Image-based biomechanical models of the musculoskeletal system

Finite element modeling is a precious tool for the investigation of the biomechanics of the musculoskeletal system. A key element for the development of anatomically accurate, state-of-the art finite element models is medical imaging. Indeed, the workflow for the generation of a finite element model includes steps which require the availability of medical images of the subject of interest: segmentation, which is the assignment of each voxel of the images to a specific material such as bone and cartilage, allowing for a three-dimensional reconstruction of the anatomy; meshing, which is the creation of the computational mesh necessary for the approximation of the equations describing the physics of the problem; assignment of the material properties to the various parts of the model, which can be estimated for example from quantitative computed tomography for the bone tissue and with other techniques (elastography, T1rho, and T2 mapping from magnetic resonance imaging) for soft tissues. This paper presents a brief overview of the techniques used for image segmentation, meshing, and assessing the mechanical properties of biological tissues, with focus on finite element models of the musculoskeletal system. Both consolidated methods and recent advances such as those based on artificial intelligence are described

Atomistic modeling of amorphous silicon carbide: An approximate first-principles study in constrained solution space

Localized basis ab initio molecular dynamics simulation within the density functional framework has been used to generate realistic configurations of amorphous silicon carbide (a-SiC). Our approach consists of constructing a set of smart initial configurations that conform essential geometrical and structural aspects of the materials obtained from experimental data, which is subsequently driven via first-principles force-field to obtain the best solution in a reduced solution space. A combination of a priori information (primarily structural and topological) along with the ab-initio optimization of the total energy makes it possible to model large system size (1000 atoms) without compromising the quantum mechanical accuracy of the force-field to describe the complex bonding chemistry of Si and C. The structural, electronic and the vibrational properties of the models have been studied and compared to existing theoretical models and available data from experiments. We demonstrate that the approach is capable of producing large, realistic configurations of a-SiC from first-principles simulation that display excellent structural and electronic properties of a-SiC. Our study reveals the presence of predominant short-range order in the material originating from heteronuclear Si-C bonds with coordination defect concentration as small as 5% and the chemical disorder parameter of about 8%.Comment: 16 pages, 7 figure

Algorithms for meshing smooth surfaces and their volumes

Ph.DDOCTOR OF PHILOSOPH

Continuum Deformation Model for Drug-Eluting Stent (DES) Medical Devices Using Finite Element Analysis (FEA)

The development of coronary stent medical devices was originally considered as a major advance in the treatment of obstructive cardiovascular disease. The implantation of stent, however, involves clinical adverse effects such as re-narrowing of arterial wall after stenting. Drug-eluting stents (DES) have been developed to prevent such adverse effects by slowly delivering anti-proliferative and/or anti-inflammatory drugs from coating composites of drug-containing polymers. One of the major issues in DES implantation is, however, that the coatings comprised of drug and polymer composite phases are often fractured or delaminated during the deployment of stent, which can lead to more serious clinical complications. In this study, we developed a computational model employing the finite element analysis (FEA) technique to predict the stress distributions of various components in DES medical devices including coating composites. This work is considered as one of the first attempts to address the stress concentrations of DES medical devices upon implantation using 2D/3D computational approaches. The ABAQUS commercial package (Hibbit Karlsson & Sorences Inc., Pawtucket, RI, USA) was used to perform computational analyses for systems with large elastic/plastic deformations. Designs of three commercial products (SYNERGY, TAXUS Express, and FLEX stents) available in the market have been modeled in this thesis. The displacement control method has been adopted in developing our model for the deployment of DES. Throughout the present thesis, the impacts of geometry and material variables such as stent strut/coating thicknesses and material contents in composites on the mechanical performance of the DES were quantitatively examined. Moreover, to predict the rate of in-stent restenosis (ISR), we developed a model to include the physiological environments, i.e., arterial wall and atherosclerotic plaque, in the system. From the results, it has been monitored that the strut thickness and coating thickness of DES are one of the major factors determining the amount of stress concentration on the inner surfaces of arterial wall and atherosclerotic plaque. The higher von-Mises stress accumulation was observed with thicker strut and coating. The findings indicate that the optimizing geometry of stent and coating is a critical variable to manipulate its mechanical performance and the rate of ISR. The computation results also demonstrate that the stress concentrations in the SYNERGY and FLEX DES are much lower than those observed in the TAXUS Express stents