reference is now made to fig1 and 2 , which schematically illustrate a device 20 for implantation in a body passage , in accordance with an embodiment of the present invention . fig1 is a pictorial illustration of the device , while fig2 is a cross - sectional view taken along a line ii - ii in fig1 . device 20 is adapted for use particularly in restricting blood flow through the coronary sinus , as described in the above - mentioned pct publication wo 01 / 72239 . alternatively , devices in accordance with the principles of the present invention may be implanted elsewhere in the vascular system , as well as in other body passages . for the sake of simplicity and clarity , however , and not limitation , embodiments of the present invention are described hereinbelow with reference to implantation of flow - constricting devices in blood vessels , such as the coronary sinus . device 20 comprises ring elements 22 and 24 , each of which comprises a resilient framework 26 . each framework defines a generally - cylindrical shape , although this shape is distorted by the mechanical constraints of the device , as described below . therefore , the cylinders tend to widen at the ends of device 20 and narrow toward the middle , as shown in fig1 . in the pictured embodiments , framework 26 comprises a wire or thin rod , which is bent into a serpentine shape . typically , the framework comprises an elastic material , which may be compressed or otherwise bent , but then returns to its original shape , as shown in the figure . super - elastic materials , such as nitinol , are useful for this purpose . alternatively , the framework may comprise a resilient , deformable material , such as a suitable metal or plastic . further alternatively or additionally , each framework 26 may comprise a mesh or coil , as is known in the art . in any case , the term “ resilient ” as used herein means that once device 20 is deployed within a body passage , framework 26 has sufficient mechanical strength to withstand normal forces exerted by the wall of the passage and by fluid flow within the passage , in the manner of stents known in the art . ring elements 22 and 24 are fixed to a flexible sleeve 28 , which has a generally tubular form . typically , sleeve 28 comprises a biocompatible fabric , such as gore - tex or dacron , which is stitched or otherwise fastened to framework 26 . alternatively , other sleeve materials may be used , such as thin plastic or rubber materials . the sleeve is fixed to the ring elements in such a way as to form a lumen 32 ( fig2 ) through device 20 . the sleeve is supported at each end of the lumen by one of the ring elements , while leaving a longitudinal gap in , the sleeve , typically several millimeters long , between the inner ends of the two ring elements . while the ring elements themselves are relatively stiff ( due to the resilience of framework 26 ); device 20 can be bent and deformed freely within the gap region of the sleeve . a constricting element 30 is fitted around sleeve 28 within the gap region . as can be seen in fig2 , the effect of this constricting element is to reduce the diameter of lumen 32 to a predetermined size , less than the expanded diameter of ring elements 22 and 24 . constricting element 30 may simply comprise a thread , which is tied around the sleeve , or it may alternatively comprise a closed ring , made of plastic or metal . a constricting ring of this latter type is shown in fig9 a and described hereinbelow with reference thereto . fig3 is a schematic side view of device 20 after implantation inside a blood vessel 40 . typically , device 20 is passed through the vascular system to the appropriate location ( such as the coronary sinus ), using a suitable percutaneous catheter ( not shown in the figures ). suitable methods of catheterization for this purpose are known in the art . during the insertion procedure , device 20 is compressed radially , so that its outer diameter is substantially smaller than the blood vessels through which it must pass . as noted above , device 20 is able to bend freely in the area of the gap between ring elements 22 and 24 , where constricting element 30 is located . this bending capability generally makes it easier for the physician operating the catheter to pass the device through bends in the blood vessels . upon reaching the desired location in blood vessel 40 , device 20 is released from the catheter . if framework 26 is made of an elastic material , such as nitinol , the device will expand by itself , due to its own elasticity , as soon as it is released . alternatively , if framework 26 comprises a malleable material , a balloon may be inflated within each of ring elements 22 and 24 , or other means known in the art may be used , in order to expand the framework . the above - mentioned pct publication describes special types of balloons that may be used for this purpose . as can be seen in fig1 and 3 , the serpentine shape of framework 26 creates elongated “ fingers ” that protrude at the ends of device 20 . once the ring elements have expanded , these fingers press outward against the wall of the blood vessel , thus anchoring device 20 in place . blood in vessel 40 flows through lumen 32 , but flow is restricted by the constriction at constricting element 30 . if device 2 q is deployed in the coronary sinus , for example , the flow restriction causes increased pressure in the coronary veins , thus promoting myocardial angiogenesis . device 20 may be left in place indefinitely , in substantially the form shown in fig3 . alternatively , it may be desirable in some cases to eliminate the flow restriction caused , by the device . in such cases , it is not necessary to remove device 20 from the body . rather , a catheter with a suitable cutting tool may be inserted percutaneously to the location of the device , and the cutting tool may then be used to cut constricting element 30 . the constriction in the diameter of lumen 32 will then open up by itself . fig4 is a schematic side view of an implantable device 50 after implantation inside blood vessel 40 , in accordance with another embodiment of the present invention . blood in vessel 40 is assumed to flow from left to right in the view of the figure . device 50 is substantially identical to device 20 , as described above , except for the shape of sleeve 28 . in device 20 , sleeve 28 is trimmed so that the ends of the sleeve have the same general shape as the “ fingers ” of framework 26 . in device 50 , however , sleeve 28 is trimmed to a generally straight edge at the upstream ( left ) end of the device , covering the interstices between the fingers , as well as the fingers themselves . the straight upstream edge can be useful in reducing blood leakage around the sides of the device , thus providing more complete and reliable flow restriction . the uneven shape of the sleeve is maintained on the downstream edge , in order to anchor device 50 securely to the walls of vessel 40 against the pressure exerted by the blood flow in the vessel . alternatively , sleeve 28 may be cut in other configurations , as mandated by medical and mechanical considerations . fig5 is a schematic , pictorial view of an implantable device 60 , in accordance with still another embodiment of the present invention . device 60 is also substantially similar to device 20 , as described above , except for the addition of longitudinal support members 62 and 65 . the support members join ring elements 22 and 24 together and thus enhance the mechanical strength and stability of device 60 . although two longitudinal support members are shown in fig5 , greater or smaller numbers of supports members may be used in like fashion . note , however , that in the gap between the ring elements , sleeve 28 is detached from the support members , so that the diameter of lumen 32 can still be reduced by constricting element 30 . fig6 a and 6b are schematic side views of a catheter 70 , in a cutaway view , which is used to deliver device 20 to a target position in blood vessel 40 , in accordance with an embodiment of the present invention . as shown in fig6 a , catheter 70 has a tubular outer shell 72 and a central lumen 74 . prior to delivery , device 20 is held inside shell 70 , with lumen 74 passing through lumen 32 of device 20 . a distal end 76 of shell 72 has a roughly conical shape , and has a small exit aperture 78 surrounding lumen 32 . typically , to implant device 20 in vessel 40 , an operator threads a guide wire 80 through a part of the patient &# 39 ; s vascular system to the target position , as is known in the art . for example , the guide wire may be passed through the jugular vein into the coronary sinus . once the guide wire is in place , the operator slides lumen 74 over the guide wire , and thus guides distal end 76 of catheter 70 to the target position . a contrast medium may be injected through lumen 74 or through another , parallel lumen ( not shown ) to aid the operator in visualizing vessel 40 during the procedure using a fluoroscope , as is known in the art . when distal end 76 has reached the target position , the operator uses an ejector 82 to push device 20 out through aperture 78 in the distal end of the catheter . distal end 76 in this embodiment is made of a material that is sufficiently elastic so that the aperture opens freely to the diameter of device 20 . once the device is ejected , it expands to the diameter of vessel 40 , as shown in fig3 , and anchors itself in place . the operator then withdraws catheter 70 , and distal end 76 contracts back roughly to its original form . fig7 a and 7b are schematic side views of another catheter 90 , which is used to deliver device 20 , in accordance with an alternative embodiment of the present invention . fig7 a shows the catheter before delivery of device 20 , while fig7 b shows the catheter , after , the delivery . in this embodiment , distal end 76 comprises a thin sheath , which tears open as ejector 82 pushes the device out of the catheter . optionally , as shown in fig7 a , the distal end is scored along lines 92 , so that as device 20 is ejected , the distal end tears cleanly , in a predictable fashion . once device 20 has been ejected , the distal end may remain open where it has torn , but the open distal does not interfere with drawal of catheter 90 along wire 80 . fig8 a , 8 b and 8 c are schematic side views of a catheter 100 for delivering device 20 , in accordance with yet another embodiment of the present invention . in this embodiment , distal end 76 has an aperture 102 that is large enough to accommodate the ( compressed ) diameter of device 20 when the device is ejected from the catheter . until the catheter reaches the target position , however , the aperture is closed by a distended portion 104 of a lumen 106 that passes through the catheter , as shown in fig8 a . the lumen is typically used to accommodate a guide wire and / or to inject contrast medium , as described above . distended portion 104 is made of a flexible material , which may be either elastic or malleable , and is shaped so as to plug aperture 102 . when distal end 76 reaches the target position , lumen 106 is advanced ( and / or catheter 100 is withdrawn ) so as to open aperture 102 , as shown in fig8 b . ejector 82 then pushes device 20 out through the aperture . as shown in fig8 c , portion 104 is sufficiently flexible so that as the narrow , gap region of lumen 32 through device 20 passes over it , portion 104 closes down so that lumen 32 can slide over it . once device 20 has been implanted at the target position , portion 104 resumes its previous shape , and lumen 106 may be pulled back in the proximal direction in order to close aperture 102 . catheter 100 is then withdrawn from the body . fig9 a is a schematic , pictorial illustration of a constricting ring 120 , in accordance with an embodiment of the present invention . this ring may be used as a constricting element in device 20 , taking the place of element 30 shown in the preceding figures . ring 120 comprises a flexible , elastic wire 122 . for example , wire 122 may comprise a super - elastic material , such as nitinol . wire 122 is formed with multiple bends , typically in a serpentine pattern , as shown in fig9 a . some of the bends are closed bends 124 , at which the wire segments on opposing sides of the bend are fixed together , thus narrowing the overall circumference of ring 120 . when ring 120 is installed in place of element 30 on device 20 , the narrowed circumference of the ring constricts the diameter of lumen 32 , as shown in fig1 and 2 . fig9 b and 9c are schematic , detail views of one of closed bends 124 in ring 120 , in accordance with two exemplary embodiments of the present invention . in the embodiment of fig9 b , the opposing segments of wire 122 are pulled together and then fastened by welding , glue or other means , at a fastening point 126 . laser micro - welding , as is known in the art , may be used for this purpose . in fig9 c , a connecting element 128 , such as a miniature ring , is welded or otherwise fastened in place between the segments of wire on either side of the bend . in either case , bends 124 are typically closed wealdy enough so that the fastening points or connecting elements will break open under outward radial pressure . fig1 is a schematic , pictorial illustration of ring 120 following opening of closed bends 124 , in accordance with an embodiment of the present invention . the closed bends may be opened in situ , after device 20 has been implanted in a blood vessel . for this purpose , for example , a balloon catheter may be inserted into lumen 32 of device 20 , and the balloon may be inflated with sufficient pressure to break open the fastening points of at least some of bends 124 . due to the elasticity of wire 122 , ring 120 will then expand to the larger diameter shown in fig1 , and lumen 32 will open up accordingly . this sort of procedure may be used , for example , to permit free flow of blood through vessel 40 when the constriction due to device 20 is no longer needed or desired . fig1 is a schematic , detail view of a part of a stent 130 , in accordance with another embodiment of the present invention . this embodiment also uses the principle of radial expansion of an intravascular implant that was described above . stent 130 comprises a structure of struts 132 with intervening openings 134 . some of the openings are bridged by narrow connecting pieces 136 . stent 130 is initially collapsed and crimped over a balloon for insertion into the target blood vessel . inflation of the balloon to a first , intermediate pressure causes the stent to expand radially outward , so that openings 134 between struts 132 open to the configuration shown in fig1 . the balloon is then withdrawn . the stent may be used in this configuration , for example , to open a blocked artery or other body lumen . it often occurs after implantation of a stent that the body lumen in question once again becomes constricted , due to accretion of material inside the stent , for example . in this case , a balloon may once more be inserted inside stent 130 and inflated to a second , higher pressure . the balloon thus exerts an outward radial force on stent 130 , causing one or more of connecting pieces 136 to break open . thus , the diameter of stent 130 ( and of the lumen it is supporting ) is increased simply and safely . although in the embodiments described above , framework 26 and sleeve 28 are shown to have certain particular shapes , alternative shapes and forms of these elements , which will , be apparent to those skilled in the art , are considered to be within the scope of the present invention . similarly , catheters of the general types described above may be used to deliver not only device 20 , but also other implantable devices as described hereinabove and as are otherwise known in the art . on the other hand , although the catheters shown here provide convenient means for delivering implants in accordance with the present invention , such implants may also be delivered by other means , both minimally invasive ( typically percutaneous ) and invasive ( i . e ., surgical ). methods for reducing the diameter or circumference of a vascular structure by surgical means are also known in the art . methods of this sort are described , for example , in u . s . pat . no . 5 , 593 , 424 and u . s . pat . no . 6 , 561 , 969 , whose disclosure are incorporated herein , by reference . these methods generally require suturing of the vascular tissue , which can be difficult and time - consuming to carry out . in contrast to these methods and to the preceding embodiments , fig1 schematically illustrates a method for constricting the diameter of a vascular structure without the use of sutures or a stent , in accordance with an alternative embodiment of the present invention . the embodiment is illustrated here with reference to reducing the diameter of a coronary sinus 140 of a patient , although this method is also applicable to other vascular structures . a catheter 142 is inserted through a right atrium 144 of the patient into coronary sinus 140 . the catheter is bent at its distal end , as shown in the figure , to permit convenient deployment of a constricting clip 146 , as described below . fig1 a - c are schematic , sectional views of coronary sinus 140 , taken along a line xiii - xiii in fig1 , showing stages in the deployment of clip 146 , in accordance with an embodiment of the present invention . clip 146 typically comprises a super - elastic material , which is formed so that in its relaxed state , it has an approximately closed form , as shown in fig1 c , for example . during insertion of catheter 142 into the coronary sinus , however , clip 146 is compressed within the distal end of catheter 142 , as shown in fig1 a . once catheter 142 has been advanced into coronary sinus 140 , a deployment mechanism , such as a pusher ( not shown ) inside the catheter , is actuated in order to advance clip 146 out of the distal end of the catheter . as a result , the clip , opens up into the configuration shown in fig1 b . ends 148 of the clip catch the tissue of coronary sinus 140 at two points that are spaced apart on the wall of the coronary sinus . the elasticity of clip 146 causes the ends of the clip to draw together as the clip is advanced further out of the catheter , as illustrated by arrows 150 . finally , when the clip has advanced completely out of the end of the catheter , ends 148 close in toward one another and pinch together the portion of the vascular tissue that is located between the clip ends . the result , as seen in fig1 c , is that the effective diameter of coronary sinus 140 is reduced . it will thus be appreciated that the embodiments described above are cited by way of example , and that the present invention is not limited to what has been particularly shown and described hereinabove . rather , the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove , as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art .
a medical implant includes first and second ring members , each including a resilient framework having a generally cylindrical form . a tubular sleeve is fixed to the first and second ring members so as to hold the ring members in mutual longitudinal alignment , thereby defining a lumen passing through the ring members . a constricting element is fit around the sleeve at a location intermediate the first and second ring members so as to reduce a diameter of the lumen at the location .