Properties and challenges in materials used as vascular and endovascular devices.

In the last decades, main evolutions in the field of vascular surgery have been correlated to the development of devices allowing more reliable and safe sustainable treatment. First devices that have been proposed were vascular prostheses made of polymeric materials. The second generation of devices was stents made of metals and alloys. The third generation, endografts, associated these both materials. Materials used as vascular and endovascular devices must meet a number of requirements based on dimensional, physical and mechanical criteria. Ideally, they should demonstrate a behavior as close as possible as that of human arteries in terms of mechanical properties such as compliance, long-term durability, and in terms of biological properties such as biocompatibility, luminal surface healing and thrombogenicity. We propose in the present manuscript a review of properties of materials currently used for the construction of vascular and endovascular devices, future challenges in the fields of new materials and scientific approaches and tests to understand and predict the behavior of the next generations of devices.

Mechanisms of rupture of knitted polyester vascular prostheses: an in vitro analysis of virgin prostheses.

OBJECTIVES: Previous explant retrieval studies have shown ruptures occurring on the remeshing line and the guide line of two types of warp-knitted grafts. The aim of our study was to characterize the mechanisms these ruptures. MATERIALS AND METHODS: We performed an in vitro study of the mechanical and chemical characteristics of virgin prostheses. We studied 2 virgin polyester warp-knitted grafts models: the Cooley Double Velour and the Microvel Double Velour constructed by Meadox (USA), using the following techniques: characterization and de-knitting of the textile structure, circumferential tensile strength, filament dynamometry, critical dissolution time of the filaments and scanning electron microscopy. RESULTS: Both prostheses were constructed in the same way but the texturized yarns of the Cooley graft included twice as many filaments (54) than the Microvel (27). There was more adsorbed tension in the Cooley structure than in the Microvel. The circumferential tensile strength test demonstrated that the Cooley graft always ruptured on the remeshing line and the Microvel graft always ruptured at the interface between the remeshing line and the standard line. Filament dynamometry demonstrated a heterogeneous behavior of the filaments inside the yarns, mainly at the remeshing line of the Cooley graft (27.1 cN/tex +/- 11.5% versus 26.1 cN/tex +/- 2.2% for the guide line and 28 cN/tex +/- 6.7% for the standard knit). Critical dissolution time of the filaments was significantly lower for the Microvel grafts (2.5 sec versus 17.2 sec for the Cooley). CONCLUSIONS: Rupture of knitted polyester prostheses are probably an underestimated phenomenon. They may occur at specific areas of the graft. Further studies are required to determine whether all grafts of this type are at risk.

Longitudinal ruptures of polyester knitted vascular prostheses.

AIM: The purpose of the study was the characterization of a type of rupture occurring on warp-knitted polyester vascular prostheses. MATERIALS AND METHODS: We studied 20 cases of warp-knitted polyester vascular prostheses that were explanted from humans that showed a longitudinal rupture as a part of a collaborative retrieval program. All the prostheses were immediately fixed in a 10% formaldehyde solution after their explantation in the operating room. The clinical data of these cases were recorded. The explants were photographed, washed to eliminate the surrounding tissues, and photographed again. The ruptures were characterized with macroscopic examination, optical stereomicroscopy, and scanning electron microscopy. RESULTS: The mean duration of implantation of the prostheses was 16.0 +/- 3.3 years (range, 9-20.7 years). The prostheses were Cooley Double Velour (n = 15) and Microvel Double Velour (n = 5). There were 16 aortobifemoral bypass grafts, 1 aorto-biiliac, 1 aorto-aortic, 1 iliofemoral, and 1 axillobifemoral. The longitudinal ruptures occurred on two specific parts of the prostheses: the guide line (6 cases) and the remeshing line (11 cases). In three cases both lines were affected. Scanning electron microscopy showed major degradation of the trilobar filaments of the velour and gradual ruptures of the flat filaments of the remeshing and guide lines. CONCLUSIONS: In this study, we have identified a specific mechanism of late (9-20 years) longitudinal rupture of knitted polyester prostheses consisting of degradation of the polyester filaments along the remeshing and guide lines that run the length of the graft.

Influence of crimping textile polyester vascular prostheses on the fluid flow kinetics. Groupe Européen de Recherche sur les Prothèses appliquées à la Chirurgie Vasculaire.

OBJECTIVES: to characterise the impact of the crimping of polyester prostheses on the fluid flow kinetics. DESIGN: an experimental in vitro study. MATERIALS AND METHODS: we investigated four models of polyester vascular prostheses in a continuous laminar flow circuit. The flow velocity was 80 ml/s for all experiments. We studied two fluids of different viscosity within the circuit. The speed of the particles was measured by a laser Doppler anemometer 2 to 52 mm from the prosthetic interface. We first established a calibrated flow-velocity profile corresponding to the study of the support inside the circuit without any prosthesis. We measured the velocity profiles for each prosthesis corresponding to four crimp densities obtained by stretching the grafts. RESULTS: the crimping of PET textile prostheses led to a decrease of flow velocity especially closer to the prosthetic surface. The decrease of flow velocity was dependent on the model of prosthesis. This decrease of flow velocity is described by the following negative exponential law: DeltaV=a times b(-x)where (a) is the crimp density and (b) the fluid viscosity. CONCLUSIONS: flow velocity near a prosthetic surface is influenced by the morphology of the crimping. The impact of crimping on the flow velocity in a vascular prosthesis can be predicted by computer simulation models. This may provide the optimal shape of crimping for each prosthesis.

Impregnated polyester arterial prostheses: performance and prospects.

Impregnated polyester arterial prostheses have gained wide acceptance by most vascular surgery teams, probably because these prostheses are easy to use, without any preclotting. We offer here a synthesis of the main studies that have appraised the experimental and clinical performance of these prostheses, and we delineate their major prospects.