iMechanica - medical implant - Comments

Fatigue Tests with Nitinol Stents in peripheral arteries

iker — Fri, 17 Aug 2007 02:31:32 -0400

Dear all,

I've read your interesting comments concerning Fatigue tests and Nitinol stent topics. As you know there are several concenrs from the medical field about which fatigue test should be performed (radial cpmpression and/or bending and/or axial compression and/or torsion) with nitinol stents for peripheral application. As, mechanical engineer working with physicians I try to get from them "real data and values". I mean, which is the bending value of the arteries, axial compression and so on.

You all, as expert in such interesting field, could you please, which Fatigue test you think should ne necessary to perform and which loading conditions

my best regards and I looking forward hearing soon from you

iker

Response of stents

Ravi-Chandar — Thu, 05 Oct 2006 12:41:06 -0400

I wrote a couple of papers in the Journal of Applied Mechanics a few years back on analytical and experimental evaluation of the response of a particular type of braided stent. Using these results to evalute the coupled response of stent and artery is a simple exercise, given the arterial response. Abstracts of the papers and links are given below.

Ravi

Paper 1: The mechanical response of a metallic stent is considered in this series of two papers. In Part I, the development of a test method for the characterization of the mechanical response of a metallic aortic stent subjected to internal or external pressure, and a model that captures the relationship between the pressure and diameter of the stent based on slender rod theory are described. The axial and radial deformation of a bare-metal stent were measured as the stent was subjected to loading ranging from an external pressure of about 80 mm of Hg to an internal pressure of about 160 mm of Hg. The pressure was applied using a polyethylene bag; the method of applying the pressure and measuring the strains was found to provide an accurate determination of the mechanical behavior of the stent. The stent was shown to exhibit two stiff limiting states corresponding to the fully collapsed and fully expanded diameters and an intermediate range between the two where the stiffness was an order of magnitude smaller than the typical stiffness of an aorta. A complete mathematical characterization of the pressure-diameter response of the wire stent was also developed; this model is a straightforward application of the theory of slender rods to the problem of the stent. Excellent agreement with the experimental measurements is indicated, opening the possibility for modeling of the coupled response of the stent and the vessel into which it is inserted. In Part II, we consider the effect of variations of pressure over the length of the stent that introduce changes in the diameter along the length of the stent which leads naturally to the formulation of the coupled problem of the stent within the blood vessel.

Paper 2: The main objective of the paper is to develop the mathematical analysis of the response of a metallic stent subject to axisymmetric loads over its length and to different boundary conditions. These situations introduce bending stresses in the stent and cannot be captured by a model of the stent that can be used to characterize the pressure-diameter relationship under axially uniform loading. The analysis presented here is based on an analogy between a thin-walled pressure vessel and a beam on elastic foundation; in the present application, we derive an equivalent beam model for the bending response of a stent. Using this model, we evaluate the shape of the stent exiting the catheter as well as the variation of the diameter along the length of the stent constrained by stiff end supports. This approach can be used to evaluate the coupled response of the stent and the blood vessel, if the mechanical properties of the blood vessel are known. The coupled problem and its implications in the design of stents are discussed.

Zhigang, This is the link to

Xiao-Yan Gong — Wed, 04 Oct 2006 09:42:56 -0400

Zhigang,

This is the link to NDC's paper on Nitinol fatigue. http://www.nitinol.info/pdf_files/001.pdf

I will upload my collection of Nitinol fatigue and papers to my web site and let you know as soon as I have it done. The poll result came from "Stent Summit 06" conference at Cleveland Clinic in August 2006.

This link takes you to the ASTM fatigue to fracture and beyond task group's file archieve and tell you the stent industry's activity on this subject. If you have trouble to access it, please let me know.

http://finance.groups.yahoo.com/group/allmembersASTMF043006/files/Fatigue%20to%20Fracture%20and%20Beyond%20task%20group/

Xiao-Yan Gong, PhD

Any good refs of industrial efforts on implant reliability?

Zhigang Suo — Tue, 03 Oct 2006 08:43:37 -0400

Xiao-Yan:

This post, as well as your earlier post, Mechanics in Medical Implant Industry, is very helpful to us at universities. They point to a rich field that few of us know much about. One of my new PhD students came in yesterday to say that he would love to work on your problems. He was showing me a printout of your post!

As you know, we at universities have limited resources and time, just like everyone else. Each one of us cannot afford to do many things at the same time. Before we commit to anything, we do what academics do: we read background materials, and try to link new problems to our old strengths.

You will find more willing and effective collaborators if you give us some references to the problem. For example, do you have a reference to the industrial poll on Nitinol fatigue, which you alluded to in your comment?

We would love to read more and in depth of your writings on this exiting topic. To save your time, perhaps you can upload some preprints. Of course, you can always make hyperlinks or cite journal paper.