ORIGINAL ARTICLE
J Appl Biomater Funct Mater 2014 ; 12 (3): 163 - 171 DOI: 10.5301/jabfm.5000189
Compliance properties of collagen-coated polyethylene terephthalate vascular prostheses Foued Khoffi1,2, Daniel Mathieu1,2, Florence Dieval1,2, Nabil Chakfé2,3, Bernard Durand1,2 Laboratory of Textile Physics and Mechanics, University of Haute Alsace, Mulhouse - France European Research Group on Grafts Used in Vascular Surgery (ERGGVS), Faculty of Medicine, Strasbourg - France 3 Departement of Vascular Surgery, University Hospitals of Strasbourg, Strasbourg - France 1
2
ABSTRACT Background: Compliance mismatch between native artery and a prosthetic graft used for infrainguinal bypass is said to be a factor for graft failure. The aim of this study was to develop a technique for measuring the compliance of collagen-coated polyethylene terephthalate (PET) vascular prostheses and to analyze the influence of several key properties on the elastic behavior of the grafts. Methods: Compliance testing was performed on 3 prostheses with and without internal compliant membrane (ICM). The principle of this test was to study the dimensional changes of prostheses submitted to internal pressure from 30 to 240 mm Hg at intervals of predetermined values. Results: We demonstrated that the ICM created links with the inner surface of the crimps and considerably modified the graft behavior when submitted to internal pressure. The results showed that compliance properties were dependent on the wall thickness and the crimping geometry of textile vascular prostheses. Mechanical analysis predicts the circumferential tensile behavior of these arterial grafts and validates tests for measuring compliance. Key words: Compliance, PET, Tensile properties, Vascular prostheses Accepted: April 22, 2013
INTRODUCTION Although synthetic prostheses made of polyethylene terephthalate (PET) have been most commonly used as material in the field of arterial replacement, failures or in vivo dysfunction of these grafts have been reported (1-4). Therefore, efforts are needed to optimize the production of prostheses for patients’ safety. Indeed, anastomotic aneurysm formation is a complication inducing dysfunction of these substitutes and morbidity for the patient (5, 6). This complication is related to the textile structure and often results from a difference in compliance between a native artery and a prosthetic graft (7-10). Stewart and Lyman (11) demonstrated that compliance mismatch disturbed blood circulation, leading to positive and negative gradients in the concentration profile at the distal anastomosis. It was seen that when the graft and artery radii were compared at zero pressure and at mean arterial pressure, low wall shear stresses were only observed in the former case. Thus, the distal intimal hyperplasia seen in noncompliant grafts may be caused partly by decreased wall shear
stress, and partly by concentration gradients of dissolved chemicals affecting chemotaxis of cells. In fact, compliance mismatch produces flow disturbance and increases mechanical stress near anastomic sites in flow models (10, 12-15). In addition, compliance mismatch causes stress concentration phenomena at the graft and host vessel suturing position and may induce vessel tissue overgrowth or vessel hyperplasia formation (7, 16, 17). Compliance is an index of conformity of the blood vessel to blood pressure waves, and has been conveniently defined as the vessel wall distensibility in relation to pressure pulses (18). Several methods of compliance measurement have been reported, most of which are for the measurement of radial compliance (19, 20). Wang et al reported that growth of the separation zone at the toe anastomosis may be related to longitudinal compliance (21), and Shu and Hwang demonstrated that longitudinal compliance alone can affect blood flow at the distal anastomosis of a bypass graft (22). Therefore, it is important that the 3 different kinds of compliance (radial, longitudinal and volumetric) of a vascular graft must be determined.
© 2014 Società Italiana Biomateriali - eISSN 2280-8000
163
Compliance properties of vascular prostheses
The first objective of this study was to develop a device to investigate the radial, longitudinal and volumetric compliance of arterial prostheses without internal compliant membrane (ICM), while the standard method uses an ICM (23, 24). However, any such process of measurement to be developed should consider essential experimental conditions and environmental prostheses in the human body. A second objective was to analyze the influence of several key properties (wall thickness and crimping geometry) on the elastic behavior of the graft. Various tests were performed to determine the circumferential tensile behavior and fabric properties of the prostheses.
Fig. 1 - Indeforma knitted structure and scanning electron microscopy (SEM) micrograph of prostheses.
MATERIALS AND METHODS Vascular grafts We studied 3 types of 8-mm nominal diameter PET collagen-coated prostheses (PH, PC and PHT) that are commercially available. All were warp knitted structures with different wall thickness values and crimping geometry (Tab. I). Knit fabric structure of 3 prostheses was Indeforma (Fig. 1). The prosthesis PH has a helical crimping geometry, but the prosthesis PC has a circular crimping geometry. The prosthesis PHT has a helical crimping geometry and a relatively thin wall. The prostheses thickness was evaluated by using the compression and thickness device of Kawabata system. This test and the test of water permeability were completed in accordance with the standard ISO 7189 Cardiovascular Implants – Tubular Vascular Prostheses (25). Tensile tests The circumferential tensile tests were carried out in accordance with the standard ISO 7189 Cardiovascular Implants – Tubular Vascular Prostheses (25). Five samples from the same prosthesis material were tested (different locations) by using a Adamel Lhomargy dynamometer (MTS/20) controlled by the software Test Works 4. The device consists of 2 stainless steel plate assemblies,
TABLE I - CHARACTERISTICS OF PROSTHESES STUDIED Prosthesis
PHT
PH
PC
Wall thickness (mm)
0.32±0.04
0.7±0.05
0.9±0.06
Crimping geometry
Helical
Helical
Circular
J Appl Biomater Funct Mater 2014 ; 12 (3): 163 - 171 DOI: 10.5301/jabfm.5000189
Compliance properties of collagen-coated polyethylene terephthalate vascular prostheses Foued Khoffi1,2, Daniel Mathieu1,2, Florence Dieval1,2, Nabil Chakfé2,3, Bernard Durand1,2 Laboratory of Textile Physics and Mechanics, University of Haute Alsace, Mulhouse - France European Research Group on Grafts Used in Vascular Surgery (ERGGVS), Faculty of Medicine, Strasbourg - France 3 Departement of Vascular Surgery, University Hospitals of Strasbourg, Strasbourg - France 1
2
ABSTRACT Background: Compliance mismatch between native artery and a prosthetic graft used for infrainguinal bypass is said to be a factor for graft failure. The aim of this study was to develop a technique for measuring the compliance of collagen-coated polyethylene terephthalate (PET) vascular prostheses and to analyze the influence of several key properties on the elastic behavior of the grafts. Methods: Compliance testing was performed on 3 prostheses with and without internal compliant membrane (ICM). The principle of this test was to study the dimensional changes of prostheses submitted to internal pressure from 30 to 240 mm Hg at intervals of predetermined values. Results: We demonstrated that the ICM created links with the inner surface of the crimps and considerably modified the graft behavior when submitted to internal pressure. The results showed that compliance properties were dependent on the wall thickness and the crimping geometry of textile vascular prostheses. Mechanical analysis predicts the circumferential tensile behavior of these arterial grafts and validates tests for measuring compliance. Key words: Compliance, PET, Tensile properties, Vascular prostheses Accepted: April 22, 2013
INTRODUCTION Although synthetic prostheses made of polyethylene terephthalate (PET) have been most commonly used as material in the field of arterial replacement, failures or in vivo dysfunction of these grafts have been reported (1-4). Therefore, efforts are needed to optimize the production of prostheses for patients’ safety. Indeed, anastomotic aneurysm formation is a complication inducing dysfunction of these substitutes and morbidity for the patient (5, 6). This complication is related to the textile structure and often results from a difference in compliance between a native artery and a prosthetic graft (7-10). Stewart and Lyman (11) demonstrated that compliance mismatch disturbed blood circulation, leading to positive and negative gradients in the concentration profile at the distal anastomosis. It was seen that when the graft and artery radii were compared at zero pressure and at mean arterial pressure, low wall shear stresses were only observed in the former case. Thus, the distal intimal hyperplasia seen in noncompliant grafts may be caused partly by decreased wall shear
stress, and partly by concentration gradients of dissolved chemicals affecting chemotaxis of cells. In fact, compliance mismatch produces flow disturbance and increases mechanical stress near anastomic sites in flow models (10, 12-15). In addition, compliance mismatch causes stress concentration phenomena at the graft and host vessel suturing position and may induce vessel tissue overgrowth or vessel hyperplasia formation (7, 16, 17). Compliance is an index of conformity of the blood vessel to blood pressure waves, and has been conveniently defined as the vessel wall distensibility in relation to pressure pulses (18). Several methods of compliance measurement have been reported, most of which are for the measurement of radial compliance (19, 20). Wang et al reported that growth of the separation zone at the toe anastomosis may be related to longitudinal compliance (21), and Shu and Hwang demonstrated that longitudinal compliance alone can affect blood flow at the distal anastomosis of a bypass graft (22). Therefore, it is important that the 3 different kinds of compliance (radial, longitudinal and volumetric) of a vascular graft must be determined.
© 2014 Società Italiana Biomateriali - eISSN 2280-8000
163
Compliance properties of vascular prostheses
The first objective of this study was to develop a device to investigate the radial, longitudinal and volumetric compliance of arterial prostheses without internal compliant membrane (ICM), while the standard method uses an ICM (23, 24). However, any such process of measurement to be developed should consider essential experimental conditions and environmental prostheses in the human body. A second objective was to analyze the influence of several key properties (wall thickness and crimping geometry) on the elastic behavior of the graft. Various tests were performed to determine the circumferential tensile behavior and fabric properties of the prostheses.
Fig. 1 - Indeforma knitted structure and scanning electron microscopy (SEM) micrograph of prostheses.
MATERIALS AND METHODS Vascular grafts We studied 3 types of 8-mm nominal diameter PET collagen-coated prostheses (PH, PC and PHT) that are commercially available. All were warp knitted structures with different wall thickness values and crimping geometry (Tab. I). Knit fabric structure of 3 prostheses was Indeforma (Fig. 1). The prosthesis PH has a helical crimping geometry, but the prosthesis PC has a circular crimping geometry. The prosthesis PHT has a helical crimping geometry and a relatively thin wall. The prostheses thickness was evaluated by using the compression and thickness device of Kawabata system. This test and the test of water permeability were completed in accordance with the standard ISO 7189 Cardiovascular Implants – Tubular Vascular Prostheses (25). Tensile tests The circumferential tensile tests were carried out in accordance with the standard ISO 7189 Cardiovascular Implants – Tubular Vascular Prostheses (25). Five samples from the same prosthesis material were tested (different locations) by using a Adamel Lhomargy dynamometer (MTS/20) controlled by the software Test Works 4. The device consists of 2 stainless steel plate assemblies,
TABLE I - CHARACTERISTICS OF PROSTHESES STUDIED Prosthesis
PHT
PH
PC
Wall thickness (mm)
0.32±0.04
0.7±0.05
0.9±0.06
Crimping geometry
Helical
Helical
Circular
