Relationship between bone tissue strain and lattice strain of HAp crystals in bovine cortical bone under tensile loading.

Cortical bone is a composite material composed of hydroxyapatite (HAp) and collagen. As HAp is a crystalline structure, an X-ray diffraction method is available to measure the strain of HAp crystals. However, HAp crystals in bone tissue have been known to have the low degree of crystallization. Authors have proposed an X-ray diffraction method to measure the lattice strain of HAp crystals from the diffusive intensity profile due to low crystallinity. The precision of strain measurement was greatly improved by this method. In order to confirm the possibility of estimating the bone tissue strain with measurements of the strain of HAp crystals, this work investigates the relationship between bone tissue strain on a macroscopic scale and the lattice strain of HAp crystals on a microscopic scale. The X-ray diffraction experiments were performed under tensile loading. Strip bone specimens of 40×6×0.8 mm in size were cut from the cortical region of a shaft of bovine femur. A stepwise tensile load was applied in the longitudinal direction of the specimen. By detecting the diffracted X-ray beam transmitted through the specimen, the lattice strain was directly measured in the loading direction. As a result, the lattice strain of HAp crystals showed lower value than the bone tissue strain measured by a strain gage. The bone tissue strain was described with the mean lattice strain of the HAp crystals and the elastic modulus

Residual stress distribution in rabbit limb bones

The presence of the residual stresses in bone tissue has been noted and the authors have reported that there are residual stresses in bone tissue. The aim of our study is to measure the residual stress distribution in the cortical bone of the extremities of vertebrates and to describe the relationships with the osteon population density. The study used the rabbit limb bones (femur, tibia/fibula, humerus, and radius/ulna) and measured the residual stresses in the bone axial direction at anterior and posterior positions on the cortical surface. The osteons at the sections at the measurement positions were observed by microscopy. As a result, the average stresses at the hindlimb bones and the forelimb bones were 210 and 149 MPa, respectively. In the femur, humerus, and radius/ulna, the residual stresses at the anterior position were larger than those at the posterior position, while in the tibia, the stress at the posterior position was larger than that at the anterior position. Further, in the femur and humerus, the osteon population densities in the anterior positions were larger than those in the posterior positions. In the tibia, the osteon population density in the posterior position was larger than that in the anterior position. Therefore, tensile residual stresses were observed at every measurement position in the rabbit limb bones and the value of residual stress correlated with the osteon population density (r=0.55, P<0.01)

Relationship between streaming potential and compressive stress in bovine intervertebral tissue.

The intervertebral disc is formed by the nucleus pulposus (NP) and annulus fibrosus (AF), and intervertebral tissue contains a large amount of negatively charged proteoglycan. When this tissue becomes deformed, a streaming potential is induced by liquid flow with positive ions. The anisotropic property of the AF tissue is caused by the structural anisotropy of the solid phase and the liquid phase flowing into the tissue with the streaming potential. This study investigated the relationship between the streaming potential and applied stress in bovine intervertebral tissue while focusing on the anisotropy and loading location. Column-shaped specimens, 5.5mm in diameter and 3mm thick, were prepared from the tissue of the AF, NP and the annulus-nucleus transition region (AN). The loading direction of each specimen was oriented in the spinal axial direction, as well as in the circumferential and radial directions of the spine considering the anisotropic properties of the AF tissue. The streaming potential changed linearly with stress in all specimens. The linear coefficients k(e) of the relationship between stress and streaming potential depended on the extracted positions. These coefficients were not affected by the anisotropy of the AF tissue. In addition, these coefficients were lower in AF than in NP specimens. Except in the NP specimen, the k(e) values were higher under faster compression rate conditions. In cyclic compression loading the streaming potential changed linearly with compressive stress, regardless of differences in the tissue and load frequency

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 ill 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

Influence of osteon area fraction and degree of orientation of HAp crystals on mechanical properties in bovine femur

Cortical bone has a hierarchical structure, spanning from the macrostructure at several millimeters or whole bone level, the microstructure at several hundred micrometers level, to the nanostructure at hydroxyapatite (HAp) crystals and collagen fibrils levels. The aim of the study is to understand the relationship between the HAp crystal orientation and the elastic modulus and the relationship between the osteon area fraction and the deformation behavior of HAp crystals in cortical bone. In the experiments, five strip specimens (40×2×1 mm3) aligned with the bone axis were taken from the cortical bone of a bovine femur. The degree of c-axis orientation of HAp crystals in the specimens was measured with the X-ray diffraction technique with the imaging plate. To measure the deformation behavior of HAp crystals in the specimens, tensile tests under X-ray irradiation were conducted. The specimens were cut at the X-ray measurement positions and osteon area fraction and porosity at the transverse cross-sections were observed. Further, the volume fraction of HAp of the specimens was measured. Results showed the degree of c-axis orientation of HAp crystals was positively correlated with the elastic modulus of the specimens (r=0.94). The volume fraction of HAp and the porosity showed no statistical correlation with the elastic modulus and the tensile strength. The HAp crystal strain εH increased linearly with the bone tissue strain ε. The average value of εH/ε was 0.69±0.13 and there was no correlation between the osteon area fraction and εH/ε (r=−0.27, p=0.33). The results suggest that the degree of c-axis orientation of HAp crystals affects the elastic modulus and the magnitude of HAp crystal strain does not depend on the osteon area fraction

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

Mechanical evaluation of hip pads to protect against fracture of elderly femurs in falls

Hip fracture in the aged easily occurs by falls and may cause these persons to become bedridden. Hip pads are effective in protecting hip fracture as they directly deflect and absorb the impact forces by falls. It is necessary for the material and the structure of hip pads to be designed to realize both high impact absorption and compliance (comfort during wearing). In this report, an impact testing system was developed to test the impact absorbing performance of hip pad with air cushions designed by the research group. The impact absorbing performance was evaluated by the impact load, collision time, and maximum load. To confirm the effectiveness in protecting against hip fracture, an impact force was applied to the greater trochanter of the human femur and the degree of fracture was measured by X-ray examination. As a result, the hip pad with air cushions had a high impact absorbing performance and was sufficiently effective to protect against hip fracture

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 a two-dimensional detector, imaging plate, 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 imaging plate 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 centre 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 imaging plate as a detector. Our approach in this work for intensity integration and radius vector positioning especially add precision in such applications