Janina M. Patsch

Increased cortical porosity in type 2 diabetic postmenopausal women with fragility fractures.

The primary goal of this study was to assess peripheral bone microarchitecture and strength in postmenopausal women with type 2 diabetes with fragility fractures (DMFx) and to compare them with postmenopausal women with type 2 diabetics without fractures (DM). Secondary goals were to assess differences in nondiabetic postmenopausal women with fragility fractures (Fx) and nondiabetic postmenopausal women without fragility fractures (Co), and in DM and Co women. Eighty women (mean age 61.3 ± 5.7 years) were recruited into these four groups (DMFx, DM, Fx, and Co; n = 20 per group). Participants underwent dual‐energy X‐ray absorptiometry (DXA) and high‐resolution peripheral quantitative computed tomography (HR‐pQCT) of the ultradistal and distal radius and tibia. In the HR‐pQCT images volumetric bone mineral density and cortical and trabecular structure measures, including cortical porosity, were calculated. Bone strength was estimated using micro–finite element analysis (µFEA). Differential strength estimates were obtained with and without open cortical pores. At the ultradistal and distal tibia, DMFx had greater intracortical pore volume (+52.6%, p = 0.009; +95.4%, p = 0.020), relative porosity (+58.1%, p = 0.005; +87.9%, p = 0.011) and endocortical bone surface (+10.9%, p = 0.031; +11.5%, p = 0.019) than DM. At the distal radius DMFx had 4.7‐fold greater relative porosity (p < 0.0001) than DM. At the ultradistal radius, intracortical pore volume was significantly higher in DMFx than DM (+67.8%, p = 0.018). DMFx also displayed larger trabecular heterogeneity (ultradistal radius: +36.8%, p = 0.035), and lower total and cortical BMD (ultradistal tibia: −12.6%, p = 0.031; −6.8%, p = 0.011) than DM. DMFx exhibited significantly higher pore‐related deficits in stiffness, failure load, and cortical load fraction at the ultradistal and distal tibia, and the distal radius than DM. Comparing nondiabetic Fx and Co, we only found a nonsignificant trend with increase in pore volume (+38.9%, p = 0.060) at the ultradistal radius. The results of our study suggest that severe deficits in cortical bone quality are responsible for fragility fractures in postmenopausal diabetic women.

Bone marrow fat composition as a novel imaging biomarker in postmenopausal women with prevalent fragility fractures

The goal of this magnetic resonance (MR) imaging study was to quantify vertebral bone marrow fat content and composition in diabetic and nondiabetic postmenopausal women with fragility fractures and to compare them with nonfracture controls with and without type 2 diabetes mellitus. Sixty‐nine postmenopausal women (mean age 63 ± 5 years) were recruited. Thirty‐six patients (47.8%) had spinal and/or peripheral fragility fractures. Seventeen fracture patients were diabetic. Thirty‐three women (52.2%) were nonfracture controls. Sixteen women were diabetic nonfracture controls. To quantify vertebral bone marrow fat content and composition, patients underwent MR spectroscopy (MRS) of the lumbar spine at 3 Tesla. Bone mineral density (BMD) was determined by dual‐energy X‐ray absorptiometry (DXA) of the hip and lumbar spine (LS) and quantitative computed tomography (QCT) of the LS. To evaluate associations of vertebral marrow fat content and composition with spinal and/or peripheral fragility fractures and diabetes, we used linear regression models adjusted for age, race, and spine volumetric bone mineral density (vBMD) by QCT. At the LS, nondiabetic and diabetic fracture patients had lower vBMD than controls and diabetics without fractures (p = 0.018; p = 0.005). However, areal bone mineral density (aBMD) by DXA did not differ between fracture and nonfracture patients. After adjustment for age, race, and spinal vBMD, the prevalence of fragility fractures was associated with −1.7% lower unsaturation levels (confidence interval [CI] −2.8% to −0.5%, p = 0.005) and +2.9% higher saturation levels (CI 0.5% to 5.3%, p = 0.017). Diabetes was associated with −1.3% (CI –2.3% to −0.2%, p = 0.018) lower unsaturation and +3.3% (CI 1.1% to 5.4%, p = 0.004) higher saturation levels. Diabetics with fractures had the lowest marrow unsaturation and highest saturation. There were no associations of marrow fat content with diabetes or fracture. Our results suggest that altered bone marrow fat composition is linked with fragility fractures and diabetes. MRS of spinal bone marrow fat may therefore serve as a novel tool for BMD‐independent fracture risk assessment.

Increased bone resorption and impaired bone microarchitecture in short-term and extended high-fat diet–induced obesity

Although obesity traditionally has been considered a condition of low risk for osteoporosis, this classic view has recently been questioned. The aim of this study was to assess bone microarchitecture and turnover in a mouse model of high-fat diet-induced obesity. Seven-week-old male C57BL/6J mice (n = 18) were randomized into 3 diet groups. One third (n = 6) received a low-fat diet for 24 weeks, one third was kept on an extended high-fat diet (eHF), and the remaining was switched from low-fat to high-fat chow 3 weeks before sacrifice (sHF). Serum levels of insulin, leptin, adiponectin, osteocalcin, and cross-linked telopeptides of type I collagen (CTX) were measured. In addition, bone microarchitecture was analyzed by micro-computed tomography; and lumbar spine bone density was assessed by dual-energy x-ray absorptiometry. The CTX, body weight, insulin, and leptin were significantly elevated in obese animals (sHF: +48%, +24%, +265%, and +102%; eHF: +43%, +52%, +761%, and +292%). The CTX, body weight, insulin, and leptin showed a negative correlation with bone density and bone volume. Interestingly, short-term high-fat chow caused similar bone loss as extended high-fat feeding. Bone volume was decreased by 12% in sHF and 19% in eHF. Bone mineral density was 25% (sHF) and 27% (eHF) lower when compared with control mice on low-fat diet. As assessed by the structure model index, bone microarchitecture changed from plate- to rod-like appearance upon high-fat challenge. Trabecular and cortical thickness remained unaffected. Short-term and extended high-fat diet-induced obesity caused significant bone loss in male C57BL/6J mice mainly because of resorptive changes in trabecular architecture.

Sclerostin serum levels correlate positively with bone mineral density and microarchitecture in haemodialysis patients

BACKGROUND Sclerostin is a soluble inhibitor of osteoblast function. Sclerostin is downregulated by the parathyroid hormone (PTH). Here, it was investigated whether sclerostin levels are influenced by intact (i) PTH and whether sclerostin is associated with bone turnover, microarchitecture and mass in dialysis patients. METHODS Seventy-six haemodialysis patients and 45 healthy controls were included in this cross-sectional study. Sclerostin, Dickkopf-1 (DKK-1), intact parathyroid hormone (iPTH), vitamin D and markers of bone turnover were analysed. A subset of 37 dialysis patients had measurements of bone mineral density (BMD) using dual-energy X-ray absorptiometry and bone microarchitecture using high-resolution peripheral quantitative computed tomography. RESULTS Dialysis patients had significantly higher sclerostin levels than controls (1257 pg/mL versus 415 pg/mL, P < 0.001). Significant correlations were found between sclerostin and gender (R = 0.41), iPTH (R = -0.28), 25-hydroxy-cholecalciferol (R = 0.27) and calcium (R = 0.25). Gender and iPTH remained significantly associated with sclerostin in a multivariate analysis. Sclerostin serum levels were positively associated with BMD at the lumbar spine (R = 0.46), femoral neck (R = 0.36) and distal radius (R = 0.42) and correlated positively mainly with trabecular structures such as trabecular density and number at the radius and tibia in dialysis patients. DKK-1 was related neither to bone measures nor to serologic parameters. CONCLUSIONS Considering that sclerostin is an inhibitor of bone formation, the observed positive correlations of serum sclerostin with BMD and bone volume were unexpected. Whether its increase in dialysis patients has direct pathogenetic relevance or is only a secondary phenomenon remains to be seen.

Multicenter precision of cortical and trabecular bone quality measures assessed by high-resolution peripheral quantitative computed tomography

High‐resolution peripheral quantitative computed tomography (HR‐pQCT) has recently been introduced as a clinical research tool for in vivo assessment of bone quality. The utility of this technology to address important skeletal health questions requires translation to standardized multicenter data pools. Our goal was to evaluate the feasibility of pooling data in multicenter HR‐pQCT imaging trials. Reproducibility imaging experiments were performed using structure and composition‐realistic phantoms constructed from cadaveric radii. Single‐center precision was determined by repeat scanning over short‐term (<72 hours), intermediate‐term (3–5 months), and long‐term intervals (28 months). Multicenter precision was determined by imaging the phantoms at nine different HR‐pQCT centers. Least significant change (LSC) and root mean squared coefficient of variation (RMSCV) for each interval and across centers was calculated for bone density, geometry, microstructure, and biomechanical parameters. Single‐center short‐term RMSCVs were <1% for all parameters except cortical thickness (Ct.Th) (1.1%), spatial variability in cortical thickness (Ct.Th.SD) (2.6%), standard deviation of trabecular separation (Tb.Sp.SD) (1.8%), and porosity measures (6% to 8%). Intermediate‐term RMSCVs were generally not statistically different from short‐term values. Long‐term variability was significantly greater for all density measures (0.7% to 2.0%; p < 0.05 versus short‐term) and several structure measures: cortical thickness (Ct.Th) (3.4%; p < 0.01 versus short‐term), cortical porosity (Ct.Po) (15.4%; p < 0.01 versus short‐term), and trabecular thickness (Tb.Th) (2.2%; p < 0.01 versus short‐term). Multicenter RMSCVs were also significantly higher than short‐term values: 2% to 4% for density and micro–finite element analysis (µFE) measures (p < 0.0001), 2.6% to 5.3% for morphometric measures (p < 0.001), whereas Ct.Po was 16.2% (p < 0.001). In the absence of subject motion, multicenter precision errors for HR‐pQCT parameters were generally less than 5%. Phantom‐based multicenter precision was comparable to previously reported in in vivo single‐center precision errors, although this was approximately two to five times worse than ex vivo short‐term precision. The data generated from this study will contribute to the future design and validation of standardized procedures that are broadly translatable to multicenter study designs.

A nonlinear finite element model validation study based on a novel experimental technique for inducing anterior wedge-shape fractures in human vertebral bodies in vitro

Vertebral compression fracture is a common medical problem in osteoporotic individuals. The quantitative computed tomography (QCT)-based finite element (FE) method may be used to predict vertebral strength in vivo, but needs to be validated with experimental tests. The aim of this study was to validate a nonlinear anatomy specific QCT-based FE model by using a novel testing setup. Thirty-seven human thoracolumbar vertebral bone slices were prepared by removing cortical endplates and posterior elements. The slices were scanned with QCT and the volumetric bone mineral density (vBMD) was computed with the standard clinical approach. A novel experimental setup was designed to induce a realistic failure in the vertebral slices in vitro. Rotation of the loading plate was allowed by means of a ball joint. To minimize device compliance, the specimen deformation was measured directly on the loading plate with three sensors. A nonlinear FE model was generated from the calibrated QCT images and computed vertebral stiffness and strength were compared to those measured during the experiments. In agreement with clinical observations, most of the vertebrae underwent an anterior wedge-shape fracture. As expected, the FE method predicted both stiffness and strength better than vBMD (R(2) improved from 0.27 to 0.49 and from 0.34 to 0.79, respectively). Despite the lack of fitting parameters, the linear regression of the FE prediction for strength was close to the 1:1 relation (slope and intercept close to one (0.86 kN) and to zero (0.72 kN), respectively). In conclusion, a nonlinear FE model was successfully validated through a novel experimental technique for generating wedge-shape fractures in human thoracolumbar vertebrae.

Bone microarchitecture in hemodialysis patients assessed by HR-pQCT.

BACKGROUND AND OBJECTIVES Dialysis patients are at high risk for low-trauma bone fracture. Bone density measurements using dual-energy x-ray absorptiometry (DXA) do not reliably differentiate between patients with and without fractures. The aim of this study was to identify differences in bone microarchitecture between patients with and without a history of fracture using high-resolution peripheral quantitative computed tomography (HR-pQCT). DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS Seventy-four prevalent hemodialysis patients were recruited for measurements of areal bone mineral density (aBMD) by DXA and bone microarchitecture by HR-pQCT. Patients with a history of trauma-related fracture were excluded. Forty healthy volunteers served as controls. Blood levels of parathyroid hormone, vitamin D, and markers of bone turnover were determined. RESULTS Dialysis patients, particularly women, had markedly impaired bone microarchitecture. Patients with fractures had significantly reduced cortical and trabecular microarchitecture compared with patients without fractures. aBMD tended to be lower in patients with fractures, but differences were statistically not significant. The strongest determinant of fracture was the HR-pQCT-measured trabecular density of the tibia, which also had the highest discriminatory power to differentiate patients according to fracture status. Radial DXA had a lower discriminatory power than trabecular density. CONCLUSIONS Bone microarchitecture is severely impaired in dialysis patients and even more so in patients with a history of fracture. HR-pQCT can identify dialysis patients with a history of low-trauma fracture.

Noninvasive imaging of bone microarchitecture

The noninvasive quantification of peripheral compartment‐specific bone microarchitecture is feasible with high‐resolution peripheral quantitative computed tomography (HR‐pQCT) and high‐resolution magnetic resonance imaging (HR‐MRI). In addition to classic morphometric indices, both techniques provide a suitable basis for virtual biomechanical testing using finite element (FE) analyses. Methodical limitations, morphometric parameter definition, and motion artifacts have to be considered to achieve optimal data interpretation from imaging studies. With increasing availability of in vivo high‐resolution bone imaging techniques, special emphasis should be put on quality control including multicenter, cross‐site validations. Importantly, conclusions from interventional studies investigating the effects of antiosteoporotic drugs on bone microarchitecture should be drawn with care, ideally involving imaging scientists, translational researchers, and clinicians.

Pathophysiology of osteoporosis

ZusammenfassungDie Osteoporose ist eine klassische altersassoziierte Erkrankung, welche Frauen häufiger als Männer betrifft. Die Hypothese, daß die Osteoporose infolge einer Östrogendefizienz auftritt, wurde bereits 1941 von Albright et al. [1] aufgestellt. Die molekularbiologischen Wirkungsmechnismen der Östrogendefizienz postmenopausaler Frauen sowie älterer Männer werden in einer Vielzahl laufender Studien untersucht. Die Östrogendefizienz hat sowohl direkte als auch indirekte Einflüsse auf den Knochenmetabolismus, welche allesamt zu einer intensivierten Osteoklastogenese führen. Diese Übersichtsarbeit beleuchtet sowohl die endokrinologischen als auch die osteoimmunologischen Mechanismen, die zur Involutionsosteoporose führen.SummaryOsteoporosis is a classical age-related disease that affects women more often than men. The hypothesis that osteoporosis is a consequence of estrogen deficiency, has been proposed as early as 1941 by Albright and colleagues [1]. The exact mechanisms of this steroid hormone deficiency in postmenopausal women as well as in the elderly men are continuously being unraveled. Collectively, estrogen deficiency has direct as well as indirect impacts on bone metabolism all of which promote osteoclastogenesis. This review aims at shedding light on the endocrine and osteoimmunological mechanisms that lead to involutional osteoporosis.

Serum miRNA Signatures Are Indicative of Skeletal Fractures in Postmenopausal Women With and Without Type 2 Diabetes and Influence Osteogenic and Adipogenic Differentiation of Adipose Tissue-Derived Mesenchymal Stem Cells In Vitro

Standard DXA measurements, including Fracture Risk Assessment Tool (FRAX) scores, have shown limitations in assessing fracture risk in Type 2 Diabetes (T2D), underscoring the need for novel biomarkers and suggesting that other pathomechanisms may drive diabetic bone fragility. MicroRNAs (miRNAs) are secreted into the circulation from cells of various tissues proportional to local disease severity and were recently found to be crucial to bone homeostasis and T2D. Here, we studied, if and which circulating miRNAs or combinations of miRNAs can discriminate best fracture status in a well‐characterized study of diabetic bone disease and postmenopausal osteoporosis (n = 80 postmenopausal women). We then tested the most discriminative and most frequent miRNAs in vitro. Using miRNA‐qPCR‐arrays, we showed that 48 miRNAs can differentiate fracture status in T2D women and that several combinations of four miRNAs can discriminate diabetes‐related fractures with high specificity and sensitivity (area under the receiver‐operating characteristic curve values [AUCs], 0.92 to 0.96; 95% CI, 0.88 to 0.98). For the osteoporotic study arm, 23 miRNAs were fracture‐indicative and potential combinations of four miRNAs showed AUCs from 0.97 to 1.00 (95% CI, 0.93 to 1.00). Because a role in bone homeostasis for those miRNAs that were most discriminative and most present among all miRNA combinations had not been described, we performed in vitro functional studies in human adipose tissue–derived mesenchymal stem cells to investigate the effect of miR‐550a‐5p, miR‐188‐3p, and miR‐382‐3p on osteogenesis, adipogenesis, and cell proliferation. We found that miR‐382‐3p significantly enhanced osteogenic differentiation (p < 0.001), whereas miR‐550a‐5p inhibited this process (p < 0.001). Both miRNAs, miR‐382‐3p and miR‐550a‐5p, impaired adipogenic differentiation, whereas miR‐188‐3p did not exert an effect on adipogenesis. None of the miRNAs affected significantly cell proliferation. Our data suggest for the first time that miRNAs are linked to fragility fractures in T2D postmenopausal women and should be further investigated for their diagnostic potential and their detailed function in diabetic bone.