- Email: [email protected]
Towards the development of composite synthetic vascular prostheses
THE EFFECTS
OF STENT-GRAFT STRUCTURE ON BLOOD FLOW Chuh K. Chong’, lhien V. How’, Peter L Harris’ ‘Department of Clinical Engineering, University of Liverpool, UK ‘Department of Vascular Surgery, Royal Liverpool University Hospital, UK INTRODUCTION: Endovascular stent-graft are increasingly being used for the treatment of aneurysmal and occlusive atetrial disease. The haemodynamic consequences of the introduction of such devices in the abdominal aortic aneurysm (AAA) were studied in vitro. The effects of stent-graft configurations on the general flow structures were also investigated using stent-grafts with endo- and exo-skeleton. MATERIALS AND METHODS: Four idea&d AAA models covering the range of anatomical features encountered in vivo were fabricated using CAD/CAM techniques. A commercially available stent-graft was used in the study. The Dacron graft was replaced by a thin-walled optically clear polyurethane graft. The graft was sutured in two different ways, (1) on the outside and (2) on the inside of the stem structure to form stent-grafts with endo- and exo-skeleton respectively. These stent-grafts were deployed just distal to the lowest renal ostium in each case. Studies were carried out under simulated normal physiological flow conditions with flow features reproduced by adjusting the resistances of the renal and the peripheral arteries, and the compliances of the aorta and the peripheral arteries. The motion of particles. diameter 75-15Opm suspended in the fluid made visible with planar laser light, was recorded with a video camera for subsequent analysis. RRSULTS AND DISCUSSION: In this study, stent-graft of the same design but with endo-skeleton resulted in greater flow instabilities generated within the proximal stent regions (where the highest density of stent structure occurred) and propagated downstream to the point of bifurcation. Flow reversal was observed near the lateral wall of the stent-graft during deceleration while prominent retrograde flow was observed throughout the cardiac cycle at the posterior side of the stent-graft region in the 60’ model. Regions of recirculations were common within the main trunk of the stent-graft not supported by the aortic wall where corrugations of the graft occurred. While the angle of the aortic neck also influences the flow (more asymmetrical entrance profile), a more organised flow pattern was observed within the stent-graft with exo-skeleton although features like recirculation regions, fIow reversal were also present. The construction of this particular stent-graft produces a local narrowing of the lumen of one of the limbs resulting in a region of recirculation downstream during the deceleration phase. Flow structures beyond the bifurcation were simihu in all models with flow separations and reversals being observed on the outer Wall.
CONCLUSION: The blood flow is disturbed by the presence of the stent-graft, although the degree of flow disturbance is different in stent-graft of the same design, but with dissimilar configurations. The clinical effect of these disturbauces is yet to be determined. However, the results of this study may be useful in the design of endovascular stent-grafts with better haemodynamic characteristics. CORRJBI’ONDENCEz Chuh K Chong Department of Clinical Engineering, 1st Floor, Duncan Building, University of Liverpool, L69 3BX UK. Fax: +44-151-7065803
156
I!“’ Conference
TOWARDS
TFiJZ DEVELOPMENT OF COMPOSITE SYNTHETIC VASCULAR PROSTHRSES C.H.G.A. van Oijen, H.A.W.M. van Damme, EN. van de Vosse and F.P.T. Baaijens Eindhoven University of Technology, The Netherlands
INTRODUCTION: Bypass or replacement of diseased medium-sized arteries (coronary, femoral and carotid) nowadays is mostly performed using autologous veins. Clinical practice reveals that commercially available synthetic arteries only can be used for arteries with large flow, small resistance and huge diameter (> 10 [mm]). Failure of medium-sized (< 10 [mm]) synthetic arteries is often contributed to a mechanical mismatch with the native artery (How et. al, 1992, Hofstm1995). The objective of this study is to obtain better knowledge of the mechanical properties of mediumsized arteries and finally to be able to develop good functioning synthetic vascular prostheses. METHODS: In order to characterize the mechanical properties of natural arteries, an appropriate constitutive model representing a non-linear anisotropic viscoelastic stress-strain relation is chosen as point of departure. The choice of the mode1 determines the requirements of the setup needed to obtain the parameters of the model. In this study, a setup to perform biaxial tensile tests of tubular specimen has been developed. To this end, the specimen is dynamically loaded with an internal pressure under static axial pre-strain. Jntemal pressure, internal diameter, length and longitudinal force of arteries subjected to physiological deformations can be measured. In order to manufacture synthetic vascular prostheses that mechanically mimic the properties of natural arteries, a dip-coating device employing a fibre winding procedure has been developed. Variation of matrix material and fibm-layout enables the production of composite vascular prostheses with different mechanical properties. RESULTS: Jn this study, an otthotropic Mooney-Rivlin mode1 has been adopted and is extended to incorporate linear visco-elasticity. Several tubes with a matrix EPDM rubber (DSM) and different Lycra (DuPont) dbre-layouts have been produced and characterized in the biaxial tensile testing set-up. The tubes were subjected to a dynamic pressure load and the response could reasonably be predicted by the theoretical Mooney-Rivlin hysteresis loops. CONCLUSION AND DISCUSSION: Both a device to produce composite vascular prostheses as well as a testing set-up to determine their anisotropic visco-elastic properties have been developed. The extended Mooney-Rivlin model could well be used to describe the mechanical properties of the different prostheses. Extended measurements of natural coronary or femoral arteries are needed to decide whether the composite prostheses are able to mimic their dynamic mechanical behavior. REFERENCES: 1. How, T.V. and Guidoin, R. and Young, S.K., Journal of Engineering in Medicine, 206.62-71, 1992. 2. Hofstm L., Jntimal hyperplasia in human vascular grafts, PhD. Thesis, University of Maastricht, 1995. CORRESPONDENCE: EN. van de Vosse Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; [email protected]
of the ESB, July 8-1 I 98. Toulouse,
France
OF STENT-GRAFT STRUCTURE ON BLOOD FLOW Chuh K. Chong’, lhien V. How’, Peter L Harris’ ‘Department of Clinical Engineering, University of Liverpool, UK ‘Department of Vascular Surgery, Royal Liverpool University Hospital, UK INTRODUCTION: Endovascular stent-graft are increasingly being used for the treatment of aneurysmal and occlusive atetrial disease. The haemodynamic consequences of the introduction of such devices in the abdominal aortic aneurysm (AAA) were studied in vitro. The effects of stent-graft configurations on the general flow structures were also investigated using stent-grafts with endo- and exo-skeleton. MATERIALS AND METHODS: Four idea&d AAA models covering the range of anatomical features encountered in vivo were fabricated using CAD/CAM techniques. A commercially available stent-graft was used in the study. The Dacron graft was replaced by a thin-walled optically clear polyurethane graft. The graft was sutured in two different ways, (1) on the outside and (2) on the inside of the stem structure to form stent-grafts with endo- and exo-skeleton respectively. These stent-grafts were deployed just distal to the lowest renal ostium in each case. Studies were carried out under simulated normal physiological flow conditions with flow features reproduced by adjusting the resistances of the renal and the peripheral arteries, and the compliances of the aorta and the peripheral arteries. The motion of particles. diameter 75-15Opm suspended in the fluid made visible with planar laser light, was recorded with a video camera for subsequent analysis. RRSULTS AND DISCUSSION: In this study, stent-graft of the same design but with endo-skeleton resulted in greater flow instabilities generated within the proximal stent regions (where the highest density of stent structure occurred) and propagated downstream to the point of bifurcation. Flow reversal was observed near the lateral wall of the stent-graft during deceleration while prominent retrograde flow was observed throughout the cardiac cycle at the posterior side of the stent-graft region in the 60’ model. Regions of recirculations were common within the main trunk of the stent-graft not supported by the aortic wall where corrugations of the graft occurred. While the angle of the aortic neck also influences the flow (more asymmetrical entrance profile), a more organised flow pattern was observed within the stent-graft with exo-skeleton although features like recirculation regions, fIow reversal were also present. The construction of this particular stent-graft produces a local narrowing of the lumen of one of the limbs resulting in a region of recirculation downstream during the deceleration phase. Flow structures beyond the bifurcation were simihu in all models with flow separations and reversals being observed on the outer Wall.
CONCLUSION: The blood flow is disturbed by the presence of the stent-graft, although the degree of flow disturbance is different in stent-graft of the same design, but with dissimilar configurations. The clinical effect of these disturbauces is yet to be determined. However, the results of this study may be useful in the design of endovascular stent-grafts with better haemodynamic characteristics. CORRJBI’ONDENCEz Chuh K Chong Department of Clinical Engineering, 1st Floor, Duncan Building, University of Liverpool, L69 3BX UK. Fax: +44-151-7065803
156
I!“’ Conference
TOWARDS
TFiJZ DEVELOPMENT OF COMPOSITE SYNTHETIC VASCULAR PROSTHRSES C.H.G.A. van Oijen, H.A.W.M. van Damme, EN. van de Vosse and F.P.T. Baaijens Eindhoven University of Technology, The Netherlands
INTRODUCTION: Bypass or replacement of diseased medium-sized arteries (coronary, femoral and carotid) nowadays is mostly performed using autologous veins. Clinical practice reveals that commercially available synthetic arteries only can be used for arteries with large flow, small resistance and huge diameter (> 10 [mm]). Failure of medium-sized (< 10 [mm]) synthetic arteries is often contributed to a mechanical mismatch with the native artery (How et. al, 1992, Hofstm1995). The objective of this study is to obtain better knowledge of the mechanical properties of mediumsized arteries and finally to be able to develop good functioning synthetic vascular prostheses. METHODS: In order to characterize the mechanical properties of natural arteries, an appropriate constitutive model representing a non-linear anisotropic viscoelastic stress-strain relation is chosen as point of departure. The choice of the mode1 determines the requirements of the setup needed to obtain the parameters of the model. In this study, a setup to perform biaxial tensile tests of tubular specimen has been developed. To this end, the specimen is dynamically loaded with an internal pressure under static axial pre-strain. Jntemal pressure, internal diameter, length and longitudinal force of arteries subjected to physiological deformations can be measured. In order to manufacture synthetic vascular prostheses that mechanically mimic the properties of natural arteries, a dip-coating device employing a fibre winding procedure has been developed. Variation of matrix material and fibm-layout enables the production of composite vascular prostheses with different mechanical properties. RESULTS: Jn this study, an otthotropic Mooney-Rivlin mode1 has been adopted and is extended to incorporate linear visco-elasticity. Several tubes with a matrix EPDM rubber (DSM) and different Lycra (DuPont) dbre-layouts have been produced and characterized in the biaxial tensile testing set-up. The tubes were subjected to a dynamic pressure load and the response could reasonably be predicted by the theoretical Mooney-Rivlin hysteresis loops. CONCLUSION AND DISCUSSION: Both a device to produce composite vascular prostheses as well as a testing set-up to determine their anisotropic visco-elastic properties have been developed. The extended Mooney-Rivlin model could well be used to describe the mechanical properties of the different prostheses. Extended measurements of natural coronary or femoral arteries are needed to decide whether the composite prostheses are able to mimic their dynamic mechanical behavior. REFERENCES: 1. How, T.V. and Guidoin, R. and Young, S.K., Journal of Engineering in Medicine, 206.62-71, 1992. 2. Hofstm L., Jntimal hyperplasia in human vascular grafts, PhD. Thesis, University of Maastricht, 1995. CORRESPONDENCE: EN. van de Vosse Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; [email protected]
of the ESB, July 8-1 I 98. Toulouse,
France