Mechanical modeling of stents deployed in tapered arteries

Abstract

The biomechanical interaction of stents and the arteries into which they are deployed is a potentially important consideration for long-term success. Adverse arterial reactions to excessive stress and the resulting damage have been linked to the development of restenosis. Complex geometric features often encountered in these procedures can confound treatment. In some cases, it is desirable to deploy a stent across a region in which the diameter decreases significantly over the length of the stent. This study aimed to assess the final arterial diameter and circumferential stress in tapered arteries into which two different stents were deployed (one stiff and one less stiff). The artery wall was assumed to be made of a strain stiffening material subjected to large deformations, with a 10% decrease in diameter over the length of the stent. A commercially available finite element code was employed to solve the contact problem between the two elastic bodies. The stiffer stent dominated over arterial taper, resulting in a nearly constant final diameter along the length of the stent, and very high stresses, particularly at the distal end. The less stiff stent followed more closely the tapered contour of the artery, resulting in lower artery wall stresses. More compliant stents should be considered for tapered artery applications, perhaps even to the exclusion of tapered stents.

Figures

Figure 1. Stent Design Parameters

Illustrations of the stent designs employed, showing the three parameters used in their construction: h is connector bar length (or strut spacing), ρ is the radius of curvature at the crown junctions, and f is the axial amplitude.

Figure 2. Final Radial Position

Deformed radial position values were averaged around the circumference of the vessel at diastolic pressure. With the stiff 1Z1 stent design, the vessel essentially assumes the shape of the stent, i.e. the taper of the vessel is no longer evident through the stented region. With the more compliant 2B3 design a gradual transition in compliance from stent to artery is evident near the ends of the stent.

Figure 3. Circumferential Stress Distribution

Stent 1Z1 imposed high stresses (> 600 kPa) across larger portions of the artery, whereas stent 2B3 (less rigid) only imposed such high stresses in the most narrow part of the artery. Overall, the 2B3 design imposed stress values that were 100 – 200 kPa lower than the 1Z1 design.

Figure 4. Average Circumferential Stress

Intimal artery wall circumferential stress (averaged over the circumference) versus axial position plots reveal the dominance of the 1Z1 stent over the taper resulting in high stresses at the distal end. The less stiff 2B3 stent more naturally follows the arterial taper, resulting in lower stresses. Note circumferential stress values at the intimal are most likely to affect the internal elastic lamina, and restenosis rates have been linked to the disruption of this structure.,