Abstract:
A stent ( 10 ) formed from cannula and having flexible segments ( 11 ) and high hoop strength segments ( 12 ) alternating therealong. Axial tie bars ( 21,22 ) interconnect the segments; minimal length reduction would occur upon expansion. In the high hoop strength segment ( 12 ), struts ( 16 ) are initially parallel in the unexpanded strut condition, while in the flexible segment ( 11 ), struts ( 14 ) extend from a respective bight ( 15 ) to converge at distal ends when unexpanded. In one embodiment, certain adjacent struts ( 16 ) of the hoop segment are spaced apart by a large gap ( 20 ) while others are spaced apart by a small gap ( 18 ). In another embodiment all the struts ( 16 ) of the hoop segment are spaced apart uniformly from each other (or from an axial tie bar) by a small gap ( 23 ), resulting in minimizing the occurrence of fatigue of high-stress sites upon expansion/contraction cycles from continuous pulsatile events.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of provisional application Serial No. 60/112,939 filed Dec. 18, 1998. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to medical devices and in particular to a stent made from cannula. 
     BACKGROUND OF THE INVENTION 
     Stents of both the balloon expandable and the self-expanding type are known that have been cut from metal cannula and expanded for placement, for example, in the vessels of a patient. In a number of designs, the stent can be comprised of first and second segments, one of which exhibits radial strength greater than that of the other. The lesser radial strength segment is then formed to have lateral flexibility greater than that of the first segment. This combination of segments provides a stent having both good radial strength as well as lateral flexibility. 
     One known stent is disclosed in U.S. patent application Ser. No. 09/160,122 filed Sep. 24, 1998 and assigned to the assignee hereof. The stent is fabricated from cannula and is defined by one or more longitudinal segments of laterally interconnected closed cells. Each closed cell is defined laterally by a pair of longitudinal struts that are interconnected at each end by a circumferentially adjustable member that deform to permit circumferential expansion while the length of the cell is maintained by the longitudinal struts. Adjacent ones of the longitudinal segments are joined by flexible interconnection segments that permit the stent to bend laterally and tat are comprised of curvilinear struts that form a series of serpentine bends that distribute lateral bending forces while only allowing a slight overall shortening of the stent. A short strut interconnects a longitudinal segment and an adjacent interconnection segment. 
     Other cannula stents are known from U.S. Pat. No. 5,421,955; U.S. Pat. No. 5,102,417; and U.S. Pat. No. 5,195,984. A wire frame stent having a number of stent segments is disclosed in U.S. Pat. No. 5,104,404. 
     However, a problem associated with certain multiple segment stents is that relatively high tensile strains are produced therein that cause areas of metal fatigue. As a result, after these stents have been subjected to pulsatile expansion and contraction due to blood flow, the high tensile strain areas will eventually fracture. In addition, bending and torsional loads to which the stent is subjected when the patient changes physical position, can also cause metal fatigue and subsequent fracture. By way of example, these multiple segment stents have various peripheral vessel applications such as in the carotid of the patient. In addition, these peripheral stents can be subjected to external forces such as the patient having external pressure applied to a vessel and causing its collapse or deformation. 
     It is desired to provide a stent from cannula, that includes segments of high radial strength alternating with segments of lesser radial strength but greater flexibility. 
     It is further desired to provide a stent that is especially useful in applications wherein it is subjected to pulsatile expansion and contraction due to blood flow, such as in arterial applications and particularly the aorta and the carotid artery. 
     SUMMARY OF THE INVENTION 
     The foregoing problems are solved and the technical advances achieved in the illustrative cannula stent of the present invention, wherein the stent has segments of high flexibility alternating with segments of high radial or hoop strength. The flexible segments have a serpentine configuration that loops back and forth upon itself, with struts projecting in spaced apart pairs from respective bights and then in the unexpanded stent condition, converging at distal ends that each join to other bights to connect with adjacent strut pairs, thus eventually forming a circumferential band. The hoop segments also have a serpentine configuration but have struts that in the unexpanded stent condition are parallel projecting in pairs from respective bights, and also join bights at distal ends to connect with adjacent strut pairs, with axial tie bars positioned between certain ones thereof that join to bights of strut pairs of the adjacent segment, to interconnect the hoop segment with flexible segments at least at one end of the hoop segment. 
     In a first embodiment, the hoop segments of the stent have a small gap between most pairs of adjacent parallel struts when the stent is unexpanded; certain pairs of adjacent parallel struts have a relatively large gap therebetween; the axial tie bars are each positioned in certain ones of the large gaps but with only small gaps between itself and the parallel struts along both sides thereof. 
     In a second embodiment of the present invention, the stent has hoop segments with uniformly spaced struts to provide a radial strength or hoop strength greater than that of the flexible segments of prior art stents. Advantageously, the struts in the flexible segments of the stent are orientated and spaced so as to provide tensile strains less than those in the higher radial or hoop strength segment. Furthermore, this uniformly spaced strut segment has axial interconnecting bars that remain in a longitudinal configuration during expansion and compression and eliminate twisting and deformation thereof as with conventional non-uniformly spaced struts. The uniform spacing of the struts significantly and advantageously reduces the high tensile strain areas and provides a stent with a superior fatigue resistance and life. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example with reference to the accompanying drawing, in which: 
     FIG. 1 depicts a flat view of the stent of the present invention cut from a cylindrical piece of cannula; 
     FIG. 2 depicts an enlarged view of the segments of the stent of FIG. 1; 
     FIG. 3 depicts a side view of the stent of FIG. 1 when in an expanded state; 
     FIG. 4 depicts a flat view of alternative embodiment of the stent of the present invention cut from a cylindrical piece of cannula; 
     FIG. 5 depicts an enlarged view of the segments of the stent of FIG. 5, with uniformly spaced axial tie bars; 
     FIG. 6 depicts an expanded side view of the stent of FIG. 5 with the hoop cell segment and the flexible interconnection segment interconnected by axial tie bars; 
     FIG. 7 depicts a flat view of the stent of respective FIGS. 4 and 5 with eyelets at the end cell of the stent; and 
     FIGS. 8 and 9 illustrate the stents of FIGS. 1 and 4 respectively in their expanded state. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a flat view of stent  10  cut from a cylindrical piece of cannula. The stent includes a plurality of flexible interconnection segments  11  and higher hoop or radial strength cell segments  12 , with end cell segment  13  preferably having high hoop strength. By way of example, the cannula can be comprised of Series 303 or 304 stainless steel that has applications for balloon expandable stents. In another application, the cannula can be formed of a nickel titanium alloy such as nitinol which can be employed for self-expanding stents. These nickel titanium self-expanding stents normally employ the superelastic properties of nitinol. By way of example, the stent is cut from a piece of cannula when in its compressed condition and then is expanded to its larger diameter expanded state. In the larger diameter expanded state, the nitinol material is heat set so that the stent retains its expanded configuration. The stent is then collapsed and introduced into a guiding catheter for deployment at the placement site. 
     As depicted, the flexible segments  11  are comprised of a serpentine configuration that loops back and forth upon itself with spacing between the struts  14  that varies from one longitudinal end of the segment to the other. Struts  14  project in spaced apart pairs from respective bights  15  and then, in the unexpanded stent condition, converge at distal ends that each join to other bights  15  to connect with adjacent strut pairs, thus eventually forming a circumferential band. 
     The hoop segments  12  also have a serpentine configuration and are comprised of a series of longitudinal struts  16  that are radially positioned with spacing therebetween that can vary circumferentially. Each pair of adjacent struts  16  extends in parallel from a respective bight  17  and are closely spaced to define narrow gaps  18 , or in parallel from a respective bight  19  more generously spaced apart to define large gaps  20 . Distal ends of the struts  16  of each pair join to other bights of adjacent strut pairs. Axial tie bars  21  extend from certain bights  19  within large gaps  20  to the right to connect with bights  15  of the adjacent flexible segment  11  to the right leaving narrow gaps between the axial tie bar and the adjacent struts  16  that may be equal in width to narrow gaps  18 ; similarly, axial tie bars  22  extend from certain bights  19  within large gaps  20  to the left to connect with bights  15  of another adjacent flexible segment  11  disposed on the left of hoop segment  12 . 
     FIG. 2 depicts an enlarged view of segments  11  and  12  of stent  10  of FIG.  1 . In particular, and by way of example, longitudinal struts  16  are approximately 0.142 mm in width (w), and narrow gaps  18  therebetween are approximately 0.026 mm wide (g 1 ). Large gap  20  between selected longitudinal struts  16  is approximately 0.194 mm wide (g 2 ). The length and width of the struts can be varied depending on the diameter of the overall stent. By way of further example, the starting cannula diameter of a stent is approximately 1.93 mm and may have a metal wall thickness of 0.007″. In this configuration, the hoop cell segments are connected to the flexible segments by axial tie bars  21 , 22 . With a configuration as described and shown, the expanded state of the stent is shown in FIG. 3 with non-uniform spacing between the struts of the hoop cell segment. 
     In FIG. 2, axial tie bars  21  are spaced circumferentially from each other approximately 1.512 mm (C). The axial tie bars  21  interconnecting hoop cell segment  12  with the adjacent flexible interconnection segment  11  extending to the right, are alternated circumferentially with respect to the axial tie bars  22  interconnecting it with the adjacent flexible interconnection segment  11  to the left. However, as shown, the distance A 1  between the midlines of axial tie bars  21 , 22  connecting right adjacent flexible interconnection segment  11  with left adjacent flexible interconnection segment  11  is 0.84 mm. This circumferential distance A 1  includes a large gap  20 . Midline distance B 1  interconnecting adjacent flexible interconnection segments including substantially only narrow gaps  18  of minimal width, is 0.672 mm. As a result, distance A 1  is greater than distance B 1  with non-uniform spacing between circumferential segments. The total of distance A 1  and B 1  is approximately 1.512 mm. 
     FIG. 4 depicts a flat view of an alternative embodiment of stent  10  of FIG. 1 cut from a cylindrical piece of cannula. The stent includes a plurality of flexible interconnection segments  11  and higher hoop or radial strength segments  12 . As depicted, flexible segments are comprised of a serpentine configuration that loops back and forth upon itself with spacing between the struts  14  that varies from one longitudinal end of the segment to the other. Hoop cell segments  12  are comprised of a series of longitudinal struts  16  that are axially positioned with spacing therebetween that is uniform around the circumference thereof. Best seen in FIG. 5, hoop cell segment  12  has longitudinal struts  16  with medium gaps  23 , for example, of 0.047 mm (g 3 ). The width of medium gap  23  is between the widths of narrow gap  18  and large gap  20  of the stent of FIG.  1 . As a result, the tensile strains of the stent in FIG. 4 is significantly lower than the tensile strains of the stent in FIG.  1 . 
     In FIG. 5, uniformly spaced gaps  23  provide for uniform radial expansion of the stent. Axial tie bars  21 , 22  are spaced circumferentially and uniformly from each other approximately 0.756 mm. The axial tie bars  21 , 22  interconnecting hoop cell segment  12  with adjacent flexible interconnection segments  11  are alternated circumferentially. However, as shown, the distance A 2  between the midline of axial tie bars  21 , 22  connecting right adjacent flexible interconnection segment  11  with left adjacent flexible interconnection segment  11  is 0.756 mm. This circumferential distance A 2  includes medium gaps  23  of approximately 0.047 mm. Midline distance B 2  interconnecting adjacent flexible interconnection segments including all uniform medium gaps  23  is again 0.756 mm. As a result, distance A 2  and B 2  is the same with uniform spacing between circumferential bars. The total of distance A 2  and B 2  is again approximately 1.512 mm. 
     FIG. 6 depicts an expanded side view of stent  10  of FIGS. 4 and 5 with the hoop cell segment  12  and flexible interconnection segment  11  interconnected by axial tie bars  21 , 22 . The widths of all struts and tie bars in both FIGS. 3 and 6 is sufficiently small, that were the stent to be positioned at a vessel location which is the site of a branch, that flow into or from the branch would not be obstructed to a substantial extend by the stent. In comparison with axial tie bars  21 , 22  of FIG. 3, the axial tie bars  21 , 22  of FIG. 6 all remain in a longitudinal orientation. The axial tie bars  21 , 22  of the stent of FIG. 3 are twisted and are not all longitudinally oriented in the same direction and form various angles of inclination with respect to the longitudinal axis and cause a certain amount of twisting and flexing of the struts during expansion and contraction. This twisting of the axial tie bars provides for excessive fatigue and premature fracture. As previously pointed out, the non-uniform spacing between the longitudinal struts of hoop cell segment  12  of the stent of FIG. 1 causes the twisting of axial tie bars  21 , 22  and the undesired fatigue therein were the stent of FIG. 1 to be used in an arterial application wherein it would be continuously subjected to pulsatile activity. 
     FIG. 7 depicts the flat view of FIG. 4 with eyelets  24  at the end cell  13  of stent  10 . By way of example, eyelets  24  are approximately 0.23 mm. These eyelets can be filled with various radiopaque materials such as gold sphere which are crushed into the aperture. 
     With reference to FIGS. 8 and 9, the stents of FIGS. 1 and 4 have each been expanded to a diameter of about 7.00 mm. Table I below provides a summary of the maximum principle tensile strains, taken at points A, B, and C of high radial strength segment  12 , which are the sites of maximum strain. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 STENT 
                   
                   
                   
               
               
                 DESIGN 
                 STRAIN AT “A” 
                 STRAIN AT “B” 
                 STRAIN AT “C” 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 FIG. 8 
                 0.0119 
                 0.0100 
                 .00816 
               
               
                 FIG. 9 
                 0.00665 
                 0.00772 
                 0.00772 
               
               
                   
               
             
          
         
       
     
     By way of example, the maximum value for the stent of FIGS. 1 and 8 is about mm/mm while the maximum value for the stent of FIGS. 4 and 9 is 0.0078 about mm/mm, which is approximately 33% lower than that of the tensile strain of the stent of FIG.  1 . Thus the stent of FIGS. 4 and 9 is appropriate for applications in vessels having substantial pulsatile events whereby the stent is subject to continuous expansion and contraction cycling. 
     In FIG. 1 to FIG. 7, it is seen that bights  15  of flexible segment  12  are semicircular, and that bights  17  of hoop segment  11  are also semicircular, but having a smaller radius. It may be seen in FIGS. 8 and 9 that bights  17  of hoop segment  12  are semicircular where not joined by an axial segment  21 , 22 .