What are the main factors we need to account for mathematically predicting (FEM) fatigue life and reliability of Mechanical heart valve?
In addition to the factors like "a)surface finish b)size c)temperature d)load factor e)fatigue strength reduction factor f)Stress avg and max" along with "Goodmann relation equations" what are the other critical factors needed to numerically predict the life in number of cycles.
Also how body fluids like blood effect the fatigue of materials, as the SN curve we generate is at room temperature and at air surroundings. So can we use the SN curve at air for predicting life of an implant socked in blood by using any correction factor?
Hi, I'm no expert in heart valve mechanics, but the following names are:
Michael Sacks at UT Austin, Yoganathan at Georgia Tech, Frank Baajiens at Eindhoven.
Those people do some simulation but probably more experiments. There was a recent publication about FEM simulation by Sun (also at Georgia Tech), the link to his list of publications is here:
http://www.tml.gatech.edu/publications.html
Hope this helps
Adrian
Keep in mind that the values tend not to failure due to mechanical wear. The build up of plague (the bodies response to a foreign object) which occurs for some patients and not so much with others leads to degradation of the values function.
Studies that do not include the bodies response generally over estimate the durability of the valves.
Cheers,
Fred
Dear scientist,
I guess one should not calculate/test a system brake down (here prosthesis dysfunction) under conditions, which are different to the conditions after implantation. That means
1.) Test conditions: air surroundings + room temperature.
Real condition: blood/fluid surroundings + at least 37 ° C or even more. The temperature as well as the fluid/blood surroundings will effect the material behaviour.
2.) The prosthesis´ orientation differ from case to case when implanted into the specific location in the heart, therefore the blood current and of course also the stress to different parts of the occluders is different from case to case.
3.) The frequency of circles is changing in a heart (let’s say from 60 to 140 b/min), which will have an impact on the material of the occluder and the complete prosthesis. Higher alternation of load increases not only the counts of completed circles but increases also the opening and closure force on the material of the occluder and prosthesis in every single circle.
4.) Last but not least the real stroke volume through the valve is not only pulsatile, but is changing severely and is often arrhythmic (see: atrial fibrillation, ES etc.), which may also change stress components to the material.
Only conditions, which simulate the real conditions as accurate as possible may give some imagination of the fatigue at most.
At the end several factors, which will be present in implanted prostheses and which are responsible for most of the dysfunctions of “mechanical” prostheses are also important, like rheology of blood (thrombus formation may cause dysfunction), tissue ingrowths into the prosthesis etc. A construction, which reduces these issues will definitely reduce the amount of dysfunctional prostheses, because most dysfunctions occur due to theses issues, not because of a broken part of the prosthesis.
Kind regards
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