Intervertebral disc characterization by shear wave elastography: an in-vitro preliminary study

Patient-specific numerical simulation of the spine is a useful tool both in clinic and research. While geometrical personalization of the spine is no more an issue, thanks to recent technological advances, non-invasive personalization of soft tissue’s mechanical properties remains a challenge. Ultrasound elastography is a relatively recent measurement technique allowing the evaluation of soft tissue’s elastic modulus through the measurement of shear wave speed (SWS). The aim of this study was to determine the feasibility of elastographic measurements in intervertebral disc (IVD). An in-vitro approach was chosen to test the hypothesis that SWS can be used to evaluate IVD mechanical properties and to assess measurement repeatability. Eleven oxtail IVDs were tested in compression to determine their stiffness and apparent elastic modulus at rest and at 400 N. Elastographic measurements were performed in these two conditions and compared to these mechanical parameters. The protocol was repeated six times to determine elastographic measurement repeatability. Average SWS over all samples was 5.3 ± 1.0 m/s, with a repeatability of 7 % at rest and 4.6 % at 400 N; stiffness and apparent elastic modulus were 266.3 ± 70.5 N/mm and 5.4 ± 1.1 MPa at rest, respectively, while at 400 N they were 781.0 ± 153.8 N/mm and 13.2 ± 2.4 MPa. Correlations were found between elastographic measurements and IVD mechanical properties; these preliminary results are promising for further in-vivo application.The authors are grateful to the ParisTech BiomecAM chair program on subject-specific musculoskeletal modelling for funding (with the support of Proteor, ParisTech and Yves Cotrel Foundations)

3-D visualization and prediction of spine fractures under axial loading

Thesis (Ph.D.)--Boston UniversityVertebral fractures are the hallmark of osteoporosis, yet the failure mechanisms involved in these fractures are not well understood. Current approaches to predicting fracture risk rely on average measures of bone mineral density in the vertebra, which are imperfect predictors of vertebral strength and poor predictors of fracture risk. Prior research has established that substantial regional variations in density exist throughout the vertebra and has suggested several biomechanical consequences of these variations. The overall goal of this dissertation was to characterize failure mechanisms in human vertebrae, with specific emphasis on the role of intra-vertebral heterogeneity in density and microstructure and on identifying clinically feasible techniques for predicting fracture risk. Using images obtained from micro-computed tomography (μCT) and quantitative computed tomography (QCT), the intra-vertebral heterogeneity in bone density was quantified in cadaveric specimens. Quantitative measures of this heterogeneity improved predictions of vertebral strength as compared to predictions based only on mean density. Subsequently, the intra-vertebral heterogeneity in density was measured via QCT in a cohort of post-menopausal women and was found to be lower in those who had sustained a vertebral fracture vs. in age-matched individuals without fracture. The next set of studies focused on assessing the accuracy of finite element (FE) models for predicting vertebral failure. Digital volume correlation (DVC) was used to measure the deformations sustained throughout the vertebra during compression tests. These results were compared against deformation patterns predicted using FE models created from QCT images of the vertebrae. Good agreement was found between predicted and measured deformations when the boundary conditions were accurately defined, despite simplifications made in representing material properties. The outcomes from this dissertation demonstrate that the intra-vertebral heterogeneity in density contributes to bone strength and has promise as a clinically feasible indicator of fracture risk. OCT-based FE models, which by definition account for this heterogeneity, are another promising technique, yet will likely require non-invasive techniques for estimating vertebral loading to provide the requisite accuracy in failure predictions. These two engineering approaches that account for the spatial heterogeneity in density within the vertebra may lead to more sensitive and specific indicators of fracture risk

Clinical observation of diminished bone quality and quantity through longitudinal HR-pQCT-derived remodeling and mechanoregulation.

High resolution peripheral quantitative computed tomography (HR-pQCT) provides methods for quantifying volumetric bone mineral density and microarchitecture necessary for early diagnosis of bone disease. When combined with a longitudinal imaging protocol and finite element analysis, HR-pQCT can be used to assess bone formation and resorption (i.e., remodeling) and the relationship between this remodeling and mechanical loading (i.e., mechanoregulation) at the tissue level. Herein, 25 patients with a contralateral distal radius fracture were imaged with HR-pQCT at baseline and 9-12 months follow-up: 16 patients were prescribed vitamin D3 with/without calcium supplement based on a blood biomarker measures of bone metabolism and dual-energy X-ray absorptiometry image-based measures of normative bone quantity which indicated diminishing (n = 9) or poor (n = 7) bone quantity and 9 were not. To evaluate the sensitivity of this imaging protocol to microstructural changes, HR-pQCT images were registered for quantification of bone remodeling and image-based micro-finite element analysis was then used to predict local bone strains and derive rules for mechanoregulation. Remodeling volume fractions were predicted by both average values of trabecular and cortical thickness and bone mineral density (R2 > 0.8), whereas mechanoregulation was affected by dominance of the arm and group classification (p < 0.05). Overall, longitudinal, extended HR-pQCT analysis enabled the identification of changes in bone quantity and quality too subtle for traditional measures

Mapping Trabecular Bone Fabric Tensor by in Vivo Magnetic Resonance Imaging

The mechanical competence of bone depends upon its quantity, structural arrangement, and chemical composition. Assessment of these factors is important for the evaluation of bone integrity, particularly as the skeleton remodels according to external (e.g. mechanical loading) and internal (e.g. hormonal changes) stimuli. Micro magnetic resonance imaging (µMRI) has emerged as a non-invasive and non-ionizing method well-suited for the repeated measurements necessary for monitoring changes in bone integrity. However, in vivo image-based directional dependence of trabecular bone (TB) has not been linked to mechanical competence or fracture risk despite the existence of convincing ex vivo evidence. The objective of this dissertation research was to develop a means of capturing the directional dependence of TB by assessing a fabric tensor on the basis of in vivo µMRI. To accomplish this objective, a novel approach for calculating the TB fabric tensor based on the spatial autocorrelation function was developed and evaluated in the presence of common limitations to in vivo µMRI. Comparisons were made to the standard technique of mean-intercept-length (MIL). Relative to MIL, ACF was identified as computationally faster by over an order of magnitude and more robust within the range of the resolutions and SNRs achievable in vivo. The potential for improved sensitivity afforded by isotropic resolution was also investigated in an improved µMR imaging protocol at 3T. Measures of reproducibility and reliability indicate the potential of images with isotropic resolution to provide enhanced sensitivity to orientation-dependent measures of TB, however overall reproducibility suffered from the sacrifice in SNR. Finally, the image-derived TB fabric tensor was validated through its relationship with TB mechanical competence in specimen and in vivo µMR images. The inclusion of trabecular bone fabric measures significantly improved the bone volume fraction-based prediction of elastic constants calculated by micro-finite element analysis. This research established a method for detecting TB fabric tensor in vivo and identified the directional dependence of TB as an important determinant of TB mechanical competence

Book of Abstracts 15th International Symposium on Computer Methods in Biomechanics and Biomedical Engineering and 3rd Conference on Imaging and Visualization

In this edition, the two events will run together as a single conference, highlighting the strong connection with the Taylor & Francis journals: Computer Methods in Biomechanics and Biomedical Engineering (John Middleton and Christopher Jacobs, Eds.) and Computer Methods in Biomechanics and Biomedical Engineering: Imaging and Visualization (JoãoManuel R.S. Tavares, Ed.). The conference has become a major international meeting on computational biomechanics, imaging andvisualization. In this edition, the main program includes 212 presentations. In addition, sixteen renowned researchers will give plenary keynotes, addressing current challenges in computational biomechanics and biomedical imaging. In Lisbon, for the first time, a session dedicated to award the winner of the Best Paper in CMBBE Journal will take place. We believe that CMBBE2018 will have a strong impact on the development of computational biomechanics and biomedical imaging and visualization, identifying emerging areas of research and promoting the collaboration and networking between participants. This impact is evidenced through the well-known research groups, commercial companies and scientific organizations, who continue to support and sponsor the CMBBE meeting series. In fact, the conference is enriched with five workshops on specific scientific topics and commercial software.info:eu-repo/semantics/draf

Intervertebral disc characterization by shear wave elastography: An in vitro preliminary study.

Published onlineJOURNAL ARTICLEAuthor's accepted (post-print) manuscriptThe final version of record is available at http://dx.doi.org/10.1177/0954411914540279Patient-specific numerical simulation of the spine is a useful tool both in clinic and research. While geometrical personalization of the spine is no more an issue, thanks to recent technological advances, non-invasive personalization of soft tissue's mechanical properties remains a challenge. Ultrasound elastography is a relatively recent measurement technique allowing the evaluation of soft tissue's elastic modulus through the measurement of shear wave speed. The aim of this study was to determine the feasibility of elastographic measurements in intervertebral disc. An in vitro approach was chosen to test the hypothesis that shear wave speed can be used to evaluate intervertebral disc mechanical properties and to assess measurement repeatability. In total, 11 oxtail intervertebral discs were tested in compression to determine their stiffness and apparent elastic modulus at rest and at 400 N. Elastographic measurements were performed in these two conditions and compared to these mechanical parameters. The protocol was repeated six times to determine elastographic measurement repeatability. Average shear wave speed over all samples was 5.3 ± 1.0 m/s, with a repeatability of 7% at rest and 4.6% at 400 N; stiffness and apparent elastic modulus were 266.3 ± 70.5 N/mm and 5.4 ± 1.1 MPa at rest, respectively, while at 400 N they were 781.0 ± 153.8 N/mm and 13.2 ± 2.4 MPa, respectively. Correlations were found between elastographic measurements and intervertebral disc mechanical properties; these preliminary results are promising for further in vivo application.ParisTech BiomecAM chair programProteorParisTechYves Cotrel Foundation