Abstract:
A prosthesis that resiliently engages a body passage includes an annular clamping ring which may be folded along a diametric axis for insertion into the body passage. The clamping ring is adapted to resiliently spring outwardly, once in position inside the body passage, and to be continually resiliently biased against the interior surface of the body passage. One or more of the clamping rings may be attached to opposed ends of a tubular graft. The rings and connected graft may be positioned in the body passage using a applicator which selectively permits expansion and/or in some embodiments contraction of the annular ring in position within a body passage. Alternatively a retaining member may be used to retain the annular ring in a compressed condition until it is in a desired position within a body passage. Among other potential uses, the present invention may be useful as a vascular stent for treating abdominal aortic aneurysms.

Description:
This application is a continuation-in-part of application Ser. No. 09/365,860 filed Aug. 3, 1999, which is a continuation of application Ser. No. 08/878,908 filed on Jun. 19, 1997. 
    
    
     BACKGROUND 
     The present invention relates to devices that are retained inside a body passage and in one particular application to vascular stents for the repair of arterial dilations known as aneurysms. 
     As a result of arteriosclerosis, portions of blood vessels may become weakened and extremely dilated. These dilated vessels may be treated by bridging the dilation or weakened extended area using a vascular tubular prosthesis. In this way the diseased portion of the vessel is effectively isolated from the pressure inside blood vessels. 
     Vascular tubular prostheses may be inserted into the diseased portion of the vessel by surgically opening the vessel and suturing the prosthesis into position. However, it may be preferred to insert the prosthesis from a remote opening, such as the femoral artery, adjacent the groin, using a catheter system. This is because the elimination of the need to open a major body cavity may diminish the potential surgical complications. 
     Generally it is desirable to insert the prosthesis, using a catheter, in a collapsed or compressed condition and then to expand the prosthesis when in position. One reason for this is that it is desirable to avoid substantially occluding the blood flow during the insertion process. Therefore, by collapsing the prosthesis, the prosthesis may be readily positioned inside the vessel, in some cases without substantially occluding the blood flow. 
     There are generally two techniques for expanding the prosthesis once in position at the location to be repaired. One technique uses a malleable metal prosthesis which has two configurations. One configuration has a relatively smaller diameter and the other has a relatively radially expanded configuration contacting and securing to a neck portion on either side of the diseased vessel region. The prosthesis may be a malleable metal ring which may be expanded by a balloon catheter to set the prosthesis in its expanded diameter, inside the neck portion, proximate to the diseased portion of the vessel. 
     Another general approach is to use a self-expandable prosthesis which may be compressed against a resilient bias. Once in position, the prosthesis is allowed to resiliently expand into contact with the vessel wall. 
     While a wide variety of solutions have been proposed to the problem of effectively bypassing diseased tissue, various existing prosthetic device designs may have certain deficiencies. For example, in some cases, the neck portion on either side of the diseased vessel portion may be relatively short. This makes it difficult for prosthetic devices to adequately engage the narrow neck on either side of the aneurysm. 
     In addition, some of the existing prostheses may cause blockage of the blood flow during insertion of the prosthesis, which can have physiologically adverse affects. Still another issue is that many existing prostheses do not adequately seal against the internal surface of a vessel, allowing leakage of blood past the prosthesis into the region between the prosthesis and the weakened blood vessel. The consequences of this type of leakage can be traumatic. In some designs, the device may not be adaptable to non-circular or irregularly shaped neck regions. 
     Still another issue with some known prostheses is that they may require the hospital to stock a variety of prosthesis sizes for different situations and different patient physiologies. Also some designs may require that the prosthesis be custom fitted for each particular patient. 
     Another difficulty may arise with regard to accurately positioning the prosthesis once it has been expanded. In some cases inaccurate positioning may be problematic. Similarly, in many existing prostheses it is possible that the prosthesis may be dislodged from its desired position so that it does not effectively accomplish its function of protecting the weakened vessel. 
     Thus, for these and other reasons, there is a continuing need for enhanced solutions to the problem of repairing diseased vessels and in general to the problem of effectively securing prosthetic devices to the internal walls of body passages. 
     SUMMARY 
     According to one aspect of the present invention, a device for retaining a prosthesis in a body passage includes an annular, resilient element. The element has an undeformed diameter greater than the diameter of the body passage. 
     According to another aspect of the present invention, a prosthesis for insertion into a body passage includes an annular, resilient spring element and a tubular graft. The graft may be attached to the element. The element has an undeformed diameter greater than the diameter of the graft. 
     According to still another aspect of the present invention, a vascular prosthesis for repairing a diseased first vessel includes a resilient, annular ring having a first pair of loops extending in one direction, and a second pair of loops, extending in the opposite direction. The first and second pairs of loops are connected together. A tubular graft is connected to the ring. The graft is arranged to extend along the length of the first vessel and the first pair of loops are arranged to extend at least partially past the point where a second vessel intersects the first vessel. One of the second pair of loops defines an opening to permit communication between the first and second vessels, at least partially past the prosthesis. 
     According to yet another aspect of the present invention, a method of securing a prosthetic device in a body passage includes the step of folding a resilient annular ring to assume a first configuration having a cross-sectional area smaller than the cross-sectional area of the undeformed ring. The ring is positioned at a desired location within a body passage and allowed to resiliently deform to a second configuration, having a larger diameter then the first configuration, but still having a cross-sectional area smaller than that of the undeformed ring. 
     According to but another aspect of the present invention, a method for repairing a diseased vessel includes the step of folding an annular ring on its diametric axis to assume a smaller cross-sectional configuration and forming a pair of loops extending away from the axis. The ring is arranged in the vessel with its diametric axis proximate to an intersecting vessel such that the loops extend at least partially past the intersecting vessel without occluding the intersecting vessel. 
     According to yet another aspect of the present invention, a method for securing a prosthetic device inside a body passage includes the step of deforming an annular resilient spring by folding the spring along its diametric axis. The spring is positioned inside a body passage. The spring expands resiliently against the body passage. The spring continuously presses outwardly against the body passage. 
     According to but another aspect of the present invention, a prosthetic device includes a prosthetic heart valve, a flexible tubular sleeve having a first end connectable to the valve and a second end. A deformable, resilient annular ring is connected to the second end and arranged to connect the graft to the interior surface of a portion of the ascending aorta. 
     According to yet another aspect of the present invention, a prosthesis for insertion into a body passage includes at least two annular resilient spring elements and a flexible, tubular graft attached to each of the elements. A rigid member longitudinally connects the elements. The rigid member is less flexible than the graft. 
    
    
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is generalized top plan view of a clamping ring in accordance with one embodiment of the present invention; 
     FIG. 2 is a reduced, perspective view of the embodiment of FIG. 1 in place within an idealized body passage; 
     FIG. 3 is a front elevational view of a clamping ring before insertion into a body passage; 
     FIG. 4 is a front elevational view of a clamping ring after introduction into a body passage; 
     FIG. 5 is a side elevational view of the embodiment shown in FIG. 4; 
     FIG. 6 is a side elevational view of a prosthesis with an application apparatus; 
     FIG. 7 is a cross-sectional view taken generally along the line  7 — 7  in FIG. 6; 
     FIG. 8 is an enlarged, partially sectioned view of the retention device shown in FIG. 6; 
     FIG. 9 is a cross-sectional view taken generally along line  9 — 9  in FIG. 8; 
     FIG. 10 is a cross-sectional view taken generally along line  10 — 10  in FIG. 8; 
     FIG. 11 is an enlarged front elevational view of a prosthesis retained by a retention loop; 
     FIG. 12 is an enlarged, front elevational view of a portion of the retention loop; 
     FIG. 13 is a side elevational view of another embodiment of the prosthesis and insertion device; 
     FIG. 14 is an enlarged view of the prosthesis shown in FIG. 13; 
     FIG. 15 is an enlarged cross-sectional view of the embodiment shown in FIG. 11 prior to insertion into a body passage; 
     FIG. 16 is a cross-sectional view of the embodiment shown in FIG. 13 prior to insertion into a body passage; 
     FIG. 17 is a front elevational view of another embodiment in place within a sectioned aortic bifurcation; 
     FIG. 18 is a front elevational view of still another embodiment in place within a sectioned aortic bifurcation; 
     FIG. 19 is an enlarged front elevational view of a module shown in FIG. 18; 
     FIG. 20 is a cross-sectional view taken generally along the line  20 — 20  in FIG. 19; 
     FIG. 21 is a cross-sectional view taken generally along the line  21 — 21  in FIG. 19; 
     FIG. 22 is a front elevational view corresponding to FIG. 14 showing an alternate embodiment; 
     FIG. 23 is a partially sectioned front elevational view of another embodiment; 
     FIG. 24 is a front elevational view of a prosthetic device positioned within a sectioned heart; 
     FIG. 25 is a front elevational view of another embodiment; 
     FIG. 26 is a partial front elevational view of still another embodiment; and 
     FIG. 27 is an enlarged cross-sectional view taken generally along the line  27 — 27  in FIG.  26 . 
     FIG. 28 is an alternative of the embodiment of FIG.  25 . 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing wherein like reference characters are used for like parts throughout the several views, an annular, resilient clamping ring  30  may be formed of a plurality of strands  32  of resilient wire as shown in FIGS. 1,  8  and  10 . One embodiment of the ring  30  may be formed by a wrapping a single length of wire around the mandrel (not shown) having a central axis “C” and then securing the strands into a bundle using ties  34 . The ties  34  may be formed from surgical suture material. Of course, the ring  30  may be formed by a variety of other techniques including the use of a single strand of wire, the use of multiple strands of helically intertwined wire, as in multi-strand wire rope, or any other suitable technique which forms a highly resilient annular ring. 
     The number of coils or strands  32  can be varied according to the wire utilized and the particular application involved. However, in one embodiment, the number of strands  32  utilized is approximately 8 to 10 as shown in FIG.  10 . However, the number of coils or strands  32  may vary from as few as 2 to as many as 100 or possibly more. 
     While a variety of different wire diameters may be utilized, the individual strands  32  may have a diameter of from about 0.05 to 1 mm. In one advantageous embodiment a wire strand  32  diameter of about 0.1 mm may be used. 
     The strands  32  may be made of any highly resilient metal or plastic material, including a nickel titanium alloy such as Nitinol. Generally the resilient or superelastic or martensitic form of Nitinol is utilized. This material is generally superelastic at body temperature. 
     The diameter D K  of the ring  30  is subject to considerable variation depending on the particular body passage involved. In connection with an aortic vascular graft, a ring diameter of about 30 mm. may be adequate and in other situations ring diameters (D K ) of from about 6 to 50 mm. may be suitable. 
     Referring to FIG. 1, the ring  32 , before compression, may have a diameter, D K , which is considerably greater then the diameter, D R , of the body passage  36  to be treated. As indicated in FIG. 1, two diametrically opposed points “A” on the undeformed ring  30  may be deflected towards one another. As indicated by the arrows, this causes the ring  30  to fold along its diametric axis “B”. In this configuration, the ring  30  may be inserted into the body passage  36  in a reduced cross-sectional configuration. 
     As a result of the folding along the diametric axis “B,” the loops  38 , which include the folded tips “A,” extend proximally relative to the points “B” which are along the diametric axis of folding. As used herein, the term “proximal” refers to the direction upstream with respect to blood flow and the term “distal” refers to the direction downstream with respect to blood flow. 
     Once in position inside the body passage  36 , the ring  30  makes continuous contact with the internal vessel  36  wall even though the ring  30  may take a generally sinusoidal shape. To a first approximation, the height H, indicated in FIG. 2, is a quadratic function of the radial compression. 
     The smallest permissible bending diameter without plastic deformation, D B , shown in FIG. 2, depends on the material, the thickness of the clamping ring  30  and the individual strands  32  which may make up the ring  30 . According to Hooke&#39;s law, the strands  32  can be regarded as parallelly connected springs whose deflection characteristic values are additive and whose individual low radial tension forces add up to a total tension force which depends on the number of strands  32 . When the entire ring  30  is compressed, each individual strand  32  has a bending diameter approximately corresponding to the minimum bending diameter D B  of the individual strand  32 . 
     As an approximation, the minimum bending diameter D B  is approximately ten times the wire diameter. This suggests that the ring  30  wire diameter be kept low. However, the ring&#39;s clamping force on the body passage  36  is a function of its diameter, suggesting conversely that the wire diameter be increased. This tradeoff can be optimized by using a plurality of strands  32 , whose diameter controls the minimum bending diameter, to form a bundle whose composite diameter controls the clamping force. Thus a clamping ring  30  with a high tension force can be shaped to a relatively small compressed configuration. After being released from a catheter, for example having a conventional diameter of from 4 to 6 mm. the ring  30  may return to its original shape and by means of sufficient tension force, securely presses the ring  30  along the wall of the body passage  36 . 
     A prosthesis  40  may include an annular ring  30  and a graft  42 , as shown in FIG.  3 . The graft  42  may be generally tubular and made of a fabric or film secured on one end to the ring  30 . The graft  42  may have a diameter D p  which is smaller then the diameter D K  of the clamping ring  30 . Due to the connection between the clamping ring  30  and the end of the graft  42 , there is a diameter D Kp  at the junction point between the clamping ring and the graft  42 . The clamping ring  30  may expand the end of the tubular graft  42  to a stop or deformation limit, after which no further expansion occurs. Thus, the ring  30  may expand upon the graft  42  in the region proximate to the ring  30  so that the diameter of the graft  42  gradually tapers in the region  44  down to a relatively constant diameter region  46 , terminating in a free end  47 . Alternatively, the graft  42  could be preformed in the flared shaped shown in FIG.  3 . 
     Any one of a variety of fabric materials compatible with human implantation may be utilized to form the graft  42 . For example, the graft  42  may be formed of flexible woven or knitted textiles made of Dacron, Teflon, or other materials. It is advantageous if the tubular graft  42  is made of a material which does not change its circumference readily. It is also advantageous if the portion  46  of the graft  42  has a diameter D P  which is approximately the same as the diameter D R  of the body passage  36  to be repaired. 
     The ring  30  can be connected with the region  44  by means of sutures or bonding. It is advantageous if the clamping ring  30  is arranged on the interior surface of the graft  42  so that when the ring  30  extends against the body passage  36  wall, the graft  42  intervenes between the passage  36  and the ring  30 . Thus, it may be advantageous that the diameter D K  of the ring  30  be considerably greater than the diameter of the portion  46  of the graft  42 . 
     Referring to FIG. 4, the prosthesis  40  may be positioned within the abdominal aorta  48  proximate to the left renal artery  50  and the right renal artery  52 . The loops  38  extend past the arteries  50  and  52  while the portion  53  is located just distally of the openings to the arteries  48  and  50 . Thus, as shown in FIG. 5, the openings to the arteries  48  and  50  are not in any way occluded by the positioning of the annular ring  30  proximate thereto because of the generally C-shaped configuration in cross-section of the ring  30 . 
     Because of this configuration, the ring  30  may be secured to a substantially undeformed neck region  54  of relatively short height bounding an aneurysm  55 . This is because at least part of the ring  30  extends proximally beyond the neck  54  without in any way affecting the flow through the arteries  48  and  50 . Moreover, because the clamping ring  30  never completely expands to its unfolded configuration (shown in FIG.  1 ), it is adaptable to irregularly configured neck  54  cross-sections. 
     For example, if the neck  54  is non-circular in cross-section, the sinusoidally shaped ring  30 , in compression, can adapt to the irregular body passage shape. By making the ring  30  with an uncompressed diameter (D K ) greater than the diameter of the body passage (D R ) which it is designed to engage, a continuing resilient engagement occurs between the ring  30  and the body passage  36  which may continue even if the body passage becomes distended over time. This may occur regularly due to normally pulsing blood pressure or due to vasodilation over time. 
     Further by making the diameter of the ring  30  (D KP ) greater than the diameter of the graft  42  (D P ), the graft diameter in use will correspond closely to the compressed cross-sectional diameter (D K ) of the ring  30 , in position within the body passage  36 . This lessens any unnecessary bunching of the graft  42  around the neck  54 . 
     Turning now to a method of positioning the prosthesis  40  in a desired location within a passage, a retention device  56 , shown in FIG. 6, may be secured to the ring  30  on at least two diametrically opposed orientations so that the device  56  extends generally parallel to the axis of the prosthesis  40 . The devices  56  may include a passage  58  in one end and a bracket  60  which secures the device  56  to the ring  30 . Alternatively the passage  58  may be replaced by wire restraining brackets (not shown). In some cases barbs  62  may be included on one end of the device  56 . However, in many cases, the barbs  62  may be unnecessary. 
     The device  56  may be engaged by a wire  64  which extends into the passage  58  and by a tube  66  which encircles the wire  64 , as indicated in FIG.  7 . Advantageously, the device  56  and the tube  66  are made of sufficiently rigid material that pushing against the device  56  by the wire  64  or the tube  66  results in displacement of the prosthesis  40  within the passage  36 . The wire  64  may have a diameter of about 0.3 to 1 mm. 
     The prosthesis  40  may be compressed to fit into the tubular catheter  68 , for transferring the prosthesis from a remote entry point to the repair site. The catheter  68  may be inserted into an incision in the femoral artery, for example, and passed to a position within the abdominal aorta, for example, where one may wish to position the annular ring  30 . Once in position, the prosthesis  40  may be pushed out of the catheter  68  using the tubes  66 . Particularly, the tubes  66  are extended inwardly from the exterior of the body by the surgeon while maintaining the catheter  68  in a fixed position so that the prosthesis  40  is left in position as the catheter  68  is backed away. If desired, the brackets  60  may be made of X-ray opaque material such as platinum, iridium or gold to serve as an X-ray marker. 
     While the above described procedure for placing the prosthesis  40  may be useful in some applications, it would be desirable to further facilitate accurate and controllable placement of the prosthesis  40  in a particular location. Once the ring  30  is allowed to expand against the passage wall, re-positioning must be done against the resistant force of the ring  30 . Thus, it is advantageous to continue to confine the ring  30  after the prosthesis  40  leaves the catheter  68 , until the prosthesis  40  is accurately positioned. To this end, a Bowden tube  70  telescopically retains a wire loop  72 , as shown in FIGS. 11 and 12. The loop  72  extends axially through the tube  70 , forms an annular ring  74  and passes through a hole  76  in the proximal free end of the Bowden tube  72 . At this point, the looped end  78  of the wire loop  72  receives a blocking wire  80 , where the loop  78  extends out of the hole  76 . 
     Referring to FIG. 11, the Bowden tube  70  extends along the exterior of the prosthesis  40  to a point proximate to the loops  38 . The annular ring  74  extends around the periphery of the loops  38  at a relatively central location along their length and is engaged in eyelets  82  secured to the ring  30 . In this way, the blocking wire  80  may be withdrawn axially, releasing the looped end  78  so that the wire loop  72  may be withdrawn, releasing the ring  30  and allowing it to spring open at a desired location. The blocking wire  80  may return, inside the Bowden tube  70 , to the entry point or it may exit the Bowden tube  70  through a gap  71 , as shown in FIG.  11 . 
     Referring to FIG. 15, the catheter  68  encircles the prosthesis  40  which in turn encircles a pair of tubes  66  with wires  64  extending through them. If necessary, a guide wire  104  may be included which may be used initially to guide the catheter to the desired location and to maintain a path for returning to the same location with additional elements, if necessary. The Bowden tube  70  with the looped wires  72  and blocking wire  80  also extends inside the catheter  68  between the catheter and the prosthesis  40 . 
     In still another embodiment, a retaining mechanism  84 , shown in FIGS. 13 and 14, retains the prosthesis  40  in a compressed configuration to accurately locate it at the desired position within a passage. The mechanism  84  may control a prosthesis  40 ′ having a pair of rings  30 , connected by a graft  42 , in a compressed position inside a catheter  68 . A guide wire catheter  86  extends axially through the prosthesis  40 ′. A plurality of ringlets  88  extend off of the catheter  86 . Each of the ringlets  88  connects to wire loops  90  which in turn connect to eyelets  92  at the free ends of the loops  38 . 
     Referring to FIG. 14, each of the wire loops  90  slidably and releasably extends through the eyelet  92  and forms a loop end  94 . A blocking wire  96  extends through the loop ends  94 . A portion of each ring  30  along its folding axis “B” is wrapped by a wire loop  98  which is engaged on its free end by blocking wire  100 . The wire loop  98  may wrap around and over the ring  30 , over the outside of the guide wire catheter  86  and into the interior of the catheter  86  through an opening  102 . Each of the rings  30  on opposed ends of the graft  42  includes the same parts and may be operated in the same way. 
     Thus, to adjust the extent of folding or the proximal-distal height of the rings  30  in the orientation shown in FIG. 14, it is simply necessary to pull outwardly on the wires  98  which may be connected together to a single wire  103  that extends to the exterior of the patient. To decrease the height and to decrease the compression of the ring  30 , the tension on the wire loop  98  may be relaxed, allowing the natural spring forces of the rings  30  to cause the bending of the ring  30  to be relieved and the ring height to be reduced. 
     After the catheter  68  is positioned in the desired location, the assembly may be ejected from the catheter using the techniques described previously. The amount of compression of the ring  30  may be adjusted so that the apparatus  84  can be temporarily positioned at a desired location. If it is determined that the location is not precisely correct, the apparatus can be re-compressed, by operating the loops  98 , to allow repositioning of the apparatus  84  to a new location. In this way, it is possible to selectively adjust the position of the prosthesis  40 ′, even after the prosthesis has previously been released to engage the body passage. If an error is initially made, it is easy to reposition the prosthesis, as necessary. Once the prosthesis is located at the desired location, the blocking wires  100  and  96  can simply be pulled out of the assembly through the catheter  68 . This allows the prosthesis  40 ′ to expand, irreversibly. The catheter  86  may be removed thereafter. 
     If desired, each of the loops  98  can be connected by an independent wire to the exterior of the patient. Or as described previously, the wires  98  may be connected so that only one single wire extends outwardly. 
     Referring now to FIG. 16, illustrating the catheter bundle for the embodiment illustrated in FIGS. 13 and 14 prior to release from the catheter  68 , the catheter  68  encircles the prosthesis  40 ′. In the interior of the prosthesis  40 ′ is the guide wire catheter  86 , with one or more wires  103  which may be used to control the position of the folded portion of the annular rings  30 . Outside of the guide wire catheter  86  are a pair of wires corresponding to the blocking wires  96  and  100 . 
     In accordance with another embodiment of the invention, the prosthesis  40  may be supplemented by one or more additional modules such as the prosthesis  106 , shown in FIG.  17 . The second prosthesis  106  telescopically engages the first prosthesis  40  using an annular ring  30  which expands outwardly against the resistance provided by the graft  42 . The second prosthesis  106  includes an upper annular ring  30 ′ and a lower annular ring  30 ″. It is the upper annular ring  30 ′ which engages the graft  42  while the lower annular ring  30 ″ engages in the distal neck  54   b . Because of the amount of telescopic extension of the second prosthesis  106  into the first prosthesis  40  may be adjusted, a wide arrangement of different vessel longitudinal sizes can be accommodated. 
     The prosthesis  106  including a pair of rings  30 ′ and  30 ″ may have a longitudinal torsion preventing wire  31  as shown in FIG.  23 . The wire  31  may be wrapped around the rings  30 ′ and  30 ″ to control torsion or twisting of the prosthesis  106  about its longitudinal axis and to provide additional longitudinal support. The wire  31  is covered by the graft  42 , either by positioning the wire  31  in the interior of the prosthesis  106  or by weaving the wire  31  through a graft  42  made of fabric. If desired, one or more additional wires  31  may be provided around the circumference of the rings  30 ′ and  30 ″. 
     The second prosthesis  106  may be located inside the first prosthesis using the guide wire  104  which stays in position after all of the wires utilized to position the first prosthesis have been removed. Thereafter the second prosthesis  106  may be run back to the same location using the guide wire  104  which stayed in place after the first prosthesis  40  was positioned. 
     The guide wire  104  maintains the opening of the graft as well. However, in practice the blood flow through the prosthesis  40  causes it to act like an open, expanded, windsock. Therefore, using the guiding action of the guide wire  104 , the second prosthesis  106  can engage the interior surface of the graft  42 . Thus, the combination of the two prostheses  40  and  106  can adjustably span between the necks  54   a  and  54   b  by altering the extension of the prosthesis  106  into the prosthesis  40 . 
     The prostheses  40  and  106  may also be positioned using the mechanism  84 , as shown in FIG.  22 . The prosthesis  106  may be inserted into the patient, already located within the prosthesis  40 , using an additional set of blocking wires  96 ′. The blocking wires  96 ′ extend through the lower loops  94  and through the interior of the prosthesis  40 . In this way, the prosthesis  106  may be manipulated independently, adjustably telescoping within the prosthesis  40 . In this case, the wires  98  and  98 ″ may run separately to the exterior of the patient to facilitate independent manipulation of the prostheses  40  and  106 . 
     A prosthesis similar to those described above may also be used to provide a bifurcated stent  120 , shown in FIG. 18, which extends from the abdominal aorta  48  and its associated neck  54   a  past the lower neck  54   b  and into the iliac or pelvic arteries  108  and  110 . Again, a prosthesis  40  as described above is provided for engagement with the neck  54   a . Instead of the prosthesis  106  described previously, a specially configured prosthesis  112  may be utilized next. The prosthesis  112 , shown in FIG. 19, may include a ring  30  on its upper end and a pair of rings  114  on its lower end. The rings  114  need not be compressed since they simply maintain the lower end of the prosthesis  112  in an open configuration. 
     As shown in FIGS. 20 and 21, the upper end of the prosthesis  112  may have a circular configuration of the type described previously. The lower end may have a double tubular configuration including a pair of passages  116  defined by a connection  118  which extends axially along the prosthesis  112  to form two separate chambers  116  terminated by the rings  114 . The rings  114  may be oriented at an angle to the axis of the prosthesis  112  to allow easy entrance from the iliac arteries  108  and  110 . 
     A pair of smaller diameter prostheses  120  are bilaterally inserted through each iliac artery  108  or  110  for engagement with the prosthesis  112 . Particularly, the upper rings  30 ″ enter through rings  114  and pass into the interior of the passages  116  where they expand outwardly against the graft  42 . At the same time the other end  122  of each prosthesis  120  engages the neck  54   b  at the iliac artery  108  or  110 . One of the prostheses  120  may be inserted using the same guide wire utilized to position the previously positioned prostheses. However, the other prosthesis  120  must be positioned independently of that guide wire. For this purpose, x-ray proof, that is, radiopaque, materials may be utilized on the rings  30 ″ and  114  to facilitate location of the rings  114  and passage through them by the prosthesis  120  which is inserted without the previously located guide wire. 
     With the apparatus and techniques described above, it should be apparent that the prostheses  40 ,  40 ′,  120  may be positioned without substantially blocking the flow of blood even during the surgical procedure. Moreover, the prostheses  40 ,  40 ′ or  120  are configured so as not to substantially interfere with intersecting vessels such as the renal arteries. At the same time a modular approach may be utilized to adjust for different physiologies. This in combination with the fact that the annular ring  30  need never extend to its fully undeformed configuration, means that it is not necessary to stock a variety of different stents. Instead it is possible to have a relatively limited or even a single set of sizes which can be adapted to a variety of patient conditions. 
     Because of the fact that the rings  30  have a C-shaped configuration in position in the body passage, it is possible to locate the prosthesis in a relatively narrow neck  54  region. Since the ring  30  remains in its compressed configuration in use, it adapts for short term and long term distension of the treated passage. Moreover, because of the constantly applied spring bias pressure of the rings  30 , good sealing contact is maintained between the rings  30  (and the prostheses) and the wall of body passage even if the passage is irregularly shaped. 
     With the positioning techniques described above it is possible to accurately position the prosthesis as desired within a body passage. This is because the prosthesis is maintained in a first compressed configuration as it is loaded and transported to the desired location so that it may be positioned without having to overcome friction between the prosthesis and the vessel passage. Once in its desired position, the prosthesis can be activated to engage the wall. It is also possible to reposition the prosthesis after the wall has been engaged if desired. This facilitates accurate positioning and avoids the need to attempt to reposition the prosthesis after it has irreversibly assumed the expanded configuration. In this way the surgeon has considerable control (through guide wire and tubes, for example) to accurately position a prosthesis at its most effective position. 
     The prosthesis  40  may also be utilized to replace a diseased portion of the ascending aorta as indicated in FIG.  24 . An annular ring  30  may be positioned in the remaining portion of the ascending aorta “D” after a portion of the aorta has been surgically removed. The clamping ring  30  secures itself to the inside surface of the aorta “D” as described previously. The clamping ring is connected to a tubular, flexible sleeve or graft  42  and the graft  42  in turn connects to a sewing ring  130  which facilitates connection to a mechanical heart valve  132 . The details of the valve and the graft will be known to those of skill in the art and are described in U.S. Pat. No. 5,123,919, issued to Sauter et al., which is hereby expressly incorporated by reference herein. 
     The graft  42  may be any of a variety of lengths depending on the amount of tissue involved. The graft  42  could extend further than is illustrated and may be considerably shorter. For example, where it is only necessary to replace the heart valve, the graft  42  may amount to little more than a short flexible sleeve connecting the mechanical valve  132  to the ring  30 . 
     Referring now to FIG. 25, a prosthesis  240  which is an alternate embodiment to the prosthesis  40  described previously, includes an upper annular ring  230  and a lower annular ring  214 . The rings  230  and  214  may be of a configuration similar or identical to that of the ring  30  described previously. Similarly, the prosthesis  240  includes a tubular graft  242  which may be the same or similar to the tubular graft  42  described previously. 
     The prosthesis  240  includes a through stent  206  which may a single continuous filament of wire which weaves around the graft  242 . For example, the stent  206  may be over sewn onto the outside of the graft  242 . The wire forming the stent  206  may be a resilient Nitinol wire, for example. The wire of the stent may be formed of multiple strands and may be comprised of less turns of wire or strands than the primary annular ring  230 . For example, the ring  230  may include twelve strands or wraps of wire while the continuous wire of the stent  206  may include ⅓ that number or four strands of wire. 
     The stent  206  forms a series of ring saddles having peaks  208  and troughs  212 . The first ring saddle  232  adjacent a primary annular ring  230  is preferably spaced a selected uniform distance Y from the primary annular ring  230 . In one embodiment, the first ring saddle  232  is not attached to the primary annular ring except by the material forming the graft  242 . The spacing between the primary annular ring  230  and the first ring saddle  232  may be adjusted to improve the stability of the overall structure. Advantageously, the distance Y, shown in FIG. 25, is less than one diameter of the prosthesis, more preferably not more than one third of one diameter of the prosthesis. Such relatively close spacing will improve the stability of the structure. Most preferably the distance Y is equal to 0.09 times the prosthesis  242  circumference. Thus, with a prosthesis  242  having a diameter D k  of  30  mm., the distance X would be approximately 5.8 mm. 
     A connecting strut  210  extends from the concave side of a peak  208  of one ring saddle to concave side of a trough  212  of the next adjacent ring saddle. The strut  210  therefore connects points on adjacent ring saddles which are longitudinally farthest apart from each other. A set of adjacent struts form a series wherein each strut  210   b  is offset radially from the previous strut  210   a , preferably by a uniform amount. The struts thereby advance around the circumference of the graft  242  in a regular fashion. In one embodiment, each ring saddle is comprised of two peaks and two troughs which alternate and are evenly spaced around the graft  242 . Consequently, adjacent struts  210   a ,  210   b  are offset from each other by 180°. This provides increased flexibility longitudinally coupled with increased strength radially. In this embodiment, the graft or prosthesis will be somewhat more flexible in bending in the plane orthogonal to the plane containing the struts as compared to the plane containing the struts. More troughs and peaks could be provided in each saddle and adjacent struts could be offset by different angles. For example, saddles of three peaks and three troughs might have adjacent struts separated by 120°, measured in the direction of the smaller angle between the two struts. 
     In one embodiment, adjacent saddles are congruent, that is, peaks are substantially co-linearly arranged along the length of the graft  242 . The troughs are also substantially co-linearly arranged along the length of the graft  242 . Expressed differently, each point on a saddle is substantially equidistant from a corresponding point on an adjacent saddle. 
     In the illustrated embodiment, the saddles  212  and struts  210  of the stent  206  are formed of a single continuous wire or bundle of wires  207 . Of course, discontinuous wires may also be used in certain aspects of the invention. Where a continuous wire is used for at least some of the adjacent saddles  212  and connecting struts  210 , the wire is doubled along a segment  244  of a saddle from the point in a trough where a strut  210   a  joins the saddle to a point on a peak where the next strut  210   b  leaves the saddle. 
     The doubled segment  244  of wire is illustrated in FIG. 25 with the wire in contact with itself along the doubled segment. An additional improvement is achieved, however, if the wire is slightly spaced apart from itself along the doubled segment. This configuration is illustrated in FIG.  28 . In the double segment there is a gap  250 , such that the wire is generally parallel to itself in the doubled segment, but not in actual contact. The gap  250  should be relatively small compared to the distance between two adjacent saddles. For example, if the distance between two linearly adjacent peaks on different saddles were five units (five millimeters, for example), the gap would be about one unit wide (one millimeter, for example). It is believed that this configuration has good flexibility and radial expansion force, but still requires less force to draw through a catheter than the embodiment of FIG.  25 . 
     The prosthesis  240  may be installed and utilized in the same fashion as the previously described prosthesis  40 . However, with the through stent  206 , the structural integrity of the overall prosthesis may be improved. For example, the ability of the prosthesis  240  to remain open is enhanced by the integrity provided by the through stent  206 . 
     Referring now to FIG. 26, a prosthesis  340  is an alternate embodiment to the prosthesis  40  described previously. The prosthesis  340  may include a primary annular ring  330  which may correspond in structure to the annular ring  30  described previously. The primary annular ring  330  is connected to a tubular graft  342  which may correspond to the tubular graft  42  described previously. 
     The prosthesis  340  includes a secondary annular ring  302 . The secondary annular ring  302  gives further stability to the primary annular ring  330 . Typically, the secondary annular ring  302  would include less turns of wire then the primary annular ring  330 . For example, the ring  330  may include twelve strands or wraps of wire  304  while the secondary annular ring  302  may include ⅓ that number or four strands of wire, as shown in FIG.  27 . 
     The spacing between the rings  330  and  302  may be adjusted to improve the stability of the overall structure. Advantageously, the distance X, shown in FIG. 26, is less than one diameter of the prosthesis, more preferably not more than one third of one diameter of the prosthesis. Such relatively close spacing will improve the stability of the structure. Most preferably the distance X is equal to 0.09 times the prosthesis  340  circumference. Thus, with a prosthesis  340  having a diameter D k  of 30 mm., the distance X would be approximately 8.5 mm. 
     While the present invention has been described with respect to a limited number of preferred embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. For example, while the device has been described in some instances as a vascular stent for treating aneurysms, the invention may be applicable to securing any device to an internal passage. In addition, it should be appreciated that certain embodiments of the present invention may have only one or more of the advantages described above or may instead have other advantages not specifically mentioned herein. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the appended claims.