iMechanica - stent - Comments

fatigue-driven potential for increased thrombosis not likely

Scott Russell — Mon, 05 Feb 2007 14:00:56 -0500

I would not expect there to be an issue of increased probability of blood coagulation during implant service due to fatigue loading of the stent for a couple of reasons:

1) Stents are generally endoltheliazed (i.e., completely encapsulated by a thin layer of tissue) within about 14 to 30 days, so the stent surface is no longer exposed to the blood stream after this time period. The American College of Cardiology recommends prescribing anticoagulation medication (aspirin plus clopidogrel) for at least one month following bare metal stent implantation and even longer for drug-eluting stent implantation. During this healing phase, I suspect that the disruption in blood flow caused by the pattern of the stent structure itself combined with the presence of the exposed metallic surface of the stent would both be much more likely to cause thrombosis issues than any potential micro-roughness caused by fatigue loading of the stent. After endotheliazation, it is a complete non-issue, unless a stent fracture occurs as Xiao-yan mentioned that may expose a broken end of a stent strut to the bloodstream.

2) In the short-term period (prior to endotheliazation), the surface roughness of a Nitinol stent may more likely change due to the increased presence of twinned martensite at the surface (very minor effect) rather than fatigue striations. The typical cyclic loading of an implanted Nitinol stent is much less than 0.4% half amplitude strain, reflecting less than +/-5% change in the amount of twinned martensite at the surface. I don't expect that to be significant enough to promote thrombosis. Further, 30 days of implantation are roughly 3 million pulsatile cardiac cycles. Approved stents are designed to withstand 400 million such cycles. Non-pulsatile loading conditions can occur, but are at a significantly lower cyclic rate (several orders of magnitude). Within this time frame, I would expect one of two things to happen: i) either the non-pulsatile conditions are so significant that the stent fractures, or ii) such conditions are not significant enough to warrant worrying about. In any case, the scale of any possible fatigue-generated surface roughness is so much less than the scale of the geometric disturbance caused by the structure of the stent itself as to be completely irrelevant in my opinion.

The more important issue by far with respect to stent fatigue is long-term durability. As stated before, the biggest issue with respect to properly designing stents to have appropriate durability is understanding the physiological loading conditions. This has always been the biggest challenge. If you have proper design inputs, the appropriate use of mechanics and fundamental material property models should yield a suitably durable design. Perhaps there is an increasingly important role for mechanics academicians and professionals to work closely with physiological researchers to more accurately and completely characterize the physiological loading conditions to improve the quality of the design inputs. This was recently done as part of the RESIStent consortium for SFA stenting.

Scott M. Russell
Benchmark Nitinol Device Technologies

cause of thromboembolism

Henry Tan — Sun, 04 Feb 2007 11:08:36 -0500

The cause of thromboembolism, a lot of problems involve both fluid mechanics and solid mechanics.

The new forum of Mechanics in Medicine

Zhigang Suo — Sun, 04 Feb 2007 09:09:21 -0500

Xiaoyan:

Thank you very much for pointing out the opportunity for mechanicians in the field of medical devices. Although related to the field of biological mechanics, the two fields seem to represent two distinct opportunities. The mechanics of medical devices seems to have the potential of making direct links between mechanics and an important and growing industry. Thank you for being persistent and proactive to seek out academic mechanicians.

To educate academic mechanicians, it would be helpful if you can point to a place where a mechanician can learn how a stent works. It would also be helpful if you can make a list of mechanics issues related to stent. Some problems may be better solved by devising suitable tests, and others may require progress in conceptual or computational mechanics. But we need to understand issues first.

"To run, you must walk first". That is true. What is also true is that we academics would like to know why we need to run with stent and, once convinced of the need to run, we don't mind to crawl first.

In the list of Who's new, I saw our old friend Hengchu Cao. It would be wonderful if the forum Mechanics in Medicine becomes a meeting place for industrial researchers and academics.

Surface roughness and blood coagulation

Xiao-Yan Gong — Sat, 03 Feb 2007 13:32:35 -0500

Relation between surface roughness and fatigue loading has not been established. Typically stent is formed by metal meshes of sub-milimeter dimensions, they may not directly contribute to the blood coagulation, but the fractured struts may.

Xiao-Yan Gong, PhD

roughness and blood coagulation

Henry Tan — Sat, 03 Feb 2007 10:51:49 -0500

Will the fatigue loading of stent and nitinol material increases the surface roughness of the implant devices, thus may increase the probability of blood coagulation during the implant service?

A focused session at McMat 2007

Rui Huang — Tue, 30 Jan 2007 22:35:11 -0500

Xiaoyan:

Thanks for bringing up this topic for discussion. For those who are interested, I would like to mention that we plan to have a focused session on Mechanics in Medical Devices as part of the Symposium on Mechanics of Integrated Structures and Materials for Advanced Technology at the 2007 ASME Mechanics and Materials Conference (McMat 2007). So far we have two abstracts, one from Xiaoyan and the other from K. Ravi-chandar. We would need 2 more talks to make a full session on this topic. The deadline for abstracts is February 16. Instructions to submit abstracts can be found from the above links.

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.