This document summarizes finite element analysis simulations of nitinol stents performed by MSC Software. Two stent designs - helical and non-helical - were modeled and subjected to pulsatile pressure, bending, and twisting in straight and realistic artery geometries. Fatigue safety factors and estimated life cycles were calculated to evaluate the designs' performance under various loading conditions. The helical design showed higher safety factors and longer predicted life compared to the non-helical design under all loading scenarios. The simulations provided insights that can help stent manufacturers develop safer medical devices.

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Comprehensive Nitinol Stent Finite Element
Numerical Simulations: From Shape Setting and
Deployment, through Fatigue Life Predictions in a
Realistic Peripheral Human Artery Subjected to
Pulsatile and Articulation Loading Conditions.
Sean Harvey
MSC Software Corp.
SMST 2010 May 20, 2010

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Motivation
• Performing stent analyses with FEA
started many years ago and was
limited to analyzing small repeatable
portions of stents.
• Increasing requests from stent
manufacturers to understand
stent subjected to articulation
loading, and stent deployment
into more realistic torturous
vessel shapes.
• Help companies develop safe
medical devices.
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Introduction
• Nitinol self-expanding stents are used to treat peripheral
occluded vessels such as the superficial femoral artery or the
carotid.
• The complex vessel articulation requires a stent device that is
flexible and kink resistant yet durable.
• Two stent geometries are evaluated in this study:
– Helical type stent design, and
– “Traditional” straight strut, with multiple crowns design.
• The two stent designs shown in this study are generic (customer
non-proprietary), but are roughly based on actual customer
designs, which could not be shared .
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Introduction (cont.)
• Two vessel configurations are evaluated
– Straight uniform diameter vessel
– Realistic peripheral artery, the superficial femoral artery (SFA),
taken from CTA scan data.
• Software used is MSC Marc 2010
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Nitinol Superelastic Material Model in MSC Marc
• Superelastic Stress Strain Curve
Ref: F. Auricchio, R. L. Taylor Ref: C. Kleinstreuer
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Vessel Material Model
• 5% compliant/ 100 mmHg
• 2 Term Mooney Rivlin
• C10 = .221 MPa and C01 = 1.33E-2 MPa
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Stent Model - Helical
• Helical Stent Design 6 x 26mm
• 15,830 elements
• Meshed at 5mm
• Expanded to 6mm and shape
set.
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Stent Models – Non-Helical
• Non-Helical Stent Design 6 x
29mm
• 17,760 elements
• Meshed at 5mm
• Expanded to 6mm and shape
set.
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Vessel Models
• Straight Vessel - 4.97mm ID, .7mm wall thickness, and
47mm in length.
• SFA model from CTA of right leg of a 68 year old male
human subject using Mimics software from Materialise.
– 55mm length, 4.9mm to 6.2mm ID, .68mm wall, 5%
compliant/100mmHg.
SFA Meshed Vessel Model
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Helical Simulation Setup
• Pulsatile 80 to 160 mmHg
• Bending 48 ° (center of bending offset 12mm from center of
vessel, Ref: Nikanorov et al. 2008)
• Twisting 20° about bent configuration
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Non-Helical Simulation Setup
• Pulsatile 80 to 160 mmHg
• Bending 48 ° (center of bending offset 12mm from center of
vessel, Ref: Nikanorov et al.)
• Twisting 20° about bent configuration
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RESULTS
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Non-Helical Stent during Deployment
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Non-Helical Deployment Movie
Click image to play movie.
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Non-Helical Deployment Strains in SFA
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Non-Helical Bending
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Non-Helical Bending Close Up
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Non-Helical Twist
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Helical Bending Movie
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Helical Twisting Movie
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Helical Crush Movie
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Fatigue Safety Factor Calculations
• Use Python script
• Extract strain tensor at every integration pt of every element.
• Calculate alternating and mean strains.
• Calculate safety factor*
• Plot Constant Life Diagram.
• Map safety factor back onto element for contour post-
processing.
* Ref:A.R. Pelton, V. Schroeder, M.R. Mitchell, Xiao-Yan Gong, M. Barney, S.W. Robertson,
Fatigue and Durability of Nitinol Stents, J Mech Behavior Biomedl Mater, 1, 2008, pp. 153-164.
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Fatigue Life Cycle Prediction Calculation
• Option in Python script
• Extract strain tensor at every integration pt of every element.
• Calculate alternating and mean strains.
• Calculate life using strain-life equation*
(estimated from plot)
• Map log(Nf) onto element for contour post-processing.
* Ref:A.R. Pelton, V. Schroeder, M.R. Mitchell, Xiao-Yan Gong, M. Barney, S.W. Robertson,
Fatigue and Durability of Nitinol Stents, J Mech Behavior Biomedl Mater, 1, 2008, pp. 153-164.
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Nitinol Strain-Life Curve
* Ref:A.R. Pelton, V. Schroeder, M.R. Mitchell, Xiao-Yan Gong, M. Barney, S.W. Robertson,
Fatigue and Durability of Nitinol Stents, J Mech Behavior Biomedl Mater, 1, 2008, pp. 153-164.
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Results Summary
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Non-Helical Pulsatile Fatigue Results
Min SF = .76
Log(Nf) = 4.3
Nf = 19500 cycles to failure
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Non-Helical Bending Fatigue Results
Min SF = .42
Log(Nf) = 3.6
Nf = 4070 cycles to failure
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Non-Helical Twisting Fatigue Results
Min SF = .53
Log(Nf) = 3.8
Nf = 6310 cycles to failure
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Helical Pulsatile Fatigue Results
Min SF = 2.66
No cycles to failure plot as Nf > 107
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Helical Bending Fatigue Results
Min SF = 1.58
No cycles to failure plot as Nf > 107
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Helical Twisting Fatigue Results
Min SF = 1.39
No cycles to failure plot as Nf > 107
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Discussion and Conclusions
• FEA of deploying longer to full length stents in more realistic arteries is
possible.
• Using a realistic vessel can reveal insight into stent apposition, and the
ability of the device to conform to variable diameter and torturous vessels.
• In present study, it is very clear the strong influence the stent design has
on fatigue safety factors.
• Articulation loading conditions are the critical loading for the two stent
designs evaluated.
• Simulation of vessel articulation can reveal limitations in stent design
early in the design cycle.
• These computer runs are generally 1 to several days, and in some very
long stents models, weeks.
• Cycles to failure < 107 is not desirable, yet life cycle predictions can be
helpful in design comparisons and test to analysis comparisons.
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Acknowledgements
• Michael Lawrenchuk and Todd Pietila of Materialise for
providing CTA vessel model.
• Doug Malcolm and Dr. Kim Parnell at MSC Software for their
expertise.
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References
1. F. Auricchio, A robust integration-algorithm for a finite-strain shape-memory-alloy
superelastic model, Int. J. Plasticity, Vol.17, 2001, pp.971-990.
2. F. Auricchio, R. L. Taylor, Shape-memory alloy: modeling and numerical simulations of the
finite-strain superelastic behavior, Comput. Methods Appl. Mech. Engrg., Vol. 143, 1997,
pp.175-194.
3. C. Kleinstreuer, Z. Li, C.A. Basciano, S. Seelecke, M. A. Farber, Computational mechanics
of Nitinol stent grafts, J Biomech, 41, 2008, pp. 2370-2378.
4. A. R. Pelton, X. Y. Gong, T. Duerig, Fatigue Testing of Diamond Shaped Specimens,
SMST-2003: Proceedings of the International Conference on Shape Memory and
Superelastic Technologies, A. R. Pelton, T. Duerig, May 5-8 2003, Pacific Grove, SMST
Society Inc., 2004, pp. 293-302.
5. A.R. Pelton, V. Schroeder, M.R. Mitchell, Xiao-Yan Gong, M. Barney, S.W. Robertson,
Fatigue and Durability of Nitinol Stents, J Mech Behavior Biomedl Mater, 1, 2008, pp. 153-
164.
6. A. Nikanorov, H. B. Smouse, K. Osman, M. Bialas, S. Shrivastava, L. B. Schwartz, Fracture
of self-expanding nitinol stents stressed in vitro under simulated intravascular conditions, J
Vasc Surg, August, 2008, pp. 435-440.
7. N. R. Dowling, Mechanical Behavior of Materials, Prentice Hall, 1993, pp. 378-380.
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THANK YOU!
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