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TECHNICAL PAPERS: Fluids/Heat/Transport

Numerical Analysis of the Hemodynamics and Embolus Capture of a Greenfield Vena Cava Filter

[+] Author and Article Information
T. N. Swaminathan

Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, 229 Towne Building, 220 S. 33rd Street, Philadelphia, PA 19104-6315

Howard H. Hu

Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, 229 Towne Building, 220 S. 33rd Street, Philadelphia, PA 19104-6315hhu@seas.upenn.edu

Aalpen A. Patel

Department of Radiology, Interventional Radiology, 3400 Spruce Street One Siverstein, University of Pennsylvania, Philadelphia, PA 19104-6315aalpen.patel@uphs.upenn.edu

J Biomech Eng 128(3), 360-370 (Dec 09, 2005) (11 pages) doi:10.1115/1.2187034 History: Received February 08, 2005; Revised December 09, 2005

Background: Vena Cava filters are used to prevent pulmonary embolism in patients with deep vein thrombosis who are unresponsive to anticoagulation therapy. Various filter designs exist in the market with different characteristics distinguishing them. An understanding of the characteristics of these filters is desirable in order to develop better designs. Methods: A computational fluid dynamical study of the flow over an unoccluded stainless steel Greenfield Vena Cava filter (Boston Scientific, Watertown, MA) to determine its properties has been performed. Simulation of flow over a filter placed axisymmetrically in a rounded inferior vena cava has been performed at a Reynolds numbers of 1000 and the consequences of the flow (by studying parameters like shear stress and stagnation zones) have been discussed. Furthermore, a new finite element based numerical method has been developed that allows the study of capturing properties of Inferior Vena Cava filters. The key idea is the introduction of a thin-wire-model (TWM) that enables a drastic reduction in the computational cost while still maintaining control on the physics of the problem. This numerical technique has been applied to evaluate the embolus capture characteristic of a Greenfield filter. Results: The flow around the unoccluded filter is found to be steady and laminar at the conditions studied. A recirculation/stagnation zone develops immediately downstream of the filter head. This zone is significantly larger when the central hole is occluded. The shear stress and stagnation zone properties for such a flow over a Greenfield filter are compared with existing literature (in vitro studies). A graph showing the regions wherein clots escape or get captured has been determined by a means of numerical simulations. The data has further been analyzed to determine the probability of clot capture as function of the clot size. Conclusions: The stagnation zone formed behind the head of the Greenfield filter is found to be smaller in size when compared to that of the same filter with the central hole occluded. A map of the shear stress distribution shows a small region having the potential for thrombogenesis. The non-Newtonian properties of blood are not seen to cause much variation in the flow field when compared to the Newtonian model. However variation in the cava size leads to a significant change in the shear stresses. This study also establishes a novel method wherein computational means are used to determine the efficacy of clot capturing of filters. These techniques can further be used to compare the different characteristics among filters.

Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 6

Flow field around a stainless steel Greenfield filter inside a cava of diameter 20mm at Re=1000. A constant viscosity model has been used in this simulation. The contour plots indicate the magnitude of the axial component of the velocity. Flow is from the left to the right. The velocity profile is fully developed (axisymmetric and parabolic) at the inlet. The shading has been normalized with the velocity profile of the inlet with the lighter shade indicating a low velocity. The slices (1) to (7) indicate the velocity profiles at various cross-sections along the length of the cylinder. The plot in the middle shows the velocity profile at the center plane cutting through the axis of the tube.

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Figure 7

Variation of the normalized shear stress on the surface of the tube. The value of 1.0 indicates the shear without the filter, which is equivalent to a shear rate of 49.4s−1 and the darker shade indicates regions of low shear.

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Figure 8

Variation of the normalized values of the shear stress at a longitudinal cross section cutting through the axis of the tube

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Figure 17

Capture/escape statistics for clots of various sizes introduced upstream of the filter at the indicated locations. The darker dots indicate the starting position of an escaped clot while a lighter dot indicates the starting position of a captured particle. The lines are the projected filter wires as seen from a front-on angle.

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Figure 18

Variation of probability of capture as a function of the radius of the clot. The vertical lines indicate error bars.

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Figure 1

Greenfield IVC filter. Actual photo of the filter and two views of the MATLAB generated geometric model.

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Figure 16

Setup for determining the clot capturing efficacy of a Greenfield filter. Depending on the final position of the clot, a captured/escaped state can be defined.

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Figure 19

Variation of the normalized values of the shear force at a longitudinal cross section for the case of no hole in the filter

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Figure 20

Flow field near the tip of the filter with and without a hole

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Figure 2

Surface and volume finite element meshes for the Greenfield filter with magnified view of its head and one leg. The diameter of the cava is 20mm. The insets (1)–(4) are the projected volume meshes at various cross sections along the cylinder as indicated. The finite element volume mesh near the legs is very dense, which allows accurate resolution of the flow fields there.

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Figure 3

Two-dimensional finite element mesh around a circular cylinder (wire) inside a channel. A uniform flow is introduced at the left and passes the cylinder in the middle of the channel.

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Figure 4

Effective mesh size around the node representing the wire in the thin-wire filter model. The mesh size has been normalized with the cylinder radius.

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Figure 5

Greenfield IVC filter. Two views of the generated thin-wire model.

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Figure 9

Variation of the normalized values of the average shear stress on the wall as a function of the distance from the inlet. The base value of 1.0 corresponds to a shear rate of 49.4s−1. The distance from the inlet is measured in mm.

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Figure 10

Fully developed velocity profile for a Newtonian and non-Newtonian fluid with constant flow rate. The radial position varies from zero to the radius of the tube and the velocity varies from zero the maximum velocity at the center.

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Figure 11

Variation of the normalized values of the shear stress at a longitudinal cross section for the case of non-Newtonian fluid

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Figure 12

Mesh on a filter deployed on a 28mm diameter tube

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Figure 13

Mesh on a filter deployed on a 10mm diameter tube. The figure has been truncated to just show half the length of the tube.

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Figure 14

Variation of the normalized shear stress on the wall for different caval sizes. The normalization for each case has been done with the corresponding value of 4μU∕R. The distance from the inlet is measured in mm.

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Figure 15

Variation of the normalized values of the average shear stress on the wall as a function of the distance from the inlet

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