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The cortical bone is a hierarchical composite material that, at the microscale, is segmented in an interstitial matrix, cement line, osteons, and Haversian canals. The cracking of the structure at this scale directly influences the macro behavior, and, in this context, the cement line has a protagonist role. In this sense, this work aims to simulate the crack initiation and propagation processes via cortical bone microstructure modeling with a two-dimensional mesh fragmentation technique that captures the mechanical relevance of its constituents. In this approach, high aspect ratio elements are inserted between the regular constant strain triangle finite elements to define potential crack paths a priori. The crack behavior is described using a composed damage model with two scalar damage variables, which is integrated by an implicit-explicit (Impl-Ex) scheme to avoid convergence problems usually found in numerical simulations involving multiple cracks. The approach's capability of modeling the failure process in cortical bone microstructure is investigated by simulating four conceptual problems and one example based on a digital image of an experimental test. The results obtained in terms of crack pattern and failure mechanisms agree with those described in the literature, demonstrating that the numerical tool is promising to simulate the complex failure mechanisms in cortical bone, considering the properties of its distinct phases. [ABSTRACT FROM AUTHOR]

Bone microstructure governs microcrack propagation complexity. Current research, relying on linear elastic fracture mechanics, inadequately considers authentic multi-level structures, like cement lines and osteons, impacting stress intensity at cracks. This study, by constructing models encompassing single or multiple osteons, delves into the influence of factors like crack length, osteon radius, and modulus ratio on the stress intensity factor at the crack tip. Employing a fracture mechanics phase-field approach to simulate crack propagation paths, it particularly explores the role of cement lines as weak interfaces in crack extension. The aim is to comprehensively and systematically elucidate the critical factors of bone microstructure in the context of crack propagation. [ABSTRACT FROM AUTHOR]

The success of titanium dental implants relies on osseointegration, the load‐bearing connection between bone tissue and the device that, in contact osteogenesis, comprises the deposition of bony cement line matrix onto the implant surface. Titanium dioxide nanotubes (NTs) are considered a promising surface for improved osseointegration, yet the mechanisms of cement line integration with such features remains elusive. Herein, we illustrate cement line deposition into NTs on the surface of titanium implants with two underlaying microstructures: a machined surface or a blasted/acid etched surface placed in the tibiae of Wistar rats. After retrieval, scanning electron microscopy of tissue reflected from the implant surface indicated minimal incursion of the cement line matrix into the NTs. To investigate this further, focused ion beam was utilized to prepare cross‐sectional samples that could be characterized using scanning transmission electron microscopy. The cement line matrix covered NTs regardless of underlying microstructure, which was further confirmed by elemental analysis. In some instances, cement line infiltration into the NTs was noted, which reveals a mechanism of nanoscale anchorage. This study is the first to demonstrate cement line deposition into titanium NTs, suggesting nano‐anchorage as a mechanism for the success of the NT modified surfaces in vivo. [ABSTRACT FROM AUTHOR]

Background: The current research on crack propagation resistance characteristics of osteons is based on the single osteon model, which cannot reflect the interaction among osteons and the influence of material-property changes on the crack propagation. Moreover, the Haversian canal and the cement line were not included in the model, which could not reflect the influence of the true structural characteristics of the cortical bone on the microcracks. Objective: To investigate the effects of osteons, cement line and its material parameters on the stress intensity factor at the crack-tip based on theoretical formula calculation and finite element calculation. Results: The single osteon model, multi-osteon model and Haversian cortical bone model were established according to the microstructural characteristics of Haversian cortical bone. The calculation results of the first two models show that the "soft" osteon accelerates the crack propagation and the "hard" osteon hinders crack propagation, and the effect of multi-osteon model on a microcrack is significantly larger than that of single bone model. The calculation results of Haversian cortical bone model show that the stress intensity factors at both ends of the microcrack decreased when the distance from the "soft" osteon was within a certain range (1. 5 < d ∕ R < 2. 5). The stress intensity factor of endpoint 'a' in the crack-tip increases sharply when the microcrack is close to the osteon. Conclusions: The analysis results of the three models present that the microcrack propagation is affected by the material properties as well as the quantity of osteon, and the cement line is conductive to enhance the fracture toughness of the cortical bone. [ABSTRACT FROM AUTHOR]

Articular calcified cartilage (ACC) has been dismissed, by some, as a remnant of endochondral ossification without functional relevance to joint articulation or weight‐bearing. Recent research indicates that morphologic and metabolic ACC features may be important, reflecting knee joint osteoarthritis (OA) predisposition. ACC is less investigated than neighbouring joint tissues, with its component chondrocytes and mineralised matrix often being either ignored or integrated into analyses of hyaline articular cartilage and subchondral bone tissue respectively. Anatomical variation in ACC is recognised between species, individuals and age groups, but the selective pressures underlying this variation are unknown. Consequently, optimal ACC biomechanical features are also unknown as are any potential locomotory roles. This review collates descriptions of ACC anatomy and biology in health and disease, with a view to revealing its structure/function relationship and highlighting potential future research avenues. Mouse models of healthy and OA joint ageing have shown disparities in ACC load‐induced deformations at the knee joint. This raises the hypothesis that ACC response to locomotor forces over time may influence, or even underlie, the bony and hyaline cartilage symptoms characteristic of OA. To effectively investigate the ACC, greater resolution of joint imaging and merging of hierarchical scale data will be required. An appreciation of OA as a 'whole joint disease' is expanding, as is the possibility that the ACC may be a key player in healthy ageing and in the transition to OA joint pathology. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This work focuses on the Lacunar–Canalicular Porosity (PLC) of cortical bone which includes the osteons. Osteons are semicylindrical porous structures saturated with fluid within the bone and are approximately 250 μ m in diameter. The outer boundary of the osteon is called the cement line. Some studies suggested that the cement line is less highly mineralized and produced evidence that it has less calcium and phosphorus and more sulfur than the neighboring bone lamellae. Most authors assume that the cement line is impermeable, while others assume that some canaliculi are crossing the cement line which will make it permeable to certain degree. The objective of this work is to develop a theoretical analysis to study the leakage through the cement line and its relationship with the pore pressure distribution. The theoretical analysis is developed using our previous analysis for osteon under harmonic loading with addition of leakage parameter. The leakage parameter varies from 0 to 1, where a value of 0 indicates free flow through the cement line and a value of 1 indicates no flow through the cement line. Experimental results could be compared to this developed theoretical solution to get in depth understanding of the effect of leakage on osteon poroelastic properties. Additionally, the developed theoretical solution will give insight into sensitivity of osteon pore pressure to leakage through the cement line. [ABSTRACT FROM AUTHOR]

Introduction: Tissue separation after regenerative periodontal therapy is a common histological observation which is thought to be an artifact. However, several articles have indicated that it can be a serious problem directly related to the weak attachment of the newly formed cementum. The Hypothesis: Tissue separation after periodontal regeneration can be the consequence of losing the fibrous attachment from the cement line between new cementum and the root surface. Evaluation of the Hypothesis: A comparison was made between the cementum–dentin junction, the cement line after root resorption and the cement line after periodontal regeneration because they represent the only means by which the body binds the cementum to root surface. After losing the fibrous attachment from the cement line, the stresses may concentrate at the coronal part of the regenerated tissue leading to the development of tissue separation at that level.

Graphical abstract Highlights • Ti-PEEK surfaces provided increased hierarchical surface roughness. • Undifferentiated osteoprogenitor cells deposited cement line matrix. • Cement line deposition increased on Ti-PEEK surfaces. • Hierarchical roughness stimulates cement line deposition. Abstract Orthopedic implants benefit from a surface that encourages direct bony ongrowth (contact osteogenesis). This process is initiated as osteoprogenitor cells attach to the implant surface and deposit a calcium-enriched, collagen-deficient interfacial layer known as the cement line, which provides an anchoring foundation for the subsequent production of collagenous bone matrix from differentiated osteoblasts. Despite the importance of the cement line, the conditions affecting its deposition are incompletely understood. The current study aimed to examine cement line formation from human osteoprogenitor cells (hFOB 1.19) on a titanium plasma-sprayed PEEK (termed Ti-PEEK) surface exhibiting hierarchical roughness, compared to two relatively flat implant materials, PEEK and Ti-6Al-4 V (Ti). The hierarchical roughness of Ti-PEEK surfaces created more surface area (40% increase at the microscale) for greater cellular proliferation and stimulated significantly increased calcium deposition, which was produced by osteoprogenitor cells in their undifferentiated state. The absence of increases in alkaline phosphatase confirmed that cells remained undifferentiated, and the lack of variation in collagen measurements supported the non-collagenous composition of the cement line. Impressively, after just 24 h, the calcium deposition measured on Ti-PEEK surfaces was 305% and 470% higher than on Ti and PEEK, respectively, providing evidence that Ti-PEEK surfaces may enhance contact osteogenesis by stimulating accelerated cement line formation from undifferentiated osteoprogenitor cells. [ABSTRACT FROM AUTHOR]

Abstract Reliable tools for fracture risk assessment are necessary to handle the challenge with an aging population and the increasing occurrence of bone fractures. As it is currently difficult to measure local damage parameters experimentally, computational models could be used to provide insight into how cortical bone microstructure and material properties contribute to the fracture resistance. In this study, a model for crack propagation in 2D at the microscale in cortical bone was developed using the extended finite element method (XFEM). By combining the maximum principal strain criterion with an additional interface damage formulation in the cement line, the model could capture crack deflections at the osteon boundaries as observed in experiments. The model was used to analyze how the Haversian canal and the interface strength of the cement line affected the crack trajectory in models depicting osteons with three different orientations in 2D. Weak cement line interfaces were found to reorient the propagating cracks while models with strong interfaces predicted crack trajectories that penetrated the cement line and propagated through the osteons. The presented model is a promising tool that could be used to analyze how local, age-related material changes influence the crack trajectory and fracture resistance in cortical bone. Graphical abstract fx1 Highlights • The XFEM model can simulate crack propagation in cortical bone at the microscale. • The model is applicable to osteons with three perpendicular orientations. • The interface damage model can capture crack deflections at osteon boundaries. • The strength of the cement line interface has great influence on the crack path. [ABSTRACT FROM AUTHOR]

Microstructural and compositional changes that occur due to aging, pathological conditions, or pharmacological treatments alter cortical bone fracture resistance. However, the relative importance of these changes to the fracture resistance of cortical bone has not been quantified in detail. In this technical note, we developed an integrated experimental-computational framework utilizing human femoral cortical bone biopsies to advance the understanding of how fracture resistance of cortical bone is modulated due to modifications in its microstructure and material properties. Four human biopsy samples from individuals with varying fragility fracture history and osteoporosis treatment status were converted to finite element models incorporating specimen-specific material properties and were analyzed using fracture mechanics-based modeling. The results showed that cement line density and osteonal volume had a significant effect on crack volume. The removal of cement lines substantially increased the crack volume in the osteons and interstitial bone, representing straight crack growth, compared to models with cement lines due to the lack of crack deflection in the models without cement lines. Crack volume in the osteons and interstitial bone increased when mean elastic modulus and ultimate strength increased and mean fracture toughness decreased. Crack volume in the osteons and interstitial bone was reduced when material property heterogeneity was incorporated in the models. Although both the microstructure and the heterogeneity of the material properties of the cortical bone independently increased the fracture toughness, the relative contribution of the microstructure was more significant. The integrated experimental-computational framework developed here can identify the most critical microscale features of cortical bone modulated by pathological processes or pharmacological treatments that drive changes in fracture resistance and improve our understanding of the relative influence of microstructure and material properties on fracture resistance of cortical bone. [Display omitted] • Experimental-computational approach evaluating bone fracture resistance was developed. • Clinical specimens of human femoral cortical bone were evaluated. • Crack volume decreased with increasing cement line density and osteonal volume. • Removal of cement lines increased crack volume. • Material property heterogeneity reduced crack volume. [ABSTRACT FROM AUTHOR]

This work aims to automatically detect cement lines in decalcified cortical bone sections stained with H&E. Employed is a methodology developed previously by the authors and proven to successfully count and disambiguate the micro-architectural features (namely Haversian canals, canaliculi, and osteocyte lacunae) present in the secondary osteons/Haversian system (osteon) of cortical bone. This methodology combines methods typically considered separately, namely pulse coupled neural networks (PCNN), particle swarm optimization (PSO), and adaptive threshold (AT). In lieu of human bone, slides (at 20× magnification) from bovid cortical bone are used in this study as proxy of human bone. Having been characterized, features with same orientation are used to detect the cement line viewed as the next coaxial layer adjacent to the outermost lamella of the osteon. Employed for this purpose are three attributes for each and every micro-sized feature identified in the osteon lamellar system: (1) orientation, (2) size (ellipse perimeter) and (3) Euler number (a topological measure). From a training image, automated parameters for the PCNN network are obtained by forming fitness functions extracted from these attributes. It is found that a 3-way combination of these features attributes yields good representations of the overall osteon boundary (cement line). Near-unity values of classical metrics of quality (precision, sensitivity, specificity, accuracy, and dice) suggest that the segments obtained automatically by the optimized artificial intelligent methodology are of high fidelity as compared with manual tracing. For bench marking, cement lines segmented by k-means did not fare as well. An analysis based on the modified Hausdorff distance (MHD) of the segmented cement lines also testified to the quality of the detected cement lines vis-a-vis the k-means method.

Cement lines are known as thin peripheral boundaries of the osteons. With a thickness below 5 μm their composition of inorganic and organic compounds has been a matter of debate. Here, we hypothesized that cement lines become hypermineralized and their degree of mineralization is not constant but related to the tissue age of the osteon. Therefore, we analyzed the calcium content of osteons and their corresponding cement lines in a range of different tissue ages reflected by osteonal mineralization levels in femoral cortical bone of both postmenopausal women with osteoporosis and bisphosphonate-treated cases. Quantitative backscattered electron imaging (qBEI) showed that cement lines are hypermineralized entities with consistently higher calcium content than their corresponding osteons (mean calcium content: 29.46 ± 0.80 vs. 26.62 ± 1.11 wt%; p < 0.001). Micro-Raman spectroscopy complemented the qBEI data by showing a significantly higher phosphate/amide I ratio in the cement lines compared to the osteonal bone (8.78 ± 0.66 vs. 6.33 ± 0.58, p < 0.001), which was both due to an increased phosphate peak and a reduced amide I peak in cement lines. A clear positive correlation of cement line mineralization and the mineralization of the osteon was observed (r = 0.839, p = 0.003). However, the magnitude of the difference between cement line and osteonal calcium content decreased with increased osteonal calcium content (r = −0.709, p < 0.001), suggesting diverging mineralization dynamics in these osseous entities. The number of mineralized osteocyte lacunae per osteon bone area correlated positively with both osteonal and cement line calcium content (p < 0.01). The degree of mineralization of cement lines may represent another tissue-age related phenomenon, given that it strongly relates to the osteonal mineralization level. Understanding of the cement lines' mineralization and their changes in aging and disease states is important for predicting crack propagation pathways and fracture resistance mechanisms in human cortical bone. [ABSTRACT FROM AUTHOR]

Cortical bone consists of osteons embedded in interstitial bone tissue and there is a thin amorphous interface, named cement line, between osteon and interstitial bone. Due to fatigue and cyclic loading, the pullout or debonding phenomenon often occurs in osteonal and interstitial tissue bone. The study aims to construct a fiber-reinforced composite material debonding model for cortical bone, in which the bonding condition along the osteon, cement line and interstitial tissue bone are assumed to be imperfect. In the study, we used the complex variable method to obtain series representations for stress fields in the osteon, cement line and the interstitial tissue bone with a radial crack. The effects of material properties of osteon and cement line, crack position, and varying degrees of debonding on the fracture behavior were investigated by computing the stress intensity factor (SIF) in the vicinity of the microcrack tips. The investigation results indicated that the cement line was important for controlling the fracture toughening mechanisms and that the level of imperfect bonding among osteon, cement line and interstitial tissue bone had a pronounced effect on the crack behavior and should not be ignored. [ABSTRACT FROM AUTHOR]

In this work, a procedure is proposed to estimate the critical energy release rate G c of the so-called cement line in cortical bone tissue. Due to the difficulty of direct experimental estimations, relevant elastic and toughness material properties at bone microscale have been inferred by correlating experimental tests and finite element simulations. In particular, three-point bending tests of ovine cortical bone samples have been performed and modeled by finite elements. The initiation and growth of microcracks in the tested samples are simulated through finite elements using a damage model based on a maximum principal strain criterion, showing a good correlation with the experimental results. It is observed that microcracks evolve mainly along the cement lines and through the interstitial material but without crossing osteons. The numerical model allows the calculation of the cement line critical energy release rate G c by approximating its definition by finite differences. This way, it is possible to estimate this property poorly documented in the literature. [ABSTRACT FROM AUTHOR]

Introduction: Tissue separation after regenerative periodontal therapy is a common histological observation which is thought to be an artifact. However, several articles have indicated that it can be a serious problem directly related to the weak attachment of the newly formed cementum. The Hypothesis: Tissue separation after periodontal regeneration can be the consequence of losing the fibrous attachment from the cement line between new cementum and the root surface. Evaluation of the Hypothesis: A comparison was made between the cementum-dentin junction, the cement line after root resorption and the cement line after periodontal regeneration because they represent the only means by which the body binds the cementum to root surface. After losing the fibrous attachment from the cement line, the stresses may concentrate at the coronal part of the regenerated tissue leading to the development of tissue separation at that level. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Open-source phase field implementations are evaluated using benchmarks tests. • A new method to correct the interface fracture toughness in a phase field model. • Microscale simulations of crack growth around osteons in bone tissue are presented. • The models predict complex cracking phenomena in cement line interfaces. • Simulated crack patterns are consistent with analytical predictions from LEFM. Bone is a composite material where tubular osteon structures surrounded by thin interfaces reinforce the tissue at the microscale. These interfaces provide alternative weak paths for crack growth and give rise to potent extrinsic toughening mechanisms. Numerical models can help to gain a deeper understanding of fracture resistance in bone, however, capturing the interface mechanics is key. For this purpose, the phase field method for fracture is appealing due to its capability of capturing complex cracking phenomena. Still, it is far from well-established in the field of biomechanics. In this study, we evaluated a selection of recent open-source implementations of the phase field method to find the best approach for simulating crack growth in bone tissue. We also proposed a new method for correcting the interface fracture toughness in a phase field model. The selected implementations were compared using benchmark tests, including single edge notched tensile specimens with and without interfaces, as well as models representing typical bone geometries. We found the quasi-Newton monolithic solvers to be superior both in terms of computational speed and robustness compared to the evaluated staggered solvers. We also showed that correcting the fracture toughness of the interface is key for simulating crack patterns that are consistent with analytical predictions from linear elastic fracture mechanics. [ABSTRACT FROM AUTHOR]

This work aims to automatically detect cement lines in decalcified cortical bone sections stained with H&E. Employed is a methodology developed previously by the authors and proven to successfully count and disambiguate the micro-architectural features (namely Haversian canals, canaliculi, and osteocyte lacunae) present in the secondary osteons/Haversian system (osteon) of cortical bone. This methodology combines methods typically considered separately, namely pulse coupled neural networks (PCNN), particle swarm optimization (PSO), and adaptive threshold (AT). In lieu of human bone, slides (at 20× magnification) from bovid cortical bone are used in this study as proxy of human bone. Having been characterized, features with same orientation are used to detect the cement line viewed as the next coaxial layer adjacent to the outermost lamella of the osteon. Employed for this purpose are three attributes for each and every micro-sized feature identified in the osteon lamellar system: (1) orientation, (2) size (ellipse perimeter) and (3) Euler number (a topological measure). From a training image, automated parameters for the PCNN network are obtained by forming fitness functions extracted from these attributes. It is found that a 3-way combination of these features attributes yields good representations of the overall osteon boundary (cement line). Near-unity values of classical metrics of quality (precision, sensitivity, specificity, accuracy, and dice) suggest that the segments obtained automatically by the optimized artificial intelligent methodology are of high fidelity as compared with manual tracing. For bench marking, cement lines segmented by k-means did not fare as well. An analysis based on the modified Hausdorff distance (MHD) of the segmented cement lines also testified to the quality of the detected cement lines vis-a-vis the k-means method. [ABSTRACT FROM AUTHOR]

Microstructural features including osteons and cement lines are considered to play an important role in determining the crack growth behaviour in cortical bone. This study aims to develop a computational mechanics approach to evaluate microscale fracture mechanisms in bone. In this study, finite element models based on actual human cortical bone images that allow for arbitrary crack growth were utilised to determine the crack propagation behaviour. The simulations varied the cement line and osteon strength and fracture toughness in different bone microstructures to assess the crack propagation trajectory, stress–strain relationship and nonlinear strain energy density. The findings of this study provide additional insight into the individual influence of microstructural features and their properties on crack growth behaviour in bone using a computational approach. [ABSTRACT FROM AUTHOR]

Abstract: Bone is a hierarchical material that involves fracture mechanisms at multiple scales ranging from nano- to macroscale. The fracture behavior of bone is altered by aging, diseases or therapeutic treatments, therefore, robust predictive methods are necessary to evaluate the changes affecting the fracture risk of bone. Although cohesive modeling has been widely used in various engineering disciplines, it has recently been applied to bone fracture. In this paper, the application of cohesive modeling to the assessment of macro- and microscale fracture mechanisms in bone is presented. The macroscale simulations focus on predicting the fracture risk at the whole bone level whereas the microscale simulations evaluate the influence of microstructural features on crack propagation behavior in bone. These studies demonstrate the strength of cohesive modeling in providing insight into bone''s fracture behavior. [Copyright &y& Elsevier]

A nonlocal nonlinear multiscale field theory is briefly introduced in this paper. The theory is then reduced to a nonlocal linear elastic theory. A meshfree method is applied to solve the governing equation. The entire process of microcrack propagation in the cortical bone, including the propagation of microcrack toward the osteon, around the osteon, and out of the osteon, is simulated. The results show that the cement line between the osteanal bone and the interstitial bone serves as a protective barrier for the Haversian canal. We also observed that, with healing process incorporated in the simulation, cement line serves as a barrier for the microcracks propagation. [ABSTRACT FROM AUTHOR]

Abstract: Bone bonding is occurring in each of us and all other terrestrial vertebrates throughout life at bony remodeling sites. The surface created by the bone-resorbing osteoclast provides a three-dimensionally complex surface with which the cement line, the first matrix elaborated during de novo bone formation, interdigitates and is interlocked. The structure and composition of this interfacial bony matrix has been conserved during evolution across species; and we have known for over a decade that this interfacial matrix can be recapitulated at a biomaterial surface implanted in bone, given appropriate healing conditions. No evidence has emerged to suggest that bone bonding to artificial materials is any different from this natural biological process. Given this understanding it is now possible to explain why bone-bonding biomaterials are not restricted to the calcium–phosphate-based bioactive materials as was once thought. Indeed, in the absence of surface porosity, calcium phosphate biomaterials are not bone bonding. On the contrary, non-bonding materials can be rendered bone bonding by modifying their surface topography. This paper argues that the driving force for bone bonding is bone formation by contact osteogenesis, but that this has to occur on a sufficiently stable recipient surface which has micron-scale surface topography with undercuts in the sub-micron scale-range. [Copyright &y& Elsevier]

Background/Aims: In cortical bone, basic multicellular units (BMUs) produce secondary osteons that mediate adaptations, including variations in their population densities and cross-sectional areas. Additional important BMU-related adaptations might include atypical secondary osteon morphologies (zoned, connected, drifting, elongated, multiple canal). These variants often reflect osteonal branching that enhances toughness by increasing interfacial (cement line) complexity. If these characteristics correlate with strain mode/magnitude-related parameters of habitual loading, then BMUs might produce adaptive differences in unexpected ways. Methods: We carried out examinations in bones loaded in habitual torsion (horse metacarpals) or bending: sheep, deer, elk, and horse calcanei, and horse radii. Atypical osteons were quantified in backscattered images from anterior, posterior, medial, and lateral cortices. Correlations were determined between atypical osteon densities, densities of all secondary osteons, and associations with habitual strain mode/magnitude or transcortical location. Results: Osteon variants were not consistently associated with ‘tension’, ‘compression’, or neutral axis (‘shear’) regions, even when considering densities or all secondary osteons, or only osteon variants associated with relatively increased interfacial complexity. Similarly, marrow- and strain-magnitude-related associations were not consistent. Conclusion: These data do not support the hypothesis that spatial variations in these osteon variants are useful for inferring a habitual bending or torsional load strain history. Copyright © 2007 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Abstract: The interfacial strength of secondary osteons from the diaphysis of the Thoroughbred equine third metacarpal was evaluated using the fiber pushout test. The pushout was performed on 300–500μm sections of 4×4×15mm bone blocks machined from four anatomic regions of the cortex. Pushout strength was evaluated from proximal to distal location within the diaphysis on four osteon types classified under polarized light on adjacent histologic sections from each block. The shear strength of the interfaces were estimated from shear lag theory. Differences were found in the interfacial strength of osteons based on appearance under polarized light with bright field having the highest interfacial strength (40.3MPa). The lowest strength was found in the dark field osteons (22.8MPa). The dorsal region had the highest shear strength and toughness compared to all other regions. The cement line and interlamellar interfaces are similar in strength, but exhibit regional dependence—specifically, the palmar region strength is less (17.5MPa) than the osteon interlamellar interfaces (30.4MPa) and osteon type dependent (alternating significantly weaker than other types). Histomorphometry revealed significant regional differences () in osteon area fraction among the four osteon types as well as differences in the osteon diameter (), with dorsal regions having larger osteons (170μm) than the palmar region (151μm). Fatigue life and fracture toughness of Haversian bone are reported in the literature to be regionally dependent and are known to be associated with osteon pullout—an osteon interfacial phenomenon. Therefore, the results presented in this study are important to further the understanding of the mechanisms of fragility and damage accumulation in cortical bone. [Copyright &y& Elsevier]

Abstract: Osteonal bones are crack-tolerant due to their repair activity and special tissue structures formed by interstitial material and osteons. Crack-resistant engineering structures may be developed by mimicking bones. In this paper, bone-like composite materials were modeled by using circular inclusions as osteons and their cement lines. A two-inclusion model was investigated to see how crack location and inclusion size and property variations affect the propagation of an initial crack between the inclusions. A four-inclusion model was studied to determine conditions for attracting cracks to inclusions for arrestment. The four-inclusion model was optimized in order to arrest any crack initiated within the interstitial material between the inclusions. The optimized model was verified with the FRANC2D software. The results showed that inclusions with effective elastic moduli smaller than that of the interstitial material always tend to attract cracks. On the contrary, an inclusion with effective modulus greater than that of the interstitial material has a tendency to repel cracks. A crack’s propagation direction can be changed by varying inclusions’ sizes and elastic properties. At optimal point, any crack initiated within the interstitial material between the four inclusions can be attracted to two inclusions for arrestment. [Copyright &y& Elsevier]

Bone is an anisotropic structure which can be compared to a composite material. Discontinuities within its microstructure may provide stress concentration sites for crack initiation, but act as a barrier to its propagation. This study looks specifically at the relationship between crack length and propagation in compact bone. Beam-shaped bone samples from sheep radii were prepared and stained with fluorochrome dyes and tested in cyclic fatigue under four-point bending in an INSTRON 1341 servo-hydraulic fatigue-testing machine. Samples were tested at a frequency of 30 Hz and stress range of 100 MPa under load control. Specimens were sectioned transversely using a diamond saw, slides prepared and examined using epifluorescence microscopy. Cracks in transverse sections were classified in terms of their location relative to cement lines surrounding secondary osteons. Mean crack length, crack numerical density and crack surface density were examined. Short microcracks (100 µm or less) were stopped at the cement lines surrounding osteons, microcracks of intermediate length (100–300 µm) were deflected as they hit the cement line, and microcracks that were able to penetrate through cement lines were longer (> 400 µm). These data show that bone microstructure allows the initiation of microcracks but acts as a barrier to crack propagation. [ABSTRACT FROM AUTHOR]

The cement lines between reparative cementum and resorbed dentin or cementum in human teeth were observed by light microscopy and scanning electron microscopy combined with NaOH maceration. The NaOH maceration was used to remove interfibrillar substance and to observe the fibrillar architecture of the cement lines directly. Light microscopy showed that the cement lines were rich in proteoglycans with mucopolysaccharides, but deficient in collagen fibrils. The cement lines were artificially broken after treatment with hyaluronidase, which digests some of the mucopolysaccharides, but digests no collagen fibril. Scanning electron microscopy showed that fibril intermingling occurred only in some places between reparative cementum and resorbed tissue. These findings suggested that the proteoglycans in cement lines mediate the attachment between new and old mineralized tissue. [ABSTRACT FROM AUTHOR]