CA2278575A1

CA2278575A1 - Heart valve prosthesis

Info

Publication number: CA2278575A1
Application number: CA002278575A
Authority: CA
Inventors: David John Wheatley; John Fisher; David Williams
Original assignee: Individual
Current assignee: Aortech International PLC
Priority date: 1997-01-24
Filing date: 1998-01-22
Publication date: 1998-07-30
Also published as: AU5675298A; DE69827016D1; DE69827016T2; CN1244107A; PT1006949E; US6171335B1; AU725504B2; ES2231956T3; JP4067128B2; CN1147275C; WO1998032400A1; JP2001508681A; ATE279164T1; BR9806985A; EP1006949B1; EP1006949A1; GB9701479D0

Abstract

The invention provides a prosthetic valve having a generally annular frame with three post and three scallops. The frame is tri-symmetric with an axis of symmetry defined by the axis of blood flow through the valve. The external surface of the frame is generally cylindrical with diameter D. Each leaflet has a truncated spherical surface adjacent to its free edge. The spherical surface is joined tangentially to a truncated conical surface. The half angle of the truncated cone is approximately 37.5~. The radius of the sphere is approximately D/2 - 0.5 (mm). The leaflet surface is axi-symmetrical with the axis of symmetry being perpendicular to the axis of the valve frame and blood flow.

Description

3 The present invention relates to medical implants, 4 particularly cardiac and vascular implants and prostheses.

7 In mammals the heart is a vital organ responsible for 8 maintaining an adequate flow of blood (and hence oxygen 9 and nutrients) to all parts of the body. The blood is prevented from flowing backwards through the heart by 11 valves.

13 Dysfunction of one or more of the valves in the heart 14 can have serious medical consequences. Dysfunction of heart valves may be the result of a congenital defect, 16 or of disease-induced damage or degeneration.
17 Dysfunction results from stenosis or reguritation (or a 18 combination) of the valve, leading to high pressure 19 upstream of the valve.
21 To date, the only solution to treat some heart valve 22 dysfunctions is to replace the malfunctioning valve.
23 Such a valve replacement operation is expensive and 24 requires specialised facilities for open-heart surgery.
Replacement of failed artificial valves carries 1 increased risk and there are practical limits on the 2 number of times that reoperation can be undertaken.
3 This makes the design and operational lifetime of any 4 replacement valve extremely important.
6 Porcine aortic valves have been used for many years in 7 human patients and it has been proposed (see for 8 example EP-A-0,402,036 of Pro Medica International Inc) 9 to use porcine pulmonary valves in human patients;
however, valves derived from biological material have a 11 finite lifetime and must generally be replaced within 12 10 years of implantation in younger patients.

14 In third world countries where rheumatic fever is still common, the problems of valve replacement in young 16 patients are considerable. Anticoagulants (required 17 for mechanical valves) are often impractical;
18 accelerated calcification (a problem of biological 19 valves in the young) precludes the use of biological alternatives.

22 In the Western world, increasing life expectancy for 23 humans results in a corresponding rise in patients 24 requiring cardiac valve replacement. There is thus an increasing need for cardiac valve prostheses having 26 both an extended useful lifetime and also a low risk of 27 inducing thrombosis in a recipient.

29 Conventional flexible leaflet heart valves are known to comprise an annular frame disposed parallel to the 31 blood flow. The annular frame generally has three posts 32 extending in the downstream direction defining three 33 generally U-shaped openings or scallops between the 34 posts. The leaflets are generally attached to the frame between the posts along the edges of the scallops 36 and are unattached at the free edges of the leaflets i_ _ _._._... ,.~~_... . T _..__._._ _~._ ~_._ 1 adjacent to the downstream ends of the posts.

3 According to the present invention there is provided a 4 cardiac valve prosthesis comprising a frame and two or more leaflets attached to the frame, wherein at least 6 one of the leaflets comprises a first portion which has 7 a generally spherical surface, and/or a second portion 8 which has a generally conical surface.

The respective surfaces are preferably partially 11 conical or spherical.

13 The prosthesis may be an artificial valve and may be 14 oriented in a particular direction in a heart (or other vascular tissue) to allow flow of blood in one 16 direction through the tissue but prevent back-flow.
17 The frame preferably has a generally circular cross 18 section with two or more posts (in an equal number to 19 the number of leaflets) extending in the same direction from a base. The prothesis is preferably oriented with 21 the posts of the frame extending in the downstream 22 direction such that the mouth of the valve formed by 23 the base is held open. The leaflets are attached to 24 the frame between the posts and each has a free edge adjacent to_the ends of the posts which can seal 26 together to close the valve at the ends of the posts.

28 The conical portion is preferably located adjacent to 29 the base of the prosthesis, and the spherical portion is preferably located adjacent to the free edge. This 31 is advantageous in that the spherical surfaces at the 32 leaflet edges seal more effectively than planar or 33 conical surfaces, and the conical portion at the base 34 of the valve opens more readily upon the increase of blood pressure in that vicinity than an equivalent 36 spherical portion.

1 A valve embodying the invention has low opening 2 resistance owing to the conical portion reacting first 3 to the increased pressure on the upstream side of the 4 valve. When closed, the increased pressure on the downstream side of the valve forces the free edges of 6 the leaflets together in a substantially parallel 7 arrangement thereby enhancing the seal between the 8 leaflets and reducing the backflow of blood through the 9 valve.
11 The spherical portion adjacent to the base of the 12 leaflets also confers advantages in the stress 13 distribution when the valve is closed and the pressure 14 is greater downstream than upstream.
16 The leaflets may (but need not) be identical.

18 The leaflets preferably number three and the frame 19 comprises three posts.
21 The leaflets are preferably flexible.

23 The leaflets may have a defined boundary between the 24 first (spherical) portion and the second (conical) portion, or alternatively, the boundary between these 26 two portions may be phased, for example by adopting a 27 sphere of gradually increasing radius merging with the 28 conical portion. This is acceptable provided that the 29 free edge of the leaflets (or a portion thereof) has a generally spherical surface.

32 In one embodiment the leaflets extend beyond the top of 33 the posts of the frame.

The leaflets can comprise any biostable, biocompatible 36 thermoplastic elastomer including but not limited to _ .~..._.~___ _. __ _.. _._._~ .. _ ... _..._ T T

1 any polyurethane or silicone elastomer or any copolymer-2 or blend based on these elements.

4 The fabrication route can be any appropriate method, 5 including not only dip moulding but also injection 6 moulding, transfer moulding and similar procedures.

8 Preferably the leaflets comprise a biostable 9 polyurethane, such as ELASTEON-CSIRO, CHRONOFLEX or TECOTHANE and are dip moulded thereby integrating the II leaflets to the supporting frame and posts.

13 The leaflets may be approximately 100-200 Vim, but the 14 thickness can vary with the material used. The leaflets can themselves vary in thickness, so as to 16 incorporate thick-walled areas and adjacent thin-walled 17 areas. Ridges and/or smooth progressions from thick to 18 thin walled areas are envisaged.

The leaflet surface is preferably axi-symmetrical, with 21 the axis of symmetry being perpendicular to the axis of 22 the valve frame and the intended direction of blood 23 flow. Where the diameter of the frame is distance 24 D(mm), the radius of the sphere preferably lies between D/2 (mm) and (D/2 )-2 (mm) .

27 The conical portion is generally truncated and has a 28 half angle within the range 30° to 45° (eg preferably 29 37.5°).
31 The frame can be parallel or slightly tapered on the 32 inside and outside, so as to allow a slightly diverged 33 flow.

The pressure required to open the valve is defined by 36 the equation ~ where E is the elastic modulus, t is 1 the leaflet thickness and R is the radius of curvature.

3 Reversal of the curvature in the centre of the 4 leaflets) may also facilitate an opening of the valve.
6 The prothesis may have incorporate an escape path for 7 trapped air, eg a bleed hole in the frame and/or in one 8 or more leaflets, optionally near the base of each 9 leaflet leading through the frame to the inflow aspect for de-airing of the sub-leaflet space.

12 Means for protecting the valve from post ensnarement 13 with an implanting suture is useful. This could take 14 the form of a simple extractable suture linking the tips of the posts, or a more sophisticated umbrella-16 like flexible polyurethane shield (not shown) which 17 could be collapsed and withdrawn through the mitral 18 prosthesis.

A metal frame may be used and the frame can be dip 21 coated with polymer and with facilities for enhancing 22 metal-polymer adhesion. The metal may be titanium or 23 titanium-alloy although any implantable metallic 24 material may be appropriate such as stainless steel or cobalt-chromium alloys.

27 Alternatively a polymer material may be used for the 28 frame. Two preferred options are a rigid polyurethane 29 and PEEK, polyetheretherketone. Alternative polymers are Delrin (a polyacetal), polyethylene and 31 polysulphone. Any rigid or semi-rigid thermoplastic 32 polymer such as a polyurethane, PEEK, polyacetal, 33 polyethylene, polysulphone, acrylic or similar 34 materials may be used.
36 Surface modifications to improve biocompatibility may __ . ...~.._T. . .. _._ T.. _. ~.._ _ _._..~~_ ~ _ _ 1 include any of these useful in relation to medical 2 device technology in general.

4 Surface modifications may be to control the interactions between the valve material and blood in 6 order to prevent protein adsorption, platelet 7 attachment and activation, activation of the clotting 8 cascade and calcification. It is preferable to coat 9 any surface of the valve, primarily including but not limited to the leaflet material.

12 The surface modification most likely to result in 13 reduced protein adsorption is that of the attachment of 14 phospholipids to the polymer. The principle is that a phospholipid, such as phosphorylcholine, is attached to 16 the polymer surface, this layer mimicking the surface 17 of cells and being resistant to the adsorption of 18 plasma proteins. Since this adsorption is the first 19 event in blood-polymer interactions that triggers all reactions with the clotting cascade and platelets, the 21 inhibition of the process delays or prevents these 22 other effects. Known technologies can be used to coat 23 any type of synthetic prosthetic heart valve. The 24 polymer used for the construction of the valve may be coated with any biomimetic substance, such as a 26 protein, glycoprotein or phospholipid analogue, for the 27 purpose of minimising plasma protein adsorption onto 28 its surface.

A further possibility involves the attachment of 31 antibodies to a surface in order to control the nature 32 of a protein that is adsorbed. For example, it is 33 known that surfaces covered with a layers of albumin 34 are far less thrombogenic than surfaces covered with fibrinogen. Attempts have been made to coat polymers 36 with these proteins but there are many immunological 1 problems associated with the use of proteins derived _ 2 from sources other than the host. The better concept 3 is to employ anti-albumin antibodies which can be 4 attached to the surface such that when the material comes into contact with the patient's blood, their own 6 albumin becomes strongly attached to the bound 7 antibody.

9 Platelets have a tendency to interact with all foreign surfaces but this process can be minimised by control 11 of the surface composition and characteristics. It is 12 important to prevent platelets from attaching to the 13 surface but also to prevent any attached platelets.from 14 being activated at or near that surface. A surface modification process that could be beneficial involves 16 the attachment of hydrophilic molecules onto the 17 polymer surface. Polyethylene glycol or other similar 18 substances may be covalently attached to polymers such 19 as polyurethane and the imparted hydrophilicity will reduce the tendency for cellular attachment.

22 Platelet attachment may also be resisted by the use of 23 pharmacologically active agents attached to the 24 surface. Drugs such as prostaglandin, heparin, hirudin, t-plasminogen activator and urokinase have 26 been attached to functionalised polymer surfaces or 27 otherwise incorporated as leachable or diffusable 28 components of polymers for this purpose. These 29 molecules are known to have anti-platelet activity through their effect on platelet membranes and/or their 31 effect on components of the clotting cascade which 32 interact with these membranes and it is possible to 33 reduce platelet attachment and activation.

One or more parts of the prosthesis can be transparent.

____ _ _.. _~__. _ . ._....
i ...___.~~~ _T --WO 98!32400 PCT/GB98/00211 1 Particularly preferred materials for use in fabrication 2 of prosthetic valves according to the present invention ~
3 are based on those disclosed in US Patent Nos 5,393,858 4 and 5,403,912, International Patent Application No PCT/AU97/00619 and Australian provisional Patent 6 Application Nos P07002, P07616 and P07878.

8 An embodiment of the invention will now be described by 9 way of example with reference to the accompanying drawings in which:

12 Fig. la and b show a valve in perspective view;
13 Fig. 2 shows a perspective view of a Fig. 1 valve 14 showing the spherical and conical portions;
Fig. 3 shows a sectional view through a leaflet of 16 the Fig. 1 and Fig. 2 valves;
17 Fig. 4 shows a side sectional view of the Fig. 1 18 valve;
19 Fig. 5 shows a perspective view of the valve when open;
21 Fig. 6 shows a plan and a perspective view of the 22 frame;
23 Fig. 7 shows a perspective view of a second valve;
24 Fig. 8 shows a perspective view of the frame of the Fig. 7 valve;
26 Fig. 9 shows a sleeve of the Fig. 7 valve in 27 perspective view;
28 Fig. 10 is a perspective view of a sewing ring of 29 the Fig. 7 valve;
Fig. 11 shows a perspective view of the Fig. 8 31 frame partially cut-away;
32 Fig. 12 shows a side sectional view of the 33 leaflets of the Fig. 7 valve;
34 Fig. 13 shows plan views (a, b, c and d) and a cross section (e) of the leaflets indicating 36 possible ribbing configurations; and 1 Fig. 14 shows a perspective view of integrally 2 moulded leaflets of the Fig. 7 valve.

5 Referring now to the drawings, a prosthesis according 6 to the invention has a generally annular frame 1 with 7 three posts 1P and three scallops 1S. The valve frame 8 1 is preferably formed from a rigid polymer such as 9 polyetheretheketone or high modulus polyurethane, and 10 is tri-symmetric with an axis of symmetry defined by 11 the axis of blood flow through the valve. The external 12 surface of the frame 1 is generally cylindrical with 13 diameter D, and diverging to the tips of the posts P.

14 For a given diameter D, the thickness of the valve frame 1 is typically 0.05 D.

17 The three scallops 1S and posts 1P are equally spaced 18 at 120° intervals around the frame. A leaflet 10 is 19 attached along the free edge of each scallop 1S and is supported by adjacent posts 1P. The three leaflets 10 21 are thus also spaced at 120° intervals around the frame 22 1.

24 Each leaflet 10 is identical, and has a truncated spherical surface lOS adjacent to its free edge. The 26 spherical surface lOS is joined tangentially to a 27 truncated conical surface lOC. The half angle of the 28 truncated cone is 37.5°, but can be any angle in the 29 range from 30° to 45° as shown in Fig. 3. The radius of the sphere is approximately D/2 - 0.5(mm), but can 31 be between D/2 and D/2-2(mm). The leaflet surface is 32 axi-symmetrical with the axis of symmetry being 33 perpendicular to the axis of the valve frame and blood 34 flow.
36 The free edge of each leaflet lies in a plane XY
~.~.__.. . _ T _ _ .__._._._.~ .~

1 perpendicular to the axis of intended blood flow 2 through the valve (Z). Fig. 3b shows the leaflet 3 geometry in the XY plane, and Fig. 4 shows the leaflet 4 geometry in the XZ plane.
6 The valve is disposed eg in vascular tissue with the 7 post 31 and free edges of the leaflets pointing 8 downstream. The leaflet geometry is designed to 9 encourage the opening of the valve leaflet from the base of the valve. An increase in pressure upstream of 11 the valve causes the conical portions lOC at the base 12 of the leaflets to diverge first. The conical surface 13 can buckle to an open position very easily with minimal 14 resistance, and thus the valve can open under very low upstream pressures. The divergence of the conical 16 sections lOC initiates divergence of the spherical 17 portions lOS.

19 The spherical portions lOS of the leaflets 10 are easily opened following the divergence under upstream 21 pressure of the conical portions lOC, and under 22 increased downstream pressure, seal against one another 23 more effectively than a conical or a flat surface.

The sealing_of the leaflets and competence of the 26 valves may be further enhanced by extending the 27 leaflets 1 to 2mm above the top of the valve posts, 28 varying the leaflet geometry above the post slightly to 29 bring the leaflets into direct opposition.
31 Figs. 7-13 show a second embodiment of a valve 32 according to the invention. The second valve 20 has 33 three leaflets 30 of flexible polyurethane located on a 34 support frame 21, a protective shield 24 for the leaflets 30, and a sewing ring 25 for surgical 36 insertion.

1 The frame 21 has 3 posts 21P, each tapering to a point _ 2 from a base 21B. The posts and the base define 3 3 scallops 215. The lower (upstream) edge of the base 4 21B is scalloped to conform generally to the scallops 21S receiving the leaflets 30.

7 A metal frame 21 is preferred and can provide maximum 8 strength and minimum frame thickness; the frame 21 9 could be dip coated with polymer. Apertures, grids or a mesh surface could enhance metal/polymer adhesion.

12 The primary function of the frame 21 is~to support the 13 base of the leaflets 30, giving a stable and 14 predictable geometry to the base of the leaflets 30.
The origin of the leaflets from the frame should be at 16 an optimised angle to minimise flexion stresses during 17 leaflet motion, and to spread the zone of transition 18 from full flexibility to full rigidity as widely as 19 possible. A seamless attachment of leaflet 30 to frame 21 is desirable to minimise the possibility of 21 separation of leaflet 30 from frame 21.

23 A degree of flexibility of the frame 21 will be 24 desirable to reduce stress on the leaflets, but resistance to creep is important.

27 An outer sleeve 24 is provided to surround the posts 28 and frame, and to provide protection to the leaflets 30 29 from contact with adjacent tissues, particularly ventricular myocardium in the case of the mitral valve, 31 and aortic wall in the case of the aortic valve. The 32 sleeve extends to beyond the edges of the posts.

34 The frame 21 also provides a secure anchorage for a sewing ring 25 to allow surgical insertion.
36 Additionally, the frame can provide a temporary support _.~..~.~__.. __.__. _ T___ __.._ _T...._ 1 for a mounting system to allow surgical handling during_ 2 implantation of the valve.

4 Ideally, the frame 21 should be attached to the sewing ring 25 in a manner which allows the implanted valve 20 6 to be rotated by the surgeon to optimise the position 7 of the frame posts.

9 Ideally, to minimise the risk of injury to the leaflets 30 during surgical implantation, and to facilitate 11 accurate and secure placement of the sewing ring 25, 12 the frame 21 should be separable from the sewing ring 13 25, and be readily and securely attachable at the time 14 of surgery following completion of sewing ring 25 insertion.

17 Overall height of the valve should be as low as is 18 compatible with good leaflet stability and reasonable 19 stress. The base of the leaflets 30 should be located as close to the inflow aspect of the valve as possible, 21 and the sewing ring 25 should be mounted a distance 22 from the inflow aspect to reduce post protrusion as 23 much as possible.

The geometry of the leaflets 30 is preferably optimised 26 for even spread of stress during opening and closing, 27 and there should be substantial zones of leaflet 30 28 apposition. The leaflets 30 should preferably open at 29 low transvalvar pressure levels to allow satisfactory use in small sizes in the mitral position, as well as 31 to gain optimal haemodynamic function. Hydrodynamic 32 performance in terms of pressure drop should rival that 33 of bileaflet mechanical valves rather than 34 bioprosthetic valves, and that of bioprosthetic valves in terms of regurgitant flow.

1 Flexible three leaflet valves have essentially two _ 2 stable positions for the leaflets - open and closed.
3 The transition between the open and closed positions 4 involves a process of rapid buckling, which inflicts rapid changes in shape on the leaflet accompanied by 6 abrupt angulation of the leaflet material and areas of 7 high stress concentration. It is possible to minimise 8 this source of transient, repetitive high stress by 9 careful leaflet geometric design.
11 The leaflet 30 can be formed with "memory" for the 12 optimised mid-buckling position allowing minimal 13 internal stress at the most vulnerable part of its 14 movement cycle. The leaflets 30 can be dip moulded in a "mid-buckling" position. This offers a solution to 16 the problem of dip moulding three leaflets 30 within a 17 complete frame 21. However, it also helps to ensure 18 that the buckling process is predictable and 19 controlled, with minimisation and distribution of stress during buckling. To ensure that the valve 21 assumes a closed position when unloaded a second dip 22 could be applied while the valve was in the closed 23 position. The same effect may be achievable by heat 24 annealing in the closed position. Whichever method is used, only sufficient memory should be induced in the 26 leaflet to allow closure, but not so much as to require 27 the level of opening transvalve pressure gradient that 28 would be present if the leaflet were moulded in the 29 closed position. It may also be helpful to carry out a third dip mould in the open position (or further heat 31 annealing) to impose a uniform, uncrimped geometry on 32 the open valve. The thickness of additional dip coats 33 would be controlled by adjusting the concentration of 34 the dipping solution..
36 A further option for both strengthening the leaflets 30 _~.._.~ -_ . _... _.... T_....-_... _.... _.~_. .

1 and controlling buckling is provided by incorporating _ 2 reinforcing ribs 26 in the polyurethane leaflets 30.
3 This has the effect of making the leaflet 30 stiffer in 4 one direction (the direction of the ribs 26) than in 5 the perpendicular direction. The anisotropic 6 properties of the native aortic valve (and porcine 7 bioprosthetic valves) could be mimicked through 8 circumferential ribbing on a polyurethane leaflet. The 9 concept can be extended to the use of grid-like ribs 26 10 or even concentrically placed circular or oval ribs 26 11 which would influence leaflet buckling in a predictable 12 fashion. Such ribs 26 can be formed in a dip moulded 13 valve, for example, by carefully etching the leaflet 30 14 dipping formers. In order to avoid potential flow 15 disturbance, it would be desirable to form the ribs 26 16 on the leaflet outflow rather than on the inflow 17 surface.

19 The leaflet 30 may be dip-moulded separately, to facilitate an adequate surface area for the leaflets 21 30, as well as the ribbing pattern of polyurethane as 22 an inherent part of the leaflet 30 (protruding from the 23 outflow aspect of the leaflets), and may be assembled 24 onto a frame 21 using locating pins and holes.
Alternatively, it is possible to dip mould all three 26 leaflets as a complete unit which could be bonded or 27 fixed onto a frame eg with the aid of locating pins and 28 corresponding holes in the frame. The sleeve 24 can 29 include a clamp and could extend beyond the posts to assist in shielding the leaflets from myocardial or 31 aortic wall impingement.

33 Example 1 A valve was manufactured as shown in Figure 2. The 36 base has an approximate outer diamter of 23.8mm with an WO 98/32400 PCTlGB98/00211 1 inner diameter of 22.4mm.

3 The posts extend approximately 17mm from the base of 4 the frame and in this embodiment the width of the top of each post is 1.4mm with a thickness of 0.7mm 7 The valve frame is manufactured from 8 polyetheretherketone and coated with ELASTEON CSIRO at 9 a thickness of 0.2mm.
11 To fabricate the coated valve frame is placed over a 12 solid mound and leaflets are formed by dip moulding 13 thereby integrating them to the frame. The leaflet 14 material is ELASTEON CSIRO polyurethane with a thickness of between 100 to 200 Eun.

17 Alternative examples of a prosthetic valve according to 18 the present invention involve using a high modulus 19 polyurethane frame (E > 500 MPa) or using CHRONOFLEX or TECOTHANE polyurethanes with an elastic modulus in the 21 range 5-15 MPa.

23 Modifications and improvements can be incorporated 24 without departing from the scope of the invention. For instance, the frame can be made of a biocompatible 26 polymer, metal, or composite. The frame can be coated 27 with polyurethane to allow integration of the leaflets, 28 and can be flexible so as to allow the post to deflect 29 (eg by approximately 0.05D) on closure of the valve under pressure.

i _ ...~...-.,~.._ __._.~_ ._ T _.~. _...__~

Claims

17

1. A cardiac valve prosthesis comprising a frame and two or more leaflets attached to the frame, wherein at least one of the leaflets comprises a first portion which has a generally spherical surface, and a second portion which has a generally conical surface.

2. A prosthesis as claimed in claim 1 wherein the surfaces of the first and second portions are respectively partially spherical or conical.

3. A prosthesis as claimed in claim 1 or claim 2 wherein the frame has a generally circular cross section with two or more posts (in an equal number to the number of leaflets) extending in the same direction from a base such that the mouth of the valve formed by the base is held open.

4. A prosthesis as claimed in any of the preceding claims wherein the leaflets are attached to the frame between the posts and each have a free edge adjacent to the ends of the posts which can seal together at the ends of the posts.

5. A prosthesis as claimed in any of the preceding claims wherein the conical portion is located adjacent to the base of the prosthesis, and the spherical portion is located adjacent to the free edge.

6. A prosthesis as claimed in any of the preceding claims wherein the leaflets are identical.

7. A prosthesis as claimed in any of the preceding claims wherein the prosthesis comprises three leaflets and three posts.

8. A prosthesis as claimed in any of the preceding claims wherein the leaflets are flexible.

9. A prosthesis as claimed in any of the preceding claims wherein the leaflets have a defined boundary between the first (spherical) portion and the second (conical) portion.

10. A prosthesis as claimed in any of claims 1 to 8 wherein the boundary between the first and second portions is phased by adopting a sphere of gradually increasing radius merging with the conical portion and the free edge of the leaflets (or a portion thereof) has a generally spherical surface.

11. A prosthesis as claimed in any of the preceding claims wherein the leaflets comprise a biostable material, such as biostable polyurethane CSIRO, and are dip moulded thereby integrating the leaflets to the supporting frame and posts.

12. A prosthesis as claimed in any of the preceding claims wherein the leaflets are approximately 100-200 µm.

13. A prosthesis as claimed in any of the preceding claims wherein the leaflets vary in thickness, so as to incorporate thick-walled areas and adjacent thin-walled areas.

14. A prosthesis as claimed in any of the preceding claims wherein the leaflet surface is axi-symmetrical, with the axis of symmetry being perpendicular to the axis of the valve frame and the intended direction of blood flor.

15. A prosthesis as claimed in any of the preceding claims wherein the diameter of the frame is distance D and the radius of the sphere lies between D/2 and D/2-2(mm).

16. A prosthesis as claimed in any of the preceding claims wherein the conical portion is truncated and has a half angle within the range 30° to 45°.

17. A prosthesis as claimed in any of the preceding claims wherein the pressure required to open the valve is defined by the equation where E is ~
the elastic modulus, t is the leaflet thickness and R is the radius of curvature.

18. A prosthesis as claimed in any of the preceding claims wherein the prothesis incorporates an escape path for trapped air.

19. A prosthesis as claimed in any of the preceding claims wherein the prosthesis further comprises means for protecting the prosthesis from post ensnarement with an implanting suture.