Patent Publication Number: US-2020297487-A1

Title: Prosthetic heart valve delivery apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 15/868,741, filed on Jan. 11, 2018, which is a continuation of U.S. application Ser. No. 14/283,056, filed on May 20, 2014, now U.S. Pat. No. 9,867,700, which claims priority to, and the benefit of, U.S. Provisional Patent Application Ser. No. 61/825,476, filed May 20, 2013, all of which applications are incorporated herein by reference. 
    
    
     FIELD 
     The present invention concerns embodiments of a prosthetic valve (e.g., prosthetic heart valve) and a delivery apparatus for implanting a prosthetic valve. 
     BACKGROUND 
     Prosthetic cardiac valves have been used for many years to treat cardiac valvular disorders. The native heart valves (such as the aortic, pulmonary and mitral valves) serve critical functions in assuring the forward flow of an adequate supply of blood through the cardiovascular system. These heart valves can be rendered less effective by congenital, inflammatory or infectious conditions. Such damage to the valves can result in serious cardiovascular compromise or death. For many years the definitive treatment for such disorders was the surgical repair or replacement of the valve during open heart surgery, but such surgeries are prone to many complications. More recently a transvascular technique has been developed for introducing and implanting a prosthetic heart valve using a flexible catheter in a manner that is less invasive than open heart surgery. 
     In this technique, a prosthetic valve is mounted in a crimped state on the end portion of a flexible catheter and advanced through a blood vessel of the patient until the prosthetic valve reaches the implantation site. The prosthetic valve at the catheter tip is then expanded to its functional size at the site of the defective native valve such as by inflating a balloon on which the prosthetic valve is mounted. Alternatively, the prosthetic valve can have a resilient, self-expanding stent or frame that expands the prosthetic valve to its functional size when it is advanced from a delivery sheath at the distal end of the catheter. 
     Balloon-expandable prosthetic valves typically are preferred for replacing calcified native valves because the catheter balloon can apply sufficient expanding force to anchor the frame of the prosthetic valve to the surrounding calcified tissue. On the other hand, self-expanding prosthetic valves sometimes are preferred for replacing a defective, non-stenotic (non-calcified) native valve, although they also can be used to replace stenotic valves. One drawback associated with implanting a self-expanding prosthetic valve is that as the operator begins to advance the prosthetic valve from the open end of the delivery sheath, the prosthetic valve tends to “jump” out very quickly from the end of the sheath; in other words, the outward biasing force of the prosthetic valve&#39;s frame tends to cause the prosthetic valve to be ejected very quickly from the distal end of the delivery sheath, making it difficult to deliver the prosthetic valve from the sheath in a precise and controlled manner and increasing the risk of trauma to the patient. 
     Another problem associated with implanting a percutaneous prosthetic valve in a non-stenotic native valve is that the prosthetic valve may not be able to exert sufficient force against the surrounding tissue to resist migration of the prosthetic valve. Typically, the stent of the prosthetic valve must be provided with additional anchoring or attachment devices to assist in anchoring the prosthetic valve to the surrounding tissue. Moreover, such anchoring devices or portions of the stent that assist in anchoring the prosthetic valve typically extend into and become fixed to non-diseased areas of the vasculature, which can result in complications if future intervention is required, for example, if the prosthetic valve needs to be removed from the patient. 
     SUMMARY 
     Certain embodiments of the present disclosure provide a prosthetic valve (e.g., a prosthetic heart valve) and a valve delivery apparatus for delivery of the prosthetic valve to a native valve site via the human vasculature. The delivery apparatus is particularly suited for advancing a prosthetic valve through the aorta (i.e., in a retrograde approach) for replacing a diseased native aortic valve. The delivery apparatus in particular embodiments is configured to deploy a prosthetic valve from a delivery sheath in a precise and controlled manner at the target location within the body. 
     In an aspect, a delivery assembly comprises a prosthetic valve, an elongate shaft located proximal to the prosthetic valve, a suture-retention member located distal to the shaft, a slidable release member, and an outer sheath. The prosthetic valve can comprise a self-expandable stent having a plurality of apices spaced circumferentially around a first end portion of the stent, wherein each apex has an aperture. The suture-retention member can comprise a proximal portion and a distal portion spaced from the proximal portion, the proximal portion being coupled to the shaft. The at least one slidable release member can extend through the proximal portion and the distal portion of the suture-retention member and a plurality of suture loops extending from the proximal portion of the suture-retention member. The plurality of suture loops can extend through the apertures in the apices of the stent and around the release member at a location between the proximal and distal portions of suture-retention member, such that at least one of the suture loops extends through the aperture of every apex. The outer sheath can be advanced over the prosthetic valve to retain the prosthetic valve in a radially compressed state, and can be retracted relative to the prosthetic valve to permit radial expansion of the prosthetic valve while the stent remains connected to the suture-retention member via the suture loops. After the entirety of the prosthetic valve is deployed from the sheath, the sheath can be advanced distally back over the prosthetic valve to cause the prosthetic valve to radially collapse as it is recaptured by the sheath. 
     In some embodiments, the suture loops are formed from a single length of suture material. 
     In some embodiments, the shaft comprises a first shaft and the assembly further comprises a second shaft extending at least partially through the first shaft, wherein the outer sheath can be advanced or retracted relative to the prosthetic valve by rotating the second shaft relative to the first shaft. 
     In some embodiments, the at least one release member is slidable relative to the suture-retention member, and when the release member is retracted proximally such that a distal end of the release member is proximal to the distal portion of the suture-retention member, the suture loops can slide off the distal end of the release member, thereby releasing the prosthetic valve from the suture-retention member. 
     In some embodiments, at least a portion of the outer sheath comprises a slotted metal tube. 
     In some embodiments, a distal end portion of the outer sheath comprises a delivery capsule connected to a distal end of the slotted metal tube, the delivery capsule configured to extend over and retain the prosthetic valve in the radially compressed state. 
     In some embodiments, the delivery capsule comprises a polymer sleeve. 
     In some embodiments, the sheath is at least about 3-10 cm is length and no greater than about 40 cm in length. 
     In some embodiments, at least one of the suture loops has a greater thickness than others of the suture loops. 
     In another aspect, a delivery apparatus for implanting a prosthetic valve comprises a first elongated shaft having a proximal end portion and a distal end portion, a second elongated shaft extending through the first shaft and having a proximal end portion and a distal end portion, and a delivery sheath having a distal end portion configured to receive and retain a prosthetic valve in a compressed delivery state and a proximal end portion connected to the distal end portion of the second elongated shaft. The second shaft can be rotatable relative to the first shaft but fixed against axial movement relative to the first shaft. The proximal end portion of the delivery sheath can be more flexible than the distal end portion of the sheath. The delivery sheath can be, without limitation, at least about 3-5 cm in length and no greater than about 40 cm in length. The second shaft can be configured to be rotatable relative to the first shaft such that rotation of the second shaft causes the delivery sheath to move axially relative to the first and second shafts. 
     In some embodiments, the delivery apparatus further comprises a screw connected to a distal end of the second shaft, and a nut mounted on the screw and connected to the delivery sheath such that rotation of the second shaft and the screw causes axial movement of the nut relative to the screw, thereby producing axial movement of the delivery sheath. 
     In some embodiments, the proximal end portion of the delivery sheath is between about 5 cm and about 30 cm in length. 
     In some embodiments, the distal end portion of the first shaft extends through the delivery sheath and comprises a slotted metal tube. 
     In some embodiments, the delivery apparatus further comprises a suture-retention member connected to the distal end portion of the first shaft, a plurality of suture loops extending from the suture-retention member and configured to extend through openings in a frame of the prosthetic valve, and at least one slidable release member configured to extend through the suture-retention member and the suture loops to releasably secure the prosthetic valve to the suture-retention member. 
     In some embodiments, the suture-retention member comprises a proximal portion and a distal portion spaced axially apart from the first portion and the release member is slidable relative to the suture-retention member, between a first position extending through the proximal and distal portions of the suture-retention member and a second position in which the release member is retracted to a location proximal of the distal portion of the suture-retention member. When the release member is in the first position and the suture loops extend through the openings of the frame and around the release member at a location between the proximal and distal portions, the prosthetic valve is secured to the suture-retention member. When the release member is in the second position, the suture loops can slide off a distal end of the release member to release the prosthetic valve from the suture-retention member. 
     In some embodiments, the at least one release member comprises a plurality of release members extending through the suture-retention member. 
     In some embodiments, the proximal portion of the outer sheath comprises a slotted metal tube. 
     In some embodiments, the distal end portion of the outer sheath comprises a delivery capsule connected to a distal end of the slotted metal tube. The delivery capsule can be configured to extend over and retain the prosthetic valve in the compressed delivery state. In some embodiments, the delivery capsule comprises a polymer sleeve. 
     In another aspect, a method for delivering a prosthetic valve to the aortic annulus of the heart comprises inserting an elongated delivery apparatus into a femoral artery of a patient, the delivery apparatus comprising a delivery sheath containing the prosthetic valve in a radially compressed state. The method can further comprise advancing the delivery apparatus through the aorta until the prosthetic valve is at an implantation location within the aortic annulus, wherein when the prosthetic valve is at the implantation location, the delivery sheath extends through the ascending aorta and the aortic arch, and a proximal end of the delivery sheath is within the descending aorta. The method can further comprise retracting the delivery sheath relative to the prosthetic valve to deploy the prosthetic valve from a distal end of the delivery sheath. 
     In some embodiments, the delivery sheath is at least about 3-5 cm and no greater than 40 cm in length. 
     In some embodiments, the delivery sheath comprises a distal end portion and a proximal end portion that is more flexible than the distal end portion. The distal end portion of the sheath can extend over and retain the prosthetic valve in the radially compressed state during the acts of the inserting and advancing the delivery apparatus, and the proximal end portion can extend through the ascending aorta, the aortic arch and into the descending aorta when the prosthetic valve is at the implantation location. 
     In some embodiments, the prosthetic valve is releasably secured to the delivery apparatus via a plural of suture loops. 
     In some embodiments, the act of retracting the delivery sheath comprises deploying the entire prosthetic valve from the delivery sheath and allowing the prosthetic valve to radially expand while still secured to the delivery apparatus via the suture loops. 
     In some embodiments, the method further comprises, after deploying the entire prosthetic valve from the delivery sheath, recapturing the prosthetic valve by advancing the delivery sheath distally back over the prosthetic valve. 
     In another aspect, a method for delivering a prosthetic valve to a native valve annulus of the heart comprises inserting an elongated delivery apparatus into the vasculature of a patient, the delivery apparatus comprising a delivery sheath containing the prosthetic valve in a radially compressed state, wherein the prosthetic valve is releasably secured to the delivery apparatus via a plural of suture loops. The method can further comprise retracting the delivery sheath relative to the prosthetic valve to deploy the entire prosthetic valve from the delivery sheath, allowing the prosthetic valve to radially expand while still secured to the delivery apparatus via the suture loops. The method can further comprise, after deploying the entire prosthetic valve from the delivery sheath, recapturing the prosthetic valve by advancing the delivery sheath distally back over the prosthetic valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prosthetic valve that can be used to replace the native aortic valve of the heart, according to one embodiment. 
         FIG. 2  is a perspective view of a portion of the prosthetic valve of  FIG. 1  illustrating the connection of two leaflets to the support frame of the prosthetic valve. 
         FIG. 3  is side elevation view of the support frame of the prosthetic valve of  FIG. 1 . 
         FIG. 4  is a perspective view of the support frame of the prosthetic valve of  FIG. 1 . 
         FIG. 5A  is a cross-sectional view of the heart showing the prosthetic valve of  FIG. 1  implanted within the aortic annulus. 
         FIG. 5B  is an enlarged view of  FIG. 5A  illustrating the prosthetic valve implanted within the aortic annulus, shown with the leaflet structure of the prosthetic valve removed for clarity. 
         FIG. 6  is a perspective view of the leaflet structure of the prosthetic valve of  FIG. 1  shown prior to being secured to the support frame. 
         FIG. 7  is a cross-sectional view of the prosthetic valve of  FIG. 1 . 
         FIG. 8  is a cross-sectional view of an embodiment of a delivery apparatus that can be used to deliver and implant a prosthetic valve, such as the prosthetic valve shown in  FIG. 1 . 
         FIGS. 8A-8C  are enlarged cross-sectional views of sections of  FIG. 8 . 
         FIG. 9  is an exploded view of the delivery apparatus of  FIG. 8 . 
         FIG. 10  is a side view of the guide catheter of the delivery apparatus of  FIG. 8 . 
         FIG. 11  is a perspective, exploded view of the proximal end portion of the guide catheter of  FIG. 10 . 
         FIG. 12  is a perspective, exploded view of the distal end portion of the guide catheter of  FIG. 10 . 
         FIG. 13  is a side view of the torque shaft catheter of the delivery apparatus of  FIG. 8 . 
         FIG. 14  is an enlarged side view of the rotatable screw of the torque shaft catheter of  FIG. 13 . 
         FIG. 15  is an enlarged perspective view of a coupling member disposed at the end of the torque shaft. 
         FIG. 16  is an enlarged perspective view of the threaded nut used in the torque shaft catheter of  FIG. 13 . 
         FIG. 17  is an enlarged side view of the distal end portion of the nose cone catheter of the delivery apparatus of  FIG. 8 . 
         FIG. 17A  is an enlarged, cross-sectional view of the nose cone of the catheter shown  FIG. 17 . 
         FIG. 17B  is an enlarged cross-sectional view of the distal end portion of the delivery apparatus of  FIG. 8  showing the stent of a prosthetic valve retained in a compressed state within a delivery sheath. 
         FIG. 18  is an enlarged side view of the distal end portion of the delivery apparatus of  FIG. 8  showing the delivery sheath in a delivery position covering a prosthetic valve in a compressed state for delivery into a patient. 
         FIG. 19  is an enlarged cross-sectional view of a section of the distal end portion of the delivery apparatus of  FIG. 8  showing the valve-retaining mechanism securing the stent of a prosthetic valve to the delivery apparatus. 
         FIG. 20  is an enlarged cross-sectional view similar to  FIG. 19 , showing the inner fork of the valve-retaining mechanism in a release position for releasing the prosthetic valve from the delivery apparatus. 
         FIGS. 21 and 22  are enlarged side views of distal end portion of the delivery apparatus of  FIG. 8 , illustrating the operation of the torque shaft for deploying a prosthetic valve from a delivery sheath. 
         FIGS. 23-26  are various views of an embodiment of a motorized delivery apparatus that can be used to operate the torque shaft of the delivery apparatus shown in  FIG. 8 . 
         FIG. 27  is a perspective view of an alternative motor that can be used to operate the torque shaft of the delivery apparatus shown in  FIG. 8 . 
         FIG. 28A  is an enlarged view of a distal segment of the guide catheter shaft of  FIG. 10 . 
         FIG. 28B  shows the cut pattern for forming the portion of the shaft shown in  FIG. 28A , such as by laser cutting a metal tube. 
         FIG. 29A  is an enlarged view of a distal segment of a guide catheter shaft, according to another embodiment. 
         FIG. 29B  shows the cut pattern for forming the shaft of  FIG. 29A , such as by laser cutting a metal tube. 
         FIG. 30  is a side elevation view of a support stent for use in a prosthetic valve. 
         FIG. 31A  is an enlarged view an exemplary delivery assembly having a plurality of suture loops for reversibly engaging the support stent of  FIG. 30 . 
         FIG. 31B  is a side view of an exemplary suture-retention member for use in the delivery assembly of  FIG. 31A . 
         FIG. 31C  is a proximal end view of the suture-retention member of  FIG. 31B , showing a proximal end of a first (proximal) disc member with suture loops extending distally outward. 
         FIG. 31D , is a distal end view of the suture-retention member of  FIG. 31B , showing a distal end view of the first disc member with suture loops extending distally outward. The second (distal) disc member and the shaft member of the suture-retention member are omitted from  FIG. 31D  for clarity. 
         FIG. 32  is a side elevation view of an exemplary delivery assembly comprising the delivery catheter of  FIG. 31A , with a suture loop shown engaging the stent of  FIG. 30 . 
         FIG. 33  is an enlarged view of the delivery assembly of  FIG. 32  engaging the stent of  FIG. 30 . 
         FIG. 34  is a side elevation view of the delivery assembly of  FIG. 32  holding the stent of  FIG. 30 , with suture loops engaging each apex of the stent. 
         FIG. 35  is a side elevation view of the delivery assembly of  FIG. 32 , with a sheath component of the delivery assembly advanced over a portion of the stent of  FIG. 30 . 
         FIG. 36  is a side elevation view of the delivery assembly of  FIG. 32 , with a sheath component of the delivery assembly fully advanced over the stent of  FIG. 30 . 
         FIG. 37  is a side elevation view of the delivery catheter of  FIG. 32 , with suture loops disengaged from the stent of  FIG. 30 . 
         FIG. 38  is a top view of another exemplary delivery assembly, showing a delivery cylinder and a screw mechanism. The delivery assembly can have a single, continuous outer sleeve portion (not shown) covering the components. 
         FIG. 39A  is a top view of a delivery cylinder and screw mechanism for use in the delivery assembly of  FIG. 38 . The screw mechanism can be used to advance and retract the delivery cylinder. The delivery cylinder, screw member and nut are shown separately. 
         FIG. 39B  is a top view of the delivery cylinder and screw mechanism of  FIG. 39A , with the nut is mounted on the screw member and the delivery cylinder shown separately. 
         FIG. 39C  is a top view of the delivery cylinder and screw mechanism of  FIG. 39A , with a tab portion of the nut (mounted on the screw member) extending through a proximally-located window of the delivery cylinder. 
         FIG. 40  is a top view of the distal end portion of the first catheter of the delivery apparatus of  FIG. 38 . 
         FIG. 41  is a top view of a section of the delivery apparatus of  FIG. 38 , showing a screw mechanism coupled to a delivery sheath. 
         FIG. 42  is a top view of the distal end portion of the delivery apparatus of  FIG. 38 , showing the delivery sheath retracted to a proximal position. 
         FIG. 43  is a detailed view of the distal end portion of the delivery apparatus of  FIG. 38 , with the delivery sheath advanced to its distal-most position for delivery of a prosthetic valve. 
         FIG. 44  is a front elevation view of a wire coil and washer assembly that can be incorporated in a torque shaft in place of the screw and nut assembly shown in  FIG. 13  or the screw and nut assembly shown in  FIG. 39A . 
         FIG. 45  is a side view of the wire coil and washer assembly of  FIG. 44  shown partially in section. 
         FIG. 46  is an enlarged, cross-sectional view of the distal end portion of a delivery sheath, according to one embodiment. 
         FIG. 47  is a side view of an alternative slotted metal tube that can be used in the delivery apparatus of  FIG. 38   
         FIG. 48  is a side view of a portion of a delivery apparatus incorporating the slotted metal tube shown in  FIG. 47 . 
         FIG. 49  is an enlarged view of the distal end portion of the slotted metal tube of  FIG. 47 , shown connected to a suture-retention member. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , there is shown a prosthetic aortic heart valve  10 , according to one embodiment. The prosthetic valve  10  includes an expandable frame member, or stent,  12  that supports a flexible leaflet section  14 . The prosthetic valve  10  is radially compressible to a compressed state for delivery through the body to a deployment site and expandable to its functional size shown in  FIG. 1  at the deployment site. In certain embodiments, the prosthetic valve  10  is self-expanding; that is, the prosthetic valve can radially expand to its functional size when advanced from the distal end of a delivery sheath. Apparatuses particularly suited for percutaneous delivery and implantation of a self-expanding prosthetic valve are described in detail below. In other embodiments, the prosthetic valve can be a balloon-expandable prosthetic valve that can be adapted to be mounted in a compressed state on the balloon of a delivery catheter. The prosthetic valve can be expanded to its functional size at a deployment site by inflating the balloon, as known in the art. 
     The illustrated prosthetic valve  10  is adapted to be deployed in the native aortic annulus, although it also can be used to replace the other native valves of the heart. Moreover, the prosthetic valve  10  can be adapted to replace other valves within the body, such venous valves. 
       FIGS. 3 and 4  show the stent  12  without the leaflet section  14  for purposes of illustration. As shown, the stent  12  can be formed from a plurality of longitudinally extending, generally sinusoidal shaped frame members, or struts,  16 . The struts  16  are formed with alternating bends and are welded or otherwise secured to each other at nodes  18  formed from the vertices of adjacent bends so as to form a mesh structure. The struts  16  can be made of a suitable shape memory material, such as the nickel titanium alloy known as Nitinol, that allows the prosthetic valve to be compressed to a reduced diameter for delivery in a delivery apparatus (such as described below) and then causes the prosthetic valve to expand to its functional size inside the patient&#39;s body when deployed from the delivery apparatus. If the prosthetic valve is a balloon-expandable prosthetic valve that is adapted to be crimped onto an inflatable balloon of a delivery apparatus and expanded to its functional size by inflation of the balloon, the stent  12  can be made of a suitable ductile material, such as stainless steel. 
     The stent  12  has an inflow end  26  and an outflow end  27 . The mesh structure formed by struts  16  comprises a generally cylindrical “upper” or outflow end portion  20 , an outwardly bowed or distended intermediate section  22 , and an inwardly bowed “lower” or inflow end portion  24 . The intermediate section  22  desirably is sized and shaped to extend into the Valsalva sinuses in the root of the aorta to assist in anchoring the prosthetic valve in place once implanted. As shown, the mesh structure desirably has a curved shape along its entire length that gradually increases in diameter from the outflow end portion  20  to the intermediate section  22 , then gradually decreases in diameter from the intermediate section  22  to a location on the inflow end portion  24 , and then gradually increases in diameter to form a flared portion terminating at the inflow end  26 . 
     When the prosthetic valve is in its expanded state, the intermediate section  22  has a diameter D 1 , the inflow end portion  24  has a minimum diameter D 2 , the inflow end  26  has a diameter D 3 , and the outflow end portion  20  has a diameter D 4 , where D 2  is less than D 1  and D 3 , and D 4  is less than D 2 . In addition, D 1  and D 3  desirably are greater than the diameter of the native annulus in which the prosthetic valve is to be implanted. In this manner, the overall shape of the stent  12  assists in retaining the prosthetic valve at the implantation site. More specifically, and referring to  FIGS. 5A and 5B , the prosthetic valve  10  can be implanted within a native valve (the aortic valve in the illustrated example) such that the lower section  24  is positioned within the aortic annulus  28 , the intermediate section  24  extends above the aortic annulus into the Valsalva&#39;s sinuses  56 , and the lower flared end  26  extends below the aortic annulus. The prosthetic valve  10  is retained within the native valve by the radial outward force of the lower section  24  against the surrounding tissue of the aortic annulus  28  as well as the geometry of the stent. Specifically, the intermediate section  24  and the flared lower end  26  extend radially outwardly beyond the aortic annulus  28  to better resist against axial dislodgement of the prosthetic valve in the upstream and downstream directions (toward and away from the aorta). Depending on the condition of the native leaflets  58 , the prosthetic valve typically is deployed within the native annulus  28  with the native leaflets  58  folded upwardly and compressed between the outer surface of the stent  12  and the walls of the Valsalva sinuses, as depicted in  FIG. 5B . In some cases, it may be desirable to excise the leaflets  58  prior to implanting the prosthetic valve  10 . 
     Known prosthetic valves having a self-expanding frame typically have additional anchoring devices or frame portions that extend into and become fixed to non-diseased areas of the vasculature. Because the shape of the stent  12  assists in retaining the prosthetic valve, additional anchoring devices are not required and the overall length L of the stent can be minimized to prevent the stent upper portion  20  from extending into the non-diseased area of the aorta, or to at least minimize the extent to which the upper portion  20  extends into the non-diseased area of the aorta. Avoiding the non-diseased area of the patient&#39;s vasculature helps avoid complications if future intervention is required. For example, the prosthetic valve can be more easily removed from the patient because the stent is primarily anchored to the diseased part of the native valve. Furthermore, a shorter prosthetic valve is more easily navigated around the aortic arch. 
     In particular embodiments, for a prosthetic valve intended for use in a 22-mm to 24-mm annulus, the diameter D 1  is about 28 mm to about 32 mm, with 30 mm being a specific example; the diameter D 2  is about 24 mm to about 28 mm, with 26 mm being a specific example; the diameter D3 is about 28 mm to about 32 mm, with 30 mm being a specific example; and the diameter D4 is about 24 mm to about 28 mm, with 26 mm being a specific example. The length L in particular embodiments is about 20 mm to about 24 mm, with 22 mm being a specific example. 
     Referring to  FIG. 1 , the stent  12  can have a plurality of angularly spaced retaining arms, or projections, in the form of posts  30  (three in the illustrated embodiment) that extend from the stent upper portion  20 . Each retaining arm  30  has a respective aperture  32  that is sized to receive prongs of a valve-retaining mechanism that can be used to form a releasable connection between the prosthetic valve and a delivery apparatus (described below). In alternative embodiments, the retaining arms  30  need not be provided if a valve-retaining mechanism is not used. 
     As best shown in  FIGS. 6 and 7 , the leaflet assembly  14  in the illustrated embodiment comprises three leaflets  34   a ,  34   b ,  34   c  made of a flexible material. Each leaflet has an inflow end portion  60  and an outflow end portion  62 . The leaflets can comprise any suitable biological material (e.g., pericardial tissue, such as bovine or equine pericadium), bio-compatible synthetic materials, or other such materials, such as those described in U.S. Pat. No. 6,730,118, which is incorporated herein by reference. The leaflet assembly  14  can include an annular reinforcing skirt  42  that is secured to the outer surfaces of the inflow end portions of the leaflets  34   a ,  34   b ,  34   c  at a suture line  44  adjacent the inflow end of the prosthetic valve. The inflow end portion of the leaflet assembly  14  can be secured to the stent  12  by suturing the skirt  42  to struts  16  of the lower section  24  of the stent (best shown in  FIG. 1 ). As shown in  FIG. 7 , the leaflet assembly  14  can further include an inner reinforcing strip  46  that is secured to the inner surfaces of the inflow end portions  60  of the leaflets. 
     Referring to  FIGS. 1 and 2 , the outflow end portion of the leaflet assembly  14  can be secured to the upper portion of the stent  12  at three angularly spaced commissure attachments of the leaflets  34   a ,  34   b ,  34   c . As best shown in  FIG. 2 , each commissure attachment can be formed by wrapping a reinforcing section  36  around adjacent upper edge portions  38  of a pair of leaflets at the commissure formed by the two leaflets and securing the reinforcing section  36  to the edge portions  38  with sutures  48 . The sandwiched layers of the reinforcing material and leaflets can then be secured to the struts  16  of the stent  12  with sutures  50  adjacent the outflow end of the stent. The leaflets therefore desirably extend the entire length or substantially the entire length of the stent from the inflow end  26  to the outflow end  27 . The reinforcing sections  36  reinforces the attachment of the leaflets to the stent so as to minimize stress concentrations at the suture lines and avoid “needle holes” on the portions of the leaflets that flex during use. The reinforcing sections  36 , the skirt  42 , and the inner reinforcing strip  46  desirably are made of a bio-compatible synthetic material, such as polytetrafluoroethylene (PTFE), or a woven fabric material, such as woven polyester (e.g., polyethylene terephtalate) (PET)). 
       FIG. 7  shows the operation of the prosthetic valve  10 . During diastole, the leaflets  34   a ,  34   b ,  34   c  collapse to effectively close the prosthetic valve. As shown, the curved shape of the intermediate section  22  of the stent  12  defines a space between the intermediate section and the leaflets that mimics the Valsalva sinuses. Thus, when the leaflets close, backflow entering the “sinuses” creates a turbulent flow of blood along the upper surfaces of the leaflets, as indicated by arrows  52 . This turbulence assists in washing the leaflets and the skirt  42  to minimize clot formation. 
     The prosthetic valve  10  can be implanted in a retrograde approach where the prosthetic valve, mounted in a crimped state at the distal end of a delivery apparatus, is introduced into the body via the femoral artery and advanced through the aortic arch to the heart, as further described in U.S. Patent Publication No. 2008/0065011, which is incorporated herein by reference. 
       FIGS. 8 and 9  show a delivery apparatus  100 , according to one embodiment, that can be used to deliver a self-expanding prosthetic valve, such as prosthetic valve  10  described above, through a patient&#39;s vasculature. The delivery apparatus  100  comprises a first, outermost or main catheter  102  (shown alone in  FIG. 10 ) having an elongated shaft  104 , the distal end of which is coupled to a delivery sheath  106  ( FIG. 18 ; also referred to as a delivery cylinder). The proximal end of the main catheter  102  is connected to a handle of the delivery apparatus.  FIGS. 23-26  show an embodiment of a handle mechanism having an electric motor for operating the delivery apparatus. The handle mechanism is described in detail below. During delivery of a prosthetic valve, the handle can be used by a surgeon to advance and retract the delivery apparatus through the patient&#39;s vasculature. Although not required, the main catheter  102  can comprise a guide catheter that is configured to allow a surgeon to guide or control the amount the bending or flexing of a distal portion of the shaft  104  as it is advanced through the patient&#39;s vasculature, such as further described below. Another embodiment of a guide catheter is disclosed in U.S. Patent Publication No. 2008/0065011, which is incorporated herein by reference. 
     As best shown in  FIG. 9 , the delivery apparatus  100  also includes a second, intermediate catheter  108  (also referred to herein as a torque shaft catheter) having an elongated shaft  110  (also referred to herein as a torque shaft) and an elongated screw  112  connected to the distal end of the shaft  110 . The shaft  110  of the intermediate catheter  108  extends coaxially through the shaft  104  of the main catheter  102 . The delivery apparatus  100  can also include a third, nose-cone catheter  118  having an elongated shaft  120  and a nose piece, or nose cone,  122  secured to the distal end portion of the shaft  120 . The nose piece  122  can have a tapered outer surface as shown for atraumatic tracking through the patient&#39;s vasculature. The shaft  120  of the nose-cone catheter extends through the prosthetic valve  10  (not shown in  FIGS. 8-9 ) and the shaft  110  of the intermediate catheter  108 . In the illustrated configuration, the innermost shaft  120  is configured to be moveable axially and rotatably relative to the shafts  104 ,  110 , and the torque shaft  110  is configured to be rotatable relative to the shafts  104 ,  120  to effect valve deployment and release of the prosthetic valve from the delivery apparatus, as described in detail below. Additionally, the innermost shaft  120  can have a lumen for receiving a guide wire so that the delivery apparatus can be advanced over the guide wire inside the patient&#39;s vasculature. 
     As best shown in  FIG. 10 , the outer catheter  102  can comprise a flex control mechanism  168  at a proximal end thereof to control the amount the bending or flexing of a distal portion of the outer shaft  104  as it is advanced through the patient&#39;s vasculature, such as further described below. The outer shaft  104  can comprise a proximal segment  166  that extends from the flex control mechanism  168  and a distal segment  126  that comprises a slotted metal tube that increases the flexibility of the outer shaft at this location. The distal end portion of the distal segment  126  can comprises an outer fork  130  of a valve-retaining mechanism  114  that is configured to releasably secure a prosthetic valve  10  to the delivery apparatus  100  during valve delivery, as described in detail below. 
       FIG. 28A  is an enlarged view of a portion of the distal segment  126  of the outer shaft  104 .  FIG. 28B  shows the cut pattern that can be used to form the distal segment  126  by laser cutting the pattern in a metal tube. The distal segment  126  comprises a plurality of interconnected circular bands or links  160  forming a slotted metal tube. A pull wire  162  can be positioned inside the distal segment  126  and can extend from a location  164  of the distal segment  126  ( FIGS. 10 and 12 ) to the flex control mechanism. The distal end of the pull wire  162  can be secured to the inner surface of the distal segment  126  at location  164 , such as by welding. The proximal end of the pull wire  162  can be operatively connected to the flex control mechanism  168 , which is configured to apply and release tension to the pull wire in order to control bending of the shaft, as further described below. The links  160  of the shaft and the gaps between adjacent links are shaped to allow bending of the shaft upon application of light pulling force on the pull wire  162 . In the illustrated embodiment, as best shown in  FIG. 12 , the distal segment  126  is secured to a proximal segment  166  having a different construction (e.g., one or more layers of polymeric tubing). In the illustrated embodiment, the proximal segment  166  extends from the flex control mechanism  168  to the distal segment  126  and therefore makes up the majority of the length of the outer shaft  104 . In alternative embodiments, the entire length or substantially the entire length of the outer shaft  104  can be formed from a slotted metal tube comprising one or more sections of interconnected links  160 . In any case, the use of a main shaft having such a construction can allow the delivery apparatus to be highly steerable, especially when use in combination with a torque shaft having the construction shown in  FIGS. 40 and 41  (described below). 
     The width of the links  160  can be varied to vary the flexibility of the distal segment along its length. For example, the links within the distal end portion of the slotted tube can be relatively narrower to increase the flexibility of the shaft at that location while the links within the proximal end portion of the slotted tube can be relatively wider so that the shaft is relatively less flexible at that location. 
       FIG. 29A  shows an alternative embodiment of a distal segment, indicated at  126 ′, which can be formed, for example, by laser cutting a metal tube. The segment  126 ′ can comprise the distal segment of an outer shaft of a delivery apparatus (as shown in  FIG. 12 ) or substantially the entire length of an outer shaft can have the construction shown in  FIG. 29A .  FIG. 29B  shows the cut pattern for forming the segment  126 ′. In another embodiment, a delivery apparatus can include a composite outer shaft comprising a laser-cut metal tube laminated with a polymeric outer layer that is fused within the gaps in the metal layer. In one example, a composite shaft can comprise a laser cut metal tube having the cut pattern of  FIGS. 29A and 29B  and a polymeric outer layer fused in the gaps between the links  160  of the metal tube. In another example, a composite shaft can comprise a laser cut metal tube having the cut pattern of  FIGS. 28A and 28B  and a polymeric outer layer fused in the gaps between the links  160  of the metal tube. A composite shaft also can include a polymeric inner layer fused in the gaps between the links  160  of the metal tube. 
     Referring to  FIGS. 8A and 11 , the flex control mechanism  168  can comprise a rotatable housing, or handle portion,  186  that houses a slide nut  188  mounted on a rail  192 . The slide nut  188  is prevented from rotating within the housing by one or more rods  192 , each of which is partially disposed in a corresponding recess within the rail  192  and a slot or recess on the inside of the nut  188 . The proximal end of the pull wire  162  is secured to the nut  188 . The nut  188  has external threads that engage internal threads of the housing. Thus, rotating the housing  186  causes the nut  188  to move axially within the housing in the proximal or distal direction, depending on the direction of rotation of the housing. Rotating the housing in a first direction (e.g., clockwise), causes the nut to travel in the proximal direction, which applies tension to the pull wire  162 , which causes the distal end of the delivery apparatus to bend or flex. Rotating the housing in a second direction (e.g., counterclockwise), causes the nut to travel in the distal direction, which relieves tension in the pull wire  162  and allows the distal end of the delivery apparatus to flex back to its pre-flexed configuration under its own resiliency. 
     As best shown in  FIG. 13 , the torque shaft catheter  108  includes an annular projection in the form of a ring  128  (also referred to as an anchoring disc) mounted on the distal end portion of the torque shaft  110  adjacent the screw  112 . The ring  128  is secured to the outer surface of the torque shaft  110  such that it cannot move axially or rotationally relative to the torque shaft. The inner surface of the outer shaft  104  is formed with a feature, such as a slot or recess, that receives the ring  128  in such a manner that the ring and the corresponding feature on the inner surface of the outer shaft  104  allow the torque shaft  110  to rotate relative to the outer shaft  104  but prevent the torque shaft from moving axially relative to the outer shaft. The corresponding feature on the outer shaft  104  that receives the ring  128  can be inwardly extending tab portions formed in the distal segment  126 , such as shown at  164  in  FIG. 12 . In the illustrated embodiment (as best shown in  FIG. 14 ), the ring  128  is an integral part of the screw  112  (i.e., the screw  112  and the ring  128  are portions of single component). Alternatively, the screw  112  and the ring are separately formed components but are both fixedly secured to the distal end of the torque shaft  110 . 
     The torque shaft  110  desirably is configured to be rotatable relative to the delivery sheath  106  to effect incremental and controlled advancement of the prosthetic valve  10  from the delivery sheath  106 . To such ends, and according to one embodiment, the delivery apparatus  100  can include a sheath retaining ring in the form of a threaded nut  150  mounted on the external threads of the screw  112 . As best shown in  FIG. 16 , the nut  150  includes internal threads  152  that engage the external threads of the screw and axially extending legs  154 . Each leg  154  has a raised distal end portion that extends into and/or forms a snap fit connection with openings  172  in the proximal end of the sheath  106  (as best shown in  FIG. 18 ) so as to secure the sheath  106  to the nut  150 . As illustrated in  FIGS. 17B and 18 , the sheath  106  extends over the prosthetic valve  10  and retains the prosthetic valve in a radially compressed state until the sheath  106  is retracted by the user to deploy the prosthetic valve. 
     As best shown in  FIGS. 21 and 22 , the outer fork  130  of the valve-retaining mechanism comprises a plurality of prongs  134 , each of which extends through a region defined between two adjacent legs  154  of the nut so as to prevent rotation of the nut relative to the screw  112  upon rotation of the screw. As such, rotation of the torque shaft  110  (and thus the screw  112 ) causes corresponding axial movement of the nut  150 . The connection between the nut  150  and the sheath  106  is configured such that axially movement of the nut along the screw  112  (in the distal or proximal direction) causes the sheath  106  to move axially in the same direction relative to the screw and the valve-retaining mechanism.  FIG. 21  shows the nut  150  in a distal position wherein the sheath  106  (not shown in  FIG. 21 ) extends over and retains the prosthetic valve  10  in a compressed state for delivery. Movement of the nut  150  from the distal position ( FIG. 21 ) to a proximal position ( FIG. 22 ) causes the sheath  106  to move in the proximal direction, thereby deploying the prosthetic valve from the sheath  106 . Rotation of the torque shaft  110  to effect axial movement of the sheath  106  can be accomplished with a motorized mechanism (such as shown in  FIGS. 23-26  and described below) or by manually turning a crank or wheel. 
       FIG. 17  shows an enlarged view of the nose cone  122  secured to the distal end of the innermost shaft  120 . The nose cone  122  in the illustrated embodiment includes a proximal end portion  174  that is sized to fit inside the distal end of the sheath  106 . An intermediate section  176  of the nose cone is positioned immediately adjacent the end of the sheath in use and is formed with a plurality of longitudinal grooves, or recessed portions,  178 . The diameter of the intermediate section  176  at its proximal end  180  desirably is slightly larger than the outer diameter of the sheath  106 . The proximal end  180  can be held in close contact with the distal end of the sheath  106  to protect surrounding tissue from coming into contact with the metal edge of the sheath. The grooves  178  allow the intermediate section to be compressed radially as the delivery apparatus is advanced through an introducer sheath. This allows the nose cone to be slightly oversized relative to the inner diameter of the introducer sheath.  FIG. 17B  shows a cross-section the nose cone  122  and the sheath  106  in a delivery position with the prosthetic valve retained in a compressed delivery state inside the sheath  106  (for purposes of illustration, only the stent  12  of the prosthetic valve is shown). As shown, the proximal end  180  of the intermediate section  176  can abut the distal end of the sheath  106  and a tapered proximal surface  182  of the nose cone can extend within a distal portion of the stent  12 . 
     As noted above, the delivery apparatus  100  can include a valve-retaining mechanism  114  ( FIG. 8B ) for releasably retaining a stent  12  of a prosthetic valve. The valve-retaining mechanism  114  can include a first valve-securement component in the form of an outer fork  130  (as best shown in  FIG. 12 ) (also referred to as an “outer trident” or “release trident”), and a second valve-securement component in the form of an inner fork  132  (as best shown in  FIG. 17 ) (also referred to as an “inner trident” or “locking trident”). The outer fork  130  cooperates with the inner fork  132  to form a releasable connection with the retaining arms  30  of the stent  12 . 
     The proximal end of the outer fork  130  is connected to the distal segment  126  of the outer shaft  104  and the distal end of the outer fork is releasably connected to the stent  12 . In the illustrated embodiment, the outer fork  130  and the distal segment  126  can be integrally formed as a single component (e.g., the outer fork and the distal segment can be laser cut or otherwise machined from a single piece of metal tubing), although these components can be separately formed and subsequently connected to each other. The inner fork  132  can be mounted on the nose catheter shaft  120  (as best shown in  FIG. 17 ). The inner fork  132  connects the stent to the distal end portion of the nose catheter shaft  120 . The nose catheter shaft  120  can be moved axially relative to the outer shaft  104  to release the prosthetic valve from the valve-retaining mechanism, as further described below. 
     As best shown in  FIG. 12 , the outer fork  130  includes a plurality of angularly-spaced prongs  134  (three in the illustrated embodiment) corresponding to the retaining arms  30  of the stent  12 , which prongs extend from the distal end of distal segment  126 . The distal end portion of each prong  134  includes a respective opening  140 . As best shown in  FIG. 17 , the inner fork  132  includes a plurality of angularly-spaced prongs  136  (three in the illustrated embodiment) corresponding to the retaining arms  30  of the stent  12 , which prongs extend from a base portion  138  at the proximal end of the inner fork. The base portion  138  of the inner fork is fixedly secured to the nose catheter shaft  120  (e.g., with a suitable adhesive) to prevent axial and rotational movement of the inner fork relative to the nose catheter shaft  120 . 
     Each prong of the outer fork cooperates with a corresponding prong of the inner fork to form a releasable connection with a retaining arm  30  of the stent. In the illustrated embodiment, for example, the distal end portion of each prong  134  is formed with an opening  140 . When the prosthetic valve is secured to the delivery apparatus (as best shown in  FIG. 19 ), each retaining arm  30  of the stent  12  extends inwardly through an opening  140  of a prong  134  of the outer fork and a prong  136  of the inner fork is inserted through the opening  32  of the retaining arm  30  so as to retain the retaining arm  30  from backing out of the opening  140 .  FIG. 42  also shows the prosthetic valve  10  secured to the delivery apparatus by the inner and outer forks before the prosthetic valve is loaded into the sheath  106 . Retracting the inner prongs  136  proximally (in the direction of arrow  184  in  FIG. 20 ) to remove the prongs from the openings  32  is effective to release the prosthetic valve  10  from the retaining mechanism. When the inner fork  132  is moved to a proximal position ( FIG. 20 ), the retaining arms  30  of the stent can move radially outwardly from the openings  140  in the outer fork  130  under the resiliency of the stent. In this manner, the valve-retaining mechanism  114  forms a releasable connection with the prosthetic valve that is secure enough to retain the prosthetic valve relative to the delivery apparatus to allow the user to fine tune or adjust the position of the prosthetic valve after it is deployed from the delivery sheath. When the prosthetic valve is positioned at the desired implantation site, the connection between the prosthetic valve and the retaining mechanism can be released by retracting the nose catheter shaft  120  relative to the outer shaft  104  (which retracts the inner fork  132  relative to the outer fork  130 ). 
     Techniques for compressing and loading the prosthetic valve  10  into the sheath  106  are described below. Once the prosthetic valve  10  is loaded in the delivery sheath  106 , the delivery apparatus  100  can be inserted into the patient&#39;s body for delivery of the prosthetic valve. In one approach, the prosthetic valve can be delivered in a retrograde procedure where delivery apparatus is inserted into a femoral artery and advanced through the patient&#39;s vasculature to the heart. Prior to insertion of the delivery apparatus, an introducer sheath can be inserted into the femoral artery followed by a guide wire, which is advanced through the patient&#39;s vasculature through the aorta and into the left ventricle. The delivery apparatus  100  can then be inserted through the introducer sheath and advanced over the guide wire until the distal end portion of the delivery apparatus containing the prosthetic valve  10  is advanced to a location adjacent to or within the native aortic valve. 
     Thereafter, the prosthetic valve  10  can be deployed from the delivery apparatus  100  by rotating the torque shaft  110  relative to the outer shaft  104 . As described below, the proximal end of the torque shaft  110  can be operatively connected to a manually rotatable handle portion or a motorized mechanism that allows the surgeon to effect rotation of the torque shaft  110  relative to the outer shaft  104 . Rotation of the torque shaft  110  and the screw  112  causes the nut  150  and the sheath  106  to move in the proximal direction toward the outer shaft ( FIG. 22 ), which deploys the prosthetic valve from the sheath. Rotation of the torque shaft  110  causes the sheath to move relative to the prosthetic valve in a precise and controlled manner as the prosthetic valve advances from the open distal end of the delivery sheath and begins to expand. Hence, unlike known delivery apparatuses, as the prosthetic valve begins to advance from the delivery sheath and expand, the prosthetic valve is held against uncontrolled movement from the sheath caused by the expansion force of the prosthetic valve against the distal end of the sheath. In addition, as the sheath  106  is retracted, the prosthetic valve  10  is retained in a stationary position relative to the ends of the inner shaft  120  and the outer shaft  104  by virtue of the valve-retaining mechanism  114 . As such, the prosthetic valve  10  can be held stationary relative to the target location in the body as the sheath is retracted. Moreover, after the prosthetic valve is partially advanced from the sheath, it may be desirable to retract the prosthetic valve back into the sheath, for example, to reposition the prosthetic valve or to withdraw the prosthetic valve entirely from the body. The partially deployed prosthetic valve can be retracted back into the sheath by reversing the rotation of the torque shaft, which causes the sheath  106  to advance back over the prosthetic valve in the distal direction. 
     In known delivery devices, the surgeon must apply push-pull forces to the shaft and/or the sheath to unsheathe the prosthetic valve. It is therefore difficult to transmit forces to the distal end of the device without distorting the shaft (e.g., compressing or stretching the shaft axially), which in turn causes uncontrolled movement of the prosthetic valve during the unsheathing process. To mitigate this effect, the shaft and/or sheath can be made more rigid, which is undesirable because the device becomes harder to steer through the vasculature. In contrast, the manner of unsheathing the prosthetic valve described above eliminates the application of push-pull forces on the shaft, as required in known devices, so that relatively high and accurate forces can be applied to the distal end of the shaft without compromising the flexibility of the device. In certain embodiments, as much as 20 lbs. of force can be transmitted to the end of the torque shaft without adversely affecting the unsheathing process. In contrast, prior art devices utilizing push-pull mechanisms typically cannot exceed about 5 lbs. of force during the unsheathing process. 
     After the prosthetic valve  10  is advanced from the delivery sheath and expands to its functional size (the expanded prosthetic valve  10  secured to the delivery apparatus is depicted in  FIG. 42 ), the prosthetic valve remains connected to the delivery apparatus via the retaining mechanism  114 . Consequently, after the prosthetic valve is advanced from the delivery sheath, the surgeon can reposition the prosthetic valve relative to the desired implantation position in the native valve such as by moving the delivery apparatus in the proximal and distal directions or side to side, or rotating the delivery apparatus, which causes corresponding movement of the prosthetic valve. The retaining mechanism  114  desirably provides a connection between the prosthetic valve and the delivery apparatus that is secure and rigid enough to retain the position of the prosthetic valve relative to the delivery apparatus against the flow of the blood as the position of the prosthetic valve is adjusted relative to the desired implantation position in the native valve. Once the surgeon positions the prosthetic valve at the desired implantation position in the native valve, the connection between the prosthetic valve and the delivery apparatus can be released by retracting the innermost shaft  120  in the proximal direction relative to the outer shaft  104 , which is effective to retract the inner fork  132  to withdraw its prongs  136  from the openings  32  in the retaining arms  30  of the prosthetic valve ( FIG. 20 ). Slightly retracting of the outer shaft  104  allows the outer fork  130  to back off the retaining arms  30  of the prosthetic valve, which slide outwardly through openings  140  in the outer fork to completely disconnect the prosthetic valve from the retaining mechanism  114 . Thereafter, the delivery apparatus can be withdrawn from the body, leaving the prosthetic aortic valve  10  implanted within the native valve (such as shown in  FIGS. 5A and 5B ). 
     The delivery apparatus  100  has at its distal end a semi-rigid segment comprised of relatively rigid components used to transform rotation of the torque shaft into axial movement of the sheath. In particular, this semi-rigid segment in the illustrated embodiment is comprised of the prosthetic valve and the screw  112 . An advantage of the delivery apparatus  100  is that the overall length of the semi-rigid segment is minimized because the nut  150  is used rather than internal threads on the outer shaft to affect translation of the sheath. The reduced length of the semi-rigid segment increases the overall flexibility along the distal end portion of the delivery catheter. Moreover, the length and location of the semi-rigid segment remains constant because the torque shaft does not translate axially relative to the outer shaft. As such, the curved shape of the delivery catheter can be maintained during valve deployment, which improves the stability of the deployment. A further benefit of the delivery apparatus  100  is that the ring  128  prevents the transfer of axial loads (compression and tension) to the section of the torque shaft  110  that is distal to the ring. 
     In an alternative embodiment, the delivery apparatus can be adapted to deliver a balloon-expandable prosthetic valve. As described above, the valve retaining mechanism  114  can be used to secure the prosthetic valve to the end of the delivery apparatus. Since the stent of the prosthetic valve is not self-expanding, the sheath  106  can be optional. The retaining mechanism  114  enhances the pushability of the delivery apparatus and prosthetic valve assembly through an introducer sheath. 
       FIGS. 23-26  illustrate the proximal end portion of the delivery apparatus  100 , according to one embodiment. The delivery apparatus  100  can comprise a handle  202  that is configured to be releasably connectable to the proximal end portion of a catheter assembly  204  comprising catheters  102 ,  108 ,  118 . It may be desirable to disconnect the handle  202  from the catheter assembly  204  for various reasons. For example, disconnecting the handle can allow another device to be slid over the catheter assembly, such as a valve-retrieval device or a device to assist in steering the catheter assembly. It should be noted that any of the features of the handle  202  and the catheter assembly  204  can be implemented in any of the embodiments of the delivery apparatuses disclosed herein. 
       FIGS. 23 and 24  show the proximal end portion of the catheter assembly  204  partially inserted into a distal opening of the handle  202 . The proximal end portion of the main shaft  104  is formed with an annular groove  212  (as best shown in  FIG. 24 ) that cooperates with a holding mechanism, or latch mechanism,  214  inside the handle. When the proximal end portion of the catheter assembly is fully inserted into the handle, as shown in  FIGS. 25 and 26 , an engaging portion  216  of the holding mechanism  214  extends at least partially into the groove  212 . One side of the holding mechanism  214  is connected to a button  218  that extends through the housing of the handle. The opposite side of the holding mechanism  214  is contacted by a spring  220  that biases the holding mechanism to a position engaging the main shaft  104  at the groove  212 . The engagement of the holding mechanism  214  within the groove  212  prevents axial separation of the catheter assembly from the handle. The catheter assembly can be released from the handle by depressing button  218 , which moves the holding mechanism  214  from locking engagement with the main shaft. Furthermore, the main shaft  104  can be formed with a flat surface portion within the groove  212 . The flat surface portion is positioned against a corresponding flat surface portion of the engaging portion  216 . This engagement holds the main shaft  104  stationary relative to the torque shaft  110  as the torque shaft is rotated during valve deployment. 
     The proximal end portion of the torque shaft  110  can have a driven nut  222  ( FIG. 26 ) that is slidably received in a drive cylinder  224  ( FIG. 25 ) mounted inside the handle. The nut  222  can be secured to the proximal end of the torque shaft  100  by securing the nut  222  over a coupling member  170  ( FIG. 15 ).  FIG. 26  is a perspective view of the inside of the handle  202  with the drive cylinder and other components removed to show the driven nut and other components positioned within the drive cylinder. The cylinder  224  has a through opening (or lumen) extending the length of the cylinder that is shaped to correspond to the flats of the nut  222  such that rotation of the drive cylinder is effective to rotate the nut  222  and the torque shaft  110 . The drive cylinder can have an enlarged distal end portion  236  that can house one or more seals (e.g.,  0 -rings  246 ) that form a seal with the outer surface of the main shaft  104  ( FIG. 25 ). The handle can also house a fitting  238  that has a flush port in communication with the lumen of the torque shaft and/or the lumen of the main shaft. 
     The drive cylinder  224  is operatively connected to an electric motor  226  through gears  228  and  230 . The handle can also house a battery compartment  232  that contains batteries for powering the motor  226 . Rotation of the motor in one direction causes the torque shaft  110  to rotate, which in turn causes the sheath  106  to retract and uncover a prosthetic valve at the distal end of the catheter assembly. Rotation of the motor in the opposite direction causes the torque shaft to rotate in an opposite direction, which causes the sheath to move back over the prosthetic valve. An operator button  234  on the handle allows a user to activate the motor, which can be rotated in either direction to un-sheath a prosthetic valve or retrieve an expanded or partially expanded prosthetic valve. 
     As described above, the distal end portion of the nose catheter shaft  120  can be secured to an inner fork  132  that is moved relative to an outer fork  130  to release a prosthetic valve secured to the end of the delivery apparatus. Movement of the shaft  120  relative to the main shaft  104  (which secures the outer fork  130 ) can be effected by a proximal end portion  240  of the handle that is slidable relative to the main housing  244 . The end portion  240  is operatively connected to the shaft  120  such that movement of the end portion  240  is effective to translate the shaft  120  axially relative to the main shaft  104  (causing a prosthetic valve to be released from the inner and outer forks). The end portion  240  can have flexible side panels  242  on opposite sides of the handle that are normally biased outwardly in a locked position to retain the end portion relative to the main housing  244 . During deployment of the prosthetic valve, the user can depress the side panels  242 , which disengage from corresponding features in the housing and allow the end portion  240  to be pulled proximally relative to the main housing, which causes corresponding axial movement of the shaft  120  relative to the main shaft. Proximal movement of the shaft  120  causes the prongs  136  of the inner fork  132  to disengage from the apertures  32  in the stent  12 , which in turn allows the retaining arms  30  of the stent to deflect radially outwardly from the openings  140  in the prongs  134  of the outer fork  130 , thereby releasing the prosthetic valve. 
       FIG. 27  shows an alternative embodiment of a motor, indicated at  400 , that can be used to drive a torque shaft (e.g., torque shaft  110 ). In this embodiment, a catheter assembly can be connected directly to one end of a shaft  402  of the motor, without gearing. The shaft  402  includes a lumen that allows for passage of an innermost shaft (e.g., shaft  120 ) of the catheter assembly, a guide wire, and/or fluids for flushing the lumens of the catheter assembly. 
     Alternatively, the power source for rotating the torque shaft  110  can be a hydraulic power source (e.g., hydraulic pump) or pneumatic (air-operated) power source that is configured to rotate the torque shaft. In another embodiment, the handle can have a manually movable lever or wheel that is operable to rotate the torque shaft  110 . 
     In another embodiment, a power source (e.g., an electric, hydraulic, or pneumatic power source) can be operatively connected to a shaft, which is turn is connected to a prosthetic valve  10 . The power source is configured to reciprocate the shaft longitudinally in the distal direction relative to a valve sheath in a precise and controlled manner in order to advance the prosthetic valve from the sheath. Alternatively, the power source can be operatively connected to the sheath in order to reciprocate the sheath longitudinally in the proximal direction relative to the prosthetic valve to deploy the prosthetic valve from the sheath. 
       FIGS. 44-45  show an alternative configuration for the screw  112  and nut  150  of the delivery apparatus  100  or delivery apparatus  600  (described below). In this embodiment, the screw  112  is replaced with a helical coil  700  (which can be, for example, a metal compression or tension spring), and the nut  150  is replaced with a sheath retaining ring in the form of a washer, or blade,  702  mounted on the coil  700 . The proximal end of the coil is fixedly secured to the distal end of the torque shaft  110  (for example by welding or a suitable adhesive). The coil  700  can be made of any of various suitable metals (e.g., stainless steel, Nitinol, etc.) or polymeric materials. 
     The washer  702  has a central aperture  704  that receives the coil  700  and an internal tooth  706  that engages the grooves defined on the outer surface of the coil and desirably extends radially inwardly between adjacent turns or loops of the coil. The outer circumferential edge of the washer  702  can be formed with a plurality of recesses, or grooves,  708 , each of which is sized to receive a prong  134  of the outer fork  130 , which prevents rotation of the washer upon rotation of the torque shaft  110 . The sheath  106  can be secured to the outer circumferential edge of the washer  702  in any convenient manner. For example, the portions between recesses  708  can extend into the openings  172  of the sheath ( FIG. 18 ) to fix the sheath axially and rotationally relative to the washer. Alternatively, the washer can be welded or adhesively secured to the sheath. 
     When incorporated in the delivery apparatus  100 , the coil  700  and washer  702  operate in a manner similar to the screw  112  and nut  150 . Thus, when the torque shaft  110  is rotated, the washer  702  is caused to move axially along the length of the coil  700  to effect corresponding axial movement of the sheath, either to deploy a prosthetic valve or recapture a prosthetic valve back into the sheath. An advantage of the coil and washer configuration is that it allows the distal portion of the delivery apparatus occupied by the coil to bend or flex to facilitate tracking through the patient&#39;s vasculature, especially in patients with relatively small aortic arches and short ascending aortas. The coil also allows the sheath to be moved (proximally or distally) upon rotation of the torque shaft when the coil is in a flexed or curved state inside the patient&#39;s vasculature. In particular embodiments, the distal portion of the delivery apparatus occupied by the coil can be flexed from a straight configuration to a curved configuration having a radius of curvature of about 1 cm. In addition, the coil can change its pitch under dynamic loading (compression or tension), which reduces the build-up of tensile forces along the length of the delivery apparatus and avoids galling of the washer when subjected to bending forces. 
     The coil and washer configuration can be implemented in other delivery apparatuses that are used to implant various other types of prosthetic implants within body ducts. For example, the coil and washer configuration can be incorporated in a delivery apparatus used to implant stents or similar implants within the coronary sinus. The coil and washer configuration can also be utilized in various non-medical applications to replace a screw and nut assembly where the screw is subjected to bending forces. 
       FIG. 30  shows another exemplary stent  300 , for use in a prosthetic heart valve. For purposes of illustration, only the bare stent  300  is shown while the other components of the prosthetic valve, including the leaflets and the skirt, are omitted. However, it should be understood that the prosthetic valve can include leaflets  34   a ,  34   b ,  34   c  and a skirt  42  mounted to the stent  300 , as described above in connection with the prosthetic valve  10 . The stent  300  can have the same overall shape and configuration as the stent  12  of prosthetic valve  10  described above, except that all apices  302  at the outflow end of the stent  300  have respective apertures  304 . The stent  300  can further comprise three commissure posts  306  (which are also referred to as “apices” herein) with eyelets  308 , also at the outflow end. The delivery apparatuses  500 ,  600  (described below for use with stent  300 ) can be used to deliver the stent  10  (or any other stent with apices that lack apertures). In this case, the delivery apparatus can engage the stent by wrapping the suture loops around the apices at one end of the stent (e.g., the outflow end). In some embodiments, the stent can have notches, channels or other narrowed portions formed in or adjacent to the apices, for stably holding the suture loops against their respective apices. 
       FIGS. 31A-37  show an exemplary delivery apparatus  500  for delivering the stent  300 . The delivery apparatus  500  is similar to the delivery apparatus  100  except that the delivery apparatus  500  includes a different mechanism for releasably securing a prosthetic valve to the delivery apparatus. The delivery apparatus  500  in the illustrated embodiment comprises a main shaft  502 , a sheath  504  mounted to the distal end of the shaft  502 , an inner shaft  506  that extends co-axially through the main shaft  502 , and a nose cone  508  mounted to the distal end of the inner shaft  506 . The inner shaft  506  can have a guidewire lumen configured to receive a guidewire  509 . As best shown in  FIGS. 31A , a suture-retention member  510  can extend distally from the distal end of the main shaft  502 . The inner shaft  506  can extend co-axially through the suture-retention member  510 . 
     Although not shown, the delivery apparatus  500  can also include a torque shaft that is effective to move the sheath  504  in the proximal and distal directions relative to the main shaft  502  and relative to a prosthetic valve secured to the distal end of the delivery apparatus. The distal end portion of the main shaft  502  can have the same configuration as the distal segment  126  of the shaft  104  of the delivery apparatus  100  described above. 
     The suture-retention member  510  comprises a proximal disc member  512 , a distal disc member  516 , and a shaft  514  extending between and connecting the proximal and distal disc members  512 ,  516 , respectively. As best shown in  FIG. 33 , the proximal disc member  512  can be fixed inside of the main shaft  502 . Each disc member  512 ,  516  is formed with one or more axially extending openings  518  ( FIGS. 31C and 31D ), each of which is sized to receive the distal end portion of a suture release member  520  ( FIG. 32 ). The release member  520  can be, for example, a stiff wire, and therefore is referred to below as a release wire. In the illustrated embodiment, the delivery apparatus includes a single release wire  520  that extends distally through corresponding openings  518  in the disc members  512 ,  516  and proximally through the main shaft  502  along the length of the delivery apparatus toward a handle (not shown) of the delivery apparatus. The proximal end of the release wire (not shown) can be exposed at the proximal end of the delivery apparatus for being manipulated by a user or can be coupled to an actuator on the handle of the delivery apparatus that can control axial movement of the release wire. 
     The release wire  520  is slidable in the proximal and distal directions relative to the suture-retention member  510  to secure the stent  300  to the suture-retention member  510  via a plurality of suture loops  522  and to release the stent  300  from the suture-retention member, as further described below. In some embodiments, the delivery apparatus can include a plurality of such release wires  520  (such as two or three release wires  520 ), each of which extends through corresponding openings  518  in the disc members  512 ,  516 . These release wires  520  can each interact with one or more suture loops  522 , and can aid in balancing load distribution. 
     As noted above, the stent  300  can be releasably connected to the suture-retention member  510  using a plurality of suture loops  522 . For that purpose, the proximal disc member  512  can include a plurality of openings  528  and  530  (in addition to opening  518  for the release wire) for threading the suture loops through the proximal disc member ( FIGS. 31A and 31B ). The suture loops  522  can be formed from a single piece of suture material that is folded multiple times so as to form multiple loops  522  extending distally from openings in the proximal disc member  512 , as depicted in  FIG. 31A . In alternative embodiments, each loop  522  can be formed from a separate piece of suture material. In some cases, each suture loop  522  consists entirely of a loop of suture material, whereas in other cases, one or more of the suture loops  522  can comprise a non-looped portion (such as a linear segment of suture material) proximal to the looped portion. As such, a “suture loop” can be characterized as “extending from” a given location, even if the looped portion itself does not originate or extend through that location, so long as the suture material comprising the looped portion extends from that location. However, where a “suture loop” is described as wrapping or extending around a given structure and/or residing at a given location, this specifically refers to the looped portion of the suture loop. 
     As shown in  FIGS. 31A-31D , multiple loops  522  (e.g., two or three loops) can extend outwardly from each opening in the proximal disc member  512 , although in other embodiments each suture loop  522  can extend from a separate opening. As shown in  FIG. 31D , the proximal disc member  512  can have six openings. Three suture loops  522  can extend from each of four openings  528  in the proximal disc member  512 , and through apertures  304  in the apices  302 . A fifth opening  530  can have one or more suture loops extending therefrom (such as three suture loops) to engage the commissure post eyelets  308  of the stent  300 . Finally, the release wire  520  can extend distally from out of the sixth opening  518 , toward the second disc member  516 . In the illustrated embodiment, a single suture loop extends from the fifth opening  530  through the eyelets  308  of each of the commissure posts. In some cases, having a single suture loop extending through the commissure post eyelets  308  provides better tension control, resulting in more controlled release and/or recapture of the prosthetic valve. The suture loop(s) extending through the commissure post eyelets  308  may be thicker than the suture loops  522  that extend through the other stent apices  302 . For example, in one embodiment, the suture loop(s) extending through the commissure posts are 4-0 sutures, whereas the suture loops extending through the apices are 3-0 sutures. The six openings can be arranged in an annular pattern as shown ( FIG. 31D ), and the suture loops  522  can be configured to extend outward to engage stent apices  302  in accordance with their relative positions within this annular pattern, such that the suture loops  522  do not cross past one another to reach their respective stent apices. In other embodiments, the suture loops can be configured to engage stent apices  302 , such that the suture loops cross one another to reach respective stent apices. 
     Referring to  FIGS. 32 and 33 , when loading the stent  300  onto the delivery apparatus, the apices  302  of the stent  300  are placed adjacent the distal disc member  516 , and each suture loop  522  is threaded through a respective aperture  304  in one of the apices  302 . By having a respective suture loop  522  extend through every apex  302  (including every commissure post  306 ), the prosthetic valve may be fully retrievable (while connected to the delivery apparatus), as the apices  302 ,  306  can be collapsed radially inward using the sutures  522 . In certain embodiments, the functioning of the prosthetic valve can be assessed after deploying the valve from the sheath  504  and prior its recapture. In various embodiments, the number of apices and corresponding suture loops can vary, so long as a sufficient number of apices are connected such that the end of the prosthetic valve is collapsed when the apices are collapsed radially inward. In the embodiment shown, there are twelve suture loops threaded through twelve respective apices. The end  524  of each suture loop  522  is then placed in the area between the proximal and distal disc members  512 ,  516  and the release wire  520  is slid axially through the loop and a respective opening  519  in the distal disc member  516  ( FIG. 31A ) so as to retain the end  524  of the loop on the release wire, as depicted in  FIG. 33 . For purposes of illustration,  FIGS. 32 and 33  show just a single suture loop  522  releasably connecting one of the apices  302  of the stent to the release wire  520 . Desirably, a suture loop  522  is inserted through each of the apertures/eyelets  304 ,  308  in the apices  302 ,  306  of the stent and retained by the release wire  520 .  FIG. 34  shows the stent  300  after suture loops  522  are inserted through all of the apices of the stent and retained on the release wire  520 . As noted above, while only one release wire  520  is shown in the illustrated embodiment, the delivery apparatus can be provided with a plurality of release wires  520  for retaining the suture loops  522 . 
     When threading the suture loops  522  through the openings  304 ,  308  of the apices, the suture loops  522  can be threaded sequentially through each of the openings  304 ,  308  moving in a circumferential direction around the stent. In another embodiment, the suture loops  522  can be inserted through every second or third or fourth opening  304 ,  308  and placed on the release wire  520 , moving in a circumferential direction around the stent several times until a suture loop is inserted through each of the openings, so as to balance the stent attachment relative to the release wire  520 . 
     After the stent  300  is connected to the suture-retention member  510  ( FIG. 34 ), the sheath  504  is advanced distally (e.g., by rotating the torque shaft of the delivery apparatus) to load the prosthetic valve into the sheath. As the sheath  504  is advanced over the suture loops  522 , tension in the suture loops causes the apices  302 ,  306  to collapse radially inward toward the main shaft  502 . The sheath  504  is further advanced, causing the sheath  504  to extend over and collapse the stent  300  (as shown in  FIG. 35 ), until the distal end of the sheath  504  abuts the nose cone  508  ( FIG. 36 ). As best shown in  FIG. 33 , the apices  302  can bear against the distal surface of the distal disc member  516 , which prevents the prosthetic valve from sliding proximally and maintains tension in the suture loops  522  as the sheath is retracted. 
     When the prosthetic valve is delivered to the desired implantation site within the body, the sheath  504  is retracted (e.g., by rotating the toque shaft) to deploy the prosthetic valve. After the prosthetic valve is fully deployed from the sheath, the stent  300  is still connected to the stent-retention member  510  by the suture loops  522 , as depicted in  FIG. 34 . Thus, if it becomes necessary to retrieve the prosthetic valve such as for removal or re-positioning, the sheath  504  is advanced distally to draw the prosthetic valve back into the sheath. On the other hand, if it is determined that the prosthetic valve is accurately positioned at the desired implantation site, the release wire  520  can be pulled proximally to release the ends  524  of the suture loops  522 . Slight retraction of the main shaft  502  is effective to pull the suture loops out of the openings  302  in the stent  300 , as depicted in  FIG. 37 . The proximal end of the release wire  520  can be exposed at the proximal end of the delivery apparatus so that the user can manually pull the release wire to release the prosthetic valve. Alternatively, the handle can have an actuator or switch that is configured to effect proximal movement of the release wire. 
     The sheath  504  can be made of a polymeric material, such as PEEK or nylon- 12 , and can have a reinforced distal tip portion, such as by securing a metal ring to the distal end portion of the sheath, to better resist the expansion force of the stent as it is drawn into the sheath. Alternatively, the sheath  504  can comprise a metal cylinder having a polymeric soft tip portion reflowed or molded to the distal end portion of the cylinder. 
       FIGS. 38-43  shows another delivery apparatus  600  generally comprising a first catheter  602  and a second catheter  604  extending coaxially through the first catheter  602 , and a delivery sheath or cylinder  612  coupled to the distal ends of the catheters  602 ,  604 . The proximal ends of the catheters  602 ,  604  can be coupled to a handle (e.g., a handle  202  such as shown in  FIG. 23 ). As best shown in  FIG. 40 , the first catheter  602  comprises an elongated shaft  606  that extends distally from the handle, an intermediate section  608  extending distally from the distal end of the shaft  606 , and a distal end portion  610  extending distally from the intermediate section  608 . The intermediate section  608  comprises a plurality of angularly spaced rails  613  that extend longitudinally from the shaft  606  to the distal end portion  610 . The rails  613  cooperate with a nut  640  to inhibit rotation of the nut yet allow longitudinal movement of the nut upon rotation of the second catheter  604 . In this manner, the rails  613  serve the same purpose of the prongs  134  in preventing rotation of the nut  150 . The distal end portion  610  in the illustrated embodiment comprises a slotted metal tube to enhance the flexibility of this section of the first catheter  602 . 
     As best shown in  FIGS. 39A-39C , the second catheter  604  can comprise a elongated shaft  614  (which can be referred to as a “torque shaft”), a coupling member  616  connected to the proximal end of the shaft  614 , and a threaded screw  618  connected to the distal end of the shaft  614 . The coupling member  616  is configured to be connected to a handle as described above (e.g., a handle  202 ). The screw  618  has external threads that engage internal threads of the nut  640 . As best shown in  FIG. 41 , when the apparatus is assembled, the elongated shaft  614  of the second catheter  604  extends coaxially through the elongated shaft  606  of the first catheter  602 , and the screw  618  extends coaxially through the railed section  608  of the first catheter  602 . The nut  640  is mounted on the screw  618  and is connected to the proximal end portion of delivery cylinder  612 . The distal end portion  610  of the first catheter  602  extends coaxially through the delivery cylinder  612 . 
     As best shown in  FIGS. 40-42 , a suture-retention member  626  can be connected to the distal end of the slotted tube  610 . The suture-retention member  626  can have features similar to as described above for suture-retention member  510 , including a proximal disc member  638  connected to the distal end of the slotted tube  610 , a distal disc member  636 , and at least one release member or release wire  628  extending through the proximal and distal disc members for interacting with one or more suture loops  522 . 
     Returning to  FIGS. 39A-39C , the delivery cylinder  612  in the illustrated embodiment comprises a relatively more flexible proximal portion  630  and a relatively less flexible distal end portion  632 . The proximal portion  630  can comprise a slotted metal tube or cylinder to enhance the flexibility of this section of the delivery cylinder  612 . The distal end portion  632  comprises a sleeve or sheath (also referred to as a “valve holding portion”) that is configured to extend over and retain a prosthetic valve in a radially compressed state during delivery. In some embodiments, the sheath  632  can extend over the prosthetic valve and the suture-retention member  626  during delivery ( FIG. 43 ). Alternatively, the suture-retention member  626  can be (at least partially) housed within the proximal portion  630  during delivery. The sheath  632  can be made of a suitable polymeric material, such as PEEK, nylon-12, and/or PEBAX, or a metal having a polymeric inner liner. When made of polymeric materials, the sheath  632  can be thermally bonded to the slotted tube  630 . A distal end segment  634  of the sheath  632  can be flared radially outward to enhance recapturability of the prosthetic valve. The distal end segment  634  can comprise a polymeric and/or elastomeric material, such as PEEK, nylon, and/or PEBAX. In particular embodiments, the distal end segment  634  is more flexible and/or elastomeric than the remaining section of the distal end portion. In a working embodiment, the distal end segment  634  comprises PEBAX and the remaining portion of the distal end portion  632  comprises nylon. As shown in  FIG. 46 , the distal end segment  634  can include a radially projecting, annular bump  650  to facilitate loading and recapturing of a prosthetic valve. During recapture, the bump  650  presses the sutures  522  inwardly, which causes the apices  302 ,  306  to collapse inwardly, allowing the sheath to slide over the frame. 
     As shown in  FIG. 42 , the delivery apparatus  600  can further comprise a nose cone  620  connected to the distal end of a nose cone shaft  622 , which extends through the distal shaft portion  610  of the first catheter  602 , the suture retention member  626 , and the screw  618  and the shaft  614  of the second catheter  604 . The nose cone shaft  622  can include a guidewire lumen and can extend proximally to the handle of the delivery apparatus. 
     The delivery cylinder  612  cooperates with the screw  618  and the nut  640  to allow for longitudinal (i.e., proximal and/or distal) movement of the delivery cylinder  612  relative to the distal shaft portion  610  and the suture-retention member  626 . Rotational motion of the screw  618  (initiated by the user rotating the torque shaft  614 ) can be converted into translational movement of the delivery cylinder  612  via the nut  640  positioned along external threads of the screw  618  ( FIG. 39B ). The nut  640  can have internal threading configured to compatibly engage the external threads of the screw member  618 . The nut  640  can further comprise one or more tabs  642  protruding radially outward, and the delivery cylinder  612  can comprise one or more receiving areas (such as one or more windows  644 ) adjacent a proximal end of the cylinder  612  for engaging with these tabs  642 . In particular, upper portions of the tab(s)  642  can extend through the window(s)  644  to produce a secure fit (e.g., a snap fit) with the delivery cylinder  612 . 
     As noted above, the first catheter  602  includes a section  608  that includes a plurality of angularly spaced rails  613 , which cooperate with the tab(s)  642  of the nut. As best shown in  FIG. 41 , the screw  618  extends coaxially through the rails  613  and the nut  640  is disposed on the screw  618  with each tab  642  positioned in the space between two adjacent rails  613 . To produce movement of the delivery cylinder  612 , the screw  618  can be rotated using a torque shaft  614 , as described above with respect to delivery apparatus  100 . Placement of the tab(s)  642  between the rails  613  prevents the nut  640  from rotating along with the screw  618 . With rotation of the nut  640  restricted, rotation of the screw  618  produces translational movement of the nut  640  along the screw  618 . Axial movement of the nut  640  along the screw  618  (in the distal or proximal direction) causes the cylinder  612  to also move axially, and in the same direction as the nut  640  (relative to the screw  618 ). Thus, as the nut  640  moves along the screw  618  longitudinally, the delivery cylinder  612  (connected to the nut at windows  644 ) is carried along-with. 
     An outer sleeve portion  648  can be positioned over the first and second catheters  602 ,  604  ( FIG. 41 ) and the delivery cylinder  612 , and thereby form an outermost layer of the delivery apparatus  600 . This sleeve portion  648  allows a user to effectively flush the delivery apparatus  600  with fluid to, for example, eliminate air bubbles. In some embodiments, the sleeve portion  648  can comprise an elastomeric material and/or may be affixed to the delivery cylinder  612  at one or more locations. In particular, a sleeve portion  648  having elastomeric properties can be affixed to both the delivery cylinder  612  and the elongated shaft  606  of the first catheter  602  (proximal to the intermediate section  608 ). In this case, the sleeve portion  648  can stretch, between the cylinder  612  and the shaft  606 , as the nut  640  and delivery cylinder  612  are advanced, and relax when these components are retracted. In some embodiments, the sleeve portion  648  is substantially rigid and/or is only affixed to the delivery cylinder  612 . In such cases, the entire sleeve portion  648  can be advanced distally or retracted proximally along with the delivery cylinder  612  relative to the first catheter. 
     In the case of a screw  618  and a nut  640  with standard-type threading, clockwise rotation of the screw  618  can result in proximal movement of the nut  640  along the screw  618 . Conversely, counter-clockwise rotation of the standard screw  618  can result in distal movement of the nut  640 . In this manner, rotation of the screw  618  can cause proximal or distal movement of the delivery cylinder  612  connected to the nut  640 . Alternatively, the threads of the screw can be reversed such that counter-clockwise rotation of the screw causes proximal movement of the nut and clockwise movement of the nut causes distal movement of the nut. 
       FIG. 43  shows the delivery cylinder  612  advanced forward to its distal-most position for delivery. In the delivery configuration, the distal end portion  632  extends over a prosthetic valve (not shown), which is retained in a radially compressed state and releasably connected to the suture retention member  626  with a plurality of sutures  522 . The distal end of the delivery cylinder  612  can abut an annular shoulder of the nose cone  620  (as shown in  FIG. 43 ) when the delivery cylinder is in the delivery configuration.  FIG. 42  shows the delivery cylinder  612  in a deployment configuration, with the delivery cylinder  612  retracted to a proximal position. In this position, the distal end portion  632  is retracted proximally past the prosthetic valve (allowing the prosthetic valve to expand) and the distal disc member  636  of the suture retention member  626 . To release the prosthetic valve from the suture retention member  626 , the release wire  628  is retracted such that its distal end is proximal to the second disc member  636 , thereby freeing the distal ends  524  of the suture loops  522  from the prosthetic valve. 
     Replacing the metal-metal connection between the stent and the delivery apparatus with suture loops allows for lower deployment and recapture torques. These reduced torques allow for relocation of the screw mechanism further away from the distal end of the delivery apparatus. Increasing the spacing between the screw  618  and the prosthetic valve advantageously decreases the relatively stiff section of the delivery apparatus occupied by the prosthetic valve at the distal end of the delivery apparatus. Referring to  FIG. 43 , the portion of the delivery cylinder  612  extending over the prosthetic valve has a length L 3  and the overall relatively stiff section of the delivery system  600  (which does not include the length of the nose cone) has a length L 4 , which in this embodiment corresponds to the length of the delivery cylinder  612  extending over the prosthetic valve and the suture-retention member  626 . For example, in some embodiments, L 4  is about 1.3×the length of L 3 . In various other embodiments, the ratio of L4 to L 3  is about  1 . 6  or less, about  1 . 5  or less, or about  1 . 4  or less. 
     Referring to  FIGS. 40-41 and 43 , the portion of the delivery apparatus  600  extending from the proximal end of the suture retention member  626  to the distal end of the screw  618  (which is equal to the length L 2  of the distal shaft portion  610 ) can be more flexible than the stiff section housing the prosthetic valve (which is equal to the length L 4  of the delivery cylinder  612 ) Desirably, the relatively more flexible section is long enough such that when the delivery system  600  is advanced through the aorta to implant a prosthetic valve at the aortic valve of a subject, the relatively stiff section is positioned in the ascending aorta, the screw  618  is positioned in the descending aorta, and the relatively more flexible portion extending therebetween is positioned in the aortic arch. This greatly facilitates steering of the delivery apparatus through the aortic arch and proper positioning of the prosthetic valve at the aortic annulus. 
     In various embodiments, for example, a distal end of the screw  618  can be located at least about 5 cm, at least about 10 cm, at least about 15 cm, at least about 20 cm, or at least about 30 cm away from the distal end of the suture-retention member  626  (and a prosthetic valve releasably connected to the suture-retention member  626 ). In various embodiments, the delivery cylinder  612  can have an overall length L 1  between about 3 cm and about 40 cm, between about 5 cm and about 40 cm, between about 10 cm and about 35 cm, between about 15 cm and about 30 cm, or between about 18 cm and about 25 cm. In various embodiments, the distal shaft portion  610  can have an overall length L 2  between about 0 cm and about 30 cm, between about 5 cm and about 25 cm, between about 10 cm and about 22 cm, or between about 15 cm and about 20 cm. 
     In alternative embodiments, the length L 1  of the deliver cylinder  612  can be longer than 40 cm, and in some embodiments, it can extend proximally to the handle of the delivery apparatus. 
       FIG. 47  shows an alternative slotted tube  652  that can be used in place of slotted tube  610  in the delivery apparatus  600 . The slotted tube  652  has a plurality of teeth or projections  654  formed in each turn or coil that extend into respective recesses in adjacent coils to increase torque resistance. A distal end of the tube can be formed with one or more longitudinal openings  656 , forming rails  658  between adjacent openings for cooperating with the projections  642  of the nut  640 . At the distal and proximal ends of the rails  658 , the tube can be formed with openings  660  to allow a pull wire  662  to extend through the openings and alongside the screw  618  on the outside of the slotted tube  652 . A proximal end of the tube  652  can be formed with a plurality of inwardly projecting tabs  664 . As shown in  FIG. 49 , the tabs  664  can engage an annular recessed portion  666  on the outer surface of the proximal member  638  of the suture-retention member  626 . The tabs  664  can be configured to form a snap-fit connection with the proximal member  638  sufficient to secure the suture-retention member to the slotted tube. 
     In alternative embodiments, the slotted tube  610  and the slotted tube  630  can have other patterns or configurations, such as any of those shown in  FIGS. 12, 28A, 28B, 29A , or  29 B. 
     General Considerations 
     For purposes of this description, certain aspects, advantages, and novel features of the embodiments of this disclosure are described herein. The disclosed methods, devices, and systems should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and nonobvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The methods, devices, and systems are not limited to any specific aspect or feature or combination thereof, nor do the disclosed embodiments require that any one or more specific advantages be present or problems be solved. 
     Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
     Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed methods can be used in conjunction with other methods. As used herein, the terms “a”, “an” and “at least one” encompass one or more of the specified element. That is, if two of a particular element are present, one of these elements is also present and thus “an” element is present. The terms “a plurality of” and “plural” mean two or more of the specified element. 
     As used herein, the term “and/or” used between the last two of a list of elements means any one or more of the listed elements. For example, the phrase “A, B, and/or C” means “A,” “B,” “C,” “A and B,” “A and C,” “B and C” or “A, B and C.” 
     As used herein, the term “coupled” generally means physically coupled or linked and does not exclude the presence of intermediate elements between the coupled items absent specific contrary language. 
     In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. 
     In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Moreover, additional embodiments are disclosed in U.S. Patent Application Publication No. 2010/0049313 (U.S. application Ser. No. 12/429,040), which is incorporated herein by reference. Accordingly, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.