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The effects of immobilization on vascular canal orientation in rat cortical bone
Abstract
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.
... Relatively smaller or larger osteon and Haversian canal area and diameter measurements represent transverse cross-sectional surfaces of bone microstructure, and may indicate how fast or slow, and/ or frequently cortical bone is filled by basic multicellular units [22]. Indeed, previous human and non-human animal research demonstrated higher osteon and osteocyte lacunae densities, and smaller osteons and Haversian canals at bone sites associated with larger strain, mechanical stress, or type (direction) of mechanical load [e.g., [22][23][24][25][26][27][28]. ...
... Descriptive statistics for histology data were previously published in [2] and partly in [23,37] substantial portions of data (<10%), although some general trends in data were still identified. Subsequent analyses where femoral robusticity calculated from Ct.Wi was assessed against the histology variables (repeated 99 tests) revealed 75 (~ 76%) statistically significant (p range from 0.000−0.046) ...
... Through a series of positive and negative correlations, along with tests for allometry, the present study supports this functional adaptation of structure in the midshaft human femur. These results agree with basic engineering predictions, and support previous studies of cortical histomorphometric change in relation to strain or mechanical load [e.g., 7,[22][23][24][25][26][27][28]44]. However, it is noted that the sample utilized here relies on mechanical loading inferences through simple measurements of bone robusticity. ...
Recent quantitative analyses of human bone microanatomy, as well as theoretical models that propose bone microstructure and gross anatomical associations, have started to reveal insights into biological links that may facilitate remodeling processes. However, relationships between bone size and the underlying cortical bone histology remain largely unexplored. The goal of this study is to determine the extent to which static indicators of bone remodeling and vascularity, measured using histomorphometric techniques, relate to femoral midshaft cortical width and robusticity. Using previously published and new quantitative data from 450 adult human male (n = 233) and female (n = 217) femora, we determine if these aspects of femoral size relate to bone microanatomy. Scaling relationships are explored and interpreted within the context of tissue form and function. Analyses revealed that the area and diameter of Haversian canals and secondary osteons, and densities of secondary osteons and osteocyte lacunae from the sub-periosteal region of the posterior midshaft femur cortex were significantly, but not consistently, associated with femoral size. Cortical width and bone robusticity were correlated with osteocyte lacunae density and scaled with positive allometry. Diameter and area of osteons and Haversian canals decreased as the width of cortex and bone robusticity increased, revealing a negative allometric relationship. These results indicate that microscopic products of cortical bone remodeling and vascularity are linked to femur size. Allometric relationships between more robust human femora with thicker cortical bone and histological products of bone remodeling correspond with principles of bone functional adaptation. Future studies may benefit from exploring scaling relationships between bone histomorphometric data and measurements of bone macrostructure.
... In the distal diaphyseal cortex, only the longitudinal orientation of the vascular canals in LS1 were different. The impact of vascular canal orientation on bone strength is poorly understood [31][32][33] . However, considering that all other diaphyseal indices of cross-sectional geometry were similar between the lines, including porosity and vascular canal thickness, we think it unlikely that the modest change in the average orientation of LS1's vascular canals (6-10°) would have a significant impact on its inferred bone strength. ...
... For each sample, a sub-region equivalent to 1 mm (526 slices) of cortical bone, starting 0.5 mm distal to the tibia-fibula junction, was imported into Fiji (ImageJ v1.50 e) 56 for processing using a custom-written macro, as follows: Stacks were first converted to binary images using a global threshold. Next, the "Analyze Particles" function in Fiji was used to digitally fill open spaces in the bone under 1000 μm 3 (~145 voxels), representing osteocyte lacunae and other noise, leaving a stack containing larger spaces representing canals 32,57 . This stack was duplicated, and the function was run once more to fill the canals, leaving a solid block in which all spaces within the cortex had been filled. ...
... These data were exported to Matlab 2016 (Mathworks, Natick, MA), where a custom script was used to remove branches under 100 μm in length 33 . We calculated the orientation of the canals relative to the longitudinal axis passing through the centroids of each slice, as described 32,33 . ...
Bone strength is influenced by mineral density and macro- and microstructure. Research into factors that contribute to bone morphology and strength has focused on genetic, environmental and morphological factors (e.g., body mass index), but little is known regarding the impact of rates of skeletal elongation on adult skeletal morphology and strength. Using micro-CT, we examined the impact of rates of skeletal elongation on bone cortical and trabecular morphology, and on rates of estrogen-dependent bone loss in the tibia in CD-1 mice, and in mice with accelerated skeletal growth (Longshanks). Groups of adult mice (n = 7/group) were subjected to ovariectomy or sham surgeries, scanned for 6 weeks, and indices of bone morphology were collected. Results show that Longshanks mice had significantly less trabecular bone at skeletal maturity, characterized by fewer, thinner trabeculae, and furthermore lost trabecular bone more slowly in response to ovariectomy. Artificial selection for rapid skeletal growth relative to somatic growth thus had a significant impact on trabecular bone morphology in Longshanks. Our data do not unequivocally demonstrate a causal relationship between rapid bone growth and reduced trabecular bone quality, but suggest that rapid linear bone growth may influence the risk of cancellous bone fragility.
... However, their work was limited to analysis of cut samples, and they did not identify the morphological axes of the bone. Previous work in Cooper's bone imaging group has measured the orientation of canals with respect to bone's long axis (Britz et al., 2012), showing the ability to differentiate between radial and longitudinal canals, and significant differences in orientation between normal and immobilized rats. The currently described method builds on that work, by adding the ability to measure orientation in the orthogonal plane, thus providing the full 3D orientations of each canal in the cortical network. ...
... The length of the canal segments is a user choice, but it is recommended to keep the canal length at approximately 100-150 m, the thickness of a typical histological ground section. Using shorter canal segments provides a more robust estimate of the orientation of curved canal sections, though with diminishing returns (Britz et al., 2012). This lineset will form the basis for the orientation measurements (see Fig. 1). ...
... The category with the lowest proportion of canals was the phi 0-22.5 • , theta 67.5-90 • group at 3.2%, which is circumferential in the plane and horizontal in the long axis. Overall the rat's cortical network is a combination of radial and longitudinal canals, with mostly oblique radial/longitudinal canals, corroborating the previous phi measurements by Britz et al. (2012). ...
The orientation of vascular canals in cortical bone can reveal information about the growth rate and loading environment of a bone. For example, in birds it has been proposed that a high proportion of circumferential canals (a laminar cortex) is related to fast growth or torsional loading related to active flight. In this paper we present a method to measure the three dimensional (3D) orientation of vascular canals. Image data are obtained from micro-CT and two angles are measured: phi, determining how longitudinal a canal is; and theta, determining whether a canal is radial or circumferential. This method can measure the orientation of each canal contained in the scanned images. Here we demonstrate the approach on two samples − a rat tibia and a hawk humerus. This method offers a direct (3D) method for quantifying features of canal orientation, such as the degree of laminarity, and can be applied easily and non-destructively to multiple species and bones. The growth and development of the cortical canal network and its impact on factors such as bone strength and bone quality remains relatively unexplored. Our method provides a new tool to examine the impact of the orientation of cortical bone canals on bone and explore the origins of cortical canals formed during modelling and remodeling. This method has applications in comparative bone biology, small animal models, and human bone studies.
... Then, a sweep method discarded all but the largest object. Bone porosity was highlighted by inverting the image, and then classified using a despeckle filter: objects with an area smaller than 20 pixels (i.e., corresponding to a diameter of about 6 µm in two-dimensional cross-sections), were considered to be either noise or osteocyte lacunae 46,66,67 . Finally, another sweep operator was applied to keep only the connected porosity, therefore ensuring to extract the subchondral channel network without interruptions. ...
... Bone adaptation to mechanical loading has been extensively investigated at the organ and tissue level, but much less at the level of the subchondral channel network. However, there is evidence that loading affects channel orientation even in primary cortical bone as immobilized bones show more radially oriented channels than loaded bones 46 . Here, we have highlighted that tendon loading may as well have an impact on the three-dimensional organization of the channel network. ...
The enthesis allows the insertion of tendon into bone thanks to several remarkable strategies. This complex and clinically relevant location often features a thin layer of fibrocartilage sandwiched between tendon and bone to cope with a highly heterogeneous mechanical environment. The main purpose of this study was to investigate whether mineralized fibrocartilage and bone close to the enthesis show distinctive three-dimensional microstructural features, possibly to enable load transfer from tendon to bone. As a model, the Achilles tendon-calcaneus bone system of adult rats was investigated with histology, backscattered electron imaging and micro-computed tomography. The microstructural porosity of bone and mineralized fibrocartilage in different locations including enthesis fibrocartilage, periosteal fibrocartilage and bone away from the enthesis was characterized. We showed that calcaneus bone presents a dedicated protrusion of low porosity where the tendon inserts. A spatially resolved analysis of the trabecular network suggests that such protrusion may promote force flow from the tendon to the plantar ligament, while partially relieving the trabecular bone from such a task. Focusing on the tuberosity, highly specific microstructural aspects were highlighted. Firstly, the interface between mineralized and unmineralized fibrocartilage showed the highest roughness at the tuberosity, possibly to increase failure resistance of a region carrying large stresses. Secondly, fibrochondrocyte lacunae inside mineralized fibrocartilage, in analogy with osteocyte lacunae in bone, had a predominant alignment at the enthesis and a rather random organization away from it. Finally, the network of subchondral channels inside the tuberosity was highly anisotropic when compared to contiguous regions. This dual anisotropy of subchondral channels and cell lacunae at the insertion may reflect the alignment of the underlying collagen network. Our findings suggest that the microstructure of fibrocartilage may be linked with the loading environment. Future studies should characterize those microstructural aspects in aged and or diseased conditions to elucidate the poorly understood role of bone and fibrocartilage in enthesis-related pathologies.
... oriented along the long bone axis) (Fig. 4), because radial vascularity was not consistently observed in all the ROIs. Including radial vessels might have also 'duplicated' the count of neighbouring longitudinal vessels and introduced a new variable of 'connectivity' (Cooper et al. 2003), though we do acknowledge that bone vascular orientation is of importance when examining bone biomechanics (Britz et al. 2012). The following were included in our analyses: simple primary vessels scattered across parallelfibered bone, as well as occasional primary and secondary osteon structures seen in lamellar bone (see Ziv et al. 1996). ...
... Because we only focused on longitudinal canals, we also suggest future examination of vessel orientation and connectivity (e.g. Britz et al. 2012). Kangaroos are marsupials whose forelimb developmental pathway differs from that of humans as joeys need forelimb strength early in ontogeny to be able to climb out of the mother's pouch (Cooper and Steppan 2010). ...
Human ribs are thought to be less affected by mechanical strain at the microscopic level than limb bones, implying that rib remodelling better reflects bone physiological homeostasis. Here, we test the hypothesis that rib tissue will be well vascularized and thus enhance susceptibility to metabolic influence. An intra-skeletal comparison of bone vascular canal density was conducted using a macropod animal model adapted to bipedal habitual hopping. The right humerus, ulna, radius, femur, tibia, fibula, a mid-thoracic and upper-thoracic rib of an eastern grey kangaroo (Macropus giganteus) were sectioned at the midshaft, from which histological sections were prepared. Bone vascularity from a maximum of 12 mm2 of sub-periosteal parallel-fibred and lamellar bone was recorded, resulting in a total of 2047 counted vessels. Vascular canal density data were corrected by cortical width, maximum length, and midshaft circumference robusticity indices computed for each bone. The fibula consistently had the highest vascular canal density, even when corrected for maximum length, cortical width and midshaft circumference robusticities. This was followed by the mid- and upper-thoracic ribs. Vascularity differences between bones were relatively consistent whether vascular canal density was controlled for by cortical width or midshaft circumference robusticities. Vascular canal density and robusticity indices were also positively and negatively correlated (p < 0.05). Results confirm that the ribs are well vascularized, which facilitates bone metabolic processes such as remodelling, but the fibula also appears to be a well vascularized bone. Future research investigating human bone metabolism will benefit from examining thoracic rib or fibula samples.
... In human long bones, they are roughly cylindrical, of varying individual lengths, around 250-300 μm in cross-section, and longitudinally oriented within the diaphysis. Together, they constitute a complex network of frequent branching and transverse connections (Britz et al., 2012, Cohen and Harris, 1958, Koltze, 1951, Maggiano et al., 2016, Tappen, 1977, in which their longitudinal alignment correlates with stress fields (Baca et al., 2007, Hert et al., 1994, Martínez-Reina et al., 2014. In microscopic analyses, their variability in size and shape is traditionally investigated through measurements of their diaphyseal area and roundness, or osteon circularity (On.Cr.) (Dominguez and Crowder, 2012, Goliath et al., 2016, Keenan et al., 2017, Miszkiewicz and Mahoney, 2016, Crescimanno and Stout, 2012. ...
... Detailed descriptive morphological analyses of osteon shape, although highly informative, are extremely rare in the literature (see Pazzaglia et al., 2013, Robling and Stout, 1999. With improving technologies for three-dimensional analyses and a growing recognition of the structural complexity of Haversian networks (Arhatari et al., 2011, Britz et al., 2012, Cohen and Harris, 1958, Cooper et al., 2003, Cooper et al., 2007, Harrison and Cooper, 2015, Hennig et al., 2015, Koltze, 1951, Maggiano et al., 2016, Mohsin et al., 2002, Pratt et al., 2018, Pratt and Cooper, 2017, Tappen, 1977, it has become clear that osteon shape and size are highly dynamic, responding to current physiological needs, as well as local microstructural history. Recent work (Maggiano et al., 2016), utilizing synchrotron micro-CT to create reconstructions of three-dimensional osteon morphology within complex Haversian networks, has opened the door to new inquiries regarding osteon structure and permits new perspectives that could better inform two-dimensional analyses. ...
Objectives
Haversian systems result from bone remodeling, and show variation in size and shape among differing ages, body weights, mechanical environments, and species. While variables such as osteon circularity (On.Cr.) are generally studied in single transverse cross-sections, little is known about On.Cr. variation along an osteon’s length, investigated here, in order to strengthen our understanding of bone microstructure.
Materials and Methods
Up to 875 measurements of On.Cr. were generated for 41 osteonal segments from the proximal anterior diaphysis of femoral human cortical bone of three adult male samples (ages 46, 62, 74). We employed four hypotheses to investigate On.Cr. variability, in cross-section and longitudinally. H1: There is no difference in On.Cr. among osteons comprising single cross-sections, H2: There is no difference in On.Cr. among individuals when single cross-sections are compared, H3: There is no difference in On.Cr. among measurements taken from an osteon along the longitudinal axis, and H4: There is no discernable pattern in an osteon’s deviation from circularity.
Results
Quantitative analysis of single cross-sections revealed relatively consistent On.Cr. measurements within individual cross-sections and among individuals, supporting both, H1 and H2. Along individual osteonal segments, substantial degrees of dispersion from central tendencies were observed in 27 out of 41 analyzed osteons (despite relatively low overall standard deviations and interquartile ranges), leading to a rejection of H3. Qualitative characterization of morphological deviation from a “typical” circularity suggests a patterned deviation, leading also to a rejection of H4.
Discussion
On.Cr. variation is discussed in the context of both, phenomena intrinsic to a given osteon (including repetitive, small perturbations at roughly 45 μm intervals), and extrinsic (including shared reversal sheaths, osteonal branching, transverse connections, and osteonal repathing). Interesting associations between On.Cr. and other characteristics of the local Haversian network emphasize the role of Haversian systems as integrated parts of a greater morphological complex.
... 19.189 ing successive periods of bone remodeling [19]. Deterioration of bone microarchitectures such as cortical porosity and disorganization of osteonal canals could influence to maintenance of bone homeostasis and mechanical properties of appendicular bones [1,[20][21][22]. ...
... Osteon aligns themselves parallel to the loading direction corresponds to habitual principal stress direction aimed at improving bone's ability to endure the loading [33]. Several studies proposed that the prevailing oblique orientation of the secondary osteon correlate with bending to the medial side and to an external rotation torsional loading causes more circumferential canals, whereas compression and tension resulted in more longitudinal canals [21,[34][35][36]. Cortical mass at the long bone of rats are known to continues expansion until about 10-12 month old, and a decrease in bone strength was not apparent in femoral mid-diaphysis until 15-month old post ovariectomy [12]. ...
Osteoporosis is a major disease in aged women, increasing the risk for fractures accompanied by changes in the microarchitecture. The aim of this study was to investigate the three-dimensional (3D) histomorphology of femur diaphysis in the animal model for postmenopausal osteoporosis. The cortical bone of femur diaphysis of the rat was serially sectioned at a thickness of 5 μm and evaluated age-associated changes of the intracortical (osteonal) canal networks three-dimensionally. Cortical microstructures of 10-month old rats were not affected by ovariectomy. Intracortical canal networks were radial toward endosteal aspect and frequently interconnected across the neighboring canals with short arciform and irregular canals reminiscent for resorption spaces in ovarectomized 16-month old rats, contrary to intact canals in 16-month old control rat. Increased proportion of the periosteal circumference lamella and deformed endosteal regions with rare cortical canals hampered reconstructive histomorphology in ovarectomized rats of 26 month age. We have shown that 3D reconstruction of rat femur of the aged model over 16-month old is suitable methods that evaluate and microstructural change of the intracortical canals and cortical bone porosity by estrogen depletion.
... They proposed that this laminar arrangement was an adaptation to better resist torsional loading, the predominant loading regime in active flight (Pennycuick, 1967;Swartz et al. 1992;de Boef, 2008). Other evidence for canal orientation responding to loading patterns exists in humans (Hert et al. 1994;Petrt yl et al. 1996) where slight deviations to longitudinal orientations of canals are thought to reflect bending as the dominant loading force, and in rats where a 6-month period of paralysis of a limb (removing all loading) dramatically alters the orientation of vascular canals (Britz et al. 2012). Following these lines, Rothschild & Panza (2007) investigated whether the degree of laminarity was a possible risk factor in bird lower limb bones with osteoarthritis, although they found no link present. ...
... This study is the first to use micro-CT to test hypotheses about canal orientation in 3D. Previous studies on canal orientation using micro-CT have only been able to differentiate between longitudinal canals and transverse canals (Britz et al. 2012;Jast & Jasiuk, 2013) and have not been able to differentiate between radial and circumferential canals until . Our study relied on the synchrotron system to provide a wider field of view than most laboratory-based micro-CT systems, allowing the imaging of the full bone cross-section non-destructively. ...
Cortical bone porosity and specifically the orientation of vascular canals is an area of growing interest in biomedical research and comparative/paleontological anatomy. The potential to explain microstructural adaptation is of great interest. However, the determinants of the development of canal orientation remain unclear. Previous studies of birds have shown higher proportions of circumferential canals (called laminarity) in flight bones than in hindlimb bones, and interpreted this as a sign that circumferential canals are a feature for resistance to the torsional loading created by flight. We defined the laminarity index as the percentage of circumferential canal length out of the total canal length. In this study we examined the vascular canal network in the humerus and femur of a sample of 31 bird and 24 bat species using synchrotron micro-computed tomography (micro-CT) to look for a connection between canal orientation and functional loading. The use of micro-CT provides a full three-dimensional (3D) map of the vascular canal network and provides measurements of the 3D orientation of each canal in the whole cross-section of the bone cortex. We measured several cross-sectional geometric parameters and strength indices including principal and polar area moments of inertia, principal and polar section moduli, circularity, buckling ratio, and a weighted cortical thickness index. We found that bat cortices are relatively thicker and poorly vascularized, whereas those of birds are thinner and more highly vascularized, and that according to our cross-sectional geometric parameters, bird bones have a greater resistance to torsional stress than the bats; in particular, the humerus in birds is more adapted to resist torsional stresses than the femur. Our results show that birds have a significantly (P = 0.031) higher laminarity index than bats, with birds having a mean laminarity index of 0.183 in the humerus and 0.232 in the femur, and bats having a mean laminarity index of 0.118 in the humerus and 0.119 in the femur. Counter to our expectation, the birds had a significantly higher laminarity index in the femur than in the humerus (P = 0.035). To evaluate whether this discrepancy was a consequence of methodology we conducted a comparison between our 3D method and an analogue to two-dimensional (2D) histological measurements. This comparison revealed that 2D methods significantly underestimate (P < 0.001) the amount of longitudinal canals by an average of 20% and significantly overestimate (P < 0.001) the laminarity index by an average of 7.7%, systematically mis-estimating indices of vascular canal orientations. In comparison with our 3D results, our approximated 2D measurement had the same results for comparisons between the birds and bats but found significant differences only in the longitudinal index between the humerus and the femur for both groups. The differences between our 3D and pseudo-2D results indicate that differences between our findings and the literature may be partially based in methodology. Overall, our results do not support the hypothesis that the bones of flight are more laminar, suggesting a complex relation between functional loading and microstructural adaptation.
... To date, loading has been shown to affect bone in terms of length [4] [5], shape [6], mass [7] [8] [9], cross sectional area [10] [11] and strength [12] [13]. More recently, studies have examined how loading affects bone at the tissue-level with a focus on primary and secondary canal measures [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]. Regardless of the level of adaptation studied, mechanotransduction, the phenomenon whereby cells sense mechanical stimuli and translate them into a signal that can potentially elicit a response [29], lies at the core of skeletal dynamics. ...
... These specimens were derived from animals utilized in a previous study conducted on the femora by one of the co-authors (TJ) at the University of Tampere, Finland [46]. These specimens were also utilized in a previous study examining the effect unloading has on vascular canal orientation [28]. Ethics approval for collection and study was granted by the Ethics Committee for Animal Experiments of the University of Tampere and the Provincial Government of Western Finland Department of Social Affairs and Health, Finland. ...
... Tommasini et al., 2012;Sharma et al., 2018;Karunaratne et al., 2016;Kerckhofs et al., 2016; Ay et al., 1038;Matsumoto and Ando, 2011), disuse(Britz et al., 2012;Matsumoto et al., 2007;Tanaka et al., 2019) and its effect on mechanical properties (Schneider et al., 2013a;Fig. 4. HR-micro-CT based analysis of (a) canal orientation and (b) canal tortuosity. ...
Intracortical canals are a major contributor to cortical bone porosity and influence its mechanical response. Canal networks act as stress concentrators and the magnitude of which depends on the size and spatial distribution of canals. In the present study, we investigated site-dependent variation in intracortical canal network morphological indices and their effect on the mechanical response of bone. For this, mid-diaphysis of rat tibia bones were scanned using high-resolution micro-CT and morphological indices were measured for four main anatomical sites-anterior, posterior, medial and lateral. Further, a micro-finite element (μFE) model was developed to quantify the stress concentration regions in different cortices. The fracture risk was assessed using an effective strain approach. Results show that canal porosity, canal orientation and canal length are site-dependent whereas canal diameter and canal number density are independent of the site. The lateral cortex has significantly higher porosity compared to the posterior cortex (p < 0.05). The orientation of canals is found significantly different between endosteal and periosteal regions for anterior and medial quadrants. Canals are inclined at higher angles with bone axis in the endosteal region as compare to the periosteal region. The μ-FE results show that the regions with higher effective strain are concentrated around the canals. Further, failed element volume per unit bone volume is found highest for medial cortex whereas lowest for posterior cortex. The higher failed volume is associated with more radial canals in the medial cortex as compare to other cortices. The linear regression analysis shows that the volume of overstrained elements strongly depends on canal orientation (R² = 0.73, p < 0.0001) and canal porosity (R² = 0.61, p < 0.0001). The findings from this study suggest that along with vascular canal porosity, canal orientation and canal diameter can further improve the bone fracture risk assessment.
... It was also noticed that behind those macroscopic changes whose appearance corresponds to C(b), actually two different microarchitectural patterns were actually evident (Figure 7). In the case of axial loading, increased loading would be expected to reduce cortical porosity in line with the bone adaptation principle (Roux, 1881;Wolff, 1892), as confirmed in experimental studies (Bouvard, Mabilleau, Legrand, Audran, & Chappard, 2012;Britz, Jokihaara, Leppanen, Jarvinen, & Cooper, 2012). However, our microarchitectural findings in all entheseal stages at the gluteus maximus insertion site showed an increased porosity of the cortical bone. ...
Objectives:
Macroscopic entheseal forms show two main features: predominant signs of bony formation or resorption. To understand the development of these forms, we investigated microarchitectural differences between the macroscopic proliferative and resorptive forms of the gluteus maximus enthesis.
Materials and methods:
The macromorphological analysis of entheseal changes (EC) was based on the Villotte, visual scoring system for fibrous entheses. Gluteal tuberosity specimens of different stages of Villote's system were harvested from 16 adult males derived from an archaeological context and scanned using microcomputed tomography.
Results:
The microarchitectural analyzes of cortical bone demonstrated a trend of higher porosity in the resorptive compared to the proliferative phase in Stage B, whereas a 30% porosity reduction was detected in the resorptive compared to proliferative phase of Stage C. In terms of the trabecular bone between the resorptive and proliferative entheseal phases, there was a trend of increased connectivity density, whereas the structural model index decreased in B and increased in C. The assessment of the entire specimen showed an increase in porosity from the proliferative to the resorptive phase in the Stage B, in contrast to a decrease in the Stage C.
Discussion:
The results suggest that from an initial flat entheses, two directions of EC development are possible: (a) a bony prominence may form and, subsequently, it is subjected to trabecularization of the cortical bone inside the prominence, such cortical trabecularization can lead to visible porosity on the cortical external surface; (b) the cortical bone defect may develop with the regular underlying cortical bone.
... Following SR CT, the intracortical canal network and osteocyte lacunae were extracted as a negative imprint of the calcified tissue from CT data sets 17,21,[33][34][35][36][37] . Using individual separation thresholds detailed (Supplementary methods 1 and supplementary table 3 ...
Osteoblast (OB) lineage cells are an important source of vascular endothelial growth factor (VEGF), which is critical for bone growth and repair. During bone development, pubertal differences in males and females exist, but little is known about whether VEGF signalling contributes to skeletal sexual dimorphism. We have found that in mice, conditional disruption of VEGF in osteocalcin expressing cells (OcnVEGFKO) exerts a divergent influence on morphological, cellular, and whole bone properties between sexes. Furthermore, we describe an underlying sexual divergence in VEGF signalling in OB cultures in vitro independent of circulating sex‐hormones. High‐resolution synchrotron computed tomography and backscattered scanning electron microscopy revealed, in males, extensive unmineralised osteoid encasing enlarged blood vessel canals and osteocyte lacunae in cortical bone following VEGF deletion, which contributed to increased porosity. VEGF was deleted in male and female long bone‐derived OBs (OBVEGKO) in vitro and Raman spectroscopic analyses of mineral and matrix repertoires highlighted differences between male and female OBVEGFKO cells, with increased immature phosphate species prevalent in male OBVEGFKO cultures versus WT. Further sexual dimorphism was observed in bone marrow endothelial cell gene expression in vitro following VEGF deletion and in sclerostin protein expression, which was increased in male OcnVEGFKO bones versus WT. The impact of altered OB matrix composition following VEGF deletion on whole bone geometry was assessed between sexes, although significant differences between OcnVEGFKO and WT were identified only in females. Our results suggest that bone‐derived VEGF regulates matrix mineralisation and vascularisation distinctly in males and females which results in divergent physical bone traits.
... Disuse-mediated changes in vascular canal organization assessed in growing rats subjected to sciatic nerve resection suggest that bone loading plays a role in determining the vascular pore structure during skeletal development. 33,34 but no three-dimensional analysis of vascular pore changes in response to disuse has been previously reported in adult rats. Interestingly, recent work from our group has shown similar vascular canal changes in the proximal tibial metaphysis of skeletally mature rats in response to estrogen deficiency, a condition associated with the development of postmenopausal osteoporosis. ...
Reduced mechanical loading can lead to disuse osteoporosis, resulting in bone fragility. Disuse models report macroscopic bone loss due to muscle inactivity and immobilization, yet only recently has there been quantification of the effects of disuse on the vascular pores and osteocyte network, which are believed to play an important role in mechanotransduction via interstitial fluid flow. The goal of this study was to perform a high‐resolution analysis of the effects of muscle inactivity on intracortical porosity and osteocyte lacunar density in skeletally mature rats. Muscle paralysis was induced in 20‐week‐old female Sprague Dawley rats by injection of botulinum neurotoxin. Rats were injected in the right hindlimb muscles with either Botox (BTX, n = 8) or saline solution (CTRL, n = 8), with a third group used as baseline controls (n = 8). Four weeks after injection, Botox caused a ∼60% reduction in hindlimb muscle mass. High‐resolution micro‐CT analysis showed that Botox‐induced muscle paralysis increased vascular canal porosity and reduced osteocyte lacunar density within the tibial metaphysis cortex. Cortical thickness and other areal properties were diminished in the proximal tibial metaphysis, whereas no differences were found in the mid‐diaphysis. Within the BTX group, the injected limbs showed a lower cancellous bone volume fraction relative to the contralateral limb. These results indicate that diminished muscle activity alters the vascular canal porosity and osteocyte lacunar density in cortical bone, which could alter interstitial fluid flow, affecting molecular transport and the transmission of mechanical signals to osteocytes. This article is protected by copyright. All rights reserved
... Although conventional absorption-based µCT is not sensitive enough to provide sufficient contrast for soft tissues, such as the vasculature, it is used to provide spatial clues for the location of the blood vessels that reside within the intracortical canal network (Schneider et al., 2009). The intracortical canal network can be extracted as a negative imprint of the calcified tissue from standard µCT images and several studies apply this principle to human (Cooper et al., 2003;Cooper et al., 2007a;Cooper et al., 2007b) and rodent bones (Britz et al., 2010;Britz et al., 2012;Schneider et al., 2007;Schneider et al., 2013;Thurner et al., 2010). Imaging of osteocytes in bone has also proved problematic in the past, due to the mineralisation of the bone matrix. ...
Cortical bone is permeated by a system of pores, occupied by the blood supply and osteocytes. With ageing, bone mass reduction and disruption of the microstructure are associated with reduced vascular supply. Insight into the regulation of the blood supply to the bone could enhance the understanding of bone strength determinants and fracture healing. Using synchrotron radiation-based computed tomography, the distribution of vascular canals and osteocyte lacunae was assessed in murine cortical bone and the influence of age on these parameters was investigated. The tibiofibular junction from 15-week- and 10-month-old female C57BL/6J mice were imaged post-mortem. Vascular canals and three-dimensional spatial relationships between osteocyte lacunae and bone surfaces were computed for both age groups. At 15 weeks, the posterior region of the tibiofibular junction had a higher vascular canal volume density than the anterior, lateral and medial regions. Intracortical vascular networks in anterior and posterior regions were also different, with connectedness in the posterior higher than the anterior at 15 weeks. By 10 months, cortices were thinner, with cortical area fraction and vascular density reduced, but only in the posterior cortex. This provided the first evidence of age-related effects on murine bone porosity due to the location of the intracortical vasculature. Targeting the vasculature to modulate bone porosity could provide an effective way to treat degenerative bone diseases, such as osteoporosis.
... Alterations in the microstructure of cortical bone have been observed in several models of osteoporosis, including estrogen deficiency, aging, and disuse (Westerlind et al., 1997;Sharma et al., 2012;Tommasini et al., 2012;Britz et al., 2012). Both vascular porosity and lacunar-canalicular microarchitecture have been shown to change in response to a drop in estrogen level in a rat https://doi.org/10.1016/j.jbiomech.2017.11.011 0021-9290/Ó 2017 Elsevier Ltd. ...
Loading-induced interstitial fluid flow in the microporosities of bone is critical for osteocyte mechanotransduction and for the maintenance of tissue health, enhancing convective transport in the lacunar-canalicular system. In recent studies, our group has reported alterations of bone's vascular porosity and lacunar-canalicular system microarchitecture in a rat model of postmenopausal osteoporosis. In this work, poroelastic finite element analysis was used to investigate whether these microstructural changes can affect interstitial fluid flow around osteocytes. Animal-specific finite element models were developed combining micro-CT reconstructions of bone microstructure and measures of the poroelastic material properties. These models were used to quantify and compare loading-induced fluid flow in the lacunar-canalicular system of ovariectomized and sham-operated rats. A parametric analysis was also used to quantify the influence of the lacunar-canalicular permeability and vascular porosity on the fluid velocity magnitude. Results show that mechanically-induced interstitial fluid velocity can be significantly reduced in the lacunar-canalicular system of ovariectomized rats. Interestingly, the vascular porosity is shown to have a major influence on interstitial fluid flow, while the lacunar-canalicular permeability influence is limited when larger than 10-20m2. Altogether our results suggest that microstructural changes associated with the osteoporotic condition can negatively affect interstitial fluid flow around osteocytes in the lacunar-canalicular system of cortical bone. This fluid flow reduction could impair mechanosensation of the osteocytic network, possibly playing a role in the initiation and progression of age-related bone loss and postmenopausal osteoporosis.
... Numerous genes regulate limb bone size and shape [29,30], and are highly sitespecific even within a single bone [2]. Alterations in osteocyte density [31][32][33] and vascularity [34] may influence bone (re)modeling and mechano-response as well. Thus, it remains unclear whether local differences in the bone's response to loading are due to variations in the mechanical behavior of the bone, because of the bone's morphology or material, or whether it is due to variations of the skeleton's biological ability to sense or respond to the mechanical signals. ...
Bone has an adaptive capacity to maintain structural integrity. However, there seems to be a heterogeneous cortical (re)modeling response to loading at different regions within the same bone, which may lead to inconsistent findings since most studies analyze only one region. It remains unclear if the local mechanical environment is responsible for this heterogeneous response and whether both formation and resorption are affected. Thus, we compared the formation and resorptive response to in vivo loading and the strain environment at two commonly analyzed regions in the mouse tibia, the mid-diaphysis and proximal metaphysis. We quantified cortical surface (re)modeling by tracking changes between geometrically aligned consecutive in vivo micro-tomography images (time lapse 15 days). We investigated the local mechanical strain environment using finite element analyses. The relationship between mechanical stimuli and surface (re)modeling was examined by sub-dividing the mid-diaphysis and proximal metaphysis into 32 sub-regions. In response to loading, metaphyseal cortical bone (re)modeled predominantly at the periosteal surface, whereas diaphyseal (re)modeling was more pronounced at the endocortical surface. Furthermore, different set points and slopes of the relationship between engendered strains and remodeling response were found for the endosteal and periosteal surfaces at the metaphyseal and diaphyseal regions. Resorption was correlated with strain at the endocortical, but not the periosteal surfaces, whereas, formation correlated with strain at all surfaces, except at the metaphyseal periosteal surface. Therefore, besides mechanical stimuli, other non-mechanical factors are likely driving regional differences in adaptation. Studies investigating adaptation to loading or other treatments should consider region-specific (re)modeling differences.
... Studies of resorption cavities in human cortical bone show a potential for variable morphologies of Haversian systems from interconnected or clustered to branched. Resorption cavities have been shown to run unidirectional or simultaneously in two or more directions through several branches (Schumacher, 1935;Cohen & Harris, 1958;Vasciaveo & Bartoli, 1961;Johnson, 1964;Tappen, 1977;Cooper et al. 2003Cooper et al. , 2004Cooper et al. , 2006Basillais et al. 2004;Matsumoto et al. 2005;Schneider et al. 2007;Britz et al. 2012;Carter et al. 2013Carter et al. , 2014. Accordingly, the 3D reality of Haversian systems is quite complex; they are both branched and connected through transversely oriented canals, building complex, dynamic networks (Koltze, 1951;Cohen & Harris, 1958;Stout et al. 1999;Cooper et al. 2011). ...
This study uses synchrotron radiation-based micro-computed tomography (CT) scans to reconstruct three-dimensional networks of Haversian systems in human cortical bone in order to observe and analyse interconnectivity of Haversian systems and the development of total Haversian networks across different ages. A better knowledge of how Haversian systems interact with each other is essential to improve understanding of remodeling mechanisms and bone maintenance; however, previous methodological approaches (e.g. serial sections) did not reveal enough detail to follow the specific morphology of Haversian branching, for example. Accordingly, the aim of the present study was to identify the morphological diversity of branching patterns and transverse connections, and to understand how they change with age. Two types of branching morphologies were identified: lateral branching, resulting in small osteon branches bifurcating off of larger Haversian canals; and dichotomous branching, the formation of two new osteonal branches from one. The reconstructions in this study also suggest that Haversian systems frequently target previously existing systems as a path for their course, resulting in a cross-sectional morphology frequently referred to as 'type II osteons'. Transverse connections were diverse in their course from linear to oblique to curvy. Quantitative assessment of age-related trends indicates that while in younger human individuals transverse connections were most common, in older individuals more evidence of connections resulting from Haversian systems growing inside previously existing systems was found. Despite these changes in morphological characteristics, a relatively constant degree of overall interconnectivity is maintained throughout life. Altogether, the present study reveals important details about Haversian systems and their relation to each other that can be used towards a better understanding of cortical bone remodeling as well as a more accurate interpretation of morphological variants of osteons in cross-sectional microscopy. Permitting visibility of reversal lines, synchrotron radiation-based micro-CT is a valuable tool for the reconstruction of Haversian systems, and future analyses have the potential to further improve understanding of various important aspects of bone growth, maintenance and health.
... The increasing availability and resolution of microcomputed tomography (micro-CT) has made it possible to efficiently visualize and quantify the osteonal canals and BMU-related resorption spaces within cortical bone (Mohsin et al. 2002;Cooper et al. 2003Cooper et al. , 2004Cooper et al. , 2006Cooper et al. , 2007Tanck et al. 2006;Basillais et al. 2007;Arhatari et al. 2011), including assessment of canal orientation (Britz et al. 2012). Analysis of cortical canals has become an established approach, but visualization of osteon boundaries (e.g. by variation in mineralization and/or cement lines) remains challenging for absorption-based micro-CT. ...
The primary microstructural unit of cortical bone, the secondary osteon or Haversian system, is widely assumed to have a cylindrical shape. It is generally accepted that osteons are roughly circular in cross-section and deviations from circularity have been attributed to deviations from longitudinal orientation. To our knowledge this idealized geometric relationship, which assumes osteons are perfect cylinders, has not been rigorously explored. As such, we sought to explore two research questions: (i) Does the orientation of osteons in 3D explain variation in shapes visualized in 2D? (ii) Can differences in osteon 3D orientation explain previously reported age-related differences observed in their 2D cross-sectional shape (e.g. more circular shape and decreased area with age)? To address these questions we utilized a combination of 2D histology to identify osteon shape and superimposed micro-computed tomography data to assess osteon orientation in 3D based upon the osteonal canal. Shape was assessed by the inverse of Aspect Ratio (On.AspR(-1) , based on a fitted ellipse) - which ranged from 0 (infinitely elongated shape) to 1 (perfectly circular). A sample (n = 27) of human female anterior femoral cortical bone samples from across the human lifespan (20-87 years) were included in the analysis, which involved 1418 osteons. The overall mean measure of On.AspR(-1) was 0.703 (1.42 Aspect Ratio). Mean osteon orientation was 79.1° (90° being longitudinal). While we anticipated a positive relation between orientation and On.AspR(-1) , we found the opposite - a weak negative correlation (with more oblique 3D osteon alignment, the 2D shape became more circular as reflected by increased On.AspR(-1) ). When analysis of covariance was performed with age and orientation as covariates, the negative relation with orientation was replaced by a significant relation with age alone. This relation with age accounted for 41% of the variation of On.AspR(-1) . The results revealed that osteons, on average, are not circular in cross-section and that 3D orientation cannot account for deviation from circular shape. Osteons thus are strictly speaking not cylinders, as they tend to have elliptical cross-sections. We observed that osteons did become less elliptical in cross-section with age independent of orientation - suggesting this is a real change in morphology.
© 2015 Anatomical Society.
... This finding is in agreement with Skedros and colleagues' [42] work where Haversian canal and osteon surface area did not confirm positive (in ribs) and negative (in femora) allomet- ric relationships. Extending the present result to a biome- chanical context, it is also in line with past research where it has been demonstrated that small osteons and Haversian canals are in an inverse relationship with strain [e.g., 3-8, [21][22][23]43]. Given that previous evidence mainly derives from smaller samples of animals and human cadaveric bone, results from the present study demonstrate strong bone histomorphometric relationships in a large human sample for the first time. ...
Cortical bone histomorphometry utilised in human and animal bone biology studies has demonstrated that osteon densities and their geometric properties may be in a relationship with biomechanical load application. Further research is required to investigate mutual links between bone histological variables to elucidate their usefulness in future biomechanical studies. Here, a series of correlations exploring bone biology relationships at the human midshaft femur were performed using a large sample. Mean intact, fragmentary and total osteon population densities, Haversian canal diameter and area, osteon area, as well as osteocyte lacunae density were measured along the sub-periosteal cortex in sections removed from the posterior midshaft aspect of modern human male (n = 233) and female (n = 217) femora (total n = 450). Parametric and non-parametric correlations between the histology variables were sought in the entire sample, as well as within age and sex sub-groups. Several significant positive and negative correlations explaining a large proportion of data variation were found. Haversian canal area, diameter, and osteon area were positively correlated. As the density of osteocyte lacunae increased, Haversian canals and osteons became smaller. As osteons increased in density, so did osteocyte lacunae, but Haversian canal and osteon area became smaller. Results were consistent across age and sex groups. Findings suggest that an increased rate of bone remodelling is associated with a decrease in geometrical properties of osteons. An increased density of osteocyte lacunae and osteons indicates the involvement of bone maintenance cells in remodelling potentially induced by mechanical stimuli. Future histomorphometry studies will benefit from examining multiple bone histology variables due to many mutual bone biology relationships that exist at the human midshaft femur.
... This is supported by a second recent HR-pQCT study at the ultradistal radius in which cortical porosity predicted prevalent fractures independent of aBMD [6]. In addition to porosity, measures of pore structure including size, shape, orientation, and distribution may be critical in understanding fracture incidence [7,8]. ...
Cortical bone porosity has been demonstrated to be a major determinant of strength, stiffness, and fracture toughness of cortical tissue. The goal of this work was to investigate changes in spatial distribution and microstructure of cortical porosity associated with aging in men and women. The specific aims were to: 1) develop an automated technique for laminar analysis of cortical microstructure based on HR-pQCT data, and; 2) apply this technique to explore sex- and age-specific distribution and microstructure of porosity within the cortex. We evaluated HR-pQCT images of the distal tibia from a cross-sectional cohort of 145 individuals, characterizing detectable pores as being in the endosteal, midcortical, or periosteal layers of the cortex. Metrics describing porosity, pore number, and pore size were quantified within each layer and compared across sexes, age groups, and cortical layers. The elderly cohort (65-78 years, n=22) displayed higher values than the young cohort (20-29 years, n=29) for all parameters both globally and within each layer. While all three layers displayed significant age-related porosity increases, the greatest difference in porosity between the young and elderly cohort was in the midcortical layer (+344%, p<0.001). Similarly, the midcortical layer reflected the greatest differences between young and elderly cohorts in both pore number (+243%, p<0.001) and size (+28%, p<0.001). Females displayed greater age-related changes in porosity and pore number than males. Females and males displayed comparable small to non-significant changes with age in pore size. In summary, considerable variability exists in the spatial distribution of detectable cortical porosity at the distal tibia, and this variability is dependent on age and sex. Intracortical pore distribution analysis may ultimately provide insight into both mechanisms of pore network expansion and biomechanical consequences of pore distribution.
Copyright © 2015. Published by Elsevier Inc.
Cortical bone is the main mechanical bearing structure of bone, and the mechanical properties of materials are not only related to bone mineral density, but also largely depend on its pores microstructure which affected by blood vessels. However, the change of pores structure in cortical bone under microgravity was still unclear. In this study, in order to clear the changes of pore structure with cortical vascular pores and its effect on bone mechanical properties, rat tail-suspension was used to simulate microgravity and the changes of the microstructure in rat tibia cortices were investigated by high-resolution micro-CT (3 μm) while the bone mechanical properties were measured via three point bending test. The results showed the bone mineral density of cortical bone didn’t change in tail-suspended rats. However, the pore structure of cortical bone in tail-suspended rats changed significantly, the proportion of pores greater than 15 μm (cortical vascular pores) increased while that less than 15 μm decreased. The mechanical properties of bone (such as maximum load and maximum stress) in tail-suspended rats deteriorated. And the volume ratio of pore vessels (vessel volume/tissue volume) was negatively correlated with the mechanical properties. In conclusion, the increase of cortical vascular pores in rats caused by the simulated microgravity contributes to the decrease of mechanical properties.
La distinction entre des restes humains et animaux ou entre différentes espèces fauniques représente l’une des étapes fondamentales de l’identification de restes osseux retrouvés en contexte archéologique ou forensique. Cependant, l’état de fragmentation des vestiges osseux rend souvent difficile voire impossible leur identification sur la seule base de l’anatomie ostéologique. L’utilisation de nouveaux critères d’identification et d’une méthode d’analyse plus précise est alors nécessaire. Dans cette perspective, l’analyse de la microstructure osseuse et en particulier du réseau canalaire cortical (RCC) apparaît discriminante car ce réseau présente des différences de structure en fonction des espèces. Cette thèse a pour objectif la définition de paramètres du RCC permettant de déterminer l’origine humaine ou animale de fragments osseux ainsi que l’identification de leur espèce animale d’appartenance. Dans le cadre de cette recherche, nous avons effectué l’analyse qualitative et quantitative en 3D d’acquisitions µCT de diaphyses d’os longs (fémurs et humérus) appartenant à 3 espèces (H. sapiens, S. scrofa et B. taurus). Le test d’identification d’espèce, effectué grâce à la méthode statistique de forêts aléatoires, permet d’obtenir un taux d’identification totale de 88.82%. Les individus humains sont correctement identifiés à 100% contre 86.2% pour S. scrofa et 79.51% pour B. taurus. La longueur moyenne des canaux et la connectivité apparaissent comme les variables les plus discriminantes. L’observation et la quantification du RCC en 3D se révèle une méthode d’analyse non-invasive prometteuse pour la distinction d’espèce, appropriée en cas de spécimens rares ou fragiles. Elle s’applique dans divers domaines scientifiques, en cas de forte fragmentation osseuse. Afin de confirmer et d’améliorer ces premiers résultats, il est toutefois nécessaire de développer cette méthode sur d’autres éléments osseux et d’élargir le spectre des espèces animales testées.
How can activity reconstruction address day-to-day life in the past? What are its strengths? What are its drawbacks? One of the ways in which everyday action can be examined in the past is through the osteological examination of activity. There are several methods, including muscle attachment site and osteoarthritis analyses, which have the potential to speak to broad levels of physical activity. In this chapter, I discuss the osteological characteristics, etiology, previous bioarchaeological research, and ongoing bioarchaeological debates for both osteoarthritis and entheseal changes. Throughout the chapter I discuss how these data can illuminate everyday activities of the ancient past. It is important to note that these methods are not without drawbacks—several contributing factors, most notably of which is age, as well as an unclear progression of the conditions limit bioarchaeological interpretations of activity in the past. However, using statistical controls and robust samples sizes, bioarchaeologists can begin to overcome some of these obstacles. I also provide a case study of entheseal changes and osteoarthritis from the ancient Kerma culture (Nubia, 2500–1500 BCE). Here I compare entheseal changes and osteoarthritis for individuals of differing socioeconomic groups and conclude that this social category had a meaningful impact on the everyday lives of these individuals and these experiences were embodied by the people of Kerma.
Vascular canals in cortical bone during growth and development typically show an anisotropic pattern with canals falling into three main categories: circumferential, radial, and longitudinal. Two major hypotheses attempt to explain the preferred orientations in bone: that vascular canal orientation is optimized to resist a predominant strain direction from functional loading, or that it reflects growth requirements and velocity. We use a controlled growth experiment in broiler chickens to investigate the effect of growth rate on vascular canal orientation. Using feed restriction we set up a fast growing control group and a slow growing restricted group. We compared the microstructure in the humerus and the femur at 42 days of age using synchrotron micro‐computed tomography (micro‐CT), a three‐dimensional (3D) method that visualizes the full canal network. We measured the 3D orientation of each canal in the whole cross‐section of the bone cortex using a set of custom ImageJ scripts. Using these orientations we compute laminar, radial, and longitudinal indices that measure the proportion of circumferential, radial, and longitudinal canals, by unit of length, in the cortex. Following previous studies we hypothesized that vascular canal orientation is related to growth, with radial canals linked to a faster growth rate and related to functional loading through a high laminar index in flight bones which reflects torsional loading resulting from active flight. The control group had final body weights that were nearly twice the final weights of the restricted group and higher absolute growth rates. We found consistent patterns in the comparison between the humerus and the femur in both groups, with the humerus having higher laminar and longitudinal indices, and a lower radial index than the femur. The control group had higher radial indices and lower laminar and longitudinal indices in both the humerus and the femur than the restricted group. The higher radial indices in our control group point to a link between radial canals and faster growth, and between laminar canals and slower growth, while the higher laminar indices in the humerus point to a link between circumferential canals and torsional loading. Overall, our results indicate that the orientation of the cortical canal network in a bone is the consequence of a complex interaction between the growth rate of that bone and functional loading environment.
Objectives:
Differentiating human from nonhuman fragmented bone is often accomplished using histological methods if the observation of gross morphology proves insufficient. Linearly oriented primary and/or secondary osteonal systems, commonly referred to as osteon bands, are described as a strong indicator of nonhuman bone, particularly the occurrence of multiple bands. This phenomenon has been conventionally documented using two-dimensional (2D) histology, but such analyses are destructive and typically limited to a single cross-section. Progressive developments in high-resolution X-ray imaging, however, allow for the nondestructive three-dimensional (3D) visualization of bone microarchitecture. The primary objective of the current research was to visualize and document the occurrence of osteon banding in adult human cortical bone using high-resolution synchrotron radiation-based micro-Computed Tomography (SR micro-CT).
Materials and methods:
Synchrotron radiation-based micro-CT scanning was carried out at the Canadian Light Source (CLS) national synchrotron facility. The presence or absence of osteon banding was visualized in human skeletal elements from three adult males with representative samples from all regions of the skeleton (n = 129). If present, osteon banding was described and quantified.
Results:
Results indicated that 23 of 129 human cortical bone specimens exhibited osteon banding, representing 18% of the sample. Linear arrangements of primary and/or secondary osteons were observed in the following skeletal elements: temporal, parietal, frontal, occipital, clavicle, mandible, femur, tibia, ulna, second metatarsal, and sacrum.
Discussion:
The present work represents the first 3D examination of inter-element variation in osteon banding in adult human cortical bone. Findings indicate that the presence of multiple osteon bands in a single specimen is not diagnostic of nonhuman bone. As such, osteon banding categorically should not be taken as evidence of nonhuman bone in forensic and archaeological contexts.
This study was aimed to investigate the spatial and temporal changes of subchondral bone and its overlying articular cartilage in rats following knee immobilization. A total of 36 male Wistar rats (11–13 months old) were assigned randomly and evenly into 3 groups. For each group, knee joints in 6 rats were immobilized unilaterally for 1, 4, or 8 weeks, respectively, while the remaining rats were allowed free activity and served as external control groups. For each animal, femurs at both sides were dissected after sacrificed. The distal part of femur was examined by micro-CT. Subsequently, femoral condyles were collected for further histological observation and analysis. For articular cartilage, significant changes were observed only at 4 and 8 weeks of immobilization. The thickness of articular cartilage and chondrocytes numbers decreased with time. However, significant changes in subchondral bone were defined by micro-CT following immobilization in a time-dependent manner. Immobilization led to a thinner and more porous subchondral bone plate, as well as a reduction in trabecular thickness and separation with a more rod-like architecture. Changes in subchondral bone occurred earlier than in articular cartilage. More importantly, immobilization-induced changes in subchondral bone may contribute, at least partially, to changes in its overlying articular cartilage. Microsc. Res. Tech., 2016.
The studies of entheses in bioarchaeology attempted to reconstruct the habitual physical activities of past populations. However, the studies of microarchitecture of the underlying bone are still lacking despite well-known potential of bone internal microarchitecture to reflect mechanical loading. It is unknown whether different morphological expressions of entheseal changes (ECs) correlate with the microstructural characteristics of the underlining bone. This study analyzed bone microstructural characteristics at the entheses. Our focus was on examining the possible successive nature of the three-stage scale of entheseal macroscopic changes by comparing EC scores with the microarchitectural features at the attachment sites. The study was based on the hypothesis that mechanical loading influences the microarchitecture of the bone at the attachment site. The bone samples were taken from 24 adult male skeletons from medieval cemeteries in Serbia, with different macroscopic expression score of EC. We evaluated the macroscopic and microscopic appearance of four entheses of the lower limbs (origin of the soleus muscle and the insertions of the adductor magnus, gluteus maximus, and iliopsoas muscles). The specimens were scanned using microcomputed tomography (Scanco µCT 40). Our data showed a lack of consistent correlation between stages of the macroscopic scoring systems with microarchitecture at the entheses, only cortical thickness was significantly different between EC stages. Analyzing relationship between trabecular and cortical bone microstructure we found correlations between cortical and trabecular variables only in Stage C. Results of our study suggest that macroscopic EC might not represent distinct successive phases in bone adaptation to mechanical loading. Am J Phys Anthropol, 2014. © 2014 Wiley Periodicals, Inc.
Mechanical testing studies by Gupta et al. (2009, 2013) suggest that the extraordinary toughness of antler bone is primarily achieved by intrinsic/nanostructural mechanisms instead of extrinsic/microstructural mechanisms. However, this conclusion is based on data from extremely small specimens from one antler loaded only in tension, which impedes discernment of the relative importance of intrinsic vs. extrinsic mechanisms. In the present study we conducted analyses into the microstructural features of antler for details of potential additional microscale toughening characteristics, as suggested by recent mechanical testing studies of bulk specimens. The data are also considered in view of the above-mentioned studies concluding that extrinsic/microstructural toughening mechanisms are less important than nanoscale/intrinsic toughening mechanisms in antler. Mule deer antlers were evaluated using: (1) backscattered electron imaging for micro-mineralization, (2) circularly polarized light for osteonal interfacial complexity and collagen fiber orientation (CFO) heterogeneity, and (3) X-ray 3D micro-computed tomography for osteon/vessel orientation, density, and size. Results showed: (1) hyper-mineralized seams of approximately 3-4 microns thickness within relatively hypermineralized "zones" that course circuitously along osteonal interfaces, (2) highly heterogeneous CFO, including increased oblique-to-transverse CFO near/adjacent to osteon peripheries, and (3) osteons are often highly elongated in 2D. 3D reconstructions show that a considerable percentage of the vascular canals course obliquely with respect to the antler long axis. While results show multiple possible extrinsic-level histological characteristics in antler bone, it remains to be determined if microstructural characteristics become subsidiary to nanostructural characteristics in enhancing toughness during the majority of post-yield behavior of antler bone when loaded in a biologically relevant fashion.
Cortical bone has a hierarchical structure, spanning from the macrostructure at several millimeters or whole bone level, the microstructure at several hundred micrometers level, to the nanostructure at hydroxyapatite (HAp) crystals and collagen fibrils levels. The aim of the study is to understand the relationship between the HAp crystal orientation and the elastic modulus and the relationship between the osteon area fraction and the deformation behavior of HAp crystals in cortical bone. In the experiments, five strip specimens (40×2×1mm(3)) aligned with the bone axis were taken from the cortical bone of a bovine femur. The degree of c-axis orientation of HAp crystals in the specimens was measured with the X-ray diffraction technique with the imaging plate. To measure the deformation behavior of HAp crystals in the specimens, tensile tests under X-ray irradiation were conducted. The specimens were cut at the X-ray measurement positions and osteon area fraction and porosity at the transverse cross-sections were observed. Further, the volume fraction of HAp of the specimens was measured. Results showed the degree of c-axis orientation of HAp crystals was positively correlated with the elastic modulus of the specimens (r=0.94). The volume fraction of HAp and the porosity showed no statistical correlation with the elastic modulus and the tensile strength. The HAp crystal strain ε(H) increased linearly with the bone tissue strain ε. The average value of ε(H)/ε was 0.69±0.13 and there was no correlation between the osteon area fraction and ε(H)/ε (r=-0.27, p=0.33). The results suggest that the degree of c-axis orientation of HAp crystals affects the elastic modulus and the magnitude of HAp crystal strain does not depend on the osteon area fraction.
Regulation of the magnitude and distribution of skeletal strain is achieved in limb bones both “internally” by alterations
to bone morphology, and “externally” by coordination of muscle activity and modifications in the animal's behavior. Though
not actually minimizing functional strain, these mechanisms conspire to produce a restricted strain environment, which allows
an economical and optimized structure, and, in a variety of species over a wide range of activities, the architectural and
behavioral modifications have resulted in remarkably similar peak functional strain magnitudes. This confined range of functional
strains reflects a universal mechanism and objective of structural adaptation in bone.
According to prevailing understanding, skeletal mechano-responsiveness declines with age and this apparent failure of the mechano-sensory feedback system has been attributed to the gradual bone loss with aging (age-related osteoporosis). The objective of this study was to evaluate whether the capacity of senescent skeleton to respond to increased loading is indeed reduced as compared to young mature skeleton.
108 male and 101 female rats were randomly assigned into Exercise and Control groups. Exercise groups were subjected to treadmill training either at peak bone mass between 47-61 weeks of age (Mature) or at senescence between 75-102 weeks of age (Senescent). After the training intervention, femoral necks and diaphysis were evaluated with peripheral quantitative computed tomography (pQCT) and mechanical testing; the proximal tibia was assessed with microcomputed tomography (microCT). The microCT analysis revealed that the senescent bone tissue was structurally deteriorated compared to the mature bone tissue, confirming the existence of age-related osteoporosis. As regards the mechano-responsiveness, the used loading resulted in only marginal increases in the bones of the mature animals, while significant exercise-induced increases were observed virtually in all bone traits among the senescent rats.
The bones of senescent rats displayed a clear ability to respond to an exercise regimen that failed to initiate an adaptive response in mature animals. Thus, our observations suggest that the pathogenesis of age-related osteoporosis is not attributable to impaired mechano-responsiveness of aging skeleton. It also seems that strengthening of even senescent bones is possible--naturally provided that safe and efficient training methods can be developed for the oldest old.
This study evaluated the contributions of locomotive loading and estrogen to the development of diaphysis of rat femur. A randomized 2x2 study design was used. Altogether, 70 female Sprague-Dawley rats were used, of which 10 were euthanized at entry. Of the remaining rats, 16 served as controls, and the rest, 44, underwent a unilateral sciatic neurectomy. The effect of estrogen was removed by ovariectomizing one-half of the neurectomized rats. After 27 wk, the animals were euthanized, and the femora were excised. Irrespective of loading or estrogen, the femur length and mineral mass increased by 142 and 687%, respectively. Axial growth was not modulated either by locomotive loading or estrogen, but the loading resulted in direction-specific changes in the cross-sectional geometry. The estrogen-related gains were evident on the endocortical surface, while the loading-related gains occurred on the periosteal surface. The loading and estrogen were significantly associated with increased bone strength (21 and 15%, respectively) in the mediolateral direction, but not in the anteroposterior direction. Axial growth and accrual of bone mineral mass of the rat femur are largely independent of locomotive loading or estrogen, whereas these factors specifically account for the femur function, as either a mechanical lever or a mineral reservoir for reproduction, respectively.
To quantify cortical bone response to weight-bearing exercise, bone size, mineral content, and formation were measured at the femoral midshaft in swine. Bone formation was measured histomorphometrically on the periosteal, endosteal, and osteonal surfaces. Sedentary adult crossbred sows (3 years, 229 kg) were randomly assigned to basal (B, n = 6), control (C, n = 7), or trained (T, n = 7) groups. The basal and control groups did not exercise and were killed initially (B) or after 20 weeks (C). The trained group walked on a treadmill 20 minutes/day at 5 km/h and 5% grade, 5 days/week for 20 weeks. Bone length, area, or fat-free dry weight was not different with time (B versus C) or with training (C versus T). Periosteal modeling was stimulated by walking. Periosteal formation surface and mineral apposition rate (MAR) were greater in trained than control femora. No effects of walking were measured on the endosteal surface. Intracortical remodeling was not affected by walking. The number of labeled osteons (22.4 cm-2) was not different among groups, but osteonal MAR was greater in trained (1.18 microns/day) than control (0.96 mu/day) femora. Walking for 20 weeks in the previously sedentary sows was not a sufficient stimulus to create differences in gross measures of bone size or mineral content but did increase periosteal and intracortical MAR. The primary effect of increased exercise appeared to be osteoblast activation.
An experimental study of the periosteal bane growth in the mallard from 42 to 154 post hatching days shows: (1) a noticeable time difference in the local biological age of the diaphyseal cortices between various long bones; (2) great differences in their histological structures, at a given individual age, expressing commensurate differences in local growth rates. Those results emphasize the importance of local factors to interpret the typology of the primary (periosteal) bone tissues. Experimental results allow to quantify the relationships between bone tissue typology and the velocity of its radial deposition.
The long-term effects of sciatic nerve section on bone mineral density (BMD) were studied using dual-energy X-ray absorptiometry (DEXA) in skeletally mature rats. Unilateral sciatic neurectomy caused the rapid loss of cancellous bone in the proximal and distal femur and tibia in the ipsilateral hindlimb and, to a lesser extent, in the contralateral intact hindlimb. The reduction in BMD rapidly progressed for 4 weeks after sciatic section and then gradually stabilized with no evidence of recovery at 12 weeks. The development of osteoporosis in the contralateral intact hindlimb was a novel finding. There was no evidence of disuse in the normal contralateral hindlimb after unilateral sciatic section; grid-crossing activity over a 24-h interval was unchanged and there was no reduction in weight bearing on the contralateral normal hindpaw during the stance phase of ambulation. Unilateral peripheral nerve lesions have well-documented effects on substance P content and function in the corresponding contralateral intact nerve. We hypothesized that after sciatic section a reduction in substance P signaling might contribute to bone loss in the contralateral hindlimb. Daily administration of the substance P receptor (NK 1 ) antagonist LY303870 for 2 weeks caused significant loss of cancellous bone in the denervated and the contralateral hindlimb, evidence that substance P signaling sustained bone density after nerve section. After sciatic neurectomy there was a 33% reduction in sciatic nerve stimulation-evoked extravasation in the contralateral intact hindlimb, indicating transmedian inhibition of substance P signaling after nerve injury. Furthermore, there was a 50% reduction in the substance P content in both tibias after unilateral sciatic section. Collectively, these data support the hypothesis that a widespread reduction in substance P content in bone contributes to the osteoporotic effects of sciatic neurectomy and that residual substance P signaling maintains bone integrity after nerve section in both the denervated and contralateral intact hindlimb.
A laboratory approach to measuring neurologic impairment has been developed that is applicable to systemic disease as well as localized nerve injury. This approach compares individual parameters of the experimental rat hind footprint (walking track) with weight- and sex-matched control track parameters; classic mathematical indexing is not utilized. The normal track data obtained for the Sprague-Dawley rat showed a significant increase in print length (PL), toe spread (TS), and intermediate toe spread (TTS) with increasing rat weight. A significant difference between male and female rats above 400 gm also was noted. For a localized injury (sciatic nerve cut), this approach demonstrated that the contralateral hind footprint was a "compensated" rather than a normal track. This approach to track analysis also was capable of demonstrating progressive neurologic impairment for a sciatic nerve compression model and a systemic disease such as diabetes, as well as demonstrating reversal of these abnormal patterns when the "disease state" was treated. (C)1991American Society of Plastic Surgeons
Coefficients of correlation between certain physical properties and the histological components of the break area were calculated on an IBM 7090 computer for 56 femoral, 79 tibial and 37 fibular specimens of embalmed cortical bone of standardized size and shape. Strong positive correlations (0.01–0.02 significance level) were found between tensile strength and the percentage of interstitial lamellae in the break area; between hardness and the number of osteons/mm2; and between hardness and the percentage of osteons in the break area. Equally high negative correlations were found between tensile strength and percentage of osteons in the break area; between shearing strength and average area/osteon remnant; between elastic modulus and percentage of spaces in the break area; and an even higher correlation (0.001) between hardness and percentage of spaces in the break area. Negative correlations (at slightly more than 0.05 significance level) were found between shearing strength and modulus and average area/osteon. Osteons tend to reduce the tensile strength and elastic modulus of bone while interstitial lamellae tend to increase them. The probable reason is the relatively greater amount of cement lines, which are sites of weakness where failure can occur, in Haversian bone as compared with lamellar bone. The predominant orientation of collagen fibers and the amount and distribution of calcium may also be involved. These factors are now being investigated.
Three-dimensional (3-D) structural parameters derived from lower-dimensional measurements using indirect morphometric methods may be strongly biased if the measured objects deviate from the assumed structure model. With the introduction of 3-D microscopic measuring techniques it is possible to obtain a complete depiction of complex spatial structures. As a consequence, new 3-D methods have recently been developed for the estimation of morphometric parameters such as volume, surface area and connectivity by direct processing of the 3-D images. Structure thickness is an important morphometric parameter which is usually defined for specific structure models only. In this paper we propose a general thickness definition for arbitrary structures allowing us to calculate the mean structure thickness and the thickness distribution of 3-D objects in a direct way and independently of an assumed structure model. Additionally, an efficient implementation for the practical usage of the method is described using distance transformation. The new method is applied to trabecular bone structures measured with a 3-D micro-computed tomography system.
To determine whether immobilization acts directly on bone by alteration of mechanical loading or systemically, studies of the effects of immobilization were carried out on histomorphometry of diaphyses of tibiae and on subcutaneous implants of demineralized allogenic bone matrix of rats. The right hind leg of growing rats was denervated by severing the tibial nerve. A sham operation on the right hind leg was performed in control animals. Bone formation at the endosteal and periosteal surfaces was significantly lower in tibiae from limbs with severed nerves as compared to tibiae from the intact limbs of nerve-sectioned rats and from both limbs of sham-operated control rats. Bone formation was decreased at both 3 and 7 weeks after immobilization. The decreased formation resulted in significant reductions in cross-sectional area. At 3 weeks post denervation, the periosteal bone formation rate was lower in tibiae of intact limbs from denervated rats as compared to tibiae from intact limbs of sham-operated animals. This finding was attributed to reduced physical activity of the denervated rats. In the implants, nerve section did not alter the amount of implant matrix resorbed, the amount of bone matrix synthesized, or the amount of calcium in the implant. These findings support the hypothesis that inhibition of bone formation at the tibial diaphysis in response to immobilization resulted from altered mechanical loading and not from the production of substances acting systemically. Whereas the mean medullary area of tibiae was not altered by nerve section, it was decreased in tibiae of all groups compared to the values of basal controls, indicating that bone formation was greater than bone resorption. The findings are interpreted to mean that, in the rat, reducing the mechanical loading of an extremity inhibits bone formation at the tibial diaphysis of the affected limb and does not influence bone resorption at that site.
To determine the effects of resistance exercise on mass, strength and local turnover of bone, 50 Sprague Dawley rats, 8 weeks
of age, were assigned to five groups: a baseline control and two groups of sedentary and exercising rats. The trunk of the
rats was kept upright during electrically stimulated jumping exercise for 1 h every other day. In 4 weeks, the trabecular
mineralizing surface per bone surface (MS/BS), bone formation rate per bone surface (BFR/BS) and the compression load of the
lumbar body increased and the number of osteoclasts decreased, but bone mineral density (BMD) and structure did not increase.
In the mid femur, the cross-sectional area, the cortical bone area, the moment of inertia, the periosteal MS/BS, BFR/BS and
the bending load increased in the exercise group. In 8 weeks, the increases in BMD, structure and load values were significant
in both the lumbar and mid femur. At both 4 and 8 weeks, the MS/BS for the endocortical surface of mid femur were not increased
and mineral apposition rate (MAR) remained reduced. These results show that jumping exercise increases the mass and strength
of the lumbar vertebrae and mid femur by stimulating bone formation and accelerates cortical drift by both increasing periosteal
bone formation and reducing the endocortical MAR.
Inter-individual differences in cortical bone volumetric density (CoD), such as those related to sex, are a product of differences in remodelling rates. While cortical bone is often treated as a uniform tissue, remodelling rates also vary within individual bones. This level of adaptation has largely been overlooked in analyses of peripheral quantitative computed tomography (pQCT) images. Further, such variation in CoD has never been assessed in growing bones. We hypothesised that CoD varied significantly within the same cross-section of the mid-tibia of adolescents. We further hypothesised that due to the profound impact of oestrogen on remodelling, this variation would be different between sexes.
Subjects were 183 adolescents (99 girls and 84 boys) in grade 6 and 7 with a mean age of 12.1 years. We used age at peak height velocity to adjust for maturational differences between sexes. Image data from a mid-tibia pQCT scan of each subject were assessed regionally within eight sectors distributed about the cortex and aligned by the anterior tibial crest. We used a repeated measures general linear model to assess intra-individual variation in CoD while controlling for differences in ethnicity, maturity, height, weight, physical activity level and total cross-sectional bone area (ToA).
Sector based variation in CoD was significant (p<0.001), with the anterior cortex having lower density than the posterior cortex. The largest percentage difference (anterior vs posteromedial sectors) was 12.2%. A significant sector*sex interaction (p = 0.018) was detected; however, its impact was relatively small with girls having 1.1-3.6% denser bones than boys depending on the sector (2.7% average difference).
The magnitude of the variation in CoD across sectors within individuals of both sex was far greater than the mean differences between the sexes. This finding indicates that the microstructural variation within the mid-tibia is detectable by pQCT and its magnitude suggests an important level of adaptation to loading.
Disuse atrophy has been the subject of research studies that make the use of an animal model in which single-limb immobilization induces atrophic changes in the immobilized limb. The effect upon the nonimmobilized contralateral limb is, however, still unclear, and findings in the literature are inconclusive. We therefore performed a multidisciplinary study to clear this issue. The experimental population was 70 young male Sprague-Dawley rats. In one group of 35 rats, the left hind limb was immobilized for 3 weeks. Another group of 35 rats served as controls. limb and tibial bone weights (both "dry" and "ash") were found to be reduced in both hind limbs. Bone alkaline phosphatase (ALP) activity was likewise reduced in both limbs. We conclude that the contralateral hind limb in the rat is clearly subjected to atrophic changes that are similar in magnitude or severity to those experienced by the immobilized limb. We further discuss these findings in view of previous knowledge regarding the atrophic process.
Although micro-computed tomography (micro-CT) has become the gold standard for assessing the 3D structure of trabecular bone, its extension to cortical bone microstructure has been relatively limited. Desktop micro-CT has been employed to assess cortical bone porosity of humans, whereas that of smaller animals, such as mice and rats, has thus far only been imaged using synchrotron-based micro-CT. The goal of this study was to determine if it is possible to visualize and quantify rat cortical porosity using desktop micro-CT. Tibiae (n = 10) from 30-week-old female Sprague-Dawley rats were imaged with micro-CT (3 μm nominal resolution) and sequential ground sections were then prepared. Bland-Altman plots were constructed to compare per cent porosity and mean canal diameter from micro-CT (3D) versus histology (2D). The mean difference or bias (histology-micro-CT; ±95% confidence interval) for per cent porosity was found to be -0.15% (±2.57%), which was not significantly different from zero (P= 0.720). Canal diameter had a bias (±95% confidence interval) of -5.73 μm (±4.02 μm) which was found to be significantly different from zero (P < 0.001). The results indicated that cortical porosity in rat bone can indeed be visualized by desktop micro-CT. Quantitative assessment of per cent porosity provided unbiased results, whereas direct analysis of mean canal diameter was overestimated by micro-CT. Thus, although higher resolution, such as that available from synchrotron micro-CT, may ultimately be required for precise geometric measurements, desktop micro-CT--which is far more accessible--is capable of yielding comparable measures of porosity and holds great promise for assessment of the 3D arrangement of cortical porosity in the rat.
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.
As computational modeling becomes an increasingly common tool for probing the regulation of bone remodeling, the need for experimental data to refine and validate such models also grows. For example, van Oers et al. (R.F. van Oers, R. Ruimerman, B. van Rietbergen, P.A. Hilbers, R. Huiskes, Relating osteon diameter to strain. Bone 2008;43: 476-482.) recently described a mechanism by which osteon size may be regulated (inversely) by strain. Empirical data supporting this relation, particularly in humans, are sparse. Therefore, we sought to determine if there is a link between body weight (the only measure related to loading available for a cadaveric population) and osteon geometry in human bone. We hypothesized that after controlling for age, sex and height, weight would be inversely related to femoral osteon size (area, On.Ar; diameter, On.Dm). Secondarily we sought to describe the relation between osteon circularity (On.Cr) and these parameters. Osteons (n=12,690) were mapped within microradiographs of femoral mid-diaphyseal specimens (n=88; 45 male, 43 female; 17-97 yrs). Univariate analysis of covariance was conducted (n=87; 1 outlier) with sex as a fixed factor and height, weight and log-transformed age as covariates. Weight was negatively related to On.Ar and On.Dm (p=0.006 and p=0.004, respectively). Age was significantly related to osteon and, it was also significantly related to circularity (all p<0.001). This relation was negative for On.Ar and On.Dm and positive for On.Cr (increasing circularity with age). On.Ar and On.Dm were found to be significantly different between the sexes (p=0.021 and p=0.019, respectively), with females having smaller osteons. No relation between sex and On.Cr was detected (p=0.449). Height was not significantly related to any of the geometric parameters. Partial eta-squared values revealed that age accounted for the largest proportion (On.Ar: 28%, On.Dm: 18%, On.Cr: 30%), weight accounted for the second largest (On.Ar: 9%, On.Dm: 10%) and sex accounted for the smallest proportion (On.Ar: 6%, On.Dm: 7%) of the variance in geometry. While previous studies have reported relations between osteon size and sex/age, we believe that our findings are the first to demonstrate a link with weight. We believe that this negative relation with weight is most probably mechanical in nature; however, alternative (endocrine) links between bone and adipose tissue cannot be ruled out by our design.
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 the design of tissue engineering scaffolds, design parameters including pore size, shape and interconnectivity, mechanical properties and transport properties should be optimized to maximize successful inducement of bone ingrowth. In this paper we describe a 3D micro-CT and pore partitioning study to derive pore scale parameters including pore radius distribution, accessible radius, throat radius, and connectivity over the pore space of the tissue engineered constructs. These pore scale descriptors are correlated to bone ingrowth into the scaffolds. Quantitative and visual comparisons show a strong correlation between the local accessible pore radius and bone ingrowth; for well connected samples a cutoff accessible pore radius of approximately 100 microM is observed for ingrowth. The elastic properties of different types of scaffolds are simulated and can be described by standard cellular solids theory: (E/E(0))=(rho/rho(s))(n). Hydraulic conductance and diffusive properties are calculated; results are consistent with the concept of a threshold conductance for bone ingrowth. Simple simulations of local flow velocity and local shear stress show no correlation to in vivo bone ingrowth patterns. These results demonstrate a potential for 3D imaging and analysis to define relevant pore scale morphological and physical properties within scaffolds and to provide evidence for correlations between pore scale descriptors, physical properties and bone ingrowth.
We describe a robust method for determining morphological properties of filamentous biopolymer networks, such as collagen or other connective tissue matrices, from confocal microscopy image stacks. Morphological properties including pore size distributions and percolation thresholds are important for transport processes, e.g., particle diffusion or cell migration through the extracellular matrix. The method is applied to fluorescently labeled fiber networks prepared from rat-tail tendon and calf-skin collagen, at concentrations of 1.2, 1.6, and 2.4 mg/ml. The collagen fibers form an entangled and branched network. The medial axes, or skeletons, representing the collagen fibers are extracted from the image stack by threshold intensity segmentation and distance-ordered homotopic thinning. The size of the fluid pores as defined by the radii of largest spheres that fit into the cavities between the collagen fibers is derived from Euclidean distance maps and maximal covering radius transforms of the fluid phase. The size of the largest sphere that can traverse the fluid phase between the collagen fibers across the entire probe, called the percolation threshold, was computed for both horizontal and vertical directions. We demonstrate that by representing the fibers as the medial axis the derived morphological network properties are both robust against changes of the value of the segmentation threshold intensity and robust to problems associated with the point-spread function of the imaging system. We also provide empirical support for a recent claim that the percolation threshold of a fiber network is close to the fiber diameter for which the Euler index of the networks becomes zero.
The normal pattern of surface strain produced in the distal third of sheep tibiae was determined by attaching rosette strain gauges to the cranial (anterior), medial, and caudal (posterior) cortices of the tibia. The strain data showed that during locomotion the cranial (longitudinally concave) cortex was the tension surface and the caudal (longitudinally convex) cortex was the compression surface. (This relationship between bending and curvature is the reverse of that which occurs in the radius of the same species).The principal strain directions remained almost constant throughout the main strain period of each stride and were practically unaffected by the speed of locomotion. On neither the cranial nor the caudal surface of the tibia was the larger principal strain aligned along the long axis of the bone. On the cranial surface it formed a proximal and lateral angle of 29 degrees with this axis and on the caudal surface, a similar angle of 23 degrees. This strain pattern is consistent with a loading regimen of craniocaudal bending and torque. The mean peak principal strain on the cranial and caudal surfaces of the bone during walking in five sheep was +709 and -666 microstrain, respectively (ratio, 1.07 to one). In eight sheep the mean thickness of the cranial cortex was 3.0 millimeters and of the caudal cortex, 2.87 millimeters (ratio, 1.03 to one). The directions of secondary osteons in the cranial and caudal cortices of the tibia in these eight sheep were shown to lie between the direction of the long axis of each bone and that of the larger principal strain in that cortex during locomotion. Determinations of the transverse axes of the proximal and distal joint surfaces of the tibia in adult sheep and in sheep in late fetal life showed that during postnatal development the distal end of the bone rotates internally some 14 degrees with respect to the proximal end. The limited strain data available from the tibia of a human suggest that the tibial torque during locomotion is in the same direction in both sheep and humans. The developmental rotation of the tibia in these species, however, is in the opposite direction.
The histodynamic response to long-term "non-traumatic" immobilisation was studied in young adult Beagle dogs by means of radiomorphometry and histomorphometry, the right forelimb being encased in plaster and the left forelimb serving as a control. The dogs were killed at two, four, six, eight, twelve, sixteen, twenty, twenty-four, thirty-two and forty weeks and the third metacarpal, radius, ulna and humerus removed for analysis of the contributions of the periosteal, haversian and endosteal envelopes to the bone loss at the mid-diaphysis. The bone mass responded to long-term immobilisation in three stages. First there was a rapid initial loss of bone, reaching its maximum (some 16 per cent of original mass) at six weeks, to which all three bone envelopes, to some extent, contributed. A rapid reversal followed, the bone mass approaching the control values between eight and twelve weeks after immobilisation. A second stage of slower but longer lasting bone loss ended twenty-four to thirty-two weeks after immobilisation; the periosteal envelope was the main contributor (80 to 90 per cent of the total loss). The third stage was characterised by maintenance of the bone mass which had been reduced by some 30 to 50 per cent of original values. This pattern was qualitatively similar in all four bones but the distal bones lost more bone than the proximal bones. The extent of resorption surface and the total histologically "active" periosteal envelope increased parallel to the phases of bone loss. The linear mineralisation rate did not differ significantly between the experimental and control sides.
Rosette strain gauges were attached to the cranial and caudal aspects of the proximal half of the radius in eight skeletally mature female sheep; The sheep's radius has a slight cranially convex curvature. During walking it was deformed so that the cranial surface was subjected to tension aligned along the bone's lon axis, and the caudal surface to compression similarly aligned. The compressive strain on the caudal aspect of the bone was consistently larger (X 1-9) than the tensile strain on the cranial aspect. The thickness of the cortex did not reflect this difference but in younger animals the process of osteonal remodelling seemed further advanced in the cortex which was customarily subject to the larger deformation. The relevance of these findings is discussed in relation to the technique of internal fixation and to our understanding of the basis of the mechanical adaptability of bone.
The authors measured osteon dimensions and number in specimens of cortical bone obtained from the medial femoral neck (calcar) from nine female patients treated with hemiarthroplasty for femoral neck fractures, and from 12 (seven women, five men) age-matched cadavers without fractures. Specimens from the same location in 14 (seven women, seven men) younger patients treated with total hip arthroplasty (THA) for osteoarthrosis were also studied. There were fewer osteons per unit area and the osteons and their Haversian canals were bigger in the fracture group than in the nonfracture or osteoarthritic groups. Patients with femoral neck fractures also exhibited a decreased Singh index and an increased intracortical porosity compared with age-matched normal controls. By contrast, the osteoarthritic group showed no difference in osteon or Haversian dimensions, osteon number per unit area, Singh index, or in porosity compared with the nonfracture group. These morphometric differences found in the cortical bone of the medial femoral neck may play a role in the incidence of fractures.
The primary mechanical functions of limb bones are to resist deformation, and hence provide stiff levers against which muscles can act, and to be sufficiently strong to prevent breaking under static or dynamic loads which arise from normal and accidental activities. If bones perform these functions with a minimum amount of material, the energetic costs associated with building, maintaining and transporting the skeleton will be minimized. Appropriate skeletal architecture for minimizing mass while maximizing strength depends on forces imposed on structural elements. In the evolutionary acquisition of flight in the bat lineage, the forelimb skeleton must have come to experience locomotor-forces that differed from those engendered by the terrestrial locomotion of non-flying bat relatives. Here we successfully measure in vivo strain on the wing bones of flying mammals. Our data demonstrate that torsion and shear are unique and crucial features of skeletal biomechanics during flight, and suggest that the evolution of skeletal design in bats and other flying vertebrates may be driven by the need to resist these loads.
A laboratory approach to measuring neurologic impairment has been developed that is applicable to systemic disease as well as localized nerve injury. This approach compares individual parameters of the experimental rat hind footprint (walking track) with weight- and sex-matched control track parameters; classic mathematical indexing is not utilized. The normal track data obtained for the Sprague-Dawley rat showed a significant increase in print length (PL), toe spread (TS), and intermediate toe spread (ITS) with increasing rat weight. A significant difference between male and female rats above 400 gm also was noted. For a localized injury (sciatic nerve cut), this approach demonstrated that the contralateral hind footprint was a "compensated" rather than a normal track. This approach to track analysis also was capable of demonstrating progressive neurologic impairment for a sciatic nerve compression model and a systemic disease such as diabetes, as well as demonstrating reversal of these abnormal patterns when the "disease state" was treated.
This investigation determined the relative importance of collagen fiber orientation, porosity, density, and mineralization in determining the tensile strength of bovine cortical bone. Thirty-nine specimens were tested for failure stress and the values of eight histologic and compositional parameters: collagen fiber orientation, wet and dry apparent density, percent mineralization of the bone matrix, and several components of porosity (Haversian canals, Volkmann's canals, and plexiform vascular spaces). Linear regression analysis showed that collagen fiber orientation was consistently the single best predictor of strength. Mineralization of the bone matrix was generally a poor predictor of strength. Density and porosity ranked between these variables in importance. Multiple regression equations containing all significantly correlated variables achieved correlation coefficients of 0.607 for plexiform bone and 0.881 for osteonal bone. Also, separate analysis of plexiform and osteonal specimens showed that the latter group was weaker even though it was less porous, apparently because it had collagen fibers which were less longitudinally oriented. This study suggests it is feasible to develop better empirical formulae for the prediction of cortical bone strength than are currently available if a variety of variables is introduced. Additional data are needed to confirm these results.
Stiffness of compact bone is found to be highly and nonlinearly dependent on its porosity, its complement, bone volume fraction and apparent density. Elastic modulus decreases as a power (0.55) of increasing porosity and increases both as a power of increasing bone tissue volume (10.92) and increasing apparent density (7.4). These data indicate that small changes in the amount or density of compact bone tissue exert a more pronounced influence on its stiffness than would similar changes in trabecular bone.
The Young's modulus of elasticity, the calcium content and the volume fraction (1-porosity) of 23 tension specimens and 80 bending specimens, taken from compact bone of 18 species of mammal, bird and reptile, were determined. There was a strong positive relationship between Young's modulus and both calcium content and volume fraction. A power law model fits the data better than a linear model. Young's modulus has a roughly cubic relationship with both calcium content and volume fraction. Over 80% of the total variation in Young's modulus in this data set is explained by these two variables.
The progressive osteopenic changes in tibial compact bone in adult male monkeys (Macaca nemestrina) were examined histologically during chronic studies of immobilization. The animals were restrained in a semirecumbent position, which reduces normally occurring stresses in the lower extremities and results in bone mass loss. The longest immobilization studies were of 7 months duration. Losses of haversian bone tended to occur predominantly in the proximal tibia and were characterized by increased activation with excessive depth of penetration of osteoclastic activity. There was no apparent regulation of the size and orientation of resorption cavities. Rapid bone loss seen during 10 weeks of immobilization appeared to be due to unrestrained osteoclastic activity without controls and regulation, which are characteristic of adaptive systems. The general pattern of loss persisted throughout 7 months of immobilization. Clear-cut evidence of a formation phase in haversian bone was seen only after 2 months of reambulation. During this period osteoblasts accumulated within resorption cavities, and there was matrix apposition. Within 6 months of recovery there was increased bone turnover, and resorption cavities with diameters of 500-1500 micron were filled partially with new bone; the mean wall thickness of new bone was 2 to 3 times larger than normal. In addition there were numerous remodeling sites that were of normal size and orientation. Trabecular bone was lost during immobilization, and it is probable that losses of large trabecular plates are not replaced, and consequently original bone volume in the cross section is not recovered. In this immobilization model we observed bone resorption occurring for long periods without apparent interruption.(ABSTRACT TRUNCATED AT 250 WORDS)
The effect of 12 weeks of treadmill exercise on the mineralization of trabecular and cortical bone was studied in rats 7,
14, and 19 months of age. Bone mineralization was evaluated by measuring concentrations of Ca, Mg, and hydroxyproline as well
as uptake of 45Ca concentration in the femur, humerus, rib and calvaria. The 7-and 14-month-old rats increased mineralization in those cortical
bones directly involved in exercise. The 19-month animal responded to exercise by increasing mineralization in all bones examined,
including the nonweight bearing trabecular calvaria and cortical rib. From these data, it is apparent that the older animals
undergo a total skeletal mineralization in response to exercise compared with local adaptation in the younger animal. Further,
we provide evidence to support the use of the rat as a model in which to study mammalian bone physiology during the aging
process.
The purpose of this study was to determine the effect of Haversian remodeling on the tensile properties of human cortical bone by testing specimens containing, as far a possible, a single type of bone tissue. Fifty-one specimens were prepared from sixteen fresh tibias, removed at autopsy. Age range was 19-35. Regions were selected so that the specimens would consist almost exclusively of either primary bone or Haversian bone. The ultimate tensile strength, ultimate strain and Young's modulus of elasticity were determined at a loading rate of 0.05 mm s-1. The primary bone specimens were found to have a significantly higher ultimate tensile strength and modulus of elasticity than those formed of Haversian bone.
Compressive properties were determined for 65 standardized specimens of wet, unembalmed cortical bone from tibias of six men. Osteons were classified as dark, light or intermediate from their appearance in polarized light and as slightly, intermediately or markedly radiolucent from their appearance in microradiographs. Compressive strength had significant positive correlations with the percent of intermediate osteons or slightly radiolucent osteons in the cross section. Significant negative correlations occurred between compressive strength and percent of spaces. Compressive strain had a significant positive correlation with the percent of light osteons. Multiple correlation coefficients between compressive strength or strain and the various histologic and microradiographic variables were higher and more significant than that found with elastic modulus as the dependent variable. The significant positive correlation between compressive strength and the percent of osteons, regardless of their collagen fiber orientation in the cross-section, suggests that osteons tend to increase the compressive strength of bone.
Experiments have been performed to evaluate the effects of immobilization and of enforced activity on bone growth and development in the young animal and on skeletal mass in the mature animal. Forty growing and 40 adult rats were divided into Groups C and R. Group C underwent cast immobilization of the left hind limb. Group R was subjected to four daily treadmill runs. Animals were given tetracycline, weighed and measured, and sacrificed at 1, 3, and 6 weeks at which time vascular injections were performed. Bones subjected to immobilization and to enforced activity were compared with nonimmobilized control bones by measurements of weight, length and diameter; microradiograms; fluorescence microscopy; microangiograms; and histologic examination. In mature animals, immobilization led to a decrease in bone mass, whereas activity resulted in an increase. In contrast to these data, immobilization in growing animals led to a decrease in whole body weight and length as well as in the length, thickness, and mass of the long bones. Articular surfaces and epiphyseal lines were irregular, bone formation was retarded, and circulation to the femoral head was diminished. Increases in whole body weight and length and in the length, thickness, mass, and epiphyseal circulation of the long bones were observed as results of enforced activity.
Unlabelled:
Five one-year-old immature swine were subjected to twelve months of exercise training. Four matched swine with no training served as controls. After they were killed, four-millimeter-wide strips of bone taken from the anterior, medial, posterior, and lateral quadrants of the central femoral diaphysis were subjected to four-point bending tests to failure. It was found that although exercise did not change the mechanical properties of the cortical bone, it resulted in significant increases in the averaged femoral cross-sectional properties: 17 per cent in cortical thickness, 23 per cent in cortical cross-sectional area, and 21 per cent and 27 per cent in maximum and minimum area moments of inertia, respectively. These changes were due primarily to reduction in the diameter of the medullary canal. The analyses of bone composition showed that the bone density and biochemical contents of the control and exercised animals were similar, but the total volume and the dry, ash, and calcium weights of the exercised bone were significantly higher. These combined results suggest that prolonged exercise has a significant effect on the quantity of the bone, but not on its quality.
Clinical relevance:
It has long been recognized that stress deprivation from immobilization in plaster casts results in profound bone atrophy, and it is generally accepted that a minimum level of activity is necessary for homeostasis of bone. These results show that exercise at a level comparable to that prescribed in running fitness programs for humans (65 to 80 per cent of maximum heart rate) can not only maintain homeostasis, but produce actual hypertrophy of bone. This work further suggests the importance of graduated, prolonged, supervised rehabilitation programs in overcoming osteoporotic states.
The long-term effects of dietary calcium and phosphorus level and of forced exercise on bone were evaluated in mature female mice. The animals were fed either a control diet providing adequate Ca ( 0.6% ) and P (0.3%) or a high level of P (1.2%) with increasing levels of Ca (0.6, 1.2 or 2.4% ). Half of the mice in each group were subjected to 1 hour of forced exercise 5 days per week for 50 weeks. The non-exercised mice fed the 1.2%)P diet with 0.6% Ca had significantly lower femur and tibia weights, bone mineral content and femoral cortical thickness than the con trols fed the same level of Ca with 0.3% P. Femur fat-free dry weight was reduced from 44.6±1.9mg (mean ± .D) to 33.2±2.3mg. Femur Ca aver aged 10.57±0.86 mg in the 0.6% Ca, 0.3% P group and 6.17+0.80 mg in the 0.6% Ca, 1.2% P group. Increasing dietary Ca partially overcame the adverse effects of high dietary P. Increasing Ca from 0.6 to 2.4% in the presence of 1.2% P increased femur weight from 33.2±2.3to 41.8±2.5mg and femur calcium from 6.17±0.80to 9.85±0.84mg. These values were still significantly lower than those of the control mice (0.6% Ca, 0.3% P) even though the Ca:P ratio was 2:1 in both cases, indicating that the absolute intake of calcium and phosphorus as well as the ratio of these minerals is important in determining their effect on bone. Forced exercise tended to increase bone weight and mineral content of the mice in all groups, but the improvements were generally small and were significant only for tibia weight, femoral cortical thickness, cortical area and percent cortical area. J. Nutr. 110: 1161-1168, 1980.
It has been hypothesized that bone has the capacity to accommodate regional differences in tension and compression strain mode and/or magnitude by altering its osteonal microstructure. We examined a simple cantilevered bone to determine whether regional differences in particular strain-related features are reflected in the microstructural organization of compact bone.
The artiodactyl (e.g., sheep and deer) calcaneus has a predominant loading condition which is typified by prevailing compressive and tensile strains on opposite cortices, and variations in strain magnitudes across each of these cortices. Microscopic examination showed osteon density and cortical porosity differences between tension (caudal) and compression (cranial) cortices, averaging 11.4% more osteons in the compression cortex (P < 0.01) and 80.2% greater porosity in the tension cortex (P < 0.01). There is 43.5% more interstitial bone in the compression cortex (P < 0.01). Osteons in the compression cortex also have smaller areas in contrast to the larger area per osteon in the tension cortex. Although no definite transcortical gradient in osteonal density or cortical porosity is found, fractional area of interstitial bone is larger and osteon population density is lowest in the endocortical regions of both tension and compression cortices. The endocortical regions also have greater porosity than their corresponding middle and pericortical regions (P < 0.01).
These osteonal microstructure and cortical porosity differences may be adaptations related to regional differences in strain mode and/or strain magnitude. This may be related to the disparity in mechanical properties of compact bone in tension vs. compression. These differences may reflect a capacity of bone to process local and regional strain-related information.
Background:
Examination of a simple skeletal cantilevered beam-like bone (artiodactyl calcaneus) suggests that regional differences in strain magnitude and mode (tension vs. compression) reflect regional adaptation in the structural/material organization of bone. The artiodactyl (e.g., sheep and deer) calcaneus has a predominant loading condition typified by the unambiguous presence of prevailing compressive and tensile strains on opposite cortices. Bone habitually loaded in bending may accommodate regional disparities in loading conditions through modifications of various aspects of its organization. These include overall bone build (gross size and shape), cross-sectional shape, cortical thickness, and mineral content.
Methods & results:
Cross-sections taken along the calcaneal body exhibited cranial-caudal elongation with the compression (cranial) cortex thicker than the tension cortex (P < 0.01). Mineral content (ash fraction) was significantly greater in the compression cortex (P < 0.01), averaging 6.6% greater than in the tension cortex. Strong positive correlations were found between mineral content and section location in both the tension (r2 = 0.955) and compression (r2 = 0.812) cortices. These correlations may reflect functional adaptations to the linear increases in stress that are known to occur in the distal-to-proximal direction in simple, unidirectionally loaded cantilevered beams. According to engineering principles, the roughly triangular transverse cross-sectional geometries and thicker compression cortex are features consistent with a short cantilevered structure designed to resist unidirectional bending.
Conclusions:
Known differences in mechanical properties of bone in tension vs. compression suggest that these regional differences in cortical thickness and mineralization may be related to differences in strain mode. These structural/material dissimilarities, however, may be related to regional variations in strain magnitude, since bending and axially directed stresses in a simple cantilevered structure produce greater strain magnitudes in the compression domain. It is possible that the superimposed habitual strain magnitudes enhance strain-mode-specific adaptive responses. We hypothesize that these structural/material differences reflect the capacity of bone to process local information and produce a regionally heterogeneous organization that is appropriate for prevailing loading conditions.
The orientation of osteons has been described in the long bones of man, using a special macroscopic method, which enabled the study of the osteonal architecture throughout the extent of the diaphysis. The osteons in all bones are arranged in two helical systems of opposite directions, which lie on the opposite sides of the diaphysis. The inclination of osteon direction from the bone axis in remarkably constant, varying between 5 degrees and 15 degrees. The boundary between the two fields is sharp. This special type of osteon orientation corresponds with the directions of the maximum principal stress and/or strain in the walls of the bones, which are exposed to both the bending and torque moments. The typical orientation of osteons reflects the history of loading of the bones in the course of physiological activity. The dominant stress state of every bone could be deduced from the osteon orientation and from the position of the pressure and tension fields on the surface of the bone.
Gonadal hormone deficiency following ovariectomy and skeletal unloading by limb immobilization are useful models of osteopenia. The purpose of this study was to compare changes in cortical bone after ovariectomy (OVX) or immobilization (IMM) for 6 and 12 weeks. Comparisons were also made when rats were ovariectomized or immobilized for 6 weeks and then immobilized (OVX/IMM) and ovariectomized (IMM/OVX), respectively, for 6 more weeks. Tibias and femurs were collected and static and dynamic cortical bone indices were determined by morphometric methods. Femurs from animals OVX or IMM for 12 weeks were tested for bone stiffness by torsional testing. Six and 12 weeks after OVX, there were increases in the periosteal perimeter, cortical area, and periosteal bone formation indices, indicating that ovariectomy increased modeling-dependent bone gain on the periosteal envelope, relative to controls. Contrarily, 6 and 12 weeks after IMM, there were decreases, compared with controls, in periosteal perimeter, cortical bone area, and periosteal bone formation indices. This indicates that immobilization decreased modeling-dependent bone gain on the periosteal envelope. These differences in modeling between the animals that were OVX and IMM resulted in a smaller cortical width and minimum cortical width in the IMM compared with the OVX animals. There were significant decreases in cortical bone stiffness and minimum cortical width at the fracture site following mechanical testing in the animals IMM for 12 weeks. Both ovariectomy and immobilization increased endocortical resorption surface, endocortical perimeter and expansion of the marrow cavity. Because of suppressed periosteal bone formation with increased endocortical resorption, immobilization had a greater effect on bone loss and decreased bone stiffness than did ovariectomy.(ABSTRACT TRUNCATED AT 250 WORDS)
Tensile testing to failure was done on 235 cortical specimens that had been machined from forty-seven femora from human cadavera. The donors had ranged in age from twenty to 102 years at the time of death. After mechanical testing, the porosity, mineralization, and microstructure were determined. Linear regression analysis showed that the mechanical properties deteriorated markedly with age. Ultimate stress, ultimate strain, and energy absorption decreased by 5, 9, and 12 per cent per decade, respectively. The porosity of bone increased significantly with age, while the mineral content was not affected. Microstructural analysis demonstrated that the amount of haversian bone increased with age. Both bivariate and multivariate analyses demonstrated the importance of age-related changes in porosity to the decline in mechanical properties. Changes in porosity accounted for 76 per cent of the reduction in strength. Microstructural changes were highly correlated with porosity and therefore had little independent effect. Mineral content did not play a major role. Thus, the quantitative changes in aging bone tissue, rather than the qualitative changes, influence the mechanical competence of the bone.
An experimental study of the periosteal bone growth in the mallard from 42 to 154 post hatching days shows: (1) a noticeable time difference in the local biological age of the diaphyseal cortices between various long bones; (2) great differences in their histological structures, at a given individual age, expressing commensurate differences in local growth rates. Those results emphasize the importance of local factors to interpret the typology of the primary (periosteal) bone tissues. Experimental results allow to quantify the relationships between bone tissue typology and the velocity of its radial deposition.
The osteons in all human long bones are grouped in two helical antirotary systems of opposite oblique directions, situated in the contralateral walls of the diaphysis. Several arguments support the hypothesis that this special architecture arises from functional adaptations and that it depends on the orientation of the first principal stress. A new macroscopic method based on the filling of vascular canals of the undecalcified, polished bone with India-ink enabled us to study the spatial architecture of the haversian bone in the entire diaphysis. The osteon directions in normal and atypical femurs was compared with the direction of the first principal stresses, determined analytically in a cylindrical tube model of the diaphysis subjected to a combination of bending, torsion and compression. Under combined loading with the bending moment in the frontal plane to the medial side and with the torque moment in the sense of external rotation, the direction of the first principal stresses corresponded with the direction of osteons in the diaphysis of the femur. In both cases, the first principal stresses, as well as the osteons, were oriented in the opposite oblique direction in the medial and lateral walls of the model and of the bones. Between the two oblique fields a sharp boundary with an atypical organization of the principal stresses and osteons existed. In atypical femurs the osteonal orientation was longitudinal (likely unloaded femurs) or rotated 90 degrees (markedly anteriorly convex femurs). These observations support the hypothesis of a causal relation between the loading mode and the dominant osteonal direction. The organization of the haversian bone seems a typical example of functional adaptation.
The influence of porosity, osteon density, osteonal area, osteonal lamellar area, osteon size, and haversian canal size on the tension and shear fracture toughness, that is, the mode I and mode II strain energy release rate (GIc and GIIc), respectively, were investigated for the human femur and the tibia. The results suggest that porosity and osteon density were the best explanatory morphological parameters for GIc and GIIc. Both GIc and GIIc significantly decrease with increasing porosity. They also increase with increasing osteon density, the increase being significant for the femur only. Morphological parameters, altogether, can explain 49%-68% of the variation in fracture toughness. We concluded that, although there must be other factors such as biochemical components and microdamage, osteon morphology has an important influence on fracture resistance of the cortical bone.