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BACKGROUND: Narrative letters of recommendation are an important component of the residency application process. However, because narrative letters of recommendation are almost always positive, it is unclear whether those reviewing the letters understand the writer’s intended strength of support for a given applicant. QUESTIONS/PURPOSES: (1) Is the perception of letter readers for narrative letters of recommendation consistent with the intention of the letter’s author? (2) Is there inter-reviewer consistency in selection committee members’ perceptions of the narrative letters of recommendation? METHODS: Letter writers who wrote two or more narrative letters of recommendation for applicants to one university-based orthopaedic residency program for the 2014 to 2015 application cycle were sent a survey linked to a specific letter of recommendation they authored to assess the intended meaning regarding the strength of an applicant. A total of 247 unstructured letters of recommendation and accompanying surveys were sent to their authors, and 157 surveys were returned and form the basis of this study (response percentage 64%). The seven core members of the admissions committee (of 22 total reviewers) at a university-based residency program were sent a similar survey regarding their perception of the letter. To answer our research question about whether letter readers’ perceptions about a candidate were consistent with the letter writer’s intention, we used kappa values to determine agreement for survey questions involving discrete variables and Spearman correlation coefficients (SCCs) to determine agreement for survey questions involving continuous variables. To answer our research question regarding inter-reviewer consistency among the seven faculty members, we compared the letter readers’ responses to each survey question using intraclass correlation coefficients (ICCs). RESULTS: There was a negligible to moderate correlation between the intended and perceived strength of the letters (SCC 0.26 to 0.57), with only one of seven letter readers scoring in the moderate correlation category. When stratifying the applicants into thirds, there was only slight agreement (kappa 0.07 to 0.19) between the writers and reviewers. There were similarly low kappa values for agreement about how the writers and readers felt regarding the candidate matching into their program (kappa 0.14 to 0.30). The ICC for each question among the seven faculty reviewers ranged from poor to moderate (ICC 0.42 to 0.52). CONCLUSION: Our results demonstrate that the reader’s perception of narrative letters of recommendation did not correlate well with the letter writer’s intended meaning and was not consistent between letter readers at a single university-based urban orthopaedic surgery residency program. CLINICAL RELEVANCE: Given the low correlation between the intended strength of the letter writers and the perceived strength of those letters, we believe that other options such as a slider bar or agreed-upon wording as is used in many dean’s letters may be helpful.

Osteocytes, the bone cells embedded in the mineralized matrix, control bone modeling and remodeling through direct contact with adjacent cells and via paracrine and endocrine factors that affect cells in the bone marrow microenvironment or distant organs. Osteocytes express numerous G protein-coupled receptors (GPCRs) and mice lacking the stimulatory subunit of G-protein (Gsα) in osteocytes (Dmp1-Gsα(KO) mice) have abnormal myelopoiesis, osteopenia and reduced adipose tissue. We previously reported that the severe osteopenia and the changes in adipose tissue present in these mice were mediated by increased sclerostin, which suppress osteoblast functions and promote browning of white adipocytes. Inversely, the myeloproliferation was driven by granulocyte colony-stimulating factor (G-CSF) and administration of neutralizing antibodies against G-CSF only partially restored the myeloproliferation, suggesting that additional osteocyte-derived factors might be involved. We hypothesized that osteocytes secrete Gsα-dependent factor(s) which regulate myeloid cells proliferation. To identify osteocyte-secreted proteins, we used the osteocytic cell line Ocy454 expressing or lacking Gsα expression (Ocy454-Gsα(cont) and Ocy454-Gsα(KO)) to delineate the osteocyte “secretome” and its regulation by Gsα. Here we reported that factors secreted by osteocytes increased the number of myeloid colonies and promoted macrophage proliferation. The proliferation of myeloid cells was further promoted by osteocytes lacking Gsα expression. Myeloid cells can differentiate into bone-resorbing osteoclasts therefore we hypothesized that osteocyte-secreted factors might also regulate osteoclastogenesis in a Gsα-dependent manner. Conditioned medium (CM) from Ocy454 (both Gsα(cont) and Gsα(KO)) significanlty increased the proliferation of bone marrow mononuclear cells (BMNC) and, at the same time, inhibited their differentiation into mature osteoclasts via a Gsα-dependent mechanism. Proteomics analysis of CM from Ocy454 Gsα(cont) and Gsα(KO) cells identified neuropilin-1 (Nrp-1) and granulin (Grn) as osteocytic-secreted proteins upregulated in Ocy454-Gsα(KO) cells compared to Ocy454-Gsα(cont), whereas semaphorin3A was significantly suppressed. Treatment of Ocy454-Gsα(cont) cells with recombinant proteins or knockdown of Nrp-1 and Grn in Ocy454-Gsα(KO) cells partially rescued the inhibition of osteoclasts, demonstrating that osteocytes control osteoclasts differentiation through Nrp-1 and Grn which are regulated by Gsα signalling.

Time is a central element of the sexual dimorphic patterns of development, pathology, and aging of the skeleton. Because the transcriptome is a representation of the phenome, we hypothesized that both sex and sex-specific temporal, transcriptomic differences in bone tissues over an 18-month period would be informative to the underlying molecular processes that lead to postnatal sexual dimorphism. Regardless of age, sex-associated changes of the whole bone transcriptomes were primarily associated not only with bone but also vascular and connective tissue ontologies. A pattern-based approach used to screen the entire Gene Expression Omnibus (GEO) database against those that were sex-specific in bone identified two coordinately regulated gene sets: one related to high phosphate-induced aortic calcification and one induced by mechanical stimulation in bone. Temporal clustering of the transcriptome identified two skeletal tissue-associated, sex-specific patterns of gene expression. One set of genes, associated with skeletal patterning and morphology, showed peak expression earlier in females. The second set of genes, associated with coupled remodeling, had quantitatively higher expression in females and exhibited a broad peak between 3 to 12 months, concurrent with the animals' reproductive period. Results of phenome-level structural assessments of the tibia and vertebrae, and in vivo and in vitro analysis of cells having osteogenic potential, were consistent with the existence of functionally unique, skeletogenic cell populations that are separately responsible for appositional growth and intramedullary functions. These data suggest that skeletal sexual dimorphism arises through sex-specific, temporally different processes controlling morphometric growth and later coupled remodeling of the skeleton during the reproductive period of the animal. © 2021 The Authors.

Risk factors for poor bone quality include estrogen loss at menopause, a high fat diet and exposures to drugs/chemicals that activate peroxisome proliferator activated receptor gamma (PPARγ). We observed that the PPARγ and retinoid X receptor dual ligand, tributyltin (TBT), repressed periosteal bone formation but enhanced trabecular bone formation in female C57BL6/J mice. Here, we examined the interaction of diet, ovariectomy (OVX) and TBT exposure on bone structure. C57BL/6J mice underwent either sham surgery or OVX at 10 weeks of age. At 12 weeks of age, they were placed on a low (10% kcal) or high (45% kcal) fat, sucrose-matched diet and treated with Vh or TBT (1 or 5 mg/kg) for 14 weeks. OVX increased body weight gain in mice on either diet. TBT enhanced body weight gain in intact mice fed a high fat diet, but decreased weight gain in OVX mice. Elemental tin concentrations increased dose-dependently in bone. TBT had marginal effects on cortical and trabecular bone in intact mice fed a low- or high- fat diet. OVX caused a reduction in cortical and trabecular bone, regardless of diet. In high-fat fed OVX mice, TBT further reduced cortical thickness, bone area and total area. Interestingly, TBT protected against OVX-induced trabecular bone loss in low fat fed mice. The protective effect of TBT was nullified by the high fat diet and accompanied by a significant decrease in serum bone formation markers. Our novel observations will provide new information on basic bone biology, potential therapeutic targets and toxicological pathways.

Many scientific studies, especially in the biomedical sciences, generate data measured simultaneously over a multitude of units, over a period of time, and under different conditions or combinations of factors. Often, an important question of interest asked relates to which units behave similarly under different conditions, but measuring the variation over time complicates the analysis significantly. In this article we address such a problem arising from a gene expression study relating to bone aging, and develop a Bayesian statistical method that can simultaneously detect and uncover signals on three levels within such data: factorial, longitudinal, and transcriptional. Our model framework considers both cluster and time-point-specific parameters and these parameters uniquely determine the shapes of the temporal gene expression profiles, allowing the discovery and characterization of latent gene clusters based on similar underlying biological mechanisms. Our methodology was successfully applied to discover transcriptional networks in a microarray data set comparing the transcriptomic changes that occurred during bone aging in male and female mice expressing one or both copies of the bromodomain (Brd2) gene, a transcriptional regulator which exhibits an age-dependent sex-linked bone loss phenotype.

Bone loss is a major health concern for astronauts during long-term spaceflight and for patients during prolonged bed rest or paralysis. Growing evidence suggests that osteocytes, the most abundant cells in the mineralized bone matrix, play a key role in sensing mechanical forces applied to the skeleton and integrating the orchestrated response into subcellular biochemical signals to modulate bone homeostasis. However, the precise molecular mechanisms underlying both mechanosensation and mechanotransduction in late-osteoblast-to-osteocyte cells under microgravity (µG) have yet to be elucidated. To unravel the mechanisms by which late osteoblasts and osteocytes sense and respond to mechanical unloading, we exposed the osteocytic cell line, Ocy454, to 2, 4, or 6 days of µG on the SpaceX Dragon-6 resupply mission to the International Space Station. Our results showed that µG impairs the differentiation of osteocytes, consistent with prior osteoblast spaceflight experiments, which resulted in the downregulation of key osteocytic genes. Importantly, we demonstrate the modulation of critical glycolysis pathways in osteocytes subjected to microgravity and discovered a set of mechanical sensitive genes that are consistently regulated in multiple cell types exposed to microgravity suggesting a common, yet to be fully elucidated, genome-wide response to microgravity. Ground-based simulated microgravity experiments utilizing the NASA rotating-wall-vessel were unable to adequately replicate the changes in microgravity exposure highlighting the importance of spaceflight missions to understand the unique environmental stress that microgravity presents to diverse cell types. In summary, our findings demonstrate that osteocytes respond to µG with an increase in glucose metabolism and oxygen consumption.

Phosphate plays a critical role in chondrocyte maturation and skeletal mineralization. Studies examining the consequences of dietary phosphate restriction in growing mice demonstrated not only the development of rickets, but also a dramatic decrease in bone accompanied by increased marrow adipose tissue (MAT). Thus studies were undertaken to determine the effects of dietary phosphate restriction on bone formation and bone marrow stromal cell (BMSC) differentiation. Acute phosphate restriction of 28-day-old mice profoundly inhibited bone formation within 48 hours. It also resulted in increased mRNA expression of the early osteolineage markers Sox9 and Runt-related transcription factor 2 (Runx2), accompanied by decreased expression of the late osteolineage markers Osterix and Osteocalcin in BMSCs and osteoblasts, suggesting that phosphate restriction arrests osteoblast differentiation between Runx2 and Osterix. Increased expression of PPARγ and CEBPα, key regulators of adipogenic differentiation, was observed within 1 week of dietary phosphate restriction and was followed by a 13-fold increase in MAT at 3 weeks of phosphate restriction. In vitro phosphate restriction did not alter BMSC osteogenic or adipogenic colony formation, implicating aberrant paracrine or endocrine signaling in the in vivo phenotype. Because BMP signaling regulates the transition between Runx2 and Osterix, this pathway was interrogated. A dramatic decrease in pSmad1/5/9 immunoreactivity was observed in the osteoblasts of phosphate-restricted mice on day 31 (d31) and d35. This was accompanied by attenuated expression of the BMP target genes Id1, KLF10, and Foxc2, the latter of which promotes osteogenic and angiogenic differentiation while impairing adipogenesis. A decrease in expression of the Notch target gene Hey1, a BMP-regulated gene that governs angiogenesis, was also observed in phosphate-restricted mice, in association with decreased metaphyseal marrow vasculature. Whereas circulating phosphate levels are known to control growth plate maturation and skeletal mineralization, these studies reveal novel consequences of phosphate restriction in the regulation of bone formation and osteoblast differentiation. © 2016 American Society for Bone and Mineral Research.

The biomechanical mechanisms leading to vertebral fractures are not well understood. Clinical and laboratory evidence suggests that the vertebral endplate plays a key role in failure of the vertebra as a whole, but how this role differs for different types of vertebral loading is not known. Mechanical testing of human thoracic spine segments, in conjunction with time-lapsed micro-computed tomography, enabled quantitative assessment of deformations occurring throughout the entire vertebral body under axial compression combined with anterior flexion ("combined loading") and under axial compression only ("compression loading"). The resulting deformation maps indicated that endplate deflection was a principal feature of vertebral failure for both loading modes. Specifically, the onset of endplate deflection was temporally coincident with a pronounced drop in the vertebra's ability to support loads. The location of endplate deflection, and also vertebral strength, were associated with the porosity of the endplate and the microstructure of the underlying trabecular bone. However, the location of endplate deflection and the involvement of the cortex differed between the two types of loading. Under the combined loading, deflection initiated, and remained the largest, at the anterior central endplate or the anterior ring apophysis, depending in part on health of the adjacent intervertebral disc. This deflection was accompanied by outward bulging of the anterior cortex. In contrast, the location of endplate deflection was more varied in compression loading. For both loading types, the earliest progression to a mild fracture according to a quantitative morphometric criterion occurred only after much of the failure process had occurred. The outcomes of this work indicate that for two physiological loading modes, the vertebral endplate and underlying trabecular bone are critically involved in vertebral fracture. These outcomes provide a strong biomechanical rationale for clinical methods, such as algorithm-based qualitative (ABQ) assessment, that diagnose vertebral fracture on the basis of endplate depression. © 2015 American Society for Bone and Mineral Research.

Epidemiological and biomechanical evidence indicates that the risk of vertebral fracture differs between men and women, and that vertebral fracture frequently involves failure of the endplate region. The goal of this study was to compare the bone microstructure of the endplate region-defined as the (bony) vertebral endplate and underlying subchondral trabecular bone-between sexes and to determine whether any such sex differences are associated with vertebral strength. The bone density (volume fraction, apparent density and tissue mineral density) of the superior-most 2 mm of the vertebra, and the bone density and trabecular architecture of the next 5 mm were quantified using micro-computed tomography in human T8 (12 female, 16 male) and L1 (13 female, 12 male) vertebrae. Average density of the vertebra (integral bone mineral density (BMD)) was determined by quantitative computed tomography and compressive strength by mechanical testing. Few differences were found between male and female vertebrae in the density of the endplate region; none were found in trabecular architecture. However, whereas endplate volume fraction was positively correlated with integral BMD in male vertebrae (r = 0.654, p .001), no correlation was found in the female vertebrae (r = 0.157, p = .455). Accounting for the density of the endplate region improved predictions of vertebral strength (p .034) and eliminated sex-specificity in the strength prediction that was based on integral BMD alone. These results suggest that the density of the endplate region influences vertebral fracture and that non-invasive assessment of this region's density can contribute to predictions of vertebral strength in men and women.

The mechanical properties of bone are fundamental to the ability of our skeletons to support movement and to provide protection to our vital organs. As such, deterioration in mechanical behavior with aging and/or diseases such as osteoporosis and diabetes can have profound consequences for individuals’ quality of life. This article reviews current knowledge of the basic mechanical behavior of bone at length scales ranging from hundreds of nanometers to tens of centimeters. We present the basic tenets of bone mechanics and connect them to some of the arcs of research that have brought the field to recent advances. We also discuss cortical bone, trabecular bone, and whole bones, as well as multiple aspects of material behavior, including elasticity, yield, fracture, fatigue, and damage. We describe the roles of bone quantity (e.g., density, porosity) and bone quality (e.g., cross-linking, protein composition), along with several avenues of future research.

A targeted proteomic analysis of murine serum over a 35-day course of fracture healing was carried out to determine if serum proteomic changes could be used to monitor the biological progression of fracture healing. Transverse, closed femoral fractures where generated and stabilized with intramedullary fixation. A single stranded DNA aptamer-based multiplexed proteomic approach was used to assay 1,310 proteins. The transcriptomic profiles for genes matching the 1,310 proteins were obtained by microarray analysis of callus mRNA. Of the 1,310 proteins analyzed, 850 proteins showed significant differences among the time points (p-value

Radiographic Union Score for Tibia (RUST) and modified RUST (mRUST) are radiographic tools for quantitatively evaluating fracture healing using a cortical scoring system. This tool has high intra-class correlation coefficients (ICCs); however, little evidence has evaluated the scores against the physical properties of bone healing. Closed, stabilized fractures were made in the femora of C3H/HeJ male mice (8-12 week-old) of two dietary groups: a control and a phosphate restricted diet group. Micro-computed tomography (μCT) and torsion testing were carried out at post-operative days (POD) 14, 21, 35, and 42 (n=10-16) per group time-point. Anteroposterior and lateral radiographic views were constructed from the μCT scans and scored by five raters. The raters also indicated if the fracture were healed. ICCs were 0.71 (mRUST) and 0.63 (RUST). Both RUST scores were positively correlated with callus bone mineral density (BMD) (r = 0.85 and 0.80, p 0.318). Clinical relevance of this study is both RUST scores showed high correlation to physical properties of healing and generally distinguished healed vs. non-healed fractures. This article is protected by copyright. All rights reserved.

Vertebral fractures are common in the elderly, but efforts to reduce their incidence have been hampered by incomplete understanding of the failure processes that are involved. This study's goal was to elucidate failure processes in the lumbar vertebra and to assess the accuracy of quantitative computed tomography (QCT)-based finite element (FE) simulations of these processes. Following QCT scanning, spine segments (n = 27) consisting of L1 with adjacent intervertebral disks and neighboring endplates of T12 and L2 were compressed axially in a stepwise manner. A microcomputed tomography scan was performed at each loading step. The resulting time-lapse series of images was analyzed using digital volume correlation (DVC) to quantify deformations throughout the vertebral body. While some diversity among vertebrae was observed on how these deformations progressed, common features were large strains that developed progressively in the superior third and, concomitantly, in the midtransverse plane, in a manner that was associated with spatial variations in microstructural parameters such as connectivity density. Results of FE simulations corresponded qualitatively to the measured failure patterns when boundary conditions were derived from DVC displacements at the endplate. However, quantitative correspondence was often poor, particularly when boundary conditions were simplified to uniform compressive loading. These findings suggest that variations in trabecular microstructure are one cause of the differences in failure patterns among vertebrae and that both the lack of incorporation of these variations into QCT-based FE models and the oversimplification of boundary conditions limit the accuracy of these models in simulating vertebral failure.

Osteocytes are master orchestrators of bone remodeling; they control osteoblast and osteoclast activities both directly via cell-to-cell communication and indirectly via secreted factors, and they are the main postnatal source of sclerostin and RANKL (receptor activator of NF-kB ligand), two regulators of osteoblast and osteoclast function. Despite progress in understanding osteocyte biology and function, much remains to be elucidated. Recently developed osteocytic cell lines-together with new genome editing tools-has allowed a closer look at the biology and molecular makeup of these cells. By using single-cell cloning, we identified genes that are associated with high Sost/sclerostin expression and analyzed their regulation and function. Unbiased transcriptome analysis of high- vs. low-Sost/sclerostin-expressing cells identified known and novel genes. Dmp1 (dentin matrix protein 1), Dkk1 (Dickkopf WNT signaling pathway inhibitor 1), and Phex were among the most up-regulated known genes, whereas Srpx2, Cd200, and carbonic anhydrase III (CAIII) were identified as novel markers of differentiated osteocytes. Aspn, Enpp2, Robo2, Nov, and Serpina3g were among the transcripts that were most significantly suppressed in high-Sost cells. Considering that CAII was recently identified as being regulated by Sost/sclerostin and capable of controlling mineral homeostasis, we focused our attention on CAIII. Here, we report that CAIII is highly expressed in osteocytes, is regulated by parathyroid hormone both in vitro and in vivo, and protects osteocytes from oxidative stress.-Shi, C., Uda, Y., Dedic, C., Azab, E., Sun, N., Hussein, A. I., Petty, C. A., Fulzele, K., Mitterberger-Vogt, M. C., Zwerschke, W., Pereira, R., Wang, K., Divieti Pajevic, P. Carbonic anhydrase III protects osteocytes from oxidative stress.

Endochondral ossification is the process where cartilage forms prior to ossification and in which new bone forms during both fracture healing and ectopic bone formation. Transitioning to ossification is a highly coordinated process between hypertrophic chondrocytes, vascular endothelial cells, osteoblasts and osteoclasts. A critical biological process that is central to the interactions of these various cell types is angiogenesis. Although it is well established that angiogenesis is crucial for fracture repair, less is known pertaining to the role of angiogenesis in ectopic bone formation. Furthermore, fracture repair models are complicated by extensive trauma, subsequent inflammatory responses and concurrent repair processes in multiple tissues. In order to more definitively characterize the relationship between angiogenesis and postnatal endochondral ossification, a model of ectopic bone formation was used. Human demineralized bone matrix (DBM) was implanted in immune-deficient mice (rag null (B6.129S7-Rag1tm1/MOM/J)) to induce ectopic bone. Inhibition of angiogenesis with either a small molecule (TNP-470) or a targeted biological (Vascular Endothelial Growth Factor Receptor type 2 [VEGFR2] blocking antibody) prevented ectopic bone formation by 83% and 77%, respectively. Most striking was that the progression of chrondrogenesis was halted during very early phases of chondrocyte differentiation between condensation and prehypertrophy (TNP-470) or the proliferative phase (VEGFR2 blockade) prior to hypertrophy, while osteoclast recruitment and resorption were almost completely inhibited. Our results demonstrate angiogenesis plays a developmental role in endochondral bone formation at a much earlier phase of chondrogenesis than suggested by prior findings.

Endplate deflection frequently occurs with vertebral failure, but the relationship between the two remains poorly defined. This study examined associations between endplate deflection under compressive loading and characteristics of the neighboring subchondral bone and intervertebral disc (IVD). Ten L1 vertebrae with adjacent IVDs were dissected, compressed axially in a stepwise manner to failure, and imaged with micro-computed tomography before each loading step. From the images, deflection was measured across the surface of each endplate at each step. Trabecular microstructure and endplate volume fraction were evaluated in 5mm regions just under the superior endplate. IVDs were assessed using computed tomography and histology. A marked increase in superior endplate deflection coincided with a drop in the load-displacement curve. Endplate deflection was higher in regions with less robust bone microstructure (p

Ex vivo mechanical testing is an essential tool for study of vertebral mechanics. However, the common method of testing vertebral bodies in the absence of adjacent intervertebral discs (IVDs) may limit the physiological relevance of the results. The goal of this study was to determine the influence of IVDs on vertebral mechanical properties and failure mechanisms. Rabbit thoracic vertebral bodies were tested with and without IVDs in a stepwise fashion that incorporated a micro-computed tomography scan at each loading step. The image sequences were analyzed using digital volume correlation to quantify deformations throughout the vertebral body. The observed deformation patterns differed substantially between the groups. Specimens tested with IVDs exhibited a slow increase in strain in the inferior and posterior regions, followed by a sudden increase in strain in the anterior cortex right at the yield point. In contrast, the highest strains in the isolated vertebral bodies were in the posterior regions throughout the test. Specimens tested with IVDs had lower stiffness (507.49 ± 184.73 N/mm vs. 845.61 ± 296.09 N/mm; p = 0.044), higher ultimate displacement (2.00 ± 0.68 mm vs. 1.17 ± 0.54 mm; p = 0.043), and higher maximum shear strains (e.g. top 25th percentile: 0.19 ± 0.11 vs. 0.06 ± 0.07mm/mm; p < 0.0458), and tended to have lower ultimate force (690.28 ± 160.25 N vs. 873.81 ± 131.48 N; p = 0.056). Similar work to failure (648.15 ± 317.86 N-mm vs. 603.49 ± 437.95 N-mm; p = 0.844) was observed between the two groups. These results indicate that testing vertebral bodies in the absence of IVDs can elicit artifactual failure mechanisms. These artifacts may be more prominent than the effects on vertebral strength and toughness.

Phosphate plays a critical role in chondrocyte maturation and skeletal mineralization. Studies examining the consequences of dietary phosphate restriction in growing mice demonstrated not only the development of rickets, but also a dramatic decrease in bone accompanied by increased marrow adipose tissue (MAT). Thus studies were undertaken to determine the effects of dietary phosphate restriction on bone formation and bone marrow stromal cell (BMSC) differentiation. Acute phosphate restriction of 28-day-old mice profoundly inhibited bone formation within 48 hours. It also resulted in increased mRNA expression of the early osteolineage markers Sox9 and Runt-related transcription factor 2 (Runx2), accompanied by decreased expression of the late osteolineage markers Osterix and Osteocalcin in BMSCs and osteoblasts, suggesting that phosphate restriction arrests osteoblast differentiation between Runx2 and Osterix. Increased expression of PPARγ and CEBPα, key regulators of adipogenic differentiation, was observed within 1 week of dietary phosphate restriction and was followed by a 13-fold increase in MAT at 3 weeks of phosphate restriction. In vitro phosphate restriction did not alter BMSC osteogenic or adipogenic colony formation, implicating aberrant paracrine or endocrine signaling in the in vivo phenotype. Because BMP signaling regulates the transition between Runx2 and Osterix, this pathway was interrogated. A dramatic decrease in pSmad1/5/9 immunoreactivity was observed in the osteoblasts of phosphate-restricted mice on day 31 (d31) and d35. This was accompanied by attenuated expression of the BMP target genes Id1, KLF10, and Foxc2, the latter of which promotes osteogenic and angiogenic differentiation while impairing adipogenesis. A decrease in expression of the Notch target gene Hey1, a BMP-regulated gene that governs angiogenesis, was also observed in phosphate-restricted mice, in association with decreased metaphyseal marrow vasculature. Whereas circulating phosphate levels are known to control growth plate maturation and skeletal mineralization, these studies reveal novel consequences of phosphate restriction in the regulation of bone formation and osteoblast differentiation. © 2016 American Society for Bone and Mineral Research.

Full-field measurement of deformation in biological structures such as bones is a promising experimental approach for study of the spatial heterogeneity in mechanical behavior. With the advent of high-resolution, 3-D imaging, digital volume correlation (DVC) allows for the measurement of spatially heterogeneous, 3-D deformation fields throughout entire volumes. For bones such as the vertebra, use of DVC to detect the onset and progression of failure is of direct relevance to the study of osteoporotic fractures. Application of DVC to whole bones, as opposed to machined specimens of bone tissue, involves additional challenges such as the irregular geometry, large data sets, and decreased signal-to-noise ratio. These challenges are addressed in this paper, and the DVC method that results is used to examine yield and post-yield deformations in vertebral compression experiments.

Knowledge of the nature of the elastic symmetry of trabecular bone is fundamental to the study of bone adaptation and failure. Previous studies have classified human vertebral trabecular bone as orthotropic or transversely isotropic but have typically obtained samples from only selected regions of the centrum. In this study, the elastic symmetry of human vertebral trabecular bone was characterized using microfinite element (μFE) analyses performed on 1019 cubic regions of side length equal to 5 mm, obtained via thorough sampling of the centrums of 18 human L1 vertebrae (age = 81.17 ± 7.7 yr; eight males and ten females). An optimization procedure was used to find the closest orthotropic representation of the resulting stiffness tensor for each cube. The orthotropic elastic constants and orientation of the principal elastic axes were then recorded for each cube and were compared to the constants predicted from Cowin's fabric-based constitutive model (Cowin, 1985, “The Relationship Between the Elasticity Tensor and the Fabric Tensor,” Mech. Mater., 4(2), pp. 137–147.) and the orientation of the principal axes of the fabric tensor, respectively. Deviations from orthotropy were quantified by the “orthotropic error” (van Rietbergen et al., 1996, “Direct Mechanics Assessment of Elastic Symmetries and Properties of Trabecular Bone Architecture,” J. Biomech., 29(12), pp. 1653–1657), and deviations from transverse isotropy were determined by statistical comparison of the secondary and tertiary elastic moduli. The orthotropic error was greater than 50% for nearly half of the cubes, and the secondary and tertiary moduli differed from one another (p

Endplate deflection frequently occurs with vertebral failure, but the relationship between the two remains poorly defined. This study examined associations between endplate deflection under compressive loading and characteristics of the neighboring subchondral bone and intervertebral disc (IVD). Ten L1 vertebrae with adjacent IVDs were dissected, compressed axially in a stepwise manner to failure, and imaged with micro-computed tomography before each loading step. From the images, deflection was measured across the surface of each endplate at each step. Trabecular microstructure and endplate volume fraction were evaluated in 5 mm regions just under the superior endplate. IVDs were assessed using computed tomography and histology. A marked increase in superior endplate deflection coincided with a drop in the load-displacement curve. Endplate deflection was higher in regions with less robust bone microstructure (p 0.009), though these associations tended to weaken as loading progressed. Immediately following the ultimate point, endplate deflection was higher in regions underlying the nucleus pulposus versus annulus fibrosus (p = 0.035), irrespective of disc grade (p = 0.346). These results indicate that a sudden increase in endplate deflection signals that the mechanical competence of the vertebra has been compromised. The mechanisms of endplate failure likely relate to anatomical features of the endplate, neighboring trabecular bone, and IVD.

Bone functions as the main load-bearing component of the musculoskeletal system, making it a classic subject for the study of biomechanics. Bone has a hierarchical structure and all levels play a role in its overall behavior and function. There are multiple factors that affect bone’s mechanical behavior, such as tissue material properties and bone geometry and structure. These factors, which undergo numerous changes with age, contribute to whole bone’s structural behavior. This chapter provides a review of classic bone biomechanics, the role of bone’s structure and composition on its mechanical properties, the mechanical behavior of whole bone, and age-related changes in bone that contribute to fracture.

This study's goal was to determine associations among the intravertebral heterogeneity in bone density, bone strength, and intervertebral disc (IVD) health. Results indicated that predictions of vertebral strength can benefit from considering the magnitude of the density heterogeneity and the congruence between the spatial distribution of density and IVD health.This study aims to determine associations among the intravertebral heterogeneity in bone density, bone strength, and IVD healthRegional measurements of bone density were performed throughout 30 L1 vertebral bodies using micro-computed tomography (μCT) and quantitative computed tomography (QCT). The magnitude of the intravertebral heterogeneity in density was defined as the interquartile range and quartile coefficient of variation in regional densities. The spatial distribution of density was quantified using ratios of regional densities representing different anatomical zones (e.g., anterior to posterior regional densities). Cluster analysis was used to identify groups of vertebrae with similar spatial distributions of density. Vertebral strength was measured in compression. IVD health was assessed using two scoring systems.QCT- and μCT-based measures of the magnitude of the intravertebral heterogeneity in density were strongly correlated with each other (p 0.005). Accounting for the interquartile range in regional densities improved predictions of vertebral strength as compared to predictions based only on mean density (R (2) = 0.59 vs. 0.43; F-test p-value = 0.018). Specifically, after adjustment for mean density, vertebral bodies with greater heterogeneity in density exhibited higher strength. No single spatial distribution of density was associated with high vertebral strength. Analyses of IVD scores suggested that the health of the adjacent IVDs may modulate the effect of a particular spatial distribution of density on vertebral strength.Noninvasive measurements of the intravertebral distribution of bone density, in conjunction with assessments of IVD health, can aid in predictions of bone strength and in elucidating biomechanical mechanisms of vertebral fracture.

Osteoporosis is a disease characterized by a loss of bone mass and deterioration in trabecular architecture, leading to decreased bone strength and increased susceptibility to fracture [1]. Vertebral fractures account for more than half of the approximately 1.5 million osteoporotic fractures that occur each year in the United States [2], heightening the need for accurate, noninvasive estimates of vertebral strength and stiffness. Quantitative computed tomography-based finite element models are increasingly used for noninvasive determination of mechanical properties of vertebrae [3,4], as they provide better predictions of vertebral strength compared to methods currently used in clinical diagnosis [5]. However, the material properties in the QCT-based FE models are typically estimated solely from local variations in bone mineral density [6,7] despite evidence that the biomechanical behavior of the vertebra depends also on the architecture of the vertebral trabecular bone [8,9], that the architecture is spatially inhomogeneous [10], and that the architecture is altered with age [11]. Advances in QCT imaging allow measurement of some architectural parameters [12,13], such as trabecular anisotropy [14], at clinically permissible doses of radiation. However, whether the effect of incorporating architectural information on the accuracy of the model predictions is large enough to warrant the additional model complexity and radiation exposure has not been established. Anisotropy of the trabecular architecture is believed to result from adaption of bone to its mechanical environment (Wolff's law). Trabecular anisotropy, represented by the principal orientation of the trabecular network and by the degree of anisotropy, is influential in the mechanical behavior of vertebra [8]. With an increase in DA due to aging and osteoporosis [11,15], the number of horizontal trabeculae reduces, rendering the vertebra less capable of withstanding unusual or off-axis loads and also more prone to buckling under axial compressive loads [16]. Prior QCT-based, FE models of the vertebra, which were based solely on BMD, have assumed a constant DA and have had the principal direction always aligned with the superior-inferior (SI) direction [4,6,7] and, thus, may fail to capture differences in the biomechanical response of vertebrae with different trabecular architectures. Variations in anisotropy within and among vertebrae can be accounted for using morphology-based constitutive models [17–19]. Recent studies that have incorporated one of these types of constitutive models as well as explicit modeling of the cortical shell in high-resolution peripheral QCT (HR-pQCT)-based FE models of vertebrae have shown combined effects of these additions on predictions of vertebral stiffness and locations of damage accumulation [20,21]. Accurate, explicit modeling of the shell is not possible at present using QCT in the axial skeleton; however, a realistic representation of the specimen-specific trabecular anisotropy alone would potentially improve the accuracy of QCT-based FE models of the vertebra. The overall goal of this study was to evaluate the importance of incorporating specimen-specific trabecular anisotropy for QCT-based FE predictions of vertebral stiffness. The major aims of this study were: (1) to compare the QCT-based FE results obtained with fixed, anisotropic material properties (“generic-anisotropic”) for trabecular bone to those obtained with specimen-specific, anisotropic material properties and (2) to study the influence of DA on the FE predictions of vertebral stiffness. The FE models were evaluated under loading conditions of axial compression, pure anterior bending, and combined loading of axial compression and anterior bending (Fig. ​(Fig.11). Fig. 1 List of the FE analyses conducted to evaluate the effect of incorporating specimen-specific, trabecular anisotropy on QCT-based FE predictions of vertebral stiffness and deformation patterns: For each combination of type of model, type of material property, ...

Quantitative computed tomography (QCT)-based finite element (FE) models provide better predictions of vertebral strength compared to traditional methods currently used in clinical diagnosis [1]. In QCT-based FE models, the intra- and inter-specimen variations in trabecular anisotropy are often ignored, despite evidence that the biomechanical behavior of the vertebra depends on the architecture of the vertebral trabecular bone [2]. A realistic representation of the specimen-specific, trabecular anisotropy in the FE models of vertebrae would potentially improve predictions of vertebral failure. The overall goal of this study was to evaluate the importance of incorporating specimen-specific, trabecular anisotropy for QCT-based FE predictions of vertebral stiffness and deformation patterns. The major aims of this study were (a) to compare the QCT-based FE results obtained with a constant, anisotropic, material model (the “generic-anisotropic” model) for trabecular bone to those obtained with a specimen-specific, anisotropic, material model and (b) to study the influence of degree of anisotropy (DA) on the FE predictions of vertebral stiffness.Copyright © 2012 by ASME

Vascular formation is intimately associated with bone formation during distraction osteogenesis (DO). While prior studies on this association have focused on vascular formation locally within the regenerate, we hypothesized that this vascular formation, as well as the resulting osteogenesis, relies heavily on the response of the vascular network in surrounding muscular compartments. To test this hypothesis, the spatiotemporal sequence of vascular formation was assessed in both muscular and osseous compartments in a murine model of DO and was compared to the progression of osteogenesis. Micro-computed tomography (μCT) scans were performed sequentially, before and after demineralization, on specimens containing contrast-enhanced vascular casts. Image registration and subtraction procedures were developed to examine the co-related, spatiotemporal patterns of vascular and osseous tissue formation. Immunohistochemistry was used to assess the contributory roles of arteriogenesis (formation of large vessels) and angiogenesis (formation of small vessels) to overall vessel formation. Mean vessel thickness showed an increasing trend during the period of active distraction (p=0.068), whereas vessel volume showed maximal increases during the consolidation period (p=0.009). The volume of mineralized tissue in the regenerate increased over time (p

Silicone implants are used for prosthetic arthroplasty of metacarpophalangeal (MCP) joints severely damaged by rheumatoid arthritis. Different silicone elastomer MCP implant designs have 1 Corresponding author. 55 Queen Street East, Suite 800, Toronto, ON, M5C 1R6, Canada. Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received February 3, 2010; final manuscript received December 19, 2010; accepted manuscript posted December 22, 2010; published online January 31, 2011. Assoc. Editor: Yener N. Yeni. been developed, including the Swanson and the NeuFlex implants. The goal of this study was to compare the in vitro mechanical behavior of Swanson and NeuFlex MCP joint implants. Three-dimensional (3D) finite element (FE) models of the silicone implants were modeled using the commercial software ANSYS and subjected to angular displacement from 0 deg to 90 deg. FE models were validated using mechanical tests of implants incrementally bent from 0 deg to 90 deg in a joint simulator. Swanson size 2 and 4 implants were compared with NeuFlex size 10 and 30 implants, respectively. Good agreement was observed throughout the range of motion for the flexion bending moment derived from 3D FE models and mechanical tests. From 30 deg to 90 deg, the Swanson 2 demonstrated a greater resistance to deformation than the NeuFlex 10 and required a greater bending moment for joint flexion. For larger implant sizes, the NeuFlex 30 had a steeper moment-displacement curve, but required a lower moment than the Swanson 4, due to implant reflexion. On average, the stress generated at the implant hinge from 30 deg to 90 deg was lower in the NeuFlex than in the Swanson. On average, starting from the neutral position of 30 deg for the reflexed NeuFlex implant, higher moments were required to extend the NeuFlex implants to 0 deg compared with the Swanson implants, which returned spontaneously to resting position. Implant toggling within the medullary canals was less in the NeuFlex than in the Swanson. The differential performance of these implants may be useful in implant selection based on the preoperative condition(s) of the joint and specific patient functional needs.

Vertebral fractures are the hallmark of osteoporosis, yet the failure mechanisms involved in these fractures are not well understood. Failure patterns within whole vertebrae are typically estimated from finite element analyses or from images acquired after fracture has occurred. Experimental measurement of deformations sustained by vertebrae during injurious loading would enable direct visualization of vertebral failure mechanisms as well as the possibility of validating the results of numerical simulations of vertebral fracture. Time-lapsed micro-computed tomography (μCT) has been used previously to visualize failure processes in excised specimens of trabecular bone [1,2], and methods of digital volume correlation (DVC) can be applied to these types of image series to obtain quantitative information on the deformations that the bone undergoes [3]. However, application of DVC to whole bones has not been reported and involves additional challenges such as the irregular geometry of the volume and the large size of the sets of image data. The overall goal of this study was to develop a DVC-based method for quantifying deformations throughout entire vertebrae as these vertebrae are loaded to failure.Copyright © 2010 by ASME