Abstract:
A stent graft is deployed by a steerable catheter delivery system having a integral tip capture release mechanism. The steering mechanism provides for a locked interference with a distal lock at the distal end of the delivery catheter. The configuration allows for selective circumferentially distributed release of one half or less of the number of crowns of a proximal spring which are captured by a tip capture mechanism so that new positioning of the stent graft can be verified and assured before full release of all proximal spring crowns is done. In this way, one or more steering elements of a catheter can be maintained in tension until catheter position is verified and acceptable stent graft position is achieved. This apparatus and method is particularly useful for deploying stent graft in curved passages such a thoracic aorta.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to a steerable tip capture TAA delivery system used to deliver stents and stent grafts. 
       BACKGROUND 
       [0002]    This invention relates generally to medical devices and procedures, and more particularly to a method and system of deploying stent-grafts in a vascular system. 
         [0003]    Prostheses for implantation in blood vessels or other similar organs of the living body are, in general, well known in the medical art. For example, prosthetic vascular grafts formed of biocompatible materials (e.g., Dacron® material or expanded, porous polytetrafluoroethylene (PTFE) tubing) have been employed to replace or bypass damaged or occluded natural blood vessels. 
         [0004]    A graft material supported by a framework is known as a stent-graft or endoluminal graft. In general, the use of stent-grafts for treatment or isolation of vascular aneurysms and vessel walls which have been thinned or thickened by disease (endoluminal repair or exclusion) is well known. 
         [0005]    Many stent-grafts, are “self-expanding”, i.e., inserted into the vascular system in a compressed or contracted state, and permitted to expand upon removal of a restraint. Self-expanding stent-grafts typically employ a wire or tube configured (e.g., bent or cut) to provide an outward radial force and employ a suitable elastic material such as stainless steel or nitinol (nickel-titanium). Nitinol may additionally employ shape memory properties. 
         [0006]    The self-expanding stent-graft implanted at a particular location is typically configured in a tubular shape with a diameter slightly greater than the diameter of the blood vessel in which the stent-graft is intended to be used. In general, stent-grafts are typically deployed through a minimally invasive intraluminal delivery, i.e., cutting through the skin to access a lumen or vasculature or percutaneously via successive dilatation, at a convenient (and less traumatic) entry point, and routing the stent-graft through the lumen to the site where the prosthesis is to be deployed. 
         [0007]    Intraluminal deployment in one example is effected using a delivery catheter with coaxial inner tube and sheath, arranged for relative axial movement. The stent-graft is compressed and disposed within the distal end of the sheath proximal to the distal end of the catheter and held by a stent stop or distal stent graft tip capture mechanism attached to a catheter shaft. 
         [0008]    The catheter is then maneuvered, typically routed though a vessel until the end of the catheter (and the stent-graft contained therein) is positioned in the vicinity of the intended treatment site. The catheter shaft is then held stationary while the sheath of the delivery catheter is withdrawn. The catheter may include a stop or tip capture prevents the stent-graft from moving back as the sheath is withdrawn. 
         [0009]    As the sheath is withdrawn, the stent-graft is gradually exposed (uncovered—released—deployed) from its proximal end. As the stent-graft is exposed it radially expands so that at least a portion of the exposed stent graft is in substantially conforming surface contact with a portion of the interior of the lumen, e.g., blood vessel wall. 
         [0010]    In straight vessels, placement of the stent-graft is relatively straightforward. However, in complex vessels, e.g., in the aortic arch or other curved vessel, placement of the stent-graft is complicated by the tendency of the catheter to maintain a straight shape while the surround vessel curves. 
         [0011]    More particularly, in the aortic arch, the stiffness of the delivery catheter causes the distal tip of the delivery catheter to position itself close to (if not conforming with) the vessel wall at the outer radius of curvature of the aortic arch. This offset positioning (to the outer radius) of the distal tip of the delivery system combined with the effect blood flow forces have on the stent-graft as it is deployed, results in a high likelihood that the stent graft will be deployed asymmetrically. 
         [0012]      FIG. 1A  shows a stent graft deliver catheter  20  containing a stent graft  22  substantially conforming to the outside radius of curvature of the thoracic aorta  30 . As shown in  FIG. 1B  when the stent graft begins to deploy, the blood flow, shown by the arrow  32 , causes the initial deployment of the bare spring  24  at the proximal end of the stent graft  22  to open unevenly such that the portion of the spring closer to the inner radius of the thoracic arch bends outward (from the centerline of the stent graft) and downward. As a result, the proximal end  26  of the stent graft  22  is not orthogonal to the vessel wall (see  FIG. 1C ). 
         [0013]    To reiterate, as stent-graft  22  deployment begins, the blood flow (e.g.,  32 ) catches the initially deployed springs (e.g.,  24 ) like a sail of a sail boat and causes some springs and or stent graft portions to bend preferentially in the direction of blood flow. This causes uneven deployment such that the portion of the springs or stent graft closer to the inner radius of curvature of the aortic arch bends out (inward with respect to the radius of curvature as shown in  FIGS. 1B and 1C ) from the stent graft and downward when deployed high in the vessel as shown. As a result, the proximal end of the stent-graft is not deployed orthogonal to the wall of the aortic arch. To correct the initial asymmetrical deployment, the physician typically tries to reposition the stent-graft, which is generally undesirable (as vessel wall abrasion and more extensive injury may result) depending upon the particular stent graft and anatomical geometry involved. Further, due to the repositioning, additional cuff (extender) type stent-grafts may need to be deployed. 
         [0014]    As described herein: the proximal end of the stent-graft is the end closest to the heart by way of blood flow path whereas the distal end is the end furthest away from the heart as deployed. In contrast and of note, the distal end of the delivery catheter is usually identified to the end that is farthest from the operator (handle) while the proximal end of the catheter is the end nearest the operator (handle). For purposes of clarity of discussion, as used herein, the distal end of the delivery catheter is the end that is farthest from the operator (the end furthest from the handle) while the distal end of the stent-graft is the end nearest the operator (the end nearest the handle), i.e., the distal end of the catheter and the proximal end of the stent-graft are the ends furthest from the handle while the proximal end of the catheter and the distal end of the stent-graft are the ends nearest the handle. However, those of skill in the art will understand that depending upon the access location, the stent-graft and delivery system proximal and distal designations may be consistent or opposite in actual usage. When using femoral artery access the distal ends are opposite in the device and catheter, while when using a brachial artery access they are consistent. 
       SUMMARY OF THE INVENTION 
       [0015]    A device and method of deploying a stent-graft in a curved vessel using features of a steerable catheter and a tip release mechanism in one delivery catheter system are provided. The steering mechanism centers the distal tip of the delivery system and thereby a proximal end of the stent-graft in the curved vessel. A sheath of the stent-graft delivery system is retracted to expose the proximal end of the proximal spring of the stent-graft captured in the catheter tip capture mechanism. The stent-graft is held centered in the vessel and a fractional (less than all) portion of the crowns held by the tip capture mechanism are initially released to partially stabilize the stent graft in the curved vessel as it self expands to deploy. After further deployment of the stent-graft, the tension on the delivery system tip steering mechanism is relaxed and crowns of the proximal end of the stent-graft still captured in the delivery system tip capture mechanism are released. 
         [0016]    By steering the delivery catheter to center the end of the catheter containing the stent-graft prior to deployment and simultaneously using tip capture to prevent uncontrolled outward bending of the proximal spring of the stent graft, the initial deployment of the stent-graft (the proximal edge of it stent graft material) is substantially orthogonal to the axial centerline of the curved vessel at the deployment location. As the initial deployment of the stent-graft is symmetric, the need to reposition the stent-graft after initial deployment may be avoided. Accordingly, the initially deployed stent-graft is accurately placed within the curved vessel and the need to deploy additional stent-grafts is eliminated. 
         [0017]    These and other features will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIGS. 1A-C  show progressive schematic cross sectional views of a prior art delivery system deploying a stent graft asymmetrically. 
           [0019]      FIGS. 2A-E  show schematic cross sectional illustrations of a delivery system having a steerable element with a tip capture elements to steer the delivery catheter in the vessel and provide tip capture capabilities during deployment. 
           [0020]      FIG. 3A  shows a schematicized perspective view of a delivery catheter as shown in  FIG. 2A . 
           [0021]      FIG. 3B  shows a cut away view of  FIG. 3A  with part of the sheath removed and no stent graft shown in position within the sheath. 
           [0022]      FIG. 4  shows a close up view of the distal end of the delivery catheter  40  shown in  FIG. 3B , viewed from a tip end of the catheter. 
           [0023]      FIG. 5  shows a close up view of the distal portion of the delivery catheter  40  shown in  FIG. 4 , viewed from a handle end of the catheter. 
           [0024]      FIGS. 6A and 6B  provide a side schematicized view of the catheter of  FIG. 2A  in a straight and bent configurations and its operation with respect to its handle. 
           [0025]      FIGS. 7A-C  provide schematicized perspective views of the progressive steps of deployment of a stent from the delivery catheter as shown in  FIG. 2A-E . 
           [0026]      FIGS. 8A-D  shows schematicized cross section of the progressive steps and motions associated with deployment of the stent graft from the delivery system as shown in  FIGS. 2A-E . 
           [0027]      FIGS. 9A-B ,  10 A-B,  11 A-C, and  12 A-C show schematicized perspective, side, and end views of the elements and progressive steps of deployment as performed and observed for a particular steering member and release member control elements at the handle of the delivery catheter deploying a stent graft as shown in  FIGS. 2A-E . 
           [0028]      FIGS. 13A and 13B  show end views of an embodiment of a handle housing arrangement for the delivery of the stent graft through a using a delivery catheter as shown in  FIGS. 2A-E . 
           [0029]      FIG. 14  shows alternate embodiment of end view an alternative embodiment of a handle housing for delivery of the stent graft using a delivery catheter as shown in  FIGS. 2A-E . 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    A steerable catheter with a integral tip release mechanism is described below. An aortic arch  30  containing a delivery catheter  40  (having a tip  42  and containing a stent graft  44 ) conforming to the outside radius of curvature  34  of the arch  33 , is shown in  FIG. 2A . Since the tip  42  of the deliver catheter  40  naturally lays along the outer radius of the arch away from the center of the vessel, a steering mechanism (later described) is used to move the tip of the tip  42  of the catheter and it&#39;s distal end  46  towards the center of the vessel (as shown by the moving arrow  48  showing the distal end  46  of the delivery catheter  40  being moved to the center of the vessel in  FIG. 2B ). Once moved to the center of the vessel and held there as shown in the dashed-line representation as shown in  FIG. 2B , the sheath of the delivery system can be retracted to begin deployment of the stent graft  44 . A contrast injection catheter  50  may be used to position the longitudinal position of the delivery catheter  40  with respect to branching arteries, e.g.,  28 . When the sheath  56  is partially retracted the stent graft can be, if needed, manipulated to be positioned (longitudinally and radially) as desired. Initially as shown in  FIG. 2C  all of the crowns  52   a,b,c,d,e,f,  are captured in the tip capture mechanism  54  of the tip  42 . Once the physician is confident that the proximal end  58 , i.e., proximal edge of the graft material, of the stent graft  44  is positioned correctly (in proximity to, but not obstructing) with respect to the branch arteries and is substantially orthogonal to the center line of the blood vessel partial deployment (release) of half of the six crowns of the proximal spring  60 , i.e.,  52   a,    52   c,  and  52   e,  as shown in the  FIG. 2D  provide initial stabilizing and centering forces to radially center the proximal end  58  of the stent graft  44  and the still connected distal end of the delivery catheter in the vessel. The sheath  56  is then further retracted to fully deploy the stent graft and the remaining undeployed crowns  52   b,    52   d,  and  52   f  are released from the tip, thereby fully releasing the stent graft  44  into the thoracic aorta  30 , as shown in  FIG. 2E . 
         [0031]    The delivery mechanism performing the above described delivery of the stent graft is pictured in  FIG. 3A . The delivery catheter  40  has a tip  42  which is shown being delivered over a guidewire  64 . A handle  70  at the proximal end  68  of the delivery system provides a mechanism to steer and release the stent graft from the catheter as earlier described. 
         [0032]      FIG. 3B  shows a partial cut away view of the delivery catheter  40  with the stent graft not shown, except for a short wire representing a crown of a stent graft as it might be situated in the tip capture mechanism  54  of the delivery system  40 . A catheter shaft  80  extends from the handle  70  to the catheter tip  42  and provides a guidewire lumen therein. The catheter shaft in this configuration is made from a splined shaft which has been centerless ground to remove the splines in the area where the stent graft is held. The center shaft  80  is made of PEEK. At least one steering member  88   a  extends from the handle  70  to a distal anchor  62 , which in this embodiment is part of the tip  42 . The steering member  88   a  can be pulled by steering tabs (e.g.,  72 ) in the handle such that tension applied to the steering tab  72  causes tension in the steering member  88   a  to shorten the length of the catheter on the side of the steering member, causing the catheter to bend in that direction. 
         [0033]    Such bending is illustrated in the comparisons of the side view of the delivery catheter shown in  FIGS. 6A and 6B  where steering tab  72  is retracted causing the steering member  88   a  to create tension on the side of the catheter pulling the distal anchor  62  preferentially towards the handle on the side of tension causing a bending or deflection as shown by the arrow  90  in  FIG. 6B . 
         [0034]      FIGS. 4 and 5  show close up views of the tip and distal anchor of the delivery catheter  40 . The catheter shaft  80  having been centerless ground to accommodate the bulk of the stent graft in the space below splines  90   a,b,c,d,e,f.  The splines as shown here have through holes to accommodate steering members  88   a,    88   b,    88   c  which are hollowed tubes made of a polymer material such as PEEK tubing. The steering members  88   a,b,c,  extend into steering member receiving openings  92   a,b,c  in distal anchor  62 , which in this embodiment is part of the tip  42 . Release members  96   d,e,f  extend through alternate splines, i.e.,  90   b,    90   d,    90   f,  to provide, in this embodiment, capture and release capabilities for alternate crowns of a proximal spring of a stent graft. 
         [0035]    In  FIG. 5 , crown portions,  52   a,b,c,d,e,f,  are shown captured in the space between the top of the splines, i.e.,  90   a,b,c,d,e,f,  and the bottom of the distal anchor  62 . The surface of the distal anchor  62  facing the splines, e.g.,  52   a  have three steering member receiving openings  92   a,b,c,  and three release member receiving openings  94   a,b,c  therein. The steering member receiving openings, e.g.,  92   a,b,c,  each lead to an expanding diameter conical hole (e.g.,  93   a,b,c ) which is an enlarged diameter cavity inside the distal anchor  62  so that a distal end of the steering members  88   a,b,c,  can be placed there in an expanded to be locked in place by release members  96   a,    96   b,    96   c,  to act as a core element to create a mechanically interfering engagement to prevent the removal of an expandable locking elements such as a expanding collet as pictured in  FIG. 5  and as further described below. 
         [0036]    The progressive and partial release of crown elements of the proximal stent spring  60  of a stent graft to be deployed will be described with reference to  FIGS. 7A-C ,  8 A-D. A partially deployed stent graft and it&#39;s tip in captured condition is shown in  FIGS. 7A and 8A . The sheath  56  having been partially retracted using mechanisms known to persons of ordinarily skilled in the art (therefore will not be described herein). The crowns  52   a,b,c,d,e,f,  of the proximal spring  60  are shown captured within the space between the splines, e.g.,  90   a,b  and the distal anchor  62 . Meanwhile though the sheath  56  has been partially retracted none of the crowns are free to move from the tip capture mechanism  54 . As can be seen in the schematicized cross-section of  FIG. 8A , the crown  52   b  is captured in the space between the catheter shaft  80  and the distal anchor  62 , the spline  90   b,  and a distal portion of release member  96   d.  The stent crown  52   e  is similarly captured between spline  90   e,  distal anchor  62 , catheter shaft  80 , and steering member  88   c  which is reinforced and locked in place by release member  96   c  passing along a center line thereof to a distal end of the steering member  88   c.    
         [0037]    The release members (wires)  96   d, e, f,  can be retracted to release alternate more or less than all of the crowns of the stent or stent graft to be deployed, e.g.,  52   b,  while tension on the steering members holding distal tip of the catheters centered in the vessel is maintained. When partial deployment of the stent graft is to take place, for example, three of the six crowns of the proximal stent are to be released before releasing all of the crowns, while As can be seen in  FIG. 7B  crowns  52   b,d,f,  have been released by movement of the release members  96   d,e,f  which do not have a corresponding steering member associated with it. As can be seen in  FIGS. 8B and 7B  the crown  52   e  and alternating crowns around the circumference of the proximal spring  60  remain captured. When the physician is ready to deploy the not yet deployed crowns, e.g.,  52   e,  as is shown in  FIG. 8C , release members  96   c  and  96   a  must first be retracted to unlock the tip locking mechanism of the steering members  88   c  and  88   a,  respectively. Once the expandable locking element (collet)  86   c,    86   a  is released from interfering with the expanded portion of the steering member receiving opening  92   c  (shown),  92   a  (not shown), in  FIG. 8 . The steering members  88   c,    88   a  can be retracted easily in the same manner as release member  96   d,e,f  were retracted, as described above, hereby releasing crown  52   e  of the proximal springs  60 . A full release of the six crowns and the proximal spring  60  is shown in  FIG. 7C  which correlates to the condition observable in  FIG. 8D . The expandable locking elements (collet)  86   a,b,c,  can be constructed by over molding polymer material on the PEEK tubing of the steering member tubular material. A cross wise straight cut (slot) through the center allows the two halves of the expanded portion to move towards each other, i.e., collapse, to allow them to pass through the narrow passage of the steering member receiving openings  92   a,b,c.  The release members  96   a,b,c,d,e,f,  are constructed of a tube or wire preferable made of stainless steel. The release member exhibits a low coefficient of friction interaction with the inside of the PEEK steering member tube. The distal anchor  62  could be made of a metallic material or could also be made of a polymer or hard plastic material as is well known in the art. 
         [0038]    While the tip capture mechanism as described above describes the release member receiving openings, e.g.,  92   a,b,c,  as being located in the distal anchor  62  the tip capture mechanism could be constructed so that the through holes in the splines (e.g.,  90   a,b,c ) into which the steering members pass are configured to have an enlarged cavity (e.g., inverted cone shaped) to receive and lock the steering members. In such an embodiment the release members would extend beyond the splines to still capture the crowns of the proximal stent graft but such a capture arrangement would be in a distal capture space beyond the splines which are acting in this embodiment as the distal anchor. In this embodiment the capture space could be described as having a proximal tip capture space. 
         [0039]    While the use of splines with through holes have been described above, an alternate arrangement for using a spline like arrangement could include using a catheter shaft with side hooks which guide the steering members and release members to the distal anchor. It might also be used to allow greater flexibly of manufacturing and design. 
         [0040]    While the use of a collet-like tube expandable member has been described with respect to the distal anchor in the embodiments above, any similar configuration of side by side tubes or wires with one having an enlarged and locking portion to create a releasable mechanical interference in a distal anchor member could be used, as is known in the art. The only criteria would be that the configuration of the release member needed to unlock the steering member would generally have to be of a uniform diameter or cross section so that it can be pulled out easily and smoothly and not be subject catching on any corners, as might be the case if tube wire or tube like elements were used side by side where both had expandable portions which could interact to prevent their release. 
         [0041]    The manipulation of the numerous steering and release members requires the individual or coordinated movement of the various wires and tubes in a handle. The handle  70 , earlier mentioned, is now described in detail. Since the initial partial release of the approximately half or less crowns of the proximal spring of the stent graft to be deployed is a simple pull motion from the distal end of the catheter towards the proximal or handle end. The mechanism for pulling is a simple linear pulling motion and the three pull wires, i.e., release members  96   d,e,f,  can be connected to a single pull plate  74  or individual release member pull tabs  76   a,b,c.  Such plates and tabs are shown in  FIGS. 13A ,  13 B, and  14 . 
         [0042]    Since steering of the catheter requires tension being applied to each of the steering members individually, it is necessary that each steering member have it&#39;s own point for application of force to influence movement of the end of the catheter as desired by the physician. During the time the steering member is under tension the release member contained within (at the core of) the steering member, in the present embodiment, must maintain its locked (forward) position so that the catheter steering using the locked end steering wire can take place.  FIG. 9A  shows the steering member and release member handle elements connected to the steering member and release member. A steering member, e.g.,  88   a, b, c,  is fixed to a steering member control cylinder  71  and steering tab  72 . Contained and configured to coaxially nest or coaxially slide within and with respect to the steering member control cylinder  71 , a release member control element  98  with a control tab  99  is coaxially arranged with the steering member control cylinder  71 . 
         [0043]    The nested and coaxially arrangement of the two pieces is shown in  FIG. 9B . The control tab  99  is shown positioned adjacent to and behind steering tab  72  so that an instances where both steering member and release member needs to move simultaneously and in a coordinated motion, they do so. 
         [0044]    A handle housing  100  shown in  FIGS. 10A ,  10 B,  11 A,  11 B, and  11 C provides a housing for holding and connection to the common handle  70  at the proximal end of the delivery catheter (system)  40 . 
         [0045]    The operation of the steering member with respect to the steering tabs and control tabs will be described by referring to the side views and end views as shown in  FIGS. 10A-12C . When the physician wishes to cause the steering or sideways movement of the end of the delivery catheter  40  it is necessary to create tension in the steering member which is accomplished by pressing proximally on both the steering tab  72  and control tab  99  and moving them proximately as shown in  FIG. 10B  by the arrow  78 , and  FIGS. 11A , and  12 A. When a physician prepares to release or unlock the locked steering members, needed to retract the release member associated with each steering member, the release member control element  98  is allowed to rotate by applying force to the control tab  99  with respect to the steering member control cylinder  71  in circumferential slot  82 . When the release member control element  98  by using the control tab  99  is rotated to the location of the unlocking slot (longitudinal) as shown by the arrow  81  in  FIGS. 11B and 12B  then the control tab of the release member control element  98  can be retracted along the unlocking slot  84  using a force applied to the end of control tab  99  in the direction shown by arrow  83  in  FIG. 11C . Once the control tab  99  has reached the distal end of the unlocking slot  84 , further movement of the control tab  99  will engage the end of the slot and also cause simultaneous proximal motion of the steering member control cylinder  71  in a proximal direction. The proximal movement of the steering member control cylinder  71  will cause release of the crowns of the proximal spring  60  as described above. 
         [0046]    The embodiment described above three steering members (e.g.,  92   a, b, c ) and six release members (e.g.,  96   a,b,c,d,e,f ) are discussed and described. It is symmetrically convenient to have six release members evenly distributed around the circumference and three steering members similarly equally distributed, however, in instances where odd numbers of stent crowns are used it would be important to maintain the steering capability of the catheter so that in an configuration where five crowns are used on the proximal spring, there would be three steering members to provide three tensioning members to the steering function of the steerable delivery catheter while two crowns would be available for early or partial release. A person skilled in the art would decide how many steering and release members would most efficiently serve their purpose. In some steerable catheters it is convenient to rotate the catheter and only use one steering member, but in large diameter catheters, rotation of the catheters can be difficult because potential frictional resistance with the surrounding vascular wall structure and therefore is not recommended. So in the normal stent grafts where large French size catheters (&gt;18 Fr) are being used it would be expected that it minimum of three steering members would be used. 
         [0047]      FIGS. 13A and 13B  show a configuration with three steering member control cylinders and their associated handle housing  100 ′, where their respective steering tabs  72  and control tabs  99  are shown in an end view. This a configuration shows a single three arm (yoke) plate  74  that acts as a pull plate to simultaneously pull/retract the three release members which do not contribute to locking or unlocking of the steering members. 
         [0048]    In another configuration as shown in  FIG. 14 , the three release members which do not contribute to locking or unlocking of the steering members can be individually manipulated by pull tabs  76   a,    76   b,    76   c.  The handle housing (e.g.,  100  ( 100 ′)) are connected the catheter shaft  80  so that axial tensile and compressive forces can be transmitted along the shaft and opposed by the steering members as they steering members are tensioned and released to bend the catheter shaft as can be seen in, for example in  FIG. 3A  and  FIG. 3B . 
         [0049]    A method of delivery a stent graft in accordance with the above described embodiments includes positioning a catheter at a treatment location, manipulating the catheter laterally at the treatment location to improve the chance of symmetric deployment, deploying approximately half the crowns of a proximal spring equally distributed around the perimeter at the proximal end of a stent graft, and further deploying previously undeployed crowns of the spring at the proximal end of the stent graft. Wherein the step of manipulating may involve manipulation of a releasable steering member. Wherein the step of deploying a approximately half the crown may involve moving the release members from a crown captured position to a crown released position. Wherein the step of deploying previously undeployed crowns may involve moving steering member release wires from a locked position to a release position and moving the steering member from a crown captured position to a crown released position. Wherein the step of manipulating may involve manipulation of one of at least two or more releasable steering members, one at least three releasable steering members or one of at least four releasable steering members. 
         [0050]    It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the embodiments described. It will also be understood that each feature of each embodiment discussed herein, and of each reference cited herein, can be used in combination with the features of any other embodiment. Patents and publications discussed herein are incorporated by reference herein in their entirety.