When Cortical Bone Matrix Properties Are Indiscernible between Elderly Men with and without Type 2 Diabetes, Fracture Resistance Follows Suit

Type 2 diabetes mellitus (T2DM) is a metabolic disease affecting bone tissue and leading to increased fracture risk in men and women, independent of bone mineral density (BMD). Thus, bone material quality (i.e., properties that contribute to bone toughness but are not attributed to bone mass or quantity) is suggested to contribute to higher fracture risk in diabetic patients and has been shown to be altered. Fracture toughness properties are assumed to decline with aging and age-related disease, while toughness of human T2DM bone is mostly determined from compression testing of trabecular bone. In this case-control study, we determined fracture resistance in T2DM cortical bone tissue from male individuals in combination with a multiscale approach to assess bone material quality indices. All cortical bone samples stem from male nonosteoporotic individuals and show no significant differences in microstructure in both groups, control and T2DM. Bone material quality analyses reveal that both control and T2DM groups exhibit no significant differences in bone matrix composition assessed with Raman spectroscopy, in BMD distribution determined with quantitative back-scattered electron imaging, and in nanoscale local biomechanical properties assessed via nanoindentation. Finally, notched three-point bending tests revealed that the fracture resistance (measured from the total, elastic, and plastic J-integral) does not significantly differ in T2DM and control group, when both groups exhibit no significant differences in bone microstructure and material quality. This supports recent studies suggesting that not all T2DM patients are affected by a higher fracture risk but that individual risk profiles contribute to fracture susceptibility, which should spur further research on improving bone material quality assessment in vivo and identifying risk factors that increase bone fragility in T2DM. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Cortical and Trabecular Bone Deficit in Middle-Aged Men Living with HIV

A significant increase in the risk of hip fracture was observed in middle-aged men living with human immunodeficiency virus (MLWH), almost a decade earlier than those without infection. Data regarding cortical and trabecular bone deficit of hip, an important determinant of bone strength, in MLWH are limited. Quantitative CT was performed in consecutive MLWH aged ≥30 years between November 2017 and October 2018 at Severance Hospital, Seoul, Korea. Volumetric bone mineral density (vBMD) and cortical bone mapping parameters of hip (cortical thickness [CTh], cortical bone vBMD [CBMD], cortical mass surface density [CMSD], endocortical trabecular density [ECTD]) were compared to age-matched and body mass index (BMI)-matched controls (1:2) using a community-based healthy adults cohort. Among 83 MLWH and 166 controls (mean age: 47.2 years; BMI: 23.6 kg/m2 ), MLWH had lower total hip vBMD (280 ± 41 versus 296 ± 41 mg/cm3 ), CMSD (155 versus 160 mg/cm2 ), and ECTD (158 versus 175 mg/cm3 ) than controls that remained robust after adjustment for covariates (adjusted β: total hip vBMD, -18.8; CMSD, -7.3; ECTD, -18.0; p < 0.05 for all). Cortical bone mapping revealed localized deficit of CTh, CBMD, and CMSD in the anterolateral trochanteric region and femoral neck in MLWH compared to controls, with a more extensive ECTD deficit. In MLWH, lower CD4 T-cell count (/100 cells/mm3 decrement) and protease inhibitor (PI)-based regimen (versus non-PI regimen) at the time of antiretroviral treatment initiation were associated with lower total hip vBMD (adjusted β -7.5 for lower CD4 count; -28.3 for PI-based regimen) and CMSD (adjusted β -2.6 for lower CD4 count; -12.7 for PI-based regimen; p < 0.05 for all) after adjustment for covariates including age, BMI, smoking, alcohol use, hepatitis C virus co-infection, tenofovir exposure, and CT scanner types. MLWH had lower hip bone density with cortical and trabecular bone deficit compared to community-dwelling controls. © 2023 American Society for Bone and Mineral Research (ASBMR).

Is There a Causal Relationship between Physical Activity and Bone Microarchitecture? A Study of Adult Female Twin Pairs

The reasons for the association between physical activity (PA) and bone microarchitecture traits are unclear. We examined whether these associations were consistent with causation and/or with shared familial factors using a cross-sectional study of 47 dizygotic and 93 monozygotic female twin pairs aged 31-77 years. Images of the nondominant distal tibia were obtained using high-resolutionperipheral quantitative computed tomography. The bone microarchitecture was assessed using StrAx1.0 software. Based on a self-completed questionnaire, a PA index was calculated as a weighted sum of weekly hours of light (walking, light gardening), moderate (social tennis, golf, hiking), and vigorous activity (competitive active sports) = light + 2 * moderate + 3 * vigorous. We applied Inference about Causation through Examination of FAmiliaL CONfounding (ICE FALCON) to test whether cross-pair cross-trait associations changed after adjustment for within-individual associations. Within-individual distal tibia cortical cross-sectional area (CSA) and cortical thickness were positively associated with PA (regression coefficients [β] = 0.20 and 0.22), while the porosity of the inner transitional zone was negatively associated with PA (β = -0.17), all p < 0.05. Trabecular volumetric bone mineral density (vBMD) and trabecular thickness were positively associated with PA (β = 0.13 and 0.14), and medullary CSA was negatively associated with PA (β = -0.22), all p ≤ 0.01. Cross-pair cross-trait associations of cortical thickness, cortical CSA, and medullary CSA with PA attenuated after adjustment for the within-individual association (p = 0.048, p = 0.062, and p = 0.028 for changes). In conclusion, increasing PA was associated with thicker cortices, larger cortical area, lower porosity of the inner transitional zone, thicker trabeculae, and smaller medullary cavities. The attenuation of cross-pair cross-trait associations after accounting for the within-individual associations was consistent with PA having a causal effect on the improved cortical and trabecular microarchitecture of adult females, in addition to shared familial factors. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

G-CSF Receptor Deletion Amplifies Cortical Bone Dysfunction in Mice With STAT3 Hyperactivation in Osteocytes

Bone strength is determined by the structure and composition of its thickened outer shell (cortical bone), yet the mechanisms controlling cortical consolidation are poorly understood. Cortical bone maturation depends on SOCS3-mediated suppression of IL-6 cytokine-induced STAT3 phosphorylation in osteocytes, the cellular network embedded in bone matrix. Because SOCS3 also suppresses granulocyte-colony-stimulating factor receptor (G-CSFR) signaling, we here tested whether global G-CSFR (Csf3r) ablation altereed bone structure in male and female mice lacking SOCS3 in osteocytes, (Dmp1^Cre :Socs3^f/f mice). Dmp1^Cre :Socs3^f/f :Csf3r^-/- mice were generated by crossing Dmp1^Cre :Socs3^f/f mice with Csf3r^-/- mice. Although G-CSFR is not expressed in osteocytes, Csf3r deletion further delayed cortical consolidation in Dmp1^Cre :Socs3^f/f mice. Micro-CT images revealed extensive, highly porous low-density bone, with little true cortex in the diaphysis, even at 26 weeks of age; including more low-density bone and less high-density bone in Dmp1^Cre :Socs3^f/f :Csf3r^-/- mice than controls. By histology, the area where cortical bone would normally be found contained immature compressed trabecular bone in Dmp1^Cre :Socs3^f/f :Csf3r^-/- mice and greater than normal levels of intracortical osteoclasts, extensive new woven bone formation, and the presence of more intracortical blood vessels than the already high levels observed in Dmp1^Cre :Socs3^f/f controls. qRT-PCR of cortical bone from Dmp1^Cre :Socs3^f/f :Csf3r^-/- mice also showed more than a doubling of mRNA levels for osteoclasts, osteoblasts, RANKL, and angiogenesis markers. The further delay in cortical bone maturation was associated with significantly more phospho-STAT1 and phospho-STAT3-positive osteocytes, and a threefold increase in STAT1 and STAT3 target gene mRNA levels, suggesting G-CSFR deletion further increases STAT signaling beyond that of Dmp1^Cre :Socs3^f/f bone. G-CSFR deficiency therefore promotes STAT1/3 signaling in osteocytes, and when SOCS3 negative feedback is absent, elevated local angiogenesis, bone resorption, and bone formation delays cortical bone consolidation. This points to a critical role of G-CSF in replacing condensed trabecular bone with lamellar bone during cortical bone formation. © 2022 American Society for Bone and Mineral Research (ASBMR).

Pore-Size Distribution and Frequency-Dependent Attenuation in Human Cortical Tibia Bone Discriminate Fragility Fractures in Postmenopausal Women With Low Bone Mineral Density

Osteoporosis is a disorder of bone remodeling leading to reduced bone mass, structural deterioration, and increased bone fragility. The established diagnosis is based on the measurement of areal bone mineral density by dual‐energy X‐ray absorptiometry (DXA), which poorly captures individual bone loss and structural decay. Enlarged cortical pores in the tibia have been proposed to indicate structural deterioration and reduced bone strength in the hip. Here, we report for the first time the in vivo assessment of the cortical pore‐size distribution together with frequency‐dependent attenuation at the anteromedial tibia midshaft by means of a novel ultrasonic cortical backscatter (CortBS) technology. We hypothesized that the CortBS parameters are associated with the occurrence of fragility fractures in postmenopausal women (n = 55). The discrimination performance was compared with those of DXA and high‐resolution peripheral computed tomography (HR‐pQCT). The results suggest a superior discrimination performance of CortBS (area under the receiver operating characteristic curve [AUC]: 0.69 ≤ AUC ≤ 0.75) compared with DXA (0.54 ≤ AUC ≤ 0.55) and a similar performance compared with HR‐pQCT (0.66 ≤ AUC ≤ 0.73). CortBS is the first quantitative bone imaging modality that can quantify microstructural tissue deteriorations in cortical bone, which occur during normal aging and the development of osteoporosis. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Quantitative CT-Based Methods for Bone Microstructural Measures and Their Relationships With Vertebral Fractures in a Pilot Study on Smokers

Osteoporosis causes fragile bone, and bone microstructural quality is a critical determinant of bone strength and fracture risk. This study pursues technical validation of novel CT-based methods for assessment of peripheral bone microstructure together with a human pilot study examining relationships between bone microstructure and vertebral fractures in smokers. To examine the accuracy and reproducibility of the methods, repeat ultra-high-resolution (UHR) CT and micro-CT scans of cadaveric ankle specimens were acquired. Thirty smokers from the University of Iowa COPDGene cohort were recruited at their 5-year follow-up visits. Chest CT scans, collected under the parent study, were used to assess vertebral fractures. UHR CT scans of distal tibia were acquired for this pilot study to obtain peripheral cortical and trabecular bone (Cb and Tb) measures. UHR CT-derived Tb measures, including volumetric bone mineral density (BMD), network area, transverse trabecular density, and mean plate width, showed high correlation (r > 0.901) with their micro-CT-derived values over small regions of interest (ROIs). Both Cb and Tb measures showed high reproducibility-intra-class correlation (ICC) was greater than 0.99 for all Tb measures except erosion index and greater than 0.97 for all Cb measures. Female sex was associated with lower transverse Tb density (p < 0.1), higher Tb spacing (p < 0.05), and lower cortical thickness (p < 0.001). Participants with vertebral fractures had significantly degenerated values (p < 0.05) for all Tb measures except thickness. There were no statistically significant differences for Cb measures between non-fracture and fracture groups. Vertebral fracture-group differences of Tb measures remained significant after adjustment with chronic obstructive pulmonary disease (COPD) status. Although current smokers at baseline had more fractures-81.8% versus 63.2% for former smokers-the difference was not statistically significant. This pilot cross-sectional human study demonstrates CT-based peripheral bone microstructural differences among smokers with and without vertebral fractures. © 2021 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Bone Strength/Bone Mass Discrepancy in Glucocorticoid-Treated Adult Mice

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 < 0.02) such that LVB6 strength was lower after 120 days of glucocorticoids compared with 120 days of vehicle treatment. Whole‐femur–BW was lower with both glucocorticoid treatment (p = 0.0001) and time (p < 0.02), with a significant time × treatment interaction (p = 0.005). Glucocorticoid treatment of male BALB/cJ mice resulted in the lowering of bone strength in both cortical and trabecular bone that either appeared earlier or was greater than the treatment‐related changes in bone mass/microarchitecture. The adult mouse may be a good model for investigating the bone strength/mass discrepancy observed in glucocorticoid‐treated patients. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Ultrasound-Based Estimates of Cortical Bone Thickness and Porosity Are Associated With Nontraumatic Fractures in Postmenopausal Women: A Pilot Study

Recent ultrasound (US) axial transmission techniques exploit the multimode waveguide response of long bones to yield estimates of cortical bone structure characteristics. This pilot cross-sectional study aimed to evaluate the performance at the one-third distal radius of a bidirectional axial transmission technique (BDAT) to discriminate between fractured and nonfractured postmenopausal women. Cortical thickness (Ct.Th) and porosity (Ct.Po) estimates were obtained for 201 postmenopausal women: 109 were nonfractured (62.6 ± 7.8 years), 92 with one or more nontraumatic fractures (68.8 ± 9.2 years), 17 with hip fractures (66.1 ± 10.3 years), 32 with vertebral fractures (72.4 ± 7.9 years), and 17 with wrist fractures (67.8 ± 9.6 years). The areal bone mineral density (aBMD) was obtained using DXA at the femur and spine. Femoral aBMD correlated weakly, but significantly with Ct.Th (R = 0.23, p < 0.001) and Ct.Po (R = -0.15, p < 0.05). Femoral aBMD and both US parameters were significantly different between the subgroup of all nontraumatic fractures combined and the control group (p < 0.05). The main findings were that (1) Ct.Po was discriminant for all nontraumatic fractures combined (OR = 1.39; area under the receiver operating characteristic curve [AUC] equal to 0.71), for vertebral (OR = 1.96; AUC = 0.84) and wrist fractures (OR = 1.80; AUC = 0.71), whereas Ct.Th was discriminant for hip fractures only (OR = 2.01; AUC = 0.72); there was a significant association (2) between increased Ct.Po and vertebral and wrist fractures when these fractures were not associated with any measured aBMD variables; (3) between increased Ct.Po and all nontraumatic fractures combined independently of aBMD neck; and (4) between decreased Ct.Th and hip fractures independently of aBMD femur. BDAT variables showed comparable performance to that of aBMD neck with all types of fractures (OR = 1.48; AUC = 0.72) and that of aBMD femur with hip fractures (OR = 2.21; AUC = 0.70). If these results are confirmed in prospective studies, cortical BDAT measurements may be considered useful for assessing fracture risk in postmenopausal women. © 2019 American Society for Bone and Mineral Research.

Reduction of Cortical Bone Turnover and Erosion Depth After 2 and 3 Years of Denosumab: Iliac Bone Histomorphometry in the FREEDOM Trial

Denosumab, a RANKL inhibitor, reduced the risk of vertebral, hip, and nonvertebral fractures in the Fracture REduction Evaluation of Denosumab in Osteoporosis every 6 Months (FREEDOM) trial of postmenopausal women with osteoporosis compared with placebo. Previous bone histomorphometric analysis in FREEDOM showed decreased bone resorption and turnover in cancellous bone after 2 and 3 years. The purpose of the present study was to evaluate the effects of denosumab compared with placebo in the cortical compartment from transiliac bone biopsies obtained during FREEDOM. A total of 112 specimens were evaluable for cortical histomorphometry, including 67 obtained at month 24 (37 placebo, 30 denosumab) and 45 at month 36 (25 placebo, 20 denosumab). Eroded surface, osteoclast surface, erosion depth, and wall thickness were measured on the endocortical surface. Cortical thickness and cortical porosity were also measured. Dynamic parameters of bone formation were assessed for endocortical, periosteal, and intracortical envelopes. Endocortical osteoclast surface, eroded surface, and mean and maximum erosion depth were significantly lower in the denosumab group versus placebo at months 24 and 36 (p < 0.0001 to p = 0.04). Endocortical wall thickness and intracortical measures (cortical porosity and cortical thickness) were not different between the two groups. Dynamic parameters were low with tetracycline labels in cortical bone observed in 13 (43%) and 10 (50%) of denosumab biopsies at months 24 and 36, respectively, reflecting a marked decrease in bone turnover. In conclusion, our data reveal the mechanism of action of denosumab on cortical bone: inhibition of osteoclastic resorption and reduced activation of new remodeling sites. In addition, reduced endocortical erosion depth with no change of wall thickness may contribute to increased bone strength by reducing the bone loss and fragility associated with deep resorption cavities and may likely contribute to the greater BMD gain with denosumab than with other antiresorptive agents. © 2018 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.

Intracortical Bone Mechanics Are Related to Pore Morphology and Remodeling in Human Bone

During aging and in osteoporosis, cortical bone becomes more porous, making it more fragile and susceptible to fractures. The aim of this study was to investigate the intracortical compression- induced strain energy distribution, and determine whether intracortical pores associated with high strain energy density (SED) in the surrounding bone matrix have a different morphology and distribution, as well as different remodeling characteristics than matrix with normal SED. Fibular diaphyseal specimens from 20 patients undergoing a jaw reconstruction (age range 41 to 75 years; 14 men and 6 women) were studied. Bone specimens were µCT-scanned, plastic embedded, and sectioned for histology. Three-dimensional microfinite element models of each specimen were tested in compression, and the SED of the bone immediately surrounding the intracortical pores was calculated within a plane of interest corresponding to the histological sections. The SED of a pore, relative to the distribution of the SED of all pores in each specimen, was used to classify pores as either a high or normal SED pore. Pores with high SED were larger, less circular, and were located closer to the endosteal surface of the cortex than normal SED pores (p < 0.001). Histological analysis of the remodeling events generating the pores revealed that the high SED pores compared with normal SED pores had 13.3-fold higher odds of being an erosive (70%) or formative (7%) pore versus a quiescent pore (p < 0.001), 5.9-fold higher odds of resulting from remodeling upon existing pores (type 2 pore) versus remodeling generating new pores (type 1 pore) (p < 0.001), and 3.2-fold higher odds of being a coalescing type 2 pore versus a noncoalescing type 2 pore (p < 0.001). Overall, the study demonstrates a strong relationship between cortical bone mechanics and pore morphology, distribution, and remodeling characteristics in human fibular bone. © 2018 American Society for Bone and Mineral Research.

The Deletion of Hdac4 in Mouse Osteoblasts Influences Both Catabolic and Anabolic Effects in Bone

Histone deacetylase 4 (Hdac4) is known to control chondrocyte hypertrophy and bone formation. We have previously shown that parathyroid hormone (PTH) regulates many aspects of Hdac4 function in osteoblastic cells in vitro; however, in vivo confirmation was previously precluded by preweaning lethality of the Hdac4-deficient mice. To analyze the function of Hdac4 in bone in mature animals, we generated mice with osteoblast lineage-specific knockout of Hdac4 (Hdac4^ob-/- ) by crossing transgenic mice expressing Cre recombinase under the control of a 2.3-kb fragment of the Col1a1 promoter with mice bearing loxP-Hdac4. The Hdac4^ob-/- mice survive to adulthood and developed a mild skeletal phenotype. At age 12 weeks, they had short, irregularly shaped and stiff tails due to smaller tail vertebrae, with almost no growth plates. The tibial growth plate zone was also thinned, and Mmp13 and Sost mRNAs were increased in the distal femurs of Hdac4^ob-/- mice. Immunohistochemistry showed that sclerostin was elevated in Hdac4^ob-/- mice, suggesting that Hdac4 inhibits its gene and protein expression. To determine the effect of PTH in these mice, hPTH (1-34) or saline were delivered for 14 days with subcutaneously implanted devices in 8-week-old female Hdac4^ob-/- and wild-type (Hdac4^fl/fl ) mice. Serum CTX, a marker of bone resorption, was increased in Hdac4^ob-/- mice with or without PTH treatment. Tibial cortical bone volume/total volume (BV/TV), cortical thickness (Ct.Th), and relative cortical area (RCA) were decreased in Hdac4^ob-/- mice, but PTH caused no further decrease in Hdac4^ob-/- mice. Tibial trabecular BV/TV and thickness were not changed significantly in Hdac4^ob-/- mice but decreased with PTH treatment. These results indicate that Hdac4 inhibits bone resorption and has anabolic effects via inhibiting Mmp13 and Sost/sclerostin expression. Hdac4 influences cortical bone mass and thickness and knockout of Hdac4 prevents the catabolic effect of PTH in cortical bone. © 2018 American Society for Bone and Mineral Research.

Understanding Age-Induced Cortical Porosity in Women: The Accumulation and Coalescence of Eroded Cavities Upon Existing Intracortical Canals Is the Main Contributor

Intracortical bone remodeling normally ensures maintenance of the cortical bone matrix and strength, but during aging, this remodeling generates excessive porosity. The mechanism behind the age-induced cortical porosity is poorly understood and addressed in the present study. This study consists of a histomorphometric analysis of sections of iliac bone specimens from 35 women (age 16-78 years). First, the study shows that the age-induced cortical porosity reflects an increased pore size rather than an increased pore density. Second, it establishes a novel histomorphometric classification of the pores, which is based on the characteristics of the remodeling sites to which each pore is associated. It takes into consideration (i) the stage of the remodeling event at the level where the pore is sectioned, (ii) whether the event corresponds with the generation of a new pore through penetrative tunneling (type 1 pores) or with remodeling of an existing pore (type 2 pores), and (iii) in the latter case, whether or not the new remodeling event leads to the coalescence of pores. Of note, the advantage of this classification is to relate porosity with its generation mechanism. Third, it demonstrates that aging and porosity are correlated with: a shift from type 1 to type 2 pores, reflecting that the remodeling of existing pores is higher; an accumulation of eroded type 2 pores, reflecting an extended resorption-reversal phase; and a coalescence of these eroded type 2 pores into enlarged coalescing type 2 cavities. Collectively, this study supports the notion, that age-related increase in cortical porosity is the result of intracortical remodeling sites upon existing pores, with an extended reversal-resorption phase (eroded type 2 pores) that may likely result in a delayed or absent initiation of the subsequent bone formation. © 2017 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.

Cathepsin K Controls Cortical Bone Formation by Degrading Periostin

Although inhibitors of bone resorption concomitantly reduce bone formation because of the coupling between osteoclasts and osteoblasts, inhibition or deletion of cathepsin k (CatK) stimulates bone formation despite decreasing resorption. The molecular mechanisms responsible for this increase in bone formation, particularly at periosteal surfaces where osteoclasts are relatively poor, remain unclear. Here we show that CatK pharmacological inhibition or deletion (Ctsk^-/- mice) potentiates mechanotransduction signals mediating cortical bone formation. We identify periostin (Postn) as a direct molecular target for degradation by CatK and show that CatK deletion increases Postn and β-catenin expression in vivo, particularly at the periosteum. In turn, Postn deletion selectively abolishes cortical, but not trabecular, bone formation in CatK-deficient mice. Taken together, these data indicate that CatK not only plays a major role in bone remodeling but also modulates modeling-based cortical bone formation by degrading periostin and thereby moderating Wnt-β-catenin signaling. These findings provide novel insights into the role of CatK on bone homeostasis and the mechanisms of increased cortical bone volume with CatK mutations and pharmacological inhibitors. © 2017 American Society for Bone and Mineral Research.

Evidence of Staphylococcus Aureus Deformation, Proliferation, and Migration in Canaliculi of Live Cortical Bone in Murine Models of Osteomyelitis

Although Staphylococcus aureus osteomyelitis is considered to be incurable, the major bacterial reservoir in live cortical bone has remained unknown. In addition to biofilm bacteria on necrotic tissue and implants, studies have implicated intracellular infection of osteoblasts and osteocytes as a mechanism of chronic osteomyelitis. Thus, we performed the first systematic transmission electron microscopy (TEM) studies to formally define major reservoirs of S. aureus in chronically infected mouse (Balb/c J) long bone tissue. Although rare, evidence of colonized osteoblasts was found. In contrast, we readily observed S. aureus within canaliculi of live cortical bone, which existed as chains of individual cocci and submicron rod-shaped bacteria leading to biofilm formation in osteocyte lacunae. As these observations do not conform to the expectations of S. aureus as non-motile cocci 1.0 to 1.5 μm in diameter, we also performed immunoelectron microscopy (IEM) following in vivo BrdU labeling to assess the role of bacterial proliferation in canalicular invasion. The results suggest that the deformed bacteria: (1) enter canaliculi via asymmetric binary fission; and (2) migrate toward osteocyte lacunae via proliferation at the leading edge. Additional in vitro studies confirmed S. aureus migration through a 0.5-μm porous membrane. Collectively, these findings define a novel mechanism of bone infection, and provide possible new insight as to why S. aureus implant-related infections of bone tissue are so challenging to treat. © 2016 American Society for Bone and Mineral Research.

Current Physical Activity Is Independently Associated With Cortical Bone Size and Bone Strength in Elderly Swedish Women

Physical activity is believed to have the greatest effect on the skeleton if exerted early in life, but whether or not possible benefits of physical activity on bone microstructure or geometry remain at old age has not been investigated in women. The aim of this study was to investigate if physical activity during skeletal growth and young adulthood or at old age was associated with cortical geometry and trabecular microarchitecture in weight-bearing and non-weight-bearing bone, and areal bone mineral density (aBMD) in elderly women. In this population-based cross-sectional study 1013 women, 78.2 ± 1.6 (mean ± SD) years old, were included. Using high-resolution 3D pQCT (XtremeCT), cortical cross-sectional area (Ct.CSA), cortical thickness (Ct.Th), cortical periosteal perimeter (Ct.Pm), volumetric cortical bone density (D.Ct), trabecular bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), and trabecular separation (Tb.Sp) were measured at the distal (14% level) and ultra-distal tibia and radius, respectively. aBMD was assessed using DXA (Hologic Discovery A) of the spine and hip. A standardized questionnaire was used to collect information about previous exercise and the Physical Activity Scale for the Elderly (PASE) was used for current physical activity. A linear regression model (including levels of exercise during skeletal growth and young adulthood [10 to 30 years of age], PASE score, and covariates) revealed that level of current physical activity was independently associated with Ct.CSA (β = 0.18, p < 0.001) and Ct.Th (β = 0.15, p < 0.001) at the distal tibia, Tb.Th (β = 0.11, p < 0.001) and BV/TV (β = 0.10, p = 0.001) at the ultra-distal tibia, and total hip aBMD (β = 0.10, p < 0.001). Current physical activity was independently associated with cortical bone size, in terms of thicker cortex but not larger periosteal circumference, and higher bone strength at the distal tibia on elderly women, indicating that physical activity at old age may decrease cortical bone loss in weight-bearing bone in elderly women. © 2016 American Society for Bone and Mineral Research.

In Vivo UTE-MRI Reveals Positive Effects of Raloxifene on Skeletal-Bound Water in Skeletally Mature Beagle Dogs

Raloxifene positively affects mechanical properties of the bone matrix in part through modification of skeletal-bound water. The goal of this study was to determine if raloxifene-induced alterations in skeletal hydration could be measured in vivo using ultra-short echotime magnetic resonance imaging (UTE-MRI). Twelve skeletally mature female beagle dogs (n = 6/group) were treated for 6 months with oral doses of saline vehicle (VEH, 1 mL/kg/d) or raloxifene (RAL, 0.5 mg/kg/d). After 6 months of treatment, all animals underwent in vivo UTE-MRI of the proximal tibial cortical bone. UTE-MRI signal intensity versus echotime curves were analyzed by fitting a double exponential to determine the short and long relaxation times of water with the bone (dependent estimations of bound and free water, respectively). Raloxifene-treated animals had significantly higher bound water (+14%; p = 0.05) and lower free water (-20%) compared with vehicle-treated animals. These data provide the first evidence that drug-induced changes in skeletal hydration can be noninvasively assessed using UTE-MRI.

Establishing biomechanical mechanisms in mouse models: practical guidelines for systematically evaluating phenotypic changes in the diaphyses of long bones

Mice are widely used in studies of skeletal biology, and assessment of their bones by mechanical testing is a critical step when evaluating the functional effects of an experimental perturbation. For example, a gene knockout may target a pathway important in bone formation and result in a "low bone mass" phenotype. But how well does the skeleton bear functional loads; eg, how much do bones deform during loading and how resistant are bones to fracture? By systematic evaluation of bone morphological, densitometric, and mechanical properties, investigators can establish the "biomechanical mechanisms" whereby an experimental perturbation alters whole-bone mechanical function. The goal of this review is to clarify these biomechanical mechanisms and to make recommendations for systematically evaluating phenotypic changes in mouse bones, with a focus on long-bone diaphyses and cortical bone. Further, minimum reportable standards for testing conditions and outcome variables are suggested that will improve the comparison of data across studies. Basic biomechanical principles are reviewed, followed by a description of the cross-sectional morphological properties that best inform the net cellular effects of a given experimental perturbation and are most relevant to biomechanical function. Although morphology is critical, whole-bone mechanical properties can only be determined accurately by a mechanical test. The functional importance of stiffness, maximum load, postyield displacement, and work-to-fracture are reviewed. Because bone and body size are often strongly related, strategies to adjust whole-bone properties for body mass are detailed. Finally, a comprehensive framework is presented using real data, and several examples from the literature are reviewed to illustrate how to synthesize morphological, tissue-level, and whole-bone mechanical properties of mouse long bones.

Odanacatib treatment affects trabecular and cortical bone in the femur of postmenopausal women: results of a two-year placebo-controlled trial

Odanacatib, a selective cathepsin K inhibitor, increases areal bone mineral density (aBMD) at the spine and hip of postmenopausal women. To gain additional insight into the effects on trabecular and cortical bone, we analyzed quantitative computed tomography (QCT) data of postmenopausal women treated with odanacatib using Medical Image Analysis Framework (MIAF; Institute of Medical Physics, University of Erlangen, Erlangen, Germany). This international, randomized, double-blind, placebo-controlled, 2-year, phase 3 trial enrolled 214 postmenopausal women (mean age 64 years) with low aBMD. Subjects were randomized to odanacatib 50 mg weekly (ODN) or placebo (PBO); all participants received calcium and vitamin D. Hip QCT scans at 24 months were available for 158 women (ODN: n = 78 women; PBO: n = 80 women). There were consistent and significant differential treatment effects (ODN-PBO) for total hip integral (5.4%), trabecular volumetric BMD (vBMD) (12.2%), and cortical vBMD (2.5%) at 24 months. There was no significant differential treatment effect on integral bone volume. Results for bone mineral content (BMC) closely matched those for vBMD for integral and trabecular compartments. However, with small but mostly significant differential increases in cortical volume (1.0% to 1.3%) and thickness (1.4% to 1.9%), the percentage cortical BMC increases were numerically larger than those of vBMD. With a total hip BMC differential treatment effect (ODN-PBO) of nearly 1000 mg, the proportions of BMC attributed to cortical gain were 45%, 44%, 52%, and 40% for the total, neck, trochanter, and intertrochanter subregions, respectively. In postmenopausal women treated for 2 years, odanacatib improved integral, trabecular, and cortical vBMD and BMC at all femur regions relative to placebo when assessed by MIAF. Cortical volume and thickness increased significantly in all regions except the femoral neck. The increase in cortical volume and BMC paralleled the increase in cortical vBMD, demonstrating a consistent effect of ODN on cortical bone. Approximately one-half of the absolute BMC gain occurred in cortical bone.

Exercise during growth and young adulthood is independently associated with cortical bone size and strength in old Swedish men

Previous studies have reported an association between exercise during youth and increased areal bone mineral density at old age. The primary aim of this study was to investigate if exercise during growth was independently associated with greater cortical bone size and whole bone strength in weight-bearing bone in old men. The tibia and radius were measured using both peripheral quantitative computed tomography (pQCT) (XCT-2000; Stratec) at the diaphysis and high-resolution pQCT (HR-pQCT) (XtremeCT; Scanco) at the metaphysis to obtain cortical bone geometry and finite element-derived bone strength in distal tibia and radius, in 597 men, 79.9 ± 3.4 (mean ± SD) years old. A self-administered questionnaire was used to collect information about previous and current physical activity. In order to determine whether level of exercise during growth and young adulthood or level of current physical activity were independently associated with bone parameters in both tibia and radius, analysis of covariance (ANCOVA) analyses were used. Adjusting for covariates and current physical activity, we found that men in the group with the highest level of exercise early in life (regular exercise at a competitive level) had higher tibial cortical cross-sectional area (CSA; 6.3%, p < 0.001) and periosteal circumference (PC; 1.6%, p = 0.011) at the diaphysis, and higher estimated bone strength (failure load: 7.5%, p < 0.001; and stiffness: 7.8%, p < 0.001) at the metaphysis than men in the subgroup with the lowest level of exercise during growth and young adulthood. Subjects in the group with the highest level of current physical activity had smaller tibial endosteal circumference (EC; 3.6%, p = 0.012) at the diaphysis than subjects with a lower current physical activity, when adjusting for covariates and level of exercise during growth and young adulthood. These findings indicate that exercise during growth can increase the cortical bone size via periosteal expansion, whereas exercise at old age may decrease endosteal bone loss in weight-bearing bone in old men.

Inhibition of cathepsin K increases modeling-based bone formation, and improves cortical dimension and strength in adult ovariectomized monkeys

Treatment with the cathepsin K (CatK) inhibitor odanacatib (ODN) protects against bone loss and maintains normal biomechanical properties in the spine and hip of ovariectomized (OVX) preclinical models. Here, we characterized the effects of ODN on the dynamics of cortical modeling and remodeling, and dimension and strength of the central femur in adult OVX-rhesus monkeys. Animals were treated with vehicle or ODN (6 or 30 mg/kg, once per day [q.d., p.o.]) in prevention mode for 21 months. Calcein and tetracycline double-labeling were given at 12 and 21 months, and the femoral cross-sections were subjected to dynamic histomorphometric and cement line analyses. ODN treatment significantly increased periosteal and endocortical bone formation (BFR/BS), accompanied with an increase in endocortical mineralizing surface (102%, p < 0.01) with the 6 mg/kg dose. ODN at both doses reduced remodeling hemiosteon numbers by 51% and 66% (p < 0.05), respectively, and ODN 30 mg/kg numerically reduced activation frequency without affecting wall thickness. On the same endocortical surface, ODN increased all modeling-based parameters, while reducing intracortical remodeling, consistent with the observed no treatment effects on cortical porosity. ODN 30 mg/kg markedly increased cortical thickness (CtTh, p < 0.001) and reduced marrow area (p < 0.01). Lastly, ODN treatment increased femoral structural strength (p < 0.001). Peak load was positively correlated with the increases in bone mineral content (BMC) (r(2)  = 0.9057, p < 0.0001) and CtTh (r2  = 0.6866, p < 0.0001). Taken together, by reducing cortical remodeling-based and stimulating modeling-based bone formation, ODN significantly improved cortical dimension and strength in OVX monkeys. This novel mechanism of CatK inhibition in stimulating cortical formation suggests that ODN represents a novel therapeutic approach for the treatment of osteoporosis.

Rapid cortical bone loss in patients with chronic kidney disease

Chronic kidney disease (CKD) patients may have high rates of bone loss and fractures, but microarchitectural and biochemical mechanisms of bone loss in CKD patients have not been fully described. In this longitudinal study of 53 patients with CKD Stages 2 to 5D, we used dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HRpQCT), and biochemical markers of bone metabolism to elucidate effects of CKD on the skeleton. Median follow-up was 1.5 years (range 0.9 to 4.3 years); bone changes were annualized and compared with baseline. By DXA, there were significant declines in areal bone mineral density (BMD) of the total hip and ultradistal radius: -1.3% (95% confidence interval [CI] -2.1 to -0.6) and -2.4% (95% CI -4.0 to -0.9), respectively. By HRpQCT at the distal radius, there were significant declines in cortical area, density, and thickness and increases in porosity: -2.9% (95% CI -3.7 to -2.2), -1.3% (95% CI -1.6 to -0.6), -2.8% (95% CI -3.6 to -1.9), and +4.2% (95% CI 2.0 to 6.4), respectively. Radius trabecular area increased significantly: +0.4% (95% CI 0.2 to 0.6), without significant changes in trabecular density or microarchitecture. Elevated time-averaged levels of parathyroid hormone (PTH) and bone turnover markers predicted cortical deterioration. Higher levels of serum 25-hydroxyvitamin D predicted decreases in trabecular network heterogeneity. These data suggest that significant cortical loss occurs with CKD, which is mediated by hyperparathyroidism and elevated turnover. Future investigations are required to determine whether these cortical losses can be attenuated by treatments that reduce PTH levels and remodeling rates.