Laurent Pothuaud

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.

A New Computational Efficient Approach for Trabecular Bone Analysis using Beam Models Generated with Skeletonized Graph Technique

Micro-finite element (FE) analysis is a well established technique for the evaluation of the elastic properties of trabecular bone, but is limited in its application due to the large number of elements that it requires to represent the complex internal structure of the bone. In this paper, we present an alternative FE approach that makes use of a recently developed 3D-Line Skeleton Graph Analysis (LSGA) technique to represent the complex internal structure of trabecular bone as a network of simple straight beam elements in which the beams are assigned geometrical properties of the trabeculae that they represent. Since an enormous reduction of cputime can be obtained with this beam modeling approach, ranging from approximately 1,200 to 3,600 for the problems investigated here, we think that the FE modeling technique that we introduced could potentially constitute an interesting alternative for the evaluation of the elastic mechanical properties of trabecular bone.

In vivo application of 3D-line skeleton graph analysis (LSGA) technique with high-resolution magnetic resonance imaging of trabecular bone structure

Over the last several years magnetic resonance (MR) imaging has emerged as a means of measuring in vivo 3D trabecular bone structure. In particular, MR based diagnosis could be used to complement standard bone mineral density (BMD) methods for assessing osteoporosis and evaluating longitudinal changes. The aim of this study was to demonstrate the feasibility of using the 3D-LSGA technique for the evaluation of trabecular bone structure of high-resolution MR images, particularly for assessing longitudinal changes, in vivo. First, the reproducibility of topological 3D-LSGA based measurements was evaluated in a set of seven volunteers, and coefficients of variations ranged from 3.5% to 6%. Second, high-resolution MR images of the radius in 30 postmenopausal women from a placebo controlled drug study (Idoxifene), divided into placebo (n=9) and treated (n=21) groups, were obtained at baseline (BL) and after 1 year of treatment (follow-up, FU). In addition, dual X-ray absorptiometry (DXA) measures of BMD were obtained in the distal radius. Standard morphological measurements based on the mean intercept length (MIL) technique as well as 3D-LSGA based measurements were applied to the 3D MR images. Significant changes from BL to FU were detected, in the treated group, using the topological 3D-LSGA based measurements, morphological measures of volume of connected trabeculae and App Tb.N from MIL analysis. The duration of the study was short, and the number of patients remaining in the study was small, hence these results cannot be interpreted with regard to a true therapeutic response. Furthermore, the site (wrist) and the drug (idoxifene) are not optimal for follow-up study. However, this paper demonstrated the feasibility of using 3D-LSGA based evaluation coupled with in vivo high-resolution MR imaging as a complementary approach for the monitoring of trabecular bone changes in individual subjects.

Long‐Term Corticosteroid Therapy Induces Mild Changes in Trabecular Bone Texture

The relative roles of bone mineral density (BMD) decrease and of microarchitectural changes in corticosteroid‐induced osteoporosis (CIOP) are debated. Our objective has been to evaluate both bone microarchitecture (by a fractal analysis of texture on radiographs) and BMD in corticosteroid (CS)‐treated patients. In this study, 60 patients from a rheumatology unit with a mean age of 60.6 ± 14.8 years taking CS therapy for more than 6 months and a cumulative dose of prednisone over 1 g and 57 controls among age‐matched patients and hospital staff were recruited. Bone diseases and bone‐modifying drugs (except calcium, vitamin D, and hormonal replacement therapy [HRT]) were considered as exclusion criteria. A fractal analysis of trabecular bone texture was performed on calcaneus radiographs after an oriented analysis in 18 directions. The fractal analysis was based on the fractional Brownian motion model Results were expressed by H parameter (H = 2 – fractal dimension) in each direction, Hmean being the average of 18 directions, Hmini the minimum, and Hmaxi the maximum. BMD was measured by double‐energy X‐ray absorptiometry (DEXA) at the femoral neck (FN) and lumbar spine (LS). The odds ratios (OR) were calculated for a variation of 1 SD. The mean duration and dose of CS therapy was 5.6 ± 6.6 years and 16.9 ± 19.7 g. CS therapy was significantly correlated to a decrease in FN or LSBMD: OR = 1.95,95% confidence interval (CI, 1.29–2.97) and OR = 3.19 (CI, 1.80–5.66), respectively. The Hmean and Hmaxi were significantly lower in the cases than in the controls: P = 0.03 and P = 0.02; OR = 1.67 (CI, 1.10–2.54) and OR = 1.75 (CI, 1.05–2.37). A similar trend was observed with Hmini but the difference did not reach the level of statistical significance: P = 0.06, OR = 1.57 (CI, 1.05–2.37). This study was repeated among cases and controls who had never taken HRT (respectively, n = 40 and n = 39). The results were similar. Among patients taking CS therapy, the presence of nontraumatic fractures was inversely related to BMD values but not to texture parameters. These data have shown that long‐term CS therapy induces both BMD decrease and trabecular bone texture changes. The effect of CS therapy was much stronger on BMD than on the fractal H parameter. These results are in accordance with previous studies showing a lower effect of CS therapy on bone microarchitecture than on bone mass. These results can be contrasted with those observed in women with postmenopausal osteoporosis and vertebral crush fractures in which the variations in the fractal parameters are more significant than the BMD variations.

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