Collagen, one of the main building blocks for various tissues, derives its mechanical properties directly from its structure of cross-linked tropocollagen molecules. The cross-links are considered to ...be a key component of collagen fibrils as they can change the fibrillar behavior in various ways. For instance, enzymatic cross-links (ECLs), one particular type of cross-links, are known for stabilizing the structure of the fibril and improving material properties, while cross-linking AGEs (Advanced-Glycation Endproducts) have been shown to accumulate and impair the mechanical properties of collageneous tissues. However, the reasons for whether and how a given type of cross-link improves or impairs the material properties remain unknown, and the exact relationship between the cross-link properties and density, and the fibrillar behavior is still not well understood. Here, we use coarse-grained steered molecular models to evaluate the effect of AGEs and ECLs cross-links content on the deformation and failure properties of collagen fibrils. Our simulations show that the collagen fibrils stiffen at high strain levels when the AGEs content exceeds a critical value. In addition, the strength of the fibril increases with AGEs accumulation. By analyzing the forces within the different types of cross-links (AGEs and ECLs) as well as their failure, we demonstrate that a change of deformation mechanism is at the origin of these observations. A high AGEs content reinforces force transfer through AGEs cross-links rather than through friction between sliding tropocollagen molecules, which leads to failure by breaking of bonds within the tropocollagen molecules. We show that this failure mechanism, which is associated with lower energy dissipation, results in more abrupt failure of the collagen fibril. Our results provide a direct and causal link between increased AGEs content, inhibited intra-fibrillar sliding, increased stiffness, and abrupt fibril fracture. Therefore, they explain the mechanical origin of bone brittleness as commonly observed in elderly and diabetic populations. Our findings contribute to a better understanding of the mechanisms underlying impaired tissue behavior due to elevated AGEs content and could enable targeted measures regarding the reduction of specific collagen cross-linking levels.
•High contents of AGEs cause stiffening of the collagen fibril in higher strain ranges.•AGEs cross-links dominate force transfer between tropocollagen molecules.•Load transfer to tropocollagen molecules is causing stiffening of the fibril.•Reduced sliding and stiffening lead to less energy dissipation in the tissue•Strengthening and stiffening of the collagen fibril may cause impaired tissue behavior.
The denaturation of collagen at the molecular level in bone and dentin can impact their structure and properties, leading to increased brittleness in pathological diseases such as osteogenesis ...imperfecta, dentinogenesis imperfecta, diabetes, and cancer. This study investigates the relationship between collagen denaturation and the macroscale resistance of bone and dentin. Through heat treatment at
160
∘
C
on bovine bone and human dentin, the effects of collagen denaturation on macroscale flexural strength, scanning electron microscopy, and transmission electron microscopy imaging of micro- and nanostructure were studied. The results show that collagen denaturation decreases the resistance of bone and dentin to fracture, even though collagen denaturation did not impact the mineral organization around and inside collagen fibrils. This is attributable to (1) a reduction in bone and dentin ability to deform (e.g., 40–75% decrease in strain to failure) and to resist fracture (e.g., 83–95% decrease in work to fracture) properties and (2) to a smoother crack path with less crack deflection around microstructural features. Reduction in deformation and toughness not only removed plastic deformation but also drastically decreased elastic deformation and elastic work to fracture in all tissues. However, the elastic modulus was only affected in radial-oriented bone samples where collagen fibrils are oriented perpendicularly to crack opening forces. This study highlights the crucial role of collagen molecule integrity and orientation in bone/dentin deformability and resistance.
In this study, the fracture behavior of ribosylated bovine cortical bone is investigated under loading conditions simulating a fall event. Single edge notched specimens, separated into a control ...group (n = 11) and a ribosylated group (n = 8), were extracted from the mid-diaphysis of a single bovine femur harvested from a mature cow. A seven-day ribosylation process results in the accumulation of Advanced-Glycation End Products (AGEs) cross-links and AGE adducts. Specimens were subjected to symmetric three point bending (opening mode) and an impact velocity of 1.6 m/s using a drop tower. Near-crack displacement fields up to fracture initiation are determined from high-speed images post-processed using digital image correlation. A constrained over-deterministic least squares regression and orthotropic material linear elastic fracture mechanics theory are used to extract the in-plane critical stress intensity factors at fracture initiation (i.e., fracture initiation toughness values). Statistically significant differences were not observed when comparing the in-plane fracture initiation toughness values (p≥0.96) or energy release rate (p=0.90) between the control and seven-day ribosylated groups. The intrinsic variability of bone may require high sample numbers in order to achieve an adequately powered experiment when assessing dynamic fracture behavior. While there are no detectable differences due to the ribosylation treatment investigated, this is likely due to the limited sample sizes utilized.
Through a process called perilacunar remodeling, bone-embedded osteocytes dynamically resorb and replace the surrounding perilacunar bone matrix to maintain mineral homeostasis. The vital canalicular ...networks required for osteocyte nourishment and communication, as well as the exquisitely organized bone extracellular matrix, also depend upon perilacunar remodeling. Nonetheless, many questions remain about the regulation of perilacunar remodeling and its role in skeletal disease. Here, we find that suppression of osteocyte-driven perilacunar remodeling, a fundamental cellular mechanism, plays a critical role in the glucocorticoid-induced osteonecrosis. In glucocorticoid-treated mice, we find that glucocorticoids coordinately suppress expression of several proteases required for perilacunar remodeling while causing degeneration of the osteocyte lacunocanalicular network, collagen disorganization, and matrix hypermineralization; all of which are apparent in human osteonecrotic lesions. Thus, osteocyte-mediated perilacunar remodeling maintains bone homeostasis, is dysregulated in skeletal disease, and may represent an attractive therapeutic target for the treatment of osteonecrosis.
The fracture resistance of bone has been attributed to a competition of sub-micron lengthscale intrinsic mechanisms, including plasticity conferred by collagen stretching and intermolecular sliding ...and much larger lengthscale extrinsic mechanisms such as crack deflection and bridging. In this study, the contribution of intrinsic toughening mechanisms on the dynamic fracture behavior of bovine cortical bone is investigated. Single edge notched cortical bone specimens were extracted from the mid-diaphysis of a bovine femur with dimensions in accordance with ASTM E399. Four specimen groups are studied, a control group, and groups subjected to two-hour heat treatments of 130
∘
C, 160
∘
C and 190
∘
C, respectively. Using a trypsin-hydroxyproline assay to determine the percent of denatured collagen achieved by each heat treatment, it is shown that the 160
∘
C and 190
∘
C groups have accumulated substantial collagen network damage compared to the 130
∘
C and control groups. Three-point bend drop tower experiments with impact velocities of 1.6m/s. The selected impact velocity results in a nominal stress intensity factor rate of
K
˙
=
1.5
×
10
5
M
P
a
m
1
/
2
/
s
.Specimen’s speckled surfaces were imaged at 500,000 fps during deformation and post-processed using digital image correlation to determine the in-plane displacement fields. Using an orthotropic material linear elastic fracture mechanics formulation and over-deterministic least-squares analysis, the critical mode-I and mode-II stress intensity factors (i.e., fracture initiation toughness) were determined immediately proceeding fracture. As the heat treatment temperature increases (and the damaged collagen content increases), a weak but decreasing trend in fracture toughness was observed. Of particular note, for the 160
∘
C and 190
∘
C heat treatments, it was observed that the mode-II fracture initiation toughness is larger than the mode-I fracture initiation toughness. Regardless of the heat treatment condition, the mode-II fracture initiation toughness was comparatively less affected. For the specific case of Haversian bovine cortical bone whose collagen network has been denatured using heat treatment, a trend is observed pointing to collagen primarily conferring mode-I fracture initiation toughness, opposed to mode-II fracture initiation toughness, for the transverse fracture orientation.
Chronic kidney disease (CKD) is a common disease of aging and increases fracture risk over advanced age alone. Aging and CKD differently impair bone turnover and mineralization. We thus hypothesize ...that the loss of bone quality would be greatest with the combination of advanced age and CKD. We evaluated bone from young adult (6 mo.), middle-age (18 mo.), and old (24 mo.) male C57Bl/6 mice three months following either 5/6th nephrectomy, to induce CKD, or Sham procedures. CKD exacerbated losses of cortical and trabecular microarchitecture associated with aging. Aging and CKD each resulted in thinner, more porous cortices and fewer and thinner trabeculae. Bone material quality was also reduced with CKD, and these changes to bone material were distinct from those due to age. Aging reduced whole-bone flexural strength and modulus, micrometer-scale nanoindentation modulus, and nanometer-scale tissue and collagen strain (small-angle x-ray scattering SAXS. By contrast, CKD reduced work to fracture and variation in bone tissue modulus and composition (Raman spectroscopy), and increased percent collagen strain. The increased collagen strain burden was associated with loss of toughness in CKD. In addition, osteocyte lacunae became smaller, sparser, and more disordered with age for Sham mice, yet these age-related changes were not clearly observed in CKD. However, for CKD, larger lacunae positively correlated with increased serum phosphate levels, suggesting that osteocytes play a role in systemic mineral homeostasis. This work demonstrates that CKD reduces bone quality, including microarchitecture and bone material properties, and that loss of bone quality with age is compounded by CKD. These findings may help reconcile why bone mass does not consistently predict fracture in the CKD population, as well as why older individuals with CKD are at high risk of fragility.
•CKD and aging differentially affected bone material properties.•CKD altered bone collagen nanomechanics.•CKD and aging both reduced cortical and trabecular microarchitecture.•Osteocyte lacunae may participate in phosphate mineral homeostasis in CKD.
All levels of the unique hierarchical structure of bone, consisting of collagen and hydroxyapatite crystals at the nanoscale to osteon/lamellae structures at the microscale, contribute to its ...characteristic toughness and material properties. Elements of bone's density and size contribute to bone quantity (or bone mass), whereas elements of bone's material composition, material properties, internal structure, and organization describe bone quality. Furthermore, bone quantity and quality can be degraded by factors such as aging, disease, treatments, and irradiation, compromising its ability to resist fracture and sustain loading. Accessing the morphology and architecture of bone at the microscale to quantify microstructural features and assess the degree of mineralization and path of crack propagation in bone provides crucial information on how these factors are influencing bone quantity and quality. Synchrotron radiation micro-computed tomography (SRμCT) was first used to assess bone structure at the end of the 1990's. One of the main advantages of the technique is that it enables accurate three-dimensional (3D), non-destructive quantification of structure while traditional histomorphometry on histological sections is inherantly destructive to the sample and two-dimensional (2D). Additionally, SRμCT uses monochromatic, high-flux X-ray beams to provide high-resolution and high-contrast imaging of bone samples. This allows the quantification of small microstructural features (e.g. osteocyte lacunae, canals, trabeculae, microcracks) and direct gray value compositional mapping (e.g. mineral quantification, cement lines) with greater speed and fidelity than lab-based micro-computed tomography. In this article, we review how SRμCT has been applied to bone research to elucidate the mechanisms by which bone aging, disease, and other factors affect bone fragility and resistance to fracture.
All levels of the unique hierarchical structure of bone, consisting of collagen and hydroxyapatite crystals at the nanoscale to osteon/lamellae structures at the microscale, contribute to its ...characteristic toughness and material properties. Elements of bone's density and size contribute to bone quantity (or bone mass), whereas elements of bone's material composition, material properties, internal structure, and organization describe bone quality. Bone quantity and quality can be degraded by factors such as aging, disease, treatments, and irradiation, compromising its ability to resist fracture and sustain loading. Accessing the morphology and architecture of bone at the microscale to quantify microstructural features and assess the degree of mineralization and path of crack propagation in bone provides crucial information on how these factors are influencing bone quantity and quality. Synchrotron radiation micro-computed tomography (SRμCT) was first used to assess bone structure at the end of the 1990's. One of the main advantages of the technique is that it enables accurate three-dimensional (3D), non-destructive quantification of structure while traditional histomorphometry on histological sections is inherantly destructive to the sample and two-dimensional (2D). Additionally, SRμCT uses monochromatic, high-flux X-ray beams to provide high-resolution and high-contrast imaging of bone samples. This allows the quantification of small microstructural features (e.g. osteocyte lacunae, canals, trabeculae, microcracks) and direct gray value compositional mapping (e.g. mineral quantification, cement lines) with greater speed and fidelity than lab-based micro-computed tomography. In this article, we review how SRμCT has been applied to bone research to elucidate the mechanisms by which bone aging, disease, and other factors affect bone fragility and resistance to fracture.
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