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Researchers use CFD and FEA tools to analyze the human body.

WHILE MRI CAN REVEAL THE DETAILS OF STRUCTURAL DAMAGE TO YOUR BODY, SOFTWARE CAN TELL YOU WHY IT HAPPENED AND WHAT TO DO ABOUT IT.

Computer modeling of the human body from a structural standpoint has recently been introduced into practice with the availability of improved finite element analysis (FEA) tools and computational fluid dynamics (CFD) systems.

The human spine has been a particular focus of these analyses. In one recent application, CFD tools were used by U.S. Food and Drug Administration (FDA) researchers to understand why patients were partially paralyzed following the administration of anesthesia through spinal catheters. In another case, FEA tools were used by French researchers as a simulation tool for surgical technique evaluation before surgery and to enable design optimization of implants.

FDA researchers chose CFD analysis of the spinal paralysis problem because CFD provides fluid velocity, pressure, and solute concentration values throughout a complex geometry and boundary condition environment. Users can change the geometry or boundary conditions of a CFD analysis and immediately view the effect on the fluid flow patterns or concentrations. CFD also can provide detailed parametric studies that can significantly reduce the amount of experimentation necessary to develop a prototype medical device and thus reduce design cycle times and costs.

FDA researchers chose FIDAP CFD software from Fluent Inc., Lebanon, N.H., as their modeling tool. This tool uses finite element analyses, which gives it an advantage of using non-structured grids. These grids provide considerably greater flexibility in modeling.

The FDA researchers approximated the cross section of the subarchoid space (where anasthesia is delivered through a catheter or needle) with an ellipse, and the vertical shape of the spinal column was obtained from MRI images. The images were digitized and used to guide the extrusion of the cross sectional shape to create a finite element mesh with about 70,000 nodes.

The majority of paralysis cases involved catheters with 0.02-cm dia needles. Researchers wanted to investigate the sensitivity of flow within the cavity to the relationship between catheter and cavity size. To study this relationship, two meshes were constructed with cross-sectional heights of 0.5 and 1.2 cm. The diameter of the catheter was held constant at 0.04-cm dia.

Simulations were performed using the small and large models, at two injection rates and various catheter orientations. The results revealed that the amount of anesthetic delivered in the sacral direction (away from the head) was much higher for high cavity height to catheter diameter ratios. Pooling of the anesthetic in the sacral direction was initially considered as a potential cause of the paralysis.

These results validated the reports that injuries nearly always occurred with smaller catheters. Optimal injection speeds were also studied. Graphical analysis clearly revealed why smaller catheters tend to produce uneven distributions. Future work will strive to increase the accuracy and geometrical range of the model.

Sofamore Danek Group in Memphis, Tenn., is a company dedicated to developing and manufacturing state-of-the-art products that increase stability during healing of spinal trauma and devices used in the surgical treatment of spinal disorders. Sofamore Danek's European division, in collaboration with the Biomechanical Laboratory of Ecole Nationale Superieure d'Arts et Metiers in Paris, has conducted unique research using FEA software from ANSYS, Canonsburg, Pa., to generate 3-D models of the spine.

The researchers needed to describe both the geometry and the mechanical behavioral laws for each component, including bones and ligaments. They started by performing morphological analyses on several vertebral segments.

Spine geometry was determined by 3-D measurements of the location of points distributed over the surface of numerous vertebrae. These were then connected with the intervertebral discs and ligaments, in accordance with medical pictures to ensure accuracy.

Finally, a program was written to construct a finite element model of a spine by digitizing x-rays and creating an appropriate data file for the ANSYS finite element code.

The research team modeled each vertebrae with eight-node isoparametric elements for each bone. Nonhomogeneous structures of the discs took into account different characteristics than were given to the annulus and the nucleus. Discal fibers were simulated with cable elements, which are active only in traction.

Evaluation of the Sofamor Danek model was performed using implant devices used to stabilize a burst fracture, most frequently found at the thoraco-lumbar junction and in the lumbar spine. Validation of the burst-fracture model is complex because such a model is difficult to create experimentally.

Finite element modeling easily allowed the researchers to simulate
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Title Annotation:computational fluid dynamics systems and finite element analysis tools
Author:Studt, Tim
Publication:R & D
Date:Jul 1, 1997
Words:745
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