WO1996032077A1

WO1996032077A1 - Artificial graft prosthesis

Info

Publication number: WO1996032077A1
Application number: PCT/AU1996/000209
Authority: WO
Inventors: David M. Stump; Vincent G. Hart
Original assignee: The University Of Queensland
Priority date: 1995-04-11
Filing date: 1996-04-11
Publication date: 1996-10-17
Also published as: AUPN228395A0

Abstract

There is provided an artificial graft prosthesis (100), having a tubular elastic body defining a lumen (105), wherein at least a portion of the body comprises a plurality of longitudinal corrugations (120). The artificial graft prosthesis of the invention is useful in surgery in mammals, including humans, especially in the formation of grafts in vascular tissue such as the formation of an arteriovenous fistula for haemodialysis access.

Description

Artificial Graft Prosthesis

Technical Field

The present invention relates to artificial graft prostheses, and in particular artificial vascular graft prostheses. The invention also relates to the use of artificial graft prostheses in surgery, especially in the treatment of humans.

Background Art

The successful clinical application of artificial vascular grafts has hitherto been limited to vessels with lumen diameters of 6 mm or greater. Smaller diameter grafts are associated with disappointing patency rates, with failure often due to acute thrombosis resulting from the development of neointimal hyperplasia at or near the anastomoses of the graft to the natural artery. A number of causes have been postulated for the development of neointimal hyperplasia, including thrombogenicity of the graft material, haemodynamic turbulence, and the stresses in the tissue resulting from the mismatch in circumferential compliance between the tissue and the graft (which can be large). As an illustration of the difference in circumferential compliance which can exist between human vascular tissue and the material of a typical prosthetic graft of the prior art, it is known that under normal pulsatile flow conditions the human carotid arterial radius distends approximately 5% at a systolic pressure of 120 mm of Hg by a circumferential stretching (or "breathing") axially symmetrical mode of deformation. However, a typical polyester or polytetrafluoroethylene (PTFE) cylindrical graft possesses a circular cross-section which may only stretch elastically in the circumferential direction by about 1 % at blood pressures of between 80-120 mm of Hg. Other graft materials may stretch elastically to different extents, which may be higher or lower than the extent of distension of the arterial radius. As a result of the mismatch in circumferential stretching stiffnesses between an artery and grafts of the prior art, pulsatile flow in the artery is significantly impacted at the anastomoses of conventional cross-section grafts, with the reflection of elastic waves in the tissue and the development of turbulent blood flow as a pulse enters and exits the graft. It is conjectured that adjacent arterial tissue is damaged over time by both of these effects and that this results in the development of neointimal hyperplasia.

In general the problem of reducing the level of circumferential stresses near the anastomoses of a vascular graft has been addressed in two ways: by design modifications to existing structures, and by the development of new materials with elastic constitutive laws that approximate those of a vascular tissue of interest. For example, U.S. Patent No. 4,938,740 discloses elliptical rather than the conventional circular graft-arterial anastomoses as a method of stress reduction. This is accomplished by joining the artery and the graft in a skew plane (i.e. not perpendicular) to the central axis. The rationale for the modification is that an elliptical ring is more compliant to deflection than a circular ring when subjected to an internal pressure. However, this design is of limited effectiveness since the cylindrical graft adjacent to the anastomoses is a three-dimensional cylindrical shell that remains much more rigid to stretching than the arterial tissue and the graft may not provide sufficient flexibility over a large enough region adjacent to the anastomoses due to the inextensibility of the cylindrical shell.

U.S. Patent No. 4,969,896 discloses a circular cross-section graft with ribs running along the exterior in the longitudinal direction in order to promote bending flexibility between the ribbed segments. This design has several disadvantages: it depends upon biocompatible silicone-based materials; the method of manufacture is extremely difficult; and the adhesive fixture of the ribs onto the interior cylinder is a site for fatigue failure.

The second approach to solving the circumferential mismatch problem has been the development of an artificial material that displays the elastic constitutive characteristics of arterial tissue and also has good biocompatibility. For example, Kira (U.S. Patent No. 4,954,127) has proposed a process for constructing a polystyrene tube with stress-strain behaviour that is similar to vascular tissue. While the analysis presented in U.S. Patent No. 4,954,127 is based on strain levels that greatly exceed 5%, it is claimed that limited canine tests with the graft have shown patency of over two months and excellent anti-thrombogenetic properties.

This approach suffers from the disadvantage that it requires the certification by regulatory authorities, such as the United States Food and Drug Administration, of the new artificial material for implantation. The process of obtaining certification for a new material for implantation can take a long time, typically a number of years, and can be expensive.

Accordingly there is a need for an improved artificial graft prosthesis for vascular and other tissue, particularly one made from an approved material.

It is an object of the present invention to provide an artificial graft prosthesis which ameliorates one or more of the disadvantages of vascular grafts of the prior art.

Disclosure of the Invention

Surprisingly, the present inventors have discovered that the greater compliance of a graft prosthesis having a plurality of longitudinal corrugations along at least a part of its length, compared to existing circular cross-section grafts, can reduce elastic mismatch between the graft and tissue into which it is inserted while not altering the blood flow in a deleterious way.

The present invention therefore provides a novel prosthetic graft type which may be adapted for use with a variety of tissues. According to a first broad form of the invention, there is provided a graft prosthesis having a tubular elastic body defining a lumen, wherein at least a portion of the body comprises a plurality of longitudinal corrugations.

According to a second broad form of the invention, there is provided a graft prosthesis for grafting to a living tissue in a mammal, the graft prosthesis having a tubular elastic body defining a lumen, the graft prosthesis being configured so as to provide an anatomically acceptable match between the stiffness of the graft prosthesis and the stiffness of the tissue.

Typically, in this form of the invention the graft prosthesis is configured by having a plurality of longitudinal corrugations along at least a part of its body. Usually, the anatomically acceptable match of stiffnesses is such that the stress in tissue adjacent to the graft prosthesis, when the graft prosthesis has been grafted to the tissue, is not elevated to a degree at which pathological changes are induced in the tissue adjacent to the graft, either proximally or distally. In one embodiment of the invention, the corrugations extend over a part of the length of the body of the graft prosthesis. In this embodiment, the corrugations typically extend from an end of the graft prosthesis and usually become shallower with increasing distance from the end until they merge with a portion of the body of the graft prosthesis which is non-corrugated. Alternatively, the corrugations may end abruptly at the portion of the body of the graft which is non-corrugated. The non-corrugated portion is typically substantially circular or elliptical, more typically substantially circular in cross-section.

Generally in this embodiment, corrugations extend from both ends of the graft prosthesis.

More generally, corrugations extend from both ends of the graft prosthesis and merge into a medial portion of the graft prosthesis which is non-corrugated and substantially circular or elliptical in cross-section.

In another form of this embodiment, the corrugations are remote from the ends of the graft prosthesis. For example, the corrugations in this form may extend over a medial portion of the body of the graft prosthesis and merge into non-corrugated portions at both ends or terminate abruptly where the non-corrugated portions begin. In this form also, the non-corrugated portions typically have a substantially circular or elliptical cross- section.

In an alternative embodiment, the corrugations extend the entire length of the graft prosthesis.

Generally, the elastic body of the graft prosthesis of the invention is resilient and/or flexible.

Where the body of the graft prosthesis of the invention comprises a non-corrugated portion, this may be provided with additional external support, for example in the form of one or more reinforcing ribs or an external reinforcing wrap, to provide resistance to kinking. As indicated above, the amplitude of the corrugations may vary along their length. That is, they may become shallower or deeper with distance from an end of the corrugated portion. Alternatively, the corrugations may be of uniform depth for their complete length. Similarly, the width of the corrugations may be substantially constant over the length of the corrugated portion, or it may vary. For example, the corrugations may become more widely spaced apart, or more narrowly spaced apart, with distance from an end of the corrugated portion of the body of the graft prosthesis.

Typically the lumen of the graft prosthesis has a longitudinal axis. More typically, the corrugations are parallel to the longitudinal axis of the lumen, or they are twisted relative to the longitudinal axis so as to define a portion of a curve such as a helix. The corrugations may be arranged symmetrically or asymmetrically around the body of the graft prosthesis. For example, the corrugations may differ in width or amplitude. Typically, however, the corrugations are arranged symmetrically around the body of the graft prosthesis. The lumen of the graft prosthesis according to the invention may have a diameter which is substantially the same along the entire length of the body of the graft prosthesis, or its diameter may vary. Typically, the lumen of the graft prosthesis tapers from one end to the other, or it tapers from each of its ends towards a medial portion of the lumen. Alternatively, the lumen of the graft prosthesis may widen from either end towards a medial portion. Where the lumen tapers or widens, the tapering or widening may be either symmetrical or non-symmetrical about the midpoint of the longitudinal axis of the body of the graft prosthesis.

Typically, the thickness of the tubular body of a graft prosthesis of the invention is in the range of from about 0.1 to about 1 mm, more typically 0.15 to 0.9 mm, still more typically 0.2 mm to 0.8 mm, yet more typically 0.25 mm to 0.7 mm, even more typically about 0.35 to about 0.5 mm. The thickness of the tubular body of the graft prosthesis may be substantially constant over its length, or it may vary. For example the body of the graft prosthesis may thicken or thin substantially uniformly from its ends to its centre or from one end to the other. The body of the graft prosthesis of the invention may further comprise a portion having a plurality of circumferential corrugations; that is, corrugations which extend around the circumference of the body of the graft prosthesis rather than along the body of the graft prosthesis. Typically, the circumferential corrugations are located in a portion of the body which has no longitudinal corrugations and are provided to confer a greater degree of radial distensibility to the graft prosthesis so that it may be bent more easily and with less distortion than may a graft prosthesis without circumferential corrugations. Usually, the number of circumferential corrugations ranges from about 3 to 30, more usually from 3 to about 20, still more usually from 3 to about 10. The number and depth of the longitudinal corrugations is selected depending on the diameter and stiffness of the tissue into which the prosthesis is to be grafted and the amount of expansion it is desired for the graft prosthesis to accommodate. Typically, the number of longitudinal corrugations ranges from 3 to about 30, more usually from about 3 to about 20, still more usually from about 3 to about 12, even more usually from about 4 to about 8. The amplitude (depth) of the longitudinal corrugations, as explained below, is usually a function of the number of corrugations but generally does not exceed about 20% of the radius of the lumen of the graft prosthesis. The basis on which the number and depth of the longitudinal corrugations may be selected is described in more detail below.

The graft prosthesis of the invention is typically an arterial or venous graft prosthesis. However, the graft prosthesis of the invention may also be used in other tissues, such as tissues of the digestive tract, the urinary tract, the reproductive tract and the respiratory tract, etc. The graft prosthesis of the invention will usually be utilised in surgery for the treatment of a human patient, but may also be used for the treatment of other mammals such as horses, cattle, sheep, goats, pigs, dogs or cats.

Thus, according to a third broad form of the invention there is provided the use of a graft prosthesis of the present invention in surgery in a mammal.

The graft prosthesis of the present invention may be attached by means of one or more sutures to the tissue to be treated by methods generally known in the art. For example, it may be attached to the tissue in end-to-end splice arrangement. More typically, the graft prosthesis will be attached to the tissue in a sideways configuration to form an alternate pathway. Generally, the graft prosthesis will be sutured in an approximately elliptical anastomosis. Thus, the graft prosthesis of the invention will typically be cut at approximately 45° at each end and sutured into place to form a side-to- end anastomosis, for example an arteriovenous fistula for haemodialysis access. It will be understood, however, that the graft prosthesis of the invention may be used in other treatments, including but not limited to the formation of a coronary artery bypass, a femoral artery bypass or a renal artery bypass. In a further broad form of the present invention there is therefore provided a method of forming a graft in a tissue of a mammal in need thereof, comprising surgically attaching a graft prosthesis of the invention to the tissue. As noted above, the tissue is typically a vascular tissue but may be other tissue such as, for example, tissue of the digestive tract, the urinary tract, the reproductive tract or the respiratory tract. The graft prosthesis of the invention may be manufactured from any material known to be compatible with the tissue into which the graft is to be inserted. Examples of suitable materials are knitted fabrics, woven fabrics, polymeric materials and combinations thereof, laminated materials, and biologically coated materials (for example collagen infiltrated fabrics). The graft prosthesis of the invention may also be manufactured from a layered or single sheet polymeric or non-polymeric material fabricated in a woven or non-woven form. As a further possibility, the graft prosthesis may comprise two or more segments constructed from different materials and joined together endwise. Generally, the material from which the graft prosthesis is manufactured is Dacron polyester, polyurethane, or polytetrafluoroethylene (PTFE).

The graft prosthesis of the invention may be manufactured by methods generally known for the production of graft prostheses. Suitable such methods include casting, extrusion, injection moulding, electrosp inning, weaving and knitting. Typically, the graft prosthesis has an external coating such as an external coating known for use with graft prosthesis of the prior art. Usually, the graft prosthesis of the invention is manufactured by casting in a mould of appropriate shape.

The graft prosthesis of the present invention provides a number of advantages over previously known graft prostheses. Existing prostheses, exemplified by the prosthesis disclosed in U.S. Patent No. 4,933,740, which proposes to splice a graft prosthesis directly between two arterial segments at an oblique angle, and other conventional grafts of either cylindrical or concertina structure, only allow very limited circumferential bending deformation of the prosthesis due to the elliptic anastomoses. However, since the base cross-sectional shape of these grafts is still cylindrical, the region where significant bending is possible is relatively small, typically on the order of a fraction of one lumen radius. Outside this region, the structure must deform in the more rigid stretching mode that is associated with the circular cross-section. As a result, any reduction in tissue stress at the anastomoses is severely limited.

By contrast, the graft prosthesis of the present invention takes advantage of the relative weakness of a noncircular cross-section to bending type deformations which allows the graft prosthesis to possess a structural stiffness that more closely approximates that of an existing vessel. The advantage of this type of graft prosthesis over those of the prior art is in the longitudinally oriented corrugations which offer a much reduced stiffness in comparison to existing grafts, due to the increased ability for cross-sectional bending down a longer portion of the graft from the anastomoses. These features can act to further reduce hoop stress elevation in tissue near the anastomoses.

In addition, the longitudinal corrugations in the direction of the flow in a graft prosthesis of the present invention can reduce the turbulent shear drag on the fluid flowing through the prosthesis. The distensibility of the walls of the graft of the invention delays the onset of turbulence, and any turbulent shear drag which occurs will be reduced by the presence of the corrugations.

Brief Description of the Drawings

Figure 1 is a graph illustrating the cross-sectional profiles of a number of rings having four corrugations of varying depth. Figure 2 is a graph in which the internal pressure in the rings illustrated in Figure 1 is plotted against the area contained within the rings.

Figure 3 is a graph in which the initial slopes of the curves shown in Figure 2 are plotted against a parameter related to the depths of the corrugations shown in Figure 1 for various numbers of corrugations in the ring.

Figure 4 is a perspective view of a graft prosthesis in accordance with the invention.

Figure 5A and Figure 5B are two examples of possible cross-sections of a corrugated portion of a graft prosthesis according to the invention. Figure 6 is a perspective view of an alternative graft prosthesis in accordance with the invention.

Figure 7 is a diagrammatic representation of another graft prosthesis in accordance with the invention, having longitudinal corrugations at either end and a medial portion having a circular cross-section. Figure 8 is a perspective view of another graft prosthesis in accordance with the invention, having longitudinal corrugations at either end and a medial region comprising circumferential corrugations.

Figure 9 is a diagrammatic representation of a further alternative graft prosthesis according to the invention, having a central region with longitudinal corrugations and having non-corrugated regions at either end, the graft prosthesis tapering from either end to the centre.

Figure 10 is a diagrammatic representation of a graft prosthesis similar to that illustrated in Figure 9 but having a greater number of corrugations in the central region. Figure 11A and 11B are representations of an end cross-section and a central cross-section of the graft prosthesis illustrated in Figure 10.

Figure 12A and 12B are diagrammatic representations of two possible profiles for the corrugations of a graft prosthesis in accordance with the invention.

Figure 13 is a perspective view of yet a further alternative graft prosthesis in accordance with the invention, having two sections such as illustrated in Figure 9, with a section comprising circumferential corrugations between them.

Best Mode and Other Modes of Carrying out the Invention

The graft prosthesis of the invention has a body including a portion having longitudinal corrugations which provide an improved circumferential compliance. Typically, the number and depth of the corrugations are selected in order to yield a substantially improved circumferential compliance compared to previously known graft prostheses, where the circumferential compliance is defined as the derivative of the internal pressure p with respect to the normalised area increase dA/A. That is, the circumferential compliance may be expressed as A dp/dA. The corrugations in the body of the graft prosthesis of the invention form a series of "ridges" and "valleys" running longitudinally for at least a portion of the length of the graft. The number and amplitude of the corrugations are typically selected so as to provide an acceptable match between the stiffness of the graft prosthesis and the stiffness characteristics of the tissue into which the prosthesis is to be grafted for the proposed application.

As discussed above, conventional vascular prosthetic grafts that are fabricated in the shape of circular cylinders or concertina-type cylinders (that is having circumferential corrugations only) can suffer from excessive stiffness in the circumferential direction. This is particularly the case for grafts with lumen diameters less than 6 mm. The mismatch in circumferential stiffness between the tissue and the prosthetic graft results in a significant elevation of the circumferential stresses in the tissue adjacent to the anastomoses. These stresses may be up to two to three times the values which wou^d be present in the intact homogeneous tissue. Importantly, the stress elevation ,s an increasing function of the ratio of prosthetic to tissue stiffness. Any design or material alterations which can reduce the structural stiffness of the graft in proximity to the anastomoses will reduce the degree of stress elevation in the adjacent vascular tissue and should promote a smoother blood flow pattern.

The graft prosthesis of the present invention reduces the stiffness mismatch in the circumferential direction of the graft by promoting the flexure displacement of the prosthesis in the radial direction through a bending mode of deformation as opposed to a stretching mode. This may be illustrated by considering the inflation of a bellows-shaped ring which approximates the cross-section of the graft prosthesis of the invention. Figure 1 shows a family of thin planar rings subjected to an internal pressure p. (It will be appreciated that the shapes shown are only some of an infinite number of possible cross- sectional shapes having four corrugations around the circumference, and have been selected for illustrational purposes only.) The parameters X_Q and yø are respectively the positive abscissa and ordinate intercept of the base circular shape. The radius of the base circular shape is a. The wall thickness t of the rings is assumed very much smaller than a, which is typically the case in practice. The parameter c controls the deviation of the unstressed ring shape from circular, and n is the number of symmetric corrugations. The value c = 0 corresponds to a circle. All the shapes illustrated in Figure 1 enclose the same area πα².

It can be shown that the resistance of the rings to stretching is many times greater than the resistance to bending. Specifically, the ratio of stretching to bending resistances is proportional to (alt)², which can often exceed 100. Noncircular rings show a two stage deformation process: bending followed by stretching. When the noncircular rings shown in Figure 1 are inflated by uniform internal pressure, they first expand out in bending deformation until they are almost circular. Next, they deform by stretching in a "breathing mode" of expansion through straining in a tangential direction, which is the only deformation mode of the initially circular ring. The difference between bending and stretching resistances means that the first stage bending deformation of the circular ring occurs at pressure levels that of order (t/ά)² compared with the pressures for the stretching deformation.

If the rings of Figure 1 are viewed as inextensible, that is incapable of stretching in the tangential direction (a reasonable assumption in may situations), then the response during the bending stage of deformation may be analysed mathematically. Let the coordinate s measure the length in the tangential direction from the intersection of the ring dy with the jc-axis, and define the tangent angle φ, where tan(φ) = — and where the shape of dx the ring is described in terms of the Cartesian coordinates x(s) and y(s) fixed at the centre of the ring.

It can be shown that a normalised set of equations which govern the deflection of the rings is φ^m'φ^,-φ"^,φ^M+φ"{α +(φ')³ -_Jp}-α"(φ')³ -φ'α""= 0,

y= sin(φ), where the symbol ' denotes differentiation with respect to s and α'(_>) is the curvature of the unstressed ring. All lengths are normalised by the circular ring radius a, and the forces are normalised by B/a, where B is the bending rigidity per unit thickness (units of N-m). Integration of these equations with the appropriate boundary conditions for given stress free shapes and fixed values of/? provides a series of curves depending on the value of c, such as shown in Figure 2, in which B is the bending rigidity per unit thickness of the material of the graft prosthesis (in units of N-m) and A is the area enclosed by the expanded ring.

For c = 0, the circular ring is almost rigid due to the very high stretching stiffness and this is indicated by the line parallel to the ordinate axis in Figure 2. As the rings become progressively more deeply corrugated in their unstressed configurations they are easier to inflate. (It will be appreciated that Figure 2 shows area increases far in excess of what is needed for a typical graft; however, the principles remain the same when applied to smaller area increases applicable to typical graft prostheses.) The initial slopes of the curves shown in Figure 2 are good measures of the structural stiffness of the rings and these can be plotted as a function of the shape parameter c for various values of n, as is done in Figure 3. In Figure 3, S , the stiffness parameter, is defined by the expression πa dp

5= lim p→∞ ^~B ^{' ~}dλ A diagram like Figure 3 may be used to select appropriate combinations of n and c for a given application by drawing a straight line parallel to the abscissa on the Figure at the level of the stiffness of the arterial tissue.

Alternatively, a suitable combination of n and c may be determined more simply by selecting a cross-sectional shape such that the circumference of the cross-section (that is, the arc length of the cross-section) is about 15 percent greater than the circumference of the tissue into which it is desired to insert the graft prosthesis.

The magnitude of the expansions discussed above with reference to Figures 1-3 are well in excess of what is required for an artificial vascular prosthetic graft. Arterial radial expansion under normal pulsatile flow is about 5% so that the area increases by only about 10% . The area of interest from a practical standpoint is the lower left hand corner of Figure 2. The example clearly illustrates the reduced bending stiffness of corrugated sections of a graft prosthesis however.

Figures 4-13 illustrate various examples of graft prostheses according to the invention.

Figure 4 is a perspective view of a graft prosthesis in accordance with the invention. Referring to Figure 4, graft prosthesis 100 has lumen 105 of generally circular cross-section, longitudinal axis 110 and eight corrugations consisting of ridges 120 and valleys 125 which extend the entire length of graft prosthesis 100 substantially parallel to longitudinal axis 110. Amplitude 130 of the corrugations at the end of graft prosthesis 100 is approximately one-fifth of the mean radius of lumen 105.

Figure 5 represents two possible cross-sections of a corrugated portion of a graft prosthesis according to the invention. Figure 5A is a typical symmetrical cross-section having eight corrugations which have rotational symmetry about the longitudinal axis of the graft prosthesis. Figure 5B is an example of an alternative asymmetric cross-section in which one of the corrugations shown in Figure 5a has in effect been omitted. The cross-section shown in Figure 5B does not have rotational symmetry about the longitudinal axis of the graft prosthesis.

Figure 6 is a perspective view of an alternative graft prosthesis in accordance with the invention. Referring to Figure 6, graft prosthesis 150 has lumen 155 of generally circular cross-section and eight corrugations consisting of ridges 160 and valleys 165 which extend the entire length of graft prosthesis 150 but which are twisted relative to the longitudinal axis (not shown) of graft prosthesis 150 so that, for example, the peaks of each of ridges 160 and the troughs of each of valleys 165 define a portion of a helix from one end of graft prosthesis 150 to the other.

Figure 7 is a diagrammatic representation of a further alternative graft prosthesis according to the present invention. Referring to Figure 7, graft prosthesis 200 comprises ends 210, 211 having corrugations 220, 221 therein, corrugations 220 and 221 extending only part way along graft prosthesis 200 and ending at a region of graft prosthesis 200 which has a substantially circular cross-section 240. As shown, graft prosthesis 200 is formed with an elbow 245 approximately centrally located. Corrugations 220, 221 typically extend at least two to three times the diameter of lumen 205 of graft prosthesis 200 from either end. Figure 8 is a perspective view of a further alternative graft prosthesis in accordance with the invention. Referring to Figure 8, graft prosthesis 250 has corrugations 270, 271 extending from either end 260, 261 in a similar way to the graft prosthesis shown in Figure 7. A central region 275 of the body of graft prosthesis 250 has a substantially circular cross-section and includes a portion 280 comprising circumferential corrugations 290, the presence of which confers greater lateral flexibility on graft prosthesis 250.

Figure 9 is a diagrammatic representation of yet a further graft prosthesis in accordance with the invention. Referring to Figure 9, graft prosthesis 300 has ends 310, 311 having substantially circular cross-section and a medial portion 320 having corrugations 330 therein. Corrugations 330 extend longitudinally over medial portion 320 of graft prosthesis 300. It is noted that in Figure 9 only a cross-section of the corrugated portion is shown. Corrugations 330 decrease in depth towards ends 310, 311 and thus merge with the portions of circular cross-section. The diameter of lumen 305 tapers from either end of graft prosthesis 300 towards the medial portion thereof. Figure 10 is a diagrammatic representation of a graft prosthesis similar to that shown in Figure 9, but having a medial portion with six corrugations, rather than four as shown in Figure 9.

Figure 11A is a representation of an end cross-section of the graft prosthesis of Figure 9 or Figure 10. Figure 1 IB is a diagrammatic representation of a cross-section of a corrugated portion of the graft prosthesis shown in Figure 10, showing corrugations 430.

Figure 12 is a diagrammatic representation of two examples of possible profiles for the corrugations of a graft prosthesis in accordance with the invention. Thus, Figure 12A diagrammatically represents a corrugation 500 having a continuous curve profile. Additional corrugations may be visualised by reflecting corrugation 500 about symmetry axes 510, 515. Figure 12B diagrammatically represents a corrugation 520 having a "bellows" profile, the profile comprising curved portions 521, 523 and 525 and straight portions 522 and 524. Axes 530, 535 are symmetry axes. Thus in Figure 12A the corrugations have a radius of curvature which is a continuous function around the exterior, while in Figure 12B the cross-sectional shape of the corrugations is comprised of a succession of straight and curved segments. The shapes of these examples of cross- sections are independent of the lumen diameter of the graft prosthesis.

Figure 13 diagrammatically represents yet a further graft prosthesis in accordance with the present invention. Referring to Figure 13, graft prosthesis 600 comprises sections 610, 611 which are substantially as illustrated in Figure 9, having medial longitudinally corrugated regions 620, 621 and ends 630, 631 with substantially circular cross-sections. Between sections 610 and 611 is a laterally flexible section which consists a number of circumferential corrugations 640.

Claims

1. A graft prosthesis having a tubular elastic body defining a lumen, wherein at least a portion of said body comprises a plurality of longitudinal corrugations.

2. A graft prosthesis according to claim 1, wherein said longitudinal corrugations extend from an end of said graft prosthesis.

3. A graft prosthesis according to claim 2, wherein said longitudinal corrugations extend from both ends of said graft prosthesis.

4. A graft prosthesis according to claim 2 or claim 3, wherein a portion of said body is non-corrugated.

5. A graft prosthesis according to claim 1, wherein said portion of said body is remote from the ends of said graft prosthesis.

6. A graft prosthesis according to claim 5, wherein a portion of said body is non- corrugated.

7. A graft prosthesis according to claim 2, wherein said longitudinal corrugations extend the entire length of said graft prosthesis.

8. A graft prosthesis according to claim 1, wherein said lumen has a longitudinal axis and said longitudinal corrugations are parallel to said longitudinal axis.

9. A graft prosthesis according to claim 1, wherein said lumen has a longitudinal axis and each of said longitudinal corrugations is twisted relative to said longitudinal axis.

10. A graft prosthesis according to claim 1, wherein a portion of said body comprises a plurality of circumferential corrugations.

11. A graft prosthesis according to any one of claims 1 to 3 wherein said lumen tapers from one end thereof to the other end thereof.

12. A graft prosthesis according to any one of claims 1 to 3 wherein said lumen tapers from each end thereof to a medial portion thereof.

13. A graft prosthesis according to claim 8 or claim 9 wherein said longitudinal corrugations have a rotational symmetry about said longitudinal axis.

14. A graft prosthesis according to claim 8 or claim 9 wherein said corrugations do not have rotational symmetry about said longitudinal axis.

15. A graft prosthesis according to claim 1 wherein the thickness of said tubular elastic body varies over the length of said tubular elastic body.

16. Use of a graft prosthesis according to claim 1 in surgery in a mammal.

17. Use according to claim 16 comprising the formation of an arteriovenous fistula for haemodialysis access.

18. Use according to claim 16 comprising the formation of a coronary artery bypass.

19. Use according to claim 16 comprising the formation of a renal artery bypass.

20. Use according to claim 16 comprising the formation of a femoral artery bypass.

21. Use according to any one of claims 16-20 wherein said mammal is a human.