VASCULAR PROSTHESIS

Abstract: A vascular prosthesis (20) comprises a flexible tube which accommodates pulsatile flow by increasing its cross-sectional area by deformation of its cross-sectional shape.

Description: VASCULAR PROSTHESIS This invention concerns a vascular prosthesis. Conventional vascular prostheses have a circular cross-section. A major problem of such prostheses, once implanted in a patient, is the risk of the prosthesis kinking, causing occlusion of the vessel and blood flow to cease. This causes obvious problems for the patient and may even cause death. Vascular prostheses accomodate pulsatile flow by transiently increasing in cross-sectional area under arterial pressure and this is achieved by uniform extension of the circular prosthesis wall. The measure of the ability of the material of the vessel wall to permit this extension under pressure is the compliance of the artificial vessel. The formation of perigraft fibrosis following implantation will considerably reduce compliance by imparing the ability of the material of the vessel wall to extend under pressure or by resisting radial extension. The present invention provides vascular prostheses which overcome, to some extent, the problems aforesaid. According to the invention there is provided a vascular prosthesis comprising a flexible tube which accomodates pulsatile flow by increasing its cross- sectional area by deformation of the cross-sectional shape. The prosthesis may have a non-circular cross- section when unpressurised or under diastolic pressure. The non-circular cross-section may have rotational symmetry. The cross-sectional shape of the prosthesis may be a tricuspid epitrochoid. The cross-sectional area of the prosthesis may be a deltoid. The lumen of the vascular prosthesis may be designed so that it can never totally occlude and may thereby be kink resistant (By "never" is meant, of course, under the most extreme conditions it will normally encounter - any tube may ultimately be squashed flat by a large enough force) . The vascular prosthesis may comprise a bio- compatible material, which may be an elastomer, such as a segmented polyurethaneurea. According to the invention there is also provided a method of making a vascular prosthesis by coagulation casting onto a profiled mandrel a solution of coagulatable polymer dissolved in a solution comprising an organic solvent. The polymer solution may contain a pore-forming agent, such as sodium hydrogen carbonate, soluble in a coagulant for the solution to leave a porous cast. The pore-forming agent may be ground to an average particle size of 60 microns, and may be present in an amount between 10 and 60 percent by weight. The polymer solution may contain a surfactant, which may be present in an amount between 1 and 10 percent by weight. The polymer may comprise polyurethane, which may be a linear segmented poly(ether)urethane with a number average molecular weight in the region 20,000 to 60,000. The solvent may be aprotic, and may comprise N,N-Dimethylacetamide or N,N-Dimethylformamide. The concentration of polymer in the solution may be between 10 and 30 grams/decilitre. The mandrel may have a tricuspid epitrochoid or a deltoid cross-section. The mandrel may be dip coated in the polymer solution but in a preferred method of production, the polymer solution is extruded through an extrusion head onto the mandrel to provide a prosthesis of uniform wall thickness. The invention will be further apparent from the following description with reference to the figures of the accompanying drawings, which show by way of example only, two forms of the vascular prosthesis embodying same. Of the drawings :- Figure 1 shows a cross-section through a first embodiment of the invention; Figure 2 shows a cross-section of a second embodiment of the invention; and Figure 3 shows cross-sections of the mandrels used to produce the prostheses of Figures 1 and 2. Referring now to the drawings, Figures 1 and 2 illustrate, in cross-section, vascular prostheses comprising flexible tubes which accomodate pulsatile flow by increasing their cross-sectional area by deformation of their cross-sectional shape. In both cases, the prostheses have non-circular cross-sections as illustrated when unpressurised or under diastolic pressure. Figure 1 illustrates a tricuspid epitrochoid cross-section; Figure 2 illustrates a deltoid cross-section. Both vascular prostheses have an increased cross-sectional area over conventional vascular prostheses having circular cross sections, as demonstrated in the following Tables. Table 1, below, shows the percentage gains in cross-sectional area of the vascular prostheses of Figure 1 as the epitrochoid shape becomes more exaggerated, ie. as the arc angle θ 21 decreases. TABLE 1 Arc Angle θ (radians) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 The increase in cross-sectional area is calculated on the basis of the difference between the cross-sectional areas of an epitrochoid and a circle whose perimeter and circumference, respectively, are equal. - Table 2, below, shows the percentage area gains as the rounding index, N, increases, N being the ratio of the internal radius 22 over the external radius 23 of the vascular prosthesis of Figure 2. TABLE 2 N % area increase 0.2 80.8 0.4 47.5 0.6 35.1 0.8 28.7 1.0 24.8 2.0 17.1 3.0 14.6 4.0 13.3 5.0 12.6 10.0 -11.1 The increase in cross-sectional area is calculated on the basis of the difference between the cross-sectional areas of a deltoid and a circle whose perimeter and circumference, respectively, are equal. In addition to the vascular prostheses of Figures 1 and 2 having an increased cross-sectional area and thus increased volume compliance over conventional prostheses, these nori-circular prostheses are extremely kink-resistant because, however much bent z the lumen 24 of these prostheses will never totally occlude. The property of increased volume compliance of these non-circular prostheses relies primarily upon bending of the vessel wall as well as stretching it, ie. on flexural rigidity in addition to elasticity. The prosthesis 20, is made from a flexible material which is capable of bending or stretching under normal blood pressures and is suitable for long term implantation, ie, is bio-compatible. Examples of such materials include elastomers, polytetrafluoroethylene (PTFE) or a combination of elastomer and PTFE. Preferred elastomers include polyurethane, polyurethaneurea, segmented polyurethanes and segmented polyurethaneureas. These materials give the prosthesis 20 the property of tending to be r self-sealing' after needle puncture, even after multiple needle puncture. A solution of coagulatable polymer, such as a linear segmented polyetherurethane with a number average molecular weight of 20,000 to 60,000, is dissolved in a solution comprising an organic solvent, such as N,N-Dimethyl- acetamide or N,N-Dimethylformamide, at a concentration of polymer in the solution of between 10 and 30 grams/decilitre. In addition, the polymer solution contains a pore-forming agent, soluble in a coagulant to leave a porous cast, such as sodium hydrogen carbonate ground to an average particle size of 60 microns in an amount between 10 and 60 percent by weight. The polymer solution also contains a surfactant, such as sodium dodecyl sulphate, in an amount between 1 and 10 percent by weight. A prosthesis is produced either by dip coating a mandrel in the polymer solution, or, to more easily produce a prosthesis having a uniform wall thickness, by extruding the polymer solution through an extrusion head as described in GB-A-2,204,873. The polymer-coated mandrel is then immersed in a coagulant and allowed to coagulate. The preferred coagulant is water which is maintained at a constant temperature throughout the coagulation process, usually 40°C. The coagulation process normally takes 1 to 2 hours. The mandrel is not limited in its cross-section. Figure 3 shows two examples of mandrels. Mandrel 25 has a tricuspid epitrochoid cross-section and is used to produce the prosthesis of Figure 1. Mandrel 26 has a deltoid cross-section and is used to produce the prosthesis of Figure 2. It will be appreciated that it is not intended to limit the invention to the above example only, many variations, such as might readily occur to one skilled in the art, being possible, without departing from the scope thereof as defined by the appended claims.

Claims: CLAIMS 1. A vascular prosthesis comprising a flexible tube which accomodates pulsatile flow by increasing its cross- sectional area by deformation of its cross-sectional shape. 2. A vascular prosthesis according to claim 1, which has a non-circular cross-section when unpressurised or under diastolic pressure. 3. A vascular prosthesis according to claim 2, wherein the non-circular cross-section has rotational symmetry. 4. A vascular prosthesis according to claim 3, wherein the cross-section is a tricuspid epitrochoid. 5. A vascular prosthesis according to claim 3, wherein the cross-section is a deltoid. 6. A vascular prosthesis according to any one of claims 1 to 4, wherein the lumen will not totally occlude, thereby preventing kinking. 7. A vascular prosthesis according to any preceding claim, comprising bio-compatible material. 8. A vascular prosthesis according to claim 7, wherein said material comprises an elastomer. 9. A vascular prosthesis according to claim 8, wherein said elastomer comprises a segmented poly¬ urethane urea. 10. A method of making a vascular prosthesis by coagulation casting onto a profiled mandrel a solution of coagulatable polymer dissolved in a solution comprising an organic solvent. 11. A method according to claim 10, wherein the polymer comprises polyurethane. 12. A method according to claim 11, wherein the polyurethane is a linear segmented poly(ether)urethane with a number average molecular weight in the region 20,000 to 60,000. 13. A method according to any one of claims 10 to 12, wherein the solvent is aprotic. 14. A method according to claim 13, wherein the solvent comprises N,N-Dimethylacetamide or N,N-Dimethyl- formamid . 15. A method according to any one of claims 10 to 14, wherein the concentration of polymer in the solution is between 10 and 30 grams/decilitre. 16. A method according to claim 10, wherein the mandrel has a tricuspid epitrocohoid cross-section. 17. A method according to claim 10, wherein the mandrel has a deltoid cross-section. 18. A method according to any preceding claim, wherein the mandrel is dip coated in the polymer solution to provide a prosthesis of uniform wall thickness. 19. A method according to claims 10 to 18, wherein the polymer solution is extruded through an extrusion head onto the mandrel to provide a prosthesis of uniform wall thickness.

Inventors: CHARLESWORTH DAVID, CHIAN KERM SIN, UNDERWOOD CHRISTOPHER JOHN, CHARLESWORTH, DAVID, CHIAN, KERM, SIN, UNDERWOOD, CHRISTOPHER, JOHN
Applicants: NEWTEC VASCULAR PRODUCTS LIMIT, NEWTEC VASCULAR PRODUCTS LIMITED
Application year: 1992
Main classification: A61L27
IPC codes: A61F2, A61L27, B29C39, B29C41, B29C47, B29C67