Abstract:
A highly flexible tissue-type heart valve is disclosed having a structural stent in a generally cylindrical configuration with cusps and commissures that are permitted to move radially. The stent commissures are constructed so that the cusps are pivotably or flexibly coupled together at the commissures to permit relative movement therebetween. The stent may be cloth-covered and may be a single element or may be made in three separate elements for a three cusp valve, each element having a cusp portion and two commissure portions; adjacent commissure portions for each pair of adjacent stent element combining to form the stent commissures. If the stent has separate elements their commissure portions may be pivotably or flexible coupled, or may be designed to completely separate into independent leaflets at bioresorbable couples. The cloth covering may have an outwardly projecting flap that mates with valve leaflets (e.g., pericardial leaflets) along the cusps and commissures. A connecting band may be provided that follows the cusps and commissures and extends outwardly. The valve is connected to the natural tissue along the undulating connecting band using conventional techniques, such as sutures. The connecting band may be a cloth-covered silicon member and attaches to the underside of the valve at the cusps to provide support to the stent and to the outer side of the valve at the commissures. A multi-legged holder is used to implant the valve, with the legs serving to maintain an implant shape to the valve. The holder may have six legs with one releasably connected to each cusp and one releasably connected to each commissure. A method of implantation of the flexible valve using the holder is also disclosed.

Description:
RELATED APPLICATION 
     The present application claims priority under 35 U.S.C §119(e) to provisional application No. 60/117,445, filed on Jan. 26, 1999 under the same title. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to prosthetic heart valves, and, more particularly, to a prosthetic tissue valve having increased flexibility enabling it to follow the motions of the annulus and sinus regions. 
     BACKGROUND OF THE INVENTION 
     Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way outflow valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. The valves of the heart separate chambers therein, and are each mounted in an annulus therebetween. The annuluses comprise dense fibrous rings attached either directly or indirectly to the atrial and ventricular muscle fibers. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves are most common because they reside in the left side of the heart where pressures are the greatest. In a valve replacement operation, the damaged leaflets are excised and the annulus sculpted to receive a replacement valve. 
     The four valves separate each ventricle from its associated atrium, or from the ascending aorta (left ventricle) or pulmonary artery (right ventricle). After the valve excision, the annulus generally comprises a ledge extending into and defining the orifice between the respective chambers. Prosthetic valves may attach on the upstream or downstream sides of the annulus ledge, but outside of the ventricles to avoid interfering with the large contractions therein. Thus, for example, in the left ventricle a prosthetic valve is positioned on the inflow side of the mitral valve annulus (in the left atrium), or on the outflow side of the aortic valve annulus (in the ascending aorta). 
     Two primary types of heart valve replacements or prostheses are known. One is a mechanical-type heart valve that uses a ball and cage arrangement or a pivoting mechanical closure to provide unidirectional blood flow. The other is a tissue-type or “bioprosthetic” valve which is constructed with natural-tissue valve leaflets which function much like a natural human heart valve, imitating the natural action of the flexible heart valve leaflets which seal against each other to ensure the one-way blood flow. 
     Prosthetic tissue valves comprise a stent having a rigid, annular ring portion and a plurality of upstanding commissures to which an intact xenograft valve or separate leaflets of, for example, bovine pericardium are attached. The entire stent structure is typically cloth-covered and a sewing ring is provided around the periphery for attaching to the natural annulus. Because of the rigidity of the material used in the stent and/or wireform, conventional valves have a diameter that is minimally affected by the natural motion of the heart orifice. In the aortic position, the commissures extend in the downstream direction a spaced distance from the walls of the downstream aortic wall. Movement of the aortic wall or sinuses does not directly affect movement of the cantilevered commissures, though fluid flow and pressures generated by movement of the walls ultimately does cause the commissures to dynamically flex to some extent (i.e., they are cantilevered downstream in the aorta). Because of the inherent rigidity in conventional heart valves, the natural dilatation of the annulus is restricted, imposing an artificial narrowing of the orifice, and increasing the pressure drop therethrough. 
     Accordingly, there is a need for a more flexible heart valve that responds to the natural motions of the annulus and downstream vessel walls. 
     SUMMARY OF THE INVENTION 
     The present invention allows the prosthesis to follow the aortic wall motion as well as that of the annulus during systole and diastole phases, thus reducing the loss in pressure caused by restriction of such motions. The solution is a heart valve having a plurality of leaflets, preferably three, directly sutured to the aortic wall, replacing the native valve. 
     The present invention provides a heart valve including a flexible wireform or stent that allows relative cusp movement or pivoting. The continuous maintenance of leaflet orientation at the commissures provides durability and predictability. Though the leaflets are not wholly independent, they are allowed to move in regions of greatest anatomical motion. 
     The present invention differs in another respect from bioprosthetic tissue valves of the prior art because it does not include a conventional sewing ring with attendant rigid stent. Alternating peripheral cusps and commissures of the prosthetic valve are attached to the annulus region and the sinus region of the ascending aorta of the host (in the aortic valve version), downstream from the location of the natural leaflets (typically excised). 
     In accordance with one aspect of the present invention, a prosthetic heart valve is provided including a flexible, generally cylindrical stent having alternating cusps and commissures. A plurality of flexible leaflets is attached to the stent so as to form a one-way valve within the cylinder. A flexible band is attached along the stent and has a free edge extending away from the stent along the alternating cusps and commissures for connecting the heart valve to an anatomical orifice. 
     Another aspect of the present invention is a highly flexible heart valve including a stent/leaflet subassembly having a peripheral stent and a plurality of leaflets disposed therewithin. The stent/leaflet subassembly defines alternating cusps and the commissures. A connecting band is attached to the stent/leaflet subassembly and follows the alternating cusps and commissures. The band includes a free edge extending from the stent for connecting the heart valve to an anatomical orifice. 
     In a still further aspect of present invention, a prosthetic heart valve comprises a plurality of flexible leaflets, each having an arcuate cusp edge and a coapting edge. The heart valve includes a stent with a plurality of cusps connected to each other at upstanding commissures to generally define a substantially cylindrical volume therebetween. The leaflets are attached to the stent within the cylindrical volume and the cusps are free to move with respect to one another about the commissures. 
     In another embodiment, the present invention provides a prosthetic heart valve comprising a stent having a plurality of stent members adjacently disposed generally around a circle to define a substantially cylindrical volume therebetween. The stent includes a plurality of alternating cusps and commissures. Preferably, the stent members each have a cusp and two commissure regions, with adjacent commissure regions of the stent members together defining each of the commissures of the stent. The stent members may be coupled together to pivot or flexibly move with respect to one another. The coupling may be permanent, or may comprise a bio-resorbable structure that permits the stent members and associated leaflets to move independently from one another. 
     Desirably, the stent of the prosthetic heart valve of the present invention is configured to permit the cusps and commissures to move radially in and out. In one embodiment, the stent comprises a cloth covered rod-like structure. The cloth covering closely surrounds the stent and includes a flap projecting therefrom substantially the entire length of the cusps and commissures for connecting the stent to both the flexible band and the leaflets. The band preferably comprises a suture-permeable inner member, such as silicone, covered by cloth. The cusps of the stent may be pivotally or flexibly coupled to each other at the commissures. Preferably, the stent comprises separate cloth-covered stent members that each define a cusp region and two commissure regions, adjacent commissure regions of the stent members together defining each of the commissures of the stent. The commissure regions of the separate stent members desirably remain spaced apart, with the leaflets extending therethrough to be attached between the cloth covering and the outer connecting band. In this manner, the leaflets are connected to separate stent members, and not to each other to facilitate flexing of the valve. 
     In another aspect of the present invention, a holder is provided for mounting the flexible heart valve. The holder includes a central hub with a plurality of radially outward upper legs, and a plurality of lower legs angled downward and outward. The upper and lower legs are adapted to connect to the alternating cusps and commissures of a flexible valve so as to maintain the position of the valve during implantation. 
     The present invention further provides a combination of a flexible prosthetic heart valve and a rigid holder. The flexible heart valve includes alternating cusps and commissures in a generally cylindrical configuration adapted to move radially in and out with respect to one another. The holder includes structure for maintaining a fixed shape of the flexible prosthetic heart valve during implantation. 
     In a still further aspect of the present invention, a heart valve leaflet is provided comprising a flexible, planar body having an arcuate cusp edge terminating at outer tips. The planar body includes a coapting edge that is defined by two relatively angled lines joined at an apex directed away from the cusp edge midway between the two tips. Desirably, the leaflet is made of pericardial tissue. 
     The present invention further provides a method of implantation of a heart valve, including the steps of: providing a flexible heart valve having alternating cusps and commissures in a generally cylindrical configuration and adapted to move radially in out with respect to one another; attaching a holder to the valve that restricts relative movement of the cusps and commissures; positioning the heart valve in proximity to an anatomical orifice; implanting the heart valve; and, disconnecting the holder from heart valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view through the left half of a human heart showing a systolic phase of left ventricular contraction; 
     FIG. 2 is a sectional view through the left half of a human heart showing a diastolic phase of left ventricular expansion; 
     FIG. 3 is an exploded perspective view illustrating sub-assemblies of a prosthetic heart valve of the present invention; 
     FIG. 4A is a top plan view of an internal stent of the prosthetic heart valve of the present invention; 
     FIG. 4B is an elevational view of the internal stent of FIG. 4A; 
     FIG. 5 is an elevational view of a stent assembly of the prosthetic heart valve; 
     FIGS. 6A and 6B are sectional views through two locations of the stent assembly, taken along lines  6 A— 6 A and  6 B— 6 B of FIG. 5; 
     FIGS. 7A,  7 B, and  7 C are plan views of leaflets suitable for use in the prosthetic heart valve of the present invention; 
     FIG. 8 is an exploded perspective view of a stent/leaflet sub-assembly and a connecting band of the prosthetic heart valve of the present invention; 
     FIG. 9 is an elevational view of an inner member of the connecting band; 
     FIG. 10 is a cross-sectional view through a cusp of the connecting band shown in FIG. 8; 
     FIG. 11 is a perspective view of an assembled prosthetic heart valve of the present invention; 
     FIG. 12A is a cross-sectional view through a cusp region of the prosthetic heart valve of the present invention, taken along line  12 A— 12 A of FIG. 11, and showing a portion of the host annulus in phantom; 
     FIG. 12B is a cross-sectional view through a commissure region of the prosthetic heart valve of the present invention, taken along line  12 B— 12 B of FIG. 11, and showing a portion of the host aortic wall in phantom; 
     FIG. 13 is a schematic view showing relative movement of the aortic and annulus walls during systolic flow; 
     FIG. 14A is a plan view of only the stent members of the prosthetic valve flexed in accordance with the anatomical motions during systole shown in FIG. 13; 
     FIG. 14B is an elevational view of the stent members flexed in accordance with the anatomical motions during systole shown in FIG. 13; 
     FIG. 15 is a schematic view showing relative movement of the aortic and annulus walls during diastolic flow; 
     FIG. 16A is a plan view of only the stent members of the prosthetic valve flexed in accordance with the anatomical motions during diastole shown in FIG. 15; 
     FIG. 16B is an elevational view of the stent members flexed in accordance with the anatomical motions during diastole shown in FIG. 15; 
     FIG. 17 is a perspective view of an alternative stent assembly for use in a prosthetic heart valve in accordance with the present invention; 
     FIG. 18 is a perspective view of an internal stent of the stent assembly of FIG.  17 ; 
     FIG. 19 is an exploded view of a commissure tip region of the stent assembly of FIG. 17; 
     FIGS. 20A-20E are elevational views of alternative stents for use in a prosthetic heart valve in accordance with the present invention; 
     FIG. 21 is a detailed view of a commissure region of the alternative stent of FIG. 20E; 
     FIG. 22 is a detailed view of a commissure region of a still further alternative stent accordance with the present invention; 
     FIG. 23 is an exploded perspective view of the prosthetic heart valve of the present invention and a holder used during implantation of the valve; 
     FIG. 24 is a perspective view of the holder coupled to the valve; 
     FIG. 25 is a top plan view of the holder coupled to the valve; 
     FIG. 26 is a cross-sectional view through the holder and valve, taken along line  26 — 26  of FIG. 25; and 
     FIGS. 27A and 27B are perspective views of an alternative holder for the prosthetic heart valve of the present invention used during implantation of the valve. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a highly flexible aortic heart valve that is attached generally along a scalloped or undulating perimeter downstream from where the natural leaflets were originally attached. The natural leaflets include arcuate cusp portions separated by common commissure portions. If the natural valve has three leaflets, and has a vertically oriented flow axis, the leaflets are evenly distributed circumferentially 120° apart with lower cusp portions and upstanding commissure portions. The commissure portions are connected between the cusp portions and are generally axially aligned along the aortic wall. The annular root of an aortic valve is composed of fibrous tissue and generally conforms to the undulating perimeter of the valve to support the leaflets. In this respect, implanting the aortic heart valve of the present invention involves excising the natural leaflets and attaching the prosthetic heart valve proximate the fibrous annulus, but also in part up the aortic wall. Because of the particular construction of the present heart valve, as will be described below, the attachment means, be it sutures, staples, adhesives, or otherwise, may be anchored into the aortic wall itself, adjacent to the fibrous annulus. 
     Anatomy 
     To better illustrate the advantages of the flexible heart valve of the present invention, an understanding of the movement of the annulus and aorta is helpful. In this regard, FIGS. 1 and 2 illustrate the two phases of left ventricular function; systole and diastole. Systole refers to the pumping phase of the left ventricle, while diastole refers to the resting or filling phase. FIGS. 1 and 2 illustrate in cross section the left chamber of the heart with the left ventricle  20  at the bottom, and the ascending aorta  22  and left atrium  24  diverging upward from the ventricle to the left and right, respectively. FIG. 1 illustrates systole with the left ventricle  20  contracting, while FIG. 2 illustrates diastole with the left ventricle dilating. The aortic valve  28  is schematically illustrated here as having leaflets  30 . Contraction of the ventricle  20  causes the mitral valve  26  to close and the aortic valve  28  to open, and ejects blood through the ascending aorta  22  to the body&#39;s circulatory system, as indicated in FIG. 1 by the arrows  32 . Dilation of the ventricle  20  causes the aortic valves  28  to close and mitral valve  26  to open, and draws blood into the ventricle from the left atrium  24 , as indicated in FIG. 2 by the arrows  33 . 
     The walls of the left chamber of the heart around the aortic valve can be generally termed the annulus region  34  and the sinus region  36 . The annulus region  34  generally defines an orifice that is the narrowest portion between the ventricle  20  and ascending aorta  22 , which as noted above is composed of generally fibrous tissue. The sinus region  36  is that area just downstream from the annulus region  34  and includes somewhat elastic, less fibrous tissue. Specifically, the sinus region  36  typically includes three identifiable, generally concave sinuses (formally known as Sinuses of Valsalva) in the aortic wall intermediate the upstanding commissures of the valve  28 . The sinuses are relatively elastic and are constrained by the intermediate, more fibrous commissures of the aortic annulus. Those of skill in the art will understand that the annulus region  34  and sinus region  36  are not discretely separated into either fibrous or elastic tissue, as the fibrous commissures of the annulus extend into the sinus region  36 . 
     The sinuses tend to move in and out to facilitate fluid dynamics of the blood in conjunction with systole and diastole. During systole, as seen in FIG. 1, the sinus region  36  expands somewhat to a diameter A. This facilitates blood flow through the ascending aorta  22  to the rest of the body. In contrast, during the diastolic phase as seen in FIG. 2, the sinus region  36  contracts somewhat to a smaller diameter B. The diameters A and B are intended to be a measurement of the radial movement of the commissure regions of the valve  28 . In this regard it will be understood that the cross-sections shown are not taken in a single plane, but instead are taken along two planes angled apart 120° with respect one another and meeting at the midpoint of the aorta  22 . The sinus region  36  has a neutral, or relaxed diameter (not shown) somewhere in between diameters A and B. 
     The annular region  34  also moves in and out during the systolic and diastolic phases. As seen in FIG. 1, the annular region  34  contracts somewhat to a diameter C during systole. In contrast, during the diastolic phase as seen in FIG. 2, the annular region  34  expands somewhat to a larger diameter D. Much like the sinus region  36 , the annular region  34  has a neutral, or relaxed diameter (not shown) somewhere in between diameters C and D. 
     As will be explained more fully below, the prosthetic valve of the present invention accommodates the in and out movements of both the annular region  34  and the sinus region  36 . That is, alternating peripheral portions of the prosthetic valve are attached to the annular region  34  and the sinus region  36  and move accordingly. It is important to point out that the preceding discussion of dynamic movement of the annulus and sinus regions is based on preliminary understanding of such movement. That is, direct measurements of these movements are problematic, and thus certain assumptions and predictions must be made. The actual dynamic movement in any particular human heart may be different, but the principles of the present invention would still apply. That is, relative movement in the annulus and sinus regions during systole and diastole does exist, and the flexible prosthetic heart valve of the present invention can accommodate any such movement. 
     Valve Subassemblies 
     With reference now to FIG. 3, the primary sub-assemblies of a preferred embodiment of the prosthetic heart valve  40  of the present invention are shown in exploded view. For purposes of discussion, the directions up and down, upper and lower, or top and bottom, are used with reference to FIG. 3, but of course the valve can be oriented in any direction both prior to and after implantation. From top to bottom, the heart valve  40  comprises a group  41  of three leaflets  42 , three angled alignment brackets  44 , a stent assembly  46 , and a connecting band  48 . Each of the sub-assemblies seen in FIG. 3 is procured and assembled separately (except for the group of leaflets, as will be explained), and then joined with the other sub-assemblies to form the fully assembled valve  40  as seen in FIG.  11 . 
     The prosthetic valve  40  is a trifoliate valve with three leaflets  42 . Although three leaflets are preferred, and mimic the natural aortic valve, the principles of the present invention can be applied to the construction of a prosthetic valve with two or more leaflets, depending on the need. 
     Each of the sub-assemblies seen in FIG. 3 include three cusps separated by three commissures. The leaflets  42  each include an arcuate lower cusp edge  50  terminating in upstanding commissure regions  52 . Each leaflet  42  includes a coapting or free edge  54  opposite the cusp edge  50 . In the assembled valve  40 , the cusp edges  50  and commissure regions  52  are secured around the periphery of the valve, with the free edges  54  permitted to meet or “coapt” in the middle. The stent assembly  46  also includes three cusps  60  separated by three upstanding commissures  62 . In like manner, the connecting band  48  includes three cusp portions  64  separated by three upstanding commissure portions  66 . Each of the sub-assemblies will now be described in detail. 
     Stent Assembly 
     Various components of a preferred stent assembly  46  are seen in FIGS. 4-6. The stent assembly  46  comprises an inner stent  70  and an outer cloth cover  72 . More specifically, the inner stent  70  desirably includes three identical and separate stent members  74 , each of which has a separate cloth covering. As seen best in FIG. 4B, each stent member  74  comprises an arcuate lower cusp region  76  and upstanding commissure regions  78  each terminating at a tip  80 . The stent members  74  comprise elongate rods or wires, preferably made out of an elastic biocompatible metal and/or plastic alloy, such as Elgiloy®, Nitinol, polypropylene, etc. The material selected for stent members  74  should be elastic to permit flexing along their lengths, but should possess a relatively high modulus of elasticity to avoid asymmetric deformation of the constructed valve  40 . The stent  70  supplies an inner frame for the valve  40  that is relatively more rigid than the other components. Therefore, the stent  70  acts to limit total flexibility of the valve  40 . 
     The stent members  74  are desirably bent into the illustrated shape, using conventional wire-forming techniques. Each of the stent members  74  is identical, and terminates in the tips  80  which are bent inward with respect to the arcuate cusp regions  76  to nearly form closed circles. As is seen in FIG. 4B, a gradual radially outward bend  82  (with respect to the cylindrical stent  70 ) is provided in the stent members  74  at a transition between each of the commissure regions  78  and the intermediate cusp region  76 . This bend  82  permits each of the stent members  74  to remain in a circular configuration, as seen from above in FIG.  4 A. That is, if the cusp regions  76  extended in a plane between each of the commissure regions  78 , the plan view would be somewhat triangular. Instead, each of the cusp regions  76  includes a lower apex  84 , and the apices of all of the cusps define a circle concentric with and having the same diameter as a circle defined by all of the tips  80 . The stent  70  thus defines a substantially cylindrical volume therewithin. Of course, other volumes may be defined by the stent  70  wherein the tips  80  define a circle that is smaller or larger than a circle defined by the apices  84 . For example, the apices  84  may be provided outward from the tips  80  so the stent  70  defines a frusto-conical volume therewithin. 
     As seen in FIG. 5, each of the stent members  74  is preferably covered with a generally tubular cloth  72  from tip to tip  80 . The cloth cover  72  is a biocompatible fabric, such as polyterephthalate, and has a varying cross sectional shape, as indicated in FIGS. 6A and 6B. More specifically, the cloth cover  72  includes a tubular portion closely conforming around each of the stent members  74  and a flap  86  extending radially outward from the stent member (with respect to the curvature of the cusp regions  76 ). The cloth cover  72  is formed by wrapping an elongated sheet of fabric around each of the stent members  74  and joining the free edges with sutures  88  to form the flaps  86 . As seen in FIG. 5, the flap  86  extends from each stent member  74  in a direction that is generally outward with respect to the cusp region  76 , and continues in the same general orientation up the commissure regions  78  to the tips  80 . The flap  86  has a dimension that is longest at the apex  84  of each cusp region  76  and shortest at the tips  80 . Indeed, the flap  86  is preferably nonexistent at the tips  80 , and gradually increases in size from the tip  80  to the apex  84 . Therefore, the cross-section of FIG. 6A taken through the commissure region  78  shows the flap  86  having a small dimension d 1 , and the cross-section of FIG. 6B taken through the apex  84  shows the flap  86  having a longer dimension d 2 . 
     The final component of the stent assembly  46  is an attachment means  90  for joining each of a cloth-covered stent members  74 . Preferably, the attachment means  90  comprises threads or sutures sewn through the central holes in each of the circular tips  80 , as shown in FIG. 5, although other suitable attachment means could be used, such as rings, cinches, or the like. The attachment means  90  may be wrapped around or sewn through the cloth cover  72 . In joining the tips  80 , the attachment means  90  are desirably not wrapped extremely tightly, but are instead provided with some slack to permit relative movement of the tips, as will be described below. When the stent members  74  are attached, as seen in FIG. 5, the stent  70  exhibits three cusps corresponding to the cusp region  76  of each member, and three upstanding commissures defined by the juxtaposition of adjacent pairs of commissure regions  78 . 
     In a preferred embodiment of the present invention the attachment means  90  comprises a non-bioresorbable material to ensure that the individual stent members  74  are maintained in the shape of the stent  70 . In an alternative configuration, however, the attachment means  90  comprises a bioresorbable material that dissolves over a period of time after implantation. In such an embodiment, the natural host tissues may have grown in and around the porous portions of the valve  40  to help retain the original shape of the stent  70 . In some instance, however, very little tissue overgrowth may have occurred prior to the attachment means  90  dissolving, and the individual stent members  74  are permitted to move radially a great deal with respect to one another. In the latter embodiment, wherein the stent members  74  are permitted to spread apart, the connecting band  48  may be re-configured to be non-continuous at the commissure portions  66  (see FIG.  3 ). As a consequence, each individual stent member  74  and associated leaflet  72  moves entirely independently of the others, albeit all oscillating with the natural contractions and expansions of the surrounding aortic wall. Such independent leaflet movement may greatly reduce any potential pressure drop across the valve. Although one embodiment is to provide a bioresorbable attachment means  90  such as the sutures shown in the embodiment of FIG. 5, those of skill in the art will understand that any of the coupling means connecting the individual stent members  74  disclosed in the present application could be modified to resorb over time. 
     The stent assembly  46  provides an inner support frame that is generally rigid along any one of stent members  74 , but which permits the stent members to move with respect to one another. In this context, “generally rigid” refers to the structural strength of the stent members  74  that is sufficient to maintain the general shape of the stent  70 , but that permits some flexing along the length of the stent members. Though the stent members  74  are generally rigid, they are able to move with respect to one another. More particularly, joining the stent members  74  with the attachment means  90  creates nodes or pivot points of the valve  40  at the commissures  62  of the stent assembly  46 . As will be more fully explained below with reference to FIGS. 13-16, the stent members  74  are permitted to pivot with respect to one another as they move radially inward and outward. Inward pivoting is permitted by spaces  94 , seen in FIG. 5, defined between adjacent cloth-covered commissure regions  78  of each stent member  74 . These regions  94  are generally triangular and gradually increase in size from the attached commissure tips to the diverging cusps. 
     Leaflet Configurations 
     FIGS. 7A,  7 B, and  7 C are plan views of various configurations of leaflets  42  suitable for use in the prosthetic heart valve  40 . FIG. 7A shows a leaflet  42  having the aforementioned cusp  50 , commissure regions  52 , and free edge  54 . It will be noted that the coapting edge  54  comprises two linear portions extending from an apex  100  to outer tips  102 . The two portions of the free edge  54  are angled with respect to one another and define sides of a triangular region  104  having as its hypotenuse an imaginary line  106  extending between the opposed tips  102 . The triangular region  104  of each leaflet  42  is under less tension during dynamic motion of the valve  40 , and helps ensure coaptation of the leaflets. That is, the leaflets  42  are generally secured along the cusp  50  and commissure regions  52 , and thus the majority of each leaflet  42  is placed in stress except in the region above imaginary line  106 . In this regard, an imaginary (dashed) fold line  108  defines an outer margin  110  of the leaflet  42  that is used to secure the leaflets into the valve  40 . As will be clear from the discussion below, the margins  110  are sutured between the stent assembly  46  and connecting band  48  (FIG.  3 ), and the free edge  54  of the leaflet extends across the cylindrical region defined within the valve  40 , and is generally free to move in that region. Because the triangular leaflet region  104  is relatively stress-free, it tends to roll over under the influence of fluid dynamic forces, thus helping the three leaflets to coapt and prevent valve insufficiency. 
     FIG. 7B shows a leaflet  112  that is substantially the same as the leaflet  42  of FIG. 7A, and thus like elements will be given the same numbers. The leaflet  112  includes a pair of generally triangular shaped commissure tabs  114  in the commissure regions  52 . The tips  102  are thus spaced farther apart than in the version shown in FIG.  7 A. The commissure tabs  114  are used to more securely fasten each of the leaflets to the commissures  62  of the stent assembly  46  (FIG.  3 ). The cloth cover  72  of the stent assembly  46  includes a flap  86  (FIG. 5) which diminishes in size in the commissure regions. The tabs  114  are thus wrapped farther around the cloth-covered stent assembly  46  in the commissure regions and sutured thereto, thus facilitating a more durable connection. 
     FIG. 7C is a further variation of a leaflet  116  which is, again, the same in all respects to the leaflets described above, except for somewhat trapezoidal-shaped commissure tabs  118 . Again, the commissure tabs  118  help to secure the leaflets  116  in the prosthetic valve  40 . 
     Stent/Leaflet Sub-assembly 
     FIG. 8 illustrates a stent/leaflet sub-assembly  120  in which the leaflets  42  are secured to the stent assembly  46 . Preferably, leaflets  42  are pre-attached to align the free edges  54 . In this manner, the free edges  54  of each two adjacent leaflets  42  extend outward in juxtaposition and are received within the triangular space  94  defined between the commissure regions  78  of the stent assembly  46  (FIG.  5 ). The group of leaflets  41  is thus “inserted” underneath the stent assembly  46  until the juxtaposed free edges  54  of the leaflets  42  are in close proximity below the attachment means  90 . The outer margin  110  of each leaflet  42  is folded underneath the corresponding cusp  60  of the stent assembly  46 . At this point, sutures or other such means attach the margins  110  to the flap  86  of the stent assembly  46 . The leaflets  42  can remain attached to one another at their adjacent tips  102  (or along the free edges  54  near the tips), or can be separated for maximum valve flexibility or when the stent is designed to separate into individual stent members by bio-resorption of a commissure couple. 
     If either the leaflet  112  or leaflet  116  of FIG. 7B or  7 C are used, the respective commissure tabs  114  or  118  are wrapped around the adjacent part of the stent assembly  46  and secured thereto. In a preferred assembly method, the leaflets  42  are simply retained in position with respect to the stent assembly  46  with temporary sutures or other such means, to permit the stent/leaflet subassembly  120  to be finally joined together with the connecting band  48  of FIG.  8 . 
     FIG. 8 also illustrates the three alignment brackets  44  and that each has a generally L-shaped cross-section and comprises a cloth-covered inner member (not separately numbered). The inner member preferably has minimum elasticity, but is relatively thin and lightweight. One preferred material for the inner member is a polyester film such as Mylar®. The brackets  44  are preferably joined to the valve  40  at the time the stent/leaflet sub-assembly  120  and connecting band  48  are joined, and thus will be described more fully below with respect to FIG.  11 . 
     Connecting Band 
     FIGS. 9 and 10 illustrate the connecting band  48  in more detail, comprising an inner member  130  surrounded by a cloth cover  132 . As mentioned previously with respect to FIG. 3, the connecting band  48  includes three cusp portions  64  alternating with commissure portions  66 , all generally formed in a tubular configuration. This shape is provided by the inner member  130 , with the cloth cover  132  simply draped and sewn thereover. In a preferred embodiment, the inner member  130  is molded of silicone rubber, and the cloth cover  132  is polyterephthalate. 
     The inner member  130  has a varying cross sectional shape along the cusps and commissures. FIG. 10 is cross-section through one of the cusp portions  64  of the connecting band  48 , and shows a region of the inner member  130  having an inner ledge  134  and upwardly angled outer free margin  136 . The cloth-covered ledges  134  extend generally radially and define three stent support regions  138  of the connecting band  48 , as seen in FIG.  8 . The ledge  134  has its greatest radial dimension at the midpoint of each of the cusp portions  64  and gradually tapers down in size toward the commissure portions  66 . Likewise, the free margins  136  form their greatest outward angle with respect to a central axis of the connecting band  48  at each cusp portion  64 , and gradually re-align to be parallel to the central axis in the commissure portions  66 . The cross-section of the inner member  130  at the commissure portions  66  is seen in FIG. 12B. A series of triangular shaped ribs  140  projects outward from the inner member  130 . The ribs  140  are formed around the entire inner member  130 , along both the cusp and commissure regions. As seen in FIG. 8, the commissure portions  66  of the connecting band  48  define generally axial gaps  142  that help permit flexing of the valve  40 . It should be noted that the connecting band  48  may be discontinuous at the commissure portions  66  if the valve has bioresorbable commissures and is designed to separate into individual “leaflets.” 
     Assembled Valve 
     FIG. 11 illustrates the assembled valve  40  in perspective, while FIGS. 12A and 12B show cross-sections through a valve cusp  150  and valve commissure  152 , respectively. The connecting band  48  is sewn or otherwise attached to the exterior of the stent/leaflet subassembly  120 . Actually, as seen in FIG. 12A, the connecting band  48  is attached underneath the stent/leaflet subassembly  120  in the cusp  150 , but the free margins  136  of the connecting band are positioned to the outside of the subassembly. In addition, the alignment brackets  44  are installed with a vertical leg  156  interposed between the commissures  62  of the stent assembly  46  and the commissure portions  66  (FIG. 3) of the connecting band  48 . A horizontal leg  154  of each of the alignment brackets  44  projects radially inward to cover the tips  80  of the stent assembly  46 . The alignment brackets  44  help hold each two adjacent tips  80  of the three-piece stent  70  together, especially helping to prevent radial mis-alignment. The brackets also provide flat surfaces which a holder can contact, as seen best in FIG.  26 . 
     With reference to the cross-section of FIG. 12A, the sandwiched configuration of the stent assembly  46 , leaflet  42 , and connecting band  48  can be seen. More specifically, the cloth flap  86  of the stent assembly  46  aligns with the leaflet margins  110 , which in turn rest on the stent supports  138 . A series of suture stitches  158  are used to secure these elements together. Preferably, the flap  86  terminates at the same location as the margin  110  of each leaflet  42 , and at the corner defined in the connecting band  48  between each ledge  134  and free margin  136 . The radially innermost wall of the ledge  134  is preferably inward from the stent member  74 . This construction helps prevent the stent  70  from migrating downward with respect to the connecting band  48 . 
     The host annulus  162  is seen in phantom with the aortic wall  164  continuing upward therefrom. It can be readily seen that the angled shape of the cusp portions  64  of the connecting band  48  conform nicely to the host annulus region. The triangular ribs  140  provide volume at the free margins  136  of the connecting band  48  to facilitate connection to the natural tissue; in other words, more volume provides more of a “bite” for the surgeon to secure the band  48  with a suture needle. Although the conventional means for attaching the valve  40  to the host tissue is with sutures, which are not shown, the present invention should not be construed as limited to being implanted with sutures and other means such as staples, adhesives, and the like could be used. 
     Now with reference to FIG. 12B, the assembly of the valve components in the commissure region is seen. The commissure edges  52  of each of the leaflets  42  are sandwiched in between the stent assembly  46  and connecting band  48 . More particularly, the commissure edges  52  are sandwiched between the flaps  86  and the generally planar commissure portions  66  of the connecting band  48  (FIG.  8 ). Sutures  170  are provided to join these elements together. Again, the commissure edges  52  preferably terminate at the same location as the flaps  86 . FIG. 12B also illustrates the gap  142  provided in the commissure regions of the connecting band  48 , and the lack of structural connection between the two sides of each valve commissure  152 . 
     FIG. 12B shows in phantom a portion of the aortic wall  172  to which the commissures  152  (seen in FIG. 11) of the valve  40  are attached. Again, the particular attachment means is not shown, but the connecting band  48  is traditionally sutured to the wall  172 . 
     Dynamic Motion of the Prosthetic Heart Valve 
     FIGS. 13 and 15 illustrate a conduit portion of a heart in the region of the aortic valve and relative motions of the conduit walls during systole and diastole, respectively. In particular, FIG. 13 shows an open valve  200  and systolic blood flow  202 , while FIG. 15 shows a closed valve  204  and diastolic back flow of blood  206 . As described with respect to FIGS. 1 and 2, the regions around the aortic valve can be generally separated into an annulus region  208  and a sinus region  210 . 
     As mentioned previously, the annulus region  208  is expected to contract during the systolic phase, as indicated by the arrows  212  in FIG. 13, and expand during the diastolic phase, as indicated by the arrows  214  in FIG.  15 . Conversely, the sinus region  210  is expected to expand during the systolic phase, as indicated by the arrows  216  in FIG. 13, and is expected to contract during the diastolic phase, as indicated by the arrows  218  in FIG.  15 . The movements of the conduit walls are shown with respect to a neutral or relaxed position  220 , and may be exaggerated from the true movements. Also, as mentioned above, these movements are educated guesses and may be different for some, if not most patients. However, the flexible heart valve of the present invention accommodates all variations of such movements. 
     FIGS. 14 and 16 schematically illustrate the synchronous movement of the prosthetic valve  40  of the present invention with respect to the movements of the host tissue in systolic and diastolic phases as seen in FIGS. 13 and 15. To simplify this explanation, FIGS. 14 and 16 only illustrate the stent  70  of the present invention, which as previously described acts as a limitation to movement of the entire valve  40  and fairly represents movement of the entire valve. 
     With reference to FIGS. 14A and 14B, during systole the valve experiences outward commissure movement, as indicated by the arrows  230 . At the same time, the valve experiences inward movement at the cusps, as indicated by the arrows  232 . During diastole, in contrast, and as seen in FIGS. 16A and 16B, the valve experiences inward commissure movement, as indicated by the arrows  234 . At the same time, the valve experiences outward movement at the cusps, as indicated by the arrows  236 . 
     Alternative Stents 
     FIGS. 17-19 illustrate an alternative stent assembly  250  comprising an inner stent  252  and an outer cloth cover  254 . As with the earlier stent assembly  46 , the stent assembly  250  includes alternating cusps  256  and commissures  258 . As best seen in FIG. 18, the stent  252  includes three separate stent members  260  having arcuate commissure tips  262  that are curved toward one another. A generally disk-shaped commissure housing  264  encompasses the adjacent commissure tips  262 , retaining the stent members  260  together while permitting relative pivoting. 
     FIG. 19 illustrates two adjacent commissure tips  262  and the commissure housing  264  exploded into a male housing portion  266  and a female housing portion  268 . The housing portions are so named because they are joined together through interference between a button  270  of the male housing portion  266  and an aperture  272  on the female housing portion  268 . Each portion of the commissure housing  264  includes a circular groove  274  for receiving the arcuate tips  262 . The grooves  274  combined to form a circular channel having an axis  276  within which the arcuate tips  262  are received and can slide. When assembled together, the commissure housings  264  thus provide nodes of rotation for each of the stent members  260 . 
     FIG. 20A illustrates an alternative stent  280  suitable for use in a heart valve of the present invention. The stent  280  includes three stent members  282 , each having commissures with a flex region  284  and tips  286 . The tips  286  of adjacent stent members  282  are secured together by sutures or other suitable means (not shown). The flex regions  284  comprise sections of each stent member  282  which are bent away from each other. The stent members  282  can thus pivot with respect to one another about the connected tips  286 . Upon inward movement of the stent members  282 , a fulcrum  288  is created by interaction between the stent members at the lower end of the flex region  284 . The relative flexibility in inward or outward movement of the stent members  282  can be modified by selection of the cross sectional size and shape of the stent members, and overall configuration of the flex region  284 . 
     FIG. 20B illustrates a second alternative stent  290  suitable for use in a heart valve of the present invention. The stent  290  includes three wires  292  and has commissure regions  294  formed by bent ends of the wires and a junction member  296 . In this embodiment, the junction member  296  either rigidly holds the terminal ends of each of the wires  292 , or permits the wires to slide or otherwise flex with respect to one another. If the wires are rigidly attached to the junction member  296  the shape of the wires in the commissure region  294  reduces stress risers in bending. 
     FIG. 20C illustrates a third alternative stent  300  suitable for use in a heart valve of the present invention. The stent  300  comprising three separate wires  302  terminating at circular commissure tips  304 . Each of the commissure tips  304  is rotatably fastened around a pin  306  provided on a junction plate  308  common to adjacent wires  302 . In this manner, the tips  304  remained located close to one another, while the cusps of the wires  302  can pivot in and out. 
     FIG. 20D illustrates a fourth alternative stent  310  suitable for use in a heart valve of the present invention. The stent  310  is made in one piece with a series of alternating cusps  312  and commissures  314 . The commissures  314  comprising a nearly 360° bend in the stent  310  which permits each cusp  312  to easily flex with respect to the other cusps. 
     FIG. 20E illustrates a fifth alternative stent  320  suitable for use in a heart valve of the present invention. The stent  320  comprises three wire-like stent members  322 , adjacent ones of which are joined together at commissure regions  324  by a U-shaped coupling  326  and a pair flexible sleeves  328 . FIG. 21 is a detail of one of the commissure regions  324  showing in hidden lines the adjacent ends of the coupling  326  and stent members  322 . The couplings  326  are preferably sized with the same diameter as the stent members  322 , and the sleeves  328  are tubular with a constant diameter lumen. The sleeves  328  may be made of silicone, or a flexible polymer such as polyurethane or the like. Other flexible interfaces such as sleeves  328  are contemplated, such as, for example, a single block of silicone into which the commissure regions  324  of the stent members  322  are molded. 
     FIG. 22 is a detailed view of a commissure region  330  of a still further alternative stent suitable for use in a heart valve of the present invention. The stent is made in one piece with adjacent cusps  332  being joined by a coil spring tip  334 . Again, great flexibility is provided by the coil spring tips  334  to enable relative motion of the cusps  332 . The amount of flexibility is selected as in any spring by varying the material, cross-sectional size and shape, and number of turns of the spring. 
     Valve Holder 
     FIGS. 23-26 illustrate a preferred holder  350  useful for implanting the flexible heart valve  40  of the present invention. As the heart valve  40  is relatively flexible, the holder  350  must provide adequate support to insure a stable platform for the surgeon to position the valve for attachment to the natural tissue. In other words, because the flexible prosthetic heart valve  40  of the present invention exhibits alternating cusps and commissures in a generally cylindrical configuration that are adapted to move radially in and out with respect to one another, the holder  350  desirably provides rigid structure for maintaining a fixed shape of the valve during implantation. In addition, the holder  350  must include structure to allow quick release from the valve  48  after the valve is implanted. 
     As seen in FIG. 23, the holder  350  comprises a proximal handle socket  352  having an inner bore  354  for receiving the distal end of a handle (not shown). The socket  352  may be provided with internal threads, or other such quick-release coupling structure to facilitate handle connection and disconnection. The holder  350  has three radially outwardly-directed commissure legs  356 , and three outwardly and downwardly angled cusp legs  358 . Consistent with the distribution of the cusps  150  and commissures  152  of the valve  40 , the commissure legs  356  are oriented 120° apart, and the cusp legs  358  are oriented 120° apart, with the three commissure legs being offset with respect to the three cusp legs by 60°. 
     As seen in FIG. 24, each of the commissure legs  356  extends outward from the handle socket  352  into proximity with one of the valve commissures  152  and is secured thereto with an upper suture  360 . Likewise, each of the cusp legs  358  extends outward and downward from the handle socket  352  into proximity with a midpoint of one of the valve cusps  150 , and is secured thereto with a lower suture  362 . The lower end of each cusp leg  358  includes a concavity for mating with the corresponding rod-like stent member  74 , as seen in FIG.  26 . In this manner, each of the cusps  150  and commissures  152  of the valve  40  is securely held in relation to the others, thus facilitating implantation by the surgeon. 
     Details of the commissure legs  356  will now being described with reference to FIGS. 23 and 26. Each commissure leg  356  extends outward from the handle socket  352  in a generally rectangular cross-section interrupted by an upwardly-facing inner notch  370  oriented cross-wise to the leg. And upwardly-facing radial channel  372  having a depth of approximately half of each commissure leg  356  extends from about the inner notch  370  to the outermost end of the leg. The inner notch  370  is not quite as deep as the channel  372 , as seen in FIG.  26 . The radial channel  372  divides the upper portion of each commissure leg  356  into two walls  374   a ,  374   b . An eyehole  376  is formed in one of the walls  374   a , and a corresponding outer notch  378  is formed in the other wall  374   b  aligned with the eyehole. The outer notch  378  is also not quite as deep as the channel  372 . 
     With reference to FIGS. 24 and 26, the upper suture  360  is preferably tied to the eyehole  376  in the first wall  374   a . The suture  360  then passes across the channel  372 , through the outer notch  378 , and is passed along the inner notch  370 , again traversing the channel  372 . The suture  368  is then passed through a suture-permeable portion of the valve commissure  152 , such as through the connecting band  48 . After passing through the commissure  152 , the suture  360  is again looped through one or both of the notches  370 ,  378  and re-tied to the eyehole  376 . By proper threading of the upper suture  360 , each commissure  152  can be secured to the commissure leg  356  and easily released by inserting a scalpel blade into the radial channel  372  to sever the portions of the suture therein. 
     Details of each cusp leg  358  can be seen in FIGS. 23 and 26. A pair of longitudinal rails  380   a ,  380   b  are provided on the outer side of each cusp leg  358 . Toward the lower end of the rails  380   a,b , a pair of aligned eyeholes  382  provide anchoring locations for the lower suture  362 . A scalpel guide or relief  384  is formed in one of the rails  380   b . As seen in FIG. 24, the lower suture  362  extends downward from the eyeholes  382 , passes through a suture-permeable portion of the cusp  150 , and is then returned and secured to the eyeholes  382 . The relief  384  exposes a portion of the lower suture  362  for severing by the surgeon using a scalpel blade. It will thus be understood that the holder  350  can be quickly released from the valve  40  by a series of six scalpel strokes, with each of the sutures  360 ,  362  remaining attached to the holder  350  and being withdrawn from the valve  40  as the holder is withdrawn. 
     FIGS. 27A and 27B illustrate an alternative holder  390  useful for implanting the flexible heart valve  40  of the present invention. The holder  390  is substantially similar to the holder  350  described above, but the ends of each of a plurality of rigid legs for attaching to the valve cusps are flared, or, more precisely, each lower leg has a width from a hub to a terminal end that is greatest at the terminal end to provide more surface area to contact the corresponding valve cusp. That is, the holder  390  includes a plurality of upper legs  392  having a generally constant width, and a plurality of lower legs  394  having flared ends  396 , the legs extending from a central hub  398 . Again, the upper legs  392  extend radially outward to connect to the valve commissures  152 , and the lower legs  394  angle radially outward and downward to connect to the valve cusps  150 . The flared ends  396  impart greater stability to the flexible valve  40  during implantation, especially helping to prevent movement of the cusps  150 . In addition, the legs  194  remain fairly narrow until the flared ends  396  to maintain good visibility through the spaces between the plurality of legs. That is, for example, the surgeon can continue to view the valve leaflets  42  between the legs as a check on valve orientation. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. In particular, though the flexible nature of the present heart valve has been described as being particularly suitable for use in the aortic position, the advantage of flexibility could equally apply to a valve implanted in other positions, such as the mitral position. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.