fig1 illustrates a narrowing vessel 52 , such as the internal carotid artery , having a wide region 56 , a narrowed region 58 , and a stenosis ( not shown ) somewhere in between , i . e ., in the cross - hatched region . the narrowing vessel of fig1 illustrates the geometry as found in an ostium at the bifurcation of the left common carotid 57 , where blood flows from the left common carotid artery 57 into the left internal carotid artery 59 . the bifurcation also opens into the left external carotid artery 60 . an ordinary stent with sufficient force to hold open the wide region 56 would have greater force than necessary to hold open the narrowed region 58 . fig2 a illustrates a plot 66a of outward radial force f along a tapered , expanded stent length l for a stent embodying the present invention . the stent has a greater force in end region 68a than at the opposite end region 70a . a tapered stent having the force curve of fig2 a is suitable for bridging a stenosis as illustrated in fig1 having sufficient force to hold open the wide region 56 of a vessel and less force in the narrow healthy tissue region 58 of the vessel , where less is required . fig3 illustrates in schematic fashion a preferred nitinol stent embodiment of the invention producing a force distribution as illustrated in fig2 . self - expanding stent 80 includes a conformable distal end 82 for contacting healthy vessel tissue , and a stiffer , closed - cell proximal region 88 for providing increased plaque support . it has upon expansion a tapered diameter as shown . for example , a 0 . 236 inch distal diameter and a 0 . 354 inch proximal diameter might be typical . these dimensions can be varied . stent 80 is positioned on the distal end of a delivery catheter , covered with a removable sheath , advanced to a stenosis to be crossed , and exposed for self - expansion by removal of the sheath . stent 80 expands radially to its memorized tapered shape pushing against the stenosis and vessel wall . fig4 illustrates in more detail the nitinol unexpanded stent embodiment of fig3 in flat plan view as a stent 100 , having a middle region 104 and end regions 106 and 108 . stent 100 has a tubular shape , shown in fig5 formed of several serpentine segments 105 , 107 , 109 , 111 and 113 , having a zig - zag pattern , each segment radially encircling a portion of stent 100 . referring again to fig4 segments 113 are longitudinally interconnected by connectors 110 , whereas the serpentine segments 105 , 107 , 109 and 111 are all interconnected as shown in fig4 a and 4b by direct connections 112 . a preferred material for constructing stent 100 is nitinol . in this embodiment , the stent is formed by laser cutting a continuous - walled nitinol tube of diameter 0 . 081 inches having a wall thickness of 0 . 006 inches , leaving only the stent structure as shown . typical dimensions of various elements of the stent are shown in the figure by way of example . referring now to fig6 the stent of fig4 is shown expanded and tapered . since nitinol is a shape memory metal it can be formed into the shape and size shown in fig4 placed over a tapered tool and expanded to a desired enlarged shape and size , such as the 0 . 236 inch distal diameter and 0 . 354 inch proximal diameter previously mentioned , heated to a high temperature such as 500 ° c . to give it the memorized size and shape on the tool . the stent is then removed from the tool and can be compressed for mounting on the delivery catheter . by starting with a stent of nitinol having the dimensions set forth in fig4 the expanded condition provides a stent having the desirable properties described hereinbefore with reference to fig3 . all dimensions in the figure are in inches . of course , this is but one example of a stent according to the invention . fig7 similarly to fig1 illustrates a narrowing vessel 52 having a wide region 56 , a narrowed region 58 , a branching vessel 55 and a stenosis ( not shown ) somewhere in between regions 56 and 58 , i . e ., the cross hatched region . again , narrowing vessel of fig7 illustrates the geometry as found at the bifurcation of the left common carotid artery 57 , where blood flows from the left common carotid artery 57 into the left internal carotid artery 59 . fig2 b illustrates a plot 66b of outward radial force f along a tapered , expandable stent length l for a stent embodying the present invention . the stent has a greater force in its middle region 67b than at its end regions 68b and 70b . a tapered stent having the force curve of fig2 b is suitable for bridging a stenosis as illustrated in fig7 having sufficient force to hold open the wide region at the ostium of internal carotid 59 and less force in healthy tissue at wide end 56 and narrow end 58 . a stent for use in this cross hatched region will have properties such as those to be described with reference to fig8 and 9 , which will be different from the stent previously described with reference to fig1 - 6 . referring now to the fig8 schematic , stent 80 includes a middle region 84 and end regions 86 and 87 . the amount of radial force exerted per unit length of stent is greater in regions having shorter and wider struts . as schematically illustrated in fig8 stent 80 has shorter and wider struts in center region 84 than in end regions 86 and 87 . thus , stent 80 has a greater outward radial force and compression resistance in center region 84 than in end regions 86 and 87 making it particularly useful for stenting in the cross - hatched region of fig7 . fig9 illustrates in more detail the nitinol unexpanded stent embodiment of fig8 in flat plan view as a stent 100 having a middle region 104 and end regions 106 and 108 . stent 100 has a tubular shape , shown in fig1 , formed of several serpentine segments 105 , 107 , 109 , 111 and 113 , having a zig - zag pattern , each segment radially encircling a portion of stent 100 . segments 111 and 113 are respectively longitudinally interconnected by several connectors 110 whereas serpentine segments 105 , 107 and 109 are all interconnected as shown in detail in fig9 a and 9b by direct connections 112 . this embodiment is also formed by laser cutting a continuous - walled nitinol tube of diameter 0 . 081 inches having a wall thickness of 0 . 006 inches , leaving only the stent structure as shown . typical dimensions of various elements of the stent are shown in fig9 by way of example . similarly to the stent embodiment of fig4 as expanded to a tapered shape shown in fig6 the stent of fig9 can be provided with a tapered memorized shape in the expanded condition . the stent will exhibit all of the desirable proportions heretofore described , particularly as discussed with reference to fig2 b . all dimensions in fig9 are in inches . the present invention provides a stent which when expanded to its tapered configuration , provides a radial force varied along stent length for use in tapered anatomies . the stent has been described , in use , as bridging stenosed vessel regions for illustrative purposes . another use in maintaining open channels through otherwise restricted body conduits . stents used for other purposes are explicitly within the scope of the invention . it should be noted that although self - expanding stents have been shown herein to illustrate the present invention , so called balloon expandable stents can also include the variable radial force feature as described herein . in the case of balloon expandable stents , however , these forces in general will be less than are necessary to expand the stent and thus the balloon will be used as known to those skilled in the art to complete the expansion of the stent . to obtain the tapered shape , two balloons of different diameter may be used to expand each end of the stent . these balloon expandable stents may be advantageously deployed in areas of a vessel such as at an ostium where a stent having more rigid or heavy members is desirable in the region of the stenosis , and enhanced flexibility in the distal portion of the stent is desired . for example , a balloon expandable stent can be made of stainless steel to the design and dimensions shown in either fig4 or fig9 . it should be understood therefore , that balloon expandable stents are also within the scope of the present invention . in use , a stent of the self - expanding type , in unexpanded form , is placed on a delivery catheter and covered with a retractable sheath . the catheter is introduced into a vessel and advanced to a region of bifurcation ( ostium or bifurcation placement ). the sheath is retracted , typically by pulling it in the proximal direction , to expose the stent . the stent then self - expands to contact the vessel wall and stenosis . in the case of a self - expanding stent such as the nitinol type described herein , the stent expands to the tapered configuration upon being exposed and exhibits the desired proportion described hereinbefore . a sheath is typically used for constraining a self - expanding stent . a balloon expandable stent is typically crimped on to the balloon and not covered by a sheath . in the case of a non - self - expanding stent , a balloon or other radial force means is inflated within the stent to expand it . in the case of the stents described herein , two balloons may be used sequentially to accomplish this . for example , a small balloon may be used to expand the stent at the small diameter end of the tapered configuration . then , a larger balloon may be used to expand the stents at the large end of the tapered configuration . the catheter ( s ) are withdrawn , leaving the stent implanted in the vessel . the method is adaptable depending on whether an ostial version or a bifurcation version of the stent is being implanted . numerous characteristics and advantages of the invention covered by this application have been set forth in the foregoing description . it will be understood , however , that this disclosure is , in many aspects , only illustrative . changes may be made in details , particularly in matters of shape , size , and arrangement of parts and in materials without exceeding the scope of the invention . the invention &# 39 ; s scope is , of course , defined in the language in which the appended claims are expressed .
a stent and method of its use , the stent in its expanded configuration , exhibiting varying outward radial force along its length . in use , the expanded stent is of a tapered configuration which provides greater force in vessel regions requiring greater force and less force in regions requiring less . in particular the stent is useful in the ostium regions and at areas of bifurcation in vessels . varying force over the length of the stent is achieved by varying the number of elements , the density of elements , the thickness of the elements making up the stent body , and maintaining a substantially metal to artery ratio in the expanded stent over its length .