Alexander Zwahlen

Abdominal Fat Is Associated With Lower Bone Formation and Inferior Bone Quality in Healthy Premenopausal Women: A Transiliac Bone Biopsy Study

CONTEXT The conventional view that obesity is beneficial for bone strength has recently been challenged by studies that link obesity, particularly visceral obesity, to low bone mass and fractures. It is controversial whether effects of obesity on bone are mediated by increased bone resorption or decreased bone formation. OBJECTIVE The objective of the study was to evaluate bone microarchitecture and remodeling in healthy premenopausal women of varying weights. DESIGN We measured bone density and trunk fat by dual-energy x-ray absorptiometry in 40 women and by computed tomography in a subset. Bone microarchitecture, stiffness, remodeling, and marrow fat were assessed in labeled transiliac bone biopsies. RESULTS Body mass index (BMI) ranged from 20.1 to 39.2 kg/m(2). Dual-energy x-ray absorptiometry-trunk fat was directly associated with BMI (r = 0.78, P < .001) and visceral fat by computed tomography (r = 0.79, P < .001). Compared with women in the lowest tertile of trunk fat, those in the highest tertile had inferior bone quality: lower trabecular bone volume (20.4 ± 5.8 vs 29.1 ± 6.1%; P = .001) and stiffness (433 ± 264 vs 782 ± 349 MPa; P = .01) and higher cortical porosity (8.8 ± 3.5 vs 6.3 ± 2.4%; P = .049). Bone formation rate (0.004 ± 0.002 vs 0.011 ± 0.008 mm(2)/mm · year; P = .006) was 64% lower in the highest tertile. Trunk fat was inversely associated with trabecular bone volume (r = -0.50; P < .01) and bone formation rate (r = -0.50; P < .001). The relationship between trunk fat and bone volume remained significant after controlling for age and BMI. CONCLUSIONS At the tissue level, premenopausal women with more central adiposity had inferior bone quality and stiffness and markedly lower bone formation. Given the rising levels of obesity, these observations require further investigation.

Increased Marrow Adiposity in Premenopausal Women with Idiopathic Osteoporosis

CONTEXT We have previously reported that premenopausal women with idiopathic osteoporosis based on fractures (IOP) or idiopathic low bone mineral density (ILBMD) exhibit markedly reduced bone mass, profoundly abnormal trabecular microstructure, and significant deficits in trabecular bone stiffness. Bone remodeling was heterogeneous. Those with low bone turnover had evidence of osteoblast dysfunction and the most marked deficits in microstructure and stiffness. OBJECTIVE Because osteoblasts and marrow adipocytes derive from a common mesenchymal precursor and excess marrow fat has been implicated in the pathogenesis of bone fragility in anorexia nervosa, glucocorticoid excess, and thiazolidinedione exposure, we hypothesized that marrow adiposity would be higher in affected women and inversely related to bone mass, microarchitecture, bone formation rate, and osteoblast number. DESIGN We analyzed tetracycline-labeled transiliac biopsy specimens in 64 premenopausal women with IOP or ILBMD and 40 controls by three-dimensional micro-computed tomography and two-dimensional quantitative histomorphometry to assess marrow adipocyte number, perimeter, and area. RESULTS IOP and ILBMD subjects did not differ with regard to any adipocyte parameter, and thus results were combined. Subjects had substantially higher adipocyte number (by 22%), size (by 24%), and volume (by 26%) than controls (P < 0.0001 for all). Results remained significant after adjusting for age, body mass index, and bone volume. Controls demonstrated expected direct associations between marrow adiposity and age and inverse relationships between marrow adiposity and bone formation, volume, and microstructure measures. No such relationships were observed in the subjects. CONCLUSIONS Higher marrow adiposity and the absence of expected relationships between marrow adiposity and bone microstructure and remodeling in women with IOP or ILBMD suggest that the relationships between fat and bone are abnormal; excess marrow fat may not arise from a switch from the osteoblast to the adipocyte lineage in this disorder. Whether excess marrow fat contributes to the pathogenesis of this disorder remains unclear.

Abnormal Bone Microarchitecture and Evidence of Osteoblast Dysfunction in Premenopausal Women with Idiopathic Osteoporosis

CONTEXT Idiopathic osteoporosis (IOP) in premenopausal women is an uncommon disorder of uncertain pathogenesis in which fragility fractures occur in otherwise healthy women with intact gonadal function. It is unclear whether women with idiopathic low bone mineral density and no history of fragility fractures have osteoporosis. OBJECTIVE The objective of the study was to elucidate the microarchitectural and remodeling features of premenopausal women with IOP. DESIGN We performed transiliac biopsies after tetracycline labeling in 104 women: 45 with fragility fractures (IOP), 19 with idiopathic low bone mineral density (Z score ≤-2.0) and 40 controls. Biopsies were analyzed by two-dimensional quantitative histomorphometry and three-dimensional microcomputed tomography. Bone stiffness was estimated using finite element analysis. RESULTS Compared with controls, affected women had thinner cortices; fewer, thinner, more widely separated, and heterogeneously distributed trabeculae; reduced stiffness; and lower osteoid width and mean wall width. All parameters were indistinguishable between women with IOP and idiopathic low bone mineral density. Although there were no group differences in dynamic histomorphometric remodeling parameters, serum calciotropic hormones, bone turnover markers, or IGF-I, subjects in the lowest tertile of bone formation rate had significantly lower osteoid and wall width, more severely disrupted microarchitecture, lower stiffness, and higher serum IGF-I than those in the upper two tertiles, suggesting that women with low turnover IOP have osteoblast dysfunction with resistance to IGF-I. Subjects with high bone turnover had significantly higher serum 1,25 dihydroxyvitamin D levels and a nonsignificant trend toward higher serum PTH and urinary calcium excretion. CONCLUSIONS These results suggest that the diagnosis of IOP should not require a history of fracture. Women with IOP may have high, normal or low bone turnover; those with low bone turnover have the most marked deficits in microarchitecture and stiffness. These results also suggest that the pathogenesis of idiopathic osteoporosis is heterogeneous and may differ according to remodeling activity.

Teriparatide for idiopathic osteoporosis in premenopausal women: A pilot study

CONTEXT Premenopausal women with idiopathic osteoporosis (IOP) have abnormal cortical and trabecular bone microarchitecture. OBJECTIVE The purpose of this study was to test the hypotheses that teriparatide increases bone mineral density (BMD) and bone formation and improves trabecular microarchitecture and stiffness in women with IOP. DESIGN This was an open-label pilot study. SETTING The setting was a tertiary care referral center. PATIENTS Participants were 21 premenopausal women with unexplained fragility fractures or low BMD. INTERVENTION Teriparatide was administered at 20 μg daily for 18 to 24 months. MAIN OUTCOME MEASURES The primary endpoint was within-subject percent change in lumbar spine BMD. Secondary endpoints included percent change in hip and forearm BMD, transiliac biopsy parameters (trabecular bone volume, microarchitecture, stiffness, and adipocytes), serum N-terminal propeptide of procollagen type 1 (P1NP), and C-telopeptide. RESULTS BMD increased at the spine (10.8 ± 8.3% [SD]), total hip (6.2 ± 5.6%), and femoral neck (7.6 ± 3.4%) (all P < .001). Serum P1NP doubled by 1 month, peaked at 6 months, and returned to baseline by 18 to 24 months. Transiliac biopsies demonstrated significant increases in cortical width and porosity and trabecular bone volume and number increased, mirrored by a 71% increase in trabecular bone stiffness (P < .02-.001). Adipocyte area, perimeter, and volume/marrow volume decreased, with no change in adipocyte number. Four women had no increase in BMD and a blunted, delayed increase in serum P1NP. Nonresponders had markedly lower baseline bone formation rate (0.002 ± 0.001 vs 0.011 ± 0.006 mm²/mm/y; P < .001) and higher serum IGF-1 (208 ± 54 vs 157± 44 ng/mL; P = .03). CONCLUSIONS Teriparatide was associated with increased spine and hip BMD and improved trabecular microarchitecture and stiffness at the iliac crest in the majority of women with IOP.

3D scanning SAXS: A novel method for the assessment of bone ultrastructure orientation

The arrangement and orientation of the ultrastructure plays an important role for the mechanical properties of inhomogeneous and anisotropic materials, such as polymers, wood, or bone. However, there is a lack of techniques to spatially resolve and quantify the materials ultrastructure orientation in a macroscopic context. In this study, a new method is presented, which allows deriving the ultrastructural 3D orientation in a quantitative and spatially resolved manner. The proposed 3D scanning small-angle X-ray scattering (3D sSAXS) method was demonstrated on a thin trabecular bone specimen of a human vertebra. A micro-focus X-ray beam from a synchrotron radiation source was used to raster scan the sample for different rotation angles. Furthermore, a mathematical framework was developed, validated and employed to describe the relation between the SAXS data for the different rotation angles and the local 3D orientation and degree of orientation (DO) of the bone ultrastructure. The resulting local 3D orientation was visualized by a 3D orientation map using vector fields. Finally, by applying the proposed 3D scanning SAXS method on consecutive bone sections, a 3D map of the local orientation of a complete trabecular element could be reconstructed for the first time. The obtained 3D orientation map provided information on the bone ultrastructure organization and revealed links between trabecular bone microarchitecture and local bone ultrastructure. More specifically, we observed that trabecular bone ultrastructure is organized in orientation domains of tens of micrometers in size. In addition, it was observed that domains with a high DO were more likely to be found near the surface of the trabecular structure, and domains with lower DO (or transition zones) were located in-between the domains with high DO. The method reproducibility was validated by comparing the results obtained when scanning the sample under different sample tilt angles. 3D orientation maps such as the ones created using 3D scanning SAXS will help to quantify and understand structure-function relationships between bone ultrastructure and bone mechanics. Beyond that, the proposed method can also be used in other research fields such as material sciences, with the aim to locally determine the 3D orientation of material components.

Strain-adaptive in silico modeling of bone adaptation — A computer simulation validated by in vivo micro-computed tomography data

Computational models are an invaluable tool to test different mechanobiological theories and, if validated properly, for predicting changes in individuals over time. Concise validation of in silico models, however, has been a bottleneck in the past due to a lack of appropriate reference data. Here, we present a strain-adaptive in silico algorithm which is validated by means of experimental in vivo loading data as well as by an in vivo ovariectomy experiment in the mouse. The maximum prediction error following four weeks of loading resulted in 2.4% in bone volume fraction (BV/TV) and 8.4% in other bone structural parameters. Bone formation and resorption rate did not differ significantly between experiment and simulation. The spatial distribution of formation and resorption sites matched in 55.4% of the surface voxels. Bone loss was simulated with a maximum prediction error of 12.1% in BV/TV and other bone morphometric indices, including a saturation level after a few weeks. Dynamic rates were more difficult to be accurately predicted, showing evidence for significant differences between simulation and experiment (p<0.05). The spatial agreement still amounted to 47.6%. In conclusion, we propose a computational model which was validated by means of experimental in vivo data. The predictive value of an in silico model may become of major importance if the computational model should be applied in clinical settings to predict bone changes due to disease and test the efficacy of potential pharmacological interventions.

Improved fracture risk assessment based on nonlinear micro-finite element simulations from HRpQCT images at the distal radius

More accurate techniques to estimate fracture risk could help reduce the burden of fractures in postmenopausal women. Although micro‐finite element (µFE) simulations allow a direct assessment of bone mechanical performance, in this first clinical study we investigated whether the additional information obtained using geometrically and materially nonlinear µFE simulations allows a better discrimination between fracture cases and controls. We used patient data and high‐resolution peripheral quantitative computed tomography (HRpQCT) measurements from our previous clinical study on fracture risk, which compared 100 postmenopausal women with a distal forearm fracture to 105 controls. Analyzing these data with the nonlinear µFE simulations, the odds ratio (OR) for the factor‐of‐risk (yield load divided by the expected fall load) was marginally higher (1.99; 95% confidence interval [CI], 1.41–2.77) than for the factor‐of‐risk computed from linear µFE (1.89; 95% CI, 1.37–2.69). The yield load and the energy absorbed up to the yield point as computed from nonlinear µFE were highly correlated with the initial stiffness (R2 = 0.97 and 0.94, respectively) and could therefore be derived from linear simulations with little loss in precision. However, yield deformation was not related to any other measurement performed and was itself a good predictor of fracture risk (OR, 1.89; 95% CI, 1.39–2.63). Moreover, a combined risk score integrating information on relative bone strength (yield load‐based factor‐of‐risk), bone ductility (yield deformation), and the structural integrity of the bone under critical loads (cortical plastic volume) improved the separation of cases and controls by one‐third (OR, 2.66; 95% CI, 1.84–4.02). We therefore conclude that nonlinear µFE simulations provide important additional information on the risk of distal forearm fractures not accessible from linear µFE nor from other techniques assessing bone microstructure, density, or mass.

The Clinical Biomechanics Award 2012 — Presented by the European Society of Biomechanics: Large scale simulations of trabecular bone adaptation to loading and treatment

BACKGROUND Microstructural simulations of bone remodeling are particularly relevant in the clinical management of osteoporosis. Before a model can be applied in the clinics, a validation against controlled in vivo data is crucial. Here we present a strain-adaptive feedback algorithm for the simulation of trabecular bone remodeling in response to loading and pharmaceutical treatment and report on the results of the large-scale validation against in vivo data. METHODS The algorithm follows the mechanostat principle and incorporates mechanical feedback, based on the local strain-energy density. For the validation, simulations of bone remodeling and adaptation in 180 osteopenic mice were performed. Permutations of the conditions for early (20th week) and late (26th week) loading of 8N or 0N, and treatments with bisphosphonates, or parathyroid hormone were simulated. Static and dynamic morphometry and local remodeling sites from in vivo and in silico studies were compared. FINDINGS For each study an individual set of model parameters was selected. Trabecular bone volume fraction was chosen as an indicator of the accuracy of the simulations. Overall errors for this parameter were 0.1-4.5%. Other morphometric indices were simulated with errors of less than 19%. Dynamic morphometry was more difficult to predict, which resulted in significant differences from the experimental data. INTERPRETATION We validated a new algorithm for the simulation of bone remodeling in trabecular bone. The results indicate that the simulations accurately reflect the effects of treatment and loading seen in respective experimental data, and, following adaptation to human data, could be transferred into clinics.

Voxel size dependency, reproducibility and sensitivity of an in vivo bone loading estimation algorithm

A bone loading estimation algorithm was previously developed that provides in vivo loading conditions required for in vivo bone remodelling simulations. The algorithm derives a bones loading history from its microstructure as assessed by high-resolution (HR) computed tomography (CT). This reverse engineering approach showed accurate and realistic results based on micro-CT and HR-peripheral quantitative CT images. However, its voxel size dependency, reproducibility and sensitivity still need to be investigated, which is the purpose of this study. Voxel size dependency was tested on cadaveric distal radii with micro-CT images scanned at 25 µm and downscaled to 50, 61, 75, 82, 100, 125 and 150 µm. Reproducibility was calculated with repeated in vitro as well as in vivo HR-pQCT measurements at 82 µm. Sensitivity was defined using HR-pQCT images from women with fracture versus non-fracture, and low versus high bone volume fraction, expecting similar and different loading histories, respectively. Our results indicate that the algorithm is voxel size independent within an average (maximum) error of 8.2% (32.9%) at 61 µm, but that the dependency increases considerably at voxel sizes bigger than 82 µm. In vitro and in vivo reproducibility are up to 4.5% and 10.2%, respectively, which is comparable to other in vitro studies and slightly higher than in other in vivo studies. Subjects with different bone volume fraction were clearly distinguished but not subjects with and without fracture. This is in agreement with bone adapting to customary loading but not to fall loads. We conclude that the in vivo bone loading estimation algorithm provides reproducible, sensitive and fairly voxel size independent results at up to 82 µm, but that smaller voxel sizes would be advantageous.

Inverse Finite Element Modeling for Characterization of Local Elastic Properties in Image-Guided Failure Assessment of Human Trabecular Bone

The local interpretation of microfinite element (μFE) simulations plays a pivotal role for studying bone structure–function relationships such as failure processes and bone remodeling.In the past μFE simulations have been successfully validated on the apparent level,however, at the tissue level validations are sparse and less promising. Furthermore,intra trabecular heterogeneity of the material properties has been shown by experimental studies. We proposed an inverse μFE algorithm that iteratively changes the tissue level Youngs moduli such that the μFE simulation matches the experimental strain measurements.The algorithm is setup as a feedback loop where the modulus is iteratively adapted until the simulated strain matches the experimental strain. The experimental strain of human trabecular bone specimens was calculated from time-lapsed images that were gained by combining mechanical testing and synchrotron radiation microcomputed tomography(SRlCT). The inverse μFE algorithm was able to iterate the heterogeneous distribution of moduli such that the resulting μFE simulations matched artificially generated and experimentally measured strains.