Increased cochlear otic capsule thickness and intracortical canal porosity in the oim mouse model of osteogenesis imperfecta.

Osteogenesis imperfecta (OI or brittle bone disease) is a group of genetic disorders of the connective tissues caused mainly by mutations in the genes encoding collagen type I. Clinical manifestations of OI include skeletal fragility, bone deformities, and severe functional disabilities, such as hearing loss. Progressive hearing loss, usually beginning in childhood, affects approximately 70% of people with OI with more than half of the cases involving the inner ear. There is no cure for OI nor a treatment to ameliorate its corresponding hearing loss, and very little is known about the properties of OI ears. In this study, we investigate the morphology of the otic capsule and the cochlea in the inner ear of the oim mouse model of OI. High-resolution 3D images of 8-week old oim and WT inner ears were acquired using synchrotron microtomography. Volumetric morphometric measurements were conducted for the otic capsule, its intracortical canal network and osteocyte lacunae, and for the cochlear spiral ducts. Our results show that the morphology of the cochlea is preserved in the oim ears at 8 weeks of age but the otic capsule has a greater cortical thickness and altered intracortical bone porosity, with a larger number and volume density of highly branched canals in the oim otic capsule. These results portray a state of compromised bone quality in the otic capsule of the oim mice that may contribute to their hearing loss.

DMP1 Ablation in the Rabbit Results in Mineralization Defects and Abnormalities in Haversian Canal/Osteon Microarchitecture.

DMP1 (dentin matrix protein 1) is an extracellular matrix protein highly expressed in bones. Studies of Dmp1 knockout (KO) mice led to the discovery of a rare autosomal recessive form of hypophosphatemic rickets (ARHR) caused by DMP1 mutations. However, there are limitations for using this mouse model to study ARHR, including a lack of Haversian canals and osteons (that occurs only in large mammalian bones), high levels of fibroblast growth factor 23 (FGF23), and PTH, in comparison with a moderate elevation of FGF23 and unchanged PTH in human ARHR patients. To better understand this rare disease, we deleted the DMP1 gene in rabbit using CRISPR/Cas9. This rabbit model recapitulated many features of human ARHR, such as the rachitic rosary (expansion of the anterior rib ends at the costochondral junctions), moderately increased FGF23, and normal PTH levels, as well as severe defects in bone mineralization. Unexpectedly, all DMP1 KO rabbits died by postnatal week 8. They developed a severe bone microarchitecture defect: a major increase in the central canal areas of osteons, concurrent with massive accumulation of osteoid throughout all bone matrix (a defect in mineralization), suggesting a new paradigm, where rickets is caused by a combination of a defect in bone microarchitecture and a failure in mineralization. Furthermore, a study of DMP1 KO bones found accelerated chondrogenesis, whereas ARHR has commonly been thought to be involved in reduced chondrogenesis. Our findings with newly developed DMP1 KO rabbits suggest a revised understanding of the mechanism underlying ARHR. © 2019 American Society for Bone and Mineral Research.

Prevalent role of porosity and osteonal area over mineralization heterogeneity in the fracture toughness of human cortical bone.

Changes in the distribution of bone mineralization occurring with aging, disease, or treatment have prompted concerns that alterations in mineralization heterogeneity may affect the fracture resistance of bone. Yet, so far, studies assessing bone from hip fracture cases and fracture-free women have not reached a consensus on how heterogeneity in tissue mineralization relates to skeletal fragility. Owing to the multifactorial nature of toughening mechanisms occurring in bone, we assessed the relative contribution of heterogeneity in mineralization to fracture resistance with respect to age, porosity, and area fraction of osteonal tissue. The latter parameters were extracted from quantitative backscattered electron imaging of human cortical bone sections following R-curve tests of single-edge notched beam specimens to determine fracture toughness properties. Microstructural heterogeneity was determined as the width of the mineral distribution (bulk) and as the sill of the variogram (local). In univariate analyses of measures from 62 human donors (21 to 101 years), local but not bulk heterogeneity as well as pore clustering negatively correlated with fracture toughness properties. With age as covariate, heterogeneity was a significant predictor of crack initiation, though local had a stronger negative contribution than bulk. When considering all potential covariates, age, cortical porosity and area fraction of osteons explained up to 50% of the variance in bone׳s crack initiation toughness. However, including heterogeneity in mineralization did not improve upon this prediction. The findings of the present work stress the necessity to account for porosity and microstructure when evaluating the potential of matrix-related features to affect skeletal fragility.

Multi-level characterization of human femoral cortices and their underlying osteocyte network reveal trends in quality of young, aged, osteoporotic and antiresorptive-treated bone.

Characterization of bone's hierarchical structure in aging, disease and treatment conditions is imperative to understand the architectural and compositional modifications to the material and its mechanical integrity. Here, cortical bone sections from 30 female proximal femurs - a frequent fracture site - were rigorously assessed to characterize the osteocyte lacunar network, osteon density and patterns of bone matrix mineralization by backscatter-electron imaging and Fourier-transform infrared spectroscopy in relation to mechanical properties obtained by reference-point indentation. We show that young, healthy bone revealed the highest resistance to mechanical loading (indentation) along with higher mineralization and preserved osteocyte-lacunar characteristics. In contrast, aging and osteoporosis significantly alter bone material properties, where impairment of the osteocyte-lacunar network was evident through accumulation of hypermineralized osteocyte lacunae with aging and even more in osteoporosis, highlighting increased osteocyte apoptosis and reduced mechanical competence. But antiresorptive treatment led to fewer mineralized lacunae and fewer but larger osteons signifying rejuvenated bone. In summary, multiple structural and compositional changes to the bone material were identified leading to decay or maintenance of bone quality in disease, health and treatment conditions. Clearly, antiresorptive treatment reflected favorable effects on the multifunctional osteocytic cells that are a prerequisite for bone's structural, metabolic and mechanosensory integrity.

Shear deformation and fracture of human cortical bone.

Bone can be viewed as a nano-fibrous composite with complex hierarchical structures. Its deformation and fracture behaviors depend on both the local structure and the type of stress applied. In contrast to the extensive studies on bone fracture under compression and tension, there is a lack of knowledge on the fracture process under shear, a stress state often exists in hip fracture. This study investigated the mechanical behavior of human cortical bone under shear, with the focus on the relation between the fracture pattern and the microstructure. Iosipescu shear tests were performed on notched rectangular bar specimens made from human cortical bone. They were prepared at different angles (i.e. 0°, 30°, 60° and 90°) with respect to the long axis of the femoral shaft. The results showed that human cortical bone behaved as an anisotropic material under shear with the highest shear strength (~50MPa) obtained when shearing perpendicular to the Haversian systems or secondary osteons. Digital image correlation (DIC) analysis found that shear strain concentration bands had a close association with long bone axis with an average deviation of 11.8° to 18.5°. The fracture pattern was also greatly affected by the structure with the crack path generally following the direction of the long axes of osteons. More importantly, we observed unique peripheral arc-shaped microcracks within osteons, using laser scanning confocal microscopy (LSCM). They were generally long cracks that developed within a lamella without crossing the boundaries. This microcracking pattern clearly differed from that created under either compressive or tensile stress: these arc-shaped microcracks tended to be located away from the Haversian canals in early-stage damaged osteons, with ~70% developing in the outer third osteonal wall. Further study by second harmonic generation (SHG) and two-photon excitation fluorescence (TPEF) microscopy revealed a strong influence of the organization of collagen fibrils on shear microcracking. This study concluded that shear-induced microcracking of human cortical bone follows a unique pattern that is governed by the lamellar structure of the osteons.

Micro-morphological properties of osteons reveal changes in cortical bone stability during aging, osteoporosis, and bisphosphonate treatment in women.

SUMMARY: We analyzed morphological characteristics of osteons along with the geometrical indices of individual osteonal mechanical stability in young, healthy aged, untreated osteoporotic, and bisphosphonate-treated osteoporotic women. Our study revealed significant intergroup differences in osteonal morphology and osteocyte lacunae indicating different remodeling patterns with implications for fracture susceptibility. INTRODUCTION: Bone remodeling is the key process in bone structural reorganization, and its alterations lead to changes in bone mechanical strength. Since osteons reflect different bone remodeling patterns, we hypothesize that the femoral cortices of females under miscellaneous age, disease and treatment conditions will display distinct osteonal morphology and osteocyte lacunar numbers along with different mechanical properties. METHODS: The specimens used in this study were collected at autopsy from 35 female donors (young group, n = 6, age 32 ± 8 years; aged group, n = 10, age 79 ± 9 years; osteoporosis group, n = 10, age 81 ± 9 years; and bisphosphonate group, n = 9, age 81 ± 7 years). Von Kossa-modified stained femoral proximal diaphyseal sections were evaluated for osteonal morphometric parameters and osteocyte lacunar data. Geometrical indices of osteonal cross-sections were calculated to assess the mechanical stability of individual osteons, in terms of their resistance to compression, bending, and buckling. RESULTS: The morphological assessment of osteons and quantification of their osteocyte lacunae revealed significant differences between the young, aged, osteoporosis and bisphosphonate-treated groups. Calculated osteonal geometric indices provided estimates of the individual osteons' resistance to compression, bending and buckling based on their size. In particular, the osteons in the bisphosphonate-treated group presented improved osteonal geometry along with increased numbers of osteocyte lacunae that had been formerly impaired due to aging and osteoporosis. CONCLUSIONS: The data derived from osteons (as the basic structural units of the cortical bone) in different skeletal conditions can be employed to highlight structural factors contributing to the fracture susceptibility of various groups of individuals.

Analysis of micro fracture in human Haversian cortical bone under compression.

A procedure to investigate local stress intensity factors in human Haversian cortical bone under compression is presented. The method combines a customised experimental setting for micro-compression tests of millimetric bone specimens and a finite element contact model conforming to the bone morphology that tracks advancing microcracks. The non-interpenetration conditions along the crack edges are ensured by penalty constraints of which the parameters are optimised for minimum contact pressure error with respect to the crack orientations. A cohesive crack opening law is implemented in the wake of the crack tips to remain consistent with the progressive tearing of collagen fibrils. The displacement solution is searched by a Newton-Raphson scheme containing a double loop first on the displacements and second on the frictional contact and cohesive condition updates at the crack interfaces. The experimental Dirichlet boundary conditions are acquired by digital image cross-correlation of bone light microscopy observations and then imported into the model. The local mechanical elastic moduli are measured by nanoindentation and microextensometry. The comparison of the macroscopic stress-strain numerical response with the experiment reveals the existence of narrow diffuse damaged zones near the major cracks where the local stress intensity factors can be calculated.

Distribution of microcrack lengths in bone in vivo and in vitro.

It is well known that bone contains small cracks; in vivo these microcracks are constantly growing and being repaired. Too rapid crack growth leads to stress fractures or fragility fractures. In vitro, changes occur in this population of microcracks when subjected to cyclic loading up to and including failure. Normally, the only parameters reported from such investigations are the number density of cracks and their average length. In the present work we examined the microcrack population in more detail. We analysed ten different sets of experimental data including in vivo and in vitro microcracks, plus two theoretical simulations. We showed for the first time that the distribution of crack lengths can be described using the two-parameter Weibull equation. The values of the two constants in the equation varied depending on bone type/species and showed consistent trends during in vitro testing. This is the most detailed study to be conducted on microcrack populations in bone; the results will be useful in future studies including the development of theoretical models and computer simulations of bone damage and failure.

Preliminary study of bipolar hip prosthesis - influence of acetabular bone interactions on bone morphology.

Periprosthetic bone changes following hip arthroplasty are yet to be completely described. The material consisted of imagistic records (X-ray films, CT and MRI scans) and of acetabular bone tissue sampled from 14 cases with femoral head prosthesis and revision of the prosthesis fixed and decalcified in Duboscq-Brazil solution and stained with Hematoxylin and Eosin, trichrome van Gieson and trichrome Masson. Acetabular bone is home of a great variety of morphological changes that can be divided in degenerative and regenerative changes seen in both compact and trabecular components but only inside the maximal pressure area of the acetabular roof. Our preliminary morphological study revealed the existence of an adaptation effort to the mechanical stress materialized through a dynamic process of bone remodeling in the maximal pressure area.

Concentrations of selected heavy metals in bones and femoral bone structure of bank (Myodes glareolus) and common (Microtus arvalis) voles from different polluted biotopes in Slovakia.

Concentrations of selected heavy metals in the femora and femoral bone structure of bank (Myodes glareolus) and common (Microtus arvalis) voles from different polluted biotopes in Slovakia (Kolínany and Nováky sites) were investigated. Length, weight, and histological structure of vole bones were also analyzed. We observed higher concentrations of lead (Pb), iron (Fe), copper (Cu), and zinc (Zn) in the bones of both species from the Kolínany site. Significant differences were observed in the concentration of Fe in bank and common voles (p<0.05) and in the concentration of Zn (p<0.05) in common voles. The animals from Nováky had higher concentrations of cadmium (Cd) and nickel (Ni) in their bones; however, the differences were not significant. The measured values for bone length and weight were higher in both species from Nováky (p<0.05). We did not identify differences in qualitative histological characteristics of the femora between the voles (M. glareolus and M. arvalis separately) between the two biotopes. In addition, no statistically significant differences for any the measured variables of primary osteons' vascular canals were observed. Correlation analysis in M. glareolus showed a strong positive relation between Cd and Ni (r=0.52), Pb and bone weight (r=0.53), Fe and bone weight (r=0.52), and Fe and perimeter size of primary osteons' vascular canals (r=0.55). In common voles, a strong positive relation was found between Fe and Cu (r=0.60) and between Fe and perimeter size of vascular canals of primary osteons (r=0.55). Our results indicate that accumulation of some heavy metals is slightly increased in the femora of both species at Kolínany.

Dependence of bone yield (volume of bone formed per unit of cement surface area) on resorption cavity size during osteonal remodeling in human rib: implications for osteoblast function and the pathogenesis of age-related bone loss.

It is both a necessary and a sufficient condition for bone to be lost with age at any surface location that during remodeling the replacement of resorbed bone is incomplete. In both the ilium and the rib, the degree of such focal imbalance is smaller on the intracortical than on the endocortical or cancellous surfaces that are adjacent to bone marrow. The reason for this difference is unknown. To further examine this question, we measured various geometric variables in 1263 osteons in rib cross sections from 65 persons, including both sexes and age ranges 20 to 30 years and 60 to 70 years (four groups). Haversian canal (HC) area did not differ significantly between sexes or age groups. Percent osteonal refilling was close to 95% in all groups and did not differ between sexes but fell slightly with age. There was a very highly significant linear relationship between osteon bone area and (osteon area + HC area) in all groups, with coefficients of determination (r(2)) greater than 0.98. The regression slopes declined slightly with age in women but not in men. There was a very highly significant quadratic relationship between osteon bone area and osteon perimeter in all groups, with r(2) values greater than 0.97. The ratio osteon bone area:osteon perimeter, an index of bone yield--the volume of bone deposited on each unit area of cement surface--was strongly related to osteon area and did not differ between sexes but was slightly less in the older groups. We conclude the following: (1) The high efficiency of intracortical remodeling in the rib is confirmed, with only trivial effects of age. (2) For HC area to be maintained within narrow limits and bone balance preserved, either initial osteoblast density or osteoblast capacity (the two determinants of bone yield) or, most likely, both must increase progressively with the size of the resorption cavity, suggesting that osteoblast recruitment (relative to available surface) and osteoblast lifespan increase with the volume of bone resorbed. (3) Intracortical remodeling in the rib is more efficient than marrow-adjacent remodeling at any site, possibly because of the different relationships to the circulation. In osteonal remodeling, all molecules released from resorbed bone must travel past the sites of osteoblast recruitment and operation, but in hemiosteonal remodeling, some molecules may not be subject to this constraint. (4) If marrow-adjacent remodeling became as efficient as rib intracortical remodeling, age-related bone loss would cease to be an important medical problem.

Cortical bone mineralization differences between hip-fractured females and controls. A microradiographic study.

The strength of bone depends on both bone quantity and bone quality. One determinant of bone quality is the degree of mineralization of bone tissue (DMB). To assess the role for DMB in osteoporotic hip fractures, we compared the degree of mineralization in femoral neck cortex from 23 women with hip fractures (age, 65-96 years) and 14 female controls (age, 75-103 years) using quantitative microradiography calibrated with an aluminum step wedge. Variables were DMB in osteons (oDMB(Al)mean) and interstitial tissue (exDMB(Al)mean). Wilcoxon signed-rank tests were used to compare oDMB(Al)mean to exDMB(Al)mean in each group, and Mann-Whitney tests to compare oDMB(Al)mean and exDMB(Al)mean between hip-fracture patients and controls. DMB was significantly lower in the osteons than in the interstitial tissue in both groups (hip-fracture group, P=0.000; control group, P=0.001). DMB values in osteons and interstitial tissue were significantly greater in the hip-fracture patients than in the controls (P=0.007 and P=0.005, respectively). These cross-sectional data suggest that bone fragility may be related to a higher degree of tissue mineralization.

Haversian cortical bone model with many radial microcracks: an elastic analytic solution.

In this study, the fracture micromechanics of Haversian cortical bone has been considered. To this effect, a two-dimensional micromechanical fibre-ceramic matrix composite tissue materials model has been presented. The interstitial tissue was modeled as a matrix and the osteon was modeled as a fibre, followed by the application of linear elastic fracture mechanics theory. The solution for edge dislocations, in terms of Green's functions, was adopted to formulate a system of singular integral equations for the radial microcracks in the matrix in vicinity of the osteon. The problem was solved for various configurations and the corresponding stress intensity factors were computed. The results of this study indicated that the interaction between microcracks and an osteon was limited to vicinity of the osteon. Furthermore, the effect of microstructure morphology and heterogeneity on the fracture behavior has been established. The interactions between microcracks were also analyzed for various configurations. These selected configurations exhibited the effects of stress amplification and stress shielding.

Indentation properties of young and old osteons.

The purpose of this study was to quantify differences in indentation modulus and microhardness between labeled osteons identified by epifluorescent microscopy and neighboring unlabeled osteons. In microradiographs and backscattered images, newly formed osteons appear more radiolucent (darker) than older osteons. This is ascribed to incomplete mineralization of the osteon. However, the mechanical properties of these young osteons are unknown. Nine femoral cross-sectional specimens were obtained from five skeletally mature dogs. Prior to death, the dogs received a pair of calcein bone labels. Labeled osteons were identified under an epiflourescent microscope. Bone specimens were transferred to a nanoindenter specimen holder, and the previous identified labeled osteons were located. Labeled (n = 102) and unlabeled (n = 101) osteons were examined by instrumented indentation testing. Indents were made to a depth of 500 nm at a loading rate of 10 nm/second. There were significant differences in the indentation modulus (P < 0.001) of labeled (10.02 +/- 3.61 gigapascal (GPa), mean +/- standard deviation) and unlabeled (15.11 +/- 3.72 GPa) osteons. Similar differences existed in microhardness measurements. Newly formed osteons had lower modulus (34%) and hardness (41%) than older osteons found in femoral cross sections. These data provide information on the indentation moduli of osteons during an early phase of mineralization compared to osteons that have completed mineralization.

Peri-implant bone organization under immediate loading state. Circularly polarized light analyses: a minipig study.

BACKGROUND: Immediate loading of dental implants is currently one of the most examined topics in implant dentistry. Using screw implants with a microstructured surface and bone-quality-adapted insertion procedures, osseointegration is achieved when implants are initially stable and when splinted with the superstructure. Despite reported success, there is a shortage of information relating to remodeling and peri-implant bone formation with immediately loaded implants. METHODS: Four to six immediately loaded and unloaded dental implants with a microstructured surface were placed in the mandible and the maxilla in seven minipigs. A total of 85 implants were placed. After a 4-month healing period, all implants were retrieved. Histomorphometry was performed using a light microscope in transmitted polarized light connected to a high-resolution video camera interfaced to a monitor and personal computer. This optical system was associated with a digitizing pad and a histomorphometry software package with image capturing capabilities. RESULTS: Implants showed osseointegration if the average insertion torque of the implants within one bridge was >35 Ncm. If the primary stability of the bridge was <35 Ncm, all implants in the quadrant were lost after 4 months. The multivariate discriminant analysis showed the highest correlation for implant stability by bridge insertion torque (BIT), localization (mandible or maxilla), and implant insertion torque (IIT) as success parameters. The loaded implants displayed collagen fibers, which were oriented in a more transverse way. In addition, a higher quantity of secondary osteons was present. In comparison, the unloaded implants had collagen fibers with a more parallel orientation, and a higher quantity of marrow spaces was present. CONCLUSIONS: When observed after 4 months, immediately loaded implants showed a higher degree of bone formation and remodeling in comparison to unloaded implants. Immediately loaded implants also demonstrated a prevalence of transversely oriented collagen fibers in the peri-implant bone. In this animal model, an average insertion torque of the implants within one bridge>35 Ncm was associated with the most successful implants.

Unexpected ultrastructral changes in bone osteiod collagens in osteogenesis imperfecta.

Osteogenesis Imperfecta (OI) is a heterogeneous, inherited bone disorder usually resulting from a defect in collagen synthesis or function. The Sillence classification recognises four OI subtypes of which type III is the severe, progressively deforming form. Here, we report distinctive ultrastructural abnormalities of bone osteoid collagen fibrils from three patients with OI type III and compared with normal controls. Collagen biochemistry of these patients showed normal alpha1(I) and alpha2(I) chains, despite the structurally abnormal collagen fibrils. The expected lamellar organisation of normal osteoid was absent in the bone biopsies of these patients. In addition their collagen fibrils had frayed edges and no periodicity was observed in most of these fibrils. These collagen fibrils were also flower like, twisted, spiralled and sparsely distributed throughout a very thick osteoid with patchy mineralisation. These structurally abnormal collagens may not be able to provide the nucleating and scaffolding sites for normal mineralisation and may lead to the bone fragility observed in OI.

Reduced osteoblastic population and defective mineralization in osteopetrotic (op/op) mice.

Osteopetrotic (op/op) mice fail to exhibit bone remodeling because of a defective osteoclast formation due to a lack of macrophage colony-stimulating factor. In this study, we investigated the femora of op/op mice to clarify whether the osteoblastic population and bone mineralization are involved in osteoclasts or their bone resorption. The op/op mice extended the meshwork of trabecular bones from the chondro-osseous junction to the diaphyseal region. In the femoral metaphyses of op/op mice, intense alkaline phosphatase (ALPase)-positive osteoblasts were observed on the metaphyseal bone in close proximity to the erosion zone of the growth plates. Von Kossa's staining revealed scattered mineralized nodules and a fine meshwork of mineralized bone matrices while the wild-type littermates developed well-mineralized trabeculae parallel to the longitudinal axis. In contrast to the metaphysis, some op/op diaphyses showed flattened osteoblasts with weak ALPase-positivity, and the other diaphyses displayed bone surfaces without a covering by osteoblasts. It is likely, therefore, that the osteoblastic population and activity were lessened in the op/op diaphyses. Despite the osteopetrotic model, von Kossa's staining demonstrated patchy unmineralized areas in the op/op diaphyses, indicating that a lower population and/or the activity of osteoblasts resulted in defective mineralization in the bone. Transmission electron microscopy disclosed few osteoblasts on the diaphyseal bones, and instead, bone marrow cells and vascular endothelial cells were often attached to the unmineralized bone. Osteocytes were embedded in the unmineralized bone matrix. Thus, osteoclasts appear to be involved in the osteoblastic population and activity as well as subsequent bone mineralization.

Geometric determinants to cement line debonding and osteonal lamellae failure in osteon pushout tests.

Cement lines are the boundaries between secondary osteons and the surrounding interstitial bone matrix in cortical bone. The interfacial properties of cement lines have been determined by osteon pushout tests. However, distinctively different material properties were obtained when osteon pushout tests were performed under different test geometries. In the present study, an axisymmetric two-dimensional finite element model was used to simulate an osteon pushout test using the test geometry of actual experiments. The results indicated that shear failure within the osteonal lamellae would occur when the osteon pushout test was performed under the condition of a thick specimen and large supporting hole. On the other hand, cement line debonding occurred when the osteon pushout test was performed using a thin specimen and small supporting hole. The finite element results were consistent with previous experiments of osteon pushout tests under different test geometries. Furthermore, the finite-element results suggest that a smoothly curved punch would most likely cause debonding at the cement line instead of osteonal lamellae.

Microcrack accumulation at different intervals during fatigue testing of compact bone.

Fatigue damage in bone occurs in the form of microcracks. This microdamage contributes to the formation of stress fractures and acts as a stimulus for bone remodelling. A technique has been developed, which allows microcrack growth to be monitored during the course of a fatigue test by the application of a series of fluorescent chelating agents. Specimens were taken from bovine tibiae and fatigue tested in cyclic compression at a stress range of 80MPa. The specimens were stained before testing with alizarin and up to three other chelating agents were applied during testing to label microcracks formed at different times. Microcracks initiated in interstitial bone in the early part of a specimen's life. Further accumulation of microcracks is then suppressed until the period late in the specimen's life. Microcracks were found to be longer in the longitudinal than in the transverse direction. Only a small proportion of cracks are actively propagating; these are longer than non-propagating cracks. These results support the concept of a microstructural barrier effect existing in bone, whereby cracks initiate easily but slow down or stop at barriers such as cement lines.

Osteon pullout in the equine third metacarpal bone: effects of ex vivo fatigue.

An important concept in bone mechanics is that osteons influence mechanical properties in several ways, including contributing to toughness and fatigue strength by debonding from the interstitial matrix so as to "bridge" developing cracks. Observations of "pulled out" osteons on fracture surfaces are thought to be indicative of such behavior. We tested the hypothesis that osteon pullout varies with mode of loading (fatigue vs. monotonic), cortical region, elastic modulus, and fatigue life. Mid-diaphseal beams from the dorsal, medial, and lateral regions of the equine third metacarpal bone were fractured in four point bending by monotonic loading to failure under deflection control, with or without 10(5) cycles of previous fatigue loading producing 5000 microstrain (15-20% of the expected failure strain) on the first cycle; or sinusoidal fatigue loading to failure, under load or deflection control, with the initial cycle producing 10,000 microstrain (30-40% of the expected failure strain). Using scanning electron microscopy, percent fracture surface area exhibiting osteon pullout (%OP.Ar) was measured. Monotonically loaded specimens and the compression side of fatigue fracture surfaces exhibited no osteon pullout. In load-controlled fatigue, pullout was present on the tension side of fracture surfaces, was regionally dependent (occurring to a greater amount dorsally), and was correlated negatively with elastic modulus and positively with fatigue life. Regional variation in %OP.Ar was also significant for the pooled (load and deflection controlled) fatigue specimens. %OP.Ar was nearly significantly greater in deflection controlled fatigue specimens than in load-controlled specimens (p=0.059). The data suggest that tensile fatigue loading of cortical bone eventually introduces damage that results in osteonal debonding and pullout, which is also associated with increased fatigue life via mechanisms that are not yet clear.