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Computational Modeling to Predict Fatigue Behavior of NiTi Stents: What Do We Need?

Dordoni E, Petrini L, Wu W, Migliavacca F, Dubini G, Pennati G - J Funct Biomater (2015)

Bottom Line: However, their effectiveness is still debated in the clinical field.However, confidence in numerical methods is only possible after verification and validation of the models used.For the case of NiTi stents, mechanical properties are strongly dependent on the device dimensions and the whole treatments undergone during manufacturing process.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering 'Giulio Natta', Politecnico di Milano, Milan 20133, Italy. elena.dordoni@hotmail.it.

ABSTRACT
NiTi (nickel-titanium) stents are nowadays commonly used for the percutaneous treatment of peripheral arterial disease. However, their effectiveness is still debated in the clinical field. In fact a peculiar cyclic biomechanical environment is created before and after stent implantation, with the risk of device fatigue failure. An accurate study of the device fatigue behavior is of primary importance to ensure a successful stenting procedure. Regulatory authorities recognize the possibility of performing computational analyses instead of experimental tests for the assessment of medical devices. However, confidence in numerical methods is only possible after verification and validation of the models used. For the case of NiTi stents, mechanical properties are strongly dependent on the device dimensions and the whole treatments undergone during manufacturing process. Hence, special attention should be paid to the accuracy of the description of the device geometry and the material properties implementation into the numerical code, as well as to the definition of the fatigue limit. In this paper, a path for setting up an effective numerical model for NiTi stent fatigue assessment is proposed and the results of its application in a specific case study are illustrated.

No MeSH data available.


Related in: MedlinePlus

Sketch of the fatigue testing condition adopted, reproducing the strain history that NiTi is subjected during stenting.
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jfb-06-00299-f003: Sketch of the fatigue testing condition adopted, reproducing the strain history that NiTi is subjected during stenting.

Mentions: During the stenting procedure, the crimping and the following self-expansion produce mainly bending deformations, having their maximum at the apex of the V strut of each ring. Assuming that tensile strain is the most dangerous solicitation for fatigue behavior and that the material is free from residual strain at the end of the production phase, the most critical points during crimping are subjected to tension and they partially recover the initial configuration because of the self-expansion during deployment. As shown in Figure 3, during the crimping and subsequent self-expansion phases, each stent point ideally describes a σ/ε curve up to an unloading strain value indicated as εm. The cyclic forces, following the deployment, superimpose an alternate strain (εa) to εm and induce small, mainly elastic loops. Accordingly, the following testing protocol for fatigue tensile tests on NiTi specimens was defined: (i) tensile force at a velocity of 0.8 mm/min up to 6%–7% strain, to replicate the stress-strain state experienced by devices during the crimping process; (ii) unloading at 0.8 mm/min up to a predefined value of mean strain, representative of device deployment inside an artery. Values in the range 1%–6% were chosen, according to different stent oversizing; and (iii) cyclic forces, at 50 Hz frequency, with different amplitude strains defined according to literature data [14]. The tests were conducted undern displacement control, in a temperature-controlled environment, using the same set-up described for the static tensile tests.


Computational Modeling to Predict Fatigue Behavior of NiTi Stents: What Do We Need?

Dordoni E, Petrini L, Wu W, Migliavacca F, Dubini G, Pennati G - J Funct Biomater (2015)

Sketch of the fatigue testing condition adopted, reproducing the strain history that NiTi is subjected during stenting.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4493513&req=5

jfb-06-00299-f003: Sketch of the fatigue testing condition adopted, reproducing the strain history that NiTi is subjected during stenting.
Mentions: During the stenting procedure, the crimping and the following self-expansion produce mainly bending deformations, having their maximum at the apex of the V strut of each ring. Assuming that tensile strain is the most dangerous solicitation for fatigue behavior and that the material is free from residual strain at the end of the production phase, the most critical points during crimping are subjected to tension and they partially recover the initial configuration because of the self-expansion during deployment. As shown in Figure 3, during the crimping and subsequent self-expansion phases, each stent point ideally describes a σ/ε curve up to an unloading strain value indicated as εm. The cyclic forces, following the deployment, superimpose an alternate strain (εa) to εm and induce small, mainly elastic loops. Accordingly, the following testing protocol for fatigue tensile tests on NiTi specimens was defined: (i) tensile force at a velocity of 0.8 mm/min up to 6%–7% strain, to replicate the stress-strain state experienced by devices during the crimping process; (ii) unloading at 0.8 mm/min up to a predefined value of mean strain, representative of device deployment inside an artery. Values in the range 1%–6% were chosen, according to different stent oversizing; and (iii) cyclic forces, at 50 Hz frequency, with different amplitude strains defined according to literature data [14]. The tests were conducted undern displacement control, in a temperature-controlled environment, using the same set-up described for the static tensile tests.

Bottom Line: However, their effectiveness is still debated in the clinical field.However, confidence in numerical methods is only possible after verification and validation of the models used.For the case of NiTi stents, mechanical properties are strongly dependent on the device dimensions and the whole treatments undergone during manufacturing process.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering 'Giulio Natta', Politecnico di Milano, Milan 20133, Italy. elena.dordoni@hotmail.it.

ABSTRACT
NiTi (nickel-titanium) stents are nowadays commonly used for the percutaneous treatment of peripheral arterial disease. However, their effectiveness is still debated in the clinical field. In fact a peculiar cyclic biomechanical environment is created before and after stent implantation, with the risk of device fatigue failure. An accurate study of the device fatigue behavior is of primary importance to ensure a successful stenting procedure. Regulatory authorities recognize the possibility of performing computational analyses instead of experimental tests for the assessment of medical devices. However, confidence in numerical methods is only possible after verification and validation of the models used. For the case of NiTi stents, mechanical properties are strongly dependent on the device dimensions and the whole treatments undergone during manufacturing process. Hence, special attention should be paid to the accuracy of the description of the device geometry and the material properties implementation into the numerical code, as well as to the definition of the fatigue limit. In this paper, a path for setting up an effective numerical model for NiTi stent fatigue assessment is proposed and the results of its application in a specific case study are illustrated.

No MeSH data available.


Related in: MedlinePlus