Abstract:
Expandable, percutaneously deployable, prosthetic heart valves and systems for minimally invasive replacement of damaged or diseased native aortic valves comprise an expandable, tubular stent body and a unidirectional valve assembly. Embodiments of the stent body comprise an annulus anchoring section, a sinus section, and an outflow section, with the outflow section flared outward from the sinus section in an expanded configuration. Embodiments of the stent body are self-expanding, comprising, for example nitinol. The valve assembly disposed within the sinus section of the stent body and sutured thereto. Embodiments of the valve assembly comprise three leaflets, each leaflet comprising a curved outer edge sutured to the sinus section of the stent body, and a coapting free edge. Embodiments of the valve leaflets comprise pericardium, for example, porcine pericardium. Embodiments of the prosthetic heart valve have a contracted configuration dimensioned for percutaneous delivery thereof.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a continuation of Ser. No. 11/749,722, filed May 16, 2007, which is a continuation of Ser. No. 10/653,843, now U.S. Pat. No. 7,276,084, filed Sep. 2, 2003, which is a continuation of Ser. No. 09/815,521, now U.S. Pat. No. 6,733,525, filed Mar. 23, 2001, all the disclosures of which are incorporated by reference herein. 
     
    
     Field of the Invention 
       [0002]    The present invention relates generally to medical devices and particularly to expandable heart valve prostheses especially for use in minimally-invasive surgeries. 
       BACKGROUND OF THE INVENTION 
       [0003]    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 valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves is most common because they reside in the left side of the heart where pressures are the greatest. 
         [0004]    Where replacement of a heart valve is indicated, the dysfunctional valve is typically cut out and replaced with either a mechanical valve, or a tissue valve. Tissue valves are often preferred over mechanical valves because they typically do not require long-term treatment with anticoagulants. The most common tissue valves are constructed with whole porcine (pig) valves, or with separate leaflets cut from bovine (cow) pericardium. Although so-called stentless valves, comprising a section of porcine aorta along with the valve, are available, the most widely used valves include some form of stent or synthetic leaflet support. Typically, a wireform having alternating arcuate cusps and upstanding commissures supports the leaflets within the valve, in combination with an annular stent and a sewing ring. The alternating cusps and commissures mimic the natural contour of leaflet attachment. Importantly, the wireform provides continuous support for each leaflet along the cusp region so as to better simulate the natural support structure. 
         [0005]    A conventional heart valve replacement surgery involves accessing the heart in the patient&#39;s thoracic cavity through a longitudinal incision in the chest. For example, a median sternotomy requires cutting through the sternum and forcing the two opposing halves of the rib cage to be spread apart, allowing access to the thoracic cavity and heart within. The patient is then placed on cardiopulmonary bypass which involves stopping the heart to permit access to the internal chambers. Such open heart surgery is particularly invasive and involves a lengthy and difficult recovery period. 
         [0006]    Some attempts have been made to enable less traumatic delivery and implantation of prosthetic heart valves. For instance, U.S. Pat. No. 4,056,854 to Boretos discloses a radially collapsible heart valve secured to a circular spring stent that can be compressed for delivery and expanded for securing in a valve position. Also, U.S. Pat. No. 4,994,077 to Dobbin describes a disk-shaped heart valve that is connected to a radially collapsible stent for minimally invasive implantation. 
         [0007]    Recently, a great amount of research has been done to reduce the trauma and risk associated with conventional open heart valve replacement surgery. In particular, the field of minimally invasive surgery (MIS) has exploded since the early to mid-1990s, with devices now being available to enable valve replacements without opening the chest cavity. MIS heart valve replacement surgery still typically requires bypass, but the excision of the native valve and implantation of the prosthetic valve are accomplished via elongated tubes or cannulas, with the help of endoscopes and other such visualization techniques. 
         [0008]    Some examples of more recent MIS heart valves are shown in U.S. Pat. No. 5,411,552 to Anderson, et al., U.S. Pat. No. 5,980,570 to Simpson, U.S. Pat. No. 5,984,959 to Robertson, et al., PCT Publication No. 00/047139 to Garrison, et al., and PCT Publication No. WO 99/334142 to Vesely. Although these and other such devices provide various ways for collapsing, delivering, and then expanding a “heart valve” per se, none of them disclose an optimum structure for tissue valves. For instance, the publication to Vesely shows a tissue leaflet structure of the prior art in  FIG. 1 , and an expandable inner frame of the invention having stent posts in  FIGS. 3A-3C . The leaflets are “mounted to the stent posts  22  in a manner similar to that shown in FIG.  1 .” Such general disclosures as in Vesely stop short of explaining how to construct a valve in a manner that maximizes long-term efficacy. In particular, the means of attaching the leaflets to the MIS stent is critical to ensure the integrity and durability of the valve once implanted. All of the prior art MIS valves are inadequate in this regard. 
         [0009]    Another problem with MIS valves of the prior art is their relatively large radial dimension during implantation. That is, these valves all utilize one or more radially-expanding stents coupled to a biological valve, and the assembly must be compressed radially and then passed through the lumen of a large bore catheter. Reducing the radial profile of the constricted valve via radial compression is problematic and conflicts with the need for sufficient circumferential length of the valve in its expanded state to fit within an adult heart valve annulus. Moreover, radial compression of the stent and biological valve must be done with great care so as not to damage the valve. 
         [0010]    Some MIS valves of the prior art are intended to be used without removing the natural valve leaflets. Sometimes the natural leaflets are heavily calcified, and their removal entails some risk of plaque particles being released in the bloodstream. Therefore some of the MIS valves are designed to expand outward within the annulus and native leaflets, and compress the leaflets against the annulus. In doing so, a relatively uneven surface against which the valve is expanded outward is created. This irregularity creates sizing problems, and also may adversely affect the circularity of the expanded valve which negatively affects the valve efficacy by impairing leaflet coaptation. 
         [0011]    Despite some advances in MIS valve design, there remains a need for a valve that can be constricted into a smaller package without damaging the biological valve within, and which can be reliably expanded generally into a tube against the relatively uneven surface of the annulus or annulus and intact native leaflets. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides an expandable prosthetic heart valve for placement in a host heart valve annulus, comprising a stent body that is rolled into a compact configuration, implanted, then unrolled into a tubular shape and secured into place in the valve annulus. The valve is small enough in its contracted state to be passed down a delivery tube, thus avoiding the need for open heart surgery. Flexible membranes attach around large apertures in the inner wall of the stent body and have sufficient play to billow inward into contact with one another and form the one-way valve occluding surfaces. The stent may be one or two pieces, and the delivery and implantation may occur in one or two steps using one or two delivery tubes. 
         [0013]    In a preferred embodiment, a prosthetic heart valve of the present invention suitable for minimally invasive delivery comprises a generally sheet-like stent body and a plurality of flexible, biocompatible membranes incorporated into the stent body to form heart valve leaflets. The stent body has a first, contracted configuration in which it is spirally-wound about an axis such that at least one winding of the stent body surrounds another winding. The stent body further has a second, expanded configuration in which it is substantially unwound and at least partly forms a tube centered about the axis and sized to engage an annulus of a patient&#39;s heart valve. In accordance with one aspect, the stent body comprises a primary stent coupled to a secondary stent that at least partially fits within the primary stent. The flexible, biocompatible membranes are incorporated into the secondary stent. Alternatively, the stent body is formed of a single stent. 
         [0014]    The stent body may have a plurality of sinus apertures with an outer edge of each biocompatible membrane fastening around the edge of an aperture. The sinus apertures may be generally semi-circular or generally oval. The outer edge of each membrane is desirably folded over to contact an inner surface of the stent body adjacent an edge of the associated aperture. 
         [0015]    One embodiment of a heart valve of the present invention includes at least one guide to insure concentricity of the sheet-like stent body about the axis during a conversion between the first, contracted configuration to the second, expanded configuration. For example, the stent body may define a pair of opposed side edges that generally mate in the second, expanded configuration, and a pair of opposed end edges that extend between the side edges, and the at least one guide comprises a tab extending generally radially along each one of the end edges. Alternatively, the at least one guide comprises a tab extending generally radially from the stent body and a cooperating slot in the stent body circumferentially spaced from and axially aligned with the tab. In the latter case, the tab enters and is retained within the slot during the conversion between the first, contracted configuration to the second, expanded configuration. 
         [0016]    In a further aspect of the present invention, the stent body defines a pair of opposed side edges that generally mate in the second, expanded configuration, and the stent body further includes lockout structure to retain the opposed side edges in mating engagement. The lockout structure may comprises tabs formed adjacent one of the side edges and apertures formed adjacent the other of the side edges that are sized to receive and retain the tabs. Desirably, the lockout structure both prevents further expansion of the stent body and contraction from the expanded tubular shape. 
         [0017]    At least one anchoring barb may be provided extending radially outward from the stent body in the second, expanded configuration. Where the stent body defines a pair of opposed side edges that generally mate in the second, expanded configuration, and a pair of opposed end edges that extend between the side edges, the anchoring barb extends from one of the end edges. 
         [0018]    Preferably, the stent body is formed of a single stent having an anchoring section on an inflow end, a sinus section, and an outflow section. The sinus section is between the anchoring section and outflow section, and has apertures for receiving flexible biocompatible membranes that form the occluding surfaces of the valve. Each biocompatible membrane fastens around the edge of an aperture, wherein the sinus apertures may be generally semi-circular and the outer edge of each membrane is folded over to contact an inner surface of the stent body adjacent an edge of an aperture. The outflow section may flare outward from the sinus section, and may include an apertured lattice, mesh or grid pattern. 
         [0019]    The present invention further provides a method of prosthetic heart valve implantation, comprising providing a prosthetic heart valve in a spirally-wound contracted configuration, delivering the prosthetic heart valve in its contracted configuration through a delivery tube to a heart valve annulus, and unfurling the prosthetic heart valve from its contracted configuration to an expanded configuration that engages the heart valve annulus. 
         [0020]    The prosthetic heart valve may comprise a single stent body having a plurality of flexible, biocompatible membranes incorporated therein that form heart valve leaflets in the expanded configuration. Alternatively, the prosthetic heart valve comprises a two-piece stent body with a primary stent and a secondary stent, wherein the steps of delivering and unfurling comprise delivering and unfurling the primary stent first and then delivering and unfurling the secondary stent within the primary stent. The secondary stent may be guided into coupling position within the primary stent using one or more guidewires. The method further may include anchoring the prosthetic heart valve in its expanded configuration to the heart valve annulus. If the native heart valve leaflets of the heart valve annulus are left in place, the step of unfurling causes the prosthetic heart valve to contact and outwardly compress the native leaflets. The step of unfurling further may include ensuring that the prosthetic heart valve remains generally concentric about a single axis, and also locking the prosthetic heart valve in its expanded configuration. 
         [0021]    A further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a perspective view of an exemplary one-piece expandable heart valve stent of the present invention; 
           [0023]      FIG. 2A  is a perspective view of an exemplary expandable heart valve of the present invention utilizing the stent of  FIG. 1 ; 
           [0024]      FIG. 2B  is a cross-sectional view taken along line  2 B- 2 B through one side of the heart valve of  FIG. 2A  showing a preferred leaflet attachment construction; 
           [0025]      FIG. 2C  is a perspective view of an alternative one-piece expandable heart valve stent of the present invention having a flared outflow end; 
           [0026]      FIG. 3A  is a perspective view of an exemplary two-piece expandable heart valve stent of the present invention having oval-shaped sinus apertures and leaflet attachment strips; 
           [0027]      FIGS. 3B and 3C  are end and side elevational views of the heart valve stent of  FIG. 3A ; 
           [0028]      FIGS. 4A and 4B  are alternative perspective views of an exemplary primary stent for use in an expandable heart valve of the present invention, particularly illustrating side tabs for alignment during unrolling; 
           [0029]      FIGS. 5A and 5B  are alternative partial perspective views of a further primary stent for use in an expandable heart valve of the present invention, particularly illustrating body tabs and slots for alignment during unrolling; 
           [0030]      FIGS. 6A-6D  are different perspective views of a further primary stent for use in an expandable heart valve of the present invention; 
           [0031]      FIG. 7  is a plan view of an exemplary secondary stent for use in an expandable heart valve of the present invention, particularly illustrating generally semi-circular sinus apertures circumscribed by leaflet attachment holes, and body tabs and slots for alignment during unrolling; 
           [0032]      FIG. 8  is a partial perspective view of a commissure/junction region of an exemplary secondary stent, particularly illustrating side tabs for alignment during unrolling; 
           [0033]      FIG. 9  is a perspective view of an exemplary expanded secondary stent of the present invention; 
           [0034]      FIG. 10  is a perspective view of a primary stent like that shown in  FIG. 6A  coupled to a secondary stent like that shown in  FIG. 10 ; 
           [0035]      FIGS. 11A-11C  are different perspective views of a further exemplary primary stent having both edge and body barbs for use in an expandable heart valve of present invention; 
           [0036]      FIGS. 11D and 11E  are end and side elevational views of the heart valve stent of  FIG. 11A ; 
           [0037]      FIG. 12  is a perspective view of a secondary stent coupled to a primary stent like that shown in  FIG. 11A ; 
           [0038]      FIG. 13A  is a perspective view of a schematic secondary stent being coupled to and unrolled within an expanded primary stent like that shown in  FIG. 6A ; 
           [0039]      FIGS. 13B and 13C  are detailed perspective views of the primary and secondary stent coupling shown in  FIG. 13A ; 
           [0040]      FIG. 14  is a schematic perspective view of an exemplary stent rolling apparatus of the present invention; 
           [0041]      FIGS. 15A-15C  are perspective views of the exemplary stent rolling apparatus illustrating details of first and second side edges of the stent; 
           [0042]      FIG. 16  is a perspective view of an alternative means for securing a second edge of a stent being rolled; 
           [0043]      FIGS. 17A and 17B  are schematic perspective views of a stent after having been rolled in accordance with the present invention; 
           [0044]      FIGS. 18A and 18B  are schematic perspective views of a rolled stent being removed from a rolling mandrel; 
           [0045]      FIG. 19  is a plan view of a still further one-piece expandable heart valve stent of the present invention having a more solid outflow section; 
           [0046]      FIG. 20A  is a plan view of another one-piece expandable heart valve stent of the present invention having a flared cage-like outflow section; 
           [0047]      FIG. 20B  is a detailed perspective view of one end of a guide slot in the heart valve stent of  FIG. 20A ; 
           [0048]      FIG. 21A  is a plan view of a heart valve having a one-piece expandable stent similar to that shown in  FIG. 20A  in several configurations from 
           [0049]      FIGS. 21B and 21C  are perspective views of the one-piece expandable heart stent of  FIG. 21A  in partially and fully unrolled configurations, respectively; 
           [0050]      FIG. 22  is a schematic perspective view of a two-piece heart valve stent assembly prior to coupling a secondary stent to a primary stent using guidewires; and 
           [0051]      FIG. 23  is a schematic perspective view of a two-piece heart valve stent assembly prior to coupling a secondary stent having a wireform structure to a primary stent using guidewires. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0052]    The present invention discloses a number of expandable heart valves for implantation in a host annulus, or host tissue adjacent the annulus. The valves may be implanted in any of the four valve positions within the heart, but are more likely to be used in replacing the aortic or mitral valves because of the more frequent need for such surgery in these positions. The patient may be placed on cardiopulmonary bypass or not, depending on the needs of the patient. 
         [0053]    A number of expandable prosthetic heart valves are disclosed that are initially rolled into a tight spiral to be passed through a catheter or other tube and then unfurled or unrolled at the implantation site, typically a valve annulus. The heart valves comprise one- or two-piece stent bodies with a plurality of leaflet-forming membranes incorporated therein. Various materials are suitable for the stent body, although certain nickel-titanium alloys are preferred for their super-elasticity and biocompatibility. Likewise, various materials may be used as the membranes, including biological tissue such as bovine pericardium or synthetic materials. It should also be noted that specific stent body configurations disclosed herein are not to be considered limiting, and various construction details may be modified within the scope of the invention. For example, the number and configuration of lockout tabs (to be described below) may be varied. 
         [0054]    Those of skill in the art will recognize that the means and techniques for delivering and implanting the prosthetic heart valves disclosed herein are numerous and not the specific focus of the present application. In general, the heart valves in a first, contracted configuration are delivered through a tube such as a percutaneously-placed catheter or shorter chest cannula and expelled from the end of the tube in the approximate implantation location. The heart valve is then expanded via a balloon, mechanical means, or self-expanded from internal elastic forces, into a second, expanded configuration that engages the native host tissue, such as the target valve annulus. Depending on the native valve being replaced, the prosthetic heart valve may have varying axial lengths. For example, in the aortic position, a portion of the valve may extend upward into and even contact the aorta to better stabilize the commissure regions of the valve. In other words, the particular design of the valve may depend on the target valve location. 
         [0055]    With reference to FIGS.  1  and  2 A- 2 B, an exemplary one-piece prosthetic heart valve  20  (complete in  FIG. 2A ) of the present invention is shown. The valve  20  comprises a stent body  22  that is shown isolated in  FIG. 1 , and a plurality of leaflet-forming membranes  24 . The stent body  22  is shown in both  FIGS. 1 and 2A  in its expanded configuration generally defining a tube centered about an axis. The membranes  24  fasten within the stent body  22  so as to form a one-way valve therewithin, and orient the valve to have an inflow end  28  and an outflow end  30 . In a preferred embodiment, there are three such membranes  24  each having a free edge  32  that extends inward from the stent body  22  and coapts or meets the other two free edges generally along radial lines spaced apart 120° with respect to each other to close the valve during the back flow cycle of blood flow, as seen in  FIG. 2A . When blood flows in the opposite direction, from the inflow to the outflow end, the free edges  32  of the membranes  24  move radially outward away from each other to open the valve. 
         [0056]    With specific reference to  FIG. 1 , the tubular stent body  22  comprises three sections, starting at the inflow end  28  and moving toward the outflow end  30 : an annulus anchoring section  40 , a sinus section  42 , and an outflow section  44 . The three sections  40 ,  42 , and  44  are desirably formed from a single generally sheet-like piece of material that can be cohesively rolled into a tight spiral and expanded into the tubular configuration shown. In this regard, the stent body  22  includes an axially-oriented first side edge  50  that mates with an axially-oriented second side edge  52  along longitudinal seam  53 . The two side edges  50 ,  52  abut or overlap and lock together using one or more, preferably two or more cooperating tabs  54  and slots  56 . In the illustrated example, two series of slots  56   a,    56   b  are provided around the circumference of the stent body  22  adjacent the first side edge  50 , while a pair of engaging tabs  54   a,    54   b  are provided adjacent the second side edge  52 . 
         [0057]    The annulus anchoring section  40  is desirably substantially solid and free of perforations so as to more reliably retain its tubular shape upon outward expansion against the native heart valve annulus. In a preferred implantation technique, the prosthetic heart valve  20  expands outward and compresses against the native leaflets which present a relatively uneven base. Even if the leaflets are excised, the circularity of the annulus depends on the skill of the surgeon. Minimizing any openings in the anchoring section  40  enhances its rigidity so as to ensure a relatively tubular support structure for the leaflet-forming membranes  24 . However, anchoring barbs  60  may be provided in the anchoring section  40 , and may be formed by integrally cut tabs as shown. In addition, a pair of openings  62  may be optionally provided in the side wall of the tubular stent body  22  to reduce the roll-up stiffness. 
         [0058]    With reference to  FIG. 2A , the sinus section  42  comprises a plurality (preferably three) of generally axially extending commissures  70  and curvilinear cusps  72  defined by relatively large sinus apertures  74  in the stent body  22 . In the illustrated embodiment, the sinus apertures  74  are generally semi-circular with a straight, circumferential edge  76  defined by the beginning of the outflow section  44 . A plurality of small attachment apertures  78  track along the edge of the sinus apertures  74 , extending around the curvilinear cusps  72  and substantially up the entire commissures  70 . 
         [0059]    The membranes  24  fasten to the stent body  22  using the attachment apertures  78 . More particularly, as seen in  FIG. 2B , an outer edge portion  80  of each membrane  24  folds upward in the outflow direction to lie against an inner surface  84  of the stent body  22 . This folded attachment helps reduce localized stresses caused by the sutures through the membrane  24 , and enhances coaptation of the free edges  32  at the commissures  70 . Fasteners such as sutures  82  secure the outer edge portion  80  flush against the inner surface  84 . The sutures typically loop through the membrane  24  twice at each attachment aperture  78  in a single mattress stitch, though various other stitching techniques are known. In a preferred embodiment, the attachment apertures  78  are spaced apart a minimum distance of about 0.004-0.0075 inches for strength. 
         [0060]    A small lip  86  of the outer edge portion  80  desirably projects beyond the sinus aperture  74  to help protect the membrane  24  from rubbing directly against the material of the stent body  22  during operation of the valve. That is, there is membrane-to-membrane cushioned contact at the sinus apertures  74  when the membranes  24  are forced outward in the opening cycle of the valve. Additionally, all exposed edges of the stent body  22  are electropolished or coated with a layer of lubricious material (e.g., PTFE or “TEFLON”) to eliminate any sharp corners and thus reduce wear on the flexible membranes  24 . 
         [0061]    The free edge  32  of each membrane  24  meets the stent body  22  at one of the commissures  70 . Because adjacent arrays of attachment apertures  78  converge in the outflow direction along each commissures  70 , the free edges  32  of adjacent membranes  24  coapt at or closely adjacent to the stent body inner surface  84 , as best seen in  FIG. 2A . This configuration eliminates leakage between the free edges  32  when the valve closes. 
         [0062]    The outflow section  44  desirably comprises at least a circular band  90  of material that joins the outflow ends of the commissures  70 . In the illustrated embodiment, the outflow section  44  further includes a second band  92  axially spaced from the first band  90  and joined thereto with a lattice, mesh or grid  94 . The outflow section  44  may not be in contact with any tissue of the heart, but rather project into the respective outflow chamber as a support for the three commissures  70 . That is, substantial inward radial loads are imposed on the commissures  70  during the closing cycle of the valve, and the outflow section  44  maintains the spacing between the commissures to ensure proper coaptation of the membrane free edges  32 . The grid  94  defines more spaces than connecting struts, and thus minimizes interference with proper blood flows in the outflow chamber. The outflow section  44  may be rigid, or may be somewhat flexible to mirror aortic wall movement. 
         [0063]    In  FIG. 2C , an alternative stent body  22 ′ has a flared outflow section  44 ′ section that conforms to and contacts the aortic wall in an aortic valve replacement setting. The aortic wall and sinuses diverge outward from the annulus, in which the annulus anchoring section  40 ′ resides. Therefore, the outward flaring of the outflow section  44 ′ permits contact with the aortic wall and better stabilizes the valve in its implantation position. Further, the backflow volume on the outflow side of the leaflets will be slightly increased which may enhance valve closing. The outflow section  44 ′ may be formed to spring open to the flared shape, or may be plastically deformed into the flared shape using a non-cylindrical expansion balloon. For example, the outflow section  44 ′ may be annealed Nitinol that self-expands to the flared shape upon being released from within a delivery tube. Further embodiments of stents having the flared outflow section are shown and described below. 
         [0064]    With reference to  FIGS. 3A-3C , an exemplary two-piece stent body  100  comprises a generally ring-shaped primary stent  102  and a tubular secondary stent  104  coupled therewithin. The primary stent  102  is shown isolated in  FIGS. 4A and 4B  and includes a first side edge  106 , a second side edge  108 , and a pair of opposed end edges  110   a,    110   b.  A pair of alignment tabs  112  projects radially outward from the end edges  110   a,    110   b  adjacent the second side edge  108 . The alignment tabs  112  provide guides for use during unfurling of the primary stent  102  to maintain concentricity about a central axis. That is, as the primary stent  102  transitions between a first, contracted configuration (i.e., a tight spiral) and a second, expanded configuration, the alignment tabs  112  prevent the stent from unrolling to form a cone. Desirably, in the first, contracted configuration, the primary stent  102  is spirally-wound about an axis such that at least one winding of the stent body  100  surrounds another winding, and preferably there are numerous windings to reduce the radial profile of the stent  102 . Desirably, the second side edge  108  resides at the center of the tightly rolled second configuration such that as the stent  102  unrolls, the end edges  110   a,    110   b  slide by and are constrained within the tabs  112 . In addition, the primary stent  102  includes lockout structure in the form of a pair of tabs  114  projecting radially inward near the first side edge  106  and a pair of notches  116  in the second side edge  108 . The tabs  114  fit within the notches  116  and lock the two side edges  106 ,  108  together. Desirably, a bi-directional locking arrangement is provided to prevent contraction of the stent but also further expansion. There are preferably two locking tabs/slots along the mating edges, desirably located symmetrically about an axial midplane of the stent. 
         [0065]    Referring to  FIGS. 3A-3C , the secondary stent  104  includes a generally solid inflow section  120 , a sinus section  122 , and an outflow band  124 . The sinus section  122  is relatively more solid than the sinus section  42  of the first embodiment, and includes a plurality, preferably three, oval-shaped sinus apertures  126 . A leaflet-forming membrane (not shown) fastens around the inflow edge of each of the sinus apertures  126  in such a manner so as to coapt within the tubular stent body  100  and define the valve occluding surfaces. More specifically, a membrane fastening strip  128  follows the edge contour of each membrane with a pair of commissure regions  130  and a curvilinear cusp region  132  and provides an anchor to which the membrane may be attached. The fastening strip  128  may be made of pericardium, and may be fastened to the inner surface of the secondary stent  104  using stitching or other suitable expedient. 
         [0066]    In an exemplary embodiment, secondary stent  104  includes at least one locking tab  140  that projects outwardly through a locking window  142  in the primary stent  102  to retain the two stents in cooperating relationship. The secondary stent  104  includes a first side edge  144  and a second side edge  146  that overlap and are locked together using suitable tabs/notches (not further described herein). In use, the primary stent  102  is first delivered and then unfurled and secured in the native annulus, after which the secondary stent  104  is delivered and then unfurled and locked within the primary stent. One or more alignment tabs  150  may be provided on the secondary stent  104  to engage alignment slots  152  and ensure the secondary stent unfurls concentrically around the axis. Further, the outwardly projecting alignment tabs  112  and locking tab(s)  140  may double as anchoring barbs projecting into the native tissue. 
         [0067]    Alternatively, a ratchet type of locking arrangement can be provided for the primary stent  102  or secondary stent  104  to enable greater size adjustment. For instance, multiple engaging teeth may be formed on either stent  102  or  104  to enable substantially continuous size adjustment beyond a minimum annulus diameter. The ratchet teeth may be on circumferentially opposed surfaces or a bent end tab may engage teeth provided on a circumferential edge of the stent. Likewise, coupling structure between the primary and secondary stents may be used other than the tabs/slots shown. For instance, a hook and loop connection may be realized by expanding the secondary stent within the primary stent. 
         [0068]      FIGS. 5A and 5B  show in greater detail exemplary alignment tabs/slots and locking tabs/notches. These figures illustrate an exemplary primary stent having a first side edge  160  and a second side edge  162 , although the same concepts may be applied to a secondary stent. A pair of alignment tabs  164  projects radially outward from the second side edge  162  and a second pair of alignment tabs  166  projects radially outward from the body of the stent. A series of circumferential slots  168  are provided along the length of the stent such that the tabs  164 ,  166  are received therein during the unfurling process. The slots  168  guide the tabs  164 ,  166  to prevent the stent from unfurling into a cone. Once the stent has fully expanded, a pair of locking tabs  170  projecting radially inward from near the first side edge  160  engages a pair of notches  172  in the second side edge  162 . 
         [0069]      FIGS. 6A-6D  illustrate a still further primary stent  180  that is similar to, but slightly axially longer than, the primary stent  102  described above. Again, the stent  180  includes overlapping first and second side edges  182   a,    182   b,  respectively, and circumferentially disposed end edges  184   a,    184   b.  As seen best in  FIGS. 6B and 6C , three alignment tabs  186  project radially outward from the second side edge  182   b  into alignment slots  188 . As before, these alignment tabs and slots prevent the primary stent  180  from unfurling unevenly to form a cone. It should be noted that the middle alignment slot  188  is circumferentially staggered with respect to the two alignment slots near the end edges  184   a,    184   b  such that at least one alignment tab  186  resides in one of the slots at all times. Additionally, two pairs of alignment tabs  190  project radially outward from the end edges  184   a,    184   b  at the second side edge  182   b,  further insuring against misalignment during the unfurling process. A pair of locking tabs  192  projects inward from the primary stent  102  near the first side edge  182   a  and engages a cooperating pair of locking notches  194  formed in the second side edge  182   b.  As can be appreciated from  FIG. 6B , the locking tabs  192  and notches  194  prevent the primary stent  180  from contracting once it has been fully expanded. Finally,  FIG. 6D  is a detail of an inwardly directed coupling tab  196  that may be used to couple a secondary stent to the primary stent  180 . In the illustrate embodiment, there are three such coupling tabs  196  distributed evenly about the stent. 
         [0070]      FIG. 7  illustrates a secondary stent  200  of the present invention in plan view, before being rolled into its contracted configuration. The stent  200  has a generally rectangular periphery defined by a first side edge  202   a,  a second side edge  202   b,  and a pair of linear end edges  204   a,    204   b . Again, the secondary stent  200  comprises a generally sheet-like body that can be rolled into a relatively tight configuration and unrolled into a tube. Three sinus apertures  206   a,    206   b,    206   c  formed in the secondary stent  200  each having a curvilinear cusp  208  and a pair of generally linear commissures  210  of either side of the cusp. The commissures  210  are joined by an outflow band  212 . A pair of combined alignment and locking tabs  216  is sized to translate within respective alignment slots  218  to insure even unfurling of stent  200 . A pair of locking notches  220  is formed at the end of the alignment slots  218  closest to the first side edge  202   a.  The locking tabs  216  have an enlarged head joined by a neck to the body of the stent  200  and the locking notches  220  also include a tapered neck  222  that permits passage of the tab neck in only one direction so as to lock it therein. 
         [0071]      FIG. 8  is a detailed isolation of overlapping side edges of a secondary stent showing alignment tabs  230  disposed on side edges of the inner layer of the stent. These alignment tabs  230  therefore can replace the alignment tabs  216  and slots  218  of the secondary stent  200  of  FIG. 7 , although alternative locking structure must be provided. 
         [0072]      FIG. 9  illustrates a still further secondary stent  250  of the present invention, and  FIG. 10  illustrates the same stent coupled with the primary stent  180  of  FIG. 6A . The secondary stent  250  includes many of the same features described above, including a generally solid inflow section  252 , a sinus section  254 , and an outflow band  256  (again, the leaflet-forming membranes are not shown to better illustrate the stent). The body of the stent  250  includes two pairs of side alignment tabs  258  that prevent the stent  250  from unfurling into a conical form. One or more lockout tabs  260  extend outward from one side edge of the stent  250  and engage one or more apertures  262  in the other side edge to secure the edges in an overlapping relationship as shown. A plurality of coupling windows  264  is located at evenly-spaced circumferential intervals around the body of the stent  250  to receive and retain coupling tabs  196  extending inward from the primary stent  180  (see  FIG. 6D ). Note in  FIG. 10  that the alignment tabs  258  closely conform to the inflow end of the primary stent  180  and further help retain the stent assembly together. Also, these alignment tabs  258  may serve as anchoring barbs to retain the valve in the host annulus. 
         [0073]      FIGS. 11A-11E  illustrate another primary stent  270  that features a plurality (at least three) of outwardly angled anchoring spikes  272 . The stent  270  includes a band-like body  274  having a first side edge  276   a  and a second side edge  276   b,  with opposed and parallel end edges  278   a ,  278   b  extending therebetween. The anchoring spikes  272  extend axially away and then radially outward from the respective end edges  278   a,    278   b  a distance of between about 1-2 mm. There are desirably at least three anchoring spikes  272  extending from each end edge  278   a,    278   b,  and more preferably six. In addition, a plurality of body anchoring barbs  280  is disposed at regular intervals around the body  274 . These barbs  280  may be small portions of the body  174  stamped into spikes and bent outward from the body  274 . The barbs  280  desirably have a length of about 1 mm.  FIGS. 11B and 11C  illustrate a two-way lockout structure on the side edges  276   a,    276   b  including tabs  282  and receptacles  284 . In addition, alignment tabs  286  and slots  288  are provided as described above. 
         [0074]      FIG. 12  shows the primary stent  270  of  FIGS. 11A-11E  coupled to an alternative secondary stent  290 . The secondary stent  290  has relatively large, semi-circular sinus apertures  292  and membrane attachment strips  294  on its inner surface. Note that the sinus apertures  292  have a curvilinear cusp edge  296  that coincides approximately with an end edge  278   b  of the primary stent  270 . This maximizes exterior reinforcement for the secondary stent  290  without interfering with the motion of the leaflet-forming membranes (not shown). 
         [0075]      FIGS. 13A-13C  schematically illustrate a secondary stent  300  unfurling within a primary stent  302 . The primary stent  302  includes coupling tabs  304  bent inward from the body of the stent that have an axially-opening notch  306  on one side. The tabs  304  are slightly circumferentially offset with respect to one another, and axially spaced nearly the entire axial dimension of the primary stent  302 . As best seen in  FIG. 13C , the secondary stent  300  has a pair of V-shaped slots  308  located on a first side edge  310  that couple with the tabs  304 . More specifically, the slots  308  terminate in a bridge  312  between the slot and a cutout  314 , and the coupling tab  304  is designed to frictionally engage the bridge by virtue of the shape of the notch  306 . The first side edge  310  is thus unrolled and the tabs  304  coupled to the slots  308  by a relative axial displacement of the secondary stent  300  and primary stent  302 . Once coupled, the secondary stent  300  is fully unfurled and locked in its expanded configuration within the primary stent  302 . The secondary stent  300  may be coupled to the primary stent  302  using relative axial and/or circumferential motion with or without a tactile feedback signaling completion of the coupling operation. 
         [0076]      FIGS. 14-18  illustrate various steps in the process of rolling a primary stent of the present invention (i.e., converting a flat sheet-like material into the first, contracted configuration of the stent). A rolling base  320  includes a raised rolling platform  322  surrounded by a pair of linear rolling tracks  324 . A stent roller  326  includes a central mandrel  328  and a pair of rolling wheels  330  that ride within the tracks  324 . 
         [0077]    An initially flat sheet-like primary stent  334  is placed on the rolling platform  322  and secured thereto at a first side edge  336 .  FIG. 15C  illustrates one means for securing the first side edge  336 , that is, angled pins  338  through holes in the first end. Alternatively, a clamp  340  as seen in  FIG. 16  may be tightened over the first side edge  336 . 
         [0078]    With reference to  FIGS. 15A and 15B , the stent roller  326  is temporarily secured to a second side edge using a pin  342  aligned with the mandrel  328 . A plurality of lockout tabs  344  are seen projecting between the pin  342  and the mandrel  328  such that rotation of the roller  326  lifts the second side edge upward from the platform  322 . The pin  342  extends through a small cavity in both rolling wheels  330  adjacent the mandrel  328  and may be easily removed once the rolling operation is complete. 
         [0079]      FIG. 17A  shows the stent  334  in its rolled configuration after the stent roller  326  has translated the length of the rolling platform  322 . The rolling tracks  324  are slightly ramped upward toward the platform  322  to accommodate the gradually increasing diameter of the stent  334  as it is rolled. A plurality of linear grooves  350  in the rolling platform  322  provide clearance for any radially outwardly projecting tabs on the stent  334 .  FIG. 17B  shows a suture  352  or other such retaining means tied around the rolled stent  334  to enable removal of the stent and roller  326  from the platform  322 . 
         [0080]    Finally,  FIGS. 18A and 18B  schematically illustrate the steps for removing the rolled stent  334  from the roller  326 . Specifically, one of the wheels  330  is removable and the rolled stent  334  is then freed for use. The inner bore illustrated may be substantially smaller if a smaller mandrel  328  is used. The same sequence of rolling may be used for both the primary and secondary stents with the membranes. The membranes lie relatively flat against the secondary stents and present little obstacle to rolling. 
         [0081]    The rolled stent  334  desirably has a diameter of less than about 20 mm. An aspect ratio of the stents of the present invention may be defined as the axial length over the final, expanded diameter. Some of the primary stents as described above may have a relatively small aspect ratio, desirably less than about 2. 
         [0082]    Once the rolled stent  334  is formed, it is loaded within a delivery tube or catheter and urged down the tube to the implantation site (of course, the suture  352  will be removed). A pusher or other such device may be used to advance the rolled stent  334 . Once at the site, the tube may be retracted and the rolled stent  334  caused to unfurl on its own, the stent may be delivered over an inflation balloon to enable plastic deformation/expansion, or the stent may be expanded with a subsequently introduced balloon or mechanical expander. 
         [0083]      FIG. 19  illustrates a still further one-piece expandable heart valve stent  400  of the present invention in its flattened configuration having a somewhat more solid or robust outflow section  402  than shown previously coupled to a sinus section  404  and anchoring section  406  on the inflow end of the stent. The stent  400  comprises a single sheet-like body  408  of a rolled superelastic metal alloy, preferably Nitinol. For orientation purpose, the body  408  is initially formed in the Y-Z plane as shown, and is elongated in the Y direction with a generally rectangular outline. The body  408  is designed to be rolled up on itself about a Z-axis into a relatively tight spiral, and later unrolled to form a tube with a first side edge  410   a  connecting to a second side edge  410   b.  In the illustrated embodiment, the left side of the stent body  408  forms the inner winding of the spiral while the right side is the outer winding. Desirably, and as mentioned above, the first side edge  410   a  and second side edge  410   b  overlap in the enlarged tubular configuration. The body  408  also defines relatively linear first and second end edges  412   a,    412   b  that form the circular outflow and inflow rims, respectively, of the tubular stent. 
         [0084]    The stent  400  includes alignment structure for ensuring proper unrolling about the central Z-axis, and also locking structure for maintaining the final tubular shape. Specifically, a pair of guide/lockout tabs  414   a,    414   b  engage a guide slot  416  that extends along the Y-axis in the outflow section, closely adjacent the sinus section  404 . A single such guide slot  416  as shown located generally in the center of the body  408  with respect to the Z-axis is believed sufficient to hold the stent in the final tubular shape, although two or more may be used as described previously. The guide/lockout tabs  414   a,    414   b  each include an enlarged generally semi-circular head  418  and a narrow neck  420  connecting the head to the body  408 . A first tab  414   a  extends from the first end edge  410   a  while a cutout in a mid-portion of the body  408  forms a second tab  414   b.    
         [0085]    The spaced tabs  414   a,    414   b  align with the guide slot  416  and are annealed out of the plane of the body  408  so as to fit within the slot. Specifically, the tabs  414   a,    414   b  are annealed so that they bend inward with respect to the rolled spiral of the stent body  408  and can then be introduced into the slot  416 . Once in the slot  416 , the head  418  of each tab  414   a,    414   b  projects through to the outside of the body  408  and retains the tabs in engagement with the slot. The neck  420  has a width that is slightly smaller than the slot width for easy longitudinal movement therewithin. As the stent body  408  unfurls from its tightly coiled contracted state to its expanded state, the tabs  414   a,    414   b  travel along the slot  416  (from the left to the right in the drawing). As this process occurs, the maintenance of the tabs  414   a,    414   b  within the slot  416  ensures that the stent body  408  will not misalign and unroll into a conical shape. Ultimately, the tabs  414   a,    414   b  travel past two pairs of similarly spaced lockout notches  422  annealed out of the plane of the body  408  toward the inside of the now tubular stent. The interference between these lockout notches  422  and the heads  418  of the tabs  414   a,    414   b  retains the stent  400  in its open, expanded configuration. 
         [0086]    A plurality of engaging pairs of bridge tabs  424  and apertures  426  maintain a uniform width of the guide slot  416  to retain the tabs  414   a,    414   b  therein during unrolling of the stent body  408 . Each tab  424  is annealed so as to bend and lock into the corresponding aperture  426 . Maintenance of the guide slot  416  width ensures a continuous engagement of the tabs  414   a,    414   b  and guide slot  416  during the unrolling process. 
         [0087]    The stent body  408  further includes a plurality of edge tabs  430  located along both end edges  412   a,    412   b  adjacent the first side edge  410   a.  Although shown flattened in the plane of the stent body  408 , the edge tabs  430  are also annealed to bend generally perpendicular to the stent body. The edge tabs  430  are disposed closely to and constrain the end edges  412   a,    412   b  during the unrolling process to further help prevent misalignment. A pair of stop slots  432  is formed in the anchor section  406  to limit the extent that the stent body  408  unrolls. One side of each slot  432  is annealed out of the plane of the stent body  408  so that they engage each other after the body has unrolled to the tubular final shape. 
         [0088]    The outflow section  402  includes an array of diamond-shaped apertures  434  forming an open lattice, mesh or grid pattern that reduces the stent surface area and thus the risk of thrombosis after implantation. The open mesh pattern is somewhat stiffer than, for example, the grid pattern shown in the stent of  FIG. 1 , and helps stabilize the valve commissures  440  about which flexible leaflet membranes  442  (shown in phantom) are attached. A plurality of triangular-shaped cutouts  444  aligned in the Y-direction in the outflow section  402  “ratchet” against one another during unrolling of the stent body  408  and thus incrementally prevent closing of the stent. 
         [0089]    Still with reference to  FIG. 19 , the sinus section  404  incorporates three membrane apertures  450  defining the aforementioned commissures  440  and intermediate curvilinear cusps  452 . A series of attachment holes  454  closely surrounds each aperture  450  and is used to suture or otherwise attach each membrane  442  to the stent  400 . The edge of each membrane  442  is folded as described above with respect to  FIG. 2B  to help prevent wear and ensure longevity. The opposed ends of the sinus section  404  are shaped to conform to the outer two membrane apertures  450 . That is, a pair of opposed extension flaps  456   a,    456   b  on the anchoring section  406  overlap and each blends along a curvilinear edge  458   a,    458   b  toward the outflow section  402 . These curvilinear edges  458   a,    458   b  provide reliefs to avoid occluding either of the outer two membrane apertures  450  when the stent is locked open and the flaps  456   a,    456   b  overlap. 
         [0090]    Although not shown, a plurality of anchoring barbs are desirably provided in at least the anchoring section  406  to secure the unrolled valve into position in the valve annulus and aortic root. Further, the outflow section  402  may be annealed so as to flare outward and contact the ascending aorta for further anchoring. 
         [0091]      FIG. 20A  illustrates a still further one-piece expandable heart valve stent  500  of the present invention in its flattened configuration with an outflow section  502  coupled to a sinus section  504  and anchoring section  506  on the inflow end of the stent. The stent  500  again comprises a single sheet-like body  508  of a rolled superelastic metal alloy, preferably Nitinol. For orientation purpose, the body  508  is initially formed in the Y-Z plane as shown, and is elongated in the Y direction with a generally rectangular outline. The body  508  is designed to be rolled up on itself about a Z-axis into a relatively tight spiral, and later unrolled to form a tube with a first side edge  510   a  connecting to a second side edge  510   b.  In the illustrated embodiment, the left side of the stent body  508  forms the inner winding of the spiral while the right side is the outer winding. That is, the stent body  508  is rolled from the left end in the direction of arrow  511 . Desirably, the first side edge  510   a  and second side edge  510   b  overlap in the enlarged tubular configuration. The body  508  also defines first and second end edges  512   a,    512   b  that form the circular outflow and inflow ends, respectively, of the tubular stent. 
         [0092]    The stent  500  includes alignment structure for ensuring proper unrolling about the central Z-axis, and also locking structure for maintaining the final tubular shape. Specifically, guide/lockout tabs  514   a,    514   b  engage guide slots  516   a,    516   b  aligned therewith along the Y-axis. A first pair of tab  514   a  and slot  516   a  is located in the outflow section, closely adjacent the sinus section  504 , while a second pair of tab  514   b  and slot  516   b  is located in the anchoring section, closely adjacent the second end edge  512   b.  The guide/lockout tabs  514   a,    514   b  are each formed with an enlarged head  518  and a pair of necks  520  on either side of the head connecting it to the body  508 . Each head  518  is annealed to bend about the necks  520  out of the plane of the stent body  508  and fits through an entrance opening  522  into the respective slot  516 . In the illustrated embodiment, the heads  518  are bent out of the page and the stent body  508  is rolled about the Z-axis out of the page so that the heads  518  project radially outwardly through the entrance openings  522 . 
         [0093]    As seen in  FIGS. 20A and 20B , each slot  516  includes a pair of lockout tabs  524  near the slot end closest to the second end edge  510   b.  Small angled cutouts  526  diverging on either side of the slot  516  form the lockout tabs  524 . Each tab  524  is annealed to bend out of the plane of the stent body  508 , in this case into the page. As the stent body  508  unrolls, the heads  518  of the tabs  514   a,    514   b  slide from left to right along the slots  516  and cam over the bent tabs  524 . The tabs  514   a,    514   b  are thus prevented by the tabs  524  from retreating along the slots  516   a,    516   b.  The maintenance of the tabs  514   a,    514   b  within the slots  516   a,    516   b  ensures that the stent body  508  will not misalign and unroll into a conical shape. 
         [0094]    A plurality of bridges  528  maintains a uniform width of the guide slots  516   a,    516   b  to retain the tabs  514   a,    514   b  therein during unrolling of the stent body  508 . Each bridge  528  crosses over the respective slot  516   a,    516   b  and is secured thereto at points  530 , such as by ultrasonic welding. Alternatively, bridges formed as an integral part of the stent body  508  are contemplated. Maintenance of the guide slot  516  width ensures a continuous engagement of the tabs  514   a,    514   b  and guide slots  516   a,    516   b  during the unrolling process. The bridges  528  are located on the inner side of the stent  508  in its rolled configuration. 
         [0095]    The outflow section  502  includes an array of cross members  534  forming a lattice, mesh or grid pattern with diamond-shaped openings that reduces the stent surface area and thus the risk of thrombosis after implantation. Adjacent the mesh pattern, a solid band  536  of the stent body  508  within which the guide slot  516   a  is formed helps stabilize the valve commissures  540  about which flexible leaflet membranes  542  (shown in phantom) are attached. 
         [0096]    Still with reference to  FIG. 20A , the sinus section  504  incorporates three membrane apertures  550  defining the aforementioned commissures  540  and intermediate curvilinear cusps  552 . A series of attachment holes  554  closely surrounds each aperture  550  and is used to suture or otherwise attach each membrane  542  to the stent  500 . The edge of each membrane  542  is folded as described above with respect to  FIG. 2B  to help prevent wear and ensure longevity. The right end of the sinus section  504  is shaped to conform to the left membrane apertures  550 . That is, a curvilinear edge  558  provides a relief to avoid occluding the left membrane aperture  550  when the stent is locked open and the end edges  510   a,    510   b  overlap. 
         [0097]    Although not shown, a plurality of anchoring barbs are desirably provided in at least the anchoring section  506  to secure the unrolled valve into position in the valve annulus and aortic root. Further, the outflow section  502  may be annealed so as to flare outward and contact the ascending aorta for further anchoring. 
         [0098]      FIGS. 21A  illustrates a heart valve  600  of the present invention having a stent  602  similar to the stent  500  described above with reference to  FIG. 20A . A pair of lockout/guide tabs  604   a,    604   b  engages an aligned pair of guide slots  604   a,    604   b  to both ensure proper unrolling and secure the unrolled valve in its expanded configuration. The tabs  604   a,    604   b  and slots  606   a,    606   b  may be configured as described above with respect to either of the embodiments of  FIG. 19  or  20 A, or may be a similar expedient. In this regard, entrance openings  608  and lockout tabs  610  may be provided to enable the tabs  604   a,    604   b  to enter the slots  606   a,    606   b  and be retained therein in an open, unrolled configuration of the valve  600 . A plurality of bridges  612  seen on the inside of the stent  602  through the slots  606   a,    606   b  maintain the width of the slots as described above. 
         [0099]    The stent  602  includes an outflow section  620  having a mesh  622  that is annealed to flare outward into contact with the aorta and increase the stiffness of valve commissures in a sinus section  624 . The sinus section  624  includes three membranes  626  attached around generally semi-circular apertures  628  to form the occluding surfaces of the valve when fully unrolled. 
         [0100]      FIG. 21B  illustrates the stent  602  by itself in a partial state of unrolling, while  FIG. 21C  shows the stent fully unrolled. Note the flared configuration of the mesh  622  on the outflow section  620  and the overlapped sides of the stent. 
         [0101]      FIGS. 22 and 23  illustrate two different two-piece expandable heart valve stents that are coupled using guide wires. In  FIG. 22 , a generally tubular primary stent  700  is first unrolled and implanted in the body. A secondary stent  702  of various configurations described above is then delivered in its contracted state into proximity with the primary stent  700  and unrolled and coupled thereto. To ensure proper rotational alignment between the primary stent  700  and secondary stent  702 , a plurality of guide wires  704  are threaded through features (not shown) within the secondary stent  702  and coupled to corresponding features on the primary stent  700 . For example, the guide wires  704  may be threaded or otherwise registered with coupling tabs (not shown) on the secondary stent  702  and also with coupling apertures  706  on the primary stent  700 . In this way, the secondary stent  702  advances along the guide wires  704  and is rotationally oriented thereby to ensure mating engagement of the coupling features. The distal end of a delivery tube  708  is illustrated through which the guide wires  704  are pulled. 
         [0102]      FIG. 23  likewise shows a generally tubular primary stent  720  being coupled to a secondary stent  722  using a plurality of guide wires  724 . The secondary stent  722  includes a tubular mesh portion  726  and a scalloped wireform portion  728  on an outflow end. Although not shown, the wireform portion  728  receives valve leaflets or an intact bioprosthetic valve as is well known in the art. The tubular mesh portion  726  fits within and couples to the tubular primary stent  720 , while the wireform portion  728  remains completely or substantially completely extended out of the outflow end of the primary stent. Again, the distal end of a delivery tube  730  is illustrated. 
         [0103]    The heart valves of the present invention may be implanted using several minimally-invasive approaches, and in one or more stages. For example, the single stent valves described herein may be delivered using a pusher or along with a balloon catheter through a large bore cannula or catheter (i.e., tube). The two piece valves may be delivered through a single tube, or through two different tubes in sequence. In one embodiment, the stent having the flexible membranes thereon may be stored in an unfurled configuration to reduce stress on and damage to the membranes, and rolled into a compact tube just prior to use. One or two balloons may be used, or the stents can be primarily self-expanding with a balloon or other expansion device used to provide a final deployment force, such as for anchoring barbs in the annulus or locking the rolled stents in the open configuration. 
         [0104]    While the foregoing describes the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Moreover, it will be obvious that certain other modifications may be practiced within the scope of the appended claims.