preclinical studies. However, the effect of combined treatment on fracture repair is still largely unexplored. In this study, we aimed to compare these drugs as single-agent and in combination in a murine closed fracture model. We wanted to assess potential differences in material properties, morphometry and in the development of the lacuno-canalicular network. A total of 40 female, 11-week-old wild type mice underwent a closed fracture on the midshaft of the tibia and were assigned to four groups (n=8-10 per group). Beginning on post-operative day 8, animals received different subcutaneous injections. Group 1 received a single injection of saline solution and Group 2 of zoledronic acid (ZA). Group 3 received daily dosing of PTH. Group 4 received a dual treatment, starting with a single dose of ZA followed by daily injection of PTH. Three weeks after fracture, all animals were euthanized and tibiae were assessed using micro-computed tomography (micro-CT), high-resolution micro-CT (HR micro-CT), Raman spectroscopy, quantitative histomorphometry, and deconvolution microscopy (DV microscopy). Combined treatment showed a significant increase of 41% in bone volume fraction and a significant decrease of 61% in the standard deviation of the trabecular spacing compared to vehicle, both known to be strong predictors of callus strength. An analysis via HR micro-CT showed similar results on all groups for lacunar numerical density, whereas mean lacuna volume was found to be higher compared to vehicle in treated groups, but only PTH mono-treatment showed a significant increase compared to vehicle (+45%). Raman spectroscopy did not reveal detectable changes in material properties of the bone calluses. Sclerostin staining, tartrate resistant acid phosphatase (TRAP) staining and canalicular analysis with DV microscopy on a subset of samples did not display distinctive difference in any of the treatments. We therefore consider PTH+ZA treatment beneficial for bone healing. No clear negative effect on bone quality was detected during this study.

Long bone maturation is driven by pore closing: A quantitative tomography investigation of structural formation in young C57BL/6 mice

During mammalian growth, long bones undergo extensive structural reorganization, transforming from primitive shapes in the limb buds into mature bones. Here we shed light on the steps involved in structural formation of the mineralized tissue in midshafts of C57BL/6 femurs, shortly after birth. By combining 3D micrometer-resolution X-ray microtomography with 2D histology, we study the transformation of the tissue from a partially-mineralized scaffold into a compact bone structure. We identify three growth phases that take place during murine long bone maturation: During a patterning phase (I) mineralized struts form a loosely connected foam-like cortical network. During a transitioning phase (II), the extensive non-mineralized tracts vanish, transforming the foam into a fully continuous mass, by 14 days of age. Concomitantly, closed porosity increases to about ∼ 1.4%, and stays at this level, also found in maturity. During a shaping phase (III), the bones gradually attain their characteristic intricate adult form. Architectured mineral depositioning--first in open foamy scaffolds, and later into solid bone material--is presumably a compromise between the mechanical needs of providing support to the body, and the biological requirements of vascularization and extensive nutritional needs in the early stages of bone formation.

Modalities for Visualization of Cortical Bone Remodeling: The Past, Present, and Future

Bone's ability to respond to load-related phenomena and repair microdamage is achieved through the remodeling process, which renews bone by activating groups of cells known as basic multicellular units (BMUs). The products of BMUs, secondary osteons, have been extensively studied via classic two-dimensional techniques, which have provided a wealth of information on how histomorphology relates to skeletal structure and function. Remodeling is critical in maintaining healthy bone tissue; however, in osteoporotic bone, imbalanced resorption results in increased bone fragility and fracture. With increasing life expectancy, such degenerative bone diseases are a growing concern. The three-dimensional (3D) morphology of BMUs and their correlation to function, however, are not well-characterized and little is known about the specific mechanisms that initiate and regulate their activity within cortical bone. We believe a key limitation has been the lack of 3D information about BMU morphology and activity. Thus, this paper reviews methodologies for 3D investigation of cortical bone remodeling and, specifically, structures associated with BMU activity (resorption spaces) and the structures they create (secondary osteons), spanning from histology to modern ex vivo imaging modalities, culminating with the growing potential of in vivo imaging. This collection of papers focuses on the theme of "putting the 'why' back into bone architecture." Remodeling is one of two mechanisms "how" bone structure is dynamically modified and thus an improved 3D understanding of this fundamental process is crucial to ultimately understanding the "why."

Altered lacunar and vascular porosity in osteogenesis imperfecta mouse bone as revealed by synchrotron tomography contributes to bone fragility

Osteogenesis imperfecta (brittle bone disease) is caused by mutations in the collagen genes and results in skeletal fragility. Changes in bone porosity at the tissue level indicate changes in bone metabolism and alter bone mechanical integrity. We investigated the cortical bone tissue porosity of a mouse model of the disease, oim, in comparison to a wild type (WT-C57BL/6), and examined the influence of canal architecture on bone mechanical performance. High-resolution 3D representations of the posterior tibial and the lateral humeral mid-diaphysis of the bones were acquired for both mouse groups using synchrotron radiation-based computed tomography at a nominal resolution of 700nm. Volumetric morphometric indices were determined for cortical bone, canal network and osteocyte lacunae. The influence of canal porosity architecture on bone mechanics was investigated using microarchitectural finite element (μFE) models of the cortical bone. Bright-field microscopy of stained sections was used to determine if canals were vascular. Although total cortical porosity was comparable between oim and WT bone, oim bone had more numerous and more branched canals (p<0.001), and more osteocyte lacunae per unit volume compared to WT (p<0.001). Lacunae in oim were more spherical in shape compared to the ellipsoidal WT lacunae (p<0.001). Histology revealed blood vessels in all WT and oim canals. μFE models of cortical bone revealed that small and branched canals, typical of oim bone, increase the risk of bone failure. These results portray a state of compromised bone quality in oim bone at the tissue level, which contributes to its deficient mechanical properties.

Modeling microdamage behavior of cortical bone

Bone is a complex material which exhibits several hierarchical levels of structural organization. At the submicron-scale, the local tissue porosity gives rise to discontinuities in the bone matrix which have been shown to influence damage behavior. Computational tools to model the damage behavior of bone at different length scales are mostly based on finite element (FE) analysis, with a range of algorithms developed for this purpose. Although the local mechanical behavior of bone tissue is influenced by microstructural features such as bone canals and osteocyte lacunae, they are often not considered in FE damage models due to the high computational cost required to simulate across several length scales, i.e., from the loads applied at the organ level down to the stresses and strains around bone canals and osteocyte lacunae. Hence, the aim of the current study was twofold: First, a multilevel FE framework was developed to compute, starting from the loads applied at the whole bone scale, the local mechanical forces acting at the micrometer and submicrometer level. Second, three simple microdamage simulation procedures based on element removal were developed and applied to bone samples at the submicrometer-scale, where cortical microporosity is included. The present microdamage algorithm produced a qualitatively analogous behavior to previous experimental tests based on stepwise mechanical compression combined with in situ synchrotron radiation computed tomography. Our results demonstrate the feasibility of simulating microdamage at a physiologically relevant scale using an image-based meshing technique and multilevel FE analysis; this allows relating microdamage behavior to intracortical bone microstructure.

Clinician approach to diagnosis of stress fractures including bisphosphonate-associated fractures

Stress fractures are repetitive strain injuries that occur in normal bones and in abnormal bones. Stress fractures share many features in common but differences depend on the status of the underlying bone. This review article for clinicians addresses aspects about stress fractures with particular respect to fatigue fractures, Looser zones of osteomalacia, atypical Looser zones, atypical femoral fractures associated with bisphosphonate therapy and stress fractures in Paget's disease of bone.

Ultrastructural characterization of the implant interface response to loading

Dynamic loading can affect the bone surrounding implants. For ultrastructural exploration of the peri-implant tissue response to dynamic loading, titanium implants were installed in rat tibiae, in which one implant was loaded while the contralateral served as the unloaded control. The loaded implants received stimulation either within 24 hrs after implantation (immediate loading) or after a 28-day healing period (delayed loading) for 4, 7, 14, 21, or 28 days. The samples were processed for histology and gene expression quantification. Compared with the unloaded control, bone-to-implant contact increased significantly by immediate loading for 28 days (p < .05), but not in case of delayed loading. No effect of loading was observed on the bone formation in the implant thread areas, on the blood vessel area, and on endosteal callus formation. Loading during healing (immediate) for 7 days induced, relative to the unloaded control, a 2.3-fold increase of Runx2 in peri-implant cortical bone (p < .01) without a change in the RANKL/Opg ratio. Loading after healing (delayed) for 7 days up-regulated Runx2 (4.3-fold, p < .01) as well as Opg (22.3-fold, p < .05) compared with the unloaded control, resulting in a significantly decreased RANKL/Opg ratio. These results indicate a stimulating effect of dynamic loading on implant osseointegration when applied during the healing phase. In addition, gene expression analyses revealed molecular adaptations favoring bone formation and, at the same time, affecting bone remodeling.

Non destructive characterization of cortical bone micro-damage by nonlinear resonant ultrasound spectroscopy

The objective of the study was to evaluate the ability of a nonlinear ultrasound technique, the so-called nonlinear resonant ultrasound spectroscopy (NRUS) technique, for detecting early microdamage accumulation in cortical bone induced by four-point bending fatigue. Small parallelepiped beam-shaped human cortical bone specimens were subjected to cyclic four-point bending fatigue in several steps. The specimens were prepared to control damage localization during four-point bending fatigue cycling and to unambiguously identify resonant modes for NRUS measurements. NRUS measurements were achieved to follow the evolution of the nonlinear hysteretic elastic behavior during fatigue-induced damage. After each fatigue step, a small number of specimens was removed from the protocol and set apart to quantitatively assess the microcrack number density and length using synchrotron radiation micro-computed tomography (SR-µCT). The results showed a significant effect of damage steps on the nonlinear hysteretic elastic behavior. No significant change in the overall length of microcracks was observed in damaged regions compared to the load-free control regions. Only an increased number of shortest microcracks, those in the lowest quartile, was noticed. This was suggestive of newly formed microcracks during the early phases of damage accumulation. The variation of nonlinear hysteretic elastic behavior was significantly correlated to the variation of the density of short microcracks. Our results suggest that the nonlinear hysteretic elastic behavior is sensitive to early bone microdamage. Therefore NRUS technique can be used to monitor fatigue microdamage progression in in vitro experiments.

Cavities in the compact bone in tetrapods and fish and their effect on mechanical properties

Bone includes cavities in various length scales, from nanoporosities occurring between the collagen fibrils and the mineral crystals all the way to macrocavities like the medullary cavity. In particular, bone is permeated by a vast number of channels (the lacunar-canalicular system), that reduce the stiffness and, more importantly, the strength of the bone that they permeate. These consequences are presumably a price worth paying for the ability of the lacunar-canalicular system to detect changes in the strain environment within the bone material and, when deleterious, to trigger processes like modeling or remodeling which 'rectify' it. Here we review the size and density of the various types of cavities in bone, and discuss their effect on the mechanical properties of cortical bone. In this respect the bones of advanced teleost fish species (probably the majority of all vertebrate species) are an unsolved conundrum because they lack bone cells (and therefore lacunae and canaliculi) in their skeleton. Yet, despite being acellular, some of these fish can undergo considerable remodeling in at least some parts of their skeleton. We address, but do not solve this mystery.

The importance of the intracortical canal network for murine bone mechanics

As shown by recent data bone strength estimation can greatly be improved by including microarchitectural parameters in the analysis. Our previous results showed that intracortical canals (the living space of the vasculature and/or remodeling units) are a major contributor to cortical tissue porosity, and therefore, can be linked to mechanical bone properties. Consequently, the goal of this study was to investigate the importance of the intracortical canal network for murine bone mechanics. To study intracortical canals within murine femoral bone, we used a mouse model, including two mouse strains, C57BL/6J-Ghrhr(lit)/J (B6-lit/+) and C3.B6-Ghrhr(lit)/J (C3.B6-lit/+) representing low and high bone mass, respectively. The intracortical canal network was assessed by synchrotron radiation-based micro-computed tomography and the mechanical bone properties were derived from three-point bending experiments. Multiple linear regression models were built to explain the variation in ultimate force, work to fracture, and stiffness in terms of the morphometric parameters. The power to explain the variation in bone mechanics was increased significantly for most mechanical measures when including morphometric parameters of intracortical canals in addition to macroscopic morphometric measures. Specifically, we could derive generalized (mouse strain-independent) models for ultimate force, where the incorporation of intracortical canals in addition to macroscopic bone measures improved the explained variation in ultimate force considerably, which was confirmed by an increase in adjusted R(2) of 73% and 8% for B6-lit/+ and C3.B6-lit/+, respectively. Further, we observed that the heterogeneity of the morphometric measures for the individual canal branches play an important role for explaining the variation in ultimate force. Finally, the current study provides strong evidence that work to fracture of murine bone, which is triggered critically by microcracks, is affected by intracortical canals. In summary, the study suggests that the intracortical canal network is important for bone mechanics.

Patient-specific modelling of bone and bone-implant systems: the challenges

In the past three decades, finite element (FE) modelling has provided considerable understanding to the area of musculoskeletal biomechanics. However, most of this understanding has been generated using generic, standardised or idealised models. Patient-specific modelling (PSM) is almost never used for making clinical decisions. Imaging technologies have made it possible to create patient-specific geometries and FE meshes for modelling. While these have brought us closer to PSM, several challenges associated with the definition of material properties, loads, boundary conditions and interaction between components still need to be overcome. This study reviews the current status of PSM with respect to defining material behaviour and prescribing boundary conditions and interactions. With regard to the constitutive modelling of bone, it is seen that imaging is being increasingly used to define elastic properties (isotropic as well as anisotropic). However, the post-elastic and time-dependent behaviour, important for several modelling situations, is mostly obtained from in vitro experiments. Strain-based plasticity, not commonly available in FE codes, appears to have the potential of reducing an element of patient-specificity in modelling the yielding behaviour of bone. PSM of real boundary conditions that include muscles and ligaments continues to remain a challenge; many clinically relevant questions can be, however, answered without their inclusion. Simulation techniques to undertake PSM of interactions between bone and uncemented implants are available. Interference fit employed in both joint replacement fracture treatments induces considerable preload whose inclusion in models is important for the prediction of interface behaviour.

Age- and gender-related differences in cortical geometry and microstructure: Improved sensitivity by regional analysis

OBJECTIVE

While the importance of cortical structure quantification is increasingly underscored by recent literature, conventional analysis techniques obscure potentially important regional variations in cortical structure. The objective of this study was to characterize the spatial variability in cortical geometry and microstructure at the distal radius and tibia using high resolution peripheral quantitative computed tomography (HR-pQCT). We show that spatially-resolved analysis is able to identify cortical sub-regions with increased sensitivity to the effects of gender and aging.

METHODS

HR-pQCT scans of 146 volunteers (92 female/54 male) spanning a wide range of ages (20-78years) were analyzed. For each subject, radius and tibia scans were obtained using a clinical HR-pQCT system. Measures describing geometry (cortical bone thickness (Ct.Th)), microstructure (porosity (Ct.Po), pore diameter (Ct.Po.Dm), and pore size heterogeneity (Ct.Po.Dm SD)), and cortical bone density were calculated from the image data. Biomechanical parameters describing load and stress distribution were calculated using linear finite element analysis. Cortical quadrants were defined based on anatomic axes to quantify regional parameter variation. Subjects were categorized by gender, and age, and menopausal status for analysis.

RESULTS

Significant regional variation was found in all geometric and microstructural parameters in both the radius and tibia. In general, the radius showed more pronounced and significant variations in all parameters as compared with the tibia. At both sites, Ct.Po displayed the greatest regional variations. Correlation coefficients for Ct.Po and Ct.Th with respect to load and stress distribution provided evidence of an association between regional cortical structure and biomechanics in the tibia. Comparing women to men, differences in Ct.Po were most pronounced in the anterior quadrant of the radius (36% lower in women (p<0.01)) and the posterior quadrant of the tibia (27% lower in women (p<0.01)). Comparing elderly to young women, differences in Ct.Po were most pronounced in the lateral quadrant of the radius (328% higher in elderly women (p<0.001)) and the anterior quadrant of the tibia (433% higher in elderly women (p<0.001)). Comparing elderly to young men, the most pronounced age differences were found in the anterior radius (205% higher in elderly men, (p<0.001)) and the anterior tibia (190% higher in elderly men (p<0.01)). All subregional Ct.Po differences provided greater sensitivity to gender and age effects than those based on the global means.

CONCLUSION

These results show significant regional variation in all geometric and microarchitectural parameters studied in both the radius and tibia. Quantification of region-specific parameters provided increased sensitivity in the analysis of age- and gender-related differences, in many cases providing statistically significant differentiation of groups where conventional global analysis failed to detect differences. These results suggest that regional analysis may be important in studies of disease and therapeutic effects, particularly where microstructural parameters based on global analyses have thus far failed to identify a response in bone quality.

Strain-adaptive in silico modeling of bone adaptation--a computer simulation validated by in vivo micro-computed tomography data

Computational models are an invaluable tool to test different mechanobiological theories and, if validated properly, for predicting changes in individuals over time. Concise validation of in silico models, however, has been a bottleneck in the past due to a lack of appropriate reference data. Here, we present a strain-adaptive in silico algorithm which is validated by means of experimental in vivo loading data as well as by an in vivo ovariectomy experiment in the mouse. The maximum prediction error following four weeks of loading resulted in 2.4% in bone volume fraction (BV/TV) and 8.4% in other bone structural parameters. Bone formation and resorption rate did not differ significantly between experiment and simulation. The spatial distribution of formation and resorption sites matched in 55.4% of the surface voxels. Bone loss was simulated with a maximum prediction error of 12.1% in BV/TV and other bone morphometric indices, including a saturation level after a few weeks. Dynamic rates were more difficult to be accurately predicted, showing evidence for significant differences between simulation and experiment (p<0.05). The spatial agreement still amounted to 47.6%. In conclusion, we propose a computational model which was validated by means of experimental in vivo data. The predictive value of an in silico model may become of major importance if the computational model should be applied in clinical settings to predict bone changes due to disease and test the efficacy of potential pharmacological interventions.