Patent Publication Number: US-2012035702-A1

Title: Stent for valve replacement

Description:
STATEMENT OF FEDERALLY SPONSORED RESEARCH 
     The U.S. Government has certain rights in the invention described herein, which was made in part with funds from NM Contract No. HHSN263200700191. 
    
    
     FIELD 
     The present document relates to stents used to deliver and position valve prostheses within the human anatomy. 
     BACKGROUND 
     Aortic stenosis and aortic regurgitation are the most common types of aortic valvular diseases. When treating aortic valvular diseases, a diseased natural valve in the body is traditionally replaced with a valve prosthesis by surgical implantation. 
     Two basic types of artificial aortic valves are available for replacement of diseased human heart valves. The first type, a mechanical valve, is constructed of synthetic rigid materials, such as polymer or metal. Its use is associated with thrombogenesis, which requires valve recipients to be on long term anti-coagulants. The second type, a tissue valve or bioprosthetic valve, includes valve leaflets of preserved animal tissue mounted on an artificial support or “stent.” 
     Presently, an aortic valve replacement procedure requires a sternotomy and the use of cardiopulmonary bypass to arrest the heart and provide a bloodless field in which to operate. The native aortic valve is resected through a large incision in the aorta and then a prosthetic valve is sutured in the place of the native valve. Due to the invasiveness of the procedure, aortic valve replacement surgery is associated with significant risk of morbidity and mortality, especially in elderly patients. 
     To decrease the risks associated with aortic valve replacement procedures, many surgeons and scientists have pursued less invasive approaches and techniques. There are two methods that are currently being investigated and developed for minimally invasive aortic valve replacement: percutaneous transcatheter valve delivery and transapical aortic valve replacement. The latter approach is emerging as a viable minimally invasive approach that consists of the placement of a bioprosthetic valve via a trocar that is inserted into the apex of the beating heart. Generally, the prosthesis used for both types of techniques includes a prosthetic valve affixed or sewn into a balloon-expandable or self-expanding stent that is surgically implanted. 
     However, the durability of bioprosthetic heart valves is limited to about 12 to 15 years. The limitations in the long term performance of bioprosthetic heart valves are believed to be due largely to the mechanical properties of the valve and the stresses imposed on the tissue leaflets by the rigidity of the stent structure while the aortic root to which the artificial valve is attached expands and contracts during the cardiac cycle. An important feature of the natural heart valve is its ability to expand in diameter by more than 10% during systole. This ability of the aortic root to expand facilitates blood flow due to a better opening of the valve during systole and contributes to minimal bending of the cusps, thus reducing possible internal flexural fatigue. In addition to the issue of expansion/contraction of the aortic root, there is also significant torsion/twisting motion that the aorta undergoes during each pulse. Ideally, this motion needs to be accounted for by any prosthetic valve design that is anchored or affixed to the aortic wall. 
     Other artificial valve designs have attempted to overcome the rigidity of artificial heart valves and accommodate the expansion of the aortic root during systole. Although these types of artificial valve designs allow for improved hemodynamics, such designs have not totally solved the problems arising from the rigidity of artificial heart valve stents. 
     Therefore, there is a need for a stent that is expandable, resilient, and durable, and that can be delivered and repositioned in a patient in need thereof, particularly a patient in need of an aortic valve replacement, while providing a better opening of the valve during systole to facilitate blood flow and contributing to minimal bending of the cusps to reduce valve failure. 
     SUMMARY 
     In one embodiment, a stent includes a tubular lattice structure having a radial direction and a longitudinal direction. The tubular lattice structure defines a middle zone in communication with a proximal end zone and a distal end zone. The proximal end zone includes a plurality of interconnected four-sided polygons and the middle zone and distal end zone includes a plurality of rods positioned substantially in the longitudinal direction of the tubular lattice structure and interconnected by a plurality of struts that collectively define a plurality of six-sided polygons with each strut defining an apex that is oriented towards the proximal end zone. 
     In another embodiment, a delivery system includes a hollow outer sheath defining an opening with a self-expandable stent disposed adjacent the opening and in a collapsed form. The self-expandable stent further includes a tubular lattice structure having a radial direction and a longitudinal direction, with the tubular lattice structure adapted to assume a fully expanded form from a collapsed form after deployment of the self-expandable stent from the outer sheath. The tubular lattice structure defines a middle zone in communication with a proximal end zone and a distal end zone. The proximal end zone includes a plurality of interconnected four-sided polygons and the middle zone and distal end zone include a plurality of rods positioned substantially in the longitudinal direction of the tubular lattice structure and interconnected by a plurality of struts that collectively define a plurality of six-sided polygons with each strut defining an apex that is oriented towards the proximal end zone. A valve prosthesis is attached to the inside of the tubular lattice structure of the self-expandable stent. 
     In yet another embodiment, a method for delivering and repositioning a stent in a lumen includes providing a stent in a delivery system with the delivery system having a hollow, retractable hollow outer sheath defining an opening with the stent disposed therein adjacent the opening; constraining the stent in a collapsed form; delivering the stent percutaneously to a location in a lumen that requires repair or replacement; retracting the outer sheath relative to the stent and permitting the stent to expand from the collapsed form to a fully expandable form in the location; and monitoring the orientation and the location of the stent in the lumen. 
     Additional objectives, advantages and novel features will be set forth in the description which follows or will become apparent to those skilled in the art upon examination of the drawings and detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevated perspective view of one embodiment of a stent; 
         FIG. 2  is a side elevation view of the stent shown in a fully expanded form; 
         FIG. 3  is partial side elevation view of the stent shown in the fully expanded form illustrating the flare defined at the proximal and distal ends of the stent; 
         FIG. 4  is a perspective view of the stent shown in a collapsed form; 
         FIG. 5  is an elevated perspective view of the stent in the fully expanded form shown attached to a valve prosthesis; 
         FIG. 6  is a perspective view of the stent in the fully expanded form shown attached to the valve prosthesis with a passive marker attached to one end of the stent; 
         FIG. 7  is a perspective view of the stent shown in the collapsed form and attached to the valve prosthesis disposed inside a delivery system; 
         FIG. 8  is a side view of the stent and valve prosthesis after deployment from the delivery system; 
         FIG. 9  is a perspective view of the stent attached to the valve prosthesis disposed inside the delivery system illustrating an active guide wire and a passive marker affixed to the stent; 
         FIG. 10  illustrates an image artifact shown in a cross-sectional view of a magnetic resonance imaging (MRI) image; 
         FIG. 11  illustrates another image artifact shown in a longitudinal sectional view of an MRI image; 
         FIG. 12  illustrates a colored trace on a cross-sectional view of an MRI image illustrating the active guide wire shown in  FIG. 11  in red highlight; 
         FIG. 13  illustrates the colored trace on another MRI image of the active guide wire shown in  FIG. 11 ; 
         FIG. 14  is a perspective view of the stent in collapsed form attached to the valve prosthesis disposed inside a manual delivery system; 
         FIG. 15  is a perspective view of the stent in collapsed form attached to the valve prosthesis disposed inside a robotic delivery system; 
         FIG. 16  is a side view showing the unfolded geometric configuration of the tubular lattice structure and a related isolated view of an embodiment of the rod with flared ends; 
         FIG. 17  is a perspective view of the stent shown in the fully expanded form; 
         FIG. 18  is a cross-sectional view of a middle zone of the stent shown in the fully expanded form; 
         FIG. 19  is a perspective view of the stent and the passive marker; 
         FIG. 19A  is an enlarged view of the stent showing the passive marker affixed thereto; 
         FIG. 20  is a side view showing the unfolded geometric configuration of the stent illustrating a plurality of grasping members affixed to the stent; 
         FIG. 21  is a perspective view of the stent shown in the fully expanded form illustrating the flared first end zone and flared second end zone; 
         FIGS. 22A and 22B  are partial perspective views of the stent showing the sequence of deployment of the stent from the delivery system; 
         FIG. 23  are radiographs of the anterior and right lateral aspects of the heart showing a well expanded stent in the aortic root; 
         FIG. 24  are images of valve sections that show widely patent coronary ostia being unobstructed by the stent or the commissures of the valve prosthesis after deployment; and 
         FIG. 25  is a side view of another embodiment of the stent. 
       Corresponding reference characters indicate corresponding elements among the view of the drawings. The headings used in the figures should not be interpreted to limit the scope of the claims. 
     
    
    
     DETAILED DESCRIPTION 
     Stents are widely used in valve replacement and other medical procedures. To function properly, stents are required to be properly positioned and attached to the orifice after deployment from a delivery system, such as a balloon catheter that expands to deploy the stent or a retractable catheter that gradually retracts to permit the stent to assume an expanded form. As such, embodiments of the stent as set forth herein include particular properties and characteristics that address issues related to deploying and positioning the stent during valve replacement. First, the stent requires little expansion upon compression such that less force need be applied to the valve prosthesis attached to the stent. The stent also provides a stable, yet flexible scaffolding platform for the valve prosthesis because of the stent&#39;s ability to resist torsion, while also being capable of expanding and contracting over long periods of time. In addition, the geometry and mechanical properties of the stent allow for more anatomically-correct placement to properly fit into the orifice when the stent is initially positioned after deployment. Further details of the stent and other related components are discussed in greater detail below. 
     Referring to the drawings, an embodiment of an expandable stent attached to a valve prosthesis  11  for implantation and deployment by a delivery system  10  are illustrated and generally indicated as  12  in  FIGS. 1-22 . The valve prosthesis  11  is typically implanted in one of the channels or lumens of the body to replace a diseased natural valve. For example, the valve prosthesis  11  is attached to the stent  12  for implantation in the aorta during a valve replacement procedure. However, it is understood that it is possible to use the stent  12  without the valve prosthesis  11  in relation with implantation in other channels of the body by using the same technique as shall be discussed below for implantation of the cardiac valve prosthesis  11 . For example, the procedure may include the implantation of: 1) a valve prosthesis  11  in the heart (for instance, a mitral valve, triscupid valve, aortic valve, or pulmonary valve) or vaculature; 2) a valve prosthesis  11  in the ureter and/or the vesica; 3) a valve prosthesis  11  in the biliary passages; and 4) a valve prosthesis  11  in the lymphatic system. 
     The following terms used in the detailed description will have the following meanings as set forth herein. As used herein, the term “chevron-shaped six-sided polygon” means a planar or non-planar figure that is bounded by a closed path or circuit, containing a sequence of six generally straight line segments, edges, or sides (i.e., by a closed polygonal chain) having six vertices or corners, wherein the interior of the polygon or body forms a generally chevron- shaped or “V” or inverted “V” shape. In the unfolded geometry of the stent  12 , the chevron-shaped six-sided polygon is planar and the segments, edges, or sides are substantially straight. However, in its folded geometry, the stent  12  includes polygons that are not planar and may have segments, edges, or sides that are generally straight but can have substantial curvature to permit good approximation of the interior of the lumen or valve that the stent  12  is supporting and replacing. 
     As used herein, the term “self-expandable” means a material that is able to deform when a load is applied and return to its original shape when the load is removed without the use of an outside force. In the context of the stent  12 , the stent  12  assumes a collapsed form to fit within the delivery system  10 , but the stent  12  is able to return to its original fully expanded form only after the stent  12  is released from the delivery system  10 . 
     As used herein, the term “expandable” shall mean a material that is able to deform when a load is applied, but will not return to its original shape when the load is removed. In the context of the stent  12 , the stent  12  assumes a collapsed form to fit within the delivery system  10 , but the stent  12  requires an exterior force, such as an expandable balloon, to exert a force to expand the stent  12 . 
     As used herein, the term “passive” when used in reference to a marker, refers to the visibility of the marker based on the susceptibility artifacts, for example, dark spots on a magnetic resonance image (“MRI”), radiopaque markers in a fluoroscopy, dense spots in an X-ray, or echo in ultrasound generated by intrinsic properties (magnetic properties in the case of MRI, fluorescence in the case of fluoroscopy, absorption of X-ray photons in radiography, and sound in the case of ultrasound), of the marker. 
     As used herein, the term “active,” when used in reference to a marker, refers to the incorporation of an MRI receiver coil (for example, an antenna or guide wire, electrically connected to a scanner) into the delivery system  10 , which is sensitive to signal only from adjacent tissue and is used to create bright spots on the MRI. 
     Referring to  FIGS. 1-3 , one embodiment of the stent  12  is shown in a fully expanded form prior to deployment from the delivery system  10 ; however, after deployment the stent  12  will expand to its environment to an expanded form that may be less than the fully expanded form. The stent  12  defines a proximal end  14  and a distal end  15  including a tubular lattice structure  13  having a radial direction and a longitudinal direction. In one embodiment, the tubular lattice structure  13  may be made from a material that has an elastic property that permits the stent  12  to self-expand from a collapsed form or bend when a force is applied to the stent  12 , while in another embodiment, the stent  12  is made from a material that has an elastic, non-self expandable property that requires an exterior force be applied to expand the stent  12 . An expandable balloon catheter (not shown) that exerts the necessary force required to expand the stent  12  may be utilized when the stent  12  is not self-expandable. 
     Referring to  FIGS. 16 and 17 , in one embodiment the tubular lattice structure  13  may define a proximal end zone  19 , a middle zone  20  and a distal end zone  21 . In another embodiment of the stent, designated  12 A, that is shown in  FIG. 25  the tubular lattice structure  13  may include only the middle zone  20  and distal zone  21  having only the six-sided polygons  24 . The proximal end zone  19  may include a plurality of diamond-shaped four-sided polygons  18  that are interconnected together to form a crown shaped end portion along the proximal end  14  of the stent  12 , while the middle and distal end zones  20  and  21  of the tubular lattice structure  13  include a plurality of interconnected chevron-shaped six-sided polygons  24 . Each of the plurality of six-sided polygons  24  is collectively defined by a respective pair of rods  16  connected together by a respective pair of struts  17  that collectively form a chevron shape. The plurality of rods  16  are positioned substantially in the longitudinal direction B ( FIG. 2 ) of the tubular lattice structure  13  and are in parallel orientation with respect to each other. As noted above, the tubular lattice structure  13  may include only the middle zone  20  and distal end zone  21  having only the chevron-shaped six-sided polygons  24  and not the four-sided polygons  18 . 
     As shown in  FIG. 3 , the struts  17  of each six sided polygon  24  define a V-shape with an apex  17 A formed by the strut  17  between each respective pair of rods  16  that may point toward the distal end zone  19  of the stent  12 . In this configuration, the struts  17  may impart a barbed feel when a user runs their hand over the tubular lattice structure  16  from the proximal end  14  to the distal end  15  of the stent  12 , while providing a relatively smooth feel when the user runs their hand from the distal end  15  to the proximal end  14  of the stent  12 . Alternatively, the apex  17 A may be oriented in the opposite direction towards the distal end zone  21 . Moreover, the apex  17 A of each strut  17  may point toward the direction of flow of fluid along longitudinal direction B ( FIG. 2 ) through the stent  12  that also assists in the stabilization of the stent  12  after deployment due to the orientation of the struts  17 . The struts  17  also provide structural reinforcement to the tubular lattice structure  13  that minimize the occurrence of fractures over time. In one embodiment, the proximal end zone  19  of the tubular lattice structure  13  may include 9 four-sided polygons  18 , which allow the stent  12  to expand and conform to the shape of the orifice (not shown), for example, the aorta, over time. However, other embodiments of the stent  12  may have more or fewer than  9  four-sided polygons  18  that form the proximal end zone  19  of tubular lattice structure  13 . 
     As shown in  FIG. 4 , the stent  12  may be placed in a collapsed form when constrained within the delivery system  10  prior to deployment and an expanded form ( FIG. 8 ) after deployment from the delivery system  10  when the stent  12  expands to the limits imposed by its environment.  FIG. 1 , illustrates the stent  12  in a fully expanded form prior to being constrained within the delivery system  10 . In one embodiment, the stent  12  in the collapsed form has substantially the same length (L C ) as the length (L E ) of the stent  12  in the fully expanded form due to the chevron-shaped six sided polygons  24  that constitute the tubular lattice structure  13 . For example, the percentage that the stent  12  shortens the longitudinal length (L E2 ) after deployment for the embodiment of the stent  12  with only the chevron shape six-sided polygons  24  is substantially 0%, while the percentage change in longitudinal length (L E ) that the embodiment of the stent  12  with both the four-sided polygons  18  and the six-sided polygons  24  is in a range between 0%-5%. In comparison, prior art stents have been found to have a change in longitudinal length of around 35% after expansion from the collapsed form. As such, the embodiments of the stent  12  do not substantially shorten when the stent  12  is deployed or lengthen when the stent  12  assumes a collapsed form when constrained within the delivery system  10 . One advantage of maintaining substantially the same length of the stent  12  in either the collapsed or expanded form is that the valve prosthesis  11  is not stressed by the lengthening of the stent  12  during compression. In one embodiment, the stent  12  is made from at least one shape memory alloy, such as nickel-titanium alloy, including those alloys sold under the trade name of NITINOL. In the embodiment of the stent  12 A that requires deployment by a balloon catheter, the stent  12 A may be made from stainless steel, platinum/iridium, and magnesium. 
     Referring to  FIGS. 5 and 6 , the stent  12  is shown in the fully expanded form with the valve prosthesis  11  disposed inside the stent  12 . In one embodiment, the valve prosthesis  11  may be an aortic valve prosthesis having three commissures  30 , wherein the commissures  30  are aligned with three of the rods  16  of the tubular lattice structure. The various embodiments of the valve prosthesis  11  may include a cardiac valve (including a mitral valve, tricuspid valve, aortic valve, or pulmonary valve), a vascular valve, a ureteral valve, a vesicle valve, a biliary passage valve, or a lymphatic system valve. In one embodiment, the stent  12  is crimped to the valve prosthesis  11 , although in other embodiments the valve prosthesis  11  may be connected, affixed, crimped, or otherwise attached to the inner side of the tubular lattice structure  13  in any manner that securely engages the valve prosthesis  11  to the stent  12 . 
     Referring to  FIGS. 16 ,  18 ,  20  and  21 , the stent  12  is shown in an unfolded geometric configuration. The values of the geometric parameters of the stent  12 , for example, diameter (D), length (L), thickness (w), flare curvature (R), and rod width (ws) provide radial force and flexibility to the stent  12 . For example, the values of the geometric parameters of the stent  12  when used for aortic valve replacement are as follows: the diameter (D) of the stent  12  needs to accommodate the typical diameters of the valve prosthesis  11  (For example, diameters of the valve prosthesis  11  of 21 mm, 23 mm, 25 mm, and 27 mm require a diameter (D) for the stent  12  to be 22 mm, 24 mm, 26 mm, and 28 mm, respectively); the length (L) has a range between 35 -37 mm); the thickness (w) is in a range of 0.35 mm-0.5 mm; the flare curvature (R) is in a range of 10-12 mm; the rod width (ws) is in a range of 0.35-0.5 mm. As shown in  FIG. 3 , each of the plurality of rods  16  define a first end  32  connected to one of the four-sided polygons  18  of the proximal end zone  19  and a second end  33  that forms a part of the distal end zone  21 . Referring back to  FIG. 16 , the isolated view of the rod  16  shows that the first and second ends  33  and  34  defined by each rod  16  may define a respective flare  27  or slight bend in the rod  16 . In addition, each rod  16  may be attached at only one point to the four-sided polygon  18  such that the rod  16  may not extend through the four-sided polygon  18  since the proximal end zone  19  of the tubular lattice structure  13  may become too rigid in such a configuration and may be less able to form the flare  27  required to properly seat the stent  12  after deployment, thereby preventing torsion of the stent  12  and valve prosthesis  11 . 
     The flares  27  may be formed when the stent  12  assumes an expanded form after deployment from the delivery system  10 . The degree that the flares  27  may bend is dependent on how much the stent  12  is allowed to expand after deployment in view of the environment, e.g., the diameter of the lumen may restrict the stent  12  from expanding to a fully expanded form. In addition, these flares  27  as well as the other geometric and mechanical parameters, such as the length (L) of the stent  12 , discussed above allow for more anatomically-correct placement of the stent  12  as well as provide more flexible reinforcement/scaffolding of the prosthetic valve  11  by the stent  12 . For example, the fracture of struts  17  may be minimized by virtue of the geometric and mechanical parameters of the tubular lattice structure  13 . 
     When good visibility and monitoring is desired during deployment and positioning of the stent  12  the following methods may be utilized. Referring to  FIGS. 16 ,  19 ,  19 A and  20 , one method involves affixing, such as by welding, a passive marker  25  to the distal end  14  of the stent  12 . The passive marker  25  may include a high-density metal-containing material selected from the group consisting of gold, platinum, tantalum, stainless steel, and combinations thereof, which may be monitored, for example, by magnetic resonance imaging, X-ray imaging, fluoroscopy, or ultrasound.  FIGS. 10 and 11  illustrate the image artifact in an MRI image that shows the location of the passive marker  25 . In one embodiment, the stent  12  may have a protective insulating layer between the stent  12  and the passive marker  25  to prevent corrosion. 
     Another method may involve use of an active marker  29 , such as an active guide wire shown in  FIGS. 9 and 15 , which is positioned along the inside wall of a retractable outer sheath  28 , of the deployment system  10 .  FIGS. 12 and 13  show the colored trace of the active marker  29  shown on an MRI image. For example, the active guide wire  29  may be a loop coil antenna, manufactured using an insulated 0.005″ magnet copper wire. The coil length was adjusted to 1.1 inches with 0.026″ outer diameter. A 0.006″ profile twisted pair was used as a transmission lie for the loop coil antenna. The whole arrangement was insulated by using medical grade polyester heat shrink tubing and embedded inside the wall of a retractable outer sheath. The loop coil antenna was matched to 50 ohm and tuned to a Larmour frequency of a 1.5T MRI scanner. 
     Referring to  FIGS. 14 and 15 , the stent  12  and valve prosthesis  11  may be adapted to be delivered, deployed, and positioned using at least two different embodiments of the delivery systems  10 . For example, one embodiment, delivery system  10  ( FIG. 14 ) may include a manually retractable outer sheath  28  that defines a sheath opening  31  with the stent  12  and valve prosthesis  11  disposed adjacent the opening  31 . The delivery system  10  is manually actuated by the user to deploy the stent  12  and valve prosthesis  11  by operation of the handle such that the outer sheath  28  is gradually retracted which incrementally deploys the stent  12  from a collapsed form shown in  FIG. 7  to partial deployment shown in  FIGS. 22A and 22B  until the stent  12  is fully deployed as illustrated in  FIG. 8 . In an alternate embodiment of the delivery system shown in  FIG. 15 , designated  10 A, a robotic delivery system  10 A operates in a similar manner as the other embodiment of the delivery system  10  except the robotic delivery system  10 A may deploy the stent  12  using a mechanism that automatically retracts the outer sheath  28  rather than manually retracting the outer sheath  28 . In yet another embodiment, the delivery system  10  may be a balloon catheter with an expandable balloon (not shown) that is disposed within the stent  12  such that expansion of the balloon by inflation causes the stent  12  to assume an expanded form during deployment. 
     Referring back to  FIG. 20 , one embodiment of the stent  12  may include one or more grasping members  34 , such as spherical beads or the like, that provide retracting capability to the stent  12  by providing one or more structural elements capable of being grasped by a loop catheter or loop snare wire (not shown) for retaining the stent  12  and valve prosthesis  11  and retract the outer sheath  28 . The loop snare wire system prevents early or accidental deployment of the valve prosthesis  11  as well as provides a means for repositioning the stent  12  if the stent  12  has not yet been completely deployed. 
     In one embodiment, the stent  12  and valve prosthesis  11  may be deployed by percutaneously delivering the stent  12  and valve prosthesis  11  disposed within the outer sheath  28  to a location in the lumen that requires either repair or replacement. The user then retracts the outer sheath  28  using the delivery system  10  such that the stent  12  is incrementally deployed from the collapsed form to the fully expanded form. Once the stent  12  is deployed, the user may then monitor the orientation and location of the stent  12  in the lumen using the passive marker  25 . If desired, the user may use a maker, such as the passive marker  25  to reposition the orientation and location of the stent  12  by engaging and manipulating the grasping member  34  of the stent  12 . However, once the stent  12  is completely deployed, the stent  12  cannot be repositioned or reoriented. 
     EXAMPLE 
     Tests were performed using the self-expandable embodiment of the stent  12  to test its structural integrity after deployment and positioning within the orifice. Specifically, one porcine heart with a prosthetic aortic valve was implanted into the aortic root for histopathologic evaluation. Referring to  FIG. 23 , the radiographs of the heart and aortic root show a widely and evenly expanded stent frame. The right lateral radiographic image also disclosed a single strut fracture on the proximal crown. Grossly, the stent appeared to be properly seated in the aortic root. The right cusp was centered on the right coronary ostium with the prosthetic annulus 0.5 cm inferior to the ostium. The left cusp was rotated posteriorly, aligning the left ostium evenly with the anterior base of the leaflet. Both coronary ostia were widely patent and unobstructed by the stent frame. The proximal end of the stent frame was covered and well seated over the native aortic valve annulus with no gaps between the stent frame, annulus or aortic root. The prosthetic annulus was covered with an opaque fibrous tissue overgrowth. One bare stent crown tip was noted on the anterior lateral wall. Distally, the stent was well apposed to the aortic wall with most struts covered with translucent neointimal overgrowth with the exception of the struts adjacent to the coronary ostia.  FIG. 24  also illustrates that only one strut  17  fracture occurred after the stent was deployed, which was an unexpected result in view of prior art stents that would have multiple struts that fractured over time. 
     The average strut fractures for a platinum-iridium stent  12  after an implantation of 6 months was 5.0±3.1 (mean±std. dev.), while the average fractures for stent  12  was 1.6±2.5 (mean±std. dev.). The fractures were due to the material fatigue of the stent  12  and the expansion, contraction, torsion forces generated between the aorta and the stent  12 . The platinum-iridium stent  12  had more strut fractures, while the NITINOL self-expanding stent  12 A had fewer or no strut fractures (p=0.046). 
     It should be understood from the foregoing that, while particular embodiments have been illustrated and described, various modifications can be made thereto without departing from the spirit and scope of the invention as will be apparent to those skilled in the art. Such changes and modifications are within the scope and teachings of this invention as defined in the claims appended hereto.