Bone tissue aging affects mineralization of cement lines

A numerical study of dehydration induced fracture toughness degradation in human cortical bone

A 2D plane strain extended finite element method (XFEM) model was developed to simulate three-point bending fracture toughness tests for human bone conducted in hydrated and dehydrated conditions. Bone microstructures and crack paths observed by micro-CT imaging were simulated using an XFEM damage model. Critical damage strains for the osteons, matrix, and cement lines were deduced for both hydrated and dehydrated conditions and it was found that dehydration decreases the critical damage strains by about 50%. Subsequent parametric studies using the various microstructural models were performed to understand the impact of individual critical damage strain variations on the fracture behavior. The study revealed the significant impact of the cement line critical damage strains on the crack paths and fracture toughness during the early stages of crack growth. Furthermore, a significant sensitivity of crack growth resistance and crack paths on critical strain values of the cement lines was found to exist for the hydrated environments where a small change in critical strain values of the cement lines can alter the crack path to give a significant reduction in fracture resistance. In contrast, in the dehydrated state where toughness is low, the sensitivity to changes in critical strain values of the cement lines is low. Overall, our XFEM model was able to provide new insights into how dehydration affects the micromechanisms of fracture in bone and this approach could be further extended to study the effects of aging, disease, and medical therapies on bone fracture.

Multiscale theoretical model shows that aging-related mechanical degradation of cortical bone is driven by microstructural changes in addition to porosity

This study aims to gain mechanistic understanding of how aging-related changes in the microstructure of cortical bone drive mechanical consequences at the macroscale. To that end, cortical bone was modeled as a bundle of elastic-plastic, parallel fibers, which represented osteons and interstitial tissue, loaded in uniaxial tension. Distinct material properties were assigned to each fiber in either the osteon or interstitial fiber "families." Models representative of mature (20-60 yrs.) bone, and elderly (60+) bone were created by modeling aging via the following changes to the input parameters: (i) increasing porosity from 5% to 15%, (ii) increasing the ratio of the number of osteon fibers relative to interstitial fibers from 40% to 50%, and (iii) changing the fiber material properties from representing mature bone samples to representing elderly bone samples (i.e., increased strength and decreased toughness of interstitial fibers together with decreased toughness of osteon fibers). To understand the respective contributions of these changes, additional models isolating one or two of each of these were also created. From the computed stress-strain curve for the fiber bundle, the yield point (ϵy, σy), ultimate point (ϵu, σu), and toughness (UT) for the bundle as a whole were measured. We found that changes to all three input parameters were required for the model to capture the aging-related decline in cortical bone mechanical properties consistent with those previously reported in the literature. In both mature and elderly bundles, rupture of the interstitial fibers drove the initial loss of strength following the ultimate point. Plasticity and more gradual rupture of the osteons drove the remainder of the response. Both the onset and completion of interstitial fiber rupture occurred at lower strains in the elderly vs. mature case. These findings point to the importance of studying microstructural changes beyond porosity, such as the area fraction of osteons and the material properties of osteon and interstitial tissue, in order to further understanding of aging-related changes in bone.

Mineralized Collagen Fibrils: An Essential Component in Determining the Mechanical Behavior of Cortical Bone

Bone comprises mechanically different materials in a specific hierarchical structure. Mineralized collagen fibrils (MCFs), represented by tropocollagen molecules and hydroxyapatite nanocrystals, are the fundamental unit of bone. The mechanical characterization of MCFs provides the unique adaptive mechanical competence to bone to withstand mechanical load. The structural and mechanical role of MCFs is critical in the deformation mechanisms of bone and the marvelous strength and toughness possessed by bone. However, the role of MCFs in the mechanical behavior of bone across multiple length scales is not fully understood. In the present study, we shed light upon the latest progress regarding bone deformation at multiple hierarchical levels and emphasize the role of MCFs during bone deformation. We propose the concept of hierarchical deformation of bone to describe the interconnected deformation process across multiple length scales of bone under mechanical loading. Furthermore, how the deterioration of bone caused by aging and diseases impairs the hierarchical deformation process of the cortical bone is discussed. The present work expects to provide insights on the characterization of MCFs in the mechanical properties of bone and lays the framework for the understanding of the multiscale deformation mechanics of bone.

Osteonal Microcracking Pattern: A Potential Vitality Marker in Human Bone Trauma

In forensic anthropology, the differential diagnosis between peri- and postmortem bone fractures is mainly based on macroscopic criteria. In contrast, studies focusing on bone histology are very scarce. In a recent publication, we showed that (perimortem) fractures in fresh human bones exhibit a different osteonal microcracking pattern than (postmortem) damage in dry bones. In the current work, we explored whether this osteonal microcracking pattern is distinctive of the vitality of (perimortem) fresh bone fractures. To this end, we compared the number, length and structural distribution of microcracks in vital humeral fractures from forensic autopsy cases with experimentally reproduced, three point-bending fractures in fresh and dry human humeri. Half of the fresh experimental bones were fractured whilst applying axial compression, i.e., attempting to simulate intra vitam conditions more accurately. The results showed a similar osteonal microcracking pattern between vital fractures and experimental fractures of fresh humeri subjected to axial compression. Interestingly, this pattern was significantly different from the one observed in the experimental fractures of fresh humeri without axial compression and dry humeri. This supports our hypothesis that the osteonal microcracking pattern can potentially be used as a marker for vital perimortem trauma, providing a histomorphometric tool for fracture timing.

Subcanalicular Nanochannel Volume Is Inversely Correlated With Calcium Content in Human Cortical Bone

The spatial distribution of mineralization density is an important signature of bone growth and remodeling processes, and its alterations are often related to disease. The extracellular matrix of some vertebrate mineralized tissues is known to be perfused by a lacunocanalicular network (LCN), a fluid-filled unmineralized structure that harbors osteocytes and their fine processes and transports extracellular fluid and its constituents. The current report provides evidence for structural and compositional heterogeneity at an even smaller, subcanalicular scale. The work reveals an extensive unmineralized three-dimensional (3D) network of nanochannels (~30 nm in diameter) penetrating the mineralized extracellular matrix of human femoral cortical bone and encompassing a greater volume fraction and surface area than these same parameters of the canaliculi comprising the LCN. The present study combines high-resolution focused ion beam-scanning electron microscopy (FIB-SEM) to investigate bone ultrastructure in 3D with quantitative backscattered electron imaging (qBEI) to estimate local bone mineral content. The presence of nanochannels has been found to impact qBEI measurements fundamentally, such that volume percentage (vol%) of nanochannels correlates inversely with weight percentage (wt%) of calcium. This mathematical relationship between nanochannel vol% and calcium wt% suggests that the nanochannels could potentially provide space for ion and small molecule transport throughout the bone matrix. Collectively, these data propose a reinterpretation of qBEI measurements, accounting for nanochannel presence in human bone tissue in addition to collagen and mineral. Further, the results yield insight into bone mineralization processes at the nanometer scale and present the possibility for a potential role of the nanochannel system in permitting ion and small molecule diffusion throughout the extracellular matrix. Such a possible function could thereby lead to the sequestration or occlusion of the ions and small molecules within the extracellular matrix. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

On the fracture behavior of cortical bone microstructure: The effects of morphology and material characteristics of bone structural components

Bone encompasses a complex arrangement of materials at different length scales, which endows it with a range of mechanical, chemical, and biological capabilities. Changes in the microstructure and characteristics of the material, as well as the accumulation of microcracks, affect the bone fracture properties. In this study, two-dimensional finite element models of the microstructure of cortical bone were considered. The eXtended Finite Element Method (XFEM) developed by Abaqus software was used for the analysis of the microcrack propagation in the model as well as for local sensitivity analysis. The stress-strain behavior obtained for the different introduced models was substantially different, confirming the importance of bone tissue microstructure for its failure behavior. Considering the role of interfaces, the results highlighted the effect of cement lines on the crack deflection path and global fracture behavior of the bone microstructure. Furthermore, bone micromorphology and areal fraction of cortical bone tissue components such as osteons, cement lines, and pores affected the bone fracture behavior; specifically, pores altered the crack propagation path since increasing porosity reduced the maximum stress needed to start crack propagation. Therefore, cement line structure, mineralization, and areal fraction are important parameters in bone fracture. The parameter-wise sensitivity analysis demonstrated that areal fraction and strain energy release rate had the greatest and the lowest effect on ultimate strength, respectively. Furthermore, the component-wise sensitivity analysis revealed that for the areal fraction parameter, pores had the greatest effect on ultimate strength, whereas for the other parameters such as elastic modulus and strain energy release rate, cement lines had the most important effect on the ultimate strength. In conclusion, the finding of the current study can help to predict the fracture mechanisms in bone by taking the morphological and material properties of its microstructure into account.

A Mild Case of Autosomal Recessive Osteopetrosis Masquerading as the Dominant Form Involving Homozygous Deep Intronic Variations in the CLCN7 Gene

Osteopetrosis is a heterogeneous group of rare hereditary diseases characterized by increased bone mass of poor quality. Autosomal-dominant osteopetrosis type II (ADOII) is most often caused by mutation of the CLCN7 gene leading to impaired bone resorption. Autosomal recessive osteopetrosis (ARO) is a more severe form and is frequently accompanied by additional morbidities. We report an adult male presenting with classical clinical and radiological features of ADOII. Genetic analyses showed no amino-acid-converting mutation in CLCN7 but an apparent haploinsufficiency and suppression of CLCN7 mRNA levels in peripheral blood mononuclear cells. Next generation sequencing revealed low-frequency intronic homozygous variations in CLCN7, suggesting recessive inheritance. In silico analysis of an intronic duplication c.595-120_595-86dup revealed additional binding sites for Serine- and Arginine-rich Splicing Factors (SRSF), which is predicted to impair CLCN7 expression. Quantitative backscattered electron imaging and histomorphometric analyses revealed bone tissue and material abnormalities. Giant osteoclasts were present and additionally to lamellar bone, and abundant woven bone and mineralized cartilage were observed, together with increased frequency and thickness of cement lines. Bone mineralization density distribution (BMDD) analysis revealed markedly increased average mineral content of the dense bone (CaMean T-score + 10.1) and frequency of bone with highest mineral content (CaHigh T-score + 19.6), suggesting continued mineral accumulation and lack of bone remodelling. Osteocyte lacunae sections (OLS) characteristics were unremarkable except for an unusually circular shape. Together, our findings suggest that the reduced expression of CLCN7 mRNA in osteoclasts, and possibly also osteocytes, causes poorly remodelled bone with abnormal bone matrix with high mineral content. This together with the lack of adequate bone repair mechanisms makes the material brittle and prone to fracture. While the skeletal phenotype and medical history were suggestive of ADOII, genetic analysis revealed that this is a possible mild case of ARO due to deep intronic mutation.

2D size of trabecular bone structure units (BSU) correlate more strongly with 3D architectural parameters than age in human vertebrae

Bone tissue is continuously remodeled. In trabecular bone, each remodeling transaction forms a microscopic bone structural unit (BSU), also known as a hemiosteon or a trabecular packet, which is bonded to existing tissue by osteopontin-rich cement lines. The size and shape of the BSUs are determined by the size and shape of the resorption cavity, and whether the cavity is potentially over- or under-filled by the subsequent bone formation. The present study focuses on the recently formed trabecular BSUs, and how their 2D size and shape changes with age and trabecular microstructure. The study was performed using osteopontin-immunostained frontal sections of L2 vertebrae from 8 young (aged 18.5-37.6 years) and 8 old (aged 69.1-96.4 years) control females, which underwent microcomputed tomography (μCT) imaging prior to sectioning. The contour of 4230 BSU profiles (181-385 per vertebra) within 1024 trabecular profiles were outlined, and their 2D width, length, area, and shape were assessed. Of these BSUs, 22 (0.5%) were generated by modeling-based bone formation (i.e. without prior resorption), while 99.5% were generated by remodeling-based bone formation (i.e. with prior resorption). The distributions of BSU profile width, length, and area were significantly smaller in the old versus young females (p < 0.005), and the median profile width, length, and area were negative correlated with age (p < 0.018). Importantly, these BSU profile size parameters were more strongly correlated with trabecular bone volume (BV/TV, p < 0.002) and structure model index (SMI, p < 0.008) assessed by μCT, than age. Moreover, the 2D BSU size parameters were positively correlated to the area of the individual trabecular profiles (p < 0.0001), which were significantly smaller in the old versus young females (p < 0.024). The BSU shape parameters (aspect ratio, circularity, and solidity) were not correlated with age, BV/TV, or SMI. Collectively, the study supports the notion that not only the BSU profile width, but also its length and area, are more influenced by the age-related bone loss and shift from plates to rods (SMI), than age itself. This implies that BSU profile size is mainly driven by changes in the trabecular microstructure, which affect the size of the resorption cavity that the BSU refills.

Ionic liquid treatment for efficient sample preparation of hydrated bone for scanning electron microscopy

This study presents a new protocol for preparing bone samples for scanning electron microscopy (SEM) using a room temperature ionic liquid (RTIL) treatment method. RTIL-based solutions can be adopted as an alternative to lengthy and laborious traditional means of preparation for SEM due to their unique low-vapour pressure and conductive properties. Applied to biological samples, RTILs can be used quickly and efficiently to observe hydrated, unfixed structures in typical SEM systems. This first-time feasibility study of the optimization of this protocol for bone was explored through various SEM modalities using two distinct ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMI][BF₄]) and 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMI][BF₄]), at varying concentrations of 5, 10, and 25 % v/v in aqueous solution through an addition-based method. Based on qualitative observations in the SEM, a 60-second solution addition treatment of 10 % v/v [BMI][BF₄] performed the best in imaging hydrated, unfixed bone samples, resulting in minimal charge buildup and no solution pooling on the surface. The treatment was applied effectively to a variety of bone samples, notably flat and polished, as well as highly topographical bone fracture surfaces of both healthy and osteoporotic human bone samples. In comparison to conventionally dehydrated bone, the RTIL treatment better preserved the natural bone structure, resulting in minimal microcracking in observed structures.

A stochastic micro to macro mechanical model for the evolution of bone-implant interface stiffness

Upon placement of an implant into living bone, an interface is formed through which various biochemical, biological, physical, and mechanical interactions take place. This interface evolves over time as the mechanical properties of peri-implant bone increase. Owing to the multifactorial nature of interfacial processes, it is challenging to devise a comprehensive model for predicting the mechanical behavior of the bone-implant interface. We propose a simple spatio-temporally evolving mechanical model - from an elementary unit cell comprising randomly oriented mineralized collagen fibrils having randomly assigned stiffness all the way up to a macroscopic bone-implant interface in a gap healing scenario. Each unit cell has an assigned Young's modulus value between 1.62 GPa and 25.73 GPa corresponding to minimum (i.e., 0) and maximum (i.e., 0.4) limits of mineral volume fraction, respectively, in the overlap region of the mineralized collagen fibril. Gap closure and subsequent stiffening are modeled to reflect the two main directions of peri-implant bone formation, i.e., contact osteogenesis and distance osteogenesis. The linear elastic stochastic finite element model reveals highly nonlinear temporal evolution of bone-implant interface stiffness, strongly dictated by the specific kinetics of contact osteogenesis and distance osteogenesis. The bone-implant interface possesses a small stiffness until gap closure, which subsequently evolves into a much higher stiffness, and this transition is reminiscent of a percolation transition whose threshold corresponds to gap closure. The model presented here, albeit preliminary, can be incorporated into future calculations of the bone-implant system where the interface is well-defined mechanically. STATEMENT OF SIGNIFICANCE: A simple, physically informed model for the mechanical characteristics of the bone-implant interface is still missing. Here, we start by extending the reported mechanical characteristics of a one cubic micrometre unit cell to a 250 µm long interface made of 1 µm thick layers. The stiffness of each cell (based on mineral content) is assigned randomly to mimic bone micro-heterogeneity. The numerical study of this interface representative structure allows for the simultaneous determination of the spatio-temporal evolution of the mechanical response at local (discrete element) and global (overall model) scales. The proposed model is the first of this kind that can easily be incorporated into realistic future models of bone-implant interaction with emphasis on implant stability and different loading conditions.

Bone quality analysis of jaw bones in individuals with type 2 diabetes mellitus-post mortem anatomical and microstructural evaluation

OBJECTIVES

With the higher risk of dental implant failure with type 2 diabetes mellitus (T2DM), there is a need to characterize the jaw bones in those individuals. The aim of this post mortem study was to compare jaw bone quality of individuals with T2DM to healthy controls.

MATERIAL AND METHODS

Bone cores from the edentulous lower first molar region and the region of mandibular angle were collected from male individuals with T2DM (n = 10, 70.6 ± 4.5 years) and healthy controls (n = 11, 71.5 ± 3.8 years) during autopsy. Within the T2DM, a subgroup treated with oral antidiabetics (OAD) and one on insulin were identified. Bone quality assessment encompassed evaluation of bone microstructure, matrix composition, and cellular activity, using microcomputed tomography (micro-CT), quantitative backscattered electron imaging (qBEI), Raman spectroscopy, and bone histomorphometry.

RESULTS

In the mandibular angle, T2DM showed 51% lower porosity of the lingual cortex (p = 0.004) and 21% higher trabecular thickness (p = 0.008) compared to control. More highly mineralized bone packets were found in the buccal cortex of the mandibular angle in insulin-treated compared to OAD-treated T2DM group (p = 0.034). In the molar region, we found higher heterogeneity of trabecular calcium content in T2DM insulin compared to controls (p = 0.015) and T2DM OAD (p = 0.019). T2DM was associated with lower osteocyte lacunar size in the trabecular bone of the molar region (vs. control p = 0.03).

CONCLUSIONS

Alterations in microstructure, mineralization, and osteocyte morphology were determined in jaw bone of individuals with T2DM compared to controls.

CLINICAL RELEVANCE

Future studies will have to verify if the mild changes determined in this study will translate to potential contraindications for dental implant placements.

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.

Focused ion beam-SEM 3D analysis of mineralized osteonal bone: lamellae and cement sheath structures

The mineralized collagen fibril is the basic building block of bone, and hence is the key to understanding bone structure and function. Here we report imaging of mineralized pig bone samples in 3D using the focused ion beam-scanning electron microscope (FIB-SEM) under conditions that reveal the 67 nm D-banding of mineralized collagen fibrils. We show that in adult pig osteons, the lamellar bone comprises alternating layers with either collagen fibrils predominantly aligned in one direction, and layers in which fibrils are predominantly aligned in two directions. The cement sheath contains thin layers of both these motifs, but its dominant structural component comprises a very complex layer of fibrils predominantly aligned in three or more directions. The degree of mineralization of the cement sheath is comparable to that of the osteon interior. The extent of alignment (dispersion) of the collagen fibrils in the osteonal lamellar bone is significantly higher than in the cement sheath. Canaliculi within the cement sheath are mainly aligned parallel to the cement sheath boundary, whereas in the lamellar bone they are mainly aligned perpendicular to the lamellar boundaries. This study further characterizes the presence of two types of collagen fibril arrangements previously identified in demineralized lamellar bone from other species. The simple sample preparation procedure for mineralized bone and the lower risk of introducing artifacts opens the possibility of using FIB-SEM to study more samples, to obtain automatic quantitative information on collagen fibril organization and to evaluate the degrees of mineralization all in relatively large volumes of bone.

Collagen Fiber Orientation Is Coupled with Specific Nano-Compositional Patterns in Dark and Bright Osteons Modulating Their Biomechanical Properties

Bone continuously adapts to its mechanical environment by structural reorganization to maintain mechanical strength. As the adaptive capabilities of bone are portrayed in its nano- and microstructure, the existence of dark and bright osteons with contrasting preferential collagen fiber orientation (longitudinal and oblique-angled, respectively) points at a required tissue heterogeneity that contributes to the excellent fracture resistance mechanisms in bone. Dark and bright osteons provide an exceptional opportunity to deepen our understanding of how nanoscale tissue properties influence and guide fracture mechanisms at larger length scales. To this end, a comprehensive structural, compositional, and mechanical assessment is performed using circularly polarized light microscopy, synchrotron nanocomputed tomography, focused ion beam/scanning electron microscopy, quantitative backscattered electron imaging, Fourier transform infrared spectroscopy, and nanoindentation testing. To predict how the mechanical behavior of osteons is affected by shifts in collagen fiber orientation, finite element models are generated. Fundamental disparities between both osteon types are observed: dark osteons are characterized by a higher degree of mineralization along with a higher ratio of inorganic to organic matrix components that lead to higher stiffness and the ability to resist plastic deformation under compression. On the contrary, bright osteons contain a higher fraction of collagen and provide enhanced ductility and energy dissipation due to lower stiffness and hardness.

The influence of microstructure on crack propagation in cortical bone at the mesoscale

The microstructure of cortical bone is key for the tissue's high toughness and strength and efficient toughening mechanisms have been identified at the microscale, for example when propagating cracks interact with the osteonal microstructure. Finite element models have been proposed as suitable tools for analyzing the complex link between the local tissue structure and the fracture resistance of cortical bone. However, previous models that could capture realistic crack paths in cortical bone were due to the required computational effort limited to idealized osteon geometries and small (<1 mm²) model domains. The objective of this study was therefore to bridge the gap between experimental and numerical analysis of crack propagation in cortical bone by introducing image-based models at the mesoscale. Tissue orientation maps from high-resolution micro-CT images were used to define the distribution and orientation of weak interfaces in the models. Crack propagation was simulated using the extended finite element method in combination with an interface damage model, previously developed to simulate crack propagation in microstructural osteon models. The results showed that image-based mesoscale models can be used to capture interactions between cracks and microstructure. The simulated crack paths predicted the general trends seen in experiments with more irregular patterns for cracks propagating perpendicular compared to parallel to the osteon orientation. In all, the proposed method enabled an efficient description of the tissue level microstructure, which is a necessity to predict realistic crack paths in cortical bone and is an important step towards simulating crack propagation in bone models in 3D.

Breaking new ground in mineralized tissue: Assessing tissue quality in clinical and laboratory studies

Mineralized tissues, such as bone and teeth, have extraordinary mechanical properties of both strength and toughness. This mechanical behavior originates from deformation and fracture resistance mechanisms in their multi-scale structure. The term quality describes the matrix composition, multi-scale structure, remodeling dynamics, water content, and micro-damage accumulation in the tissue. Aging and disease result in changes in the tissue quality that may reduce strength and toughness and lead to elevated fracture risk. Therefore, the capability to measure the quality of mineralized tissues provides critical information on disease progression and mechanical integrity. Here, we provide an overview of clinical and laboratory-based techniques to assess the quality of mineralized tissues in health and disease. Current techniques used in clinical settings include radiography-based (radiographs, dual energy x-ray absorptiometry, EOS) and x-ray tomography-based methods (high resolution peripheral quantitative computed tomography, cone beam computed tomography). In the laboratory, tissue quality can be investigated in ex vivo samples with x-ray imaging (micro and nano-computed tomography, x-ray microscopy), electron microscopy (scanning/transmission electron imaging (SEM/STEM), backscattered scanning electron microscopy, Focused Ion Beam-SEM), light microscopy, spectroscopy (Raman spectroscopy and Fourier transform infrared spectroscopy) and assessment of mechanical behavior (mechanical testing, fracture mechanics and reference point indentation). It is important for clinicians and basic science researchers to be aware of the techniques available in different types of research. While x-ray imaging techniques translated to the clinic have provided exceptional advancements in patient care, the future challenge will be to incorporate high-resolution laboratory-based bone quality measurements into clinical settings to broaden the depth of information available to clinicians during diagnostics, treatment and management of mineralized tissue pathologies.

Long-Term Immobilization in Elderly Females Causes a Specific Pattern of Cortical Bone and Osteocyte Deterioration Different From Postmenopausal Osteoporosis

Immobilization as a result of long-term bed rest can lead to gradual bone loss. Because of their distribution throughout the bone matrix and remarkable interconnectivity, osteocytes represent the major mechanosensors in bone and translate mechanical into biochemical signals controlling bone remodeling. To test whether immobilization affects the characteristics of the osteocyte network in human cortical bone, femoral diaphyseal bone specimens were analyzed in immobilized female individuals and compared with age-matched postmenopausal individuals with primary osteoporosis. Premenopausal and postmenopausal healthy individuals served as control groups. Cortical porosity, osteocyte number and lacunar area, the frequency of hypermineralized lacunae, as well as cortical bone calcium content (CaMean) were assessed using bone histomorphometry and quantitative backscattered electron imaging (qBEI). Bone matrix properties were further analyzed by Fourier transform infrared spectroscopy (FTIR). In the immobilization group, cortical porosity was significantly higher, and qBEI revealed a trend toward higher matrix mineralization compared with osteoporotic individuals. Osteocyte density and canalicular density showed a declining rate from premenopausal toward healthy postmenopausal and osteoporotic individuals with peculiar reductions in the immobilization group, whereas the number of hypermineralized lacunae accumulated inversely. In conclusion, reduced osteocyte density and impaired connectivity during immobilization are associated with a specific bone loss pattern, reflecting a phenotype clearly distinguishable from postmenopausal osteoporosis. Immobilization periods may lead to a loss of survival signals for osteocytes, provoking bone loss that is even higher than in osteoporosis states, whereas osteocytic osteolysis remains absent. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Detection and imaging of gadolinium accumulation in human bone tissue by micro- and submicro-XRF

Gadolinium-based contrast agents (GBCAs) are frequently used in patients undergoing magnetic resonance imaging. In GBCAs gadolinium (Gd) is present in a bound chelated form. Gadolinium is a rare-earth element, which is normally not present in human body. Though the blood elimination half-life of contrast agents is about 90 minutes, recent studies demonstrated that some tissues retain gadolinium, which might further pose a health threat due to toxic effects of free gadolinium. It is known that the bone tissue can serve as a gadolinium depot, but so far only bulk measurements were performed. Here we present a summary of experiments in which for the first time we mapped gadolinium in bone biopsy from a male patient with idiopathic osteoporosis (without indication of renal impairment), who received MRI 8 months prior to biopsy. In our studies performed by means of synchrotron radiation induced micro- and submicro-X-ray fluorescence spectroscopy (SR-XRF), gadolinium was detected in human cortical bone tissue. The distribution of gadolinium displays a specific accumulation pattern. Correlation of elemental maps obtained at ANKA synchrotron with qBEI images (quantitative backscattered electron imaging) allowed assignment of Gd structures to the histological bone structures. Follow-up beamtimes at ESRF and Diamond Light Source using submicro-SR-XRF allowed resolving thin Gd structures in cortical bone, as well as correlating them with calcium and zinc.

Phenomenon of osteocyte lacunar mineralization: indicator of former osteocyte death and a novel marker of impaired bone quality?

An increasing number of patients worldwide suffer from bone fractures that occur after low intensity trauma. Such fragility fractures are usually associated with advanced age and osteoporosis but also with long-term immobilization, corticosteroid therapy, diabetes mellitus, and other endocrine disorders. It is important to understand the skeletal origins of increased bone fragility in these conditions for preventive and therapeutic strategies to combat one of the most common health problems of the aged population. This review summarizes current knowledge pertaining to the phenomenon of micropetrosis (osteocyte lacunar mineralization). As an indicator of former osteocyte death, micropetrosis is more common in aged bone and osteoporotic bone. Considering that the number of mineralized osteocyte lacunae per bone area can distinguish healthy, untreated osteoporotic and bisphosphonate-treated osteoporotic patients, it could be regarded as a novel structural marker of impaired bone quality. Further research is needed to clarify the mechanism of lacunar mineralization and to explore whether it could be an additional target for preventing or treating bone fragility related to aging and various endocrine diseases.

Raman fingerprints as promising markers of cellular senescence and aging

Due to our aging population, understanding of the underlying molecular mechanisms constantly gains more and more importance. Senescent cells, defined by being irreversibly growth arrested and associated with a specific gene expression and secretory pattern, accumulate with age and thus contribute to several age-related diseases. However, their specific detection, especially in vivo, is still a major challenge. Raman microspectroscopy is able to record biochemical fingerprints of cells and tissues, allowing a distinction between different cellular states, or between healthy and cancer tissue. Similarly, Raman microspectroscopy was already successfully used to distinguish senescent from non-senescent cells, as well as to investigate other molecular changes that occur at cell and tissue level during aging. This review is intended to give an overview about various applications of Raman microspectroscopy to study aging, especially in the context of detecting senescent cells.

Multiscale bone quality analysis in osteoarthritic knee joints reveal a role of the mechanosensory osteocyte network in osteophytes

Osteophytes - bony outgrowths on joint structures - are found in healthy individuals but are specifically present in late osteoarthritis (OA). Osteophyte development and function is not well understood, yet biomechanical stimuli are thought to be critical. Bone adapts to mechanical forces via the cellular network of osteocytes. The involvement of osteocytes in osteophyte formation and maturation has not been unravelled. Forty-three osteophytes from tibias of 23 OA patients (65 ± 9 years) were analysed. The trabecular bone structure of osteophytes presented with fewer trabeculae of lower bone mineral density compared to subchondral bone. We identified 40% early stage and 60% late stage osteophytes that significantly differed in their trabecular bone characteristics. Osteophyte bone revealed a higher number of osteocytes and a lower number of empty osteocyte lacunae per bone area than the subchondral bone. We found that OA osteophytes consist of younger bone material comprised of woven and lamellar bone with the capacity to develop into a late stage osteophyte potentially via the involvement of the osteocyte network. Our analysis of OA osteophytes implies a transition from woven to lamellar bone as in physiological bone growth within a pathological joint. Therefore, osteophyte development and growth present a valuable research subject when aiming to investigate the osteogenic signalling cascade.

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.

Cellular and extracellular matrix of bone, with principles of synthesis and dependency of mineral deposition on cell membrane transport

Bone differs from other connective tissues; it is isolated by a layer of osteoblasts that are connected by tight and gap junctions. This allows bone to create dense lamellar type I collagen, control pH, mineral deposition, and regulate water content forming a compact and strong structure. New woven bone formed after degradation of mineralized cartilage is rapidly degraded and resynthesized to impart structural order for local bone strength. Ossification is regulated by thickness of bone units and by patterning via bone morphogenetic receptors including activin, other bone morphogenetic protein receptors, transforming growth factor-β receptors, all part of a receptor superfamily. This superfamily interacts with receptors for additional signals in bone differentiation. Important features of the osteoblast environment were established using recent tools including osteoblast differentiation in vitro. Osteoblasts deposit matrix protein, over 90% type I collagen, in lamellae with orientation alternating parallel or orthogonal to the main stress axis of the bone. Into this organic matrix, mineral is deposited as hydroxyapatite. Mineral matrix matures from amorphous to crystalline hydroxyapatite. This process includes at least two-phase changes of the calcium-phosphate mineral as well as intermediates involving tropocollagen fibrils to form the bone composite. Beginning with initiation of mineral deposition, there is uncertainty regarding cardinal processes, but the driving force is not merely exceeding the calcium-phosphate solubility product. It occurs behind a epithelial-like layer of osteoblasts, which generate phosphate and remove protons liberated during calcium-phosphate salt deposition. The forming bone matrix is discontinuous from the general extracellular fluid. Required adjustment of ionic concentrations and water removal from bone matrix are important details remaining to be addressed.

Microscopic study of chronic charcot arthropathy foot bones contributes to understanding pathogenesis - A preliminary report

INTRODUCTION

Charcot arthropathy (CA) is non-infective, chronic destructive condition affecting the pes architecture of long standing diabetic patients with neuropathy. Even though several theories have emerged to disclose its pathogenesis, inflammatory cytokine induced osteoclastogenesis stands as the chief culprit. Studies on micro-architecture of foot bones of acute stage CA patients, describes mainly destructive phase of bone remodelling. Increased osteoclast cell activity is reported in all studies communicated. No study has to the best of our knowledge detailed the microscopic structure of chronic stage CA foot bones.

AIM

To study the microscopic structure of foot bones in patients with chronic CA.

MATERIALS AND METHODS

Foot bones were collected from the feet of chronic CA patients (six in number) who underwent corrective foot surgery in the Department of Podiatric Surgery of a tertiary care hospital. Control samples were collected from the feet of age matched non-diabetic controls (2 in number). The samples were fixed in formalin, decalcified in 10% nitric acid, processed, sectioned and stained with haematoxylin and eosin. Histopathology and histomorphometry analysis were performed by two different pathologists.

RESULTS

Trabeculae of chronic CA foot bones exhibited mainly a lamellar architecture, with reduced number of osteocytes and plenty of empty lacunae. Trabecular connectivity was lost and trabeculae showed considerable thinning. Trabecular osteoids lined by active osteoblast cells was a remarkable observation. Bone area was also considerably reduced in chronic CA foot bones.

CONCLUSION

Chronic stage CA foot bones presented features of both healing and fragile bone. The compromised bone quality may be due to thin and fragmented trabecular structure and reduced cellularity.

Ultra-high matrix mineralization of sperm whale auditory ossicles facilitates high sound pressure and high-frequency underwater hearing

The auditory ossicles-malleus, incus and stapes-are the smallest bones in mammalian bodies and enable stable sound transmission to the inner ear. Sperm whales are one of the deepest diving aquatic mammals that produce and perceive sounds with extreme loudness greater than 180 dB and frequencies higher than 30 kHz. Therefore, it is of major interest to decipher the microstructural basis for these unparalleled hearing abilities. Using a suite of high-resolution imaging techniques, we reveal that auditory ossicles of sperm whales are highly functional, featuring an ultra-high matrix mineralization that is higher than their teeth. On a micro-morphological and cellular level, this was associated with osteonal structures and osteocyte lacunar occlusions through calcified nanospherites (i.e. micropetrosis), while the bones were characterized by a higher hardness compared to a vertebral bone of the same animals as well as to human auditory ossicles. We propose that the ultra-high mineralization facilitates the unique hearing ability of sperm whales. High matrix mineralization represents an evolutionary conserved or convergent adaptation to middle ear sound transmission.

Crack propagation in cortical bone is affected by the characteristics of the cement line: a parameter study using an XFEM interface damage model

Bulk properties of cortical bone have been well characterized experimentally, and potent toughening mechanisms, e.g., crack deflections, have been identified at the microscale. However, it is currently difficult to experimentally measure local damage properties and isolate their effect on the tissue fracture resistance. Instead, computer models can be used to analyze the impact of local characteristics and structures, but material parameters required in computer models are not well established. The aim of this study was therefore to identify the material parameters that are important for crack propagation in cortical bone and to elucidate what parameters need to be better defined experimentally. A comprehensive material parameter study was performed using an XFEM interface damage model in 2D to simulate crack propagation around an osteon at the microscale. The importance of 14 factors (material parameters) on four different outcome criteria (maximum force, fracture energy, crack length and crack trajectory) was evaluated using ANOVA for three different osteon orientations. The results identified factors related to the cement line to influence the crack propagation, where the interface strength was important for the ability to deflect cracks. Crack deflection was also favored by low interface stiffness. However, the cement line properties are not well determined experimentally and need to be better characterized. The matrix and osteon stiffness had no or low impact on the crack pattern. Furthermore, the results illustrated how reduced matrix toughness promoted crack penetration of the cement line. This effect is highly relevant for the understanding of the influence of aging on crack propagation and fracture resistance in cortical bone.

On the Origins of Fracture Toughness in Advanced Teleosts: How the Swordfish Sword's Bone Structure and Composition Allow for Slashing under Water to Kill or Stun Prey

The osseous sword of a swordfish (Xiphias gladius) is specialized to incapacitate prey with stunning blows. Considering the sword's growth and maturation pattern, aging from the sword's base to the tip, while missing a mechanosensitive osteocytic network, an in-depth understanding of its mechanical properties and bone quality is lacking. Microstructural, compositional, and nanomechanical characteristics of the bone along the sword are investigated to reveal structural mechanisms accounting for its exceptional mechanical competence. The degree of mineralization, homogeneity, and particle size increase from the base toward the tip, reflecting aging along its length. Fracture experiments reveal that crack-growth toughness vastly decreases at the highly and homogeneously mineralized tip, suggesting the importance of aging effects. Initiation toughness, however, is unchanged suggesting that aging effects on this hierarchical level are counteracted by constant mineral/fibril interaction. In conclusion, the sword of the swordfish provides an excellent model reflecting base-to-tip-wise aging of bone, as indicated by increasing mineralization and decreasing crack-growth toughness toward the tip. The hierarchical, structural, and compositional changes along the sword reflect peculiar prerequisites needed for resisting high mechanical loads. Further studies on advanced teleosts bone tissue may help to unravel structure-function relationships of heavily loaded skeletons lacking mechanosensing cells.

Inter-site Variability of the Human Osteocyte Lacunar Network: Implications for Bone Quality

PURPOSE OF REVIEW

This article provides a review on the variability of the osteocyte lacunar network in the human skeleton. It highlights characteristics of the osteocyte lacunar network in relation to different skeletal sites and fracture susceptibility.

RECENT FINDINGS

Application of 2D analyses (quantitative backscattered electron microscopy, histology, confocal laser scanning microscopy) and 3D reconstructions (microcomputed tomography and synchrotron radiation microcomputed tomography) provides extended high-resolution information on osteocyte lacunar properties in individuals of various age (fetal, children's growth, elderly), sex, and disease states with increased fracture risk. Recent findings on the distribution of osteocytes in the human skeleton are reviewed. Quantitative data highlighting the variability of the osteocyte lacunar network is presented with special emphasis on site specificity and maintenance of bone health. The causes and consequences of heterogeneous distribution of osteocyte lacunae both within specific regions of interest and on the skeletal level are reviewed and linked to differential bone quality factors and fracture susceptibility.

50 years of scanning electron microscopy of bone-a comprehensive overview of the important discoveries made and insights gained into bone material properties in health, disease, and taphonomy

Bone is an architecturally complex system that constantly undergoes structural and functional optimisation through renewal and repair. The scanning electron microscope (SEM) is among the most frequently used instruments for examining bone. It offers the key advantage of very high spatial resolution coupled with a large depth of field and wide field of view. Interactions between incident electrons and atoms on the sample surface generate backscattered electrons, secondary electrons, and various other signals including X-rays that relay compositional and topographical information. Through selective removal or preservation of specific tissue components (organic, inorganic, cellular, vascular), their individual contribution(s) to the overall functional competence can be elucidated. With few restrictions on sample geometry and a variety of applicable sample-processing routes, a given sample may be conveniently adapted for multiple analytical methods. While a conventional SEM operates at high vacuum conditions that demand clean, dry, and electrically conductive samples, non-conductive materials (e.g., bone) can be imaged without significant modification from the natural state using an environmental scanning electron microscope. This review highlights important insights gained into bone microstructure and pathophysiology, bone response to implanted biomaterials, elemental analysis, SEM in paleoarchaeology, 3D imaging using focused ion beam techniques, correlative microscopy and in situ experiments. The capacity to image seamlessly across multiple length scales within the meso-micro-nano-continuum, the SEM lends itself to many unique and diverse applications, which attest to the versatility and user-friendly nature of this instrument for studying bone. Significant technological developments are anticipated for analysing bone using the SEM.

Osseointegration and current interpretations of the bone-implant interface

Complex physical and chemical interactions take place in the interface between the implant surface and bone. Various descriptions of the ultrastructural arrangement to various implant design features, ranging from solid and macroporous geometries to surface modifications on the micron-, submicron-, and nano- levels, have been put forward. Here, the current knowledge regarding structural organisation of the bone-implant interface is reviewed with a focus on solid devices, mainly metal (or alloy) intended for permanent anchorage in bone. Certain biomaterials that undergo surface and bulk degradation are also considered. The bone-implant interface is a heterogeneous zone consisting of mineralised, partially mineralised, and unmineralised areas. Within the meso-micro-nano-continuum, mineralised collagen fibrils form the structural basis of the bone-implant interface, in addition to accumulation of non-collagenous macromolecules such as osteopontin, bone sialoprotein, and osteocalcin. In the published literature, as many as eight distinct arrangements of the bone-implant interface ultrastructure have been described. The interpretation is influenced by the in vivo model and species-specific characteristics, healing time point(s), physico-chemical properties of the implant surface, implant geometry, sample preparation route(s) and associated artefacts, analytical technique(s) and their limitations, and non-compromised vs compromised local tissue conditions. The understanding of the ultrastructure of the interface under experimental conditions is rapidly evolving due to the introduction of novel techniques for sample preparation and analysis. Nevertheless, the current understanding of the interface zone in humans in relation to clinical implant performance is still hampered by the shortcomings of clinical methods for resolving the finer details of the bone-implant interface. STATEMENT OF SIGNIFICANCE: Being a hierarchical material by design, the overall strength of bone is governed by composition and structure. Understanding the structure of the bone-implant interface is essential in the development of novel bone repair materials and strategies, and their long-term success. Here, the current knowledge regarding the eventual structural organisation of the bone-implant interface is reviewed, with a focus on solid devices intended for permanent anchorage in bone, and certain biomaterials that undergo surface and bulk degradation. The bone-implant interface is a heterogeneous zone consisting of mineralised, partially mineralised, and unmineralised areas. Within the meso-micro-nano-continuum, mineralised collagen fibrils form the structural basis of the bone-implant interface, in addition to accumulation of non-collagenous macromolecules such as osteopontin, bone sialoprotein, and osteocalcin.

A multiscale analytical approach to evaluate osseointegration

Osseointegrated implants are frequently used in reconstructive surgery, both in the dental and orthopedic field, restoring physical function and improving the quality of life for the patients. The bone anchorage is typically evaluated at micrometer resolution, while bone tissue is a dynamic composite material composed of nanoscale collagen fibrils and apatite crystals, with defined hierarchical levels at different length scales. In order to understand the bone formation and the ultrastructure of the interfacial tissue, analytical strategies needs to be implemented enabling multiscale and multimodal analyses of the intact interface. This paper describes a sample preparation route for successive analyses allowing assessment of the different hierarchical levels of interest, going from macro to nano scale and could be implemented on single samples. Examples of resulting analyses of different techniques on one type of implant surface is given, with emphasis on correlating the length scale between the different techniques. The bone-implant interface shows an intimate contact between mineralized collagen bundles and the outermost surface of the oxide layer, while bone mineral is found in the nanoscale surface features creating a functionally graded interface. Osteocytes exhibit a direct contact with the implant surface via canaliculi that house their dendritic processes. Blood vessels are frequently found in close proximity to the implant surface either within the mineralized bone matrix or at regions of remodeling.