Bijay Giri

Deformation of mineral crystals in cortical bone depending on structural anisotropy

The deformation mechanism of bone at different hierarchical levels has been of wide interest. The important features of bone, its anisotropy and orientation dependent deformation are equally important, which have also gained a long run discussion. Most of the studies are concentrated on protein-rich collagen fibres and matrix, where different deformation mechanisms at the lower length scales are proposed. But in relation to this, how the mineral particles behave depending on their distribution is yet to be revealed in detail. In the present work, we demonstrate mineral crystals deformation and arrangement characteristics on the basis of experimental outcomes. Using X-ray diffraction procedures, we quantified the mineral strains, degree of orientation of the crystallites and their evolution under different applied step-loads in bovine femoral cortical specimens having different alignment with the femoral axis direction. We provide a direct quantitative comparison of these parameters in the specimens having preferential orientations roughly at 0, 30, 45, 75 and 90 degrees with reference to the loading direction. The mineral strains in the compliant specimens, i.e. 0 and 30 degrees oriented specimens were observed to differ with the stiffer specimens, i.e. 75 and 90 degrees oriented specimens, whereas the 45 degrees oriented specimen show almost equal strains at different loads. These were explained by the degree of orientation with reference to the loading direction and the preferential orientation direction of the specimens. On the basis of observed parameters, we describe deformation phenomena of mineral particles to occur in different stages, which consist of redistribution stage, elastic strain stage and inelastic strain stage. These phenomena are expected to occur at different scales and rates depending on the orientation and distribution of crystals.

Estimating nanoscale deformation in bone by X-ray diffraction imaging method

Knowledge of internal stress-strain in bone tissue is important for clinical diagnosis and remedies. The inorganic mineral phase of apatite crystals in bone composite, because of its crystalline nature, provides a reliable way of measurement through X-ray diffraction system. Use of two-dimensional detector, imaging plate (IP), is considered to expedite the process with much more information, hence, is widely applied in the study of organization, stress, strain, etc. for crystalline substance. The distortion of Debye rings in the image obtained by IP can be directly related to the deformation in lattice plane of the crystals. Since X-ray diffraction method involves measurement at nano-level, proper focus on the extraction of data and corresponding analysis is needed. In the current work, we considered weighted average value of intensity to locate radius vectors along azimuthal direction in the diffracted rings from the primary array of digital data in steps of pixels. The widely applied approaches for profile shift measurement--peak shift and full width at half maximum (FWHM) of a peak, and shift of center of gravity of profile--were compared with a new concept of segmental shift (SS) proposed previously by the authors. We observed reliable and effective outcomes with higher precision in the consideration of SS while using IP as a detector. Our approach in this work for intensity integration and radius vector positioning especially add precision in such applications.

In situ mechanical behavior of mineral crystals in human cortical bone under compressive load using synchrotron X-ray scattering techniques

It is of great interest to delineate the effect of orientation distribution of mineral crystals on the bulk mechanical behavior of bone. Using a unique synergistic approach combining a progressive loading scheme and synchrotron X-ray scattering techniques, human cortical bone specimens were tested in compression to examine the in situ mechanical behavior of mineral crystals aligned in different orientations. The orientation distribution was quantitatively estimated by measuring the X-ray diffraction intensity from the (002) plane in mineral crystals. In addition, the average longitudinal (c-axis), transverse (a-axis), and shear strains of the subset of mineral crystals aligned in each orientation were determined by measuring the lattice deformation normal to three distinct crystallographic planes (i.e. 002, 310, and 213) in the crystals. The experimental results indicated that the in situ strain and stress of mineral crystals varied with orientations. The normal strain and stress in the longitudinally aligned mineral crystals were markedly greater than those in the transversely oriented crystals, whereas the shear stress reached a maximum for the crystals aligned in ±30° with respect to the loading direction. The maximum principal strain and stress were observed in the mineral crystals oriented along the loading axis, with a similar trend observed in the maximum shear strain and stress. By examining the in situ behavior, the contribution of mineral crystals to load bearing and the bulk behavior of bone are discussed.

X-ray diffraction as a promising tool to characterize bone nanocomposites

Abstract To understand the characteristics of bone at the tissue level, the structure, organization and mechanical properties of the underlying levels down to the nanoscale as well as their mutual interactions need to be investigated. Such information would help understand changes in the bone properties including stiffness, strength and toughness and provide ways to assess the aged and diseased bones and the development of next generation of bio-inspired materials. X-ray diffraction techniques have gained increased interest in recent years as useful non-destructive tools for investigating the nanostructure of bone. This review provides an overview on the recent progress in this field and briefly introduces the related experimental approach. The application of x-ray diffraction to elucidating the structural and mechanical properties of mineral crystals in bone is reviewed in terms of characterization of in situ strain, residual stress–strain and crystal orientation.

Understanding site-specific residual strain and architecture in bovine cortical bone

Living bone is considered as adaptive material to the mechanical functions, which continually undergoes change in its histological arrangement with respect to external prolonged loading. Such remodeling phenomena within bone depend on the degree of stimuli caused by the mechanical loading being experienced, and therefore, are specific to the sites. In the attempts of understanding strain adaptive phenomena within bones, different theoretical models have been proposed. Also, the existing literatures mostly follow the measurement of surface strains using strain gauges to experimentally quantify the strains experienced in the functional environment. In this work, we propose a novel idea of understanding site-specific functional adaptation to the prolonged load in bone on the basis of inherited residual strains and structural organization. We quantified the residual strains and amount of apatite crystals distribution, i.e., the degree of orientation, using X-ray diffraction procedures. The sites of naturally existing hole in bone, called foramen, are considered from bovine femur and metacarpal samples. Significant values of residual strains are found to exist in the specimens. Trends of residual strains noted in the specimens are mostly consistent with the degree of orientation of the crystallites. These features explain the response behavior of bone to the mechanical loading history near the foramen sites. Preferential orientation of crystals mapped around a femoral foramen specimen showed furnished tailored arrangement of the crystals around the hole. Effect of external loading at the femoral foramen site is also explained by the tensile loading experiment.

Progressive Post-Yield Behavior of Human Cortical Bone in Shear

Post-yield behavior is important for bone fragility since it accounts for the major part of energy dissipation of bone. Therefore, it is essential to study the post-yield behavior of bone to understand the different pathways for energy dissipation [1]. The post-yield behavior of bone may depend on the different loading modes. Previous studies have utilized a novel progressive loading scheme to study the post-yield behavior of cortical bone at tension [2] and compression [3]. However, few studies have reported post-yield behaviors of cortical bone in shear [4]. One of major challenges in shear tests of cortical bone is to achieve a uniform stress field over a test region. For example, the notches of the Iosipescu test may cause non-constant stress fields and locally high stresses when small amounts of bending are present [5]. The objective of this study was to develop the progressive loading scheme of shear in bone using an inclined double notch shear test, in which homogeneous shear stress fields were produced [5].Copyright

Pre-Strain and Integrity of Mineral Crystals in Human Cortical Bone Under Tensile and Compressive Loads

In this study, the pre-strain status and structural integrity of mineral crystals in bone was investigated using synchrotron X-ray scattering techniques. By measuring the pre-strain status, the interaction of mineral crystals with the surrounding matrix was determined. In addition, the load-induced changes in the lattice integrity of mineral crystallites were assessed using the value of the full-width-at-half-magnitude (FWHM). With increasing load, it was observed that the pre-strain in mineral crystals was relaxed in both tension and compression, whereas the lattice integrity of mineral crystallites was sustained until the failure of bone irrespective of loading modes.© 2012 ASME

Contribution of Mineral Crystals to the Bulk Behavior of Human Cortical Bone in Compression

In this study, human cortical bone was tested in compression to determine the in situ behavior of mineral crystals using a synergistic approach combining a progressive loading scheme and synchrotron X-ray scattering techniques. By quantifying the orientation distribution of mineral crystals, the average strain tensor of each subset of mineral crystals in the same orientation was determined based on its lattice deformation in three distinct crystallographic directions. The stress tensor of the crystals was determined based on the Hooke’s law using the stiffness tensor well derived in the literature (1). By examining the concurrent changes in the in situ and bulk behaviors, the contribution of mineral crystals to the bulk behavior of bone was discussed.Copyright

Effect of hydrogen bonding ability, dipole-dipole interactions and viscosity of extracellular matrix fluid on the bone mechanical behavior

Fragility fracture as a mode of pathologic failure in bone is a major healthcare concern and has adverse consequences with respect to morbidity, cost and to a lesser extent mortality. Understanding the structure/composition and functional relationships among the bone constituents is an important step towards prevention/treatment of fragility fractures.© 2012 ASME

Shear strain of mineral crystals calculated using wide-angle X-Ray scattering (WAXS) techniques

The post-yield properties of bone mainly provide the estimation of its toughness and are directly related to the composition and structure of the tissue at basic building unit. Combining the recently developed synchrotron X-ray scattering techniques with a novel progressive loading scheme [1], a recent study has evinced that the internal strain of mineral and collagen phases in bone varies significantly before and after yielding of the tissue [2]. The minerals as well as collagen fibrils in bone have random and preferred orientation distribution according to the location within a body. Such orientation distribution will have significant influence on the deformation mechanisms of bone. This study hence aims to develop a methodology to determine local strain tensors in the mineral phase aligned in different orientations. To do so, the deformation of different lattice planes within individual mineral crystals is taken in account using the full WAXS spectrum.Copyright