Understanding the mechanism & effects of stent fracture : a combined experimental & finite element analysis

Izadian, Mohammad Hossein

Engineering
May 2018

Thesis or dissertation


Rights
© 2018 Mohammad Hossein Izadian. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Abstract

Atherosclerosis is a common heart disease, categorised by a build-up of fatty substances (plaque) in the inner surface of the coronary arteries and causing obstruction to the blood flow to vital organs and other parts of body. Over time, the arteries become narrowed which can lead to serious complications such as angina, heart attack, and stroke. There are several treatments to slow down the progress and reduce the risk, including medication and medical procedures. Percutaneous coronary intervention (PCI) is a non-surgical procedure which reopens blocked arteries and restores the blood flow. In some cases the PCI involves a tiny mesh tube known as a stent, which is placed in the narrowed artery to widen the lumen, support the vessel wall and prevent restenosis. Whilst this is generally successful procedure, stents might cause further problems such as stent fracture, in-stent restenosis, and stent thrombosis. Stent fracture is known to be associated with a number of factors; stent length, stent overlap, vessel tortuosity, degree of calcification of lesions, stent design, and the conditions under which the stent operates.

The first part of this thesis presents a design-independent finite element analysis evaluation of the relative stresses induced in a coronary stent when placed in an angulated vessel geometry. This was achieved by idealising the stent to a thin tube, with the structural modulus of the tube representing that of a stent-like structure (this could be adapted for different types of stent structure). The artery and stent were then subjected to a displacement representing a bending movement of 20˚. Furthermore, various artery angles were modelled from 30˚ to 90˚ and each time the angle was transformed in 10˚. This series of finite element analyses computed the stress distribution associated with the displacement, hence quantifying the relationship between the vessel angle and the stress when considering the “hinge-type” movement that the vessel will undergo with each heartbeat. This constant repetitive loading constitutes the most severe mechanical loading that the stent will undergo, which far exceeds the radial expansion/contraction systolic/diastolic of the vessel or any torsional effects. It was observed that changes in stresses within the stent model are directly proportional to the vessel angulation, which stresses increased when the vessel angles became more severe. Furthermore, the bending region where was associated with the hinge-type movement experienced higher amounts of stress in the idealised stent model, and severe vessel angle caused a larger area undergo higher stress. The values increase at a greater rate once an angle of 75 degree has been exceeded, which agrees with clinical observation. Also areas of high stress corresponded to areas where fractures are seen clinically.

The second part involved the mechanical testing of 9 samples of four different stent designs; Muilti-Link Vision®, PRO-Kinetic Energy, BioMatrix NeoFlexTM and Promus PREMIER. Stents deployed at nominal pressure into physiological mock artery at initial angle of 90˚, were then subjected to a 20˚ continuous repetitive hinge-type movement, at a rate of approximately 1100rpm (cycles per minute). By 300 million cycles fractures were identified in 7 stents, and are limited to only the Biomatrix design (34.67±28.78 million cycles), exhibiting between one and four strut fractures. Fractures were first seen to occur at 13.5 million cycles, where fractures were observed in 2 stents. All fractures were seen to occur at the ring linker parts of the stent and in the areas which would undergo the most severe tensile and compressive loading.

This study shows that artery angulation has a significant impact on the stent stress, and more tortuous vessel increases the risk of stent fracture. Also in vitro experimental work illustrates that stent material and structure play an important role in stent flexibility.

Publisher
School of Engineering, The University of Hull
Supervisor
Dobson, Catherine Anne
Sponsor (Organisation)
University of Hull; Hull and East Riding Cardiac Trust
Qualification level
Doctoral
Qualification name
PhD
Language
English
Extent
5 MB
Identifier
hull:17250
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