Effects of Stent Design Parameters on Normal Artery Wall Mechanics

[+] Author and Article Information
Julian Bedoya, Clark A. Meyer, Lucas H. Timmins, Michael R. Moreno

Department of Biomedical Engineering,  Texas A&M University, 3120 TAMU, College Station, TX 77843-3120

James E. Moore

Department of Biomedical Engineering,  Texas A&M University, 3120 TAMU, College Station, TX 77843-3120jmoorejr@tamu.edu

J Biomech Eng 128(5), 757-765 (Apr 25, 2006) (9 pages) doi:10.1115/1.2246236 History: Received August 30, 2005; Revised April 25, 2006

A stent is a device designed to restore flow through constricted arteries. These tubular scaffold devices are delivered to the afflicted region and deployed using minimally invasive techniques. Stents must have sufficient radial strength to prop the diseased artery open. The presence of a stent can subject the artery to abnormally high stresses that can trigger adverse biologic responses culminating in restenosis. The primary aim of this investigation was to investigate the effects of varying stent “design parameters” on the stress field induced in the normal artery wall and the radial displacement achieved by the stent. The generic stent models were designed to represent a sample of the attributes incorporated in present commercially available stents. Each stent was deployed in a homogeneous, nonlinear hyperelastic artery model and evaluated using commercially available finite element analysis software. Of the designs investigated herein, those employing large axial strut spacing, blunted corners, and higher amplitudes in the ring segments induced high circumferential stresses over smaller areas of the artery’s inner surface than all other configurations. Axial strut spacing was the dominant parameter in this study, i.e., all designs employing a small stent strut spacing induced higher stresses over larger areas than designs employing the large strut spacing. Increasing either radius of curvature or strut amplitude generally resulted in smaller areas exposed to high stresses. At larger strut spacing, sensitivity to radius of curvature was increased in comparison to the small strut spacing. With the larger strut spacing designs, the effects of varying amplitude could be offset by varying the radius of curvature and vice versa. The range of minimum radial displacements from the unstented diastolic radius observed among all designs was less than 90μm. Evidence presented herein suggests that stent designs incorporating large axial strut spacing, blunted corners at bends, and higher amplitudes exposed smaller regions of the artery to high stresses, while maintaining a radial displacement that should be sufficient to restore adequate flow.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Design parameters . Generic stent showing the three parameters of interest: h is the connector bar length (or strut spacing), ρ is the radius of curvature at the crown junctions, and f is the axial amplitude. These three parameters were varied to test their effects on artery wall stress.

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Figure 2

Stent Designs . Renderings of the generic stent designs developed for this study. All stents were constructed by varying the three design parameters described herein.

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Figure 3

Artery Model Mesh . The artery mesh developed for this study is non-uniform with higher density in the regions of interest. The artery was divided into three regions in the axial direction. Within the end regions, a one-way bias was applied with larger elements specified at the ends of the artery and smaller elements specified at the outer edges of the central region. Within the central region a two-way bias was applied with larger elements specified in the center and smaller elements specified at the inner edges of the central region. The stent model was placed completely within the central region.

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Figure 4

Hoop Stress Distribution . For quantitative analysis three critical stress thresholds were established. Class I stresses, denoted by red in this illustration, are defined as stresses in excess of 545kPa. Class II stresses are defined as stresses in excess of 510kPa and are denoted by orange and red in this illustration. Class III stresses are defined as stresses in excess of 475kPa and are denoted by red, orange, and yellow-orange in this illustration. Note that stent designs with small strut spacing and small amplitude induced more critical stresses in diffuse areas than those with large strut spacing and amplitude.

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Figure 5

Binary Plot of Class III Critical Stress Distribution . Designs incorporating large strut spacing with large amplitude and non-zero radius of curvature (2A3 and 2B3) induced Class III stresses over less than 26% of the intima. Note also, the lower distribution near the ends of the stents with these designs, which exhibit gradual transition in compliance.

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Figure 6

Binary Plot of Class II Critical Stress Distribution . The small strut spacing with low amplitude designs induced Class II stresses over more than 86% of the inner surface area. Note the diffuse distribution with the low amplitude designs (1Z1, 1A1, and 1B1), versus the more localized distribution with the larger amplitude design (1B2).

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Figure 7

Radial Displacement Map . Stent designs that induce the highest stresses also provide the greatest radial displacement (referenced from the unstented artery at diastolic pressure) in the stented region. However, differences in minimum radial displacement between designs are small, approximately 90μm. Note that the large spacing large amplitude designs exhibit greater compliance at the ends of the stent.




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