Search

We have found that genetic ablation of the Sost gene (protein: sclerostin) prevents aortic valve stenosis in aged, Western diet mice. This is a new role for sclerostin in the cardiovascular system. To the knowledge of the authors, this is one of the first studies directly manipulating sclerostin in a cardiovascular disease model and the first to specifically study the aortic valve. We also provide a potential new role for Hox genes in cardiovascular disease, noting pan- Hox upregulation in the aortic roots of sclerostin genetic knockouts. The role of Hox genes in postnatal cardiovascular health and disease is another burgeoning field of study to which this article contributes.

Extend fast, two-dimensional (2D) methods of bound and pore water mapping in bone to arbitrary slice orientation.To correct for slice profile artifacts caused by gradient errors of half pulse 2D ultra-short echo time (UTE), we developed a library of predistorted gradient waveforms that can be used to interpolate optimized gradient waveforms for 2D UTE slice selection. We also developed a method to estimate and correct for a bulk phase difference between the two half pulse excitations used for 2D UTE signal excitation. Bound water images were acquired in three healthy subjects with adiabatic inversion recovery prepared 2D UTE, while pore water images were acquired after short-T2 signals were suppressed with double adiabatic inversion recovery preparation. The repeatability of bound and pore water imaging with 2D UTE was tested by repeating acquisitions after repositioning.The library-based interpolation of optimized slice select gradient waveforms combined with the method to estimate bulk phase between two excitations provided compact slice profiles for half pulse excited 2D UTE. Quantitative bound and pore water values were highly repeatable-the pooled SD of bound water across all three subjects was 0.38 molFast, quantitative, 2D UTE-based bound and pore water images can be acquired at arbitrary oblique orientations after correcting for errors in the slice select gradient waveform and bulk phase shift between the two half acquisitions.

Ultrashort echo time (UTE) MRI techniques can be used to image the concentration of water in bones. Particularly, quantitative MRI imaging of collagen-bound water concentration (

Three-to-four percent of children with neurofibromatosis type 1 (NF1) present with unilateral tibia bowing, fracture, and recalcitrant healing. Alkaline phosphatase (ALP) enzyme therapy prevented poor bone mineralization and poor mechanical properties in mouse models of NF1 skeletal dysplasia; but transition to clinical trials is hampered by the lack of a technique that (i) identifies NF1 patients at risk of tibia bowing and fracture making them eligible for trial enrollment and (ii) monitors treatment effects on matrix characteristics related to bone strength. Therefore, we assessed the ability of matrix-sensitive techniques to provide characteristics that differentiate between cortical bone from mice characterized by postnatal loss of Nf1 in Osx-cre

This article uses in vivo and ex vivo analysis to track the development of osteoporosis and left heart cardiovascular disease in an aged, high-cholesterol diet, mouse ovariectomy model. Mice develop early left ventricle hypertrophy without concurrent fibrosis or aortic valve stenosis. These findings allow for a new model of the study of left ventricle hypertrophy in postmenopausal osteoporosis that more closely mimics the natural progression of disease in female patients.

Glycosaminoglycans (GAGs) are responsible for preserving bone tissue toughness as well as regulating collagen formation and mineralization in the extracellular matrix. However, current methods for characterization of GAGs in bone are destructive, thus unable to capture in situ changes or differences in GAGs between experimental groups. As an alternative, Raman spectroscopy is a non-destructive method and can detect concurrent changes in GAGs and other bone constituents. In this study, we hypothesized that the two most prominent Raman peaks of sulfated GAGs (at ~1066 cm(−1) and at ~1378 cm(−1)) could be used to detect differences in GAGs content of bone. To test this hypothesis, three experimental models were utilized: an in vitro model (enzymatic removal of GAGs from human cadaver bone), an in vivo mouse model (biglycan KO vs. WT), and an ex vivo aging model (comparing cadaveric bone samples from young and old donors). All Raman measurements were compared to Alcian blue measurements to confirm the validity of Raman spectroscopy in detecting GAGs changes in bone. Irrespective of different models, it was found that the ~1378cm(−1) peak in Raman spectra of bone was uniquely sensitive to changes of GAGs content in bone when normalized with respect to the phosphate phase (~960cm(−1)); i.e., 1378cm(−1)/960cm(−1) (peak intensity ratio) or 1370-1385cm(−1)/930-980cm(−1) (integrated peak area ratio). In contrast, the 1070cm(−1) peak, which includes another major peak of GAGs (1066cm(−1)), seemed to be compromised to detect changes of GAGs in bone due to concurrent changes of carbonate (CO(3)) in the similar peak range. This study validates the ability of Raman spectroscopy to detect in situ treatment-, genotype-, and age-related changes in GAG levels of bone matrix.

Following severe injury, biomineralization is disrupted and limited therapeutic options exist to correct these pathologic changes. This study utilized a clinically relevant murine model of polytrauma including a severe injury with concomitant musculoskeletal injuries to identify when bisphosphonate administration can prevent the paradoxical decrease of biomineralization in bone and increased biomineralization in soft tissues, yet not interfere with musculoskeletal repair. Systemic and intrinsic mechanisms in bone and soft tissues help promote biomineralization to the skeleton, while preventing it in soft tissues. However, severe injury can disrupt this homeostatic biomineralization tropism, leading to adverse patient outcomes due to a paradoxical decrease of biomineralization in bone and increased biomineralization in soft tissues. There remains a need for therapeutics that restore the natural tropism of biomineralization in severely injured patients. Bisphosphonates can elicit potent effects on biomineralization, though with variable impact on musculoskeletal repair. Thus, a critical clinical question remains as to the optimal time to initiate bisphosphonate therapy in patients following a polytrauma, in which bone and muscle are injured in combination with a severe injury, such as a burn. To test the hypothesis that the dichotomous effects of bisphosphonates are dependent upon the time of administration relative to the ongoing biomineralization in reparative bone and soft tissues, this study utilized murine models of isolated injury or polytrauma with a severe injury, in conjunction with sensitive, longitudinal measure of musculoskeletal repair. This study demonstrated that if administered at the time of injury, bisphosphonates prevented severe injury-induced bone loss and soft tissue calcification, but did not interfere with bone repair or remodeling. However, if administered between 7 and 21 days post-injury, bisphosphonates temporally and spatially localized to sites of active biomineralization, leading to impaired fracture callus remodeling and permanence of soft tissue calcification. There is a specific pharmacologic window following polytrauma that bisphosphonates can prevent the consequences of dysregulated biomineralization, yet not impair musculoskeletal regeneration.

Delayed long bone fracture healing and nonunion continue to be a significant socioeconomic burden. While mechanical stimulation is known to be an important determinant of the bone repair process, understanding how the magnitude, mode, and commencement of interfragmentary strain (IFS) affect fracture healing can guide new therapeutic strategies to prevent delayed healing or nonunion. Mouse models provide a means to investigate the molecular and cellular aspects of fracture repair, yet there is only one commercially available, clinically-relevant, locking intramedullary nail (IMN) currently available for studying long bone fractures in rodents. Having access to alternative IMNs would allow a variety of mechanical environments at the fracture site to be evaluated, and the purpose of this proof-of-concept finite element analysis study is to identify which IMN design parameters have the largest impact on IFS in a murine transverse femoral osteotomy model. Using the dimensions of the clinically relevant IMN as a guide, the nail material, distance between interlocking screws, and clearance between the nail and endosteal surface were varied between simulations. Of these parameters, changing the nail material from stainless steel (SS) to polyetheretherketone (PEEK) had the largest impact on IFS. Reducing the distance between the proximal and distal interlocking screws substantially affected IFS only when nail modulus was low. Therefore, IMNs with low modulus (e.g., PEEK) can be used alongside commercially available SS nails to investigate the effect of initial IFS or stability on fracture healing with respect to different biological conditions of repair in rodents.

Raman spectroscopy (RS) is used to analyze the physiochemical properties of bone because it is non-destructive and requires minimal sample preparation. With over two decades of research involving measurements of mineral-to-matrix ratio, Type-B carbonate substitution, crystallinity, and other compositional characteristics of the bone matrix by RS, there are multiple methods to acquire Raman signals from bone, to process those signals, and to determine peak ratios including sub-peak ratios as well as the full-width at half maximum of the most prominent Raman peak, which is nu1 phosphate (ν(1)PO(4)). Selecting which methods to use is not always clear. Herein, we describe the components of RS instruments and how they influence the quality of Raman spectra acquired from bone because signal-to-noise of the acquisition and the accompanying background fluorescence dictate the pre-processing of the Raman spectra. We also describe common methods and challenges in preparing acquired spectra for the determination of matrix properties of bone. This article also serves to provide guidance for the analysis of bone by RS with examples of how methods for pre-processing the Raman signals and for determining properties of bone composition affect RS sensitivity to potential differences between experimental groups. Attention is also given to deconvolution methods that are used to ascertain sub-peak ratios of the amide I band as a way to assess characteristics of collagen type I. We provide suggestions and recommendations on the application of RS to bone with the goal of improving reproducibility across studies and solidify RS as a valuable technique in the field of bone research.

The in-orbit results and lessons learned of the first Finnish satellite Aalto-1 are briefly presented in this paper. Aalto-1, a three-unit CubeSat which was launched in June 2017, performed AaSI (Aalto Spectral Imager), Radiation Monitor (RADMON), and Electrostatic Plasma Brake (EPB) missions. The satellite partly fulfilled its mission objectives and allowed to either perform or attempt the experiments. Although attitude control was partially functional, AaSI and RADMON were able to acquire valuable measurements. EPB was successfully commissioned but the tether deployment was not successful., 35 pages

Osteogenesis imperfecta (OI), is a genetic disorder of bone fragility caused by mutations in collagen I or proteins involved in collagen processing. Previous studies in mice and human OI bones have shown that excessive activation of TGF‐β signaling plays an important role in dominant and recessive OI disease progression. Inhibition of TGF‐β signaling with a murine pan‐specific TGF‐β neutralizing antibody (1D11) was shown to significantly increase trabecular bone volume and long bone strength in mouse models of OI. To investigate the frequency of dosing and dose options of TGF‐β neutralizing antibody therapy, we assessed the effect of 1D11 on disease progression in a dominant OI mouse model (col1a2 gene mutation at G610C). In comparison with OI mice treated with a control antibody, we attempted to define mechanistic effects of 1D11 measured via μCT, biomechanical, dynamic histomorphometry, and serum biomarkers of bone turnover. In addition, osteoblast and osteoclast numbers in histological bone sections were assessed to better understand the mechanism of action of the 1D11 antibody in OI. Here we show that 1D11 treatment resulted in both dose and frequency dependency, increases in trabecular bone volume fraction and ultimate force in lumbar bone, and ultimate force, bending strength, yield force, and yield strength in the femur (p ≤ 0.05). Suppression of serum biomarkers of osteoblast differentiation, osteocalcin, resorption, CTx‐1, and bone formation were observed after 1D11 treatment of OI mice. Immunohistochemical analysis showed dose and frequency dependent decreases in runt‐related transcription factor, and increase in alkaline phosphatase in lumbar bone sections. In addition, a significant decrease in TRACP and the number of osteoclasts to bone surface area was observed with 1D11 treatment. Our results show that inhibition of the TGF‐β pathway corrects the high‐turnover aspects of bone disease and improves biomechanical properties of OI mice. These results highlight the potential for a novel treatment for osteogenesis imperfecta. © 2021 Sanofi‐Genzyme. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Resorbable bone cements are replaced by bone osteoclastic resorption and osteoblastic new bone formation near the periphery. However, the ideal bone cement would be replaced by new bone through processes similar to fracture repair, which occurs through a variable combination of endochondral and intramembranous ossification. In this study, nanocrystalline hydroxyapatite (nHA)-poly(thioketal urethane) (PTKUR) cements were implanted in femoral defects in New Zealand White rabbits to evaluate ossification at 4, 12, and 18 months. Four formulations were tested: an injectable, flowable cement and three moldable putties with varying ratios of calcium phosphate to sucrose granules. New bone formation and resorption of the cement by osteoclasts occurred near the periphery. Stevenel’s Blue and Safranin O staining revealed infiltration of chondrocytes into the cements and ossification of the cartilaginous intermediate. These findings suggest that nHA-PTKUR cements support combined intramembranous and endochondral ossification, resulting in enhanced osseointegration of the cement that could potentially improve patient outcomes.

The accumulation of advanced glycation end-products (AGEs) in the organic matrix of bone with aging and chronic disease such as diabetes is thought to increase fracture risk independently of bone mass. However, to date, there has not been a clinical trial to determine whether inhibiting the accumulation of AGEs is effective in preventing low-energy, fragility fractures. Moreover, unlike with cardiovascular or kidney disease, there are also no pre-clinical studies demonstrating that AGE inhibitors or breakers can prevent the age- or diabetes-related decrease in the ability of bone to resist fracture. In this review, we critically examine the case for a long-standing hypothesis that AGE accumulation in bone tissue degrades the toughening mechanisms by which bone resists fracture. Prior research into the role of AGEs in bone has primarily measured pentosidine, an AGE crosslink, or bulk fluorescence of hydrolysates of bone. While significant correlations exist between these measurements and mechanical properties of bone, multiple AGEs are both non-fluorescent and non-crosslinking. Since clinical studies are equivocal on whether circulating pentosidine is an indicator of elevated fracture risk, there needs to be a more complete understanding of the different types of AGEs including non-crosslinking adducts and multiple non-enzymatic crosslinks in bone extracellular matrix and their specific contributions to hindering fracture resistance (biophysical and biological). By doing so, effective strategies to target AGE accumulation in bone with minimal side effects could be investigated in pre-clinical and clinical studies that aim to prevent fragility fractures in conditions that bone mass is not the underlying culprit.

High resolution, peripheral quantitative computed tomography (HR-pQCT) scanners can now characterize an individual's trabecular architecture, cortical structure, and volumetric bone mineral density at a nominal resolution of 61 μm. While predictions of failure load of the distal radius and tibial diaphysis in compression by finite element analysis (FEA) of HR-pQCT scans have been validated against mechanical tests of cadaveric bones in compression, namely for images with nominal resolutions of 82 μm and 165 μm, the HR-pQCT parameters that best predict bending strength of cortical bone remain unknown. Therefore, we scanned cadaveric forearms from 31 elderly donors (Female: 72.8 ± 8.8 years and Male: 72.1 ± 6.3 years), and then loaded the radial diaphysis to failure in three-point bending after denuding each bone (38 in total). The cortical parameters had stronger correlations with ultimate moment than the trabecular parameters such that cortical area and estimated failure load of the distal radius had the highest Spearman correlation coefficients (r = 0.89 and r = 0.81, respectively, p 0.0001). Despite being a known determinant of bone strength, cortical porosity of the distal radius did not correlate with ultimate moment (p = 0.8537). In multivariate linear regressions with section modulus (SM) of the radial diaphysis as one of two predictors of bending strength, cortical area and cortical thickness were each significant contributors to the prediction of ultimate moment. Their contribution was one-half and one-third, respectively, of the contribution from SM. None of the HR-pQCT parameters were strongly correlated with post-yield displacement, an indicator of bone brittleness. In support of HR-pQCT imaging of the distal radius to identify individuals with osteoporosis, the present study found that parameters of the cortex and failure load predictions by linear FEA are strongly related to the bending strength of cortical bone.

Severely injured patients are beleaguered by complications during convalescence, such as dysregulated biomineralization. Paradoxically, severely injured patients experience the loss of bone (osteoporosis), resulting in diminished skeletal integrity and increased risk of fragility fractures; yet they also accrue mineralization in soft tissues, resulting in complications such as heterotopic ossification (HO). The pathophysiology leading to dysregulated biomineralization in severely injured patients is not well defined. It has been postulated that these pathologies are linked, such that mineralization is “transferred” from the bone to soft tissue compartments. The goal of this study was to determine if severe injury-induced osteoporosis and soft tissue calcification are temporally coincident following injury. Using a murine model of combined burn and skeletal muscle injury to model severe injury, it was determined that mice developed significant progressive bone loss, detectable as early as 3 days post injury, and marked soft tissue mineralization by 7 days after injury. The observed temporal concordance between the development of severe injury-induced osteoporosis and soft tissue mineralization indicates the plausibility that these complications share a common pathophysiology, though further experiments are required., Highlights • Severely injured patients can experience both the loss of bone or the accrual of mineralization in soft tissues following injury. • It has been postulated that these pathologies are linked, such that mineralization is “transferred” between compartments. • In a murine model, we observed a temporal concordance between severe injury-induced osteoporosis and soft tissue mineralization. • These results support the plausibility that these complications share a common pathophysiology.

Glucocorticoids increase bone fragility in patients in a manner that is underestimated by bone mass measurement. This study aimed to determine if the adult mouse could model this bone strength/bone mass discrepancy. Forty‐two 13‐week‐old BALB/cJ mice were randomized into vehicle and glucocorticoid groups, implanted with vehicle or 6‐methylprednisolone pellets, and necropsied after 60 and 120 days. Bone strength and bone mass/microarchitecture were assessed at the right central femur (CF; cortical‐bone–rich) and sixth lumbar vertebral body (LVB6; trabecular‐bone–rich). Bound water (BW) of the whole right femur was analyzed by proton‐nuclear magnetic resonance (1H‐NMR) relaxometry. Data were analyzed by two‐factor ANOVA with time (day 60 and day 120) and treatment (vehicle and glucocorticoid) as main effects for all data. Significant interactions were further analyzed with a Tukey's post hoc test. Most bone strength measures in the CF were lower in the glucocorticoid group, regardless of the duration of treatment, with no time × treatment interaction. However, bone mass measures in the CF showed a significant time × treatment interaction (p = 0.0001). Bone strength measures in LVB6 showed a time × treatment interaction (p

The incidence of bone fracture increases with age, due to both declining bone quantity and quality. Toward the goal of an improved understanding of the causes of the age-related decline in the fracture toughness of male cortical bone, nanoindentation experiments were performed on femoral diaphysis specimens from men aged 21–98 years. Because aged bone has less matrix-bound water and dry bone is less viscoelastic, we used a nanoindentation method that is sensitive to changes in viscoelasticity. Given the anisotropy of bone stiffness, longitudinal (n = 26) and transverse (n = 25) specimens relative to the long axis of the femur diaphysis were tested both dry in air and immersed in phosphate buffered saline solution. Indentation stiffness (storage modulus) and hardness increased with age, while viscoelasticity (loss modulus) was independent of donor age. The increases in indentation stiffness and hardness with age were best explained by increased mineralization with age. Indentation stiffness and hardness were negatively correlated with previously acquired fracture toughness parameters, which is consistent with a tradeoff between material strength and toughness. In keeping with the complex structure of bone, a combination of tissue-level storage modulus or hardness, bound water, and osteonal area in regression models best explained the variance in the fracture toughness of male human cortical bone. On the other hand, viscoelasticity was unchanged with age and was not associated with fracture toughness. In conclusion, the age-related increase in stiffness and hardness of male cortical bone may be one of the multiple tissue-level characteristics that contributes to decreased fracture toughness.

Impact microindentation is a new technique that measures the resistance of a patient’s bone to micro-indentation but has not yet been implemented in an intraoperative setting. To assess the technique’s safety and utility, we acquired microindentation measurements of bone material strength index (BMSi) using the OsteoProbe prior to distal radius fixation with a volar locking plate. Subsequently, the patients received a dual-energy x-ray absorptiometry scan to measure the areal bone mineral density of the proximal femur, lumbar spine, and contralateral distal radius. By assigning the patients to low-energy, fragility fracture (n = 17) and high-energy fracture (n = 11) groups based on clinical history, we investigated whether intraoperative BMSi was sensitive to osteoporosis. Impact microindentation added a maximum of 10 min of operative time and did not result in any intraoperative or postoperative complications. There were, however, no significant differences in BMSi at the radius between these two groups. This study demonstrates the feasibility of performing intraoperative impact microindentation to directly assess a patient’s bone quality, but additional research is necessary to establish whether intraoperative microindentation can identify patients with inferior bone matrix quality.

Lysosomal acid lipase (LAL) is essential for cholesteryl ester (CE) and triacylglycerol (TAG) hydrolysis in lysosomes. Clinically, an autosomal recessive LIPA mutation causes LAL deficiency (LAL-D), either Wolman Disease or Cholesterol Ester Storage Disease (CESD). LAL-D is associated with ectopic neutral lipid accumulation in the liver, small intestine, spleen, adrenal glands, and blood. Considering the importance of unesterified cholesterol and fatty acids in bone metabolism, we hypothesized that LAL is essential to bone formation, and ultimately, skeletal health. To investigate the role of LAL in skeletal homeostasis, we used LAL-deficient ((−/−)) mice and osteoblast cell cultures. Male LAL(−/−) mice had lower trabecular BV/TV (12%) compared to WT mice (21%), due to decreased trabecular number and increased trabecular separation; this change was not apparent in the females. While both sexes of LAL(−/−) mice displayed decreased cortical bone thickness and polar moment of inertia, only the female LAL(−/−) mice showed increased cortical porosity. Histological analyses revealed that LAL(−/−) mice tended to have less osteoblasts but no change in osteoclast numbers. In studying the cell-autonomous role of LAL, we observed impaired osteoblastogenesis of LAL(−/−) calvarial osteoblasts and in bone marrow stromal cells treated with the LAL inhibitor lalistat. Consistent with LAL’s role in other tissues, lalistat resulted in profound lipid puncta accumulation and an altered intracellular lipid profile. Finally, we analyzed a large de-identified national insurance database (i.e. 2016/2017 Optum Clinformatics®) which revealed that adults (≥18 years) with CESD (n=3,076) had a higher odds ratio (OR=1.21; 95% CI=1.03-1.41) of all-cause fracture at any location compared to adults without CESD (n=13.7 M) after adjusting for demographic variables and osteoporosis. These data demonstrate that alterations in LAL have significant clinical implications related to fracture risk and that LAL’s modulation of lipid metabolism is a critical for osteoblast function.

Hypertension affects approximately one-half of the adult population. Another common disease is osteoporosis, which affects 53 million people in the United States. Clinical studies have shown an association between hypertension and bone loss. We hypothesized that experimental hypertension would be associated with osteoporosis. In Angiotensin (Ang) II-induced hypertension, there is an increase in hematopoietic stem cells (HSCs) expressing the master myeloid transcription factor PU.1 (2.99 ± 0.92) vs. sham (0.17 ± 0.04,p=0.007). Here we reported an increase an osteoclasts number per mm 2 by Tartrate-resistant acid phosphatase staining in DOCA-salt hypertension (27.4 ± 2.3) vs. sham (12.09 ± 1.7, p=0.0002). Using microCT analysis of femoral bone, we found that Ang II-induced hypertension caused striking bone loss as reflected by decreases in cortical bone area, cortical thickness and trabecular number (see Table). We also examined bone strength using a three-point biomechanical testing. In keeping with the micro-CT data, both forms of hypertension were associated with increased bone fragility, reflected by several parameters including maximum force to failure and rigidity (see Table). Recent studies have implicated T cells in the genesis of osteoporosis, and in keeping with this, we found almost two-fold increase in both CD4 + and CD8 + T cells in the marrow of mice with DOCA-salt hypertension. Thus, in two models of experimental hypertension there is marked bone loss and the development of bone fragility, potentially related to T cell accumulation and cytokine release. These findings might explain the association between osteoporosis and hypertension.

The fracture resistance of cortical bone and matrix hydration are known to decline with advanced aging. However, the underlying mechanisms remain poorly understood, and so we investigated levels of matrix proteins and post-translational modifications (PTM) of collagen I in extracts from the tibia of 6-mo. and 20-mo. old BALB/c mice (female and male analysis done separately). Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed that the levels of collagen I deamidation at specific asparagine (Asn) and glutamine (Gln) residues significantly increased with age. Other non-enzymatic PTMs such as carboxymethylation of lysine (CML) were detected as well, but the relative abundance did not vary with age. No significant age-related differences in the abundance of hydroxylysine glycosylation sites were found, but hydroxylation levels at a few of the numerous lysine and proline hydroxylation sites significantly changed by a small amount with age. We performed molecular modeling and dynamics (MD) simulations for three triple helical fragments representing collagen I regions with prominent age-dependent increases in deamidation as identified by LC-MS/MS of male extracts. These 3 fragments included deamidated Asn and Gln residues as follows: 1) an Asn(428) site of the α2(I) chain in which deamidation levels increased from 4.4% at 6-mo. to 8.1% at 20-mo., 2) an Asn(983) site of the α2(I) chain with a deamidation increase from 18.3% to 36.8% with age and an Asn(1052) site of the α1(I) chain with consistent deamidation levels of ~60% across the age groups, and 3) a Gln(410) site of the α1(I) chain that went from no detectable deamidation at 6-mo. to 2.7% at 20-mo. and a neighboring Asn(421) site of the same chain with an age-related deamidation increase from 3.6% to 16.3%. The deamidation levels at these sites inversely correlated with an estimate of toughness determined from three-point bending tests of the femur mid-diaphysis. MD revealed that the sidechains become more negatively charged at deamidated sites and that deamidation alters hydrogen bonding with water along the collagen backbone while increasing water interactions with the aspartic and glutamic acid sidechains. Our findings suggest a new mechanism of the age-dependent reduction in the fracture resistance of cortical bone whereby deamidation of Asn and Glu residues redistributes bound water within collagen I triple helix.

Bone fractures create five problems that must be resolved: bleeding, risk of infection, hypoxia, disproportionate strain, and inability to bear weight. There have been enormous advancements in our understanding of the molecular mechanisms that resolve these problems after fractures, and in best clinical practices of repairing fractures. We put forth a modern, comprehensive model of fracture repair that synthesizes the literature on the biology and biomechanics of fracture repair to address the primary problems of fractures. This updated model is a framework for both fracture management and future studies aimed at understanding and treating this complex process. This model is based upon the fracture acute phase response (APR), which encompasses the molecular mechanisms that respond to injury. The APR is divided into sequential stages of “survival” and “repair.” Early in convalescence, during “survival,” bleeding and infection are resolved by collaborative efforts of the hemostatic and inflammatory pathways. Later, in “repair,” avascular and biomechanically insufficient bone is replaced by a variable combination of intramembranous and endochondral ossification. Progression to repair cannot occur until survival has been ensured. A disproportionate APR—either insufficient or exuberant—leads to complications of survival (hemorrhage, thrombosis, systemic inflammatory response syndrome, infection, death) and/or repair (delayed- or non-union). The type of ossification utilized for fracture repair is dependent on the relative amounts of strain and vascularity in the fracture microenvironment, but any failure along this process can disrupt or delay fracture healing and result in a similar non-union. Therefore, incomplete understanding of the principles herein can result in mismanagement of fracture care or application of hardware that interferes with fracture repair. This unifying model of fracture repair not only informs clinicians how their interventions fit within the framework of normal biological healing but also instructs investigators about the critical variables and outputs to assess during a study of fracture repair.

Bone fractures at weight-bearing sites are challenging to treat due to the difficulty in maintaining articular congruency. An ideal biomaterial for fracture repair near articulating joints sets rapidly after implantation, stabilizes the fracture with minimal rigid implants, stimulates new bone formation, and remodels at a rate that maintains osseous integrity. Consequently, the design of biomaterials that mechanically stabilize fractures while remodeling to form new bone is an unmet challenge in bone tissue engineering. In this study, we investigated remodeling of resorbable bone cements in a stringent model of mechanically loaded tibial plateau defects in sheep. Nanocrystalline hydroxyapatite-poly(ester urethane) (nHA-PEUR) hybrid polymers were augmented with either ceramic granules (85% β-tricalcium phosphate/15% hydroxyapatite, CG) or a blend of CG and bioactive glass (BG) particles to form a settable bone cement. The initial compressive strength and fatigue properties of the cements were comparable to those of non-resorbable poly(methyl methacrylate) bone cement. In animals that tolerated the initial few weeks of early weight-bearing, CG/nHA-PEUR cements mechanically stabilized the tibial plateau defects and remodeled to form new bone at 16 weeks. In contrast, cements incorporating BG particles resorbed with fibrous tissue filling the defect. Furthermore, CG/nHA-PEUR cements remodeled significantly faster at the full weight-bearing tibial plateau site compared to the mechanically protected femoral condyle site in the same animal. These findings are the first to report a settable bone cement that remodels to form new bone while providing mechanical stability in a stringent large animal model of weight-bearing bone defects near an articulating joint.

Fracture risk increases as type 2 diabetes (T2D) progresses. With the rising incidence of T2D, in particular early-onset T2D, a representative pre-clinical model is needed to study mechanisms for treating or preventing diabetic bone disease. Towards that goal, we hypothesized that fracture resistance of bone from diabetic TallyHO mice decreases as the duration of diabetes increases. Femurs and lumbar vertebrae were harvested from male, TallyHO mice and male, non-diabetic SWR/J mice at 16 weeks (n ≥ 12 per strain) and 34 weeks (n ≥ 13 per strain) of age. As is characteristic of this model of juvenile T2D, the TallyHO mice were obese and hyperglycemic at an early age (5 weeks and 10 weeks of age, respectively). The femur mid-shaft of TallyHO mice had higher tissue mineral density and larger cortical area, as determined by micro-computed tomography, compared to the femur mid-shaft of SWR/J mice, irrespective of age. As such, the diabetic rodent bone was structurally stronger than the non-diabetic rodent bone, but the higher peak force endured by the diaphysis during three-point (3pt) bending was not independent of the difference in body weight. Upon accounting for the structure of the femur diaphysis, the estimated toughness at 16 weeks and 34 weeks was lower for the diabetic mice than for non-diabetic controls, but neither toughness nor estimated material strength and resistance to crack growth (3pt bending of contralateral notched femur) decreased as the duration of hyperglycemia increased. With respect to trabecular bone, there were no differences in the compressive strength of the L6 vertebral strength between diabetic and non-diabetic mice at both ages despite a lower trabecular bone volume for the TallyHO than for the SWR/J mice at 34 weeks. Amide I sub-peak ratios as determined by Raman Spectroscopy analysis of the femur diaphysis suggested a difference in collagen structure between diabetic and non-diabetic mice, although there was not a significant difference in matrix pentosidine between the groups. Overall, the fracture resistance of bone in the TallyHO model of T2D did not progressively decrease with increasing duration of hyperglycemia. However, given the variability in hyperglycemia in this model, there were correlations between blood glucose levels and certain structural properties including peak force.

PURPOSE OF REVIEW: While thinning of the cortices or trabeculae weakens bone, age-related changes in matrix composition also lower fracture resistance. This review summarizes how the organic matrix, mineral phase, and water compartments influence the mechanical behavior of bone, thereby identifying characteristics important to fracture risk. RECENT FINDINGS: In the synthesis of the organic matrix, tropocollagen experiences various post-translational modifications that facilitate a highly organized fibril of collagen I with a preferred orientation giving bone extensibility and several toughening mechanisms. Being a ceramic, mineral is brittle but increases the strength of bone as its content within the organic matrix increases. With time, hydroxyapatite-like crystals experience carbonate substitutions, the consequence of which remains to be understood. Water participates in hydrogen bonding with organic matrix and in electrostatic attractions with mineral phase, thereby providing stability to collagen-mineral interface and ductility to bone. SUMMARY: Clinical tools sensitive to age- and disease-related changes in matrix composition that the affect mechanical behavior of bone could potentially improve fracture risk assessment.