EVALUATION OF ABDOMINAL AORTIC ANEURYSM WALL STESS BASED ON FLOW INDUCED LOAD

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 11, November 2018, pp , Article ID: IJMET_09_11_068 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EVALUATION OF ABDOMINAL AORTIC ANEURYSM WALL STESS BASED ON FLOW INDUCED LOAD R.Vinoth Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, Karnataka, India ABSTRACT The Fluid Structure Interaction (FSI) model of Abdominal Aortic Aneurysm (AAA) is developed to estimate wall stress value in abdominal aortic aneurysm. The One-way coupled transient FSI technique is used for the analysis. The geometry of AAA constructed from Computed Tomographic (CT) image data. The aortic wall is treated as linear elastic material. The pressure values from the transient Computational Fluid Dynamics (CFD) study of AAA were used as luminal pressure load on the aortic wall. The Peak Wall Stress (PWS) of psi ( N/cm 2 ) is estimated from the model. The quantitative value of wall stress from this study could assist the physician in assessment of aneurysm rupture risk. KEYWORDS: Fluid Structure Interaction, Computational Fluid Dynamics, Abdominal Aortic Aneurysm, Wall Stress Cite this Article R.Vinoth, Evaluation of Abdominal Aortic Aneurysm Wall Stess Based on Flow Induced Load, International Journal of Mechanical Engineering and Technology, 9(11), 2018, pp INTRODUCTION Aorta is the biggest blood vessel in cardiovascular system which transports blood from heart to other organs in a body. Aortic Aneurysm is a vascular pathology which involves bulging and weakening of the aorta segment. This disease happens mainly in the elderly group of people [1, 2]. Abdominal Aortic Aneurysm (AAA) is a type of aneurysm which happens in abdominal portion of the aorta. The aneurysm can grow and rupture when it is not treated in time. The maximum aneurysm diameter method is used in clinical treatment to recommend for aneurysm repair. The aneurysms with small diameter can rupture suddenly without any symptoms. The recommendation for aneurysm repair should not be made by only based on aneurysm s diameter. It should be made by more reliable components associated with risk of rupture of the aneurysm. Biomechanical methods have been used to evaluate the likelihood of aneurysm rupture. The aneurysm ruptures when the wall stress exceeds the wall s failure strength [3, 4]. The author editor@iaeme.com

2 R.Vinoth reported that, the AAA with peak wall stress value of 77 N/cm 2 was ruptured and with stress value of 33 N/cm 2 was not ruptured [5]. The authors conducted an experimental investigation to estimate wall strength of abdominal aortic aneurysm wall and declared it was 65 N/cm 2 [6, 7]. In view of these, it is decided that the estimation of AAA wall stress is very crucial to predict rupture of aneurysms. In this work, Fluid Structure Interaction (FSI) model of patient-specific AAA is developed to estimate wall stress value in AAA. 2. METHODOLOGY A male subject with age more than 50 years was considered for the analysis. The medical details were collected from the subject for diagnostic purpose. The Institutional Ethics Committee has given permission to conduct this study. The patient-specific AAA geometry was obtained using dicom files of Computed Tomographic (CT) image with the help of Mimics v17.0. The geometry of patient-specific AAA is depicted in Figure 1. The fluid domain section of the model was imported to ANSYS workbench (v15, ANSYS, Inc.) to create mesh for that model. The mesh file of the model was generated by using tetrahedral elements. The fine mesh was used for the analysis. The transient Computational Fluid Dynamic (CFD) analysis was carried out in the study to find the pressure distribution on the aorta. The velocity and pressure profiles were enforced at inlet and outlets of the model respectively. The inlet velocity condition is depicted in Figure 2 and pressure condition was shown in my previous publication [8]. The velocity profile derived from the literature [9]. The fluid (blood) was taken as a non-newtonian fluid. The yield stress was assumed as Pascal. The laminar flow was adopted for the simulation. The Reynolds number was considered as 1350 [10]. The dynamic viscosity and density of blood were to be Ns m 2 and 1050 kg m 3 respectively [11]. The wall domain was created by ANSYS Design Modeler (v15, ANSYS, Inc.). The wall was treated as linear elastic material and its property was taken from the literature [12]. Figure 1: 3D Geometry of AAA editor@iaeme.com

3 Evaluation of Abdominal Aortic Aneurysm Wall Stess Based on Flow Induced Load Figure 2: Velocity profile. The number of mesh elements for wall domain is The pressure values from the transient CFD study of AAA were used as luminal pressure load on the wall. The displacements were solved from the model using ANSYS CFX (v15, ANSYS, Inc.). 3. RESULTS AND DISCUSSION The wall stress is measured at peak pressure (t = 4.31s). Figure 3 presents wall stress distribution of aorta with AAA. Figure 4 shows wall stress distribution of AAA. Figure 3: Wall stress of aorta model with AAA in psi. Arrow indicates peak value editor@iaeme.com

4 R.Vinoth The peak value of stress occurs in aortic arch region. The peak wall stress (PWS) of psi ( N/cm 2 ) is observed in aorta model. The PWS of psi (48.85 N/cm 2 ) is observed from Figure 4 in aneurysm. The high stress in aneurysm is a major cause of aneurysm rupture. The PWS was observed at posterior-lateral surface of aneurysm model. The rupture can occur on the posterior surface of the AAAs. The authors reported that, the AAA rupture happens when the wall stress is more than the strength of the wall tissue [3, 4]. Hence, the measurement of wall stress of aneurysm is a crucial factor in assessing the rupture of aneurysm. Figure 4: Wall stress of AAA in psi. Arrow indicates peak value. 4. CONCLUSION The transient one-way coupled FSI analysis was carried out in the AAA model. The stress distribution in the aortic wall was obtained from the analysis. The PWS appeared at the posteriorlateral surface of aneurysm. This non-invasive measurement of wall stress could be clinically useful for assessing the risk of aneurysm rupture. REFERENCES [1] Blanchard, J. F, Epidemiology of abdominal aortic aneurysms. Epidemiologic Reviews, 21, 1999, pp [2] Powell, J. T and Brown, L. C, The natural history of abdominal aortic aneurysms and their risk of rupture. Advances in Surgery, 35, 2001, pp [3] Raghavan, M. L and Vorp, D. A, Toward a biomechanical tool to evaluate rupture potential of abdominal aortic aneurysm: identification of a finite strain constitutive model and evaluation of its applicability. Journal of Biomechanics, 33, 2000, pp [4] Vorp, D. A and Vande Geest, J. P, Biomechanical Determinants of Abdominal Aortic Aneurysm Rupture. Arteriosclerosis, Thrombosis and Vascular Biology, 25, 2005, pp editor@iaeme.com

5 Evaluation of Abdominal Aortic Aneurysm Wall Stess Based on Flow Induced Load [5] Fillinger, M. F, Marra, S. P, Raghavan, M. L and Kennedy, F. E, Prediction of rupture risk in abdominal aortic aneurysm during observation: wall stress versus diameter. Journal of Vascular Surgery, 37, 2003, pp [6] Raghavan, M. L, Vorp, D. A and Webster, M. W, Ex-vivo biomechanical behavior of abdominal aortic aneurysm: assessment using a new mathematical model. Annals of Biomedical Engineering, 24, 1996, pp [7] Vorp, D. A, Raghavan, M. L, Muluk, S. C, Makaroun, M. S, Steed, D. L and Shapiro, R, Wall strength and stiffness of aneurysmal and nonaneurysmal abdominal aorta. Annals of the New York Academy of Science, 800, 1996, pp [8] Vinoth, R. Kumar, D. Raviraja Adhikari and Vijay Shankar, C. S, Non-Newtonian and Newtonian blood flow in human aorta: A transient analysis. Biomedical Research, 28(7), 2017, pp [9] Olufsen, M. S, Charles Peskin, Won Yong Kim, Erik Pedersen, Ali Nadim and Jesper Larsen, Numerical Simulation and Experimental Validation of Blood Flow in Arteries with Structured-Tree Outflow Conditions. Annals of Biomedical Engineering, 28, 2000, pp [10] Liepsch, D, Moravec, S and Baumgart, R, Some flow visualization and laser-doppler velocity measurements in a true-to-scale elastic model of a human aortic arch- a new model technique. Biorheology, 29, 1992, pp [11] Fung, Y. C, Biomechanics: Circulation, 2nd Edition. New York: Springer, [12] Li, Z and Kleinstreuer, C, Analysis of biomechanical factors affecting stent-graft Migration in an abdominal aortic aneurysm model. Journal of Biomechanics, 39, 2006, pp editor@iaeme.com