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Nitinol, stent fracture and related issues
Stent and Nitinol have revolutionized the medicine. In past decades, guidewires, stents, filters and many minimumly invasive devices and implants are made of Nitinol and they proved to be very successful.
However, the fatigue behavior of Nitinol has not been well understood. As a consequences, many stent fractures have been observed in-vivo. Below is a list of misconcepts that may contribute to the widely observed in-vivo fractures on Nitinol stents:
1. The safety margin of stent is low. For historical reasons, calculation of the safety factor was not properly established at the time the device was designed.
2. Processing change when going from prototype devices to massive manufacturing. Alloy is highly thermal-mechanical coupled. Both heattreatment and surface finishing can change the fatigue resistance of the alloy.
Above all the possible causes on designs and material processes, there is a lack of understanding of the anatomy, let alone the interactions of the devices and the anatomy. Many stents were design only to survive under a straight pulsatile fatigue environment, however, the in-vivo deformation, especially in the peripheral arteries, is multiaxial. Furthermore, our knowledge on the mechanical properties of the artery, the motions of the artery, and the interaction of the artery and the stent are extremely limited.
To short, design engineers in stent industry are in desparate need for knowledge of material fatigue, loading conditions and how the stent is interacting with the anatomy to modernize the stent design.
These motivate the formation of this forum so that multiple topics and opinions can be seen here from basic fundamental material knowledge to complex human anatomy motion and the interaction of the device and the body.
As Dr. Michael Mitchell contantly reminded me that "To run, you must walk first". I will start this forum by posting an old fatigue article, please be crucial to criticize and beging to post your opinions.
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 NDC Fatigue SMST.pdf | 2.31 MB |
A focused session at McMat 2007
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.
RH
roughness and blood coagulation
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?
Surface roughness and blood coagulation
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
fatigue-driven potential for increased thrombosis not likely
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
The new forum of Mechanics in Medicine
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.
cause of thromboembolism
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