PATENT SPECIFICATION (ii)
1598112 ( 21) Application No 22665/80 ( 62) Divided out of No 1598111 ( 22) Filed 22 Feb 1978 ( 31) Convention Application Nos 771359 ( 32) Filed 23 Feb 1977 ( 33) ( 44) ( 51) 790442 United States of America (US) Complete Specification published 16 Sept 1981
INT CL 3 F 16 K 15/14 April 1977 in ( 52) Index at acceptance F 2 V J 2 A J 2 D ( 54) HEART VALVE PROSTHESIS ( 71) We, ROBERT BERNARD DAVIS, of 11, Michael Road, Framingham, Massachusetts, 01701, United States of America, JOHN SKELTON, of 11, Alden Street, Sharon, Massachusetts 02067, United States of America, RICHARD EDWIN Cy'ARK, of 6, Thorndell Drive, St Louis, Missouri 63117, United States of America, and WILBUR MILTON SWANSON, of 2301, Parkridge Avenue, St.
Louis, Missouri 63144, United States of America, all citizens of the United States of America except John Skelton who is a citizen of the United Kingdom, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-
This invention relates to a frame for a heart valve prosthesis and to a heart valve replacement comprising a fabric supported on such a frame.
The replacement of heart valves with prostheses has become a standard surgical technique However, the prostheses currently in use do not entirely satisfy the above objects Currently, most prosthetic heart valves rely for closure on the sealing of a ball or a flap against a gasket ring With this construction the ball or flap is situated within the flow channel when lifted away from the gasket in the flow configuration This is disadvantageous in two important respects.
First the pressure drop across the valve during the open or flow condition is greater than the pressure drop across the natural valve which causes a slight, but continuous and cumulative overload on the heart Second, the presence of the ball or flap creates regions of turbulent flow tending to damage the red blood cells.
With the foregoing disadvantages in mind, research has been directed to developing leaflet valves more closely approximating the structure and functions of the human valve.
The latter comprises thin, flexible membranes that fold flat against the wall of the surrounding blood vessel in the open configuration, thus causing a minimum of disturbance to the flowing blood In the closed configuration the leaflets form three contiguous pouches that are held in close and leakproof contact by the pressure of the blood As a result of the extreme lightness and flexibility of the leaflets the valve has a short response time, passing quickly from the fully closed to the fully open state, with the result that there is little energy loss in the flowing blood and a minimum of undesirable regurgitation.
These functional characteristics of the human valve result from the composite structure of the natural leaflet This comprises an arrangement of bundles of collagen fibres embedded in a softer tissue material.
The composite structure gives the leaflet good load bearing capacity, a high resistance to tear and sufficient softness and flexibility to make a good seal in the closed configuration At the peak of the pressure pulse, the leaflet withstands a load exceeding 150 gm/cm along a line therein normal to the load.
Heart valve tissue is also anisotropic in its elastic properties, that is, the load-deformation characteristic in one direction is different from that in another direction It has been found useful to define two particular directions for purposes of this description.
These directions are parallel and perpendicular to the free edge of the leaflet, and correspond respectively to the circumferential and radial directions commonly referred to in the literature.
In the direction parallel to the free edge the natural leaflet extends very readily with increased load until an elongation of ten to twelve percent is reached at a load of one to two grams per centimetre of leaflet width.
Upon further increase in load the resistance ( 19) 1,598,112 to further elongation increases greatly In the direction perpendicular to the free edge the region of easy extension with increased load continues to approximately 20 percent elongation, at which the load is about 2 gm/cm.
Upon further increases in load the resistance to further elongation, though greater than in the initial region, is not as high as it is in the parallel direction.
A recent development involves the use of stabilized pig heart valves as replacement for failing human valves These valves embody some of the characteristics of human valves discussed above However, the collection, grading, sterilizing, fixing and storing of pig valves is complicated and costly In consequence a clear need has been realised for a trileaflet heart valve made entirely from synthetic materials.
The present invention, from one aspect provides a frame system for a heart valve prosthesis comprising a first frame having at least three generally parallel legs each comprising a pair of rod portions connected at one end and diverging at the other end as lobes respectively connecting with rod portions of others of said legs, the lobes forming an aperture therebetween, said legs being adapted to receive the margin of a valve leaflet between the rod portions thereof, whereby said leaflet may be secured to two adjacent legs and the interconnecting lobe so as to have a free edge extending between said adjacent legs; and a second frame adapted to nest with said first frame comprising a rod formed to be substantially congruent with said interconnecting lobes, whereby said leaflets may be secured to and between said frames.
From another aspect the invention provides a heart valve prosthesis comprising a frame system as defined above and including a plurality of flexible leaflets each inserted between the rod portions of two adjacent legs of said first frame, secured to said legs and the interconnecting lobe and having a free edge extending between said adjacent legs, the free edges of said leaflets being deflectable into mutual contact for sealing said aperture, said leaflets being secured to and between said first and second frames.
Fabrics especially useful for, and for use with, the frame system of the present invention are described, illustrated and claimed in our copending patent application No.
6963/78 (Serial No 159811).
The foregoing and other features of the invention are described more fully in the following description with reference to the appended drawings.
In the drawings:
Figure 1 shows the main frame of the frame system of the present invention, Figure 2 shows a fabric ribbon in the configuration formed by inserting it into the main frame between the rod portions of its legs.
Figure 3 shows the second frame.
Figure 4 is a top plan view of the frame shown in Figure 1 70 Figure 5 is a top plan view of the frame shown in Figure 3.
Figure 6 shows a partially fabricated heart valve with the fabric inserted into the main frame and cut open preparatory to cement 75 ing thereto.
Figure 7 is a developed view of the partially constructed heart valve, corresponding to Figutre 6.
Figure 8 is a cross sectional view taken on 80 line 8-8 in Figure 7.
Referring to Figure 1 there is shown a main frame 22 comprising a single length of 0.1 cm diameter round polypropylene rod bent into a form having three mutually 85 equidistant, generally parallel legs 24, 26 and 28, each leg comprising a pair of rod portions slightly spaced apart, the rod portions being connected at one end and diverging at the other end The diverging rod portions form 90 three lobes 30, 32 and 34 The connected ends of the rod portions in each pair form bights 36, 38 and 40 Figure 4 is a top plan view of the main frame 22.
A second frame 42 (Figures 3 and 5) is 95 formed of a single length of 0 1 cm diameter round polypropylene rod bent into a form having three lobes 44, 46 and 48 generally congruent with the lobes 30, 32 and 34 so as to fit in close contact therewith as shown in 100 Figure 7.
The assembly is started by threading a bilaterally crimped and compacted ribbon 50, produced from a woven fabric that has been compressively shrunk in two directions 105 according to the method described and illustrated in copending application No 6963/78 (Serial No 1598111), through the three pairs of legs so as to produce the configuration shown in Figure 2 The frame 22 is shown in 110 Figure 1 in exploded relation to Figure 2 for clarity of illustration The upper selvage has no uncut yams and forms the free edges 52, 54 and 56 of valve leaflets.
Thus a double layer of the fabric is passed 115 through each pair of rod portions forming one of the legs 24, 26 and 28 It is necessary to attach the fabric firmly to these legs, and also to the connecting lobes 30, 32 and 34 To facilitate this attachment, the fabric is prefer 120 ably cut lengthwise externally of each leg as shown in Figure 6 Referring to Figure 6, adhesive such a polyurethane dissolved in tetrahydrofuran is applied to attach the fabric to each of the legs as follows Flaps 125 such as 58 and 60 are spread apart and the adhesive is applied at the external point of juncture of the flaps where they enter between the rod portions, in a continuous line extending between points a and b The 130 1,598,112 adhesive material reaches to the external surfaces of the frame by penetration through the fabric flaps along this line; that is, the adhesive contacts the rod portions of the frame only on their outer surfaces The leaflets comprise only those portions of the fabric on the inside of the frame, and these portions are not penetrated by the adhesive.
Thus local stiffening and resultant flex failure caused by such adhesive penetration is avoided.
The above method of adhesive application also distributes the stresses of flexure evenly along the margins of the leaflets, and avoids excessive stress concentrations These margins are permitted to move upon each flexure over the rounded contours of the surfaces of the rod portions that are located on the inside of the frame, and that are not penetrated by the adhesive.
Also, this method of adhesive application reduces blood contact with the adhesive.
The attachment of the fabric to the lobes 30, 32 and 34 is next accomplished by first placing the second frame 42 adjacent those lobes with the fabric pieces passing therebetween as shown in Figures 7 and 8.
Adhesive 61 is then applied through the fabric and to the surfaces of both the main frame 22 and the second frame 42, in a continuous line extending between the points b of the respective legs and connecting these three points As in the previous step, the adhesive material preferably does not penetrate any portion of the leaflet material lying within the main frame 22, and remains out of contact with blood passing through the valve.
The foregoing steps essentially complete the fabrication of the leaflet portions of the valve The remaining steps of fabrication are for the purposes of facilitating the suturing of the prosthesis within the blood vessel The excess fabric available on the outside of the frame can be rolled and consolidated along the junction line between the main and second frames to provide attachment points for stitches during surgical insertion.
The frame material is preferably polypropylene, although other materials have also been employed with success Polypropylene has excellent flex endurance and chemical stability, but is difficult to attach by adhesive to other materials To facilitate adhesion, the main and second frames 22 and 42 may be encapsulated with polyurethane by multiple dip coating The resulting encapsulated frame components have proved to demonstrate the desired characteristics of polypropylene without structural failures or breakdowns at the adhesive junctures.
Valves employing the fabric described above have been tested in an accelerated fatigue tester to assess their long term endurance characteristics Fatigue failures so induced have generally occurred in the region of greatest fabric flexure, that is, along a line in each leaflet that is perpendicular to its free edge and substantially equidistant between the contiguous legs The failures have generally occurred by breakdown of the filaments in the yarns running parallel to the free edge of the leaflets As a means of providing greater fabric strength along the last mentioned lines, woven fabrics may be provided with a greater number of load-bearing yams in this direction.