GB2169386A - Prosthetic heart valve and method of manufacture - Google Patents

Abstract

The valve comprises an element (11) of natural tissue mounted on a cloth-covered stent and suture ring assembly (13), the stent having a series of circumferentially spaced commissure posts (25',25'',25''') separated by scalloped portions (26) above a common annular base. In making the valve a substantially rectangular section of natural tissue, e.g. bovine pericardium, is cut, cleaned in saline solution, and partially fixed in a glutaraldehyde solution. The tissue section is then secured by stitching (40,42,43) onto the box and posts of the stent, the latter being mounted on a mandrel (30). The assembly is then dipped in saline solution causing the tissue to conform to depressions (33) in the mandrel and form leaves (15) of the valve element; thereafter the element is fixed in glutaraldehyde solution and the valve removed from the mandrel. <IMAGE>

Description

1 GB2169386A 1

SPECIFICATION

Prosthetic heart valve and method of preparation thereof stent, such that recessed areas in the tool extend between the commissure posts to de fine with the posts the desired leaflet configu ration to be assumed by the strip; immersing the assembled stent, leaflet-forming tool and tissue in a second solution effective to cause the tissue to assume the leaflet configuration of the leafiet-forming tool; immersing the as sembled stent, leaflet-forming tool and tissue in a third, fixative, solution for a sufficient length of time to permanently shape the strip to conform to the leaflet configuration of the leaflet-forming tool without the application of pressure; and removing the assembled stent, leaflet-forming tool and strip from the solution and removing the tool from the stent and strip.

In order that the invention may be better understood, a preferred embodiment will now be described, by way of example, with refer ence to the accompanying drawings, in which:

Figure 1 is an isometric view of the pericar dium tissue section employed in fabrication of the tissue valve of the preferred embodiment, illustrating the preliminary partial fixation pro cedure; Figure 2 is a front view on elevation of the stent and suture ring assembly together with the leaflet-forming mandrel employed in con struction of the tissue valve of the preferred embodiment; optimum hydrodynamic characteristics desir- Figure 3 is an isometric view of the valve able in a tissue valve. Bovine pericardium has illustratin the att h t f th ti th been employed in the past for enha..ced hy- drodynamic performance but has been placed 100 along the interior of the stent in order to ef fect the necessary support of the valve.

It has been the practice to employ pressure fixation in the pre-forming of the tissue valve element. However, it is desirable to avoid 105 pressure fixation so as not to affect the colle gium bundles in the tissue and in general to provide for an improved method of fixation which will preserve the fibre structure of the tissue while permitting fabrication of a smooth 110 and wrinkle-free valve element.

Representative patents of interest in the fab rication and construction of natural tissue heart valves are U.S.Letters Patent Nos.

2,548,418; 3,983,581 and 4,035,849 to W.W. Angell et al; 4,084,268 to M.L lonescu et al; and 4,172,295 to RJ. Batten.

The invention consists in a method of preparing a tissue valve element for a prosthetic heart valve in which a stent is formed having an outer fabric covering and circumferentially spaced commissure posts, comprising the steps of: forming a rectangular strip of natural tissue; partially fixing the strip in a first fixa- tive solution; positioning the strip on the stent with the length of the strip circumscribing the base of the stent and the stent being of a width to extend from the base of the stent beyond the terminal ends of the commissure posts; inserting a leaflet-forming tool into the This invention relates to prosthetic heart valves; and more particularly to those pre pared with tissue valve elements.

The construction and method of fabrication of natural tissue heart valves, having a cloth 75 or fabric covered stent which incorporates a suture ring at its base, so as to facilitate its implantation into the annulus or wall of the heart using conventional surgical procedures, has been developed. A major difficulty in the 80 construction of tissue valves is that in forming the desired valve configuration the tissue must be selected, treated and handled in such a manner that the natural hydrodynamic and ten sile properties of the tissue are preserved so 85 as to produce a valve which approximates, as nearly as possible, the structure and function of a normal human heart valve. In particular, it is critical that damage to the collegium fibre bundles in the tissue be avoided, and more over that the valve elements be streamlined and uniform in tissue thickness throughout.

In determining the source of the natural tis sue employed, it is recognised that low profile porcine valves have been devised, but none theless have not been found to possess the U --- -- - 00UM L _ stent and the commissure posts, and prior to preliminary trimming of the upper free margin edge; Figure 4 is an isometric view of the valve during the final fixation process prior to final trimming of the upper free margin edge; Figure 5 is a front view in elevation of the completed tissue valve; Figure 6 is a top plan view of the completed tissue valve; and Figure 7 is a bottom plan view of the completed valve.

Referring in more detail to the drawings, and particularly to Figs. 2 to 5, a natural tissue heart valve 10 comprises a tissue valve element 11 mounted upon and sewn to a cloth-covered stent 12 incorporating a suture ring 13 at its base. The valve 10 may be of generally circular configuration, defining three cusps or leaflets 15 which open and close in response to reversals in blood flow through the valve annulus. It will be understood, however, that the principles of the invention may be advantageously applied to different valve configurations.

Fabrication of a properly functioning valve requires careful preparation of the natural tissue prior to attachment to the stent. The tissue employed is preferably bovine pericardium which is free from blood vessels and excess fat. As illustrated in Fig. 1, the tissue is cut into a single, generally rectangular section 20 2 GB2169386A 2 of sufficient size to permit construction of three leaflets, the length of the lower edge 21 substantially corresponding to the outer circumference of the covered stent. The tissue section 20 is placed in a shallow dish D containing 0.9% saline solution for cleaning and dissection to remove fibrous and adipose tissue. The section 20 is fine dissected to a thickness of approximately.0014" to.001W (0.036 mm to 0.041 mm), depending upon the external diameter of the stent, and then inspected with a light magnifier for uniformity of thickness, nicks, cuts and the like. In the event that one edge of the tissue 20 is of slightly greater thickness this edge, designated as lower edge 21, should form the base of the tissue valve element 11. The opposite or upper edge will form the free margin edge 22 of the valve element 11.

The cleaned tissue section 20 is removed from the saline solution and placed flat in a fixative solution, such as, 0.5% glutaraldehyde, for approximately one to two minutes, depending upon the thickness of the tissue. It is essential that the section 20 remain perfectly flat during preliminary partial fixation, in order to avoid the formation of permanent wrinkles or folds in the tissue. Upon completion of the requisite fixation period the tissue 20 is immediately removed from the glutaraldehyde solution and immersed in a large container of saline solution, again avoiding folding or wrinkling, and is gently agitated for approximately three minutes to remove ai,,, remaining fixative.

Fig. 2 illustrates a stent 12 comprising a circular mounting frame provided with a series of circumferentially spaced commissure posts generally designated at 25, the latter sepa- rated by curved depressions or scalloped portions 26 above a common annular base 27. An elastomeric suture ring 13 fits in close outer concentric relation to the outer circumference of the base 27 and a unitary fabric covering, not shown, serves both to enclose the entire stent/ring assembly and to mount the ring 13 in place. The selected stent is placed over a conventional leaflet-type form mandrel 30 so that the base 27 of the stent 12 rests firmly against a circumferential stop flange or enlargement, not shown, which is typically provided on the base 31 of the leaflet-forming mandrel. The vanes 32 of the mandrel 30 are exactly aligned with the corn- missure posts 25, and the hollowed-out, cuspforming portions 33 of the mandrel 30 correspond in outline to the scalloped edges 26 of the stent 12. Prior to attachment of tissue section 20, the stent/mandrel assembly is im- mersed and thoroughly soaked in saline solution. Upon removal therefrom, the suture ring 13 is folded down to expose the stitching site at the base 27 of the stent, as shown in Fig. 2. Using needle holders and a needle threaded with suitable suture, such as, No. 5-0 single armed suture, a small pocket 35 is exposed between the suture ring 13 and the base of the stent aligned with the center of a selected one of the commissure posts 25'. A single stitch is taken downwardly, then upwardly through the cloth covering joining the stent 12 and sewing ring 13 in the pocket 35, so that both the threaded end and the "tail" of the suture extend upwardly out of the pocket 35.

Positioning of the tissue 20 on the stent 12 is accomplished by immersing both the tissue 20 and the stent/mandrel assembly in a container of saline so that the tissue 20 can float or slide onto the stent without applying me- chanical pressure. The tissue section 20 is laid flat in the saline, then carefully positioned in slightly overlapping relation to the commissure post 25', the lower edge 21 of the tissue section 20 resting just above the inner circum- ferential edge 36 of the suture ring 13.

The base or lower edge 21 of the tissue section 20 is then stitched to the cloth covering joining the stent 12 and suture ring 13 using small vertical stitches 40 and gentle ten- sion. After executing the first stitch, the entire assembly is preferably placed in the saline solution again and the tissue repositioned so that when the assembly is withdrawn from the solution the tissue makes complete, wrinkle- free contact against the leaflet-forming mandrel, particularly in the area of the hollowed cusp-forming portions 33. The lower edge 21 may partially cover the suture ring 13 at this time; this excess should be trimmed away prior to attachment of each leaflet. Stitching is then continued around the circumference of the base 27, catching, or passing into but not through, the cloth lying between the stent 12 and the suture ring 13, until the approximate center of the next commissure post 25" is reached; for right-handed persons this is most conveniently undertaken in a clockwise direction and, conversely, in a counterclockwise direction for left-handed persons. The final stitch at the second commissure post 25" is not pulled tight; but a loop of suture is left so that the first leaflet thus formed between posts 25' and 25" may be tied off with surgical square knots or the like. Without cutting the suture, the knot is tucked into a second pocket 35' formed at the base of the commissure post 25" between the stent base 27 and the sewing ring 13. Stitching of the second and third leaflets 15 proceeds in a like man- ner, first trimming the base edge 21 of the tissue section 20 as described. In tying off the final leaflet 15, however, it is not necessary to form a loop of suture, as the "tail" left on the first stitch 40 may be used to form the knots.

Fig. 3 illustrates the attachment of the tissue section 20 to the clothcovered commissure posts. The stitching, when completed, forms a continuous criss-cross pattern extend- ing approximately between the base 27 of the 3 GB2169386A 3 stent 12 and the top of the commissure post 25. Prior to stitching, however, it is advisable to trim the excess tissue at the upper free marginal edge 22 to the level of the upper surfaces 32' of vanes 32 of the leaflet-forming mandrel 30, such that leaflets 15 terminate in converging upper edges 41. Excess overlapping tissue on the side edges 23 at the first commissure post 25' must also be cut away until trimmed side edges 23 just meet at the center of the commissure post 25'. Stitching of the tissue 20 to the commissure post 25 is initiated in the manner previously described in reference to the base stitching, specifically in that a "tail" of suture is left extending from the pocket 35 for subsequent "tying off". In this step, however, it is preferable to employ a 6-0 suture. Diagonal stitches 42 angled at approximately 450 with respect to the stent base 27 are taken through the tissue, joining the side edges 23 and catching the cloth covering of the commissure post 25' thereunder. The stitches 42 gradually decrease in length as the top of the commissure post 25' is reached. At the top of the post 25' oppositely directed diagonal stitches 43 are angled in criss-cross relation to one another down the post 25', the needle entering and exiting at the same points as for the first set of stitches 42, thereby forming a cross-stitch pattern. At the base 27, the suture is woven through the cloth joining the stent 12 and suture ring 13, then tied off as previously disclosed.

Before stitching the remaining commissure posts 25", 25"', the excess tissue remaining between the tops of said posts and the upper edges 32' of vanes 32 should be vertically slit to just above the tops of the posts, as indi- cated at 44 in Fig. 4, for a purpose that will become apparent hereinbelow. In other respects, stitching is essentially the same as for the first commissure post 25': The tissue 20 is stitched to the cloth covering of each post, although in stitching posts 25" and 25... there is obviously no need to join tissue edges, and the suture is tied off in the prescribed manner, thereby completing attachment of the tissue to the stent/ring assembly.

The valve must undergo a second treatment 115 of pressure-free fixation for a total of approxi mately two hours in order to set the tissue in a uniform cusp configuration and prevent wrinkling and distortion of the tissue. The valve 10, still on the leaflet-forming mandrel 30, is inverted and dipped into saline solution; by capillary action the tissue will adhere to the hollowed-out cusp-forming portions 33 on the mandrel so as to set the leaflets 15 in the desired shape. After insuring that no wrinkles are present, particularly in the tissue at the top of each commissure post 25, the valve 10 is dipped, again with the posts facing downwardly, into a beaker of 0.5% glutaral- dehyde solution for on the order of fifteen seconds, then removed and again inspected for wrinkles or folds. The valve/mandrel assembly is then set upright on the mandrel base 31 and allowed to rest for approximately five minutes, as illustrated in Fig. 4. The immersion/resting procedure is repeated every five minutes for approximately thirty minutes with inspection after each step to insure that the leaflets 15 are wrinkle- free and positioned properly on the forming mandrel 30. The valve/mandrel assembly is then placed upright in the glutaraidehyde solution and left undisturbed for a period on the order of thirty minutes, after which time the mandrel 30 may be carefully removed from the stent 12. The valve 10 is again placed in the glutaraldehyde for approximately one hour in preparation for final trimming of the upper free margin edges 22.

The leaflets are individually trimmed above the top of the commissure posts 25 by inserting the blade of a straight scissors between the converging upper edges 41 of a pair of adjacent leaflets 15 and cutting a single leaflet edge 41 from a point at the top of the cornmissure post 25 to the center of coaptation, indicated at 50, where the leaflet edges 41 converge. It will be appreciated that the aforementioned vertical slits 44 will facilitate separ- ation and trimming of each leaflet. The cutting line will not be perfectly horizontal, but should be angled upwardly toward the coaptation point 50, on the order of 10' to 15'. This provides a greater amount of tissue at the center of the valve 10 and greater contact between the inner surfaces of the converging leaflet edges 41, thereby preventing prolapse while at the same time permitting the valve to open to maximum orifice size. The remaining leaflet edges are cut in the same manner, using the previously cut edge as a guide. After all edges 41 have been trimmed, the cut edges should be inspected for loose tissue fibres, which must be trimmed away.

The completed valve is illustra-ted in Figs. 5 to 7. It will be appreciated that since the tissue valve element 11 is formed from a single piece of tissue, there being no cutting or stitching of the leaflets 15 themselves, the leaflets 15 possess extremely smooth and symmetrical exterior and interior surfaces as well as uniform tissue thickness throughout. The downstream end of the valve element consists of a transverse edge, portions of which correspond to edges of each respective cusp. When the valve element is closed, the edges of the cusps meet, along generally radial lines. When the valve element opens, these edges separate and the cusps undergo an expansion; when fully open, the valve element defines an orifice greater than the stent orifice. A greater effective orifice area than hitherto is achieved by supporting the bovine pericardium around the exterior of the stent and fabric covering, rather than the interior of 4 GB2169386A 4 the stent as hitherto; this, with the smooth interior surface, gives a low pressure drop.

Moreover, the stitching which unites the tissue to the stent 12 is done entirely on the exte rior of the valve, thereby eliminating inwardly 70 facing seams or knots so as to maximize the inner diameter of the valve and further mini mize any tendency to cause excess pressure drop or increased turbulence or interference in blood flow. The multi-step process of glutaral- 75 dehyde fixation prior to tissue attachment and following completion of the valve avoids the usual application of mechanical or vacuum pressure and thus prevents distortion of the collegium bundles in the tissue. The fixation procedure further serves to set the leaflets in the desired cusp shape defined by the forming mandrel while resisting the formation of wrinkles or folds in the tissue, thereby provid ing an improved valve which simulates as closely as possible the natural functioning of the human heart valve it replaces.

The natural tissue heart valve is adaptable for implantation in place of the atrioventricular valves, either mitral or tricuspid; or in place of 90 the aortic or pulmonary valves.

This application is a divisional patent appli cation based on Application No.8417758.

Claims (8)

1. A method of preparing a tissue valve element for a prosthetic heart valve in which a stent is formed having an outer fabric covering and circumferentially spaced commissure posts, comprising the steps of: forming a rec tangular strip of natural tissue;;,artially fixing the strip in a first fixative solution; positioning the strip on the stent with the length of the strip circumscribing the base of the stent and the stent being of a width to extend from the base of the stent beyond the terminal ends of the commissure posts; inserting a leaflet-form ing tool into the stent, such that recessed areas in the tool extend between the commis sure posts to define with the posts the de sired leaflet configuration to be assumed by the strip; immersing the assembled stent, leaflet-forming tool and tissue in a second so lution effective to cause the tissue to assume the leaflet configuration of the leaflet-forming tool; immersing the assembled stent, leaflet forming tool and tissue in a third, fixative, so lution for a sufficient length of time to perma nently shape the strip to conform to the leaflet configuration of the leaflet-forming tool without the application of pressure; and re moving the assembled stent, leaflet-forming tool and strip from the solution and removing the tool from the stent and strip.

2. A method according to claim 1, wherein the first solution is a glutaraidehyde solution and the strip is partially fixed therein without the application of pressure.

3. A method according to claim 1 or 2, wherein the third fixative solution is a glutaral- dehyde solution and the assembly is repeatedly immersed in the fixative solution for spaced time intervals.

4. A method according to claim 3, wherein the stent and the strip are immersed in the third fixative solution following removal of the leafletforming tool therefrom.

5. A method according to any one of claim 1, 2, 3 or 4, wherein the second solution is a saline solution, the strip is immersed therein and shaped without the application of pressure.

6. A method according to claims 1 to 5, wherein the positioning of the tissue strip on the stent is accomplished without pressure by immersing the stent and the strip in a saline solution.

7. A method according to claim 2 or any claim appendant thereto, wherein the tissue strip is trimmed above the terminal ends of the commissure posts following the immersion to define upper converging edges of the tissue strip in the said leaflet configuration.

8. A method of preparing a tissue valve element for a prosthetic heart valve, substantially as described herein with reference to the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Did 8818935, 1986, 4235.

Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies may be obtained.