Types Of Osteons Research Articles

Analysis of histomorphometric variables: Proposal and validation of osteon definitions.

Histological analysis of bone tissue has been used to explore a variety of questions relating to age-at-death, habitual behaviors, health, and nutritional stress. Identification of intact and fragmentary osteons is of key interest to many researchers in these studies, yet the definitions of these features vary between researchers making cross-study comparisons problematic. Furthermore, histological variable definitions are often ambiguous or require subjective classifications by the observer. As a result, and as indicated by previous studies, observer error and misclassification of certain variables, namely intact and fragmentary osteons, can be significant. This study proposes new definitions for intact and fragmentary osteons that are designed to limit observer subjectivity and also explore efficacy of combining osteon types into one variable. A sample of 30 6th rib cross-sections from a modern forensic population was used to test the validity of the proposed definitions. Observations of intact osteon population density (OPD(I)) and fragmentary osteon population density (OPD(F)) were made by three observers for each cross-section. These observations were used to explore the interobserver error associated with the proposed definitions and determine if combining variables into one variable (OPD) mitigates persisting classification difficulties. Results indicate that the proposed definitions significantly reduce interobserver error and misclassification of intact and fragmentary osteons. However, the interobserver error associated with fragmentary osteons is still high. Evaluation of the variables independently indicates that combining variables has potential to reduce the predictive strength of an age estimation model and the ability to interpret age-related bone remodeling.

Collagen Fiber Orientation Is Coupled with Specific Nano-Compositional Patterns in Dark and Bright Osteons Modulating Their Biomechanical Properties.

Bone continuously adapts to its mechanical environment by structural reorganization to maintain mechanical strength. As the adaptive capabilities of bone are portrayed in its nano- and microstructure, the existence of dark and bright osteons with contrasting preferential collagen fiber orientation (longitudinal and oblique-angled, respectively) points at a required tissue heterogeneity that contributes to the excellent fracture resistance mechanisms in bone. Dark and bright osteons provide an exceptional opportunity to deepen our understanding of how nanoscale tissue properties influence and guide fracture mechanisms at larger length scales. To this end, a comprehensive structural, compositional, and mechanical assessment is performed using circularly polarized light microscopy, synchrotron nanocomputed tomography, focused ion beam/scanning electron microscopy, quantitative backscattered electron imaging, Fourier transform infrared spectroscopy, and nanoindentation testing. To predict how the mechanical behavior of osteons is affected by shifts in collagen fiber orientation, finite element models are generated. Fundamental disparities between both osteon types are observed: dark osteons are characterized by a higher degree of mineralization along with a higher ratio of inorganic to organic matrix components that lead to higher stiffness and the ability to resist plastic deformation under compression. On the contrary, bright osteons contain a higher fraction of collagen and provide enhanced ductility and energy dissipation due to lower stiffness and hardness.

Osteon Classification in Human Fibular Shaft by Circularly Polarized Light

Pattern of osteonal classification referred to collagen fiber orientation in a human fibula was studied. Along the length of the whole bone, serial 100-µm parallel cross sections were obtained and analyzed by circularly polarized light microscopy. The distribution of 3 different kinds of osteons (transversal, alternated and longitudinal-hooped) depending on their collagen fiber orientation were analyzed to determine the effect of length, anatomical quadrant and cortical thickness. Referring to length, the 3 types of osteons appeared with approximately the same relative frequency, but when the symmetry of the fibula was considered and the length was expressed as distance from the bone mid-section, the alternated osteons are predominant in the mid-section. On the contrary, moving toward the epiphyses, the balance is inverted, and transversal together with longitudinal-hooped types become more frequent than the alternated type. Osteonal pattern distribution was not significantly affected by the anatomical quadrant of each section or by the thickness of the cortex. This observation seems to confirm that along the fibula shaft the orientation of collagen fibers within the osteons is consistent with the loading conditions which the bone undergoes.

ASSESSMENT OF MECHANICAL PROPERTIES OF HUMAN OSTEON LAMELLAE EXHIBITING VARIOUS DEGREES OF MINERALIZATION BY NANOINDENTATION

Background: Cortical bone analysis has been investigated at the macroscopic level with mechanical tests and imaging techniques, but few studies have been done at the microscopic level (osteons). The purpose of this study is to measure the elastic modulus of thick lamellae of osteons exhibiting different degrees of mineralization. This study aims to provide clinicians with a better understanding of bone remodeling and help in assessing the different stages of bone healing. Methods: Six femoral human samples (5 mm × 5 mm × 5 mm) were cut transversally along the length of a human femur. Scanning electron micrographs were produced to reflect the composition of the microstructure. Three types of osteons were selected: white (high mineralization), gray (intermediate mineralization), and dark (low mineralization) osteons. Nanoindentation tests were performed on three locations of the thick lamella located in the middle of each osteon. The mechanical test induced three holdings and unloadings with a constant holding of 10 s. The maximal force was 2500 μN, which induced a maximal depth of about 400 nm. Results: Elastic modulus (E) and hardness (H) for the white (N = 61), gray (N = 17), and dark (N = 39) osteons were Ewhite= 21.30 GPa ± 3.00 GPa and Hwhite= 0.55 GPa ± 0.15 GPa, Egray= 19.27 GPa ± 1.78 GPa and Hgray= 0.41 GPa ± 0.09 GPa, and Edark= 12.95 GPa × 2.66 GPa and Hdark= 0.30 GPa ± 0.10 GPa, respectively. The variation of elastic properties within a lamella was approximately 2.6 GPa, depending on the level of mineralization. Conclusions: These results demonstrate the inhomogeneity of the lamella, suggesting that both the orientation of collagen fibers and the degree of mineralization may vary within the lamella. Our study shows a large range of elastic properties and hardness, reflecting different degrees of osteon mineralization.

Osteon interfacial strength and histomorphometry of equine cortical bone

The interfacial strength of secondary osteons from the diaphysis of the Thoroughbred equine third metacarpal was evaluated using the fiber pushout test. The pushout was performed on 300–500 μm sections of 4×4×15 mm bone blocks machined from four anatomic regions of the cortex. Pushout strength was evaluated from proximal to distal location within the diaphysis on four osteon types classified under polarized light on adjacent histologic sections from each block. The shear strength of the interfaces were estimated from shear lag theory. Differences were found in the interfacial strength of osteons based on appearance under polarized light with bright field having the highest interfacial strength (40.3 MPa). The lowest strength was found in the dark field osteons (22.8 MPa). The dorsal region had the highest shear strength and toughness compared to all other regions. The cement line and interlamellar interfaces are similar in strength, but exhibit regional dependence—specifically, the palmar region strength is less (17.5 MPa) than the osteon interlamellar interfaces (30.4 MPa) and osteon type dependent (alternating significantly weaker than other types). Histomorphometry revealed significant regional differences ( p < 0.0001 ) in osteon area fraction among the four osteon types as well as differences in the osteon diameter ( p = 0.01 ), with dorsal regions having larger osteons (170 μm) than the palmar region (151 μm). Fatigue life and fracture toughness of Haversian bone are reported in the literature to be regionally dependent and are known to be associated with osteon pullout—an osteon interfacial phenomenon. Therefore, the results presented in this study are important to further the understanding of the mechanisms of fragility and damage accumulation in cortical bone.

Morphology of the Drifting Osteon

Drifting osteons were followed longitudinally through the cortex of human and baboon long bones using serial sections. Direction of transverse drift was recorded at different cross-sectional levels of the same systems, and maximum angular change in drift direction was measured for each system. Most drifting osteons exhibit: (1) substantial (∼90°) variation in the direction of transverse drift along their longitudinal axes, (2) intermittent regions of concentric (type I) morphology, and (3) change in drift direction over time, evident at single cross-sectional levels. Additionally, 3-dimensional reconstruction reveals that the basic multicellular units (BMUs) responsible for creating drifting osteons are morphologically distinct from the cutting-cone–closing-cone model BMUs that produce other types of osteons. The stimulus involved in the activation and guidance of drifting BMUs is unclear, but it is likely that the complex strain environment experienced by long bone cortices exerts a significant influence on their morphology.

X-Ray Diffraction on Cyclically Loaded Osteons

The results of a study on the fine structural distortion due to the two previously observed types of degradation in cyclically loaded single osteons (i.e., stiffness degradation and pinching effect) are presented. Fully calcified longitudinal and alternate osteons were isolated from 350-microns-thick longitudinal sections of human femoral cortical bone. The samples were prepared from 500-microns-long central cylindrical portions of an osteon, whose two ends were penetrating into rectangular lugs for fixation to an electromechanical device that cyclically loaded the samples. This device was connected to a microwave micrometer and a recorder. The structural distortions induced by cyclic loading were investigated by high- and low-angle X-ray diffraction on conventional and synchrotron radiation sources. Cyclic loading results in a reduction in the degree of orientation of apatite crystallites, especially in longitudinal osteons, in which the most abundant longitudinal lamellae are not protected against buckling by transverse lamellae as they are in alternate osteons. In contrast, the degree of orientation of collagen fibrils does not seem to be affected by cycling loading in the two osteon types, possibly because the disorientation of collagen fibrils is, within limits, a reversible process. Finally, the contrast between the disorientation of inorganic crystallites and the apparently unaltered distribution of collagen fibrils suggests that the degradation of cyclically loaded osteons may be due to a separation of the crystallites from the fibrils.

Micromechanics of osteonal cortical bone fracture.

Microcracks have been associated with age-related bone tissue fragility and fractures. The objective of this study was to develop a simple osteonal cortical bone model and apply linear elastic fracture mechanics theory to understand the micromechanics of the fracture process in osteonal cortical bone and its dependence on material properties. The linear fracture mechanics of our composite model of cortical bone, consisting of an osteon and interstitial bone tissue, was characterized in terms of a stress intensity factor (SIF) near the tip of a microcrack. The interaction between a microcrack and an osteon was studied for different types of osteons and various spacing between the crack and the osteon. The results of the analysis indicate that the fracture mechanics of osteonal cortical bone is dominated by the modulus ratio between the osteon and interstitial bone tissue: A soft osteon promotes microcrack propagation toward the osteon (and cement line) while a stiff one repels the microcrack from the osteon (and cement line). These findings suggest that newly formed, low-stiffness osteons may toughen cortical bone tissue by promoting crack propagation toward osteons. A relatively accurate empirical formula also was obtained to provide an easy estimation of the influence of osteons on the stress intensity factor.

Pinching in longitudinal and alternate osteons during cyclic loading

Pinching is a degrading phenomenon which occurs during cyclic loading of certain materials. A change in the slope of the deflection curve reveals pinching; lesions, either flexural cracks or bond degradation, cause pinching. This paper investigates pinching for 20 longitudinal and 18 alternate fully calcified osteonic samples of cylindrical shape and 500 μm length. Each sample was axially loaded beyond the proportional limit using an electromechanical device acting as a transducer of the variations in length of the sample into changes in the resonance frequency of a microwave micrometer. A cubic polynomial served as a mathematical model to investigate the stress-strain diagrams at the first and last cycles through the study of strain limits, stiffness and pinching behaviors, and energy absorption. The hysteretic behavior of the two types of osteons differs and is far from ideal. The presence of pinching may derive from the existence of longitudinal fibrils, in particular the yielding of the incompletely calcified ones. In longitudinal osteons consisting mainly of longitudinal collagen fibrils, the deformation under compression is not protected by lamellae consisting of transverse fibrils, therefore the lesions inducing pinching are magnified. In contrast, in alternate osteons, where the fibrils having a longitudinal orientation are reduced and protected by lamellae containing transversely oriented fibrils, the lesions-inducing pinching are lessened.

Structural changes with aging in cortical bone of the human tibia.

Structural modifications are considered to play a significant role in the age-related alterations of bone quality and strength. Senescent compact bone is characterized by an increasing heterogeneity of aspects, including high numbers of lowly mineralized osteons as well as the presence of osteons with hypermineralized lamellae or with a notched haversian canal wall, and of double-zone osteons. These latter three types of osteons are different from the structures involved in the haversian remodeling. In the present study, blocks of midshaft tibia from 7 young men (18-39 years), 14 aged men (50-92 years) and 15 aged women (57-96 years) were embedded in methyl methacrylate in order to perform microradiographic and histomorphometric analysis of undecalcified sections. The intracortical porosity was higher in the aged men than in the young ones, as were the numbers of haversian structures and, to a lesser extent, the diameters of the haversian canals. The aged women showed the same tendency, with cortical porosity still higher than in the men. The osteons with hypermineralized lamellae, those with a notched canal and the double-zone osteons appear to constitute large subgroups of the total haversian population, even in the early adult life. Among them, only the osteons with a notched canal wall increased in frequency with age. The 3 types are much more numerous than the structures involved in the typical haversian remodeling. The correlations between their frequencies as well as their significant topographic association corroborates the hypothesis that the hypermineralized lamellae may crumble down because of their excessive brittleness, giving rise to the haversian canals with notched walls. These enlarged canals could be refilled by bone apposition and result in the double-zone osteons. The 3 types of osteons could constitute different steps of one mechanism of bone desintegration and repair occurring very progressively, which might contribute to modify the bone quality and to increase the intracortical porosity.

The torsional properties of single selected osteons

The aim of the present investigation is to determine the mechanical behavior of single selected osteons loaded by torsion along their axis. Two osteon types were chosen: (a) fully calcified ‘longitudinal’ osteons, whose fibers (and, consequently, crystallites) have a marked longitudinal spiral course in successive lamellae, (b) fully calcified ‘alternate’ osteons, whose fibers (and, consequently, crystallites) have a marked longitudinal spiral course in one lamella, and an apparently transverse or circular course in the next. The osteon samples, which were cylindrically shaped, and whose extremities fitted into two rectangular lugs, were prepared on a microturning lathe. The principle of the specially manufactured torsional device was that of a pendulum set oscillating around a very sharp wedge so as to reduce friction to negligible levels. The results indicate that longitudinal osteons are those most resistant to torsional loading. The low resistance to torsional loading of the lamellae whose fibers apparently have an almost transversal course is discussed with reference to recent views on the microstructure of this type of lamella.

The bending properties of single osteons

The bending properties of two fully calcified osteon types (longitudinal and alternate) have been investigated in 62 cylindrical samples by applying the technique of three-point bending loading. The bending of each sample was recorded using a microwave micrometer based on the cavity and pulse technique. It has been shown that alternate osteons are better able to withstand bending stress than longitudinal ones. This result offers a definitive explanation for the high concentration of transverse lamellae in pathologically bowed bone shaft.

Collagen orientation in compact bone: I. A new method for the determination of the proportion of collagen parallel to the plane of compact bone sections

Previous work has shown that different types of osteons that have different mechanical properties with regard to tension and compression can be identified and studied by polarized light microscopy. This study reports the development of an automated method for the analysis of that proportion of collagen in transverse sections of human femurs that lies parallel with the plane of section. The method is based upon the use of circularly polarized light to detect collagen with a major component lying in the plane of the section, correcting the measurement for bright collagen under these circumstances with a corresponding darkfield measurement to determine the area of bone within each field of the section analyzed. The method has been validated in several different ways. The measured value of bright collagen under circularly polarized light depends upon the section thickness, since a linear relationship was found between section thickness and bright areas for a large number of microscopic fields. Plane parallel sections are therefore recommended for this study. The pattern of distribution of bright areas was the same for different observers and different set detection levels in any one section. The pattern found in adjacent sections from the same region of the shaft was identical.

Orientation of apatite in single osteon samples as studied by pole figures.

The X-ray diffraction method based on pole figures has been applied to single osteon samples in order to obtain information about the texture of the inorganic bone fraction and the way it changes during calcification. The osteon samples were cylindrically shaped, with axes corresponding to those of the haversian canals. Selection was carried out according to the degree of calcification and the orientation of collagen bundles and inorganic particles. Osteons at both the initial and final stages of calcification were chosen. Arrangements of fiber bundles and inorganic particles in successive lamellae characteristic of three types of osteons were selected: longitudinal, alternate, and transversal. The results indicate that in all three types of osteons, the long axis of the sample is apparently the only direction of orientation because the transversally oriented crystallites give an isotropic diffuse scattering as would be expected if all the inorganic particles were irregularly oriented around the osteon axis. The number of longitudinally oriented crystallites increases progressively from transversally oriented osteons to alternately and longitudinally oriented ones. The crystallite orientation in an axial direction increases in fully calcified osteons. This last result is in agreement with the electron microscopic finding that the long needle-shaped crystallites covering much more than a major collagen period and measuring 40–45 A in width increase in number as calcification proceeds.

Relationship Between Ultrastructure and ???Pin Test??? in Osteons

In the 3 types commonly recognizable under the polarizing microscope (longitudinally, alternately and transversally structured osteons), the lamellar appearance of osteons in electron micrographs (EM) is due to the presence of layers of paralleloriented collagen bundles, with the orientation of the bundles changing abruptly through about 90 degrees in successive layers. On the EM scale, lamellae appear as completely independent entities. All 3 osteon types can be considered as consisting of a series of complete or incomplete lamellae. The quantities of collagen bundles in each of the 2 types of lamella--those with longitudinally or concentrically oriented bundles--is a distinctive feature determining the specific appearance of each type of osteon. This view has interesting implications as regards the nature of the factors which instruct and regulate osteoblasts in determining the cross-parallel arrangement of lamellar bone, and the specific characteristics of the various types of osteon. The mechanical properties of osteons are closely related to their structure. The highest values for ultimate dilating strength were obtained with transversally and alternately structured osteons, and the lowest values with longitudinally structured ones.

Relation of collagen fiber orientation to some mechanical properties of human cortical bone

The percentage of dark, light, and intermediate osteons and their fragments, as seen in polarized light, was determined for 53 cross-sections of embalmed, adult human bone. Dark osteons have collagen fibers predominantly parallel with the long axis of the osteon while in light osteons the fibers are at a sharp angle to the long axis of the osteon. Intermediate osteons are mixed. Significant positive correlations were found between dark osteons alone, as well as when combined with their fragments, and the single shearing strength. Intermediate osteons alone, and when combined with their fragments, showed significant positive correlations with the (1) single shearing strength, (2) modulus of elasticity, and (3) percent elongation. Significant negative correlations were found between light osteons alone, and when combined with their fragments, and the (1) single shearing strength and (2) modulus of elasticity. No other significant correlations were found among the various types of osteons alone, or when combined with their fragments, and the mechanical properties of the specimens.

A TETRACYCLINE-BASED EVALUATION OF THE RELATIVE PREVALENCE AND INCIDENCE OF FORMATION OF SECONDARY OSTEONS IN HUMAN CORTICAL BONE

In mineralized sections of 160 adult human bones, the relative prevalence of three major kinds of secondary osteons was compared with the relative incidence with which they were formed, the latter with the aid of bones that had been marked in vivo with tetracycline antibiotics. Relative prevalence and incidence were similar, implying that the biological lifetimes of the three osteon types are similar. Unremodeled secondary osteons were about 90% of the total osteon population, and also of the labelled-bone forming sites. Each of the 822 tetracycline-labelled foci of bone formation that were found inside the cortices of various bones belonged to one of the three osteon types that were studied, and no other kinds of labelled sites were observed. This implies that osteonal remodeling is the major mode of osteoblastic activity inside of cortical bone.