Quantitative 3D analysis of the canal network in cortical bone by micro-computed tomography

Structural analysis of cortical porosity applied to HR-pQCT data

PURPOSE

The investigation of cortical porosity is an important aspect of understanding biological, pathoetiological, and biomechanical processes occurring within the skeleton. With the emergence of HR-pQCT as a noninvasive tool suitable for clinical use, cortical porosity at appendicular sites can be directly visualized in vivo. The aim of this study was to introduce a novel topological analysis of the cortical pore network for HR-pQCT data and determine the influence of resolution on measures of cortical pore network microstructure and topology.

METHODS

Cadaveric radii were scanned using HR-pQCT at two different voxel sizes (41 and 82 μm) and also using μCT at a voxel size of 18 μm. HR-pQCT and μCT image sets were spatially coregistered. Segmentation and quantification of cortical porosity (Ct.Po) and mean pore diameter (Ct.Po.Dm) were achieved using an established extended cortical analysis technique. Topological classification of individual pores was performed using topology-preserving skeletonization and multicolor dilation algorithms. Based on the pore skeleton topological classification, the following parameters were quantified: total number of planar surface-skeleton canals (N.Slabs), tubular curve-skeleton canals (N.Tubes), and junction elements (N.Junctions), mean slab volume (Slab.Vol), mean tube volume (Tube.Vol), mean slab orientation (Slab.θ), mean tube orientation (Tube.θ), N.Slabs/N.Tubes, and integral (total) slab volume/integral tube volume (iSlab.Vol/iTube.Vol). An in vivo reproducibility study was also conducted to assess short-term precision of the topology parameters. Precision error was characterized using root mean square coefficient of variation (RMSCV%).

RESULTS

Correlations to μCT values for Ct.Po were significant for both the 41 and 82 μm HR-pQCT data (41: r(2) = 0.82, p < 0.001, 82: r(2) = 0.75, p < 0.001). For Ct.Po.Dm, only the 41 μm data were significantly predictive of μCT values (r(2) = 0.72, p < 0.01) Data at both HR-pQCT voxel sizes were strongly predictive of the μCT values for N.Slabs (41: r(2) = 0.93, p < 0.001; 82: r(2) = 0.84, p < 0.001), N.Tubes (41: r(2) = 0.94, p < 0.001; 82: r(2) = 0.84, p < 0.001), and N.Junctions (41: r(2) = 0.93, p < 0.001; 82: r(2) = 0.78, p < 0.001), though proportional bias was evident in these correlations. Weak correlations were seen for iSlab.Vol/iTube.Vol at both voxel sizes (41: r(2) = 0.52, p < 0.01; 82: r(2) = 0.39, p < 0.05). Slab.Vol was significantly correlated to μCT data at 41 μm (r(2) = 0.60, p < 0.01) but not at 82 μm, while Tube.Vol was significantly correlated at both voxel sizes (41: r(2) = 0.79, p < 0.001; 82: r(2) = 0.68, p < 0.01). In vivo precision error for these parameters ranged from 2.31 to 9.68 RMSCV%.

CONCLUSIONS

Strong correlations between μCT- and HR-pQCT-derived measurements were found, particularly in HR-pQCT images obtained at 41 μm. These data are in agreement with our previous study investigating the effect of voxel size on standard HR-pQCT metrics of trabecular and cortical microstructure, and extend our previous findings to include topological descriptors of the cortical pore network.

Normal variation in cortical osteocyte lacunar parameters in healthy young males

The most abundant cell in bone, osteocytes form an interconnected system upon which the regulation of healthy bone relies. Although the complete nature of the role of osteocytes has yet to be defined, they are generally accepted to play a part in the sensing of load and the initiation of damage repair. A previous study conducted by our group identified variation of up to 30% in osteocyte lacunar density and morphological parameters between regions of a single cross-section of human femoral shaft; that study, however, was limited to a single individual. The aim of the current study was to determine whether this pattern consistently occurs in healthy young male femora. Anterior, posterior, medial and lateral blocks were prepared from the proximal femoral shaft of seven males and synchrotron radiation micro-CT imaged. Average lacunar densities (± SD) from the anterior, posterior, medial and lateral regions were 23 394 ± 1705, 30 180 ± 4860, 35 946 ± 5990 and 29 678 ± 6081 lacunae per mm(3) of bone tissue, respectively. These values were significantly different between the anterior and both the medial and posterior regions (P < 0.05). The density of the combined anterior and posterior regions was also significantly lower (P = 0.006) than the density of the combined medial and lateral regions. Although no difference was found in predominant orientation, shape differences were found; with the combined anterior-posterior regions having lacunae that were significantly more elongated and less flat than the combined medial-lateral values (P < 0.001). As expected, in this larger study, there was a dramatic difference in lacunar density between the medial and anterior region (up to ~ 54%). The study clearly demonstrates that the high variation seen in osteocyte lacunar density as well as other lacunar parameters, noted in a number of biomechanical, age and pathology studies, are well within the range of normal variation; however, the reasons for and consequences of this variation remain unclear. Lacunar parameters including abundance and shape are being increasingly incorporated into computational modeling of bone biology and this paper represents a more comprehensive description of normal healthy lacunae.

Cortical measurements of the tibia from high resolution peripheral quantitative computed tomography images: a comparison with synchrotron radiation micro-computed tomography

High resolution-peripheral quantitative computed tomography (HR-pQCT) measurements are carried out in clinical research protocols to analyze cortical bone. Micro-computed tomography (micro-CT) is a standard tool for ex vivo examination of bone in 3D. The aim of this work was to evaluate cortical measurements derived from HR-pQCT images compared to those from synchrotron radiation (SR) micro-CT in a distal position (4.2 cm from the distal pilon). Twenty-nine tibia specimens were scanned with HR-pQCT using protocols provided by the manufacturer. The standard measured outcomes included volumetric bone density (gHA/cm(3)) of the cortical region (Dcomp), and the cortical thickness (Ct.Th, mm). New features, such as cortical porosity (Ct.Po) and mean pore diameter (Ct.Po.Dm), were measured by an auto-contouring process. All tibias were harvested from the posterior region and imaged with SR micro-CT (voxel size=7.5 μm). The cortical thickness, (Ct.Thmicro-CT), porosity (PoV/TV), pore diameter, pore spacing, pore number, and degree of mineralization of bone (DMB) were obtained for SR micro-CT images. For standard measurements on HR-pQCT images, site matched analyses with micro-CT were completed to obtain Dcomplocal and Ct.Thlocal. Dcomp was highly correlated to PoV/TV (r=-0.84, p<10(-4)) but not to DMB. Dcomplocal was correlated to PoV/TV (r=-0.72, p<10(-4)) and to DMB (r=0.40, p>0.05). Ct.Thlocal and Ct.Thmicro-CT were moderately correlated (r=0.53, p<0.01). Ct.Th and Ct.Po results from the autocontouring process are influenced by the level of trabecularization of the cortical bone and need manual correction of the endosteal contour. Distal tibia is a reliable region to study cortical bone with Dcomp as the best parameter because it reflects both the micro-porosity (Havers canals) and macro-porosity (resorption lacunae) of the cortical bone.

Modeling microdamage behavior of cortical bone

Bone is a complex material which exhibits several hierarchical levels of structural organization. At the submicron-scale, the local tissue porosity gives rise to discontinuities in the bone matrix which have been shown to influence damage behavior. Computational tools to model the damage behavior of bone at different length scales are mostly based on finite element (FE) analysis, with a range of algorithms developed for this purpose. Although the local mechanical behavior of bone tissue is influenced by microstructural features such as bone canals and osteocyte lacunae, they are often not considered in FE damage models due to the high computational cost required to simulate across several length scales, i.e., from the loads applied at the organ level down to the stresses and strains around bone canals and osteocyte lacunae. Hence, the aim of the current study was twofold: First, a multilevel FE framework was developed to compute, starting from the loads applied at the whole bone scale, the local mechanical forces acting at the micrometer and submicrometer level. Second, three simple microdamage simulation procedures based on element removal were developed and applied to bone samples at the submicrometer-scale, where cortical microporosity is included. The present microdamage algorithm produced a qualitatively analogous behavior to previous experimental tests based on stepwise mechanical compression combined with in situ synchrotron radiation computed tomography. Our results demonstrate the feasibility of simulating microdamage at a physiologically relevant scale using an image-based meshing technique and multilevel FE analysis; this allows relating microdamage behavior to intracortical bone microstructure.

Studying osteocytes within their environment

It is widely hypothesized that osteocytes are the mechano-sensors residing in the bone's mineralized matrix which control load induced bone adaptation. Owing to their inaccessibility it has proved challenging to generate quantitative in vivo experimental data which supports this hypothesis. Recent advances in in situ imaging, both in non-living and living specimens, have provided new insights into the role of osteocytes in the skeleton. Combined with the retrieval of biochemical information from mechanically stimulated osteocytes using in vivo models, quantitative experimental data is now becoming available which is leading to a more accurate understanding of osteocyte function. With this in mind, here we review i) state of the art ex vivo imaging modalities which are able to precisely capture osteocyte structure in 3D, ii) live cell imaging techniques which are able to track structural morphology and cellular differentiation in both space and time, and iii) in vivo models which when combined with the latest biochemical assays and microfluidic imaging techniques can provide further insight on the biological function of osteocytes.

Micro-channels in the mastoid anatomy. Indications of a separate blood supply of the air cell system mucosa by micro-CT scanning

The mastoid air cell system has traditionally been considered to have a passive role in gas exchange and pressure regulation of the middle ear possibly with some acoustic function. However, more evidence has focused on the mucosa of the mastoid, which may play a more active role in regulation of middle ear pressure. In this study we have applied micro-CT scanning on a series of three human temporal bones. This approach greatly enhances the resolution (40-60 μm), so that we have discovered anatomical details, which has not been reported earlier. Thus, qualitative analysis using volume rendering has demonstrated notable micro-channels connecting the surface of the compact bone directly to the mastoid air cells as well as forming a network of connections between the air cells. Quantitative analysis on 2D slices was employed to determine the average diameter of these micro-channels (158 μm; range = 40-440 μm) as well as their density at a localized area (average = 75 cm(-2); range = 64-97 cm(-2)). These channels are hypothesized to contain a separate vascular supply for the mastoid mucosa. However, future studies of the histological structure of the micro-channels are warranted to confirm the hypothesis. Studies on the mastoid mucosa and its blood supply may improve our knowledge of its physiological properties, which may have important implications for our understanding of the pressure regulation of the middle ear. This article is part of a special issue entitled "MEMRO 2012".

3D characterization of pores in the cortical bone of human femur in the elderly at different locations as determined by synchrotron micro-computed tomography images

Diaphysis, inferior, and lateral superior regions of the femoral neck are subjected to diverse mechanical loads. Using micro-CT based on synchrotron radiation, three-dimensional morphology and connectivity of the pore network are location dependent, underlying different remodeling mechanisms.

INTRODUCTION

The three-dimensional (3D) morphology and connectivity of the pore network at various locations in human femurs subjected to diverse mechanical loads were assessed using micro-CT based on synchrotron radiation.

METHODS

The cortex from 20 human femurs (mean age, 78.3 ± 12.4 years) was taken from the diaphysis (D), the inferior (IN), and the lateral superior (LS) regions of the femoral neck. The voxel size of the 3D reconstructed image was 7.5 μm. Cortical thickness and pore volume/tissue volume (Po.V/TV), pore diameter (Po.Dm) and spacing (Po.Sp) were determined. The pore surface/pore volume ratio (Po.S/Po.V), the number of pores (Po.N), the degrees of anisotropy (DA), and the connectivity density (ConnD), the degree of mineralization (DMB) were also determined.

RESULTS

The characteristics of the pore network in femoral cortical bone were found to be location dependent. There was greater porosity, Po.Dm, and Po.N, and more large (180-270 μm), extra-large (270-360 μm) and giant pores (>360 μm) in the LS compared to the IN and D. The difference in porosity in between the periosteal and endosteal layers was mostly due to an increase of Po.Dm rather than Po.N. There was a lower DMB of bone in the LS, which is consistent with a higher remodeling rate.

CONCLUSION

The results provide evidence for large variations in the structure of the internal pore network in cortical bone. These variations could involve different underlying remodeling mechanisms.

The importance of the intracortical canal network for murine bone mechanics

As shown by recent data bone strength estimation can greatly be improved by including microarchitectural parameters in the analysis. Our previous results showed that intracortical canals (the living space of the vasculature and/or remodeling units) are a major contributor to cortical tissue porosity, and therefore, can be linked to mechanical bone properties. Consequently, the goal of this study was to investigate the importance of the intracortical canal network for murine bone mechanics. To study intracortical canals within murine femoral bone, we used a mouse model, including two mouse strains, C57BL/6J-Ghrhr(lit)/J (B6-lit/+) and C3.B6-Ghrhr(lit)/J (C3.B6-lit/+) representing low and high bone mass, respectively. The intracortical canal network was assessed by synchrotron radiation-based micro-computed tomography and the mechanical bone properties were derived from three-point bending experiments. Multiple linear regression models were built to explain the variation in ultimate force, work to fracture, and stiffness in terms of the morphometric parameters. The power to explain the variation in bone mechanics was increased significantly for most mechanical measures when including morphometric parameters of intracortical canals in addition to macroscopic morphometric measures. Specifically, we could derive generalized (mouse strain-independent) models for ultimate force, where the incorporation of intracortical canals in addition to macroscopic bone measures improved the explained variation in ultimate force considerably, which was confirmed by an increase in adjusted R(2) of 73% and 8% for B6-lit/+ and C3.B6-lit/+, respectively. Further, we observed that the heterogeneity of the morphometric measures for the individual canal branches play an important role for explaining the variation in ultimate force. Finally, the current study provides strong evidence that work to fracture of murine bone, which is triggered critically by microcracks, is affected by intracortical canals. In summary, the study suggests that the intracortical canal network is important for bone mechanics.

Age-related changes in the 3D hierarchical structure of rat tibia cortical bone characterized by high-resolution micro-CT

Three-dimensional hierarchical structure of female Sprague-Dawley rat tibia cortical bone was characterized as a function of age (3, 12, 32, 42, 60, and 72 wk) using a high-resolution micro-computed tomography. At the whole bone level, 3-wk samples exhibited statistically significant differences in a mean total tissue volume, mean cortical bone volume, mean cortical bone volume density, mean periosteal perimeter, and mean cortical thickness (P < 0.05) compared with all other ages. At the tissue level, there was a statistically significant increase in a mean canal number density and a decrease in a mean canal volume and diameter between 3-wk and 12-wk samples. While no significant variations were found between mean canal lengths, there was a dependence of mean canal orientation on age. At the cell level, there were no statistically significant differences in a lacuna number density and a lacuna volume density, and all lacunae element-based parameters displayed no dependence on age across age. In addition, at the microstructural level, the cannular indexes were reported separately for anterior, posterior, medial, and lateral anatomic regions. From 3 to 32 wk of age, there existed significantly fewer canals per volume of bone in the medial region of the tibia vs. other cross-sectional quadrants. Although there were changes with age, there were no statistically significant differences in the mean canal volume, mean canal diameter, and mean canal length between the four anatomic regions.

Variation in osteocyte lacunar morphology and density in the human femur--a synchrotron radiation micro-CT study

In recent years there has been growing interest in the spatial properties of osteocytes (including density and morphology) and how these potentially relate to adaptation, disease and aging. This interest has, in part, arisen from the availability of increasingly high-resolution 3D imaging modalities such as synchrotron radiation (SR) micro-CT. As resolution increases, field of view generally decreases. Thus, while increasingly detailed spatial information is obtained, it is unclear how representative this information is of the skeleton or even the isolated bone. The purpose of this research was to describe the variation in osteocyte lacunar density, morphology and orientation within the femur from a healthy young male human. Multiple anterior, posterior, medial and lateral blocks (2 mm × 2 mm) were prepared from the proximal femoral shaft and SR micro-CT imaged at the Advanced Photon Source. Average lacunar densities (± standard deviation) from the anterior, posterior, medial and lateral regions were 27,169 ± 1935, 26,3643 ± 1262, 37,521 ± 6416 and 33,972 ± 2513 lacunae per mm(3) of bone tissue, respectively. These values were significantly different between the medial and both the anterior and posterior regions (p<0.05). The density of the combined anterior and posterior regions was also significantly lower (p=0.001) than the density of the combined medial and lateral regions. Although no difference was found in predominant orientation, shape differences were found; with the combined anterior and posterior regions having more elongated (p=0.004) and flattened (p=0.045) lacunae, than those of the medial and lateral regions. This study reveals variation in osteocyte lacunar density and morphology within the cross-section of a single bone and that this variation can be considerable (up to 30% difference in density between regions). The underlying functional significance of the observed variation in lacunar density likely relates to localized variations in loading conditions as the pattern corresponds well with mechanical axes. Lower density and more elongate shapes being associated with the antero-posterior oriented neutral axis. Our findings demonstrate that the functional and pathological interpretations that are increasingly being drawn from high resolution imaging of osteocyte lacunae need to be better situated within the broader context of normal variation, including that which occurs even within a single skeletal element.

Advances in assessment of bone porosity, permeability and interstitial fluid flow

This contribution reviews recent research performed to assess the porosity and permeability of bone tissue with the objective of understanding interstitial fluid movement. Bone tissue mechanotransduction is considered to occur due to the passage of interstitial pore fluid adjacent to dendritic cell structures in the lacunar-canalicular porosity. The movement of interstitial fluid is also necessary for the nutrition of osteocytes. This review will focus on four topics related to improved assessment of bone interstitial fluid flow. First, the advantages and limitations of imaging technologies to visualize bone porosities and architecture at several length scales are summarized. Second, recent efforts to measure the vascular porosity and lacunar-canalicular microarchitecture are discussed. Third, studies associated with the measurement and estimation of the fluid pressure and permeability in the vascular and lacunar-canalicular domains are summarized. Fourth, the development of recent models to represent the interchange of fluids between the bone porosities is described.

Prolonged unloading in growing rats reduces cortical osteocyte lacunar density and volume in the distal tibia

Bone dynamically adapts its structure to the environmental demands placed upon it. Load-related stimuli play an important role in this adaptation. It has been postulated that osteocytes sense changes in these stimuli and initiate adaptive responses, across a number of scales, through a process known as mechanotransduction. While much research has focused on gross and tissue-level adaptation, relatively little is known regarding the relation between cellular-level features (e.g. osteocyte lacunar density, volume and shape) and loading. The increasing availability of high resolution 3D imaging modalities, including synchrotron-based techniques, has made studying 3D cellular-level features feasible on a scale not previously possible. The primary objective of this study was to test the hypothesis that unloading (sciatic neurectomy) during growth results in altered osteocyte lacunar density in the tibial diaphysis of the rat. Secondarily, we explored a potential effect of unloading on mean lacunar volume. Lacunar density was significantly (p<0.05) lower in immobilized bones (49,642 ± 11,955 lacunae per mm(3); n=6) than in control bones (63,138 ± 1956 lacunae per mm(3); n=6). Mean lacunar volume for immobilized bones (209 ± 72 μm(3); n=6) was significantly smaller (p<0.05) than that for the control bones (284 ± 28 μm(3); n=6). Our results demonstrate that extreme differences in loading conditions, such as those created by paralysis, do indeed result in changes in osteocyte lacunar density and volume. Further investigation is warranted to examine relations between these measures and more subtle variation in loading as well as pathological states, which have been linked to alterations in mechanotransduction.

A comparison between micro-CT and histology for the evaluation of cortical bone: effect of polymethylmethacrylate embedding on structural parameters

Cortical bone microstructure is an important parameter in the evaluation of bone strength. The aim of this study was to validate the characterization of human cortical bone microarchitecture using microcomputed tomography. In order to do this, microcomputed tomography structural measurements were compared with those obtained through histological examination (the gold standard). Moreover, to calculate structural parameters, microcomputed tomography images have to be binarized with the separation between bone and nonbone structures throughout a global thresholding. As the effect of the surrounding medium on the threshold value is not clear, an easy procedure to find the global uniform threshold for a given acquisition condition is applied. This work also compared the structural parameters of microcomputed tomography cortical sample scan in air or embedded in polymethylmethacrylate; histology was used as a reference. For each acquisition condition, a fixed threshold value was found and was applied on the corresponding microcomputed tomography image for the parameters assessment. Twenty cortical bone samples were collected from human femur and tibia diaphyses. All samples were microcomputed tomography scanned in air, embedded in polymethylmethacrylate, rescanned by microcomputed tomography, examined by histology and finally compared. A good correspondence between the microcomputed tomography images and the histological sections was found. Paired comparisons in cortical porosity, Haversian canal diameter and Haversian canal separation between histological sections and microcomputed tomography cross sections, first in air and then embedded in PolyMethylMethAcrylate, were made: no significant differences were found. None of the comparisons showed significant differences for cortical porosity, Haversian canal diameter and Haversian separation over a three-dimensional volume of interest, between microcomputed tomography scans in air and with samples embedded in PolyMethylMethAcrylate. The very good correlation between bone structural measures obtained from microcomputed tomography datasets and from two-dimensional histological sections confirms that microcomputed tomography may be an efficient tool for the characterization of cortical bone microstructure. Moreover, when the corresponding threshold value for each condition is used, structural parameters determined by microcomputed tomography are not affected by the surrounding medium (PolyMethylMethAcrylate).

The effects of immobilization on vascular canal orientation in rat cortical bone

It is well established that bone is capable of adapting to changes in loading; however, little is known regarding how loading specifically affects the internal 3D microarchitecture of cortical bone. The aim of this study was to experimentally test the hypothesis that loading is a determinant of the 3D orientation of primary vascular canals in the rat tibial diaphysis. Left tibiae from 10 rats (30 weeks old) that had been immobilized (sciatic neurectomy) for 27 weeks, right SHAM-operated tibiae from these same rats (internal control) and right tibiae from 10 normal age-matched rats (external control) were scanned by micro-CT. Mean canal orientation (for the whole bone segment and by region), percent porosity, canal diameter and canal separation were quantitatively assessed in 3D. Canal orientation in the immobilized tibiae was significantly (P < 0.001) more radial (by 9.9°) compared to the external controls but did not differ from the internal controls (P = 0.310). Comparing the external and internal controls, orientation was significantly (P < 0.05) more radial in the internal control group (by 6.8°). No differences were found for percent porosity and canal separation. Canal diameter was significantly greater in the immobilized vs. internal (P < 0.001) and external control (P < 0.001) tibiae. The differences in orientation relative to the external controls indicated that the organization of cortical bone in the rat is affected by loading. Although the predicted difference in canal orientation was not detected between immobilized and internal control groups, the distributions of individual canal orientations, from which the mean values were derived, revealed distinctive patterns for all three groups. The internal controls exhibited an intermediate position between the immobilized and external controls, suggesting that paralysis on the contralateral side resulted in altered loading relative to the normal state represented by the external control. This was also evident in a regional analysis by quadrant. The loaded bones had the same cross-sectional shape; however, their internal structure differed. These results provide novel insights into the impact of loading on the 3D organization of primary cortical bone and have implications for understanding the relation between cortical bone adaptation, disease and mechanical properties.

High-resolution computed tomography for clinical imaging of bone microarchitecture

BACKGROUND

The role of bone structure, one component of bone quality, has emerged as a contributor to bone strength. The application of high-resolution imaging in evaluating bone structure has evolved from an in vitro technology for small specimens to an emerging clinical research tool for in vivo studies in humans. However, many technical and practical challenges remain to translate these techniques into established clinical outcomes.

QUESTIONS/PURPOSES

We reviewed use of high-resolution CT for evaluating trabecular microarchitecture and cortical ultrastructure of bone specimens ex vivo, extension of these techniques to in vivo human imaging studies, and recent studies involving application of high-resolution CT to characterize bone structure in the context of skeletal disease.

METHODS

We performed the literature review using PubMed and Google Scholar. Keywords included CT, MDCT, micro-CT, high-resolution peripheral CT, bone microarchitecture, and bone quality.

RESULTS

Specimens can be imaged by micro-CT at a resolution starting at 1 μm, but in vivo human imaging is restricted to a voxel size of 82 μm (with actual spatial resolution of ~ 130 μm) due to technical limitations and radiation dose considerations. Presently, this mode is limited to peripheral skeletal regions, such as the wrist and tibia. In contrast, multidetector CT can assess the central skeleton but incurs a higher radiation burden on the subject and provides lower resolution (200-500 μm).

CONCLUSIONS

CT currently provides quantitative measures of bone structure and may be used for estimating bone strength mathematically. The techniques may provide clinically relevant information by enhancing our understanding of fracture risk and establishing the efficacy of antifracture for osteoporosis and other bone metabolic disorders.

Serial FIB/SEM imaging for quantitative 3D assessment of the osteocyte lacuno-canalicular network

Up to now, a quantitative three-dimensional (3D) assessment of the lacuno-canalicular network (LCN) within bone has not been achieved in a comprehensive way and the LCN has mostly been investigated using two-dimensional imaging methods only. First attempts for the 3D assessment of the osteocytes and their cell processes have been reported using different imaging techniques. Nevertheless, various experimental limitations allowed for assessment of isolated or incompletely interconnected osteocytes only. On the other hand, serial focused ion beam/scanning electron microscopy (FIB/SEM) currently seems to be a promising imaging method for quantitative 3D assessment of the LCN. However, combined 3D visualization and quantification of the LCN using serial FIB/SEM imaging has not been reported so far. The aim of this study was to provide a proof of concept that serial FIB/SEM meets all requirements for quantitative 3D imaging of the LCN. To this end, we developed a new bone sample preparation protocol for serial FIB/SEM imaging providing a resolution on the order of 30nm. This technique was successfully applied to the mid-diaphysis of a mouse femur. Moreover, we devised and applied novel measures for subsequent quantitative 3D morphometry of the LCN. Briefly, serial FIB/SEM was shown to be an appropriate technique to quantify the morphology of the LCN truly in 3D. This will allow investigating bone matrix changes on an ultrastructural level, which result from aging, disease, and treatment.

Relating age and micro-architecture with apparent-level elastic constants: a micro-finite element study of female cortical bone from the anterior femoral midshaft

Homogenized elastic properties are often assumed for macro-finite element (FE) models used in orthopaedic biomechanics. The accuracy of material property assignments may have a strong effect on the ability of these models to make accurate predictions. For cortical bone, most macro-scale FE models assume isotropic elastic material behaviour and do not include variation of material properties due to bone micro-architecture. The first aim of the present study was to evaluate the variation of apparent-level (homogenized) orthotropic elastic constants of cortical bone with age and indices of bone micro-architecture. Considerable age-dependent differences in porosity were noted across the cortical thickness in previous research. The second aim of the study was to quantify the resulting differences in elastic constants between the periosteum and endosteum. Specimens were taken from the anterior femoral midshaft of 27 female donors (age 53.4 ± 23.6 years) and micro-FE (µFE) analysis was used to derive orthotropic elastic constants. The variation of orthotropic elastic constants (Young’s moduli, shear moduli, and Poisson’s ratios) with various cortical bone micro-architectural indices was investigated. The ratio of canal volume to tissue volume, Ca.V/TV, analogous to porosity, was found to be the strongest predictor ( r ave2 = 0.958) of the elastic constants. Age was less predictive ( r ave2 = 0.385) than Ca.V/TV. Elastic anisotropy increased with increasing Ca.V/TV, leading to lower elastic moduli in the transverse, typically less frequently loaded, directions. Increased Ca.V/TV led to a more substantial reduction in elastic constants at the endosteal aspect than at the periosteal aspect. The results are expected to be most applicable in similar midshaft locations of long bones; specific analysis of other sites would be necessary to evaluate elastic properties elsewhere. It was concluded that Ca.V/TV was the most predictive of cortical bone elastic constants and that considerable periosteal–endosteal variations in these constants can develop with bone loss.

Visualization of 3D osteon morphology by synchrotron radiation micro-CT

Cortical bone histology has been the subject of scientific inquiry since the advent of the earliest microscopes. Histology - literally the study of tissue - is a field nearly synonymous with 2D thin sections. That said, progressive developments in high-resolution X-ray imaging are enabling 3D visualization to reach ever smaller structures. Micro-computed tomography (micro-CT), employing conventional X-ray sources, has become the gold standard for 3D analysis of trabecular bone and is capable of detecting the structure of vascular (osteonal) porosity in cortical bone. To date, however, direct 3D visualization of secondary osteons has eluded micro-CT based upon absorption-derived contrast. Synchrotron radiation micro-CT, through greater image quality, resolution and alternative contrast mechanisms (e.g. phase contrast), holds great potential for non-destructive 3D visualization of secondary osteons. Our objective was to demonstrate this potential and to discuss areas of bone research that can be advanced through the application of this approach. We imaged human mid-femoral cortical bone specimens derived from a 20-year-old male (Melbourne Femur Collection) at the Advanced Photon Source synchrotron (Chicago, IL, USA) using the 2BM beam line. A 60-mm distance between the target and the detector was employed to enhance visualization of internal structures through propagation phase contrast. Scan times were 1 h and images were acquired with 1.4-μm nominal isotropic resolution. Computer-aided manual segmentation and volumetric 3D rendering were employed to visualize secondary osteons and porous structures, respectively. Osteonal borders were evident via two contrast mechanisms. First, relatively new (hypomineralized) osteons were evident due to differences in X-ray attenuation relative to the surrounding bone. Second, osteon boundaries (cement lines) were delineated by phase contrast. Phase contrast also enabled the detection of soft tissue remnants within the vascular pores. The ability to discern osteon boundaries in conjunction with vascular and cellular porosity revealed a number of secondary osteon morphologies and provided a unique 3D perspective of the superimposition of secondary osteons on existing structures. Improvements in resolution and optimization of the propagation phase contrast promise to provide further improvements in structural detail in the future.

Structural pattern and functional correlations of the long bone diaphyses intracortical vascular system: investigation carried out with China ink perfusion and multiplanar analysis in the rabbit femur

The intracortical vessel system of the rabbit femur has been studied after perfusion of the vascular tree with a water solution of dye (China ink) with multiplanar analysis. This method utilizes the full depth of field of the microscope objectives focusing different planes of the thick cortex. The microscopic observation even if restricted to a limited volume of cortex allowed to differentiate true 3-D nodes (54.5%) from the superimposition of vessels lying on different planes. The network model with elongated meshes preferentially oriented along the longitudinal axis of the diaphysis in his static configuration is not very different from the vascular anatomy depicted in the 2-D traditional models; however, the semi-quantitative morphometric analysis applied to the former supported the notion of a multidirectional microvascular network allowing change of flow according to the functional requirements. Other peculiar aspects not previously reported were cutting cone loops, blind-end and short-radius-bent vessels, and button-holes figures. The network design and node distribution were consistent with the straight trajectory of the secondary remodeling, with the proximal-to-distal and distal-to-proximal advancement directions of the cutting cones and with two main modes of node formation, namely bifurcation of the cutting cone and interception with pre-existing canals. The general organization of the network and its uninterrupted transformation during bone modeling and remodeling suggested a substantial plasticity of the intracortical vascular system capable to adapt itself to the changeable haemodynamic conditions.

Image processing and fractal box counting: user-assisted method for multi-scale porous scaffold characterization

Image analysis has gained new effort in the scientific community due to the chance of investigating morphological properties of three dimensional structures starting from their bi-dimensional gray-scale representation. Such ability makes it particularly interesting for tissue engineering (TE) purposes. Indeed, the capability of obtaining and interpreting images of tissue scaffolds, extracting morphological and structural information, is essential to the characterization and design of engineered porous systems. In this work, the traditional image analysis approach has been coupled with a probabilistic based percolation method to outline a general procedure for analysing tissue scaffold SEM micrographs. To this aim a case study constituted by PCL multi-scaled porous scaffolds was adopted. Moreover, the resulting data were compared with the outputs of conventionally used techniques, such as mercury intrusion porosimetry. Results indicate that image processing methods well fit the porosity features of PCL scaffolds, overcoming the limits of the more invasive porosimetry techniques. Also the cut off resolution of such IP methods was discussed. Moreover, the fractal dimension of percolating clusters, within the pore populations, was addressed as a good indication of the interconnection degree of PCL bi-modal scaffolds. Such findings represent (i) the bases for a novel approach complementary to the conventional experimental procedure used for the morphological analysis of TE scaffolds, in particular offering a valid method for the analysis of soft materials (i.e., gels); also (ii) providing a new perspective for further studies integrating to the structural and morphological data, fluid-dynamics and transport properties modelling.

Elastic anisotropy and off-axis ultrasonic velocity distribution in human cortical bone

Elastic structure in cortical bone is usually simplified as orthotropic or transversely isotropic, which allows estimates of three-dimensional technical constants from ultrasonic and density measurements. These elastic property estimates can then be used to study phenotypic changes in cortical bone structure and function, and to create finite element models of skeletal structures for studies of organismal variation and functional adaptation. This study examines assumptions of orthotropic or transversely isotropic material structure in cortical bone through the investigation of off-axis ultrasonic velocities in the cortical plane in 10 samples each from a human femur, mandible and cranium. Longitudinal ultrasonic velocities were measured twice through each bone sample by rotating the perimeter of each sample in 1 ° angular intervals between two ultrasonic transducers. The data were fit to sine curves f(x)=(A × sin(x + B) + C) and the goodness of fit was examined. All the data from the femur fit closely with the ideal sine curve model, and all three coefficients were similar among specimens, indicating similar elastic properties, anisotropies and orientations of the axes of maximum stiffness. Off-axis ultrasonic velocities in the mandible largely fit the sine curve model, although there were regional variations in the coefficients. Off-axis ultrasonic velocities from the cranial vault conformed to the sine curve model in some regions but not in others, which shows an irregular and complex pattern. We hypothesize that these variations in ultrasonic velocities reflect variations in the underlying bulk microstructure of the cortical bone, especially in the three-dimensional patterns of osteonal orientation and structure. Elastic property estimates made with ultrasonic techniques are likely valid in the femur and mandible; errors in estimates from cranial bone need to be evaluated regionally. Approximate orthotropic structure in bulk cortical bone specimens should be assessed if ultrasound is used to estimate three-dimensional elastic properties.

Regional, ontogenetic, and sex-related variations in elastic properties of cortical bone in baboon mandibles

Understanding the mechanical features of cortical bone and their changes with growth and adaptation to function plays an important role in our ability to interpret the morphology and evolution of craniofacial skeletons. We assessed the elastic properties of cortical bone of juvenile and adult baboon mandibles using ultrasonic techniques. Results showed that, overall, cortical bone from baboon mandibles could be modeled as an orthotropic elastic solid. There were significant differences in the directions of maximum stiffness, thickness, density, and elastic stiffness among different functional areas, indicating regional adaptations. After maturity, the cortical bone becomes thicker, denser, and stiffer, but less anisotropic. There were differences in elastic properties of the corpus and ramus between male and female mandibles which are not observed in human mandibles. There were correlations between cortical thicknesses and densities, between bone elastic properties and microstructural configuration, and between the directions of maximum stiffness and bone anatomical axes in some areas. The relationships between bone extrinsic and intrinsic properties bring us insights into the integration of form and function in craniofacial skeletons and suggest that we need to consider both macroscopic form, microstructural variation, and the material properties of bone matrix when studying the functional properties and adaptive nature of the craniofacial skeleton in primates. The differences between baboon and human mandibles is at variance to the pattern of differences in crania, suggesting differences in bone adaption to varying skeletal geometries and loading regimes at both phylogenetic and ontogenetic levels.

Controlled electro-implementation of fluoride in titanium implant surfaces enhances cortical bone formation and mineralization

Previous studies have shown that bone-to-implant attachment of titanium implants to cortical bone is improved when the surface is modified with hydrofluoric acid. The aim of this study was to investigate if biological factors are involved in the improved retention of these implants. Fluoride was implemented in implant surfaces by cathodic reduction with increasing concentrations of HF in the electrolyte. The modified implants were placed in the cortical bone in the tibias of New Zealand white rabbits. After 4 weeks of healing, wound fluid collected from the implant site showed lower lactate dehydrogenase activity and less bleeding in fluoride-modified implants compared to control. A significant increase in gene expression levels of osteocalcin and tartrate-resistant acid phosphatase (TRAP) was found in the cortical bone attached to Ti implants modified with 0.001 and 0.01 vol.% HF, while Ti implants modified with 0.1% HF showed only induced TRAP mRNA levels. These results were supported by the performed micro-CT analyses. The volumetric bone mineral density of the cortical bone hosting Ti implants modified with 0.001% and 0.01% HF was higher both in the newly woven bone (<100 microm from the interface) and in the older Haversian bone (>100 microm). In conclusion, the modulation of these biological factors by surface modification of titanium implants with low concentrations of HF using cathodic reduction may explain their improved osseointegration properties.

A model of the intracortical vascular system of long bones and of its organization: an experimental study in rabbit femur and tibia

The vascular anatomy of the cortical bone and the canal system are highly correlated, and the former has an important bearing on shape and microscopic lamellar structure, as it is established in the progression of the remodelling process. The classical description of a longitudinal system of canals (Havers') connected by the transversal Volkmann's canals is the generally acknowledged model of the structural organization of the cortex. However, it is remarkably difficult to study the circulation inside the compact bone in detail owing to its hard, calcified matrix, and the methods thus far applied have represented either the bone morphology and the architecture of the canal system or the injected vessel network. In the present study, the intracortical vessel network was injected with black China ink and evidenced by transillumination of full-thickness, decalcified hemicortices. By making use of the depth of field of the microscope objective, the three-dimensional architecture of the network was highlighted and the morphometry of vessel size measurements and a classification of the network nodes according to the number of arms was made possible. These observations were integrated with data obtained by routine histology on decalcified sections relevant to the connections of the intracortical canal system with the outer environment, with regard to the direction of advancement of new canals and with regard to the mode of formation of the system nodes. The formation of the intracortical vessels network involved two processes: the incorporation of the periosteal network and osteonal remodelling, the latter occurring through the advancement of cutting cones followed by their own vascular loop and by concentric lamellar apposition. The two systems could be distinguished by the diameter of the vessels (the former were significantly larger) and by the network architecture (the former convoluted, and the latter longitudinally orientated and straight). Longitudinal vessels could form branches or create connections with the periosteal derived vessels that occasionally meet on the line of their advancement. They were observed entering from either inside the cortex from the metaphyses or from the endosteal surface of the marrow cavity. The combined observations from different methods of study documented a model of intracortical canal and vessel networks formed by two initially independent systems: one derived from the external, periosteal vessels, and one from metaphyseal and marrow vessels. Connections between the two were established with the advancing of cutting cones from the extremities of the diaphysis. Analysis of the system architecture and the modalities of its progressive organization suggested that the direction of advancement of a forming canal does not necessarily correspond to the final blood flow direction of its central vessel.

In vivo evaluation of the presence of bone marrow in cortical porosity in postmenopausal osteopenic women

This is the first observational study examining cortical porosity in vivo in postmenopausal osteopenic women and to incorporate data from two different imaging modalities to further examine the nature of cortical porosity. The goal of this study was to combine high-resolution peripheral computed tomography (HR-pQCT) images, which contain high spatial resolution information of the cortical structure, and magnetic resonance (MR) images, which allow the visualization of soft tissues such as bone marrow, to observe the amount of cortical porosity that contains bone marrow in postmenopausal osteopenic women. The radius of 49 and the tibia of 51 postmenopausal osteopenic women (age 56 ± 3.7) were scanned using both HR-pQCT and MR imaging. A normalized mutual information registration algorithm was used to obtain a three-dimensional rigid transform which aligned the MR image to the HR-pQCT image. The aligned images allowed for the visualization of bone marrow in cortical pores. From the HR-pQCT image, the percent cortical porosity, the number of cortical pores, and the size of each cortical pore was determined. By overlaying the aligned MR and HR-pQCT images, the percent of cortical pores containing marrow, the number of cortical pores containing marrow, and the size of each cortical pore containing marrow were measured. While the amount of cortical porosity did not vary greatly between subjects, the type of cortical pore, containing marrow vs. not containing marrow, varied highly between subjects. The results suggest that cortical pore spaces contain components of varying composition, and that there may be more than one mechanism for the development of cortical porosity.

Morphometric analysis of the canal system of cortical bone: An experimental study in the rabbit femur carried out with standard histology and micro-CT

The osteonal pattern of cortical bone is gradually built around the intracortical vessels by the progression of the cutting cones (secondary remodelling); therefore, the central canal size can be used as index of the remodelling activity. An experimental model in the rabbit femur was used to investigate, through central canal morphometry and frequency distribution analysis, the remodelling activity, comparing the middle of the diaphysis (mid-shaft) with the extremity (distal-shaft) and at the same level sectors and layers of the cortex in transversal sections. The study documented a higher density of canals in the mid-shaft than in the distal-shaft and a higher remodelling in the distal-shaft. There were no significant differences between dorsal, ventral, medial and lateral sectors at both mid-shaft and distal-shaft levels, while the number of canals was higher in the sub-periosteal layers than in the sub-endosteal. A lower threshold of 40 microm(2) was observed in the central canal area. Sealed osteons in the midshaft were 22.43% of the total number of osteons of the central canal area between 40 and 200 microm(2) and 0.44% of those of the distal-shaft. Micro-CT allowed a 3D reconstruction of the vascular canal system, which confirmed the branched network pattern rather than the trim architecture of the traditional representation. Some aspects like the lower threshold of the central canal size and the sealed osteons documented the plasticity of the system and its capacity for adaptation to changes in the haemodynamic conditions.

Experimental determination of the permeability in the lacunar-canalicular porosity of bone

Permeability of the mineralized bone tissue is a critical element in understanding fluid flow occurring in the lacunar-canalicular porosity (PLC) compartment of bone and its role in bone nutrition and mechanotransduction. However, the estimation of bone permeability at the tissue level is affected by the influence of the vascular porosity in macroscopic samples containing several osteons. In this communication, both analytical and experimental approaches are proposed to estimate the lacunar-canalicular permeability in a single osteon. Data from an experimental stress-relaxation test in a single osteon are used to derive the PLC permeability by curve fitting to theoretical results from a compressible transverse isotropic poroelastic model of a porous annular disk under a ramp loading history (2007, "Compressible and Incompressible Constituents in Anisotropic Poroelasticity: The Problem of Unconfined Compression of a Disk," J. Mech. Phys. Solids, 55, pp. 161-193; 2008, "The Unconfined Compression of a Poroelastic Annular Cylindrical Disk," Mech. Mater., 40(6), pp. 507-523). The PLC tissue intrinsic permeability in the radial direction of the osteon was found to be dependent on the strain rate used and within the range of O(10(-24))-O(10(-25)). The reported values of PLC permeability are in reasonable agreement with previously reported values derived using finite element analysis (FEA) and nanoindentation approaches.

Anatomy of the intracortical canal system: scanning electron microscopy study in rabbit femur

The current model of compact bone is that of a system of longitudinal (Haversian) canals connected by transverse (Volkmann's) canals. Models based on histology or microcomputed tomography lack the morphologic detail and sense of temporal development provided by direct observation. Using direct scanning electron microscopy observation, we studied the bone surface and structure of the intracortical canal system in paired fractured surfaces in rabbit femurs, examining density of canal openings on periosteal and endosteal surfaces, internal network nodes and canal sizes, and collagen lining of the inner canal system. The blood supply of the diaphyseal compact bone entered the cortex through the canal openings on the endosteal and periosteal surfaces, with different morphologic features in the midshaft and distal shaft; their density was higher on endosteal than on periosteal surfaces in the midshaft but with no major differences among subregions. The circumference measurements along Haversian canals documented a steady reduction behind the head of the cutting cone but rather random variations as the distance from the head increased. These observations suggested discontinuous development and variable lamellar apposition rate of osteons in different segments of their trajectory. The frequent branching and types of network nodes suggested substantial osteonal plasticity and supported the model of a network organization. The collagen fibers of the canal wall were organized in intertwined, longitudinally oriented bundles with 0.1- to 0.5-mum holes connecting the canal lumen with the osteocyte canalicular system.

A comparative view on mechanisms and functions of skeletal remodelling in teleost fish, with special emphasis on osteoclasts and their function

Resorption and remodelling of skeletal tissues is required for development and growth, mechanical adaptation, repair, and mineral homeostasis of the vertebrate skeleton. Here we review for the first time the current knowledge about resorption and remodelling of the skeleton in teleost fish, the largest and most diverse group of extant vertebrates. Teleost species are increasingly used in aquaculture and as models in biomedical skeletal research. Thus, detailed knowledge is required to establish the differences and similarities between mammalian and teleost skeletal remodelling, and between distantly related species such as zebrafish (Danio rerio) and medaka (Oryzias latipes). The cellular mechanisms of differentiation and activation of osteoclasts and the functions of teleost skeletal remodelling are described. Several characteristics, related to skeletal remodelling, distinguish teleosts from mammals. These characteristics include (a) the absence of osteocytes in most species; (b) the absence of haematopoietic bone marrow tissue; (c) the abundance of small mononucleated osteoclasts performing non-lacunar (smooth) bone resorption, in addition to or instead of multinucleated osteoclasts; and (d) a phosphorus- rather than calcium-driven mineral homeostasis (mainly affecting the postcranial dermal skeleton). Furthermore, (e) skeletal resorption is often absent from particular sites, due to sparse or lacking endochondral ossification. Based on the mode of skeletal remodelling in early ontogeny of all teleosts and in later stages of development of teleosts with acellular bone we suggest a link between acellular bone and the predominance of mononucleated osteoclasts, on the one hand, and cellular bone and multinucleated osteoclasts on the other. The evolutionary origin of skeletal remodelling is discussed and whether mononucleated osteoclasts represent an ancestral type of resorbing cells. Revealing the differentiation and activation of teleost skeletal resorbing cells, in the absence of several factors that trigger mammalian osteoclast differentiation, is a current challenge. Understanding which characters of teleost bone remodelling are derived and which characters are conserved should enhance our understanding of the process in fish and may provide insights into alternative pathways of bone remodelling in mammals.

Percolation in insect nest networks: evidence for optimal wiring

Optimization has been shown to be a driving force for the evolution of some biological structures, such as neural maps in the brain or transport networks. Here we show that insect networks also display characteristic traits of optimality. By using a graph representation of the chamber organization of termite nests and a disordered lattice model, it is found that these spatial nests are close to a percolation threshold. This suggests that termites build efficient systems of galleries spanning most of the nest volume at low cost. The evolutionary consequences are outlined.

Anticipating bipedalism: trabecular organization in the newborn ilium

Trabecular bone structural organization is considered to be predominantly influenced by localized temporal forces which act to maintain and remodel the trabecular architecture into a biomechanically optimal configuration. In the adult pelvis, the most significant remodelling forces are believed to be those generated during bipedal locomotion. However, during the fetal and neonatal period the pelvic complex is non-weight bearing and, as such, structural organization of iliac trabecular bone cannot reflect direct stance-related forces. In this study, micro-computed tomography scans from 28 neonatal ilia were analysed, using a whole bone approach, to investigate the trabecular characteristics present within specific volumes of interest relevant to density gradients highlighted in a previous radiographic study. Analysis of the structural indices bone volume fraction, trabecular thickness, trabecular spacing and trabecular number was carried out to quantitatively investigate structural composition. Quantification of the neonatal trabecular structure reinforced radiographic observations by highlighting regions of significant architectural form which grossly parallel architectural differences in the adult pattern but which have previously been attributed to stance-related forces. It is suggested that the seemingly organized rudimentary scaffold observed in the neonatal ilium may be attributable to other non-weight bearing anatomical interactions or even to a predetermined genetic blueprint. It must also be postulated that whilst the observed patterning may be indicative of a predetermined inherent template, early non-weight bearing and late stance-related locomotive influences may subsequently be superimposed upon this scaffolding and perhaps reinforced and likely remodelled at a later age. Ultimately, the analysis of this fundamental primary pattern has core implications for understanding the earliest changes in pelvic trabecular architecture and provides a baseline insight into future ontogenetic development and bipedal capabilities.

Modularity and robustness of bone networks

Cortical bones, essential for mechanical support and structure in many animals, involve a large number of canals organized in intricate fashion. By using state-of-the art image analysis and computer graphics, the 3D reconstruction of a whole bone (phalange) of a young chicken was obtained and represented in terms of a complex network where each canal was associated to an edge and every confluence of three or more canals yielded a respective node. The representation of the bone canal structure as a complex network has allowed several methods to be applied in order to characterize and analyze the canal system organization and the robustness. First, the distribution of the node degrees (i.e. the number of canals connected to each node) confirmed previous indications that bone canal networks follow a power law, and therefore present some highly connected nodes (hubs). The bone network was also found to be partitioned into communities or modules, i.e. groups of nodes which are more intensely connected to one another than with the rest of the network. We verified that each community exhibited distinct topological properties that are possibly linked with their specific function. In order to better understand the organization of the bone network, its resilience to two types of failures (random attack and cascaded failures) was also quantified comparatively to randomized and regular counterparts. The results indicate that the modular structure improves the robustness of the bone network when compared to a regular network with the same average degree and number of nodes. The effects of disease processes (e.g., osteoporosis) and mutations in genes (e.g., BMP4) that occur at the molecular level can now be investigated at the mesoscopic level by using network based approaches.

Image-based characterization of foamed polymeric tissue scaffolds

Tissue scaffolds are integral to many regenerative medicine therapies, providing suitable environments for tissue regeneration. In order to assess their suitability, methods to routinely and reproducibly characterize scaffolds are needed. Scaffold structures are typically complex, and thus their characterization is far from trivial. The work presented in this paper is centred on the application of the principles of scaffold characterization outlined in guidelines developed by ASTM International. Specifically, this work demonstrates the capabilities of different imaging modalities and analysis techniques used to characterize scaffolds fabricated from poly(lactic-co-glycolic acid) using supercritical carbon dioxide. Three structurally different scaffolds were used. The scaffolds were imaged using: scanning electron microscopy, micro x-ray computed tomography, magnetic resonance imaging and terahertz pulsed imaging. In each case two-dimensional images were obtained from which scaffold properties were determined using image processing. The findings of this work highlight how the chosen imaging modality and image-processing technique can influence the results of scaffold characterization. It is concluded that in order to obtain useful results from image-based scaffold characterization, an imaging methodology providing sufficient contrast and resolution must be used along with robust image segmentation methods to allow intercomparison of results.

Effects of intracortical porosity on fracture toughness in aging human bone: a microCT-based cohesive finite element study

The extent to which increased intracortical porosity affects the fracture properties of aging and osteoporotic bone is unknown. Here, we report the development and application of a microcomputed tomography based finite element approach that allows determining the effects of intracortical porosity on bone fracture by blocking all other age-related changes in bone. Previously tested compact tension specimens from human tibiae were scanned using microcomputed tomography and converted to finite element meshes containing three-dimensional cohesive finite elements in the direction of the crack growth. Simulations were run incorporating age-related increase in intracortical porosity but keeping cohesive parameters representing other age-related effects constant. Additional simulations were performed with reduced cohesive parameters. The results showed a 6% decrease in initiation toughness and a 62% decrease in propagation toughness with a 4% increase in porosity. The reduction in toughnesses became even more pronounced when other age-related effects in addition to porosity were introduced. The initiation and propagation toughness decreased by 51% and 83%, respectively, with the combined effect of 4% increase in porosity and decrease in the cohesive properties reflecting other age-related changes in bone. These results show that intracortical porosity is a significant contributor to the fracture toughness of the cortical bone and that the combination of computational modeling with advanced imaging improves the prediction of the fracture properties of the aged and the osteoporotic cortical bone.

Modulation of osteoblast gap junction connectivity by serum, TNFalpha, and TRAIL

We studied the effects of serum growth factors and of TNF family proteins on osteoblast gap junction connectivity. Serum starvation of human MG63 osteosarcoma cells or nontransformed osteoblasts decreased connexin43 protein. TNFalpha or TRAIL reduced connexin43 further. Serum starvation redistributed gap junctions but did not reduce intercellular diffusion. In contrast, TNFalpha or TRAIL reduced gap junctions on cell processes and decreased intercellular diffusion. Effects of TNFs on connexin43 were mediated by lysosomal proteolysis. Activating analogs of cAMP increased connexin43 protein, but did not block effects of serum starvation, TNFalpha, or TRAIL on connexin43 protein. Connexin43 and connectivity recovered overnight if stimuli were withdrawn. Surprisingly, connexin43 mRNA increased in serum starvation and with TNFalpha or TRAIL. Since beta-catenin is a binding partner of connexin43, when connexin43 is degraded, beta-catenin activation may contribute to a reflexive increase in connexin43 transcription. We conclude that osteoblast connectivity is regulated by a multifactorial system that maintains intercellular connections. Serum starvation, TNFalpha and TRAIL augmented connexin43 degradation and connexin43 transcription. Cell-cell communication was maintained in serum starvation, which may model response to acute injury, but was sensitive to TNFs. These inflammatory agents mediated selective, reversible removal of connexin43 from cell processes.

Ultrastructural properties in cortical bone vary greatly in two inbred strains of mice as assessed by synchrotron light based micro- and nano-CT

Nondestructive SR-based microCT and nano-CT methods have been designed for 3D quantification and morphometric analysis of ultrastructural phenotypes within murine cortical bone, namely the canal network and the osteocyte lacunar system. Results in two different mouse strains, C57BL/6J-Ghrhr(lit)/J and C3.B6-Ghrhr(lit)/J, showed that the cannular and lacunar morphometry and their bone mechanics were fundamentally different.

INTRODUCTION

To describe the different aspects of bone quality, we followed a hierarchical approach and assessed bone tissue properties in different regimens of spatial resolution, beginning at the organ level and going down to cellular dimensions. For these purposes, we developed different synchrotron radiation (SR)-based CT methods to assess ultrastructural phenotypes of murine bone.

MATERIALS AND METHODS

The femoral mid-diaphyses of 12 C57BL/6J-Ghrhr(lit)/J (B6-lit/lit) and 12 homozygous mutants C3.B6-Ghrhr(lit)/J (C3.B6-lit/lit) were measured with global SR microCT and local SR nano-CT (nCT) at nominal resolutions ranging from 3.5 microm to 700 nm, respectively. For volumetric quantification, morphometric indices were determined for the cortical bone, the canal network, and the osteocyte lacunar system using negative imaging. Moreover, the biomechanics of B6-lit/lit and C3.B6-lit/lit mice was determined by three-point bending.

RESULTS

The femoral mid-diaphysis of C3.B6-lit/lit was larger compared with B6-lit/lit mice. On an ultrastructural level, the cannular indices for C3.B6-lit/lit were generally bigger in comparison with B6-lit/lit mice. Accordingly, we derived and showed a scaling rule, saying that overall cannular indices scaled with bone size, whereas indices describing basic elements of cannular and lacunar morphometry did not. Although in C3.B6-lit/lit, the mean canal volume was larger than in B6-lit/lit, canal number density was proportionally smaller in C3.B6-lit/lit, so that lacuna volume density was found to be constant and therefore independent of mouse strain and sex. The mechanical properties in C3.B6-lit/lit were generally improved compared with B6-lit/lit specimens. For C3.B6-lit/lit, we observed a sex specificity of the mechanical parameters, which could not be explained by bone morphometry on an organ level. However, there is evidence that for C3.B6-lit/lit, the larger cortical bone mass is counterbalanced or even outweighed by the larger canal network in the female mice.

CONCLUSIONS

We established a strategy to subdivide murine intracortical porosity into ultrastructural phenotypes, namely the canal network and the osteocyte lacunar system. Nondestructive global and local SR-based CT methods have been designed for 3D quantification and subsequent morphometric analysis of these phenotypes. Results in the two different mouse strains C57BL/6J-Ghrhr(lit)/J and C3.B6-Ghrhr(lit)/J showed that the cannular and lacunar morphometry and the biomechanical properties were fundamentally different.

On the way to building an integrated computational environment for the study of developmental patterns and genetic diseases

Genetic diseases and developmental patterns should be studied on several levels: from macroscale (organs and tissues) to nanoscale (cells, genes, proteins). Due to the overwhelming complexity of the life science data, it is common that disparate data pieces are meticulously stored but never fully analyzed or correlated. We have begun to develop a novel methodology based on virtual reality techniques for the study of these phenomena. Our key approach to knowledge integration is a top-down mapping of data onto visual contexts. For each organism that we want to study, a structural model is created and used as a visual “wireframe” onto which other data types are superimposed in a top-down assembly. Data analysis tools, visual controls, and queries are enabled so that users can interactively explore data. Our visualization technology gives users an opportunity to map disparate data onto a common model, and search visually for hitherto unknown patterns and correlations contained within the data. It is our goal to eventually transform genomics research from measuring various data pieces analytically into a fully interactive exploration of combined data in a 4D immersive visual environment that best matches a researcher’s intuition.

Effect of porosity on effective diagonal stiffness coefficients (cii) and elastic anisotropy of cortical bone at 1 MHz: a finite-difference time domain study

Finite-difference time domain (FDTD) numerical simulations coupled to real experimental data were used to investigate the propagation of 1 MHz pure bulk wave propagation through models of cortical bone microstructures. Bone microstructures were reconstructed from three-dimensional high resolution synchrotron radiation microcomputed tomography (SR-muCT) data sets. Because the bone matrix elastic properties were incompletely documented, several assumptions were made. Four built-in bone matrix models characterized by four different anisotropy ratios but the same Poisson's ratios were tested. Combining them with the reconstructed microstructures in the FDTD computations, effective stiffness coefficients were derived from simulated bulk-wave velocity measurements. For all the models, all the effective compression and shear bulk wave velocities were found to decrease when porosity increases. However, the trend was weaker in the axial direction compared to the transverse directions, contributing to the increase of the effective anisotropy. On the other hand, it was shown that the initial Poisson's ratio value may substantially affect the variations of the effective stiffness coefficients. The present study can be used to elaborate sophisticated macroscopic computational bone models incorporating realistic CT-based macroscopic bone structures and effective elastic properties derived from muCT-based FDTD simulations including the cortical porosity effect.