CA1220001A

CA1220001A - Tissue valve

Info

Publication number: CA1220001A
Application number: CA000457329A
Authority: CA
Inventors: Robert P. Totten
Original assignee: Mitral Medical International Inc
Current assignee: Mitral Medical International Inc
Priority date: 1983-07-12
Filing date: 1984-06-25
Publication date: 1987-04-07
Also published as: GB2169386B; GB8601661D0; GB2143306B; GB8417758D0; GB2169386A; GB2143306A

Abstract

Abstract of the Disclosure A unitary tissue element is mounted on a pre-ferred stent and suture ring assembly, the stent compred of a circular mounting frame with a series of circumferen-tially spaced commissure posts, separated by scalloped por-tions above a common annular base. A suture ring disposed in outer concentric relation to the stent base is secured thereon by a unitary tubular cloth covering stitched to enclose both stent and suture ring. The pericardium is formed from a rectangular section of uniform thickness cut from a bovine pericardial sac, cleaned in saline solution, then partially fixed in a glutaraldehyde solution. A con-ventional leaflet-forming tool is inserted through the stent annulus and the tissue is positioned in solution on the stent/ring assembly. Using small vertical stitches, the tissue is stitched continuously along its exterior lower edge to the cloth covering so as to join the base of the stent and the inner circumferential edge of the suture ring. The tissue is secured to the cloth covering of the commissure posts by a vertical pattern of continuous cross stitches. The valve is then dipped in saline to set the tissue in the desired leaflet configuration, then immersed in the fixative alternated with withdrawal and resting periods.

Description

~22~

This invention generally re:lates to tissue heart valves; and more particularly relates to a novel and improved natural tissue heart valve and to a method of making same~
This invention is directed to certain improve-ments in the construction and method of fabrication of natural tissue heart valves of the type characterized by having a cloth or fabric covered stent of the type disclosed in U.S. Letters Patent No. ~,477,930 and assigned to the assignee of the present invention, which incor-porates a suture ring at its base so as to facilitate its implantation into the annulus or wall of the heart using conventional surgical procedures. The valve element itself is composed of pericardium which is mounted upon and sewn to the stent and, for example, in a semi-circular heart valve is so conEigured as to define three cusps which will undergo opening and closing in response to reversals in the flow of blood through the annulus.
A major difficulty in the 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 tensile properties of the tissue are preserved so 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 fiber bundles in the tissue be avoided, and moreover that the valve element be streamlined and uniform in tissue thickness throughout.
In determining the source of the natural tissue employed, it is recognized that low profile porcine valves have been devised, but nonetheless have not been found to X ~ Ji~

possess the optimum hydrodynamic characteristics desirable in a tissue valve. Bovine pericarcl:ium has been employed in the pas-t for enhanced hyclrodynami.c performance but has been placed along the interior of the stent in order to effect the necessary support of the valve. A greatef effective orifice area can be achieved if -the hovine pericardium can be supported around -the exterior of the stent and fabric covering, and further if the necessary sti-tching or anchoring of the tissue or pericardium be performed between the tissue and exterior of the stent so that the leafle-ts can open as wide as possible while leaving a smooth interior thereby achieving a lower pressure drop. Thus, a closely-related consideration in the construction of the valve is that -the stitching employed between the cloth covering, stent, sewing ring and tissue be uniEorm or sym-metrical throughout and in such a way as to avoid the introduction of increased bulk or non-uniformities in thickness at any point as well as to eliminate any exposed seams or knots on the interior of the valve. In achieving the foregoing, a single piece of tissue is utilized as the valve elemen-t which can be securely mounted in place on-to the cloth-covered stent while maintaining a uniform inter-nal diameter when the tissue valve element is expanded -to its open position.
Previously, it has been the practice to ernploy pressure fixation in the pre-forming of the tissue valve element. Elowever, it is desirable to avoid pressure fixa-tion so as not -to affect the collegium bundles in -the tissue and in general to provide for an improved method of fixation which will preserve the fiber s-tructure of the ~z~

tiSsue while permitting fabrication oE a smooth and wrinkle-free valve element.
Representa-tive patents of interest in the fabrication and construction of natural tissue heart valves are U.S. Letters Patent Nos. ~,5~ 18, 3,983,581 and 4,035,849 to W. W. Angell et al, 4,08~ 2~8 to M. ~ Ionescu et al; and ~,172,295 -to R. J. Batten.
Among the desirable objectives and advantages of the present invention in the construction of natural tissue heart valves is the formation of a 10W profile valve uti-lizing a pericardium or tissue which is united as a single piece element to the stent in such a way as not to prolapse in use.
It is a further object of the present invention to provide for a novel and improved tissue valve and me-thod of making same wherein preparation and handling of -the tissue and attachment thereof to a stent is undertaken in a manner which preserves the optimurn hydrodynamic and tensile properties of the tissue so as -to produce a valve which simulates the functioning of a human heart valve.
It is yet another object of the present invention to provide a tissue valve so constructed as to maximize the effective valve orifice and thereby reduce pressure drop of the blood flow therethrough, and further which pos~esses a smooth, uniform internal diameter, and uniEorm tissue thickness throu~hout so as to minimize interference with or turbulence in the flow oE blooc1.
It is a still further object of the present invention to provide for a novel and improved method of attaching the tissue element to the stent in a minimum number of steps.

Ye-t another object of thc present invention is to provic~e -for an improved method of fixatlon for a na-tural tlssue heart valve which avoids applicatlon of vacuurn or mechanical pressure, ye-t permi-ts ~ormation oE leafle-ts in a desired configuration withou-t dis-tortion or wrinkling of the tissue.
Additionally, it is an object of the present invention to provi~e a natural tissue heart valve which is adaptable for implantation in place of the atrioventricular valves, either mitral or tricuspid, or in place of -the aor-tic or pulmonary valves.
In carrying out -the teachings of the present invention, the unitary tissue element is mounted on a pre ferred s~ent and suture ring assembly, the sten-t comprising a circular mounting frame having a series of circumferen-tially spaced commissure posts, separated by curved depressions or scalloped portions above a common annular base~ A suture ring is disposed in outer concentric rela-tion to the stent base and is secured thereon by a unitary tubular cloth covering which is stitched so as to enclose both stent and suture ring.
The pericardium is specially selected and formed into a rec-tangu]ar section of uniform thickness which is cut from a bovine pericardial sac, cleaned in saline solu-tion, then partially fixed in a suitable fixative, such as, a glutaraldehyde solution. With the aid of a conventional leaflet-forming tool inserted through the stent annulus, the tissue is positioned in solution on the stent/ring assembly. Vsing small vertical sti-tches, the tissue is stitched continuously along its exterior lower edye to -the cloth coveriny joininy the base of the stent and the inner ~Z~

circumferential edge oE the suture rlng~ The tissue is then secured to the cloth covering of each commissure post using a vertical pattern of continuous cross sti-tches. The partially completed valve is then dipped in saline -to set the tissue in the desired leaElet conEiyuration defined by the leaflet-forming tool, and is then subject -to a process of immersion in the Eixative alternated with withclrawal and resting periods, for approximately thirty minutes. The valve is then reir~nersed for a continuolls period of about thirty mintues ~efore removal of the leaflet-forming tool;
thereafter the valve is returned to the glutaraldehyde for approximately one hour prior to final shaping of the tissue element. The upper edges of the leaflets are individually trimmed upwardly toward the coaptation point at an angle of about 10 to 15 with respect to the lower tissue edge, thereby forming a slight peak or crown at the center of the valve to prevent prolapse during operation thereof. The completed valve may then be sterilized and packaged in the conventional manner.
Other objects, advantages and features of the present invention will become more readily appreciated and understood when taken together with the Eollowing detailed description in conjunction with the accompanying drawings, in which:
Figure 1 is an isometric view oE the pericardium tissue section ernployed in fabrication of the tissue valve of the present invention, illustrating the preliminary par-tial fixation procedure;
Figure 2 is a front view in elevation of the stent and suture ring assembly together with the leaflet-lZ~ 30~

forming mandrel employed in constrllction of -the tissue valve of -the present invention;
Fiyure 3 is an isometric view of the valve illus-trating the attachment of the -tissue to the stent ancl the commissure posts, and prior to preliminary trimming of the upper free maryin edge, Figure ~ is an isometric view of the valve during the final fixation process prior to final trimming of the upper free margin eclge, 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 bottom plan view of the completed valve.
Referring in more detail to the drawings, and particularly to Figures 2 to 5, the natural tissue heart valve lO of the present invention comprises a tissue valve element ll 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 aclvantageously applied to different valve configurations.
Fabrication of a proæerly Eunctioning valve requires careful preparation o the natural tissue prior to attachment to the stent. The -tissue employed is preferably bovine pericardium which is free from blood vessels ancl excess fat. As illustrated in Figure l, the tissue is cut ~z~

into a single, generally rectangular section 20 of suf--ficient size to permit construction of three leaElets, 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 0016", 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 asl 0.5~ glutaraldehyde, for approximately one to two minutes, depending upon the thickness of the tissue. It is essential that the section 20 remain perfec-tly flat during preliminary partial fixation/ in order to avoid the for-mation of permanent wrinkles or Eolds 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 any remaining Eixa-tive.
~ stent of the type disclosed in U.S. Letters Patent No. 4,477,930 is particularly well suited for fabrication of a tissue valve acco-rding to the principles ~C

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of the present invention. ~igure 2 illustrates a stent 12 comprising a circular mountlng frame provided with a series of circumferentially spaced commissure posts generally designated at 25, the latter separated 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 con-ventional leaflet-type form mandrel 30 so that the base 27 of the stent 12 rests firml~ against a circumferential stop flange or enlargement, not shown, which is typically pro-vided on the base 31 of the leaflet-forming mandrel. The vanes 32 of the mandrel 30 are exactly aligned with -the commissure posts 25, and the hollowed-out, cusp-forming portions 33 of the mandrel 30 correspond in outline to the scalloped edges 26 of the stent 12. Prior to attach-ment of tissue section 20, the stent/mandrel assembly is ~20 immersed 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 Figure 2. Using needle holders and a needle threaded with suitable 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 throuyh the cloth covering joining the stent 12 and sewing ring 13 in the pocket 35, Z~

90 that both the threaded end and the "tail" of -the su-ture extend upwardly out of the pocket 3S.

Positioning of ~he tissue 20 on -the stent 12 is accomplished by immersing both the tissue 20 and the stent/manclrel assembly in a container of saline so tha-t the -tissue 20 can float or slide onto the stent without applying mechanical pressure. The tissue section 20 is laid flat in the saline, then carefully positioned in slightly overlapping relation to ~he commissure post 25', the lower edge 21 of the tissue section 20 resting just above the inner circumferential edge 36 of the suture ring 13.
The base or lower edge 21 of the tissue section 20 is then stitch~d to the cloth covering joining the stent 12 and suture ring 13 using small vertical stitches 40 and gentle tension. 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 comple~e, wrinkle-free contact against the leaflet-forming mandrel, particularly in the area of the hollowed cusp-forming por-tions 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. Stitchiny is then continued around the circumference of the base 27, catching, or passinc~ into but not through, the cloth lying between the stent 12 and the suture ring 13, until the approximate center of the next commissure pos-t 25'' is reached; for ri~ht-handed persons this is mos-t conveniently undertaken in a clockwise direction and, conversely, in a counter-clockwice direction for left-handed persons. The final stitch at the second commissure post 25'' is not pulled tight; bu-t 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. ~ithout cutting the suture, the knot is -tucked into a seconcl pocket 35' formed at the base of the commissure post 25'' bet~een the stent base 27 and the sewing riny 13. Sti-tching of the second and third leaflets 15 proceeds in a like manner, first trimming the base edye 21 of the tissue section 20 as described. In tying off the final leaflet 15, however, lt is not necessary to form a loop of suture, as the "tail"
left on the -first stitch ~0 may be used to form the knots.
Figure 3 illustrates the attachment of the tissue section 20 to the cloth-covered commissure posts. The stitching, when completed, forms a continuous criss-cross pattern extending approximately ~etween the base 27 of the 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 ~e cut away until trimmed side edyes 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 step7 however, it is preferable to employ a 6-0 suture. Diagonal stitches 42 angled at approximately ~5 with respect to the - 1.0 '.!I.Z~

stent base 27 are taken throucJh the tissue, joining -the side edges 23 and catching the cloth covering o~ the com-missure post 25' thereunder. The sti-tches ~2 gradually decrease in length as the -top of the commissure post ~5' is reached. At the top oE the post 25' oppositely clirec-ted diagonal stitches 43 are angled ln 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-sti-tch pattern. At the base

2~, 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 indicated at 44 in Figure ~, for a purpose that will become apparent hereinbelow. In other respects, stitching is essentially the same as for the first commissure pos-t 25':

2n 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 20 to the stent/ring assembly.
The valve must undergo a second treatmen-t of pressure-free fiY.ation for a total of approximately two hours in order to se-t th~ tisswe in a uniform cusp con-figuration and prevent wrinkling and ~is-tortion 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 ~IL2~

cusp-formlng portions 33 on -the manclrel so as t.o set the leaflets 15 in -the desired shape. After insuring -that no wrin~les are present, particularly in the tissue at the top of each comm.issure post 25, t'he valve 10 is dipped, again with the posts facing clownwardly, into a beaker of 0.5%
glutaraldehyde 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 E'igure 4. The immersion/resting procedure is repeated every five minutes for appro~imately thirty minutes with inspection af-ter 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 uprigh-t in the glu-taraldehyde solution and left unclisturbed 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 cornmissure posts 25 by inserting the blade of a straight scissors between the converging upper eclges 41 of a pair of adjacent leaflets 15 ancl cutting a single leaflet edge 41 from a point at the top of the commissure post 25 to the center of coaptation, ,inclicated at 50, where the leaflet edges 41 converye. It will be appreciated that the aforementioned vertical slits 44 will facilitate separation and trimrning of each leaflet. The cutting li.ne will not be perfectly horizontal, but should be angled upwardly -toward o~

the coapta-tion po.int 50, on -the order of 10 to 1~. This provides a greater amount of tissue at -the center oE the valve 10 and greater contact between the inner surfaces of the converging leaflet edges ~1, thereby preventing pro-lapse while at the same time permit-ting the valve to open to maximum oriflce size. The remaining leaflek edges a.re cut in the same manner, using the previously Cllt edge as a guide~ After all edges 41 have been trimmed, the cut edges should be inspected for loose -tissue fibers, which must be trimmed away.
The completed valve is illustrated ln Figures 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 lea1ets 15 them-selves, the leaflets 15 possess extremely smooth and sym-metrical exterior and interior surfaces as well as uniform tissue thickness throughout. Moreover, the stitching which uni-tes the tissue to the stent 12 is done entirely on the exterior of the valve, thereby eliminating inwardly facing 2`0 seams or knots so as to maximi7.e the inner diameter of the valve and minimize any tendency to cause excess pressure drop or increased turbulence or interference in blood flow.
The multi-step process of glutaraldehyde 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 collegi~lm bundles in the tissue. The fixation procedure further ser-ves to set the leaflets in the desirecl cusp shape defined by the forming mandrel while resisting the formation of wrinkles or folds in the tissue, thereby providing an - l3 -~2~

improved valve which simulates as c].osely as possible the natural :Eunctioning of the human heart valve i.t replaces.
It is therefore to be understood -that while the present inventlon has been described with particularity relative to the foregoi.ng descrip-tion of preferred and alternate embodiments, other modifications, changes ancl adclitions may be made and will be readily apparent -to -those of ordinary skill in the art without departing :Erom the spirit and scope of the presen-t invention.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

l. In a prosthetic heart valve of the type having an annular stent defining a stent orifice and provided with upright commissure posts at spaced circumferential inter-vals at the downstream end of said stent and a suture ring anchored to the base of said stent at the upstream end thereof, the improvement comprising:
a tissue valve element of tubular con-figuration disposed in surrounding relation to said stent, said valve element having mating edges which extend midway of one of said commissure posts, an upstream end of said valve element anchored between said stent and said suture ring; and suture means extending through said valve element whereby to anchor said valve element to each of said posts.
2. In a prosthetic heart valve according to claim 1, said suture means defined by a continuous length of thread in criss-cross relation, there being a fabric covering over said stent, and said suture means extending through the thickness of said valve element and partially through said fabric covering.
3. In a prosthnetic heart valve according to claim 1, said valve element having cusps extending between said com-missure posts and terminating in upper convergent edges when in a closed position, said cusps undergoing expansion to an orifice size greater than said stent when in an open position.
4. In a prosthetic heart valve according to claim 3, said valve element formed by pressure free fixation in a fixative solution.
5. In a prosthetic heart valve according to claim 4, said valve element being composed of a bovine pericardium.
6. In a prosthetic heart valve according to claim 1, said tissue valve element formed from a single section of tissue, said section having opposite upper and lower edges, said lower edge substantially corresponding in length to the outer circumEerence of said covered stent and having a thickness greater than the thickness of said upper edge.
7. In a prosthetic heart valve according to claim 3, said upper convergent edges angled upwardly towards the center of said orifice at an angle of from 10° to 15° with respect to said upstream edge of said valve element when said cusps are in a closed position.
8. In a prosthetic heart valve according to claim 4, said fixative solution being a glutaraldehyde solution having a concentration on the order of 0.5%.
9. The 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 com-missure posts, comprising the steps of:
forming a rectangular strip of natural tissue;
partially fixing said strip in a first fixa-tive solution;

positioning said strip on said stent with the length of said strip circumscribing the base of said stent and said stent being of a width to extend from the base of said stent beyond the terminal ends of said com-missure posts;
inserting a leaflet-forming tool within said stent in which said tool has recessed areas extending bet-ween said commissure posts which conform to the desired configuration of the leaflet configuration to be assumed by said strip between said commissure posts;
immersing the assembled stent, leaflet-forming tool and tissue in a second solution effective to cause said tissue to assume the leaflet configuration of said leaflet-forming tool; and immersing said assembled stent, leaflet-forming tool and tissue in a third fixative solution for a sufficient length of time to permanently shape said strip to conform to the leaflet configuration of said leaflet-forming tool without the application of pressure, followed by removal of said assembled stent, leaflet-forming tool and strip from said solution and removal of said tool from said stent and strip.
10. The method according to claim 9, in which said first solution is a glutaraldehyde solution and said strip is partially fixed therein without the application of pressure.
11. The method according to claim 9, in which said third fixative solution is a glutaraldehyde solution and said assembly is repeatedly immersed in said fixative solu-tion for spaced time intervals.
12. The method according to claim 11, in which said stent and said strip are immersed in said third fixative solution following removal of said leaflet-forming tool therefrom.
13. The method according to claim 9, in which said second solution is a saline solution, said strip is immersed therein and shaped without the application of pressure.
14. The method according to claim 9, in which said positioning of said tissue strip on said stent is accomplished without pressure by immersing said stent and said strip in a saline solution.
15. The method according to claim 10, in which said tissue strip is trimmed above said terminal ends of said commissure posts following said immersion to define upper converging edges of said tissue strip in said leaflet con-figuration.
A prosthetic tissue heart valve comprising:
an annular stent defining a generally cir-cular valve orifice and provided at the downstream end thereof with upright commissure posts at equally spaced circumferential intervals;
a suture ring anchored to the base of said stent;

a fabric covering enclosing said stent and said suture ring;
a tissue valve element of tubular con-figuration disposed in surrounding relation to said stent and composed of a single section of tissue partially fixed in a fixative solution, said valve element having leaflet portions extending between said commissure posts and ter-minating in upper convergent edges when said valve element is in the closed position, mating edges of said tissue sec-tion extending midway of one of said commissure posts, and the lower upstream edge of said tissue section anchored between said stent and said suture ring;
first suture means anchoring said lower upstream edge of said valve element and comprising a con-tinuous length of thread extending through the thickness of said valve element and into said fabric covering between said stent and said suture ring and defining a continuous line of stitching around the circumference of said valve element, said thread being knotted at each said commissure post; and second suture means anchoring said valve element to each said commissure post, comprising a con-tinuous length of thread extending through the thickness of said valve element and partially through said fabric covering of said stent, said thread forming diagonal stitches extending in criss-cross relation to one another substantially from said base of said stent to the terminal ends of said commissure posts.
17. A prosthetic tissue heart valve according to claim 16, wherein said stent is provided with three said com-missure posts and three said leaflet portions extending therebetween, said upper converging edges of said valve element angled upwardly with respect to said lower upstream edge in a direction toward said valve orifice and coacting at the center of said valve orifice when said valve element is in a closed position.
18. A prosthetic tissue heart valve according to claim 16, wherein said fixative solution is a 0.5 glutaraldehyde solution, and said tissue section is immersed therein for one to two minutes.