Micro-computed tomography of fatigue microdamage in cortical bone using a barium sulfate contrast agent

Treatment affects load to failure and microdamage accumulation in healthy and osteolytic rat vertebrae

Bone turnover and microdamage are impacted by the presence of skeletal metastases which can contribute to increased fracture risk. Treatments for metastatic disease may further impact bone quality. This exploratory study aimed to establish an initial understanding of microdamage accumulation and load to failure in healthy and osteolytic rat vertebrae following focal and systemic cancer treatment (docetaxel (DTX), stereotactic body radiotherapy (SBRT), or zoledronic acid (ZA)). Osteolytic spine metastases were developed in 6-week-old athymic female rats via intracardiac injection of HeLa human cervical cancer cells (day 0). Additional rats served as healthy controls. Rats were either untreated, received SBRT to the T10-L6 vertebrae on day 14 (15 Gy, two fractions), DTX on day 7 or 14, or ZA on day 7. Rats were euthanized on day 21. Tumor burden was assessed with bioluminescence images acquired on day 14 and 21, histology of the excised T11 and L5 vertebrae, and ex-vivo μCT images of the T13-L4. Microstructural parameters (bone volume/total volume, trabecular number, spacing, thickness, and bone mineral density) were measured from L2 vertebrae. Load to failure was measured with axial compressive loading of the L1-L3 motion segments. Microdamage accumulation was labeled in T13 vertebrae with BaSO4 staining and was visualized with high resolution μCT imaging. Microdamage volume fraction was defined as the ratio of BaSO4 to bone volume. DTX administered on day 7 reduced tumor growth significantly (p < 0.05). Microdamage accumulation was found to be increased by the presence of metastases but was reduced by all treatments with ZA showing the largest improvement in HeLa cell injected rats. Load to failure was decreased in untreated and SBRT HeLa cell injected rats compared to healthy controls (p < 0.01). There was a moderate negative correlation between load to failure and microdamage volume fraction in vertebrae from rats injected with HeLa cells (R = -0.35, p = 0.031). Strong correlations were also found between microstructural parameters and load to failure and microdamage accumulation. Several factors, including the presence of osteolytic lesions and use of cancer therapies, influence microdamage accumulation and load to failure in rat vertebrae. Understanding the impact of these treatments on fracture risk of metastatic vertebrae is important to improve management of patients with spinal metastases.

Zoledronate reduces loading-induced microdamage in cortical ulna of ovariectomized rats

As a daily physiological mechanism in bone, microdamage accumulation dissipates energy and helps to prevent fractures. However, excessive damage accumulation might bring adverse effects to bone mechanical properties, which is especially problematic among the osteoporotic and osteopenic patients treated by bisphosphonates. Some pre-clinical studies in the literature applied forelimb loading models to produce well-controlled microdamage in cortical bone. Ovariectomized animals were also extensively studied to assimilate human conditions of estrogen-related bone loss. In the present study, we combined both experimental models to investigate microdamage accumulation in the context of osteopenia and zoledronate treatment. Three-month-old normal and ovariectomized rats treated by saline or zoledronate underwent controlled compressive loading on their right forelimb to create in vivo microdamage, which was then quantified by barium sulfate contrast-enhanced micro-CT imaging. Weekly in vivo micro-CT scans were taken to evaluate bone (re)modeling and to capture microstructural changes over time. After sacrifice, three-point-bending tests were performed to assess bone mechanical properties. Results show that the zoledronate treatment can reduce cortical microdamage accumulation in ovariectomized rats, which might be explained by the enhancement of several bone structural properties such as ultimate force, yield force, cortical bone area and volume. The rats showed increased bone formation volume and surface after the generation of microdamage, especially for the normal and the ovariectomized groups. Woven bone formation was also observed in loaded ulnae, which was most significant in ovariectomized rats. Although all the rats showed strong correlations between periosteal bone formation and microdamage accumulation, the correlation levels were lower for the zoledronate-treated groups, potentially because of their lower levels of microdamage. The present study provides insights to further investigations of pharmaceutical treatments for osteoporosis and osteopenia. The same experimental concept can be applied in future studies on microdamage and drug testing.

Segmentation of trabecular bone microdamage in Xray microCT images using a two-step deep learning method

INTRODUCTION

One of the current approaches to improve our understanding of osteoporosis is to study the development of bone microdamage under mechanical loading. The current practice for evaluating bone microdamage is to quantify damage volume from images of bone samples stained with a contrast agent, often composed of toxic heavy metals and requiring long tissue preparation. This work aims to evaluate the potential of linear microcracks detection and segmentation in trabecular bone samples using well-known deep learning models, namely YOLOv4 and Unet, applied on microCT images.

METHODS

Six trabecular bovine bone cylinders underwent compression until ultimate stress and were subsequently imaged with a microCT at a resolution of 1.95 μm. Two of these samples (samples 1 and 2) were then stained using barium sulfate (BaSO₄) and imaged again. The unstained samples (samples 3-6) were used to train two neural networks YOLOv4 to detect regions with microdamage further combined with Unet to segment the microdamage at the pixel level in the detected regions. Four different model versions of YOLOv4 were compared using the average Intersection over Union (IoU) and the mean average precision (mAP). The performance of Unet was also measured using two segmentation metrics, the Dice Score and the Intersection over Union (IoU). A qualitative comparison was finally done between the deep learning and the contrast agent approaches.

RESULTS

Among the four versions of YOLOv4, the YOLOv4p5 model resulted in the best performance with an average IoU of 45,32% and 51,12% and a mAP of 28.79% and 46.22%, respectively for samples 1 and 2. The segmentation performance of Unet provided better IoU and DICE score on sample 2 compared to sample 1. The poorer performance of the test on sample 1 could be explained by its poorer contrast to noise ratio (CNR). Indeed, sample 1 resulted in a CNR of 7,96, which was worse than the average CNR in the training samples, while sample 2 resulted in a CNR of 10,08. The qualitative comparison between the contrast agent and the deep learning segmentation showed that two different regions were segmented by the two techniques. Deep learning is segmenting the region inside the cracks while the contrast agent segments the region around it or even regions with no visible damage.

CONCLUSION

The combination of YOLOv4 for microdamage detection with Unet for damage segmentation showed a potential for the detection and segmentation of microdamage in trabecular bone. The accuracy of both neural networks achieved in this work is acceptable considering it is their first application in this specific field and the amount of data was limited. Even if the errors from both neural networks are accumulated, the two-steps approach is faster than the semantic segmentation of the whole volume.

Biological Utility of Fluorinated Compounds: from Materials Design to Molecular Imaging, Therapeutics and Environmental Remediation

The applications of fluorinated molecules in bioengineering and nanotechnology are expanding rapidly with the controlled introduction of fluorine being broadly studied due to the unique properties of C-F bonds. This review will focus on the design and utility of C-F containing materials in imaging, therapeutics, and environmental applications with a central theme being the importance of controlling fluorine-fluorine interactions and understanding how such interactions impact biological behavior. Low natural abundance of fluorine is shown to provide sensitivity and background advantages for imaging and detection of a variety of diseases with ¹⁹F magnetic resonance imaging, ¹⁸F positron emission tomography and ultrasound discussed as illustrative examples. The presence of C-F bonds can also be used to tailor membrane permeability and pharmacokinetic properties of drugs and delivery agents for enhanced cell uptake and therapeutics. A key message of this review is that while the promise of C-F containing materials is significant, a subset of highly fluorinated compounds such as per- and polyfluoroalkyl substances (PFAS), have been identified as posing a potential risk to human health. The unique properties of the C-F bond and the significant potential for fluorine-fluorine interactions in PFAS structures necessitate the development of new strategies for facile and efficient environmental removal and remediation. Recent progress in the development of fluorine-containing compounds as molecular imaging and therapeutic agents will be reviewed and their design features contrasted with environmental and health risks for PFAS systems. Finally, present challenges and future directions in the exploitation of the biological aspects of fluorinated systems will be described.

Spontaneous formation of barium sulfate crystals at liquid–liquid interfaces

Interfacial ion transfer from organic phase to aqueous phase is employed as the basis for formation of barium sulfate crystals close to the interface.

Intrinsically radiopaque biomaterial assortments: a short review on the physical principles, X-ray imageability, and state-of-the-art developments

X-ray attenuation ability, otherwise known as radiopacity of a material, could be indisputably tagged as the central and decisive parameter that produces contrast in an X-ray image. Radiopaque biomaterials are vital in the healthcare sector that helps clinicians to track them unambiguously during pre and post interventional radiological procedures. Medical imaging is one of the most powerful resources in the diagnostic sector that aids improved treatment outcomes for patients. Intrinsically radiopaque biomaterials enable themselves for visual targeting/positioning as well as to monitor their fate and further provide the radiologists with critical insights about the surgical site. Moreover, the emergence of advanced real-time imaging modalities is a boon to the contemporary healthcare systems that allow to perform minimally invasive surgical procedures and thereby reduce the healthcare costs and minimize patient trauma. X-ray based imaging is one such technologically upgraded diagnostic tool with many variants like digital X-ray, computed tomography, digital subtraction angiography, and fluoroscopy. In light of these facts, this review is aimed to briefly consolidate the physical principles of X-ray attenuation by a radiopaque material, measurement of radiopacity, classification of radiopaque biomaterials, and their recent advanced applications.

Novel contrast media based on the liquid metal gallium for in vivo digestive tract radiography: a feasibility study

The barium sulfate has been playing an important role as the contrast medium in gastrointestinal radiography and disease diagnosis. However, its application has been gradually reduced due to the limitation of its imaging effect and the progress of other imaging techniques. Here, the liquid metal gallium was proposed as an improved contrast agent, which was applied in the in vivo digestive tract radiography of the mice for the first time. Under the CT scanning, the gallium produced excellent contrast effect intuitively. According to the records of discharge time, the tissue sections of organs, the survival state and body weight, the liquid metal was proven to be capable and safe for gastrointestinal radiography. Further, the mixture of the gallium and the barium sulfate has been tested, which showed better performance in both contrast and detail. Therefore, with the characteristics of better imaging contrast effect and acceptable safety, the gallium and its mixture with the barium sulfate might be useful as potential candidates for digestive tract contrast agent in animal experiments, even possibly as alternative contrast agents for clinical use.

Ex vivo quantification of lanthanum and gadolinium in post-mortem human tibiae with estimated barium and iodine concentrations using K x-ray fluorescence

OBJECTIVES

Lanthanum (La) and gadolinium (Gd) are known to deposit in bone of exposed populations, namely those who are orally administered lanthanum carbonate (LaC, La₂(CO₃)₃) or are injected with Gd-based contrast agents, respectively. In this work, bone La and Gd concentrations from the environment and diet were measured using x-ray fluorescence in ten post-mortem human tibiae. As a secondary objective, bone barium (Ba) and iodine concentrations were estimated.

APPROACH

Two calibration lines were produced for La and Gd and the minimum detection limits (MDLs) of the system were determined using a 180° irradiation-detection geometry.

MAIN RESULTS

The MDLs of the system were 0.4 µg La g^-1 bone mineral and 0.5 µg Gd g^-1 bone mineral. The mean concentrations were  -0.02  ±  0.1 µg La g^-1 bone mineral and 0.1  ±  0.2 µg Gd g^-1 bone mineral in tibiae. The average Ba and iodine concentrations estimated from the experimental La calibration line and Monte-Carlo derived sensitivity factors were determined to be 3.4  ±  0.8 µg Ba g^-1 bone mineral and  -0.5  ±  0.3 µg iodine g^-1 bone mineral. Since it was discovered that four donors previously received an iodine-based contrast agent, the mean concentrations in these donors was 27.8  ± 28.4 µg iodine g^-1 bone mineral.

SIGNIFICANCE

The XRF system has determined baseline concentrations of these four heavy metals in trace quantities from natural exposure pathways (with the exception of iodine in four donors). This indicates that the system can measure low levels in ex vivo tibiae samples and can potentially be further developed for in vivo studies involving live subjects who are directly exposed to these metals.

Elevated Microdamage Spatially Correlates with Stress in Metastatic Vertebrae

Metastasis of cancer to the spine impacts bone quality. This study aims to characterize vertebral microdamage secondary to metastatic disease considering the pattern of damage and its relationship to stress and strain under load. Osteolytic and mixed osteolytic/osteoblastic vertebral metastases were produced in athymic rats via HeLa cervical or canine Ace-1 prostate cancer cell inoculation, respectively. After 21 days, excised motion segments (T12-L2) were µCT scanned, stained with BaSO₄ and re-imaged. T13-L2 motion segments were loaded in axial compression to induce microdamage, re-stained and re-imaged. L1 (loaded) and T12 (unloaded) vertebrae were fixed, sample blocks cut, polished and BSE imaged. µFE models were generated of all L1 vertebrae with displacement boundary conditions applied based on the loaded µCT images. µCT stereological analysis, BSE analysis and µFE derived von Mises stress and principal strains were quantitatively compared (ANOVA), spatial correlations determined and patterns of microdamage assessed qualitatively. BaSO₄ identified microdamage was found to be spatially correlated with regions of high stress in µFEA. Load-induced microdamage was shown to be elevated in the presence of osteolytic and mixed metastatic disease, with diffuse, crossed hatched areas of microdamage present in addition to linear microdamage and microfractures in metastatic tissue, suggesting diminished bone quality.

Nanocellulose templated growth of ultra-small bismuth nanoparticles for enhanced radiation therapy

An unmet need in nanomedicine is to prepare biocompatible and renal clearable nanoparticles by controlling the diameter, composition and surface properties of the nanoparticles. This paper reports cellulose nanofiber templated synthesis of ultra-small bismuth nanoparticles, and their uses in enhanced X-ray radiation therapy. The interstitial spaces between adjacent fibers are the adsorption sites of bismuth ions and also stabilize nanoparticles generated by chemical reduction. The sizes of nanoparticles are tailored in the 2-10 nm range using cellulose nanofibers with various amounts of carboxyl groups. In vitro cytotoxicity, reactive oxygen species (ROS) and in vivo animal tests with tumor-bearing mice are studied in order to enhance X-ray radiation therapy using cellulose nanofiber-templated bismuth nanoparticles. Bismuth nanoparticles show strong X-ray attenuation ability, concentration-dependent cytotoxicity and high level production of ROS upon X-ray exposure, which is consistent with enhanced cellular damage and retarded growth of tumors in animals.

Spatiotemporal characterization of microdamage accumulation in rat ulnae in response to uniaxial compressive fatigue loading

Repetitive fatigue loading can induce microdamage accumulation in bone matrix, which results in impaired mechanical properties and increased fracture susceptibility. However, the spatial distribution and time-variant process of microdamage accumulation in fatigue-loaded skeleton, especially for linear microcracks which are known to initiate bone remodeling, remain not fully understood. In this study, the time-varying process of the morphology and distribution of microcracks in rat ulnae subjected to uniaxial compressive fatigue loading was investigated. Right forelimbs of thirty four-month-old male Sprague-Dawley rats were subjected to one bout of cyclic ramp loading with 0.67 Hz at a normalized peak force of 0.055 N/g body weight for 6000 cycles, and the contralateral left ulnae were not loaded as the control samples. Ten rats were randomly euthanized on Days 3, 5, and 7 post fatigue loading. Our findings via two-dimensional histomorphometric measurements based on basic fuchsin staining and three-dimensional quantifications using contrast-enhanced micro-computed tomography (MicroCT) with precipitated BaSO₄ staining demonstrated that the accumulation of linear microcracks (increase in the amount of linear microcracks) on Day 5 was significantly higher than that on Day 3 and Day 7 post fatigue loading. Our histological and histomorphometric results revealed that linear microcrack density (Cr.Dn) in the tensile cortex at Days 3, 5 and 7 post fatigue loading was significantly higher than that in the compressive side, whereas linear microcrack length (Cr.Le) in the tensile cortex at Day 3 was significantly lower than that in the compressive cortex. Our findings revealed that microcrack accumulation exhibited a non-linear time-varying process at 3, 5 and 7 days post axial compressive fatigue loading (with observable peak Cr.Dn at Day 5). Our findings also revealed distinct distribution of microcrack density and morphology in rat ulnae with tensile and compressive strains, as characterized by more microcracks accumulated in tensile cortices, and longer cracks shown in compressive cortices.

The micro-damage process zone during transverse cortical bone fracture: No ears at crack growth initiation

OBJECTIVE

Apply high-resolution benchtop micro-computed tomography (micro-CT) to gain greater understanding and knowledge of the formation of the micro-damage process zone formed during traverse fracture of cortical bone.

METHODS

Bovine cortical bone was cut into single edge notch (bending) fracture testing specimens with the crack on the transverse plane and oriented to grow in the circumferential direction. We used a multi-specimen technique and deformed the specimens to various individual secant modulus loss levels (P-values) up to and including maximum load (Pmax). Next, the specimens were infiltrated with a BaSO₄ precipitation stain and scanned at 3.57-μm isotropic voxel size using a benchtop high resolution-micro-CT. Measurements of the micro-damage process zone volume, width and height were made. These were compared with the simple Irwin's process zone model and with finite element models. Electron and confocal microscopy confirmed the formation of BaSO₄ precipitate in micro-cracks and other porosity, and an interesting novel mechanism similar to tunneling.

RESULTS

Measurable micro-damage was detected at low P values and the volume of the process zone increased according to a second order polynomial trend. Both width and height grew linearly up to Pmax, at which point the process zone cross-section (perpendicular to the plane of the crack) was almost circular on average with a radius of approximately 550µm (approximately one quarter of the unbroken ligament thickness) and corresponding to the shape expected for a biological composite under plane stress conditions.

CONCLUSION

This study reports details of the micro-damage fracture process zone previously unreported for cortical bone. High-resolution micro-CT enables 3D visualization and measurement of the process zone and confirmation that the crack front edge and process zone are affected by microstructure. It is clear that the process zone for the specimens studied grows to be meaningfully large, confirming the need for the J-integral approach and it does not achieve steady state at Pmax in most specimens. With further development, this approach may become valuable towards better understanding the role of the process zone in cortical bone fracture and the effects of relevant modifications towards changes in fracture toughness in a cost effective way.

A robust methodology for the quantitative assessment of the rat jawbone microstructure

Micro-computed tomography can be applied for the assessment of the micro-architectural characteristics of the cortical and trabecular bones in either physiological or disease conditions. However, reports often lack a detailed description of the methodological steps used to analyse these images, such as the volumes of interest, the algorithms used for image filtration, the approach used for image segmentation, and the bone parameters quantified, thereby making it difficult to compare or reproduce the studies. This study addresses this critical need and aims to provide standardized assessment and consistent parameter reporting related to quantitative jawbone image analysis. Various regions of the rat jawbones were screened for their potential for standardized micro-computed tomography analysis. Furthermore, the volumes of interest that were anticipated to be most susceptible to bone structural changes in response to experimental interventions were defined. In the mandible, two volumes of interest were selected, namely, the condyle and the trabecular bone surrounding the three molars. In the maxilla, the maxillary tuberosity region and the inter-radicular septum of the second molar were considered as volumes of interest. The presented protocol provides a standardized and reproducible methodology for the analysis of relevant jawbone volumes of interest and is intended to ensure global, accurate, and consistent reporting of its morphometry. Furthermore, the proposed methodology has potential, as a variety of rodent animal models would benefit from its implementation.

Synchrotron Imaging Assessment of Bone Quality

Bone is a complex hierarchical structure, and its principal function is to resist mechanical forces and fracture. Bone strength depends not only on the quantity of bone tissue but also on the shape and hierarchical structure. The hierarchical levels are interrelated, especially the micro-architecture, collagen and mineral components; hence, analysis of their specific roles in bone strength and stiffness is difficult. Synchrotron imaging technologies including micro-CT and small/wide angle X-ray scattering/diffraction are becoming increasingly popular for studying bone because the images can resolve deformations in the micro-architecture and collagen-mineral matrix under in situ mechanical loading. Synchrotron cannot be directly applied in vivo due to the high radiation dose but will allow researchers to carry out systematic multifaceted studies of bone ex vivo. Identifying characteristics of aging and disease will underpin future efforts to generate novel devices and interventional therapies for assessing and promoting healthy aging. With our own research work as examples, this paper introduces how synchrotron imaging technology can be used with in situ testing in bone research.

Spectral K-edge subtraction imaging of experimental non-radioactive barium uptake in bone

PURPOSE

To evaluate the feasibility of using non-radioactive barium as a bone tracer for detection with synchrotron spectral K-edge subtraction (SKES) technique.

METHODS

Male rats of 1-month old (i.e., developing skeleton) and 8-month old (i.e., skeletally mature) were orally dosed with low dose of barium chloride (33mg/kg/day Ba²⁺) for 4weeks. The fore and hind limbs were dissected for imaging in projection and computed tomography modes at 100μm and 52μm pixel sizes. The SKES method utilizes a single bent Laue monochromator to prepare a 550eV energy spectrum to encompass the K-edge of barium (37.441keV), for collecting both 'above' and 'below' the K-edge data sets in a single scan.

RESULTS

The SKES has a very good focal size, thus limits the 'crossover' and motion artifacts. In juvenile rats, barium was mostly incorporated in the areas of high bone turnover such as at the growth plate and the trabecular surfaces, but also in the cortical bone as the animals were growing at the time of tracer administration. However, the adults incorporated approximately half the concentration and mainly in the areas where bone remodeling was predominant and occasionally in the periosteal and endosteal layers of the diaphyseal cortical bone.

CONCLUSIONS

The presented methodology is simple to implement and provides both structural and functional information, after labeling with barium, on bone micro-architecture and thus has great potential for in vivo imaging of pre-clinical animal models of musculoskeletal diseases to better understand their mechanisms and to evaluate the efficacy of pharmaceuticals.

The sensitivity of nonlinear computational models of trabecular bone to tissue level constitutive model

Microarchitectural finite element models have become a key tool in the analysis of trabecular bone. Robust, accurate, and validated constitutive models would enhance confidence in predictive applications of these models and in their usefulness as accurate assays of tissue properties. Human trabecular bone specimens from the femoral neck (n = 3), greater trochanter (n = 6), and lumbar vertebra (n = 1) of eight different donors were scanned by μ-CT and converted to voxel-based finite element models. Unconfined uniaxial compression and shear loading were simulated for each of three different constitutive models: a principal strain-based model, Drucker-Lode, and Drucker-Prager. The latter was applied with both infinitesimal and finite kinematics. Apparent yield strains exhibited minimal dependence on the constitutive model, differing by at most 16.1%, with the kinematic formulation being influential in compression loading. At the tissue level, the quantities and locations of yielded tissue were insensitive to the constitutive model, with the exception of the Drucker-Lode model, suggesting that correlation of microdamage with computational models does not improve the ability to discriminate between constitutive laws. Taken together, it is unlikely that a tissue constitutive model can be fully validated from apparent-level experiments alone, as the calculations are too insensitive to identify differences in the outcomes. Rather, any asymmetric criterion with a valid yield surface will likely be suitable for most trabecular bone models.

Multiscale imaging of bone microdamage

Bone is a structural and hierarchical composite that exhibits remarkable ability to sustain complex mechanical loading and resist fracture. Bone quality encompasses various attributes of bone matrix from the quality of its material components (type-I collagen, mineral and non-collagenous matrix proteins) and cancellous microarchitecture, to the nature and extent of bone microdamage. Microdamage, produced during loading, manifests in multiple forms across the scales of hierarchy in bone and functions to dissipate energy and avert fracture. Microdamage formation is a key determinant of bone quality, and through a range of biological and physical mechanisms, accumulates with age and disease. Accumulated microdamage in bone decreases bone strength and increases bone's propensity to fracture. Thus, a thorough assessment of microdamage, across the hierarchical levels of bone, is crucial to better understand bone quality and bone fracture. This review article details multiple imaging modalities that have been used to study and characterize microdamage; from bulk staining techniques originally developed by Harold Frost to assess linear microcracks, to atomic force microscopy, a modality that revealed mechanistic insights into the formation diffuse damage at the ultrastructural level in bone. New automated techniques using imaging modalities, such as microcomputed tomography are also presented for a comprehensive overview.

Fracture mechanics by three-dimensional crack-tip synchrotron X-ray microscopy

To better understand the relationship between the nucleation and growth of defects and the local stresses and phase changes that cause them, we need both imaging and stress mapping. Here, we explore how this can be achieved by bringing together synchrotron X-ray diffraction and tomographic imaging. Conventionally, these are undertaken on separate synchrotron beamlines; however, instruments capable of both imaging and diffraction are beginning to emerge, such as ID15 at the European Synchrotron Radiation Facility and JEEP at the Diamond Light Source. This review explores the concept of three-dimensional crack-tip X-ray microscopy, bringing them together to probe the crack-tip behaviour under realistic environmental and loading conditions and to extract quantitative fracture mechanics information about the local crack-tip environment. X-ray diffraction provides information about the crack-tip stress field, phase transformations, plastic zone and crack-face tractions and forces. Time-lapse CT, besides providing information about the three-dimensional nature of the crack and its local growth rate, can also provide information as to the activation of extrinsic toughening mechanisms such as crack deflection, crack-tip zone shielding, crack bridging and crack closure. It is shown how crack-tip microscopy allows a quantitative measure of the crack-tip driving force via the stress intensity factor or the crack-tip opening displacement. Finally, further opportunities for synchrotron X-ray microscopy are explored.

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

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

Reduced diaphyseal strength associated with high intracortical vascular porosity within long bones of children with osteogenesis imperfecta

Osteogenesis imperfecta is a genetic disorder resulting in bone fragility. The mechanisms behind this fragility are not well understood. In addition to characteristic bone mass deficiencies, research suggests that bone material properties are compromised in individuals with this disorder. However, little data exists regarding bone properties beyond the microstructural scale in individuals with this disorder. Specimens were obtained from long bone diaphyses of nine children with osteogenesis imperfecta during routine osteotomy procedures. Small rectangular beams, oriented longitudinally and transversely to the diaphyseal axis, were machined from these specimens and elastic modulus, yield strength, and maximum strength were measured in three-point bending. Intracortical vascular porosity, bone volume fraction, osteocyte lacuna density, and volumetric tissue mineral density were determined by synchrotron micro-computed tomography, and relationships among these mechanical properties and structural parameters were explored. Modulus and strength were on average 64-68% lower in the transverse vs. longitudinal beams (P<0.001, linear mixed model). Vascular porosity ranged between 3 and 42% of total bone volume. Longitudinal properties were associated negatively with porosity (P≤0.006, linear regressions). Mechanical properties, however, were not associated with osteocyte lacuna density or volumetric tissue mineral density (P≥0.167). Bone properties and structural parameters were not associated significantly with donor age (P≥0.225, linear mixed models). This study presents novel data regarding bone material strength in children with osteogenesis imperfecta. Results confirm that these properties are anisotropic. Elevated vascular porosity was observed in most specimens, and this parameter was associated with reduced bone material strength. These results offer insight toward understanding bone fragility and the role of intracortical porosity on the strength of bone tissue in children with osteogenesis imperfecta.

Bone embrittlement and collagen modifications due to high-dose gamma-irradiation sterilization

Bone allografts are often used in orthopedic reconstruction of skeletal defects resulting from trauma, bone cancer or revision of joint arthroplasty. γ-Irradiation sterilization is a widely-used biological safety measure; however it is known to embrittle bone. Irradiation has been shown to affect the post-yield properties, which are attributed to the collagen component of bone. In order to find a solution to the loss of toughness in irradiated bone allografts, it is important to fully understand the effects of irradiation on bone collagen. The objective of this study was to evaluate changes in the structure and integrity of bone collagen as a result of γ-irradiation, with the hypothesis that irradiation fragments collagen molecules leading to a loss of collagen network connectivity and therefore loss of toughness. Using cortical bone from bovine tibiae, sample beams irradiated at 33kGy on dry ice were compared to native bone beams (paired controls). All beams were subjected to three-point bend testing to failure followed by characterization of the decalcified bone collagen, using differential scanning calorimetry (DSC), hydrothermal isometric tension testing (HIT), high performance liquid chromatography (HPLC) and gel electrophoresis (SDS-PAGE). The carbonyl content of demineralized bone collagen was also measured chemically to assess oxidative damage. Barium sulfate staining after single edge notch bending (SEN(B)) fracture testing was also performed on bovine tibia bone beams with a machined and sharpened notch to evaluate the fracture toughness and ability of irradiated bone to form micro-damage during fracture. Irradiation resulted in a 62% loss of work-to-fracture (p≤0.001). There was significantly less micro-damage formed during fracture propagation in the irradiated bone. HPLC showed no significant effect on pentosidine, pyridinoline, or hydroxypyridinoline levels suggesting that the loss of toughness is not due to changes in these stable crosslinks. For DSC, there was a 20% decrease in thermal stability (p<0.001) with a 100% increase (p<0.001) in enthalpy of denaturation (melting). HIT testing also showed a decrease in thermal stability (20% lower denaturation temperature, p<0.001) and greatly reduced measures of collagen network connectivity (p<0.001). Interestingly, the increase in enthalpy of denaturation suggests that irradiated collagen requires more energy to denature (melt), perhaps a result of alterations in the hydrogen bonding sites (increased carbonyl content detected in the insoluble collagen) on the irradiated bone collagen. Altogether, this new data strongly indicates that a large loss of overall collagen connectivity due to collagen fragmentation resulting from γ-irradiation sterilization leads to inferior cortical bone toughness. In addition, notable changes in the thermal denaturation of the bone collagen along with chemical indicators of oxidative modification of the bone collagen indicate that the embrittlement may be a function not only of collagen fragmentation but also of changes in bonding.

Nanoscale examination of microdamage in sheep cortical bone using synchrotron radiation transmission x-ray microscopy

Microdamage occurs in bone through repeated and excessive loading. Accumulation of microdamage weakens bone, leading to a loss of strength, stiffness and energy dissipation in the tissue. Imaging techniques used to examine microdamage have typically been limited to the microscale. In the current study microdamage was examined at the nanoscale using transmission x-ray microscopy with an x-ray negative stain, lead-uranyl acetate. Microdamage was generated in notched and unnotched beams of sheep cortical bone (2×2×20 mm), with monotonic and fatigue loading. Bulk sections were removed from beams and stained with lead-uranyl acetate to identify microdamage. Samples were sectioned to 50 microns and imaged using transmission x-ray microscopy producing projection images of microdamage with nanoscale resolution. Staining indicated microdamage occurred in both the tensile and compressive regions. A comparison between monotonic and fatigue loading indicated a statistically significant greater amount of stain present in fatigue loaded sections. Microdamage occurred in three forms: staining to existing bone structures, cross hatch damage and a single crack extending from the notch tip. Comparison to microcomputed tomography demonstrated differences in damage morphology and total damage between the microscale and nanoscale. This method has future applications for understanding the underlying mechanisms for microdamage formation as well as three-dimensional nanoscale examination of microdamage.

A novel videography method for generating crack-extension resistance curves in small bone samples

Assessment of bone quality is an emerging solution for quantifying the effects of bone pathology or treatment. Perhaps one of the most important parameters characterising bone quality is the toughness behaviour of bone. Particularly, fracture toughness, is becoming a popular means for evaluating bone quality. The method is moving from a single value approach that models bone as a linear-elastic material (using the stress intensity factor, K) towards full crack extension resistance curves (R-curves) using a non-linear model (the strain energy release rate in J-R curves). However, for explanted human bone or small animal bones, there are difficulties in measuring crack-extension resistance curves due to size constraints at the millimetre and sub-millimetre scale. This research proposes a novel “whitening front tracking” method that uses videography to generate full fracture resistance curves in small bone samples where crack propagation cannot typically be observed. Here we present this method on sharp edge notched samples (<1 mm×1 mm×Length) prepared from four human femora tested in three-point bending. Each sample was loaded in a mechanical tester with the crack propagation recorded using videography and analysed using an algorithm to track the whitening (damage) zone. Using the “whitening front tracking” method, full R-curves and J-R curves could be generated for these samples. The curves for this antiplane longitudinal orientation were similar to those found in the literature, being between the published longitudinal and transverse orientations. The proposed technique shows the ability to generate full “crack” extension resistance curves by tracking the whitening front propagation to overcome the small size limitations and the single value approach.

On-line analysis of cracking in cortical bone under wedge penetration

Understanding the mechanism of crack propagation during bone cutting is necessary for the development of realistic bone cutting models. This article studies the on-line fractural behaviour of cortical bone caused by penetration with a sharp metallic wedge mounted on an on-line loading stage within an X-ray microfocus computed tomography system. The experimental results demonstrated anisotropy in crack propagation depending on the penetration direction with regard to the longitudinal bone axis and relate the crack growth to the extent of penetration. Scanning electron microscopy is performed to analyse the mechanism of cracking in the two phase microstructure of compact bone.

Preclinical imaging of the cellular and molecular events in the multistep process of bone metastasis

Bone metastasis is a complex process that ultimately leads to devastating metastatic bone disease. It is therefore of key interest to unravel the mechanisms underlying the multistep process of skeletal metastasis and cancer-induced bone disease, and to develop better treatment and management of patients with this devastating disease. Fortunately, novel technologies are rapidly emerging that allow real-time imaging of molecules, pathogenic processes, drug delivery and drug response in preclinical in vivo models. The outcome of these experimental studies will facilitate clinical cancer research by improving the detection of cancer cell invasion, metastasis and therapy response.

Binding affinity of surface functionalized gold nanoparticles to hydroxyapatite

Gold nanoparticles (Au NPs) have been investigated for a number of biomedical applications, including drug and gene delivery vehicles, thermal ablation therapy, diagnostic sensors, and imaging contrast agents. Surface functionalization with molecular groups exhibiting calcium affinity can enable targeted delivery of Au NPs to calcified tissue, including damaged bone tissue. Therefore, the objective of this study was to investigate the binding affinity of functionalized Au NPs for targeted delivery to bone mineral, using hydroxyapatite (HA) crystals as a synthetic analog in vitro. Au NPs were synthesized to a mean particle size of 10-15 nm and surface functionalized with either L-glutamic acid, 2-aminoethylphosphonic acid, or alendronate, which exhibit a primary amine for binding gold opposite carboxylate, phosphonate, or bisphosphonate groups, respectively, for targeting calcium. Bisphosphonate functionalized Au NPs exhibited the most rapid binding kinetics and greatest binding affinity to HA, followed by glutamic acid and phosphonic acid. All functional groups reached complete binding after 24 h. Equilibrium binding constants in de-ionized water, determined by nonlinear regression of Langmuir isotherms, were 3.40, 0.69, and 0.25 mg/L for bisphosphonate, carboxylate, and phosphonate functionalized Au NPs, respectively. Functionalized Au NPs exhibited lower overall binding in fetal bovine serum compared to de-ionized water, but relative differences between functional groups were similar.

Synchrotron radiation micro-CT at the micrometer scale for the analysis of the three-dimensional morphology of microcracks in human trabecular bone

Bone quality is an important concept to explain bone fragility in addition to bone mass. Among bone quality factors, microdamage which appears in daily life is thought to have a marked impact on bone strength and plays a major role in the repair process. The starting point for all studies designed to further our understanding of how bone microdamage initiate or dissipate energy, or to investigate the impact of age, gender or disease, remains reliable observation and measurement of microdamage. In this study, 3D Synchrotron Radiation (SR) micro-CT at the micrometric scale was coupled to image analysis for the three-dimensional characterization of bone microdamage in human trabecular bone specimens taken from femoral heads. Specimens were imaged by 3D SR micro-CT with a voxel size of 1.4 µm. A new tailored 3D image analysis technique was developed to segment and quantify microcracks. Microcracks from human trabecular bone were observed in different tomographic sections as well as from 3D renderings. New 3D quantitative measurements on the microcrack density and morphology are reported on five specimens. The 3D microcrack density was found between 3.1 and 9.4/mm3 corresponding to a 2D density between 0.55 and 0.76 /mm2. The microcrack length and width measured in 3D on five selected microcrack ranged respectively from 164 µm to 209 µm and 100 µm to 120 µm. This is the first time that various microcracks in unloaded human trabecular bone--from the simplest linear crack to more complex cross-hatch cracks--have been examined and quantified by 3D imaging at this scale. The suspected complex morphology of microcracks is here considerably more evident than in the 2D observations. In conclusion, this technique opens new perspective for the 3D investigation of microcracks and the impact of age, disease or treatment.

Functional histology of glioma vasculature by FTIR imaging

Fourier-transform infrared (FTIR) imaging has been used to investigate brain tumor angiogenesis using a mice solid tumor model and bare-gold (∅ 25 nm) or BaSO(4) (∅ 500 nm) nanoparticles (NP) injected into blood vasculature. FTIR images of 20-μm-thick tissue sections were used for chemical histology of healthy and tumor areas. Distribution of BaSO(4)-NP (using the 1,218-1,159 cm(-1) spectral interval) revealed clearly all details of blood vasculature with morphological abnormalities of tumor capillaries, while Au-NP (using the 1,046-1,002 cm(-1) spectral interval) revealed also diffusion properties of leaky blood vessels. Diffusion of Au-NP out of vascular space reached 64 ± 29 μm, showing the fenestration of "leaky" tumor blood vessels, which should allow small NP (<100 nm, as for Au-NP) to diffuse almost freely, while large NP should not (as for BaSO(4)-NP in this study). Therefore, we propose to develop FTIR imaging as a convenient tool for functional molecular histology imaging of brain tumor vasculature, both for identifying blood capillaries and for determining the extravascular diffusion space offered by vessel fenestration.

Contrast-enhanced micro-computed tomography of fatigue microdamage accumulation in human cortical bone

Conventional methods used to image and quantify microdamage accumulation in bone are limited to histological sections, which are inherently invasive, destructive, two-dimensional, and tedious. These limitations inhibit investigation of microdamage accumulation with respect to volumetric spatial variation in mechanical loading, bone mineral density, and microarchitecture. Therefore, the objective of this study was to investigate non-destructive, three-dimensional (3-D) detection of microdamage accumulation in human cortical bone using contrast-enhanced micro-computed tomography (micro-CT), and to validate micro-CT measurements against conventional histological methods. Unloaded controls and specimens loaded in cyclic uniaxial tension to a 5% and 10% reduction in secant modulus were labeled with a precipitated BaSO₄ stain for micro-CT and basic fuchsin for histomorphometry. Linear microcracks were similarly labeled by BaSO₄ and basic fuchsin as shown by backscattered electron microscopy and light microscopy, respectively. The higher X-ray attenuation of BaSO₄ relative to the bone extracellular matrix provided enhanced contrast for the detection of damage that was otherwise not able to be detected by micro-CT prior to staining. Therefore, contrast-enhanced micro-CT was able to nondestructively detect the presence, 3-D spatial location, and accumulation of fatigue microdamage in human cortical bone specimens in vitro. Microdamage accumulation was quantified on segmented micro-CT reconstructions as the ratio of BaSO₄ stain volume (SV) to total bone volume (BV). The amount of microdamage measured by both micro-CT (SV/BV) and histomorphometry (Cr.N, Cr.Dn, Cr.S.Dn) progressively increased from unloaded controls to specimens loaded to a 5% and 10% reduction in secant modulus (p < 0.001). Group means for micro-CT measurements of damage accumulation were strongly correlated to those using histomorphometry (p < 0.05), validating the new methods. Limitations of the new methods in the present study included that the precipitated BaSO₄ stain was non-specific and non-biocompatible, and that micro-CT measurements exhibited greater variability compared to conventional histology. Nonetheless, contrast-enhanced micro-CT enabled non-destructive imaging and 3-D spatial information, which are not possible using conventional histological methods.

Circumferential Arc-Shaped Microcracks in Haversian Bone: Lead-Uranyl Acetate Staining for Micro-CT Imaging

Despite much progress in recent years, the nature of microcracking in bone at the nano-meter scale is still not well understood. This is partly due to the complexity of bone's hierarchical structure, but also to the difficulty of detecting cracks at very fine scales. Bone microcracking is typically detected using fluorescent dye staining techniques followed by optical or laser microscopy examinations. However, fluorescence-based methods are limited to sub-micron resolution and do not fit three-dimensional imaging such as micro-CT or high resolution imaging such as electron microscopy. This pilot study explores the potential of a heavy metal staining technique to label nano-sized cracks in bone that could be detected by electron microscopy and, albeit at a larger scale, by micro-computed tomography. Upon further development, the method described here may lead to the nano-meter scale characterization of bone microcracking.

Guidelines for assessment of bone microstructure in rodents using micro-computed tomography

Use of high-resolution micro-computed tomography (microCT) imaging to assess trabecular and cortical bone morphology has grown immensely. There are several commercially available microCT systems, each with different approaches to image acquisition, evaluation, and reporting of outcomes. This lack of consistency makes it difficult to interpret reported results and to compare findings across different studies. This article addresses this critical need for standardized terminology and consistent reporting of parameters related to image acquisition and analysis, and key outcome assessments, particularly with respect to ex vivo analysis of rodent specimens. Thus the guidelines herein provide recommendations regarding (1) standardized terminology and units, (2) information to be included in describing the methods for a given experiment, and (3) a minimal set of outcome variables that should be reported. Whereas the specific research objective will determine the experimental design, these guidelines are intended to ensure accurate and consistent reporting of microCT-derived bone morphometry and density measurements. In particular, the methods section for papers that present microCT-based outcomes must include details of the following scan aspects: (1) image acquisition, including the scanning medium, X-ray tube potential, and voxel size, as well as clear descriptions of the size and location of the volume of interest and the method used to delineate trabecular and cortical bone regions, and (2) image processing, including the algorithms used for image filtration and the approach used for image segmentation. Morphometric analyses should be based on 3D algorithms that do not rely on assumptions about the underlying structure whenever possible. When reporting microCT results, the minimal set of variables that should be used to describe trabecular bone morphometry includes bone volume fraction and trabecular number, thickness, and separation. The minimal set of variables that should be used to describe cortical bone morphometry includes total cross-sectional area, cortical bone area, cortical bone area fraction, and cortical thickness. Other variables also may be appropriate depending on the research question and technical quality of the scan. Standard nomenclature, outlined in this article, should be followed for reporting of results.

Preparation of functionalized gold nanoparticles as a targeted X-ray contrast agent for damaged bone tissue

Conventional methods used to image and quantify microdamage in bone tissue are limited to thin histological sections. Therefore recent studies have begun to investigate methods for non-destructive, three-dimensional (3-D) detection and imaging of microdamage in bone tissue. The objective of this study was to investigate gold nanoparticles (Au NPs) as a potential damage-specific X-ray contrast agent due to their relative biocompatibility, ease of surface functionalization, colloidal stability, and high X-ray attenuation. Au NPs were prepared using a citrate reduction reaction to approximately 15 or 40 nm diameter, and functionalized with glutamic acid for targeting damaged bone tissue. As-synthesized and functionalized Au NPs were spherical, relatively monodispersed, and exhibited aqueous colloidal stability. Functionalized Au NPs were demonstrated to target damaged bovine cortical bone tissue as visually evidenced by surface scratches turning a characteristic red color after soaking in functionalized Au NP solutions. Individual Au NPs were observed on the surface of damaged tissue using backscattered electron imaging and atomic force microscopy. Therefore, functionalized Au NPs are a promising candidate for a targeted X-ray contrast agent for damaged bone tissue.