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Prediction of strength and radial recoil of various stents using FE analysis
Materials Today: Proceedings xxx (xxxx) xxx
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Prediction of strength and radial recoil of various stents using FE analysis Rahul Kumar Choubey, Sharad K. Pradhan ⇑ Department of Mechanical Engineering, National Institute of Technical Teachers’ Training and Research, Bhopal 462002, India
a r t i c l e
i n f o
Article history: Received 6 August 2019 Accepted 17 September 2019 Available online xxxx Keywords: Stent Coronary Artery Disease Equivalent Von-Mises Stress Radial recoil FEM
a b s t r a c t The recent developments in engineering and technology over the past few decades have brought many devices that makes human life very comfortable and to cure acute ailments. In current life style, the probability of Coronary Artery Diseases (CAD) is very high even at the younger age throughout the globe. Coronary artery stent implantation is one of the effective and usually used strategies for treatment of coronary heart diseases. The main purpose for stent implantation is to provide mechanical support to the blood vessel wall. Thus, various mechanical properties of different stent materials become important parameters to design and select suitable stent. Inappropriate mechanical properties may cause harm to the vessel wall and in turn human life. This study provides a guideline to understand the mechanical behaviour of various stent materials generally used in terms of selected properties. Deformation behaviour has been studied numerically for various stents using Finite element analysis (FEA) to predict Equivalent Von-Mises Stress, Radial Recoil and Factor of Safety using ANSYS work bench software. Stent of different geometry are modeled using SOLIDWORKS and then structural analysis is performed on Stents of seven different materials viz. SS 316L Stent, Cobalt Chromium L-605 Stent, Bio-Degradable Stent (PCL), Nitinol Stent (Austenite), Elgiloy Stent, Tantalum Stent, Cobalt Chromium MP35 N Stent under normal blood pressure. The ‘Radial recoil’, Equivalent ‘Von-Mises Stress’ and ‘Factor of Safety’ of various stent materials using same stent design and same boundary conditions are compared. The results reveal that the L-605 Cobalt Chromium has low radial recoil and 316 L Stainless Steel is having highest factor of safety among the selected stent materials. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction Coronary heart diseases are caused by atherosclerotic injuries that diminish blood vessel lumen measured through plaque arrangement and blood vessel thickening. Diminishing bloodstream leads to the heart and as often as possible prompting extreme difficulties like angina pectoris or myocardial infarction. The most common performed procedure to treat CAD is Percutaneous coronary intervention (PCI) which consists of Balloon Angioplasty which is usually followed by Stenting. Medical stents are one of the non-surgical ways to treat Coronary Artery Disease (CAD). Balloon or self-expanding stent are two types of stents that are essential for the treatment of blocked CAD such as myocardial infarction and angina. Although open cardiac surgery (coronary artery bypass) for treating coronary artery diseases has a better outcome in multi vessel disease, percutaneous coronary stenting ⇑ Corresponding author. E-mail address: [email protected] (S.K. Pradhan).
is a minimally invasive procedure, with lower mortality and morbidity (e.g. stroke) and therefore has a better short term outcome in the critically ill patients. During angioplasty, stents utilize its scaffolding effect to reduce any pathologic remodelling. Implanting stents in coronary artery can limit the vessel lumen shrinkage and any further restenosis. A stent is a device used in bio-medical with complex cylindrical shape and it is utilized to help blood vessel walls to reduce the blockage of blood vessel due to plaque formation [1,2]. Greater part of stents accessible in the market is made from bare metal, but bio-degradable, drug eluting stents are also available at much higher cost [3]. Stents come in various designs with wide range of mechanical properties and attributes enabling doctors to choose the appropriate one depending upon the local deposition of plague and fatty substances in arterial vessel. There is a need for well structured methodology so that doctors can select the appropriate ones according to age of patient. Finite element analysis (FEA) or Finite element method (FEM) can help in assessing the performance and examination of various stent in less time and cost. FEA studies could give bits of knowledge into differ-
https://doi.org/10.1016/j.matpr.2019.09.107 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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R.K. Choubey, S.K. Pradhan / Materials Today: Proceedings xxx (xxxx) xxx
ent parts associated with stent structures that may leads to subsequently decrease the danger of occurring restenosis and vascular injury. It additionally gives a broad measure of data under clearly defined conditions, making it doable to screen different stent design iterations before costly prototyping, testing and deployment inside human artery. The Food and Drug Administration Authority of USA (FDA) established guidelines [4] for the industry which manufacture medical devices and listed a few key clinically-relevant functional attributes for stents. Any new stent configuration ought to fulfil the prerequisites given by the FDA. The key clinical attributes include radial strength, fatigue resistance, stresses/strains, and expansion recoil, which are yet to be inspected efficiently concerning the design parameters of stent such as pattern, geometry, and hemodynamic parameters such as age. It is imperative to build up the coordinated CAD/FEA joined methodology for the stent industry, as this procedure could decrease the product development cycle and cost significantly. This can be the need for the stent industry in the near future and it may initiate a manufacturing oriented atmosphere in countries where need for stent implantation is apparently increasing day by day.
Fig. 2. Assembly of blood vessel and artery.
2. Numerical model and analysis The 3D model of stent is developed in parametric form in commercial solid modeling package. The parametric design reduces the modeling time and facilitates easy development of alternative designs. In this work, the stent geometry is developed by adapting the methods given by Nitinol Development Corporation an open source database for stent design. Fig. 1 illustrates the CAD model and parametric design of a cardiovascular stent. Biodegradable Stent Poly caprolactone (PCL), Stainless Steel 316 L Stent, Tantalum Stent, Elgiloy Stent, Nitinol (Austenite) Stent, Cobalt Chromium L-605 Stent, Cobalt Chromium MP35 N Stent material are taken for analysis which is commonly used in market. The material properties of the Bio-degradable(PCL) Stent is taken from Hughes T.J. [5] and Montgomery D.C. [6] while material properties for 316 L Stainless Steel Stent, Tantalum Stent, Elgiloy Stent, Nitinol (Austenite) Stent, L- 605 Cobalt Chromium Stent, MP35 N Cobalt Chromium Stent are taken from Trina Roy et al. [7], similarly material property of blood is taken from Niroomand et al. [8]. The material properties of all seven stent along with linear elastic model are used for modeling stent. Finite element analysis is performed using commercial FEA software ANSYS. Modelling of blood vessel is done whose inner diameter is taken as 8.0 mm which is the outer diameter of the stent and thickness of the blood vessel is taken as 0.5 mm [9] and length of blood vessel is taken slightly larger than stent length of stent to
Fig. 3. Contact defined in the model.
Fig. 4. Meshing of the model.
Fig. 1. CAD model of Stent.
avoid end effect on output results. Assembly of stent and blood vessel is shown in Fig. 2. Based on the fact that stents expand to and support the blood vessel, the ‘‘bonded” contact type is selected, which is basically linear behaviour as shown in Fig. 3. Coarse meshing of blood vessel is done in order to reduce the solution time by keeping accuracy in consideration. The mesh cells are mixed with tetrahedron and hexahedron cells; there are 125,124 nodes and 48,660 elements which are shown in Fig. 4. The pressure is applied on internal surface is 12 kPa (or 90 mm Hg), which is the average blood pressure in the human body as shown in Fig. 5. The fixed support constraints all degrees of freedom on edge, vertex or surface, which prevents translations
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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Fig. 8. Stainless Steel 316 L Stent. Fig. 5. Blood pressure applied on the model.
Fig. 9. Nitinol (Austenite) Stent. Fig. 6. Fixed supports on both sides on the blood vessel.
Fig. 10. Elgiloy Stent. Fig. 7. Biodegradable (PCL) Stent.
and rotations in X, Y and Z. The fixed supports are applied on the both sides of the blood vessel as shown in Fig. 6. 3. Equivalent Von-Mises Stress of various stent After post processing the Equivalent Von-Mises Stress of Various Stents at 12 kPa (or 90 mm Hg) are shown from Figs. 7–13. 4. Total deformation of various stent Total deformations of all the selected stents at 12 kPa (or 90 mm Hg) are shown from Figs. 14–20, which is used for finding
the radial recoil; Radial deformation can be observed in the stents as a result of the radially compressive forces exerted by the artery. As an example, the rigidity of biodegradable stents can resist the elastic recoil of the vessel wall, which can be calculated by (Change in diameter/Original diameter)*100. In this case the change in diameter is 0.7579 mm and original diameter is 8 mm, hence radial recoil would be 9.47375%. To validate the FE results the loads are increased and decreased by 20% which leads to linear variation of the FE results hence it proves that there are no singularities or errors in the model and simulation environment of the proposed analysis. The Yield Strength of various materials like for Bio-degradable stent PCL 8.2 MPa is taken from Shaun Eshraghi et al. [10]. Similarly, Yield strength of 316 L Stainless Steel Stent is taken as
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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Fig. 11. Cobalt Chromium L-605 Stent.
Fig. 12. Cobalt Chromium MP35 N Stent.
Fig. 15. Stainless Steel 316 L Stent.
Fig. 16. Nitinol (Austenite) Stent.
Fig. 13. Tantalum Stent. Fig. 17. Elgiloy Stent.
Fig. 14. Biodegradable (PCL) Stent.
Fig. 18. Cobalt Chromium L-605 Stent.
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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Fig. 19. Cobalt Chromium MP35 N Stent.
Fig. 21. Equivalent Von-Mises Stress of Various Stent materials.
Fig. 20. Tantalum Stent.
Table 1 Ultimate tensile Strength and Equivalent Von–Mises Stress comparison for finding fracture. Stent Material
Ultimate Tensile Strength (MPa)
Equivalent Von-Mises Stress (MPa)
Fracture
Biodegradable (PCL) Stainless Steel 316 L Nitinol (Austenite) Elgiloy S Tantalum Cobalt Chromium L-605 Cobalt Chromium MP35 N
10.5 595 1200 1020 285 1020 930
14.911 231.14 436.12 524.85 514.70 536.20 529.82
Yes No No No Yes No No
380 MPa, for Elgiloy Stent as 520 MPa, Tantalum Stent as 220 MPa, L-605 Cobalt Chromium Stent as 629 MPa, MP35 N Cobalt Chromium 414 MPa from Trina Roy et al. [7]. The factor of safety is calculated using Equivalent Von-Mises stress and Yield strength values. From Table 1, it is clear that Bio-Degradable (PCL) Stent and Tantalum Stent have undergone fracture, while 316 L Stainless Steel Stent, Nitinol (Austenite) Stent, Elgiloy Stent, L-605 Cobalt Chromium Stent and MP35 N cobalt Chromium Stent has not failed, with same design and under same boundary condition. Equivalent Von-Mises Stress of Bio-Degradable (PCL) Stent is 14.911 MPA, which is smallest among all and Equivalent VonMises Stress for L-605 Cobalt Chromium Stent is 536.2 MPa, which is largest among all the materials as shown in Fig. 21. Radial Recoil of Bio- Degradable (PCL) Stent is 9.473%, which is largest among all and Radial recoil for L-605 Cobalt Chromium Stent is 0.979%, which is smallest among all the materials used in simulation. As shown in Fig. 22.
Fig. 22. Radial Recoil of Different Stent Material.
Fig. 23. Factor of safety of Different Stent Materials.
Factor of Safety of Tantalum Stent is 0.4274, which is smallest among all and Factor of Safety for Stainless Steel 316 L Stent is 1.644, which is largest among all the materials used in simulation as shown in Fig. 23. Table 2 summarises the Equivalent Von-Mises Stress, Radial Recoil and Factor of safety of different materials, which is obtained from simulation results at normal blood pressure, which is 90 mm Hg (1.2e-002 MPa). Table 2: Various Stents Equivalent Von-Mises Stress, Radial Recoil and Factor of safety comparison based on Simulation results.
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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Table 2 Various Stent Equivalent Von-Mises Stress, Radial Recoil and Factor of safety comparison based on Simulation results. Stent Material
Equivalent Von-Mises Stress (MPa)
Radial Recoil (Percentage)
Factor of Safety
Bio-Degradable (PCL) Stainless Steel 316 L Nitinol (Austenite) Elgiloy Tantalum Cobalt Chromium L-605 Cobalt Chromium MP35 N
14.911 231.14 436.12 524.85 514.70 536.20 529.82
9.473% 5.20% 2.31% 1.21% 1.22% 0.979% 1.013%
0.5499 1.6440 1.2840 0.9907 0.4274 1.1730 0.7813
0.979%, which is lowest among all the materials while its radial strength is highest among all the materials used in simulation. Factor of Safety is highest for 316 L Stainless Steel Stent material among the seven materials used in simulation. The factor of safety of Nitinol (austenite) and L-605 Cobalt Chromium are also reasonably good as compare to rest of the materials except 316 L Stainless Steel Stent material. Nitinol has an advantage over other biomaterials as it has better resistance to corrosions. So it is an alternate for 316 L Stainless Steel Stent as its Factor of safety is 1.2840, which is just lower than 316 L Stainless Steel stent, but higher than L-605 Cobalt chromium stent. So Cobalt Chromium L-605 and Nitinol will be better material for stent. References
Radial recoil is a common characteristic of metallic stents and is used widely by researchers and stent manufacturers in the verification of a stent design. For ideal Stent, Radial recoil should be low and radial strength should be high for better life of stent. From Table 2 it is clear that the radial recoil of bio-degradable stent is very high 9.473% which shows that the rigidity of biodegradable stents is less than metallic stents. Radial recoil of L-605 Cobalt Chromium Stent is 0.979%, which is quite good as compare to other materials with reasonably good factor of safety as 1.1730. 5. Conclusion The chances of developing fracture are evident in Bio-Degradable (PCL) Stent and Tantalum Stent during working of stent in artery, on the other hand there is no risk of crack initiation in case of 316 L Stainless Steel, Nitinol (Austenite), Elgiloy, L-605 Cobalt Chromium Stent, MP35 N Cobalt Chromium Stent because of equivalent von-Mises Stress of this material are less than Ultimate tensile Stress of this materials. For ideal radial Stent material the radial recoil should be less and strength should be high, radial recoil of bio-degradable (PCL) Stent is very high 9.473% which shows that the rigidity of biodegradable stents is less as compared to metallic stents. Radial recoil of L-605 Cobalt Chromium Stent is
[1] C. Rogers, D.Y. Tseng, J.C. Squire, E.R. Edelman, Balloon-artery interactions during stent placement: a finite element analysis approach to pressure, compliance, and stent design as contributors to vascular injury, Circ. Res. 84 (1999) 378–383. [2] C. Dumoulin, B. Cochelin, Mechanical behaviour modelling of balloonexpandable stents, J. Biomech. 33 (11) (2000) 1461–1470. [3] S. Constantine. ‘‘Heart Attack”. Medmovie, 2017. Available: http:// medmovie.com/library_id/ 3255/topic /ahaw_0072a/. [Accessed 20 May 2016]. [4] Non-clinical engineering tests and recommended labeling for intravascular stents and associated delivery systems. Document number 1545. Rockville (MD): Food and Drug Administration. Center for Devices and Radiological Health. April 18; 2010. [5] J.R. Thomas, Hughes, Linear Static and Dynamic Finite Element Analysis, Dover Publications, The Finite Element Method, 2000. [6] C. Douglas, Montgomery, Wiley, Design and Analysis of Experiments, 2013. [7] T. Roy, A. Chanda, Computational modelling and analysis of latest commercially available coronary stents during deployment, in: International Conference on Advances in Manufacturing and Material Engineering AMME2014, Procedia Materials Science, 2014, pp. 2310–2319. [8] H. Niroomand Oscuii, M. Tafazzoli Shadpour, Ghalichi, Flow characteristics in elastic arteries using a fluid-structure interaction mode, Am. J. Appl. Sci. 4 (8) (2007) 516–524. [9] G.J. L’italien, I.G. Kidson, J. Megerman, W.M. Abbott, In vivo measurement of blood vessel wall thickness, Am. J. Physiol.-Heart Circ. Physiol. 237 (2) (1979) H265–H268. [10] S. Eshraghi, S. Das, Mechanical and microstructural properties of polycaprolactone scaffolds with one-dimensional, two-dimensional, and three-dimensional orthogonally oriented porous architectures produced by selective laser sintering, Acta Biomater. 6 (2010) 2467–2476.
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Prediction of strength and radial recoil of various stents using FE analysis Rahul Kumar Choubey, Sharad K. Pradhan ⇑ Department of Mechanical Engineering, National Institute of Technical Teachers’ Training and Research, Bhopal 462002, India
a r t i c l e
i n f o
Article history: Received 6 August 2019 Accepted 17 September 2019 Available online xxxx Keywords: Stent Coronary Artery Disease Equivalent Von-Mises Stress Radial recoil FEM
a b s t r a c t The recent developments in engineering and technology over the past few decades have brought many devices that makes human life very comfortable and to cure acute ailments. In current life style, the probability of Coronary Artery Diseases (CAD) is very high even at the younger age throughout the globe. Coronary artery stent implantation is one of the effective and usually used strategies for treatment of coronary heart diseases. The main purpose for stent implantation is to provide mechanical support to the blood vessel wall. Thus, various mechanical properties of different stent materials become important parameters to design and select suitable stent. Inappropriate mechanical properties may cause harm to the vessel wall and in turn human life. This study provides a guideline to understand the mechanical behaviour of various stent materials generally used in terms of selected properties. Deformation behaviour has been studied numerically for various stents using Finite element analysis (FEA) to predict Equivalent Von-Mises Stress, Radial Recoil and Factor of Safety using ANSYS work bench software. Stent of different geometry are modeled using SOLIDWORKS and then structural analysis is performed on Stents of seven different materials viz. SS 316L Stent, Cobalt Chromium L-605 Stent, Bio-Degradable Stent (PCL), Nitinol Stent (Austenite), Elgiloy Stent, Tantalum Stent, Cobalt Chromium MP35 N Stent under normal blood pressure. The ‘Radial recoil’, Equivalent ‘Von-Mises Stress’ and ‘Factor of Safety’ of various stent materials using same stent design and same boundary conditions are compared. The results reveal that the L-605 Cobalt Chromium has low radial recoil and 316 L Stainless Steel is having highest factor of safety among the selected stent materials. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction Coronary heart diseases are caused by atherosclerotic injuries that diminish blood vessel lumen measured through plaque arrangement and blood vessel thickening. Diminishing bloodstream leads to the heart and as often as possible prompting extreme difficulties like angina pectoris or myocardial infarction. The most common performed procedure to treat CAD is Percutaneous coronary intervention (PCI) which consists of Balloon Angioplasty which is usually followed by Stenting. Medical stents are one of the non-surgical ways to treat Coronary Artery Disease (CAD). Balloon or self-expanding stent are two types of stents that are essential for the treatment of blocked CAD such as myocardial infarction and angina. Although open cardiac surgery (coronary artery bypass) for treating coronary artery diseases has a better outcome in multi vessel disease, percutaneous coronary stenting ⇑ Corresponding author. E-mail address: [email protected] (S.K. Pradhan).
is a minimally invasive procedure, with lower mortality and morbidity (e.g. stroke) and therefore has a better short term outcome in the critically ill patients. During angioplasty, stents utilize its scaffolding effect to reduce any pathologic remodelling. Implanting stents in coronary artery can limit the vessel lumen shrinkage and any further restenosis. A stent is a device used in bio-medical with complex cylindrical shape and it is utilized to help blood vessel walls to reduce the blockage of blood vessel due to plaque formation [1,2]. Greater part of stents accessible in the market is made from bare metal, but bio-degradable, drug eluting stents are also available at much higher cost [3]. Stents come in various designs with wide range of mechanical properties and attributes enabling doctors to choose the appropriate one depending upon the local deposition of plague and fatty substances in arterial vessel. There is a need for well structured methodology so that doctors can select the appropriate ones according to age of patient. Finite element analysis (FEA) or Finite element method (FEM) can help in assessing the performance and examination of various stent in less time and cost. FEA studies could give bits of knowledge into differ-
https://doi.org/10.1016/j.matpr.2019.09.107 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
2
R.K. Choubey, S.K. Pradhan / Materials Today: Proceedings xxx (xxxx) xxx
ent parts associated with stent structures that may leads to subsequently decrease the danger of occurring restenosis and vascular injury. It additionally gives a broad measure of data under clearly defined conditions, making it doable to screen different stent design iterations before costly prototyping, testing and deployment inside human artery. The Food and Drug Administration Authority of USA (FDA) established guidelines [4] for the industry which manufacture medical devices and listed a few key clinically-relevant functional attributes for stents. Any new stent configuration ought to fulfil the prerequisites given by the FDA. The key clinical attributes include radial strength, fatigue resistance, stresses/strains, and expansion recoil, which are yet to be inspected efficiently concerning the design parameters of stent such as pattern, geometry, and hemodynamic parameters such as age. It is imperative to build up the coordinated CAD/FEA joined methodology for the stent industry, as this procedure could decrease the product development cycle and cost significantly. This can be the need for the stent industry in the near future and it may initiate a manufacturing oriented atmosphere in countries where need for stent implantation is apparently increasing day by day.
Fig. 2. Assembly of blood vessel and artery.
2. Numerical model and analysis The 3D model of stent is developed in parametric form in commercial solid modeling package. The parametric design reduces the modeling time and facilitates easy development of alternative designs. In this work, the stent geometry is developed by adapting the methods given by Nitinol Development Corporation an open source database for stent design. Fig. 1 illustrates the CAD model and parametric design of a cardiovascular stent. Biodegradable Stent Poly caprolactone (PCL), Stainless Steel 316 L Stent, Tantalum Stent, Elgiloy Stent, Nitinol (Austenite) Stent, Cobalt Chromium L-605 Stent, Cobalt Chromium MP35 N Stent material are taken for analysis which is commonly used in market. The material properties of the Bio-degradable(PCL) Stent is taken from Hughes T.J. [5] and Montgomery D.C. [6] while material properties for 316 L Stainless Steel Stent, Tantalum Stent, Elgiloy Stent, Nitinol (Austenite) Stent, L- 605 Cobalt Chromium Stent, MP35 N Cobalt Chromium Stent are taken from Trina Roy et al. [7], similarly material property of blood is taken from Niroomand et al. [8]. The material properties of all seven stent along with linear elastic model are used for modeling stent. Finite element analysis is performed using commercial FEA software ANSYS. Modelling of blood vessel is done whose inner diameter is taken as 8.0 mm which is the outer diameter of the stent and thickness of the blood vessel is taken as 0.5 mm [9] and length of blood vessel is taken slightly larger than stent length of stent to
Fig. 3. Contact defined in the model.
Fig. 4. Meshing of the model.
Fig. 1. CAD model of Stent.
avoid end effect on output results. Assembly of stent and blood vessel is shown in Fig. 2. Based on the fact that stents expand to and support the blood vessel, the ‘‘bonded” contact type is selected, which is basically linear behaviour as shown in Fig. 3. Coarse meshing of blood vessel is done in order to reduce the solution time by keeping accuracy in consideration. The mesh cells are mixed with tetrahedron and hexahedron cells; there are 125,124 nodes and 48,660 elements which are shown in Fig. 4. The pressure is applied on internal surface is 12 kPa (or 90 mm Hg), which is the average blood pressure in the human body as shown in Fig. 5. The fixed support constraints all degrees of freedom on edge, vertex or surface, which prevents translations
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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Fig. 8. Stainless Steel 316 L Stent. Fig. 5. Blood pressure applied on the model.
Fig. 9. Nitinol (Austenite) Stent. Fig. 6. Fixed supports on both sides on the blood vessel.
Fig. 10. Elgiloy Stent. Fig. 7. Biodegradable (PCL) Stent.
and rotations in X, Y and Z. The fixed supports are applied on the both sides of the blood vessel as shown in Fig. 6. 3. Equivalent Von-Mises Stress of various stent After post processing the Equivalent Von-Mises Stress of Various Stents at 12 kPa (or 90 mm Hg) are shown from Figs. 7–13. 4. Total deformation of various stent Total deformations of all the selected stents at 12 kPa (or 90 mm Hg) are shown from Figs. 14–20, which is used for finding
the radial recoil; Radial deformation can be observed in the stents as a result of the radially compressive forces exerted by the artery. As an example, the rigidity of biodegradable stents can resist the elastic recoil of the vessel wall, which can be calculated by (Change in diameter/Original diameter)*100. In this case the change in diameter is 0.7579 mm and original diameter is 8 mm, hence radial recoil would be 9.47375%. To validate the FE results the loads are increased and decreased by 20% which leads to linear variation of the FE results hence it proves that there are no singularities or errors in the model and simulation environment of the proposed analysis. The Yield Strength of various materials like for Bio-degradable stent PCL 8.2 MPa is taken from Shaun Eshraghi et al. [10]. Similarly, Yield strength of 316 L Stainless Steel Stent is taken as
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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Fig. 11. Cobalt Chromium L-605 Stent.
Fig. 12. Cobalt Chromium MP35 N Stent.
Fig. 15. Stainless Steel 316 L Stent.
Fig. 16. Nitinol (Austenite) Stent.
Fig. 13. Tantalum Stent. Fig. 17. Elgiloy Stent.
Fig. 14. Biodegradable (PCL) Stent.
Fig. 18. Cobalt Chromium L-605 Stent.
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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Fig. 19. Cobalt Chromium MP35 N Stent.
Fig. 21. Equivalent Von-Mises Stress of Various Stent materials.
Fig. 20. Tantalum Stent.
Table 1 Ultimate tensile Strength and Equivalent Von–Mises Stress comparison for finding fracture. Stent Material
Ultimate Tensile Strength (MPa)
Equivalent Von-Mises Stress (MPa)
Fracture
Biodegradable (PCL) Stainless Steel 316 L Nitinol (Austenite) Elgiloy S Tantalum Cobalt Chromium L-605 Cobalt Chromium MP35 N
10.5 595 1200 1020 285 1020 930
14.911 231.14 436.12 524.85 514.70 536.20 529.82
Yes No No No Yes No No
380 MPa, for Elgiloy Stent as 520 MPa, Tantalum Stent as 220 MPa, L-605 Cobalt Chromium Stent as 629 MPa, MP35 N Cobalt Chromium 414 MPa from Trina Roy et al. [7]. The factor of safety is calculated using Equivalent Von-Mises stress and Yield strength values. From Table 1, it is clear that Bio-Degradable (PCL) Stent and Tantalum Stent have undergone fracture, while 316 L Stainless Steel Stent, Nitinol (Austenite) Stent, Elgiloy Stent, L-605 Cobalt Chromium Stent and MP35 N cobalt Chromium Stent has not failed, with same design and under same boundary condition. Equivalent Von-Mises Stress of Bio-Degradable (PCL) Stent is 14.911 MPA, which is smallest among all and Equivalent VonMises Stress for L-605 Cobalt Chromium Stent is 536.2 MPa, which is largest among all the materials as shown in Fig. 21. Radial Recoil of Bio- Degradable (PCL) Stent is 9.473%, which is largest among all and Radial recoil for L-605 Cobalt Chromium Stent is 0.979%, which is smallest among all the materials used in simulation. As shown in Fig. 22.
Fig. 22. Radial Recoil of Different Stent Material.
Fig. 23. Factor of safety of Different Stent Materials.
Factor of Safety of Tantalum Stent is 0.4274, which is smallest among all and Factor of Safety for Stainless Steel 316 L Stent is 1.644, which is largest among all the materials used in simulation as shown in Fig. 23. Table 2 summarises the Equivalent Von-Mises Stress, Radial Recoil and Factor of safety of different materials, which is obtained from simulation results at normal blood pressure, which is 90 mm Hg (1.2e-002 MPa). Table 2: Various Stents Equivalent Von-Mises Stress, Radial Recoil and Factor of safety comparison based on Simulation results.
Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107
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Table 2 Various Stent Equivalent Von-Mises Stress, Radial Recoil and Factor of safety comparison based on Simulation results. Stent Material
Equivalent Von-Mises Stress (MPa)
Radial Recoil (Percentage)
Factor of Safety
Bio-Degradable (PCL) Stainless Steel 316 L Nitinol (Austenite) Elgiloy Tantalum Cobalt Chromium L-605 Cobalt Chromium MP35 N
14.911 231.14 436.12 524.85 514.70 536.20 529.82
9.473% 5.20% 2.31% 1.21% 1.22% 0.979% 1.013%
0.5499 1.6440 1.2840 0.9907 0.4274 1.1730 0.7813
0.979%, which is lowest among all the materials while its radial strength is highest among all the materials used in simulation. Factor of Safety is highest for 316 L Stainless Steel Stent material among the seven materials used in simulation. The factor of safety of Nitinol (austenite) and L-605 Cobalt Chromium are also reasonably good as compare to rest of the materials except 316 L Stainless Steel Stent material. Nitinol has an advantage over other biomaterials as it has better resistance to corrosions. So it is an alternate for 316 L Stainless Steel Stent as its Factor of safety is 1.2840, which is just lower than 316 L Stainless Steel stent, but higher than L-605 Cobalt chromium stent. So Cobalt Chromium L-605 and Nitinol will be better material for stent. References
Radial recoil is a common characteristic of metallic stents and is used widely by researchers and stent manufacturers in the verification of a stent design. For ideal Stent, Radial recoil should be low and radial strength should be high for better life of stent. From Table 2 it is clear that the radial recoil of bio-degradable stent is very high 9.473% which shows that the rigidity of biodegradable stents is less than metallic stents. Radial recoil of L-605 Cobalt Chromium Stent is 0.979%, which is quite good as compare to other materials with reasonably good factor of safety as 1.1730. 5. Conclusion The chances of developing fracture are evident in Bio-Degradable (PCL) Stent and Tantalum Stent during working of stent in artery, on the other hand there is no risk of crack initiation in case of 316 L Stainless Steel, Nitinol (Austenite), Elgiloy, L-605 Cobalt Chromium Stent, MP35 N Cobalt Chromium Stent because of equivalent von-Mises Stress of this material are less than Ultimate tensile Stress of this materials. For ideal radial Stent material the radial recoil should be less and strength should be high, radial recoil of bio-degradable (PCL) Stent is very high 9.473% which shows that the rigidity of biodegradable stents is less as compared to metallic stents. Radial recoil of L-605 Cobalt Chromium Stent is
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Please cite this article as: R. K. Choubey and S. K. Pradhan, Prediction of strength and radial recoil of various stents using FE analysis, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.107