Helical stent having improved flexibility and expandability

Abstract

A stent includes a central portion of helically wound undulations formed of struts, cylindrical end portions, and transition zones between the helical portion and the cylindrical portions. According to a first aspect of the invention, the torsional flexibility of the stent is maximized by having bridges connecting adjacent winding be interrupted by the maximum possible number of undulations. In one embodiment, each winding includes nineteen undulations around the circumference, bridges are provided every five undulations. According to a second aspect of the invention, uniform opening of the transition zone is achieved by altering the width, and thereby the flexibility, of a series of struts in accordance with their lengths. Specifically, the long transition zone struts are made wider.

Claims

1. A stent for insertion into a vessel of a patient, the stent comprising: a central portion comprising two ends and a plurality of helical circumferential windings, each of the windings including a plurality of undulations, each undulation including a pair of struts connected to each other; bridges extending between undulations on adjacent windings to connect adjacent helical windings to each other, the bridges arranged such that a same number of struts is traversed from any one bridge to a next bridge on each of the windings when traveling in either a right-handed or left-handed direction along each of the windings; and a cylindrical end winding connected at each end of the central portion.

2. The stent according to claim 1, wherein the pair of struts of each undulation is connected by a loop.

3. The stent according to claim 2, wherein the bridges extend between loops located one and one-half pitches away.

4. The stent according to claim 1, wherein each circumferential winding of the central portion of the stent includes sixteen to twenty undulations.

5. The stent according to claim 1, wherein each circumferential winding includes three to five bridges extending therefrom.

6. The stent according to claim 1, wherein each of the bridges in the central portion extends in a same direction in a cylindrical plane of the stent.

7. The stent according to claim 6, wherein each of the bridges extends longitudinally crosswise between adjacent windings.

8. The stent according to claim 1, wherein the stent comprises a laser cut tube.

9. A stent comprising: a central portion having: two ends; a plurality of helical circumferential windings, each of the windings including a plurality of undulations; and bridges extending between the adjacent windings, for at least one of the windings, for any given number n of undulations on the winding and any given number b of bridges connected to the windings; where n is not equal to b, a maximum number of undulations is provided between the bridges and a cylindrical end winding connected to each end of the central portion.

10. The stent according to claim 9, wherein the undulations on adjacent windings are interdigitated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a broken flattened view of a helical stent according to the invention in an unexpanded state, wherein the stent has been cut parallel to its longitudinal axis and laid flat;

(2) FIG. 2 is an enlarged broken flattened view of a central portion of the helical stent of FIG. 1;

(3) FIG. 3 is an enlarged broken flattened view of a transition zone portion of the helical stent of FIG. 1; and

(4) FIG. 4 is a schematic view of a plurality of struts of the transition zone of FIG. 3 shown in an open configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) Turning now to FIG. 1, a helical stent 10 according to the invention is shown. The stent has a collapsed configuration with a first smaller diameter for insertion into a body vessel, and self-expands to an expanded or deployed configuration with a second larger diameter for deployment within the vessel. The stent is preferably a laser-cut tubular construction of a superelastic metal alloy such as nickel-titanium.

(6) The stent 10 includes a central portion 12, a cylindrical portion 14 at each end of the stent 10, and a transition zone 16 between the central portion 12 and each cylindrical end portion 14. The central portion 12 is comprised of a plurality of helical circumferential windings (single turns of a helix) 18 of substantially like undulations (in length and width) 20, with each undulation 20 being defined by two adjacent struts, e.g., struts 22, 24, and a loop 26 connecting the struts (FIG. 2). The cylindrical end portion 14 is comprised of preferably a single cylindrical winding 28 of like undulations 30, with each such undulation 30 being defined by two adjacent struts, e.g., struts 32, 34, and a loop 36 connecting the struts. Optionally, one or more structures 37 adapted to receive or otherwise be coupled to radiopaque markers (not shown) can be provided at the ends of one or more of the undulations 30. The transition zone 16 is comprised of preferably a single winding 38 of undulations 40 that preferably progressively increase in size, with each such undulation 40 being defined by two adjacent struts, e.g., struts 42, 44, and a loop 46 connecting the struts.

(7) In each of sections 12, 14 and 16, the undulations 20, 30, 40 extend in a generally longitudinal direction. That is, when the stent is in a collapsed configuration, as shown in FIG. 1, struts of the helical portion (e.g., 22 and 24), cylindrical portion (e.g., 32 and 34) and transition zone (e.g., 42 and 44) all extend substantially parallel to the longitudinal axis AL of the stent. In the expanded configuration, adjacent struts are moved apart and angled relative to each other.

(8) Referring to FIG. 2, particularly with respect to the central portion 12, as the windings 18a, 18b, 18c are longitudinally displaced along the length of the stent, bridges, e.g. 50, 52, 54 and 56, are provided to connect together the loops 26 of undulations 20 on adjacent windings, e.g. 18a and 18b, and 18b and 18c, to prevent stent unwinding. The bridges 50, 52, 54, 56 can be seen to be arranged in right-handed and left-handed helical strings (right-handed string 60 and left-handed string 62) which extend about the stent.

(9) There are several preferred design considerations (criteria) which, according to the invention, are preferably used together to identify a desired number and placement of undulations in any winding and a number and placement of bridges 50, 52, 54, 56 which connect together loops 26 of undulations on adjacent windings (and thereby connect together the windings 18a, 18b, 18c). If the central portion 12 is designed in accord with the following criteria, the central portion 12 will have a desired torsional flexibility and expandability; i.e., be not too stiff, but also be sufficiently flexible so that the central portion 12 will not be subject to kinking.

(10) In accord with a first criterion, the pattern of bridges is as symmetric as possible. That is, the right-handed and left-handed strings 60, 62 of bridges should be as similar as possible. Further, the torsional flexibility of the stent is maximized by having each string 60, 62 of bridges be interrupted by the maximum possible number of undulations 20. This results in the bridge strings being as stretchy and compressible as possible. In any given stent design, there is a certain number of undulations which form a complete circumferential winding (single turns of the helical portion). The number of undulations 20 which separate the bridges lying along any one string depends, therefore, on the number of bridges within a complete circumferential winding. For example, if there are eighteen undulations around a circumferential winding and three bridges, and if the bridges on adjacent windings are staggered, in accord with the invention there should be three undulations separating bridges along each helical strings of bridges.

(11) In accord with a second criterion, it is preferred that the loops 26 of the undulations 20 of the central portion 12 be interdigitated between the loops of the undulations on an adjacent winding. For example, if there are eighteen undulations around the circumference, each undulation would be rotationally displaced from the undulations on the adjacent winding by one-half an undulation (i.e., one thirty-sixth of a circle or ten degrees), so that the peak of one loop is directed into the valley between two loops on an adjacent winding.

(12) In accord with a third criterion, it is necessary to observe how the number (m) of undulations between bridges and the number (n or n+) of undulations around the circumference interact to create helical strings of bridges. That is, with an increase in n for a stent of a given diameter, the stent is weakened and subject to kinking. This is because, for a stent of a given diameter, by providing more struts, narrower and weaker struts must be used. As n is decreased, the struts are increased in width and thus stiffness. However, while this may strengthen the stent, the stent is nevertheless limited in flexibility and may be undesirably stiff. In accord with the invention, for the optimum combination of strength and flexibility, it is preferred that n (i.e. the number of undulations) be sixteen to twenty, and more preferably eighteen to nineteen, where n may optionally be a non-integer. In addition, the number of bridges, m, for the preferred number of struts is most preferably three to five bridges per circumferential winding.

(13) In accord with a fourth criterion, consideration must be made as to the locations between which the bridges connect and the direction in which the bridges extend. In accord with the preferred interdigitated criterion, the bridges cannot extend parallel to the longitudinal axis AL of the stent. Rather, they preferably extend across loops located one and one-half pitches away; i.e., each bridge connects over two struts relative to directly across from the strut from which the bridge extends. In addition, the bridges extend longitudinally crosswise across the helical space separating the adjacent loops (i.e. in a short direction), as opposed circumferentially along the helical space separating the adjacent loops (i.e., in a long direction).

(14) In view of the above, a preferred exemplar central portion 12 of the stent 10 illustrating the application of the above criteria is now described. Referring to FIG. 2, the central portion 12 of the stent 10 includes repeating undulations 20 (each comprising two struts 22, 24 and a loop 26) that are helically wound in circumferential windings 18a, 18b, 18c, etc. There are preferably nineteen undulations 20 in each circumferential winding 18a, 18b, 18c and the undulations are interdigitated. With reference to windings 18b and 18c, a bridge 50, 52, 54 is located every five undulations therebetween, and each bridge joins loops of undulations on the adjacent windings 18a, 18b which are one and one-half pitches away (or two struts over from directly across) in the short direction. That is, all bridges in the central portion 12 of the stent preferably extend in the same direction, longitudinally crosswise across the helical space. This preferred exemplary embodiment provides a very symmetrical distribution of bridges. In particular, traveling from any one bridge, e.g. bridge 54, to the next bridge, e.g. bridge 56, along the right-hand string 60 of bridges, traverses exactly two and one half undulations (or five struts 70, 72, 74, 76 and 78). Moreover, traveling from any one bridge, e.g. bridge 52, to the next bridge, e.g. bridge 56, along the left-handed string 62 of bridges, also traverses exactly two and one half undulations (or five struts 80, 82, 84, 86 and 88). This design gives very even torsional flexibility and expandibility, and the stent may be twisted considerably in either direction without buckling.

(15) Referring now to FIG. 3, the transition zone 16 of the stent 10 is shown. The transition zone, as stated above, includes struts that progressively increase in length. The long transition zone struts 90, 92 are relatively wider (in the cylindrical plane) than the shorter transition zone struts 94, 96 to compensate for the greater bending moments imposed by the longer struts. Moreover, even the shortest transition zone strut 98 is preferably longer and wider than the struts 22, 24 of the central portion 12. In the embodiment shown in FIG. 3, a terminal end of a strut of the central portion 12 adjoins one longer strut between the opposing ends of the one longer strut. In this embodiment, the one longer strut is connected to the shortest transition zone strut 98.

(16) More particularly, referring to FIGS. 3 and 4, for substantially uniform expansion of the stent, it is desirable for the opening distance D (i.e., the distance between the ends of two adjacent struts, e.g. struts 32 and 34, when the stent is in an open configuration) to be approximately even throughout the transition zone 16. Accordingly, the opening angle a between pairs of longer struts in the transition zone, e.g. struts 32 and 34, must be smaller than the opening angle a between shorter struts, e.g. struts 94 and 96. In addition, the bending stiffness of the longer struts must be even greater than in proportion to their increased bending moment. The bending stiffness S of a rectangular-section beam is in proportion to the third power of the width (in the bending direction) W. As such, by way of example, in order to double the bending stiffness of a strut, the width W of the strut must be increased by the cube root of two.

(17) The bending moment M of a strut is in linear proportion to the length L of the strut. The opening angle a is proportional to the bending moment M divided by the stiffness S. The opening distance D is proportional to the product of the opening angle a multiplied by strut length L. Therefore, the opening distance D is proportional to a*L, which is equal to (MIS)*L. Since M is linearly proportional to L, the opening distance D is proportional to the square of L divided by stiffness S. In order to keep the opening distance D between adjacent struts (i.e., pairs of struts) equal throughout the transition zone 16, the stiffness of the bending segments of the struts must be in proportion to the square of their lengths. Hence, the cube of the width must be proportional to the square of the length:
W.sup.3 is proportional to L.sup.2

(18) In a preferred transition zone, the shortest strut 98 should be approximately half the length of the longest strut 42. Therefore, in order to maintain similar opening distances, the longer struts are most preferably wider by the cube root of 2 squared, or 1.59, relative to the shorter struts. The ratio may be adjusted to a value near this ratio (e.g., 25%, or 1.19 to 1.99) in order to achieve a uniform opening, giving consideration to the fact that in a transition zone two adjacent struts of unequal length both contribute to the bending moment on the flexing connection that joins them. It may also be desirable to make the opening angle a between the shortest strut pairs not exceed a certain value in order to limit the maximum strains experienced in that portion of the transition zone.

(19) As such, uniform opening is achieved in the transition zone by altering the flexibility of a series of struts in accordance with their lengths.

(20) There have been described and illustrated two aspects of a preferred stent relating to the helical central portion and the transition zone. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, the two preferred aspects (relating to the central helical portion and the transition zone) can be used together or separately. Moreover, each of the design considerations relating to the helical central portion can be used alone or together with one or more of the other considerations. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed.