Towards novel measurements of remodeling activity in cortical bone: implications for osteoporosis and related pharmaceutical treatments

Bone remodelling is performed by basic multicellular units (BMUs) that resorb and subsequently form discrete packets of bone tissue. Normally, the resorption and formation phases of BMU activity are tightly coupled spatially and temporally to promote relatively stable bone mass and bone quality. However, dysfunctional remodelling can lead to bone loss and is the underlying cause of osteoporosis. This review surveys how BMU activity is altered in postmenopausal, disuse and glucocorticoid-induced osteoporosis as well as the impact of anabolic and anti-resorptive pharmaceutical treatments. The dysfunctional remodelling observed during disease and following medical intervention bares many testable hypotheses regarding the regulation of BMU activity and may provide novel insights that challenge existing paradigms of remodelling dynamics, particularly the poorly understood BMU coupling mechanisms. Most bone remodelling research has focused on trabecular bone and 2D analyses, as technical challenges limit the direct assessment of BMU activity in cortical bone. Recent advances in imaging technology present an opportunity to investigate cortical bone remodelling in vivo. This review discusses innovative experimental methods, such as 3D and 4D (i.e. time- lapsed) evaluation of BMU morphology and trajectory, that may be leveraged to improve the understanding of the spatio-temporal coordination of BMUs in cortical bone.

Homogeneous hydroxyapatite/alginate composite hydrogel promotes calcified cartilage matrix deposition with potential for three-dimensional bioprinting

Calcified cartilage regeneration plays an important role in successful osteochondral repair, since it provides a biological and mechanical transition from the unmineralized cartilage at the articulating surface to the underlying mineralized bone. To biomimic native calcified cartilage in engineered constructs, here we test the hypothesis that hydroxyapatite (HAP) stimulates chondrocytes to secrete the characteristic matrix of calcified cartilage. Sodium citrate (SC) was added as a dispersant of HAP within alginate (ALG), and homogeneous dispersal of HAP within ALG hydrogel was confirmed using sedimentation tests, electron microscopy, and energy dispersive spectroscopy. To examine the biological performance of ALG/HAP composites, chondrocyte survival and proliferation, extracellular matrix production, and mineralization potential were evaluated in the presence or absence of the HAP phase. Chondrocytes in ALG/HAP constructs survived well and proliferated, but also expressed higher levels of calcified cartilage markers compared to controls, including Collagen type X secretion, alkaline phosphatase (ALP) activity, and mineral deposition. Compared to controls, ALG/HAP constructs also showed an elevated level of mineralized matrix in vivo when implanted subcutaneously in mice. The printability of ALG/HAP composite hydrogel precursors was verified by 3D printing of ALG/HAP hydrogel scaffolds with a porous structure. In summary, these results confirm the hypothesis that HAP in ALG hydrogel stimulates chondrocytes to secrete calcified matrix in vitro and in vivo and reveal that ALG/HAP composites have the potential for 3D bioprinting and osteochondral regeneration.

Skeletal Lead Burden of the British Royal Navy in Colonial Antigua

Lead (Pb) has been known to be a cause of human poisoning since ancient times, but despite this, it was a widely used metal in the European colonial period. In this study, the relationship between Pb exposure and the demographic variables ancestry and age was explored by comparing the bone Pb levels of individuals that were of either African or European ancestry, excavated from a British Royal Navy hospital cemetery (1793-1822 CE) at English Harbour in Antigua, West Indies. More direct comparisons of Pb levels between the two ancestral groups were possible in this study because of the unsegregated nature of this cemetery. Inductively coupled plasma mass spectrometry was used to determine bulk Pb levels in cortical bone samples from the fibular diaphyses of 23 male individuals. No significant difference was found between the distributions of the Pb levels of the ancestral groups (p = 0.94). Further, no positive correlations or significant differences were found in relation to the individuals' ages and their Pb levels (p = 0.24). Levels of Ba, Ca and rare earth elements support a largely biogenic origin of lead. This is bolstered by Pb deposition patterns, generated by synchrotron X-ray fluorescence imaging for another study. The data suggest that naval personnel, regardless of ancestry at English Harbour, had very similar experiences with regard to Pb exposure. Their exposure to the toxic metal was likely not consistent over time as steady exposure would have resulted in accumulation of Pb with age. This study contributes to addressing historical questions regarding the prevalence of Pb poisoning within the British Royal Navy during the colonial period.

Lacunar-canalicular network in femoral cortical bone is reduced in aged women and is predominantly due to a loss of canalicular porosity

The lacunar-canalicular network (LCN) of bone contains osteocytes and their dendritic extensions, which allow for intercellular communication, and are believed to serve as the mechanosensors that coordinate the processes of bone modeling and remodeling. Imbalances in remodeling, for example, are linked to bone disease, including fragility associated with aging. We have reported that there is a reduction in scale for one component of the LCN, osteocyte lacunar volume, across the human lifespan in females. In the present study, we explore the hypothesis that canalicular porosity also declines with age. To visualize the LCN and to determine how its components are altered with aging, we examined samples from young (age: 20–23 y; n = 5) and aged (age: 70–86 y; n = 6) healthy women donors utilizing a fluorescent labelling technique in combination with confocal laser scanning microscopy. A large cross-sectional area of cortical bone spanning the endosteal to periosteal surfaces from the anterior proximal femoral shaft was examined in order to account for potential trans-cortical variation in the LCN. Overall, we found that LCN areal fraction was reduced by 40.6% in the samples from aged women. This reduction was due, in part, to a reduction in lacunar density (21.4% decline in lacunae number per given area of bone), but much more so due to a 44.6% decline in canalicular areal fraction. While the areal fraction of larger vascular canals was higher in endosteal vs. periosteal regions for both age groups, no regional differences were observed in the areal fractions of the LCN and its components for either age group. Our data indicate that the LCN is diminished in aged women, and is largely due to a decline in the canalicular areal fraction, and that, unlike vascular canal porosity, this diminished LCN is uniform across the cortex.

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The lacunar-canalicular network (LCN) is reduced by 40.6% in aged women

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The lacunar density (lacunae number per given area of bone) is reduced by 21.4% in aged women

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The reduction in LCN in aged women is primarily due to a 44.6% loss of canaliculi

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No endosteal vs. periosteal regional differences were observed in the LCN and its components in either young or aged women

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A reduction in canaliculi with age may contribute to bone fragility in aged women

Cortical Bone Porosity: What Is It, Why Is It Important, and How Can We Detect It?

There is growing recognition of the role of micro-architecture in osteoporotic bone loss and fragility. This trend has been driven by advances in imaging technology, which have enabled a transition from measures of mass to micro-architecture. Imaging trabecular bone has been a key research focus, but advances in resolution have also enabled the detection of cortical bone micro-architecture, particularly the network of vascular canals, commonly referred to as 'cortical porosity.' This review aims to provide an overview of what this level of porosity is, why it is important, and how it can be characterized by imaging. Moving beyond a 'trabeculocentric' view of bone loss holds the potential to improve diagnosis and monitoring of interventions. Furthermore, cortical porosity is intimately linked to the remodeling process, which underpins bone loss, and thus a larger potential exists to improve our fundamental understanding of bone health through imaging of both humans and animal models.

Role of endocortical contouring methods on precision of HR-pQCT-derived cortical micro-architecture in postmenopausal women and young adults

Precision errors of cortical bone micro-architecture from high-resolution peripheral quantitative computed tomography (pQCT) ranged from 1 to 16 % and did not differ between automatic or manually modified endocortical contour methods in postmenopausal women or young adults. In postmenopausal women, manually modified contours led to generally higher cortical bone properties when compared to the automated method.

INTRODUCTION

First, the objective of the study was to define in vivo precision errors (coefficient of variation root mean square (CV%RMS)) and least significant change (LSC) for cortical bone micro-architecture using two endocortical contouring methods: automatic (AUTO) and manually modified (MOD) in two groups (postmenopausal women and young adults) from high-resolution pQCT (HR-pQCT) scans. Second, it was to compare precision errors and bone outcomes obtained with both methods within and between groups.

METHODS

Using HR-pQCT, we scanned twice the distal radius and tibia of 34 postmenopausal women (mean age ± SD 74 ± 7 years) and 30 young adults (27 ± 9 years). Cortical micro-architecture was determined using AUTO and MOD contour methods. CV%RMS and LSC were calculated. Repeated measures and multivariate ANOVA were used to compare mean CV% and bone outcomes between the methods within and between the groups. Significance was accepted at P < 0.05.

RESULTS

CV%RMS ranged from 0.9 to 16.3 %. Within-group precision did not differ between evaluation methods. Compared to young adults, postmenopausal women had better precision for radial cortical porosity (precision difference 9.3 %) and pore volume (7.5 %) with MOD. Young adults had better precision for cortical thickness (0.8 %, MOD) and tibial cortical density (0.2 %, AUTO). In postmenopausal women, MOD resulted in 0.2-54 % higher values for most cortical outcomes, as well as 6-8 % lower radial and tibial cortical BMD and 2 % lower tibial cortical thickness.

CONCLUSIONS

Results suggest that AUTO and MOD endocortical contour methods provide comparable repeatability. In postmenopausal women, manual modification of endocortical contours led to generally higher cortical bone properties when compared to the automated method, while no between-method differences were observed in young adults.

Does frequency of resistance training affect tibial cortical bone density in older women? A randomized controlled trial

SUMMARY

This randomized controlled trial evaluated the effect of resistance training frequency (0, 1, and 2 times/week) on cortical volumetric bone mineral density (vBMD) at the tibia in older women. There was no mean difference in change in tibial cortical vBMD in older women who engaged in resistance training (RT) one or two times/week compared with the control group over 12 months after adjusting for baseline values.

INTRODUCTION

National guidelines recommend RT two to three times/week to optimize bone health. Our objective was to determine the effect of a 12-month intervention of three different RT frequencies on tibial volumetric cortical density (CovBMD) in healthy older women.

METHODS

We randomized participants to the following groups: (1) 2×/week balance and tone group (i.e., no resistance beyond body weight, BT), (2) 1×/week RT (RT1), and (3) 2×/week RT (RT2). Treatment allocation was concealed, and measurement team and the bone data analyst were blinded to group allocation. We used peripheral quantitative computed tomography to acquire one 2.3-mm scan at the 50 % tibia, and the primary outcome was CovBMD. Data were collected at baseline, 6 and 12 months, and we used linear mixed modeling to assess the effect at 12 months.

RESULTS

We assessed 147 participants; 100 women provided data at all three points. Baseline unadjusted mean (SD) tibial CovBMD (in milligrams per cubic centimeter) at the 50 % site was 1,077.4 (43.0) (BT), 1,087.8 (42.0) (RT1), and 1,058.7 (60.4) (RT2). At 12 months, there were no statistically significant differences (-0.45 to -0.17 %) between BT and RT groups for mean difference in change in tibial CovBMD for exercise interventions (BT, RT1, RT2) after adjusting for baseline tibial CovBMD.

CONCLUSION

We note no mean difference in change in tibial CovBMD in older women who engaged in RT one or two times/week compared with the control group over 12 months. It is unknown if RT of 3× or 4×/week would be enough to promote a statistically significant difference in change of bone density.

Three dimensional mapping of strontium in bone by dual energy K-edge subtraction imaging

The bones of many terrestrial vertebrates, including humans, are continually altered through an internal process of turnover known as remodeling. This process plays a central role in bone adaptation and disease. The uptake of fluorescent tetracyclines within bone mineral is widely exploited as a means of tracking new tissue formation. While investigation of bone microarchitecture has undergone a dimensional shift from 2D to 3D in recent years, we lack a 3D equivalent to fluorescent labeling. In the current study we demonstrate the ability of synchrotron radiation dual energy K-edge subtraction (KES) imaging to map the 3D distribution of elemental strontium within rat vertebral samples. This approach has great potential for ex vivo analysis of preclinical models and human tissue samples. KES also represents a powerful tool for investigating the pharmokinetics of strontium-based drugs recently approved in many countries around the globe for the treatment of osteoporosis.

Imaging the 3D structure of secondary osteons in human cortical bone using phase-retrieval tomography

By applying a phase-retrieval step before carrying out standard filtered back-projection reconstructions in tomographic imaging, we were able to resolve structures with small differences in density within a densely absorbing sample. This phase-retrieval tomography is particularly suited for the three-dimensional segmentation of secondary osteons (roughly cylindrical structures) which are superimposed upon an existing cortical bone structure through the process of turnover known as remodelling. The resulting images make possible the analysis of the secondary osteon structure and the relationship between an osteon and the surrounding tissue. Our observations have revealed many different and complex 3D structures of osteons that could not be studied using previous methods. This work was carried out using a laboratory-based x-ray source, which makes obtaining these sorts of images readily accessible.

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

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

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

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

Cortical and trabecular bone in the femoral neck both contribute to proximal femur failure load prediction

We examined the contributions of femoral neck cortical and trabecular bone to proximal femur failure load. We found that trabecular bone mineral density explained a significant proportion of variance in failure load after accounting for total bone size and cortical bone mineral content or cortical area.

INTRODUCTION

The relative contribution of femoral neck trabecular and cortical bone to proximal femur failure load is unclear.

OBJECTIVES

Our primary objective was to determine whether trabecular bone mineral density (TbBMD) contributes to proximal femur failure load after accounting for total bone size and cortical bone content. Our secondary objective was to describe regional differences in the relationship among cortical bone, trabecular bone, and failure load within a cross-section of the femoral neck.

MATERIALS AND METHODS

We imaged 36 human cadaveric proximal femora using quantitative computed tomography (QCT). We report total bone area (ToA), cortical area (CoA), cortical bone mineral content (CoBMC), and TbBMD measured in the femoral neck cross-section and eight 45 degrees regions. The femora were loaded to failure.

RESULTS AND OBSERVATIONS

Trabecular bone mineral density explained a significant proportion of variance in failure load after accounting for ToA and then either CoBMC or CoA respectively. CoBMC contributed significantly to failure load in all regions of the femoral neck except the posterior region. TbBMD contributed significantly to failure load in all regions of the femoral neck except the inferoanterior, superoposterior, and the posterior regions.

CONCLUSION

Both cortical and trabecular bone make significant contributions to failure load in ex vivo measures of bone strength.

Anterior-posterior bending strength at the tibial shaft increases with physical activity in boys: evidence for non-uniform geometric adaptation

We investigated bone structural adaptations to a 16-month school-based physical activity intervention in 202 young boys using a novel analytical method for peripheral quantitative computed tomography scans of the tibial mid-shaft. Our intervention effectively increased bone bending strength in the anterior-posterior plane as estimated with the maximum second moment of area (I(max)).

INTRODUCTION

We previously reported positive effects of a physical activity intervention on peripheral quantitative computed tomography (pQCT)-derived bone strength at the tibial mid-shaft in young boys. The present study further explored structural adaptations to the intervention using a novel method for pQCT analysis.

METHODS

Participants were 202 boys (aged 9-11 years) from 10 schools randomly assigned to control (CON, 63 boys) and intervention (INT, 139 boys) groups. INT boys participated in 60 min/week of classroom physical activity, including a bone-loading program. We used ImageJ to process pQCT images of the tibial mid-shaft and determine the second moments of area (I(max), I(min)) and cortical area (CoA) and thickness (CTh) by quadrant (anterior, medial, lateral, posterior). We defined quadrants according to pixel coordinates about the centroid. We used mixed linear models to compare change in bone outcomes between groups.

RESULTS

The INT boys had a 3% greater gain in I(max) than the CON boys (p = 0.04) and tended to have a greater gain in I(min) ( approximately 2%, NS). Associated with the greater gain in I(max) was a slightly greater (NS) gain (1-1.4%) in CoA and CTh in the anterior, medial, and posterior (but not lateral) quadrants.

CONCLUSION

Our results suggest regional variation in bone adaptation consistent with patterns of bone formation induced by anterior-posterior bending loads.

Muscle power is related to tibial bone strength in older women

We enrolled 65 to 75 year-old community-dwelling women and measured muscle power, strength, physical activity using accelerometry and tibial bone strength using peripheral quantitative computed tomography (pQCT). Muscle power contributed 6.6% of the variance in the bone strength-strain index and 8.9% in the section modulus after accounting for age, height, weight, and physical activity; moderate to vigorous physical activity was related to muscle power in the lower extremity.

INTRODUCTION

Muscle power is associated with DXA measurements of bone mass, but it is not known whether muscle power is associated with bone strength. There are no reports of investigations that have tested the effect of muscle power on bone compartments using advanced imaging.

METHODS

We enrolled 74 community-dwelling women aged 65-75 years. We measured muscle power and strength of leg extension using Keiser air-pressure resistance equipment. All participants wore a waist-mounted Actigraph accelerometer to record physical activity. We used peripheral quantitative computed tomography (pQCT) to measure tibial mid-shaft (50% of the site) bone strength (strength-strain index, section modulus). We used Pearson correlations and multi-level linear regression to investigate the associations between muscle and bone.

RESULTS

Muscle power contributed 6.6% (p = 0.007) of the variance in the bone strength-strain index and 8.9% (p = 0.001) the variance in the section modulus in older women after accounting for age, height, weight, and physical activity. Moderate to vigorous physical activity was significantly related to muscle power in the lower extremity (r = 0.260; p = 0.041).

CONCLUSION

Muscle power significantly contributed to the variance in estimated bone strength. Whether power training will prove to be a more effective stimulus for bone strength than conventional strength training will require further studies.

Trabecular bone in the bird knee responds with high sensitivity to changes in load orientation

Wolff's law of trajectorial orientation proposes that trabecular struts align with the orientation of dominant compressive loads within a joint. Although widely considered in skeletal biology, Wolff's law has never been experimentally tested while controlling for ontogenetic stage, activity level, and species differences, all factors that may affect trabecular bone growth. Here we report an experimental test of Wolff's law using a within-species design in age-matched subjects experiencing physiologically normal levels of bone strain. Two age-matched groups of juvenile guinea fowl Numida meleagris ran on a treadmill set at either 0 degrees (Level group) or 20 degrees (Incline group), for 10 min per day over a 45-day treatment period. Birds running on the 20 degrees inclined treadmill used more-flexed knees than those in the Level group at midstance (the point of peak ground reaction force). This difference in joint posture enabled us to test the sensitivity of trabecular alignment to altered load orientation in the knee. Using a new radon transform-based method for measuring trabecular orientation, our analysis shows that the fine trabecular bone in the distal femur has a high degree of correspondence between changes in joint angle and trabecular orientation. The sensitivity of this response supports the prediction that trabecular bone adapts dynamically to the orientation of peak compressive forces.

Bisphosphonates reduce bone mineral loss at ligament entheses after joint injury

OBJECTIVE

To examine the effects of anterior cruciate ligament (ACL) insufficiency, and subsequent bisphosphonate (BP) antiresorptive therapy, on the bone mineral interface at the enthesis of remaining ligamentous restraints.

METHODS

We measured bone mineral geometry (and subsequent adaptation) at the medial collateral ligament (MCL) origin, using micro-computed tomography (muCT). Groups of normal control, 6 and 14 wk anterior cruciate ligament transected (ACLX), and 6 wk ACLX-BP (risedronate) dosed rabbits were evaluated. Samples were then processed histologically, and the results of mineral adaptation and progression of osteoarthritis (OA) compared to joint laxity values obtained from previous biomechanical testing of the MCL-complex.

RESULTS

muCT defined the MCL origin as a symmetrical, metaphyseal depression that contained soft-tissue elements, including fibrocartilage and ligament--as seen in subsequent histological sections. In contrast, the insertions from ACLX animals lost significant bone mineral, with an MCL-insertion volume 1.2 times that of normal controls at 6 wk ACLX, which further increased to 2.3 times that of normal controls at 14 wk ACLX. Significant differences were also measured between 6 and 14 wk ACLX and age-matched normal controls in volume of cortical bone containing the MCL insertion. However, there were no significant differences in the percentage of cortical bone to underlying trabecular bone at the MCL insertion. When comparing muCT mineral adaptation at the MCL-enthesis with historical MCL-complex laxity data, the values for laxity after ACLX increased proportionately as bone mineral at the insertion was lost, and subsequent use of the BP risedronate reduced both mineral loss and MCL-complex laxity.

CONCLUSION

Compared to the untreated ACLX condition, administering bisphosphonate immediately after loss of the ACL conserved bone mineral at the MCL enthesis, suggesting the potential to therapeutically influence joint-complex laxity and OA progression.

Comparison of microcomputed tomographic and microradiographic measurements of cortical bone porosity

Cortical bone is perforated by a network of canals that have a significant impact upon its material properties. Microcomputed tomography offers the possibility of noninvasively visualizing and quantifying cortical pores in both two and three dimensions. Establishing how two-dimensional (2D) microcomputed tomographic (microCT) analysis compares with conventional methods for analyzing cortical porosity is an important prerequisite for the wider adoption of this technique and the development of three-dimensional (3D) analysis. Therefore, we compared porosity-related parameters from 2D microcomputed tomographic images with those from matching microradiographic sections. Samples from five human femora were scanned at a 10-microm resolution and then sequentially sectioned and microradiographed. An average of eight image pairs were produced from each femur (total, n = 41). The repeatability and comparability of the two techniques was assessed for three parameters; cortical porosity (%), mean pore area (microm(2)), and pore density (pores/mm(2)). For repeatability, no significant difference ( P > 0.05) was found between the two methods for cortical porosity and mean pore area; however, pore density differed significantly ( P < 0.001). For comparability, the bias (+/- error) between the methods was found to be 0.51% (+/-0.31%) for cortical porosity and -155 microm(2) (+/-293 microm(2)) for mean pore area. The bias for pore density was dependent upon measurement size with microcomputed tomographic images having 14% (+/-9.3%) fewer pores per millimeter squared. The qualitative and quantitative similarities between the two techniques demonstrated the utility of 2D microcomputed tomographic for cortical porosity analysis. However, the relatively poor results for pore density revealed that a higher resolution (<10 microm) is needed to consistently visualize all cortical pores in human bone.

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

Cortical bone is perforated by an interconnected network of porous canals that facilitate the distribution of neurovascular structures throughout the cortex. This network is an integral component of cortical microstructure and, therefore, undergoes continual change throughout life as the cortex is remodeled. To date, the investigation of cortical microstructure, including the canal network, has largely been limited to the two-dimensional (2D) realm due to methodological hurdles. Thanks to continuing improvements in scan resolution, micro-computed tomography (muCT) is the first nondestructive imaging technology capable of resolving cortical canals. Like its application to trabecular bone, muCT provides an efficient means of quantifying aspects of 3D architecture of the canal network. Our aim here is to introduce the use of muCT for this application by providing examples, discussing some of the parameters that can be acquired, and relating these to research applications. Although several parameters developed for the analysis of trabecular microstructure are suitable for the analysis of cortical porosity, the algorithm used to estimate connectivity is not. We adapt existing algorithms based on skeletonization for this task. We believe that 3D analysis of the dimensions and architecture of the canal network will provide novel information relevant to many aspects of bone biology. For example, parameters related to the size, spacing, and volume of the canals may be particularly useful for investigation of the mechanical properties of bone. Alternatively, parameters describing the 3D architecture of the canal network, such as connectivity between the canals, may provide a means of evaluating cumulative remodeling related change.

Angiogenesis in the distal femoral chondroepiphysis of the rabbit during development of the secondary centre of ossification

In the developing chondroepiphyses of long bones, the avascular cartilaginous anlage is invaded by numerous blood vessels, through the process of angiogenesis. The objective of this study was to investigate the chronology of this vascular invasion with the spontaneous calcification of the cartilaginous epiphysis during development of the secondary ossification centre in the rabbit distal femur. The time-course of chondroepiphyseal vascular invasion was determined histologically and standardized for eight gestational and four postnatal intervals by plotting kit body mass against crown-rump length. Similarly, microcomputed tomography (micro-CT) helped to visualize calcification at those same gestational and postnatal intervals. To confirm the angiogenic nature of the avascular chondroepiphysis, such samples were assayed on the chick chorio-allantoic membrane (CAM). Neovascular outgrowths from the CAM were apparent 48 h following introduction of an 18-day (gestational) chondroepiphyseal sample. Chondroepiphyseal samples were assayed for the potent developmental angiogenic factors bFGF and VEGF, with the mRNA expression for both these mediators being confirmed using RT-PCR. As angiogenesis and calcification during chondroepiphyseal development occur in a defined tissue environment initially devoid of blood vessels and mineral, those processes provided a unique opportunity to study their progression without complication of injury-related inflammation or extant vasculature and mineral. Furthermore, the discovery of angiogenic, angiostatic or mineral-regulating mediators specific to developing connective tissue may prove useful for analysing the regulation of vascular and mineral pathogenesis in articular tissues.