On the effect of X-ray irradiation on the deformation and fracture behavior of human cortical bone

Bone strength and residual compressive stress in apatite crystals

Residual stresses are omnipresent in composite materials, often arising during the fabrication process. Residual compressive stresses were recently observed to develop in collagen fibrils during the process of mineralization. They have in fact been reported in a range of bony materials spanning tooth dentin to mammalian and fish bones. Treatment by heat or by irradiation have shown that compressive residual stresses up to 100 MPa can be released in the mineral by inducing damage to the protein fibers. This mini-review assembles some of the knowledge about residual stresses in bony nanocomposites and uses a composite model to argue that such stresses play a major role in enhancing the strength of bone.

Tough Cortical Bone-Inspired Tubular Architected Cement-Based Material with Disorder

Cortical bone is a tough biological material composed of tube-like osteons embedded in the organic matrix surrounded by weak interfaces known as cement lines. The cement lines provide a microstructurally preferable crack path, hence triggering in-plane crack deflection around osteons due to cement line-crack interaction. Inspired by this toughening mechanism and facilitated by a hybrid (3D-printing/casting) process, the study engineers architected tubular cement-based materials with the stepwise cracking toughening mechanism, that enables a non-brittle fracture. Using experimental and theoretical approaches, the study demonstrates the competition between tube size and shape on stress intensity factor from which engineering stepwise cracking can emerge. Two competing mechanisms, both positively and negatively affected by the growing tube size, arise to significantly enhance the overall fracture toughness by up to 5.6-fold compared to the monolithic brittle counterpart without sacrificing the specific strength. This is enabled by crack-tube interaction and engineering the tube size, shape, and orientation, which promotes rising resistance-curves (R-curve). "Disorder" curves and statistical mechanics parameters are proposed for the first time to quantitatively characterize the degree of disorder for describing the representation of the architected arrangement of materials in lieu of otherwise inadequate "periodicity" classification and misperceived disorder parameters (perturbation and Voronoi tessellation methods).

Crossing length scales: X-ray approaches to studying the structure of biological materials

Biological materials have outstanding properties. With ease, challenging mechanical, optical or electrical properties are realised from comparatively `humble' building blocks. The key strategy to realise these properties is through extensive hierarchical structuring of the material from the millimetre to the nanometre scale in 3D. Though hierarchical structuring in biological materials has long been recognized, the 3D characterization of such structures remains a challenge. To understand the behaviour of materials, multimodal and multi-scale characterization approaches are needed. In this review, we outline current X-ray analysis approaches using the structures of bone and shells as examples. We show how recent advances have aided our understanding of hierarchical structures and their functions, and how these could be exploited for future research directions. We also discuss current roadblocks including radiation damage, data quantity and sample preparation, as well as strategies to address them.

Bone collagen tensile properties of the aging human proximal femur

Despite the dominant role of bone mass in osteoporotic fractures, aging bone tissue properties must be thoroughly understood to improve osteoporosis management. In this context, collagen content and integrity are considered important factors, although limited research has been conducted on the tensile behavior of demineralized compact bone in relation to its porosity and elastic properties in the native mineralized state. Therefore, this study aims (i) at examining the age-dependency of mineralized bone and collagen micromechanical properties; (ii) to test whether, and if so to which extent, collagen properties contribute to mineralized bone mechanical properties. Two cylindrical cortical bone samples from fresh frozen human anatomic donor material were extracted from 80 proximal diaphyseal sections from a cohort of 24 female and 19 male donors (57 to 96 years at death). One sample per section was tested in uniaxial tension under hydrated conditions. First, the native sample was tested elastically (0.25 % strain), and after demineralization, up to failure. Morphology and composition of the second specimen was assessed using micro-computed tomography, Raman spectroscopy, and gravimetric methods. Simple and multiple linear regression were employed to relate morphological, compositional, and mechanical variables with age and sex. Macro-tensile properties revealed that only elastic modulus of native samples was age dependent whereas apparent elastic modulus was sex dependent (p < 0.01). Compositional and morphological analysis detected a weak but significant age and sex dependency of relative mineral weight (r = -0.24, p < 0.05) and collagen disorder ratio (I∼1670/I∼1640, r = 0.25, p < 0.05) and a strong sex dependency of bone volume fraction while generally showing consistent results in mineral content assessment. Young's modulus of demineralized bone was significantly related to tissue mineral density and Young's modulus of native bone. The results indicate that mechanical properties of the organic phase, that include collagen and non-collagenous proteins, are independent of donor age. The observed reduction in relative mineral weight and corresponding overall stiffer response of the collagen network may be caused by a reduced number of mineral-collagen connections and a lack of extrafibrillar and intrafibrillar mineralization that induces a loss of waviness and a collagen fiber pre-stretch.

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.

Automated detection and classification of concealed objects using infrared thermography and convolutional neural networks

This paper presents a study on the effectiveness of a convolutional neural network (CNN) in classifying infrared images for security scanning. Infrared thermography was explored as a non-invasive security scanner for stand-off and walk-through concealed object detection. Heat generated by human subjects radiates off the clothing surface, allowing detection by an infrared camera. However, infrared lacks in penetration capability compared to longer electromagnetic waves, leading to less obvious visuals on the clothing surface. ResNet-50 was used as the CNN model to automate the classification process of thermal images. The ImageNet database was used to pre-train the model, which was further fine-tuned using infrared images obtained from experiments. Four image pre-processing approaches were explored, i.e., raw infrared image, subject cropped region-of-interest (ROI) image, K-means, and Fuzzy-c clustered images. All these approaches were evaluated using the receiver operating characteristic curve on an internal holdout set, with an area-under-the-curve of 0.8923, 0.9256, 0.9485, and 0.9669 for the raw image, ROI cropped, K-means, and Fuzzy-c models, respectively. The CNN models trained using various image pre-processing approaches suggest that the prediction performance can be improved by the removal of non-decision relevant information and the visual highlighting of features.

High-resolution local trabecular strain within trabecular structure under cyclic loading

Trabecular bone structure is a complex microstructure consisting of rods and plates, which poses challenges for its mechanical characterization. Digital image correlation (DIC) offers the possibility to characterize the strain response on the surface of trabecular bone. This study employed DIC equipped with a telecentric lens to investigate the strain state of individual trabeculae within their trabecular structure by assessing the longitudinal strain of the trabeculae at both the middle and near the edges of the trabeculae. Due to the high-resolution of the used DIC system, local surface strain of trabeculae was analyzed too. Lastly, the correlation between longitudinal trabecular strain and the orientation and slenderness of the trabeculae was investigated. The results showed that the strain magnification close to the edge of the trabeculae was higher and reached up to 8-folds the strain along the middle of the trabeculae. On the contrary, no strain magnification was found for most of the trabeculae between the longitudinal trabecular strain along the middle of the trabeculae and the globally applied strain. High-resolution full-field strain maps were obtained on the surface of trabeculae showing heterogeneous strain distribution with increasing load. No significant correlation was found between longitudinal trabecular strain and its orientation or slenderness. These findings and the applied methodology can be used to broaden our understanding of the deformation mechanisms of trabeculae within the trabecular network.

Evaluation on the efficacy of processed hydrated and dehydrated amnion chorion membrane on the proliferation of periodontal ligament fibroblasts

The purpose of the present study was to process and assess the effect of hydrated amnion chorion membrane and dehydrated amnion chorion membrane on proliferation of periodontal ligament (PDL) fibroblast cells. The amnion chorion membrane (ACM) from placenta of 18 systemically healthy patients was obtained from the Department of Obstetrics and Gynaecology. They were processed as hydrated and dehydrated based on different processing methods. The Periodontal ligament cells were obtained from periodontal ligament of freshly extracted premolars of systemically healthy patients, due to orthodontic reasons. The PDL cells were further cultured in laboratory and were exposed to hydrated and dehydrated amnion chorion membrane. The MTT assay was performed to assess the proliferation of PDL fibroblast cells after 24 and 48 h. The hydrated and dehydrated amnion chorion membrane showed proliferation of PDL fibroblasts after 24 and 48 h. The proliferation of PDL fibroblasts in hydrated (p = 0.043) and dehydrated (p = 0.050) amnion chorion membrane was statistically significant at the end of 24 and 48 h respectively. On inter-group comparison dehydrated ACM showed significant proliferation of PDL fibroblasts after 24 (p=0.014) and 48 h (p=0.019). Within the limits of the present study, it can be concluded: both hydrated and dehydrated amnion chorion membrane showed proliferationof PDL fibroblast cells. However, dehydrated ACM showed significant proliferation of PDL fibroblasts.

Effect of gamma irradiation and supercritical carbon dioxide sterilization with Novakill™ or ethanol on the fracture toughness of cortical bone

Sterilization of structural bone allografts is a critical process prior to their clinical use in large cortical bone defects. Gamma irradiation protocols are known to affect tissue integrity in a dose dependent manner. Alternative sterilization treatments, such as supercritical carbon dioxide (SCCO2 ), are gaining popularity due to advantages such as minimal exposure to denaturants, the lack of toxic residues, superior tissue penetration, and minor impacts on mechanical properties including strength and stiffness. The impact of SCCO2 on the fracture toughness of bone tissue, however, remains unknown. Here, we evaluate crack initiation and growth toughness after 2, 6, and 24 h SCCO2 -treatment using Novakill™ and ethanol as additives on ~11 samples per group obtained from a pair of femur diaphyses of a canine. All mechanical testing was performed at ambient air after 24 h soaking in Hanks' balanced salt solution (HBSS). Results show no statistically significant difference in the failure characteristics of the Novakill™-treated groups whereas crack growth toughness after 6 and 24 h of treatment with ethanol significantly increases by 37% (p = .010) and 34% (p = .038), respectively, compared to an untreated control group. In contrast, standard 25 kGy gamma irradiation causes significantly reduced crack growth resistance by 40% (p = .007) compared to untreated bone. FTIR vibrational spectroscopy, conducted after testing, reveals a consistent trend of statistically significant differences (p < .001) with fracture toughness. These trends align with variations in the ratios of enzymatic mature to immature crosslinks in the collagen structure, suggesting a potential association with fracture toughness. Additional Raman spectroscopy after testing shows a similar trend with statistically significant differences (p < .005), which further supports that collagen structural changes occur in the SCF-treated groups with ethanol after 6 and 24 h. Our work reveals the benefits of SCCO2 sterilization compared to gamma irradiation.

Relative Effects of Radiation-Induced Changes in Bone Mass, Structure, and Tissue Material on Vertebral Strength in a Rat Model

The observed increased risk of fracture after cancer radiation therapy is presumably due to a radiation-induced reduction in whole-bone strength. However, the mechanisms for impaired strength remain unclear, as the increased fracture risk is not fully explained by changes in bone mass. To provide insight, a small animal model was used to determine how much of this whole-bone weakening effect for the spine is attributable to changes in bone mass, structure, and material properties of the bone tissue and their relative effects. Further, because women have a greater risk of fracture after radiation therapy than men, we investigated if sex had a significant influence on bone's response to irradiation. Fractionated in vivo irradiation (10 × 3 Gy) or sham irradiation (0 Gy) was administered daily to the lumbar spine in twenty-seven 17-week-old Sprague-Dawley rats (n = 6-7/sex/group). Twelve weeks after final treatment, animals were euthanized, and lumbar vertebrae (L4 and L5 ) were isolated. Using a combination of biomechanical testing, micro-CT-based finite element analysis, and statistical regression analysis, we separated out the effect of mass, structural, and tissue material changes on vertebral strength. Compared with the sham group (mean ± SD strength = 420 ± 88 N), the mean strength of the irradiated group was lower by 28% (117 N/420 N, p < 0.0001). Overall, the response of treatment did not differ with sex. By combining results from both general linear regression and finite element analyses, we calculated that mean changes in bone mass, structure, and material properties of the bone tissue accounted for 56% (66 N/117 N), 20% (23 N/117 N), and 24% (28 N/117 N), respectively, of the overall change in strength. As such, these results provide insight into why an elevated clinical fracture risk for patients undergoing radiation therapy is not well explained by changes in bone mass alone. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

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

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

A Cross-Sectional Study of Bone Nanomechanics in Hip Fracture and Aging

Bone mechanics is well understood at every length scale except the nano-level. We aimed to investigate the relationship between bone nanoscale and tissue-level mechanics experimentally. We tested two hypotheses: (1) nanoscale strains were lower in hip fracture patients versus controls, and (2) nanoscale mineral and fibril strains were inversely correlated with aging and fracture. A cross-sectional sample of trabecular bone sections was prepared from the proximal femora of two human donor groups (aged 44-94 years): an aging non-fracture control group (n = 17) and a hip-fracture group (n = 20). Tissue, fibril, and mineral strain were measured simultaneously using synchrotron X-ray diffraction during tensile load to failure, then compared between groups using unpaired t-tests and correlated with age using Pearson's correlation. Controls exhibited significantly greater peak tissue, mineral, and fibril strains than the hip fracture (all p < 0.05). Age was associated with a decrease in peak tissue (p = 0.099) and mineral (p = 0.004) strain, but not fibril strain (p = 0.260). Overall, hip fracture and aging were associated with changes in the nanoscale strain that are reflected at the tissue level. Data must be interpreted within the limitations of the observational cross-sectional study design, so we propose two new hypotheses on the importance of nanomechanics. (1) Hip fracture risk is increased by low tissue strain, which can be caused by low collagen or mineral strain. (2) Age-related loss of tissue strain is dependent on the loss of mineral but not fibril strain. Novel insights into bone nano- and tissue-level mechanics could provide a platform for the development of bone health diagnostics and interventions based on failure mechanisms from the nanoscale up.

In situ synchrotron radiation μCT indentation of cortical bone: Anisotropic crack propagation, local deformation, and fracture

The development of treatment strategies for skeletal diseases relies on the understanding of bone mechanical properties in relation to its structure at different length scales. At the microscale, indention techniques can be used to evaluate the elastic, plastic, and fracture behaviour of bone tissue. Here, we combined in situ high-resolution SRμCT indentation testing and digital volume correlation to elucidate the anisotropic crack propagation, deformation, and fracture of ovine cortical bone under Berkovich and spherical tips. Independently of the indenter type we observed significant dependence of the crack development due to the anisotropy ahead of the tip, with lower strains and smaller crack systems developing in samples indented in the transverse material direction, where the fibrillar bone ultrastructure is largely aligned perpendicular to the indentation direction. Such alignment allows to accommodate the strain energy, inhibiting crack propagation. Higher tensile hoop strains generally correlated with regions that display significant cracking radial to the indenter, indicating a predominant Mode I fracture. This was confirmed by the three-dimensional analysis of crack opening displacements and stress intensity factors along the crack front obtained for the first time from full displacement fields in bone tissue. The X-ray beam significantly influenced the relaxation behaviour independent of the tip. Raman analyses did not show significant changes in specimen composition after irradiation compared to non-irradiated tissue, suggesting an embrittlement process that may be linked to damage of the non-fibrillar organic matrix. This study highlights the importance of three-dimensional investigation of bone deformation and fracture behaviour to explore the mechanisms of bone failure in relation to structural changes due to aging or disease. STATEMENT OF SIGNIFICANCE: : Characterising the three-dimensional deformation and fracture behaviour of bone remains essential to decipher the interplay between structure, function, and composition with the aim to improve fracture prevention strategies. The experimental methodology presented here, combining high-resolution imaging, indentation testing and digital volume correlation, allows us to quantify the local deformation, crack propagation, and fracture modes of cortical bone tissue. Our results highlight the anisotropic behaviour of osteonal bone and the complex crack propagation patterns and fracture modes initiating by the intricate stress states beneath the indenter tip. This is of wide interest not only for the understanding of bone fracture but also to understand other architectured (bio)structures providing an effective way to quantify their toughening mechanisms in relation to their main mechanical function.

The elasto-plastic nano- and microscale compressive behaviour of rehydrated mineralised collagen fibres

The hierarchical design of bio-based nanostructured materials such as bone enables them to combine unique structure-mechanical properties. As one of its main components, water plays an important role in bone's material multiscale mechanical interplay. However, its influence has not been quantified at the length-scale of a mineralised collagen fibre. Here, we couple in situ micropillar compression, and simultaneous synchrotron small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) with a statistical constitutive model. Since the synchrotron data contain statistical information on the nanostructure, we establish a direct connection between experiment and model to identify the rehydrated elasto-plastic micro- and nanomechanical fibre behaviour. Rehydration led to a decrease of 65%-75% in fibre yield stress and compressive strength, and 70% in stiffness with a 3x higher effect on stresses than strains. While in agreement with bone extracellular matrix, the decrease is 1.5-3x higher compared to micro-indentation and macro-compression. Hydration influences mineral more than fibril strain with the highest difference to the macroscale when comparing mineral and tissue levels. The effect of hydration seems to be strongly mediated by ultrastructural interfaces while results provide insights towards mechanical consequences of reported water-mediated structuring of bone apatite. The missing reinforcing capacity of surrounding tissue for an excised fibril array is more pronounced in wet than dry conditions, mainly related to fibril swelling. Differences leading to higher compressive strength between mineralised tissues seem not to depend on rehydration while the lack of kink bands supports the role of water as an elastic embedding influencing energy-absorption mechanisms. STATEMENT OF SIGNIFICANCE: Characterising structure-property-function relationships in hierarchical biological materials helps us to elucidate mechanisms that enable their unique properties. Experimental and computational methods can advance our understanding of their complex behaviour with the potential to inform bio-inspired material development. In this study, we close a gap for bone's fundamental mechanical building block at micro- and nanometre length scales. We establish a direct connection between experiments and simulations by coupling in situ synchrotron tests with a statistical model and quantify the behaviour of rehydrated single mineralised collagen fibres. Results suggest a high influence of hydration on structural interfaces, and the role of water as an elastic embedding by outlining important differences between wet and dry elasto-plastic properties of mineral nanocrystals, fibrils and fibres.

Fracture in porous bone analysed with a numerical phase-field dynamical model

A dynamic phase-field fracture finite element model is applied to discretized high-resolution three-dimensional computed tomography images of human trabecular bone to analyse rapid bone fracture. The model is contrasted to quasi-static experimental results and a quasi-static phase-field finite element model. The experiment revealed complex stepwise crack evolution with multiple crack fronts, and crack arrests, as the global tensile displacement load was incrementally increased. The quasi-static phase-field fracture model captures the fractures in the experiment reasonably well, and the dynamic model converges towards the quasi-static model when mechanically loaded at low rates. At higher load rates, i.e., at larger impulses, inertia effects significantly contribute to an increased initial global stiffness, higher peak forces and a larger number of cracks spread over a larger volume. Since the fracture process clearly is different at large impulses compared to small impulses, it is concluded that dynamic fracture models are necessary when simulating rapid bone fracture.

Comparison of linear and nonlinear stepwise μFE displacement predictions to digital volume correlation measurements of trabecular bone biopsies

Digital volume correlation (DVC) enables to evaluate the ability of μFE models in predicting experimental results on the mesoscale. In this study predicted displacement fields of three different linear and materially nonlinear μFE simulation methods were compared to DVC measured displacement fields at specific load steps in the elastic regime (Step_El) and after yield (Step_Ult). Five human trabecular bone biopsies from a previous study were compressed in several displacement steps until failure. At every compression step, μCT images (resolution: 36 μm) were recorded. A global DVC algorithm was applied to compute the displacement fields at all loading steps. The unloaded 3D images were then used to generate homogeneous, isotropic, linear and materially nonlinear μFE models. Three different μFE simulation methods were used: linear (L), nonlinear (NL), and nonlinear stepwise (NLS). Regarding L and NL, the boundary conditions were derived from the interpolated displacement fields at Step_El and Step_Ult, while for the NLS method nonlinear changes of the boundary conditions of the experiments were captured using the DVC displacement field of every available load step until Step_El and Step_Ult. The predicted displacement fields of all μFE simulation methods were in good agreement with the DVC measured displacement fields (individual specimens: R²>0.83 at Step_El and R²>0.59 at Step_Ult; pooled data: R²>0.97 at Step_El and R²>0.92 at Step_Ult). At Step_El, all three simulation methods showed similar intercepts, slopes, and coefficients of determination while the nonlinear μFE models improved the prediction of the displacement fields slightly in all Cartesian directions at Step_Ult (individual specimens: L: R²>0.59 and NL, NLS: R²>0.68; pooled data: L: R²>0.92 and NL, NLS: R²>0.94). Damaged/overstrained elements in L, NL, and NLS occurred at similar locations but the number of overstrained elements was overestimated when using the L simulation method. Considering the increased solving time of the nonlinear μFE models as well as the acceptable performance in displacement prediction of the linear μFE models, one can conclude that for similar use cases linear μFE models represent the best compromise between computational effort and accuracy of the displacement field predictions.

Effect of radiation-induced damage of trabecular bone tissue evaluated using indentation and digital volume correlation

Exposure to X-ray radiation for an extended amount of time can cause damage to the bone tissue and therefore affect its mechanical properties. Specifically, high-resolution X-ray Computed Tomography (XCT), in both synchrotron and lab-based systems, has been employed extensively for evaluating bone micro-to-nano architecture. However, to date, it is still unclear how long exposures to X-ray radiation affect the mechanical properties of trabecular bone, particularly in relation to lab-XCT systems. Indentation has been widely used to identify local mechanical properties such as hardness and elastic modulus of bone and other biological tissues. The purpose of this study is therefore, to use indentation and XCT-based investigative tools such as digital volume correlation (DVC) to assess the microdamage induced by long exposure of trabecular bone tissue to X-ray radiation and how this affects its local mechanical properties. Trabecular bone specimens were indented before and after X-ray exposures of 33 and 66 h, where variation of elastic modulus was evaluated at every stage. The resulting elastic modulus was decreased, and micro-cracks appeared in the specimens after the first long X-ray exposure and crack formation increased after the second exposure. High strain concentration around the damaged tissue exceeding 1% was also observed from DVC analysis. The outcomes of this study show the importance of designing appropriate XCT-based experiments in lab systems to avoid degradation of the bone tissue mechanical properties due to radiation and these results will help to inform future studies that require long X-ray exposure for in situ experiments or generation of reliable subject-specific computational models.

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

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

Primary radiation damage in bone evolves via collagen destruction by photoelectrons and secondary emission self-absorption

X-rays are invaluable for imaging and sterilization of bones, yet the resulting ionization and primary radiation damage mechanisms are poorly understood. Here we monitor in-situ collagen backbone degradation in dry bones using second-harmonic-generation and X-ray diffraction. Collagen breaks down by cascades of photon-electron excitations, enhanced by the presence of mineral nanoparticles. We observe protein disintegration with increasing exposure, detected as residual strain relaxation in pre-stressed apatite nanocrystals. Damage rapidly grows from the onset of irradiation, suggesting that there is no minimal 'safe' dose that bone collagen can sustain. Ionization of calcium and phosphorous in the nanocrystals yields fluorescence and high energy electrons giving rise to structural damage that spreads beyond regions directly illuminated by the incident radiation. Our findings highlight photoelectrons as major agents of damage to bone collagen with implications to all situations where bones are irradiated by hard X-rays and in particular for small-beam mineralized collagen fiber investigations.

Application of phase-field fracture theories and digital volume correlation to synchrotron X-ray monitored fractures in human trabecular bone: A case study

Fracture processes of trabecular bone have been studied using various approaches over the years. However, reliable methods to analyse fracture at the single trabecula level are limited. In this study, a digital volume correlation (DVC) and a phase-field fracture model are applied and contrasted for human trabecular bone to analyse its failure under global compression at high resolution. A human trabecular bone sample was fractured in situ under synchrotron-based X-ray micro computed tomography (CT). Reconstructed CT data was then used in DVC algorithms to obtain high-resolution displacement fields in the bone at different load steps. A high-resolution specimen-specific structural mesh was discretized from the CT data and used for the phase-field simulation of the fracturing bone. The DVC analysis showed opening mode cracks as well as shear mode cracks. Strains in cracked regions were analysed. The load distribution in the trabecular structure resulted in two completely separated fracture regions in the sample body. A phenomenon that was also captured in the phase-field model. The results encourage us to believe improvements in boundary conditions and material models are worthwhile pursuing. Findings in this study support further development of a phase-field method to analyse fracture in samples with complex morphology, such as trabecular bone, and the capacity of DVC to quantify strains and slowly growing stable fractures during step-wise loading of trabecular bone.

A practical guide for in situ mechanical testing of musculoskeletal tissues using synchrotron tomography

Musculoskeletal tissues are complex hierarchical materials where mechanical response is linked to structural and material properties at different dimensional levels. Therefore, high-resolution three-dimensional tomography is very useful for assessing tissue properties at different scales. In particular, Synchrotron Radiation micro-Computed Tomography (SR-microCT) has been used in several applications to analyze the structure of bone and biomaterials. In the past decade the development of digital volume correlation (DVC) algorithms applied to SR-microCT images and its combination with in situ mechanical testing (four-dimensional imaging) have allowed researchers to visualise, for the first time, the deformation of musculoskeletal tissues and their interaction with biomaterials under different loading scenarios. However, there are several experimental challenges that make these measurements difficult and at high risk of failure. Challenges relate to sample preparation, imaging parameters, loading setup, accumulated tissue damage for multiple tomographic acquisitions, reconstruction methods and data processing. Considering that access to SR-microCT facilities is usually associated with bidding processes and long waiting times, the failure of these experiments could notably slow down the advancement of this research area and reduce its impact. Many of the experimental failures can be avoided with increased experience in performing the tests and better guidelines for preparation and execution of these complex experiments; publication of negative results could help interested researchers to avoid recurring mistakes. Therefore, the goal of this article is to highlight the potential and pitfalls in the design and execution of in situ SR-microCT experiments, involving multiple scans, of musculoskeletal tissues for the assessment of their structural and/or mechanical properties. The advice and guidelines that follow should improve the success rate of this type of experiment, allowing the community to reach higher impact more efficiently.

Mechanical response and in-situ deformation mechanism of cortical bone materials under combined compression and torsion loads

Bone fracture is an extremely dangerous health risk to human. Actually, cortical bone is often subjected to the complicated loading patterns. The mechanical properties and deformation mechanism under the complicated loading pattern could provide a more precise understanding for the bone fracture. For this purpose, the mechanical response and multi-scale deformation mechanism of cortical bone material were investigated by in-situ experimental research using the compression-torsion coupling loads as an example. It was found that the torsion strength and shear modulus all decreased under the compression-torsion coupling loads than single torsion load. This indicated bone would suffer greater risk of fracture under the compression-torsion coupling loads. Based on in-situ observation, it was found that the rapid reduction of the anisotropy of bone material under the compression load was the potential influencing factor. Because of the redistribution of the principal strain and the variations of cracks propagation, the comprehensive fracture pattern containing both transverse and longitudinal fracture was shown under the coupling loads, and finally resulted in the reduction of the torsion properties. This research could provide new references for researches on mechanical properties of cortical bone material under complicated loading patterns.

Degenerations in Global Morphometry of Cancellous Bone in Rheumatoid Arthritis, Osteoarthritis and Osteoporosis of Femoral Heads are Similar but More Severe than in Ageing Controls

We have recently revealed significant differences in microarchitectural properties (i.e. global and local morphometries) and mechanical properties between rheumatoid arthritis (RA), osteoarthritis (OA) and osteoporosis (OP) in cancellous bones. This study compared these properties with those of ageing controls by matching bone volume fraction (BV/TV), the most important determinant for bones' mechanical properties, to investigate whether these bones have similar properties and degenerative potentials. RA, OA and OP femoral heads were harvested from patients undergoing total hip replacement surgery. The selected patients were matched by similar cancellous bone BV/TV, with seven patients in each group. Four samples were prepared from each femoral head and scanned with micro-CT to quantify microarchitectural properties and compression tested to determine mechanical properties. In terms of global morphometry, no significant differences were observed between these diseased bones. In terms of local morphometry, the number of plates in the RA group was significantly greater than that of the OP and control groups. Plate volume density in the RA group was significantly greater than in the control group. Interestingly, the ultimate stresses in the three diseased groups were 77% to 195% lower than in the control group (p < 0.001). Degenerations of global morphometry of cancellous bones in these diseased femoral heads are similar but more severe than in ageing controls matched by BV/TV, as evidenced by pronounced reduction in bone strength. This phenomenon suggests that some local morphometric parameters, along with other factors, such as abnormal collagen, mineralisation, erosion and microdamage, may contribute to further compromising mechanical properties.

Digital volume correlation for the characterization of musculoskeletal tissues: Current challenges and future developments

Biological tissues are complex hierarchical materials, difficult to characterise due to the challenges associated to the separation of scale and heterogeneity of the mechanical properties at different dimensional levels. The Digital Volume Correlation approach is the only image-based experimental approach that can accurately measure internal strain field within biological tissues under complex loading scenarios. In this minireview examples of DVC applications to study the deformation of musculoskeletal tissues at different dimensional scales are reported, highlighting the potential and challenges of this relatively new technique. The manuscript aims at reporting the wide breath of DVC applications in the past 2 decades and discuss future perspective for this unique technique, including fast analysis, applications on soft tissues, high precision approaches, and clinical applications.

Influence of X-rays and gamma-rays on the mechanical performance of human bone factoring out intraindividual bone structure and composition indices

Doses of irradiation above 25 ​kGy are known to cause irreversible mechanical decay in bone tissue. However, the impact of irradiation doses absorbed in a clinical setting on the mechanical properties of bone remains unclear. In daily clinical practice and research, patients and specimens are exposed to irradiation due to diagnostic imaging tools, with doses ranging from milligray to Gray. The aim of this study was to investigate the influence of irradiation at these doses ranges on the mechanical performance of bone independent of inter-individual bone quality indices.

Therefore, cortical bone specimens (n ​= ​10 per group) from a selected organ donor were irradiated at doses of milligray, Gray and kilogray (graft tissue sterilization) at five different irradiation doses. Three-point bending was performed to assess mechanical properties in the study groups.

Our results show a severe reduction in mechanical performance (work to fracture: 50.29 ​± ​11.49 Nmm in control, 14.73 ​± ​1.84 Nmm at 31.2 ​kGy p ​≤ ​0.05) at high irradiation doses of 31.2 ​kGy, which correspond to graft tissue sterilization or synchrotron imaging. In contrast, no reduction in mechanical properties were detected for doses below 30 ​Gy. These findings are further supported by fracture surface texture imaging (i.e. more brittle fracture textures above 31.2 ​kGy).

Our findings show that high radiation doses (≥31.2 ​kGy) severely alter the mechanical properties of bone. Thus, irradiation of this order of magnitude should be taken into account when mechanical analyses are planned after irradiation. However, doses of 30 ​Gy and below, which are common for clinical and experimental imaging (e.g., radiation therapy, DVT imaging, CT imaging, HR-pQCT imaging, DXA measurements, etc.), do not alter the mechanical bending-behavior of bone.

Opportunities for biomineralization research using multiscale computed X-ray tomography as exemplified by bone imaging

Biominerals typically have complex hierarchical structures traversing many length scales. This makes their structural characterization complicated, since it requires 3D techniques that can probe full specimens at down to nanometer-resolution, a combination that is difficult - if not impossible - to achieve simultaneously. One challenging example is bone, a mineralized tissue with a highly complex architecture that is replete with a network of cells. X-ray computed tomography techniques enable multiscale structural characterization through the combination of various equipment and emerge as promising tools for characterizing biominerals. Using bone as an example, we discuss how combining different X-ray imaging instruments allow characterizing bone structures from the nano- to the organ-scale. In particular, we compare and contrast human and rodent bone, emphasize the importance of the osteocyte lacuno-canalicular network in bone, and finally illustrate how combining synchrotron X-ray imaging with laboratory instrumentation for computed tomography is especially helpful for multiscale characterization of biominerals.

An Investigation of the Effect of Irradiation on the Biomechanical Properties of Fibular Grafts

PURPOSE

Irradiation of fibular grafts for jaw reconstruction following mandibulectomy can be associated with increased graft failure and implant instability. The objective of this study is to investigate the effect of isolated irradiation on the biomechanical properties of ex-vivo porcine fibula grafts. It was hypothesized that the isolated irradiation (without biological response) will not significantly influence the biomechanical properties of the fibular grafts.

METHODS

Forty porcine fibular grafts (n = 20 irradiated and n = 20 nonirradiated) were obtained. The irradiated group was subjected to a single dose of 60 Gy of irradiation with cesium 137. Both groups underwent 3-point bending tests with cyclic loading for 1500 cycles (50 to 500 N at 2 Hz) followed by push to failure. Rate of damage (mm/s), stiffness (N/mm), modulus of elasticity (MPa), maximum load (N), maximum displacement (mm), and maximum stress (MPa) were compared between the 2 groups using independent t tests (P < .05).

RESULTS

No significant differences were found between the irradiated and nonirradiated groups for any of the biomechanical parameters (all P values > 0.05).

CONCLUSIONS

The results of this study support our hypothesis that even high levels of isolated irradiation do not significantly affect the biomechanical properties of fibular bone grafts. This work allows us to exclude acute structural changes in the bone due to irradiation as a possible factor leading to bone/implant instability following jaw reconstruction with fibular grafts. Further studies need to be conducted to better understand the range of factors that may lead to implant instability, including the biological response after radiation therapy.

A combined experimental and numerical method to estimate the elastic modulus of single trabeculae

The elastic modulus at the single trabecular level is an important parameter for the understanding of the mechanical behavior of trabecular bone. Current methods are commonly limited by the irregular trabecular shape and the accuracy of displacement measurement. The aim of this study was to propose a method to estimate the trabecular modulus overcoming some of these limitations. For high-precision displacement measurements, in-situ compression within a synchrotron radiation based X-ray tomograph was used. Trabecular displacements were subsequently estimated by a global digital volume correlation algorithm, followed by high-resolution finite element analyses to account for the irregular geometry. The trabecular elastic moduli were then estimated by comparing the loads from the finite element analyses with those of the experiments. With this strategy, the average elastic modulus was estimated to 3.83 ± 0.54 GPa for three human trabeculae samples. Though limited by the sample size, the demonstrated method shows a potential to estimate the mechanical properties at the single trabecular level.

Time-resolved in situ synchrotron-microCT: 4D deformation of bone and bone analogues using digital volume correlation

Digital volume correlation (DVC) in combination with high-resolution micro-computed tomography (microCT) imaging and in situ mechanical testing is gaining popularity for quantifying 3D full-field strains in bone and biomaterials. However, traditional in situ time-lapsed (i.e., interrupted) mechanical testing cannot fully capture the dynamic strain mechanisms in viscoelastic biological materials. The aim of this study was to investigate the time-resolved deformation of bone structures and analogues via continuous in situ synchrotron-radiation microCT (SR-microCT) compression and DVC to gain a better insight into their structure-function relationships. Fast SR-microCT imaging enabled the deformation behaviour to be captured with high temporal and spatial resolution. Time-resolved DVC highlighted the relationship between local strains and damage initiation and progression in the different biostructures undergoing plastic deformation, bending and/or buckling of their main microstructural elements. The results showed that SR-microCT continuous mechanical testing complemented and enhanced the information obtained from time-lapsed testing, which may underestimate the 3D strain magnitudes as a result of the stress relaxation occurring in between steps before image acquisition in porous biomaterials. Altogether, the findings of this study highlight the importance of time-resolved in situ experiments to fully characterise the time-dependent mechanical behaviour of biological tissues and biomaterials and to further explore their micromechanics under physiologically relevant conditions. STATEMENT OF SIGNIFICANCE: Time-resolved synchrotron X-ray tomography in combination with in situ mechanical testing provided the first four-dimensional analysis of the mechanical deformation of bone and bone analogues. To unravel the interplay of damage initiation and progression with local deformation, digital volume correlation was used to map the local strain field while microstructural changes were tracked with high temporal and spatial resolution. The results highlighted the importance of fast imaging and time-resolved in situ experiments to capture the real deformation of complex porous materials to fully characterize the local strain-damage relationship. The findings are notably improving the understanding of time-dependent mechanical behaviour of bone tissue, with the potential to be extend to highly viscoelastic biomaterials and soft tissues.

Dual-path deep learning reconstruction framework for propagation-based X-ray phase-contrast computed tomography with sparse-view projections

Propagation-based X-ray phase-contrast computed tomography (PB-PCCT) can serve as an effective tool for studying organ function and pathologies. However, it usually suffers from a high radiation dose due to the long scan time. To alleviate this problem, we propose a deep learning reconstruction framework for PB-PCCT with sparse-view projections. The framework consists of dual-path deep neural networks, where the edge detection, edge guidance, and artifact removal models are incorporated into two subnetworks. It is worth noting that the framework has the ability to achieve excellent performance by exploiting the data-based knowledge of the sample material characteristics and the model-based knowledge of PB-PCCT. To evaluate the effectiveness and capability of the proposed framework, simulations and real experiments were performed. The results demonstrated that the proposed framework could significantly suppress streaking artifacts and produce high-contrast and high-resolution computed tomography images.

Photoacoustic computed tomography for functional human brain imaging [Invited]

The successes of magnetic resonance imaging and modern optical imaging of human brain function have stimulated the development of complementary modalities that offer molecular specificity, fine spatiotemporal resolution, and sufficient penetration simultaneously. By virtue of its rich optical contrast, acoustic resolution, and imaging depth far beyond the optical transport mean free path (∼1 mm in biological tissues), photoacoustic computed tomography (PACT) offers a promising complementary modality. In this article, PACT for functional human brain imaging is reviewed in its hardware, reconstruction algorithms, in vivo demonstration, and potential roadmap.

Influence of non-enzymatic glycation on the mechanical properties of cortical bone

Poor bone quality induced by non-enzymatic glycation (NEG) of bone tissue in patients with type 2 diabetes mellitus (T2DM) is regarded as the major factor of bone fragility and affecting bone mechanical properties. A comprehensive and systemic mechanical investigation for evaluating the effect of NEG on bone was still lacking. In order to provide additional information for the bone quality of T2DM, the effects of NEG on mechanical properties of cortical bone were investigated in terms of elastoplasticity, fracture toughness and viscoelasticity. All samples of cortical bone, including the samples of strength test (n = 20), fracture toughness test (n = 40, quasi-static and fall-like conditions with displacement rates of 10^-3 mm/s and 10 mm/s, respectively) and stress relaxation test (n = 20), were harvested from bovine tibiae. The samples of each test were equally divided into incubated-control group and ribose-incubated group. All mechanical tests were performed after incubating all samples for 15 days. Post-yield strain (p = 0.014), post-yield energy (p < 0.0001) and damage fraction (p = 0.040) of ribose-incubated group were significantly lower than those of incubated-control group, but secant modulus (p = 0.029) of ribose-incubated group was significantly higher than that of incubated-control group. In quasi-static condition, the plastic contribution J_pl of fracture toughness (p = 0.043) of ribose-incubated group was significantly lower than that of incubated-control group. In fall-like condition, there were no differences in J_pl, elastic contribution J_el and J-integral in both two groups. The quasi-static J_el (p < 0.0001, p < 0.0001) of incubated-control and ribose-incubated groups and J-integral (p = 0.007) of incubated-control group were all significantly higher than those of fall-like condition. In stress relaxation test, initial modulus E₀ (p = 0.040) and equilibrium modulus (p = 0.029) of ribose-incubated group were significantly higher than those of incubated-control group. Reductions of relaxation modulus, which were the differences between two adjacent time points within 700 s-3000 s for ribose-incubated group, were significantly lower than those of incubated-control group. NEG could decrease the post-yield properties and quasi-static facture toughness of cortical bone, especially the plastic contribution of quasi-static fracture toughness. It could also decrease the viscoelasticity of cortical bone. The present study confirmed the negative effects of NEG on the mechanical properties of cortical bone in terms of elastoplasticity, fracture toughness and viscoelasticity, but NEG had no significant effect on the fracture toughness of cortical bone at fall-like loading. These results provided more evidence for increased fragility of cortical bone in patients with T2DM.

Tomographic X-ray scattering based on invariant reconstruction: analysis of the 3D nanostructure of bovine bone

Small-angle X-ray scattering (SAXS) is an effective characterization technique for multi-phase nanocomposites. The structural complexity and heterogeneity of biological materials require the development of new techniques for the 3D characterization of their hierarchical structures. Emerging SAXS tomographic methods allow reconstruction of the 3D scattering pattern in each voxel but are costly in terms of synchrotron measurement time and computer time. To address this problem, an approach has been developed based on the reconstruction of SAXS invariants to allow for fast 3D characterization of nanostructured inhomogeneous materials. SAXS invariants are scalars replacing the 3D scattering patterns in each voxel, thus simplifying the 6D reconstruction problem to several 3D ones. Standard procedures for tomographic reconstruction can be directly adapted for this problem. The procedure is demonstrated by determining the distribution of the nanometric bone mineral particle thickness (T parameter) throughout a macroscopic 3D volume of bovine cortical bone. The T parameter maps display spatial patterns of particle thickness in fibrolamellar bone units. Spatial correlation between the mineral nano-structure and microscopic features reveals that the mineral particles are particularly thin in the vicinity of vascular channels.

Altered mechanical behavior of demineralized bone following therapeutic radiation

Post-radiotherapy (RTx) bone fragility fractures are a late-onset complication occurring in bone within or underlying the radiation field. These fractures are difficult to predict, as patients do not present with local osteopenia. Using a murine hindlimb RTx model, we previously documented decreased mineralized bone strength and fracture toughness, but alterations in material properties of the organic bone matrix are largely unknown. In this study, 4 days of fractionated hindlimb irradiation (4 × 5 Gy) or Sham irradiation was administered in a mouse model (BALB/cJ, end points: 0, 4, 8, and 12 weeks, n = 15/group/end point). Following demineralization, the viscoelastic stress relaxation, and monotonic tensile mechanical properties of tibiae were determined. Irradiated tibiae demonstrated an immediate (day after last radiation fraction) and sustained (4, 8, 12 weeks) increase in stress relaxation compared to the Sham group, with a 4.4% decrease in equilibrium stress (p < .017). While tensile strength was not different between groups, irradiated tibiae had a lower elastic modulus (-5%, p = .027) and energy to failure (-12.2%, p = .012) with monotonic loading. Gel electrophoresis showed that therapeutic irradiation (4 × 5 Gy) does not result in collagen fragmentation, while irradiation at a common sterilization dose (25 kGy) extensively fragmented collagen. These results suggest that altered collagen mechanical behavior has a role in postirradiation bone fragility, but this can occur without detectable collagen fragmentation. Statement of Clinical Significance: Therapeutic irradiation alters bone organic matrix mechanics and which contribute to diminished fatigue strength, but this does not occur via collagen fragmentation.

Shear behavior of human skull bones

A shear-punch test (SPT) experimental method was developed to address the lack of shear deformation and failure response data for the human skull as a function of local bone microarchitecture. Improved understanding of skull deformation and fracture under varying stress-states helps implement mechanism-based, multi-axial material models for finite element analysis for optimizing protection strategies. Shear-punch coupons (N = 47 specimens) were extracted from right-parietal and frontal bones of three fresh-frozen-thawed human skulls. The specimens were kept as full through-thickness or segmented into the three skull constituent layers: the inner and outer cortical tables and the middle porous diploë. Micro-computed x-ray tomography (μCT) before and after SPT provided the bone volume fraction (BVF) as a function of depth for correlation to shear mechanisms in the punched volumes. Digital image correlation was used to track displacement of the punch above the upper die to minimize compliance error. Five full-thickness specimens were subjected to partial indentation loading to investigate the process of damage development as a function of BVF and depth. It was determined that BVF dominates the shear yield and ultimate strength of human skull bone, but the imposed uniaxial loading rate (0.001 and 0.1 s^-1) did not have as strong a contribution (p = 0.181-0.806 > 0.05) for the shear yield and ultimate strength of the skull bone layer specimens. Shear yield and ultimate strength data were highly correlated to power law relationships of BVF (R² = 0.917-0.949). Full-thickness and partial loaded SPT experiments indicate the diploë primarily dictates the shear strength of the intact structure.

Epoxy based sandwich composite using three-dimensional integrally woven fabric as core strengthened with additional carbon face-sheets

Sandwich composites are three-dimensional (3D) composite structures that offer higher stiffness with overall low density. However, they suffer from low strength; thus, not suited for load bearing applications. In this work, an attempt is made to develop a high strength lightweight sandwich composite suited for load-bearing applications. A sandwich composite based on 3D integrally woven fabric with thickness 3 mm as the core and strengthened with additional 2x2 twill woven carbon fabric face-sheets is reported. The samples were manufactured by wet hand lay co-lamination process using Araldite® LY 1564 epoxy as the matrix polymer and with fiber fraction of 50% by weight. The number of additional carbon face-sheets over the core was varied from two to eight in steps of two. The composite samples were experimented under three-point bending and edgewise compression tests to determine the flexural and compressive strengths in both warp and weft directions. The weft direction samples yielded higher flexural and compressive strengths due to the continuous arrangement of the core pile yarn. The samples with six carbon face-sheets tested along the weft direction offered the highest specific strengths of ~409 kN m/kg and 259 kN m/kg in bending and compression tests. Similarly, the flexural strength was ~340 MPa, and compressive strength was ~217 MPa. A detailed fractography study revealed no core crushing or compression failure of the core during bending tests.

Radiation Disrupts the Protective Function of the Spinal Meninges in a Mouse Model of Tumor-induced Spinal Cord Compression

BACKGROUND

Recent advances in multidisciplinary treatments for various cancers have extended the survival period of patients with spinal metastases. Radiotherapy has been widely used to treat spinal metastases; nevertheless, long-term survivors sometimes undergo more surgical intervention after radiotherapy because of local tumor relapse. Generally, intradural invasion of a spinal tumor seldom occurs because the dura mater serves as a tissue barrier against tumor infiltration. However, after radiation exposure, some spinal tumors invade the dura mater, resulting in leptomeningeal dissemination, intraoperative dural injury, or postoperative local recurrence. The mechanisms of how radiation might affect the dura have not been well-studied.

QUESTIONS/PURPOSES

To investigate how radiation affects the spinal meninges, we asked: (1) What is the effect of irradiation on the meningeal barrier's ability to protect against carcinoma infiltration? (2) What is the effect of irradiation on the meningeal barrier's ability to protect against sarcoma infiltration? (3) What is the effect of irradiation on dural microstructure observed by scanning electron microscopy (SEM)? (4) What is the effect of irradiation on dural microstructure observed by transmission electron microscopy (TEM)?

METHODS

Eighty-four 10-week-old female ddY mice were randomly divided into eight groups: mouse mammary tumor (MMT) implantation 6 weeks after 0-Gy irradiation (nonirradiation) (n = 11), MMT implantation 6 weeks after 20-Gy irradiation (n = 10), MMT implantation 12 weeks after nonirradiation (n = 10), MMT implantation 12 weeks after 20-Gy irradiation (n = 11), mouse osteosarcoma (LM8) implantation 6 weeks after nonirradiation (n = 11), LM8 implantation 6 weeks after 20-Gy irradiation (n = 11), LM8 implantation 12 weeks after nonirradiation (n = 10), and LM8 implantation 12 weeks after 20-Gy irradiation (n = 10); female mice were used for a mammary tumor metastasis model and ddY mice, a closed-colony mice with genetic diversity, were selected to represent interhuman diversity. Mice in each group underwent surgery to generate a tumor-induced spinal cord compression model at either 6 weeks or 12 weeks after irradiation to assess changes in the meningeal barrier's ability to protect against tumor infiltration. During surgery, the mice were implanted with MMT (representative of a carcinoma) or LM8 tumor. When the mice became paraplegic because of spinal cord compression by the growing implanted tumor, they were euthanized and evaluated histologically. Four mice died from anesthesia and 10 mice per group were euthanized (MMT-implanted groups: MMT implantation occurred 6 weeks after nonirradiation [n = 10], 6 weeks after irradiation [n = 10], 12 weeks after nonirradiation [n = 10], and 12 weeks after irradiation [n = 10]; LM8-implanted groups: LM8 implantation performed 6 weeks after nonirradiation [n = 10], 6 weeks after irradiation [n = 10], 12 weeks after nonirradiation [n = 10], and 12 weeks after irradiation [n = 10]); 80 mice were evaluated. The spines of the euthanized mice were harvested; hematoxylin and eosin staining and Masson's trichrome staining slides were prepared for histologic assessment of each specimen. In the histologic assessment, intradural invasion of the implanted tumor was graded in each group by three observers blinded to the type of tumor, presence of irradiation, and the timing of the surgery. Grade 0 was defined as no intradural invasion with intact dura mater, Grade 1 was defined as intradural invasion with linear dural continuity, and Grade 2 was defined as intradural invasion with disruption of the dural continuity. Additionally, we euthanized 12 mice for a microstructural analysis of dura mater changes by two observers blinded to the presence of irradiation. Six mice (three mice in the 12 weeks after nonirradiation group and three mice in the 12 weeks after 20-Gy irradiation group) were quantitatively analyzed for defects on the dural surface with SEM. The other six mice (three mice in the 12 weeks after nonirradiation group and three mice in the 12 weeks after 20-Gy irradiation group) were analyzed for layer structure of collagen fibers constituting dura mater by TEM. In the SEM assessment, the number and size of defects on the dural surface on images (200 μm × 300 μm) at low magnification (× 2680) were evaluated. A total of 12 images (two per mouse) were evaluated for this assessment. The days from surgery to paraplegia were compared between each of the tumor groups using the Kruskal-Wallis test. The scores of intradural tumor invasion grades and the number of defects on dural surface per SEM image were compared between irradiation group and nonirradiation group using the Mann-Whitney U test. Interobserver reliabilities of assessing intradural tumor invasion grades and the number of dural defects on the dural surface were analyzed using Fleiss'κ coefficient. P values < 0.05 were considered statistically significant.

RESULTS

There was no difference in the median (range) time to paraplegia among the MMT implantation 6 weeks after nonirradiation group, the 6 weeks after irradiation group, the 12 weeks after nonirradiation group, and the 12 weeks after irradiation group (16 days [14 to 17] versus 14 days [12 to 18] versus 16 days [14 to 17] versus 14 days [12 to 15]; χ2 = 4.7; p = 0.19). There was also no difference in the intradural invasion score between the MMT implantation 6 weeks after irradiation group and the 6 weeks after nonirradiation group (8 of 10 Grade 0 and 2 of 10 Grade 1 versus 10 of 10 Grade 0; p = 0.17). On the other hand, there was a higher intradural invasion score in the MMT implantation 12 weeks after irradiation group than the 12 weeks after nonirradiation group (5 of 10 Grade 0, 3 of 10 Grade 1 and 2 of 10 Grade 2 versus 10 of 10 Grade 0; p = 0.02). Interobserver reliability of assessing intradural tumor invasion grades in the MMT-implanted group was 0.94. There was no difference in the median (range) time to paraplegia among in the LM8 implantation 6 weeks after nonirradiation group, the 6 weeks after irradiation group, the 12 weeks after nonirradiation group, and the 12 weeks after irradiation group (12 days [9 to 13] versus 10 days [8 to 13] versus 11 days [8 to 13] versus 9 days [6 to 12]; χ2 = 2.4; p = 0.50). There was also no difference in the intradural invasion score between the LM8 implantation 6 weeks after irradiation group and the 6 weeks after nonirradiation group (7 of 10 Grade 0, 1 of 10 Grade 1 and 2 of 10 Grade 2 versus 8 of 10 Grade 0 and 2 of 10 Grade 1; p = 0.51), whereas there was a higher intradural invasion score in the LM8 implantation 12 weeks after irradiation group than the 12 weeks after nonirradiation group (3 of 10 Grade 0, 3 of 10 Grade 1 and 4 of 10 Grade 2 versus 8 of 10 Grade 0 and 2 of 10 Grade 1; p = 0.04). Interobserver reliability of assessing intradural tumor invasion grades in the LM8-implanted group was 0.93. In the microstructural analysis of the dura mater using SEM, irradiated mice had small defects on the dural surface at low magnification and degeneration of collagen fibers at high magnification. The median (range) number of defects on the dural surface per image in the irradiated mice was larger than that of nonirradiated mice (2 [1 to 3] versus 0; difference of medians, 2/image; p = 0.002) and the median size of defects was 60 μm (30 to 80). Interobserver reliability of assessing number of defects on the dural surface was 1.00. TEM revealed that nonirradiated mice demonstrated well-organized, multilayer structures, while irradiated mice demonstrated irregularly layered structures at low magnification. At high magnification, well-ordered cross-sections of collagen fibers were observed in the nonirradiated mice. However, disordered alignment of collagen fibers was observed in irradiated mice.

CONCLUSION

Intradural tumor invasion and disruptions of the dural microstructure were observed in the meninges of mice after irradiation, indicating radiation-induced disruption of the meningeal barrier.

CLINICAL RELEVANCE

We conclude that in this form of delivery, radiation is associated with disruption of the dural meningeal barrier, indicating a need to consider methods to avoid or limit Postradiation tumor relapse and spinal cord compression when treating spinal metastases so that patients do not experience intradural tumor invasion. Surgeons should be aware of the potential for intradural tumor invasion when they perform post-irradiation spinal surgery to minimize the risks for intraoperative dural injury and spinal cord injury. Further research in patients with irradiated spinal metastases is necessary to confirm that the same findings are observed in humans and to seek irradiation methods that prevent or minimize the disruption of meningeal barrier function.

XRD and ATR-FTIR techniques for integrity assessment of gamma radiation sterilized cortical bone pretreated by antioxidants

Terminal sterilization of bone allograft by gamma radiation is required to reduce the risk of infection. Free radical scavengers could be utilized to minimize the deteriorating effects of gamma radiation on bone allograft mechanical properties. The objective of this research is to assess the changes in structural and chemical composition induced by hydroxytyrosol (HT) and alpha lipoic acid (ALA) free radical scavengers in gamma sterilized cortical bone. Bovine femurs specimens were soaked in different concentrations of HT and ALA for 7 and 3 days respectively before irradiation with 35 KGy gamma radiation. The attenuated total reflection-Fourier transform infrared spectroscopy and the X-ray diffraction techniques were utilized to analyze the changes in chemical composition induced by irradiation in the presence of free radical scavengers. A significant increase in the proportion of amide I and amide II to phosphate was noticed in the irradiated group, while in the pretreated groups with ALA and HT this effect was minimized. In addition, gamma radiation reduced the mature to immature cross links while ALA and HT alleviated this reduction. No significant changes were noticed in the mineral crystallinity or crystal size. Bone chemical structure has been changed due to gamma irradiation and these changes are mainly relevant to amide I, amide II proportions and collagen crosslinks. The deteriorating effects of gamma sterilization dose (35 kGy) on chemical structure of bone allograft can be alleviated by using (HT) and (ALA) free radical scavengers before irradiation.

CT based quantitative measures of the stability of fractured metastatically involved vertebrae treated with spine stereotactic body radiotherapy

Mechanical instability secondary to vertebral metastases can lead to pathologic vertebral compression fracture (VCF) mechanical pain, neurological compromise, and the need for surgical stabilization. Stereotactic body radiation therapy (SBRT) as a treatment for spinal metastases is effective for pain and local tumor control, it has been associated with an increased risk of VCF. This study quantified computed tomography (CT) based stability measures in metastatic vertebrae with VCF treated with spine SBRT. It was hypothesized that semi-automated quantification of VCF based on CT metrics would be related to clinical outcomes. 128 SBRT treated spinal metastases patients were identified from a prospective database. Of these, 18 vertebral segments were identified with a VCF post-SBRT. A semi-automated system for quantifying VCF was developed based on CT imaging before and after SBRT. The system identified and segmented SBRT treated vertebral bodies, calculated stability metrics at single time points and changes over time. In the vertebrae that developed a new (n = 7) or progressive (n = 11) VCF following SBRT, the median time to VCF/VCF progression was 1.74 months (range 0.53-7.79 months). Fractured thoracolumbar vertebrae that went on to be stabilized (cemented and/or instrumented), had greater fractured vertebral body volume progression over time (12%) compared to those not stabilized (0.4%, p < 0.05). Neither the spinal instability neoplastic score (SINS) or any single timepoint stability metrics in post-hoc analyses correlated with future stabilization. This pilot study presents a quantitative semi-automated method assessing fractured thoracolumbar vertebrae based on CT. Increased fractured vertebral body volume progression post-SBRT was shown to predict those patients who were subsequently stabilized, motivating study of methods that assess temporal radiological changes toward augmenting existing clinical management in the metastatic spine.

Effects of long-term in vivo micro-CT imaging on hallmarks of osteopenia and frailty in aging mice

In vivo micro-CT has already been used to monitor microstructural changes of bone in mice of different ages and in models of age-related diseases such as osteoporosis. However, as aging is accompanied by frailty and subsequent increased sensitivity to external stimuli such as handling and anesthesia, the extent to which longitudinal imaging can be applied in aging studies remains unclear. Consequently, the potential of monitoring individual mice during the entire aging process-from healthy to frail status-has not yet been exploited. In this study, we assessed the effects of long-term in vivo micro-CT imaging-consisting of 11 imaging sessions over 20 weeks-on hallmarks of aging both on a local (i.e., static and dynamic bone morphometry) and systemic (i.e., frailty index (FI) and body weight) level at various stages of the aging process. Furthermore, using a premature aging model (PolgA(D257A/D257A)), we assessed whether these effects differ between genotypes. The 6th caudal vertebrae of 4 groups of mice (PolgA(D257A/D257A) and PolgA(+/+)) were monitored by in vivo micro-CT every 2 weeks. One group was subjected to 11 scans between weeks 20 and 40 of age, whereas the other groups were subjected to 5 scans between weeks 26-34, 32-40 and 40-46, respectively. The long-term monitoring approach showed small but significant changes in the static bone morphometric parameters compared to the other groups. However, no interaction effect between groups and genotype was found, suggesting that PolgA mutation does not render bone more or less susceptible to long-term micro-CT imaging. The differences between groups observed in the static morphometric parameters were less pronounced in the dynamic morphometric parameters. Moreover, the body weight and FI were not affected by more frequent imaging sessions. Finally, we observed that longitudinal designs including baseline measurements at young adult age are more powerful at detecting effects of in vivo micro-CT imaging on hallmarks of aging than cross-sectional comparisons between multiple groups of aged mice subjected to fewer imaging sessions.

Sub-trabecular strain evolution in human trabecular bone

To comprehend the most detrimental characteristics behind bone fractures, it is key to understand the material and tissue level strain limits and their relation to failure sites. The aim of this study was to investigate the three-dimensional strain distribution and its evolution during loading at the sub-trabecular level in trabecular bone tissue. Human cadaver trabecular bone samples were compressed in situ until failure, while imaging with high-resolution synchrotron radiation X-ray tomography. Digital volume correlation was used to determine the strains inside the trabeculae. Regions without emerging damage were compared to those about to crack. Local strains in close vicinity of developing cracks were higher than previously reported for a whole trabecular structure and similar to those reported for single isolated trabeculae. Early literature on bone fracture strain thresholds at the tissue level seem to underestimate the maximum strain magnitudes in trabecular bone. Furthermore, we found lower strain levels and a reduced ability to capture detailed crack-paths with increased image voxel size. This highlights the dependence between the observed strain levels and the voxel size and that high-resolution is needed to investigate behavior of individual trabeculae. Furthermore, low trabecular thickness appears to be one predictor of developing cracks. In summary, this study investigated the local strains in whole trabecular structure at sub-trabecular resolution in human bone and confirmed the high strain magnitudes reported for single trabeculae under loading and, importantly extends its translation to the whole trabecular structure.

Biomechanical properties enhancement of gamma radiation-sterilized cortical bone using antioxidants

Gamma radiation sterilization is the method used by the majority of tissue banks to reduce disease transmission from infected donors to recipients through bone allografts. However, many studies have reported that gamma radiation impairs the structural and mechanical properties of bone via formation of free radicals, the effect of which could be reduced using free radical scavengers. The aim of this study is to examine the radioprotective role of hydroxytyrosol (HT) and alpha lipoic acid (ALA) on the mechanical properties of gamma-sterilized cortical bone of bovine femur, using three-point bending and microhardness tests. Specimens of bovine femurs were soaked in ALA and HT for 3 and 7 days, respectively, before being exposed to 35-kGy gamma radiation. In unirradiated samples, both HT and ALA pre-treatment improved the cortical bone bending plastic properties (maximum bending stress, maximum bending strain, and toughness) without affecting microhardness. Irradiation resulted in a drastic reduction of the plastic properties and an increased microhardness. ALA treatment before irradiation alleviated the aforementioned reductions in maximum bending stress, maximum bending strain, and toughness. In addition, under ALA treatment, the microhardness was not increased after irradiation. For HT treatment, similar effects were found. In conclusion, the results indicate that HT and ALA can be used before irradiation to enhance the mechanical properties of gamma-sterilized bone allografts.

Investigation of bone matrix composition, architecture and mechanical properties reflect structure-function relationship of cortical bone in glucocorticoid induced osteoporosis

Glucocorticoid induced osteoporosis (GIOP) is the most common negative consequence of long-term glucocorticoid treatment, leading to increased fracture risk followed by loss of mobility and high mortality risk. These biologically induced changes in bone quality at molecular level lead to changes both in bone matrix architecture and bone matrix composition. However, the quantitative details of changes in bone quality - and especially their link to reduced macroscale mechanical properties are still largely missing. In this study, a mouse model for glucocorticoid-induced osteoporosis (GIOP) was used to investigate mechanical and material alterations in bone cortex (natural nanocomposite) at different scale. By combining quantitative backscattered electron (qBSE) imaging, nanoindentation and high brilliance synchrotron X-ray nanomechanical imaging on a genetically modified mouse model of GIOP, we were able to quantify the local indentation modulus, mineralization distribution and the alterations of nanoscale structures and deformation mechanisms in the mid-diaphysis of femur, and relate them to the macroscopic mechanical changes. Our results showed clear and significant changes in terms of material quality of bone at nanoscale and microscale, which manifests itself in development of spatial heterogeneities in mineralization and indentation moduli across the bone organ, with potential implications for increased fracture risk.

A method for fracture toughness measurement in trabecular bone using computed tomography, image correlation and finite element methods

The fracture resistance of load-bearing trabecular bone is adversely affected by diseases such as osteoporosis. However, there are few published measurements of trabecular bone fracture toughness due to the difficulty of conducting reliable tests in small specimens of this highly porous material. A new approach is demonstrated that uses digital volume correlation of X-ray computed tomographs to measure 3D displacement fields in which the crack shape and size can be objectively identified using a phase congruency analysis. The criteria for crack propagation, i.e. fracture toughness, can then be derived by finite element simulation, with knowledge of the elastic properties.

Full-Field Strain Uncertainties and Residuals at the Cartilage-Bone Interface in Unstained Tissues Using Propagation-Based Phase-Contrast XCT and Digital Volume Correlation

A deeper understanding of the cartilage-bone mechanics is fundamental to unravel onset and progression of osteoarthritis, enabling better diagnosis and treatment. The aim of this study is therefore to explore the capability of X-ray computed (XCT) phase-contrast imaging in a lab-based system to enable digital volume correlation (DVC) measurements of unstained cartilage-bone plugs from healthy adult bovines. DVC strain uncertainties were computed for both articular cartilage and mineralized tissue (calcified cartilage and subchondral bone) in the specimens at increasing propagation distances, ranging from absorption up to four times (4× such effective distance. In addition, a process of dehydration and rehydration was proposed to improve feature recognition in XCT of articular cartilage and mechanical properties of this tissue during the process were assessed via micromechanical probing (indentation), which was also used to determine the effect of long X-ray exposure. Finally, full-field strain from DVC was computed to quantify residual strain distribution at the cartilage-bone interface following unconfined compression test (ex situ). It was found that enhanced gray-scale feature recognition at the cartilage-bone interface was achieved using phase-contrast, resulting in reduced DVC strain uncertainties compared to absorption. Residual strains up to ~7000 µε in the articular cartilage were transferred to subchondral bone via the calcified cartilage and micromechanics revealed the predominant effect of long phase-contrast X-ray exposure in reducing both stiffness and hardness of the articular cartilage. The results of this study will pave the way for further development and refinement of the techniques, improving XCT-based strain measurements in cartilage-bone and other soft-hard tissue interfaces.

Investigation of the mechanism of gamma irradiation effect on bovine bone

Radiation sterilization is an effective method of bone sterilization prior to bone graft transplantation. Gamma irradiation affects the biological and mechanical properties of bone; depending on the dose of radiation. The effect of gamma irradiation on bone mechanical properties is an unwanted phenomenon. However the mechanism of the effect of irradiation on bone mechanical properties is not properly understood. In this research paper the mechanism of the effect of gamma irradiation on bovine bone is investigated using scanning electron microscopy, energy-dispersive X-rays spectroscopy and Fourier transform infrared spectroscopy techniques. Gamma irradiation affects the mineral and fiber composition of bovine bone. The mineral content of bone especially calcium, magnesium and phosphorus decrease with increasing dose of gamma radiation. At Nano-level gamma irradiation alter amide I, amide II and amide III collagen contents. High dose gamma irradiation induces collagen cross-linking reaction in bone and degrades bone properties.