Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Heart valve – Flexible leaflet
Reexamination Certificate
1999-11-10
2001-01-09
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Heart valve
Flexible leaflet
C623S002330, C623S002120
Reexamination Certificate
active
06171335
ABSTRACT:
The present invention relates to medical implants, particularly cardiac and vascular implants and prostheses.
In mammals the heart is a vital organ responsible for maintaining an adequate flow of blood (and hence oxygen and nutrients) to all parts of the body. The blood is prevented from flowing backwards through the heart by valves.
Dysfunction of one or more of the valves in the heart can have serious medical consequences. Dysfunction of heart valves may be the result of a congenital defect, or of disease-induced damage or degeneration. Dysfunction results from stenosis or reguritation (or a combination) of the valve, leading to high pressure upstream of the valve.
To date, the only solution to treat some heart valve dysfunctions is to replace the malfunctioning valve. Such a valve replacement operation is expensive and requires specialised facilities for open-heart surgery. Replacement of failed artificial valves carries increased risk and there are practical limits on the number of times that reoperation can be undertaken. This makes the design and operational lifetime of any replacement valve extremely important.
Porcine aortic valves have been used for many years in human patients and it has been proposed (see for example EP-A-0,402,036 of Pro Medica International Inc) to use porcine pulmonary valves in human patients; however, valves derived from biological material have a finite lifetime and must generally be replaced within 10 years of implantation in younger patients.
In third world countries where rheumatic fever is still common, the problems of valve replacement in young patients are considerable. Anticoagulants (required for mechanical valves) are often impractical; accelerated calcification (a problem of biological valves in the young) precludes the use of biological alternatives.
In the Western world, increasing life expectancy for humans results in a corresponding rise in patients requiring cardiac valve replacement. There is thus an increasing need for cardiac valve prostheses having both an extended useful lifetime and also a low risk of inducing thrombosis in a recipient.
Conventional flexible leaflet heart valves are known to comprise an annular frame disposed parallel to the blood flow. The annular frame generally has three posts extending in the downstream direction defining three generally U-shaped openings or scallops between the posts. The leaflets are generally attached to the frame between the posts along the edges of the scallops and are unattached at the free edges of the leaflets adjacent to the downstream ends of the posts.
According to the present invention there is provided 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/or a second portion which has a generally conical surface.
The respective surfaces are preferably partially conical or spherical.
The prosthesis may be an artificial valve and may be oriented in a particular direction in a heart (or other vascular tissue) to allow flow of blood in one direction through the tissue but prevent back-flow. The frame preferably 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. The prothesis is preferably oriented with the posts of the frame extending in the downstream direction such that the mouth of the valve formed by the base is held open. The leaflets are attached to the frame between the posts and each has a free edge adjacent to the ends of the posts which can seal together to close the valve at the ends of the posts.
The conical portion is preferably located adjacent to the base of the prosthesis, and the spherical portion is preferably located adjacent to the free edge. This is advantageous in that the spherical surfaces at the leaflet edges seal more effectively than planar or conical surfaces, and the conical portion at the base of the valve opens more readily upon the increase of blood pressure in that vicinity than an equivalent spherical portion.
A valve embodying the invention has low opening resistance owing to the conical portion reacting first to the increased pressure on the upstream side of the valve. When closed, the increased pressure on the downstream side of the valve forces the free edges of the leaflets together in a substantially parallel arrangement thereby enhancing the seal between the leaflets and reducing the backflow of blood through the valve.
The spherical portion adjacent to the base of the leaflets also confers advantages in the stress distribution when the valve is closed and the pressure is greater downstream than upstream.
The leaflets may (but need not) be identical.
The leaflets preferably number three and the frame comprises three posts.
The leaflets are preferably flexible.
The leaflets may have a defined boundary between the first (spherical) portion and the second (conical) portion, or alternatively, the boundary between these two portions may be phased, for example by adopting a sphere of gradually increasing radius merging with the conical portion. This is acceptable provided that the free edge of the leaflets (or a portion thereof) has a generally spherical surface.
In one embodiment the leaflets extend beyond the top of the posts of the frame.
The leaflets can comprise any biostable, biocompatible thermoplastic elastomer including but not limited to any polyurethane or silicone elastomer or any copolymer or blend based on these elements.
The fabrication route can be any appropriate method, including not only dip moulding but also injection moulding, transfer moulding and similar procedures.
Preferably the leaflets comprise a biostable polyurethane, such as ELASTEON-CSIRO, CHRONOFLEX or TECOTHANE and are dip moulded thereby integrating the leaflets to the supporting frame and posts.
The leaflets may be approximately 100-200 &mgr;m, but the thickness can vary with the material used. The leaflets can themselves vary in thickness, so as to incorporate thick-walled areas and adjacent thin-walled areas. Ridges and/or smooth progressions from thick to thin walled areas are envisaged.
The leaflet surface is preferably axi-symmetrical, with the axis of symmetry being perpendicular to the axis of the valve frame and the intended direction of blood flow. Where the diameter of the frame is distance D (mm), the radius of the sphere preferably lies between D/2 (mm) and (D/2)−2 (mm).
The conical portion is generally truncated and has a half angle within the range 30° to 45° (eg preferably 37.5°).
The frame can be parallel or slightly tapered on the inside and outside, so as to allow a slightly diverged flow.
The pressure required to open the valve is defined by the equation Et
3
/R where E is the elastic modulus, t is the leaflet thickness and R is the radius of curvature.
Reversal of the curvature in the centre of the leaflet(s) may also facilitate an opening of the valve.
The prothesis may have incorporate an escape path for trapped air, eg a bleed hole in the frame and/or in one or more leaflets, optionally near the base of each leaflet leading through the frame to the inflow aspect for de-airing of the sub-leaflet space.
Means for protecting the valve from post ensnarement with an implanting suture is useful. This could take the form of a simple extractable suture linking the tips of the posts, or a more sophisticated umbrella-like flexible polyurethane shield (not shown) which could be collapsed and withdrawn through the mitral prosthesis.
A metal frame may be used and the frame can be dip coated with polymer and with facilities for enhancing metal-polymer adhesion. The metal may be titanium or titanium-alloy although any implantable metallic material may be appropriate such as stainless steel or cobalt-chromium alloys.
Alternatively a polymer material may be used for the frame. Two preferred options are a rigid polyurethane and PEEK, polyetheretherketone. Alternative polymers are Delrin (a polyacetal
Fisher John
Wheatley David John
Williams David
Aortech Europe Limited
Jackson Suzette J.
Ratner & Prestia
Willse David H.
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