$404 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

the arterial wall and that this effect depends on both arterial geometry and the prevailing flow conditions. We performed computational simulations in which the coupled Navier-Stokes and advection-diffusion equations were solved to determine the flow field and drug concentration in the vicinity of model drug- eluting stents positioned within either straight or curved arterial segments. Steady and pulsatile flow simulations with Reynolds numbers ranging from 200400 were performed, and stent designs ranging from a simple series of wires to more realistic configurations consisting of multiple intertwined rings were tested. Drug elution was modeled as a constant source emitted from the entire body of the stent. The results demonstrate that for steady flow in straight arterial segments within which stents idealized as a series of wires are deployed, the in-stent drug concentration at the arterial wall increases with stent wire thickness. In curved vessels, drug concentration at the wall de- creases as Reynolds number and vessel curvature increase. In pulsatile flow, the in-stent drug concentrations are lower during flow acceleration than during deceleration. Results using stents modeled as intertwined rings reveal intricate flow field and drug concentration sensitivity to the details of stent design. The present results enhance our understanding of the performance of drug-eluting stents and point to design strategies for optimizing this performance.

6020 Th, 09:45-10:00 (P39) A sequential porohyperelastic-transport framework for simulating drug-eluting stents

P.H. Feenstra 1 , C.A. Taylor 2. 1Mechanical Engineering, Stanford University, Stanford, USA, 2Bioengineering and Surgery, Stanford University, Stanford, USA

Drug-eluting stents (DES) have had a significant impact on the treatment of coronary artery disease by minimizing in-stent restenosis. Due to the complexity of drug transport in diseased coronary arteries, simulation tools have important applications in improving the design of these complex devices. Compared to conventional stents, DES have a dual purpose: the first is mechanical, to provide support to the injured vessel; the second is therapeutic, to deliver drug locally to the vessel wall. The objective of this paper is to present a simulation framework for DES that takes the dual purpose into consideration. Stent deployment is simulated first, a mechanical porohyperelastic (PHE) contact analysis. This analysis calculates the interstitial fluid velocity as the result of pore pressure gradients and me- chanical deformations of the vessel wall. The deformed geometry of stent and vessel, interstitial pore fluid velocity field, and porosity field are extracted and used as input for the drug release simulation: a reaction-advection-diffusion (PAD) transport analysis calculating the spatial and temporal drug distribution. A novel feature of the framework is that stent deployment is simulated prior to simulating drug release. The advantage of this approach is that the deformed geometry and interstitial fluid velocity field are not assumed a-priori, but are actually calculated using a stent deployment simulation. The framework is demonstrated simulating a DES in an idealized, 3D vessel. Varying mechanical and transport properties based on literature review are assigned to each of the three layers in the wall and Kedem-Katchalsky conditions are explicitly modeled to account for varying porosity of the three layers. The stent deployment simulation results in a realistic pressure-radial expansion relationship of the stent and a realistic deformed geometry. The results of the drug release simulation for a period of 13 weeks show that a relative constant concentration field can be sustained. Future work includes simulating drug release with different parameters, and applying the framework to specific stent designs and realistic, image-based vessel geometries.

T1.4 Computational Modelling and Mechanobiology of Vascular Anastomosis 4884 Th, 11:00-11:15 (P42) Transmural stress during bypass surgery: A patient-specific computational analysis

F. Cacho 1,2, M. Doblar 2, G.A. Holzapfel 1,3. 1Graz University of Technology, Computational Biomechanics, Graz, Austria, 2 University of Zaragoza, Group of Structural Mechanics and Materials Modeling (GEMM), Aragon Institute of Engineering Research (13A), Zaragoza, Spain, 3Royal Institute of Technology, School of Engineering Sciences, Stockholm, Sweden

Vascular mechanics at branching locations and junctions such as the carotid sinus or bypass anastomoses have been traditionally studied by researches in the field of fluid mechanics. They mainly focused on the analysis of the complex blood flow patterns and related their alterations to clinical reports (for a recent review see [1]). These studies provide significant contributions for the explanation of, for example, certain long-term anatomical alterations observed in bypassed vessels. Frequently, in these studies the arterial wall is assumed to be rigid. The most sophisticated simulations consider (only) elastic arteries with (too) simplified constitutive models. Given the highly nonlinear behavior

of vascular tissue, and specific cases for bypass geometries in which small geometrical changes may lead to significant flow alterations, it seems to be important to consider appropriate constitutive descriptors for the arterial walls. In this work, we present the simulation of a coronary artery bypass procedure with the focus on the wall mechanics. We consider both the native artery and the venous graft. The complex, multilayered vascular morphology of an individual patient is reconstructed, and the corresponding material behavior is imposed by means of an appropriate constitutive model. The related material parameters are obtained by fitting the model to the experimental data of the (same) specimen from which the geometry was taken. Our results suggest that the transmural stresses depend heavily on the incision length, which is related to the graft insertion angle. In addition, the graft undergoes large displacements and is subject to high stresses under arterial flow conditions at sites where intimal thickening has been reported in the medical literature. Acknowledgement: Financial support for this research was partly provided by the Cooperative Research EU-Project DISHEART; Call Identifier: FP6-2002- SME-I.

References [1] E Migliavacca, G. Dubini. Computational modeling of vascular anastomoses.

Biomech Model Mechanobiol 2005; 3: 235-250.

6261 Th, 11 : 15-11:30 (P42) That the end-to-end anastomosis is superior to the end-to-side anastomosis for peripheral bypass surgery: Observations from computational studies T.P. O'Brien 1 , M.T. Walsh 1 , P. Grace 2, P.D. Devereux 1 , S.M. O'Callaghan 1 , P. Burke 2, T.M. McGIoughlin 8 . 1 Centre forApplied Biomechanical Engineering Research and Materials and Surface Science Institute, University of Limerick, Limerick, Ireland, 2Department of Vascular Surgery, Mid-Western Regional Hospital Limerick, Ireland

Introduction: The end-to-side anastomosis has traditionally been the standard choice amongst surgeons implanting peripheral artery bypass grafts. However, the end-to-side bypass graft has been characterised by poor long-term patency rates resulting in ambitious surgical endeavours to buffer graft/artery material mismatch and attempts to reduce hemodynamic abnormality at the junction. Research to date has suggested that the end-to-end anastomosis may offer significant hemodynamic and wall stress advantages, albeit with a loss in function due to elimination of the host artery proximal to the anastomosis. Materials and Methods: The objective of this study was to assess the performance advantages of the end-to-end anastomosis over that of the end- to-side anastomosis and to develop a surgical technique which would enable end-to-end suturing without loss of function. Studies of both anastomoses were performed using computational fluid dynamics (CFD) and finite element analysis (FEA). The results of these studies were then used to specify design criteria for a novel bypass graft. This novel design was then tested using the same methods to ensure that the desired performance was achieved. Results: Fluid induced wall shear stresses and pressure induced wall stresses were the primary results of interest. The end-to-side anastomosis was found to produce more abnormal hemodynamic patterns, wall shear stresses and wall stresses than the end-to-end anastomosis. Summary results from an in vive study demonstrate how the hemodynamics and wall stresses contribute to dis- ease development in the end-to-side anastomosis and how these contributory factors may be alleviated using the new bypass procedure. Conclusion: The end-to-end anastomosis offers significant hemodynamic and wall stress advantages over the conventional end-to-side anastomosis and it is possible to enable its implementation in a vascular bypass graft without loss of function.

5963 Th, 11:30-11:45 (P42) Non-Newtonian effects of pulsatile blood flow in non-planar distal end-to-side anastomosis W. Wang, Q. Wang. Medical Engineering Division, Department ef Engineering, Queen Mary, University of London, London, UK

A number of factors are known to affect flow patterns and wall shear stress (WSS) distribution in blood vessels. They include the geometry of the vessel, rheological properties of the blood and pulsatility of the circulation. In this study, non-Newtonian effects of blood are examined in a distal end-to-side anastomosis. Shear-thinning behaviour of the blood is depicted by a modified Power-Law (PL) and Carreau-Yasuda models (CY). To explore the effect of non-planarity in vessel geometry, a curved bypass graft is connected to a host tube with out-of-plane layout [1,2]. Pulsatile flow waveform in the femoral artery is applied [3]. Flow velocities, WSS and oscillatory shear index (OSI) are calculated using Fluent and softwares developed in house. Results are given at times of systolic acceleration, peak systole, systolic deceleration, end systole and peak diastole. Effects of rheological properties of the blood is analysed by comparing results to those of a Newtonian flow. Non-planarity serves to