Pierre Levitz

Combination of topological parameters and bone volume fraction better predicts the mechanical properties of trabecular bone

Trabecular bone structure may complement bone volume/total volume fraction (BV/TV) in the prediction of the mechanical properties. Nonetheless, the direct in vivo use of information pertaining to trabecular bone structure necessitates some predictive analytical model linking structure measures to mechanical properties. In this context, the purpose of this study was to combine BV/TV and topological parameters so as to better estimate the mechanical properties of trabecular bone. Thirteen trabecular bone mid-sagittal sections were imaged by magnetic resonance (MR) imaging at the resolution of 117 x 117x 300 microm(3). Topological parameters were evaluated in applying the 3D-line skeleton graph analysis (LSGA) technique to the binary MR images. The same images were used to estimate the elastic moduli by finite element analysis (FEA). In addition to the mid-sagittal section, two cylindrical samples were cored from each vertebra along vertical and horizontal directions. Monotonic compression tests were applied to these samples to measure both vertical and horizontal ultimate stresses. BV/TV was found as a strong predictor of the mechanical properties, accounting for 89-94% of the variability of the elastic moduli and for 69-86% of the variability of the ultimate stresses. Topological parameters and BV/TV were combined following two analytical formulations, based on: (1) the normalization of the topological parameters; and on (2) an exponential fit-model. The normalized parameters accounted for 96-98% of the variability of the elastic moduli, and the exponential model accounted for 80-95% of the variability of the ultimate stresses. Such formulations could potentially be used to increase the prediction of the mechanical properties of trabecular bone.

Three‐Dimensional‐Line Skeleton Graph Analysis of High‐Resolution Magnetic Resonance Images: A Validation Study From 34‐μm‐Resolution Microcomputed Tomography

The resolution achievable in vivo by magnetic resonance imaging (MRI) techniques is not sufficient to depict precisely individual trabeculae and, thus, does not permit the quantification of the “true” trabecular bone morphology and topology. Nevertheless, the characterization of the “apparent” trabecular bone network derived from high‐resolution MR images (MRIs) and their potential to provide information in addition to bone mineral density (BMD) alone has been established in studies of osteoporosis. The aim of this work was to show the ability of the three‐dimensional‐line skeleton graph analysis (3D‐LSGA) to characterize high‐resolution MRIs of trabecular bone structure. Fifteen trabecular bone samples of the distal radius were imaged using the high‐resolution MRI (156 × 156 × 300 μm3) and microcomputed tomography (μCT; 34 × 34 × 34 μm3). After thresholding, the 3D skeleton graph of each binary image was obtained. To remove the assimilated‐noise branches of the skeleton graph and smooth this skeleton graph before it was analyzed, we defined a smoothing length criterion (lc), such that all “termini” branches having a length lower than lc were removed. Local topological and morphological LSGA measurements were performed from MRIs and μCT images of the same samples. The correlations between these two sets of measurements were dependent on the smoothing criterion lc, reaching R2 = 0.85 for topological measurements and R2 = 0.57–0.64 for morphological measurements. 3D‐LSGA technique could be applied to in vivo high‐resolution MRIs of trabecular bone structure, giving an indirect characterization of the microtrabecular bone network.

Adsorption of nonionic surfactants at the solid—solution interface and micellization: A comparative fluorescence decay study

Abstract The fluorescence decay of the phenoxy group of two monodisperse and one polydisperse (Triton X-100) polyoxyethylene alkyl phenols has been studied in aqueous solution, and in the adsorbed state on a hydrophilic (hydroxylated) or hydrophobic (methylated) silica surface. In aqueous solution, below the CMC, one single exponential decay is observed with a lifetime of 4.5 nsec. Above the CMC, a second exponential component with a lifetime of 37 nsec appears with the monodisperse compounds, but not with Triton X-100. This long decay is assigned to dimeric species and its amplitude is quantitatively correlated to the fraction of molecules involved in micellar aggregates. On the hydrophilic silica surface, the same long decay appears from the very beginning of the adsorption isotherm and its amplitude remains constant up to the plateau of the isotherm, close to the value observed in the pure liquid surfactant. This is taken as evidence that, from the outset of adsorption, the majority of molecules is involved in molecular aggregates. On the hydrophobic surface, the constant amplitude regime is observed only at high surface coverage. At low surface coverage, the amplitude increases linearly with θ, suggesting that, in this regime, the adsorbed phase can be described as a dilute gas.

Influence of surface heterogeneity on the luminescence decay of probe molecules in heterogeneous systems Ru(bpy)2+3 on clays

The decay of a luminescent probe [Ru(bpy)2+3] adsorbed on solids (clays) containing quenching impurities (Fe3+) has been examined. The time law is a multi-exponential function which stems from (i) the quasi-total translational immobility of the probe on the microsecond scale, and (ii) the heterogeneous nature of the surface, which is influenced by the local concentration of iron in the clay lattice. An elementary model has been proposed based on a randomly decorated 2-dimensional lattice for the probe and a second underlying and randomly decorated 2-dimensional lattice for the quencher ions. Assuming that the quenching probability for an excited probe is linearly related to the number of neighbouring quenchers, a decay function has been derived which, for very low quencher concentrations, reduces to the Infelta–Gratzel–Thomas equation for quenching in micellar solutions. The parameters of the decay function have been correlated to the chemical composition of the clays and to their swelling properties.

Sol/gel phase diagrams of industrial organo-bentones in organic media

Abstract The gelation process of industrial organo-bentone in organic solvents is a complex phenomenon due to various parameters, such as, the crystallographic nature of the clay, the nature of adsorbed surfactant, the dielectric constant of medium. Sol-gel phase diagrams have been established by varying the dielectric constant versus solid-liquid ratio of the mixture (mass percentage). To modify the dielectric constant, different polar agents, such as, alcohols are added to organo-clay suspensions dispersed in non polar organic solvent. With trioctahedral clays, we showed that the alkyl chain length of alcohols has a relevant influence on gelation. Addition of short alkyl chain alcohols increases the gelation ability, whereas, the addition of long alkyl chain alcohols delays the gel formation. In the case of these trioctahedral clays, gelation process was explained by a polarisability change of the organo-clay particles and of the solvent molecules. The influence of water has also been investigated because the polar agents used contain small amounts of water (

A NMR investigation of adsorption/desorption hysteresis in porous silica gels.

The purpose of this study is to investigate possible differences in geometry and connectivity of the liquid phase in a partially water-satured porous medium between the adsorption and the desorption branches, using a series of silica gels. This was done by comparing the T1 and T2 relaxation times (in 1H and 2H nuclear magnetic resonance (NMR) relaxation) as well as the water self-diffusion coefficient D (in 1H) along the two branches of the adsorption/desorption isotherms. The results show that the two relaxation times and the diffusion coefficient are strongly dependent on the water content (saturation level). When plotted in normalized coordinates, the T1 and T2 vs. P/Po curves fit closely the adsorption/desorption isotherms, which validates the two-population, fast-exchange model. Furthermore, because at equivalent saturation levels, there is no difference between the relaxation times and diffusion coefficients obtained along the adsorption branch and those obtained along the desorption branch, one is led to the conclusion that despite different equilibrium conditions, the geometry and connectivity of the liquid phase are statistically the same along the two branches.

A Scaling Model of the Microstructural Evolution in C3S/C-S-H Pastes

In order to build a reliable and multiscale morphological model for the solid-void interface in hardened cement pastes, the textural evolution of hardened C-S-H pastes were characterized at all length scales between the size of the elementary colloidal gel particles and the size of the largest voids, using several nondestructive or mildly destructive structural techniques. The starting samples were all prepared at the same W/C ratio (0.4). Their evolution was monitored between one day and one year. The C3S/C-S-H ratio was measured by X-ray diffraction. Mercury intrusion was used as a standard reference method in order to measure the pore-size distribution and the pore-scaling properties between 1 μm and 10 nm. The specific surface area accessible to water molecules in saturated conditions was measured by proton NMR. Small-angle X-ray scattering was used to determine the scaling properties between 45 nm and 3 nm. A multifractal analysis was performed on digitized transmission electron micrographs obtained from ultrathin (50 nm) slices after stepwise solvent exchange and inclusion.

Pattern formation in particulate complex fluids: A guided tour

Dispersion of colloidal particles in a liquid leads to the formation of complex fluids which are most often non-Newtonian and viscoelastic. When larger particles are used, plastic pastes may be obtained. Without dispersion medium, the particles just form a powder. Each of these systems exhibit peculiar flow or fracture behaviors which allows for a rich variety of fractal patterns, including viscous fingers, viscoelastic fracturing patterns, rough fracture surfaces, and granular displacement patterns.

SIMULATION OF OSTEOPOROSIS BONE CHANGES: EFFECTS ON THE DEGREE OF ANISOTROPY

Osteoporosis is characterized by several kinds of microarchitecture changes, such as thinning or disconnection of trabeculae, or increase of resorption lacunae.l-3The first characterization of these trabecular bone alterations has been obtained by histomorphometry.4-7Several parameters characterizing the 3D microarchitecture of the trabecular bone network are available.8-12The most relevant parameters following the last years’ studies are the volume fraction of solid (Bv/Tv: Bone Volume/Total Volume)13-15the connectivity12,16and the anisotropy.17-20These three parameters determine the mechanical properties of the trabecular bone micro-architecture.21