Intercrossed Microstructures of Hydroxyapatite Sheets of Tibia Bone

The observation of Scanning Electron Microscope (SEM) shows that a tibia bone is a kind of bioceramic composite consisting of hydroxyapatite layers and collagen protein matrix. All the hydroxyapatite layers are parallel with the surface of the bone and consist of numerous hydroxyapatite sheets. The observation also shows that there is a kind of intercrossed microstructure of the hydroxyapatite sheets. In this kind of microstructure, the hydroxyapatite sheets in an arbitrary hydroxyapatite layer make a large intercrossed angle with the hydroxyapatite sheets in their adjacent hydroxyapatite layers. The maximum pullout force of the intercrossed microstructure, which is closely related to the fracture toughness of the bone, is investigated and compared with that of the parallel microstructure of the sheets through their representative models. The result of the investigation indicates that the maximum pullout force of the intercrossed microstructure is markedly larger than that of the parallel microstructure, which is experimentally verified.     

Processing and characterization of laser sintered hydroxyapatite scaffold for tissue engineering

The sintering processing of hydroxyapatite (HAP) powder was studied using selective laser sintering for bone tissue engineering. The effect of laser energy density on the microstructure, phase composition and mechanical properties of the sintered samples was investigated. The results indicate that the average grain size increases from 0.211 ± 0.039 to 0.979 ± 0.133 μm with increasing the laser energy density from 2.0 to 5.0 J/mm². The maximum value of Vickers hardness and fracture toughness were 4.0 ± 0.13 Gpa and 1.28 ± 0.033 MPam^1/2, respectively, when the laser energy density was 4.0 J/mm². The XRD results indicated that the nano-HAP was decomposed into TCP with the laser energy density of above 4.0 J/mm². In vitro bioactivity after soaking in simulated body fluid (SBF) for 3 ～ 12 days showed that a bone-like apatite layer on the surface of the sintered samples. It indicated that the HAP scaffold possesses favorable mechanical properties and bioactivity, and may be used for bone tissue engineering.     

Equine cortical bone exhibits rising R-curve fracture mechanics

Previous studies of the fracture properties of cortical bone have suggested that the fracture toughness increases with crack length, which is indicative of rising R-curve behavior. Based on this indirect evidence and the similarity of bone to ceramic matrix composites, we hypothesized that bone would exhibit rising R-curve behavior in the transverse orientation and that the characteristics of the R-curves would be regionally dependent within the cortex due to variations in bone microstructure and toughening mechanisms. To test these hypotheses, we conducted R-curve experiments on specimens from equine third metacarpal bones using standard fracture mechanics testing methods. Compact type specimens from the dorsal and lateral regions in the middle of the diaphysis were oriented for crack propagation transverse to the longitudinal axis of the bone.The test results demonstrate that equine cortical bone exhibits rising R-curve behavior during transverse crack propagation as hypothesized. Statistical analyses of the crack growth initiation toughness, K0, the peak toughness, Kpeak, and the crack extension at peak toughness, deltaa, revealed significant regional differences in these characteristics. Specifically, the lateral cortex displayed higher crack growth initiation and peak toughnesses. The dorsal cortex exhibited greater crack extension at the peak of crack growth resistance. Scanning electron microscopy revealed osteon pullout on fracture surfaces from the dorsal cortex and but not in the lateral cortex. Taken together, the significant differences in R-curves and the SEM fractography indicate that the fracture mechanisms acting in equine cortical bone are regionally dependent.     

Microstructure and crystallographic-texture of giant barnacle (Austromegabalanus psittacus) shell

Barnacle shell is a very complex and strong composite bioceramic composed of different structural units which consist of calcite 15 microcrystals of very uniform size. In the study reported herein, the microstructural organization of these units has been examinated in detail with optical and scanning electron microscopy, and X-ray diffraction techniques. These analyses showed that the external part of the shell has a massive microstructure consisting of randomly oriented crystals. Toward the interior, the shell became organized in mineral layers separated by thin organic sheets. Each of these mineral layers has a massive microstructure constituted by highly oriented calcite microcrystals with their c-axes aligned [(001) fibre texture] perpendicular to the organic sheets and the shell surface. Interestingly, in another structural unit, the shell shield, the orientation of the c-axis calcite crystals shifts from being perpendicular to being parallel to the shell surface across its thickness. This study provides evidence that the organic matrix is responsible for the organization of the shell mineral and exterts strong a strict control on the polymorphic type, size and orientation of shell-forming crystals.     

Evaluation of fracture toughness of human dentin using elastic-plastic fracture mechanics

Dentin, the mineralized tissue forming the bulk of the tooth, lies between the enamel and the pulp chamber. It is a rich source of inspiration for designing novel synthetic materials due to its unique microstructure. Most of the previous studies investigating the fracture toughness of dentin have used linear-elastic fracture mechanics (LEFM) that ignores plastic deformation and could underestimate the toughness of dentin. With the presence of collagen (approximately 30% by volume) aiding the toughening mechanisms in dentin, we hypothesize that there is a significant difference between the fracture toughness estimated using LEFM (Kc) and elastic-plastic fracture mechanics (EPFM) (KJc). Single-edge notched beam specimens with in-plane (n=10) and anti-plane (n=10) parallel fractures were prepared following ASTM standard E1820 and tested in three-point flexure. KJc of the in-plane parallel and anti-plane parallel specimens were found to be 3.1 and 3.4 MPa m 1/2 and Kc were 2.4 and 2.5 MPa m 1/2, respectively. The fracture toughness estimated based on KJc is significantly greater than that estimated based on Kc (32.5% on average; p<0.001). In addition, KJc of anti-plane parallel specimens is significantly greater than that of in-plane parallel specimens. We suggest that, in order to critically evaluate the fracture toughness of human dentin, EPFM should be employed.     

Cortical bone failure mechanisms during screw pullout

An experimental and computational study of screw pullout from cortical bone has been conducted. A novel modification of standard pullout tests providing real time image capture of damage mechanisms during screw pullout was developed. Pullout forces, measured using the novel test rig, have been validated against standard pullout tests. Pullout tests were conducted, considering osteon alignment, to investigate the effect of osteons aligned parallel to the axis of the orthopaedic screw (longitudinal pullout) as well as the effect of osteons aligned perpendicular to the axis of the screw (transverse pullout). Distinctive alternate failure mechanisms, for longitudinally and transversely orientated cortical bone during screw pullout, were uncovered. Vertical crack propagation, parallel to the axis of the screw, was observed for a longitudinal pullout. Horizontal crack propagation, perpendicular to the axis of the screw, was observed for a transverse pullout. Finite element simulation of screw pullout, incorporating material damage and crack propagation, was also performed. Simulations revealed that a homogenous material model for cortical bone predicts vertical crack propagation patterns for both longitudinal and transverse screw pullout. A bi-layered composite model representing cortical bone microstructure was developed. A unique set of material and damage properties was used for both transverse and longitudinal pullout simulations, with only layer orientations being changed. Simulations predicted: (i) higher pullout forces for transverse pullout; (ii) horizontal crack paths perpendicular to screw axis for transverse pullout, whereas vertical crack paths were computed for longitudinal pullout. Computed results agreed closely with experimental observations in terms of pullout force and crack propagation.     

Rising crack-growth-resistance behavior in cortical bone: implications for toughness measurements

Fracture mechanics studies have characterized bone's resistance to fracture in terms of critical stress intensity factor and critical strain energy release rate measured at the onset of a fracture crack. This approach, although useful, provide a limited insight into fracture behavior of bone because, unlike classical brittle materials, bone is a microcracking solid that derives its resistance to fracture during the process of crack propagation from microfracture mechanisms occurring behind the advancing crack front. To address this shortfall, a crack propagation-based approach to measure bone toughness is described here and compared with crack initiation approach. Post hoc analyses of data from previously tested bovine and antler cortical bone compact specimens demonstrates that, in contrast to crack initiation approach, the crack propagation approach successfully identifies the superior toughness properties of red deer's antler cortical bone. Propagation-based slope of crack growth resistance curve is, therefore, a more useful parameter to evaluate cortical bone fracture toughness.     

Contribution, development and morphology of microcracking in cortical bone during crack propagation

A fracture mechanics study of cortical bone is presented to investigate the contribution, development morphology of microcracking in cortical bone during crack propagation. Post-hoc analyses of microcrack orientation, crack propagation velocity and fracture surface roughness were conducted on previously tested human and bovine bone compact tension specimens. It was found that, consistent with its higher toughness, bovine bone formed significantly more longitudinal, transverse and inclined microcracks than human bone. However, in human bone more of the microcracks that formed were longitudinal than transverse or inclined, a feature that would optimise bone's toughness. Crack propagation velocity in human and bovine bone displayed the same characteristic pattern with crack extension, where an increase in velocity is followed by a consequent decrease and vice versa. On the basis of this pattern, a model or crack propagation has been proposed. It provides a detailed account of mocrocrack formation and contribution towards the propagation of a fracture crack. Analyses of fracture surfaces indicated that, consistent with its higher toughness, bovine bone displays a rougher surface than human bone but they both have the same basic fractured element, i.e. a mineralised collagen fibril.     

Augmented bone-matrix formation and osteogenesis under magnetic field stimulation in vivo XRD, TEM and SEM investigations

Bone is a composite biomaterial, which is formed, when proteins constituting collagen fibers attract calcium, phosphate and hydroxide ions in solution to nucleate atop the fibers. It grows into a hard structure of tiny crystallites of hydroxyapatite, aligned along the long axis of collagen fibers. The present work reports the stimulating effect of static magnetic field on microstructure and mineralization process of bone repair. A unilateral transverse fracture of mid-shaft of metacarpal was surgically created in healthy goats under thiopental sedation and xylocaine analgesia. Two bar magnets (approximately 800 gauss/cm2 field strength) were placed across the fracture line at opposite pole alignment immobilized in Plaster of Paris (POP) splint bandage for static magnetic field stimulation. Radiographs were taken at weekly intervals up to 45 days. Results show that formation of extra-cellular matrix and its microstructure can be influenced by non-invasive physical stimulus (magnetic field) for achieving an enhanced osteogenesis, leading to quicker regeneration of bone tissue in goats. X-ray diffraction (XRD) patterns of treated (magnetic field-exposed) and control samples revealed the presence and orientation of crystalline structures. Intensity of diffraction peaks corresponding to 310 and 222 planes were enhanced with respect to 211 families of reflections, indicating preferential alignment of the crystals. Also, the percent crystallinity and crystal size were increased in treated samples. The study provides a biophysical basis for augmented fracture healing under the influence of semi-aligned static magnetic field applied across the fracture line.     

Application of fracture mechanics to failure in manatee rib bone

BACKGROUND: The Florida manatee (Trichechus manatus latirostris) is listed as endangered by the U.S. Department of the Interior. Manatee ribs have different microstructure from the compact bone of other mammals. Biomechanical properties of the manatee ribs need to be better understood. Fracture toughness (K(C)) has been shown to be a good index to assess the mechanical performance of bone. Quantitative fractography can be used in concert with fracture mechanics equations to identify fracture initiating defects/cracks and to calculate the fracture toughness of bone materials. METHOD OF APPROACH: Fractography is a standard technique for analyzing fracture behavior of brittle and quasi-brittle materials. Manatee ribs are highly mineralized and fracture in a manner similar to quasi-brittle materials. Therefore, quantitative fractography was applied to determine the fracture toughness of manatee ribs. RESULTS: Average fracture toughness values of small flexure specimens from six different sizes of manatees ranged from 1.3 to 2.6 MPa(m)(12). Scanning electron microscope (SEM) images show most of the fracture origins were at openings for blood vessels and interlayer spaces. CONCLUSIONS: Quantitative fractography and fracture mechanics can be combined to estimate the fracture toughness of the material in manatee rib bone. Fracture toughness of subadult and calf manatees appears to increase as the size of the manatee increases. Average fracture toughness of the manatee rib bone materials is less than the transverse fracture toughness of human and bovine tibia and femur.     

Pullout strength of suture anchors: Effect of mechanical properties of trabecular bone

This study investigated the relationships between trabecular microstructure and elastic modulus, compressive strength, and suture anchor pullout strength. Twelve fresh-frozen humeri underwent mechanical testing followed by micro-computed tomography (μCT). Either compression testing of cylindrical bone samples or pullout testing using an Arthrex 5 mm Corkscrew was performed in synthetic sawbone or at specific locations in the humerus such as the greater tuberosity, lesser tuberosity, and humeral head. Synthetic sawbone underwent identical mechanical testing and μCT analysis. Bone volume fraction (BVF), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbSp), trabecular number (TbN), and connectivity density were compared against modulus, compressive strength, and pullout strength in both materials. In cadaveric bone, modulus showed correlations to all of the microstructural properties, while compressive and pullout strength were only correlated to BVF, SMI, and TbSp. The microstructure of synthetic bone differed from cadaveric bone as SMI and TbTh showed little variation across the densities tested. Therefore, SMI and TbTh were the only microstructural properties that did not show correlations to the mechanical properties tested in synthetic bone. This study helps identify key microstructure–property relationships in cadaveric and synthetic bone as well as illustrate the similarities and differences between cadaveric and synthetic bone as biomechanical test materials.     

The arrangement and function of octopus arm musculature and connective tissue

The morphology of the musculature and connective tissues of the arms of Octopus bimaculoides was analyzed with light microscopy. We also studied O. briareus and O. digueti, which possess relatively more elongate and less elongate arms, respectively. The morphology of the arms was found to be remarkably uniform among species. The arms consist of a densely packed three-dimensional arrangement of muscle fibers and connective tissue fibers surrounding a central axial nerve cord. Three primary muscle fiber orientations were observed: 1) transverse muscle fibers oriented in planes perpendicular to the long axis of the arm; 2) longitudinal muscle fibers oriented parallel to the long axis; and 3) oblique muscle fibers arranged in helixes around the arm. The proportion of the arm cross section occupied by each of these muscle fiber groups (relative to the total cross sectional area of the musculature) remains constant along the length of the arm, even though the arm tapers from base to tip. A thin circular muscle layer wraps the arm musculature on the aboral side only. Much of this musculature has its origin and insertion on several robust connective tissue sheets including a layer surrounding the axial nerve cord and crossed-fiber connective tissue sheets located on the oral and the aboral sides of the arm. An additional thin layer of connective tissue wraps the arm musculature laterally and also serves as a site of origin and insertion of some of the muscle fibers. The fibers of the oral and aboral crossed-fiber connective tissue sheets are arranged oblique to the long axis of the arm with the same fiber angle as the oblique muscle layers that originate and insert on the sheets. The oblique muscle layers and the crossed-fiber connective tissue sheets thus form composite right- and left-handed helical fiber arrays. Analysis of arm morphology from the standpoint of biomechanics suggests that the transverse musculature is responsible for elongation of the arms, the longitudinal musculature is responsible for shortening, and the oblique muscle layers and associated connective tissues create torsion. Arm bending may involve unilateral contraction of longitudinal muscle bundles in combination with resistance to arm diameter increase due to contraction of the transverse musculature or passive stiffness of the arm tissues. The arms may also be bent by a combination of decrease in diameter due to contraction of the transverse musculature and maintenance of constant length on one side of the arm by unilateral activity of longitudinal muscle bundles. An increase in flexural stiffness of the arm may be achieved by cocontraction of the transverse and longitudinal muscle. Torsional stiffness may be increased by simultaneous contraction of both the right- and left-handed oblique muscle layers.     

Anisotropy of age-related toughness loss in human cortical bone: a finite element study

A mechanistic understanding of the role of bone quality on fracture processes is essential for determining the underlying causes of age-related changes in the mechanical response of the human bone. In this study, a previously developed cohesive finite element model was used to investigate the effects of age-related changes and the orientation of crack growth on the toughening behavior of human cortical bone. The change in the anisotropy of toughening mechanisms with age was also studied. Finite element method (FEM) simulations showed that the initiation toughness decreased by 3% and 8%/decade for transverse and longitudinal crack growth, respectively. In contrast, fracture resistance curve slope for transverse and longitudinal crack growth decreased by 2% and 3%/decade, respectively. Initiation fracture toughness values were higher for the transverse than for the longitudinal for a given age. On the other hand, propagation fracture toughness values were higher for longitudinal than for transverse crack growth for a given age. With respect to age, the toughness ratio for crack initiation decreased by 6%/decade, but that for propagation showed almost no change (less than 1%). In light of these findings, an analytical model evaluating the crack arresting feature of cement lines, is proposed to explain the factors that determine crack penetration into osteons or its deflection by cement lines.     

Advances in the fracture mechanics of cortical bone

As cortical bone is a semi-brittle solid, its fracture is dependent not only on the magnitude of the applied stress, but also on the nature of any intrinsic or introduced cracks. Consequently a variety of fracture mechanics techniques have been utilised to evaluate the fracture toughness of cortical bone, including the single edge notched, centre notched cylindrical and compact tension methods, and values have been established for the critical stress intensity factor (Kc) and the critical strain energy release rate (Gc). The Kc and Gc values obtained depend on the orientation of the cortical bone, as well as on bone density, the velocity of crack propagation and specimen geometry. The significance of these fracture mechanics parameters for cortical bone is critically reviewed.     

Novel way of measuring the fracture toughness of leaves and other thin films using a single inclined razor blade

A new test for measuring leaf fracture toughness by cutting with a single inclined razor blade is described here, this having been developed to overcome some of the inadequacies of conventional double-bladed cutting tests, such as scissoring and shearing. The accuracy and precision of this test were determined by measuring the fracture toughness of various leaf types and homogeneous films, and comparing the results with those obtained by scissoring. The new test was found to display a low friction of cutting with great precision in measurements. Fracture toughness measurements of the specimens were considerably lower for the new test than those obtained by scissoring, owing to greater blade sharpness and reduced damage to the specimens during cutting. Despite this, the rankings of fracture toughness measurements for the specimens are similar for both the new test and scissoring, thus demonstrating the test's consistency with scissoring. The new test was found to be successful in measuring the fracture toughness of leaf blades and other thin, film-like materials. It was also able to overcome some of the difficulties of conventional double-bladed cutting tests, especially the estimation of energy expenditure that is extraneous to the work of cutting.     

The effect of the trabecular microstructure on the pullout strength of suture anchors

This study investigates how the microstructural properties of trabecular bone affect suture anchor performance. Seven fresh-frozen humeri were tested for pullout strength with a 5 mm Arthrex Corkscrew in the greater tuberosity, lesser tuberosity, and humeral head. Micro-computed tomography analysis was performed in the three regions of interest directly adjacent to individual pullout experiments. The morphometric properties of bone mineral density (BMD), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbS), trabecular number (TbN), and connectivity density were compared against suture anchor pullout strength. BMD (r=0.64), SMI (r=?0.81), and TbTh (r=0.71) showed linear correlations to the pullout strength of the suture anchor with p-values<0.0001. A predictive model was developed to explain the variances in the individual BMD, SMI, and TbTh correlations. The multi-variant model of pullout strength showed a stronger relationship (r=0.86) compared to the individual experimental results. This study helps confirm BMD is a major influence on the pullout strength of suture anchors, but also illustrates the importance of local microstructure in pullout resistance of suture anchors.     

Bone material properties and mineral matrix contributions to fracture risk or age in women and men

The strength of bone is related to its mass and geometry, but also to the physical properties of the tissue itself. Bone tissue is composed primarily of collagen and mineral, each of which changes with age, and each of which can be affected by pharmaceutical treatments designed to prevent or reverse the loss of bone. With age, there is a decrease in collagen content, which is associated with an increased mean tissue mineralization, but there is no difference in cross-link levels compared to younger adult bone. In osteoporosis, however, there is a decrease in the reducible collagen cross-links without an alteration in collagen concentration; this would tend to increase bone fragility. In older people, the mean tissue age (MTA) increases, causing the tissue to become more highly mineralized. The increased bone turnover following menopause may reduce global MTA, and would reduce overall tissue mineralization. Bone strength and toughness are positively correlated to bone mineral content, but when bone tissue becomes too highly mineralized, it tends to become brittle. This reduces its toughness, and makes it more prone to fracture from repeated loads and accumulated microcracking. Most approved pharmaceutical treatments for osteoporosis suppress bone turnover, increasing MTA and mineralization of the tissue. This might have either or both of two effects. It could increase bone volume from refilling of the remodeling space, reducing the risk for fracture. Alternatively, the increased MTA could increase the propensity to develop microcracks, and reduce the toughness of bone, making it more likely to fracture. There may also be changes in the morphology of the mineral crystals that could affect the homogeneity of the tissue and impact mechanical properties. These changes might have large positive or negative effects on fracture incidence, and could contribute to the paradox that both large and small increases in density have about the same effect on fracture risk. Bone mineral density measured by DXA does not discriminate between density differences caused by volume changes, and those caused by changes in mineralization. As such, it does not entirely reflect material property changes in aging or osteoporotic bone that contribute to bone's risk for fracture.     

High resolution transmission electron microscopy of developing enamel in the Australian lungfish, Neoceratodus forsteri (Osteichthyes: Dipnoi)

The permanent tooth plates of the Australian lungfish, Neoceratodus forsteri, are covered by enamel that develops initially in a similar manner to that of other vertebrates. As the enamel layer matures, it acquires several unusual characteristics. It has radially oriented protoprismatic structures with the long axes of the protoprisms perpendicular to the enamel surface. Protoprisms can be defined as aggregations of hydroxyapatite crystals that lack the highly ordered arrangement of the rods of mammalian enamel but are not without a specific structure of their own. The protoprisms are arranged in layers of variable thickness that are deposited sequentially as the tooth plate grows. They may be confined to the separate layers, or may cross the boundary between each layer. Crystals within the protoprisms are long and thin with hydroxyapatite c-axis dimensions of between 30 and 350 nm, and with typical a-b axis dimensions of 5-10 nm. The hydroxyapatite crystals of lungfish enamel have no centre dark lines of octacalcium phosphate, an unusual character among vertebrates. As each crystal develops, arrays of atoms may change direction, and regions exist where dislocations and extra lattice planes are inserted into the long crystal. The resulting hydroxyapatite crystal is not straight, and has a rough surface. The crystals are arranged in tangled structures with their crystallographic c-axes closely aligned with the long axis of the protoprism. Lungfish enamel differs from the enamel of higher vertebrates in that the hydroxyapatite crystals are of different shape, and, in mature enamel, the protoprisms remain as protoprisms and do not develop into the conventional prismatic structures characteristic of mammalian enamel.     

Effect of temperature on the fracture toughness of compact bone

Bone is a composite composed mainly of organics, minerals, and water. Many researchers have studied effects such as crack velocity, density, orientation, storage media, porosity, and age on the fracture toughness (K(C), also called critical stress intensity factor) of compact bone. Most of these studies were conducted at room temperature. Considering that the body temperature of animals is greater than room temperature, and that bone has a large volumetric percentage of organics and water (generally, 55-65%), it is hypothesized that temperature has a significant effect on the fracture toughness of compact bone. Single-edge V-notched (SEVN) specimens were prepared to measure the fracture toughness of bovine femur and manatee rib in water at 0, 10, 23, 37, and 50 degrees C in four-point flexure. The fracture toughness values of bovine femur and manatee rib were found to decrease from 7.0 to 4.3MPam(1/2) and from 5.5 to 4.0MPam(1/2), respectively, as temperature increased over a temperature range of 50 degrees C. The results support the hypothesis that temperature has a significant effect on the fracture toughness of compact bone. Therefore, we suggest that study on fracture toughness of bone should be done at physiologically relevant temperatures.     

Elastic properties of woven bone: effect of mineral content and collagen fibrils orientation

Woven bone is a type of tissue that forms mainly during fracture healing or fetal bone development. Its microstructure can be modeled as a composite with a matrix of mineral (hydroxyapatite) and inclusions of collagen fibrils with a more or less random orientation. In the present study, its elastic properties were estimated as a function of composition (degree of mineralization) and fibril orientation. A self-consistent homogenization scheme considering randomness of inclusions’ orientation was used for this purpose. Lacuno-canalicular porosity in the form of periodically distributed void inclusions was also considered. Assuming collagen fibrils to be uniformly oriented in all directions led to an isotropic tissue with a Young’s modulus \(E = 1.90\) GPa, which is of the same order of magnitude as that of woven bone in fracture calluses. By contrast, assuming fibrils to have a preferential orientation resulted in a Young’s modulus in the preferential direction of 9–16 GPa depending on the mineral content of the tissue. These results are consistent with experimental evidence for woven bone in foetuses, where collagen fibrils are aligned to a certain extent.