Extending ribbon stent

The present invention concerns an improved stent for placement in corporeal lumens. More particularly, the present invention concerns an improved stent for placement in a corporeal lumen comprising a coiled ribbon which is rigid enough to maintain the relative position of each turn of the coiled ribbon during its expansion within the lumen after placement.

FIELD OF THE INVENTION 
The present invention relates to an improved stent for placement in 
corporeal lumens. More particularly, the present invention relates to an 
improved stent for placement in a corporeal lumen comprising a coiled 
ribbon which is rigid enough to maintain the relative position of each 
turn of the coil during its expansion within the lumen after placement. 
BACKGROUND OF THE INVENTION 
The term stent generally refers to a prosthesis, which can be introduced 
into a corporeal lumen and expanded to support that lumen or attach a 
conduit to the inner surface of that lumen. Since most lumens are roughly 
circular in cross-section, so are most stents. But the stent should not be 
a rigid cylinder, because few lumens are straight; all but the shortest 
stents need some element of flexibility. The long axis of the stent must 
be able to bend in order to follow the long axis of the lumen. 
The flexibility of any tubular structure depends on the ease with which the 
stent structure can accommodate changes in the relative lengths of its 
inner and outer walls; a stent is no exception. If the stent bends 
smoothly, its outer wall will always be longer than its inner wall, to a 
degree which is proportional to the width of the stent and inversely 
proportional to the radius of curvature. The flexibility of a stent is 
determined by the extent to which its structure can accommodate the 
necessary length discrepancy. 
In general, a stent with many longitudinally oriented structural elements 
is rigid because the structural elements impose their fixed length on the 
length of the stent wall. More transversely oriented structural elements 
do not have this effect, because small changes in the angles between them 
can produce large changes in stent length. In addition, any joining of 
stent elements, that limits their freedom in a direction parallel to the 
long axis of the stent, will also reduce stent flexibility. Therefore, the 
most flexible stents are those in which the structural elements are 
oriented transversely, and there are no longitudinal connections. 
Since all stents expand by changing the orientation of their structural 
elements from longitudinal to transverse, it is possible to maximize 
flexibility of a stent by expanding it to the highest degree, thereby 
maximizing the transverse orientation of the structural elements. 
Unfortunately, this degree of stent expansion may compromise the 
structural integrity of the stent by over-deforming the junctions between 
stent elements. The only stents that are not subject to this constraint 
consist of one or more coil shaped elements, which may be intertwined, but 
are not joined. Examples include the stents described by Willard, (U.S. 
Pat. No. 5,222,971); Harada et al, (U.S. Pat. No. 5,037,427); Dotter, 
(U.S. Pat. No. 4,503,569); Wilkoff, (U.S. Pat. No. 4,990,155); Maas, et al 
(U.S. Pat. No. 4,553,545); Wallsten, (U.S. Pat. No. 4,655,771). When fully 
expanded, with the adjacent coils almost touching, these stents are as 
flexible as a Slinky. 
Unfortunately, this flexibility comes at a price. Since the structural 
element that forms the coil has a fixed length, and high degrees of stent 
expansion must be accompanied by large reductions either in the number of 
turns or in the length of the stent. Winding the stent into a compressed 
state is possible, but is limited by the resultant tension. Moreover, the 
unwinding of a stent during delivery precludes pre-attachment to an 
unwound prosthesis such as a fabric, which is often deemed desirable by 
the user. Such unwinding is also difficult to control and can be damaging 
to the inner surface of the corporeal lumen. For these reasons, stent 
shortening tends to be the primary means of stent expansion for this type 
of stent. Therefore, the more flexible coil stents tend to be much shorter 
when expanded than they are in the fully compressed state. This degree of 
shortening complicates stent delivery, particularly if the stent is to be 
used inside a lumen of uncertain diameter, or in conjunction with a 
prosthesis of a fixed length, such as a fabric. 
A third alternative is to make the structural element of the coil variable 
in length. The additional length required to encircle the expanded stent 
comes from extension of the coil element. Several manifestations of this 
approach currently exist. They include the stents described by Hillstead, 
(U.S. Pat. No. 5,019,085) and Wiktor (U.S. Pat. No. 5,133,732). All stents 
of this type have the same basic structure, and all suffer from the same 
basic problem, irrespective of whether expansion is driven by a balloon or 
by the elasticity of the material; the zig-zag wire that forms the coil 
can twist and bend in any direction, making the stent structurally 
unstable. The resultant gaps and luminal impingement can be prevented by 
linking adjacent turns of the coil, but his limits the flexibility of the 
stent. 
A number of prior art references are available in the art, each of which 
references are directed to some specific discreet elements of the system 
which is described and claimed in the present invention, however, none of 
which is directed to the totality of the combination, or its use and 
function in the manner described and claimed herein. 
The following prior art references are known to the inventor: 
U.S. Pat. No. 4,503,569, which issued to Dotter on Mar. 12, 1985 describes 
a transluminally placed endovascular graft prosthesis which includes a 
helically wound coil having a generally tubular shape and is made of a 
shape memory Nitinol alloy having a transition temperature in the range of 
115.degree.-125.degree. F. After placement within a body blood vessel and 
upon heating of the prosthesis to its transition temperature, the 
prosthesis expands so as to become firmly anchored to the inside wall of 
the body blood vessel. 
U.S. Pat. No. 4,553,545, which issued to Maas et al in November 1985, is 
directed to a spiral stent composed of a flat (non-extending) ribbon, or 
parallel connected ribbons of steel. 
U.S. Pat. No. 4,655,771, which issued to Wallsten on Apr. 7, 1987, teaches 
a prosthesis for transluminal implantation comprising a flexible tubular 
body which has a diameter that is variable by axial movement of the ends 
of the body relative to each other and which is composed of several 
individual rigid but flexible thread elements each of which extends in 
helix configuration with the centerline of the body as a common axis; 
U.S. Pat. No. 4,739,762, which issued to Palmaz on Apr. 26, 1988, teaches 
an expandable intraluminal graft for use within a body passageway or duct 
which is particularly useful for repairing blood vessels which have been 
narrowed or occluded by disease; 
U.S. Pat. No. 4,990,155, which issued to Wilkoff on Feb. 5, 1991 is 
directed to a method for preventing arterial restenosis after angioplasty 
which includes the steps of providing a base coil of a substantially 
uniform diameter, inducing in the base coil an elastic memory that 
provides an inherent tendency to return to the given diameter after any 
distortion, forming from the base coil a coil stent with a substantially 
uniform predetermined diameter substantially less than the given diameter, 
releasably coupling the coil stent to an elongated delivery device adapted 
to pass through a blood carrying vessel, inserting the coil stent and 
delivery device into a vessel, and manipulating the delivery device within 
the vessel so as to position the coil stent at a desired location therein. 
U.S. Pat. No. 5,019,085, which issued to Hillstead on May 28, 1991, relates 
to a stent delivery system and method wherein a delivery wire is routed 
out an opening in a delivery catheter and looped over a portion of the 
stent and then routed back inside the delivery catheter. At an extreme 
distal end of the stent, the wire again exits the delivery catheter center 
passageway. This compresses the stent into a form whereby the delivery 
catheter can be maneuvered through a vessel to position the stent. To 
release the stent from the delivery catheter, the delivery wire is 
retracted so that its distal end passes out both pair of openings in the 
delivery catheter allowing the stent to expand into engagement with the 
vessel wall. 
U.S. Pat. No. 5,037,427, which issued to Harada et al on Aug. 6, 1991, 
teaches an instrument for expanding a tubular organ such as a blood vessel 
and for keeping the tubular organ expanded for a predetermined period of 
time, and a catheter for mounting said instrument at a desired position 
within the tubular organ, said catheter being capable of moving and 
recovering the instrument mounted within the tubular organ. The instrument 
is formed of a two-way shape memory alloy and expands or shrinks in the 
radial direction, in accordance with changes in temperature. 
U.S. Pat. No. 5,133,732, which issued to Wiktor on Jul. 28, 1992, discloses 
a stent for implantation into a body vessel comprising a cylindrical stent 
body which has been coiled from a generally continuous wire which has been 
imparted with a deformable zig-zag structure; 
U.S. Pat. No. 5,222,971, which issued to Willard et al on Jul. 29, 1993 
teaches a temporary stent for supporting a region of a vessel in a body 
comprising a stent portion and an actuator portion and methods for the use 
and manufacture thereof. The stent portion is comprised of an elongate 
perfusable vessel supporting portion adapted to be configurable between a 
reduced size for placement in the vessel and between a reduced size for 
placement in the vessel and removal therefrom and an expanded size for 
structurally supporting the vessel and perfusable end portions connected 
to and forming ends of the vessel supporting portion and adapted to allow 
fluid flow therethrough. 
U.S. Pat. No. 5,370,683, which issued to Fontaine on Dec. 6, 1994, is 
directed to a vascular stent for reducing hemodynamic disturbances caused 
by angioplasty, said stent being formed from a single filament of low 
memory biocompatible material having a series of U-shaped bends. The 
filament is wrapped about a mandril in a circular fashion in order to 
align the curved portions of each bend which may then be connected; 
None of the foregoing references in any way teaches the advantages to be 
achieved by utilizing the coiled ribbon stent of the present invention. 
It is, therefore, an objective of the present invention to provide for an 
improved stent which exhibits superior structural stability, as compared 
to other available prior art stents, when inserted into a corporeal lumen. 
It is a further object of the present invention to provide for an improved 
stent which comprises an extending ribbon wherein the ribbon maintains the 
relative position of each turn of each coil during the process of 
expansion. 
It is a further object of the present invention to provide for an extending 
ribbon stent which allows for an increase in the diameter of the stent 
without any appreciable change in the overall length of the stent or in 
the number of turns of the coiled stent structure. 
Lastly, it is an object of the present invention to provide for an improved 
stent which comprises a coiled ribbon structure which may be either 
self-expanding or balloon expanding. 
These and other objects of the invention will become apparent from the 
following discussion of the invention. 
SUMMARY OF THE INVENTION 
The present invention provides for an improved stent for placement in 
corporeal lumens. The instant invention relies upon utilization of a 
coiled ribbon which has sufficient structural rigidity to extend when 
placed within a corporeal lumen and, at the same time, retain the relative 
position of each turn or curve which has been incorporated into the coiled 
ribbon structure. 
The problems discussed above are solved by making a coiled stent from an 
extending ribbon, which is rigid enough to maintain the relative position 
of each turn, of each coil during expansion within the lumen. The diameter 
of the extending coiled ribbon stent of the present invention increases 
without any substantial change in the overall length of the stent, or in 
the number of turns of the coil. The only change is in the length of the 
ribbon itself. Ribbon elongation is accomplished by the same kind of 
configurational change through which any other stent expands; structural 
elements are rotated away from the long axis of the stent into a more 
transverse orientation. 
The construction and obvious advantages of the system provided for by the 
present invention will be more clearly understood from the following 
description of the various specific embodiments when read in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to an improved "stent" for placement in 
corporeal lumens. More particularly, the present invention is directed to 
an improved stent for placement in a corporeal lumen comprising a coiled 
ribbon which is rigid enough to maintain the relative position of each 
turn of the coil during its expansion within the lumen after placement. 
With reference to FIGS. 1-7, it will be appreciated that the longitudinal 
stability of the stent of the present invention depends on each turn of 
the coil maintaining its approximate position relative to its neighbors. 
To do this, the axial orientation of the ribbon must be substantially 
constant relative to the long axis of the stent. There can be little 
bending of the ribbon up or down the surface of the stent. In order for 
the ribbon to bend in this direction, its two edges would have to be 
capable of differential elongation. Therefore, limitation of differential 
elongation between these edges is a key design feature, in that it imbues 
the ribbon with the necessary rigidity, and the stent with the necessary 
stability. To achieve this goal, a single strip of metal may be etched to 
form a lattice, or zig-zag wires may be braided to form a flat mesh. 
With reference to FIGS. 1(a)-1(b), which depict the stent 10 of the present 
invention in compressed and expanded views respectively, which stent 
comprises a coiled ribbon 12 having a defined length which defines the 
overall length 14 of the coiled stent, which overall length of the coiled 
stent remains relatively constant from the compressed position to the 
expanded position. In order to maintain the aforementioned overall length 
relatively constant from the compressed to the expanded position, each 
turn 16 of the coil ribbon must retain its approximate relative position 
with relation to each of its adjacent neighboring turns. 
With reference to FIGS. 2(a)-2(c), which depict a preferred embodiment of 
the structure of the ribbon element of the coiled ribbon stent of the 
present invention, a portion of the ribbon 12 element is shown in FIG. 
2(a) in the compressed position and appears as an etched lattice 18 having 
an alternating offset series of elongated slots in the compressed 
configuration. In the expanded configuration depicted in FIG. 2(b), it can 
be seen that the lattice 18 in this embodiment consists of four zig-zag 
chains 20, a portion of one of which chains is depicted in FIG. 2c. Each 
chain comprises a series of struts 22 which are joined at their points of 
contact in three lines of connections 24. The etching and method of 
expansion, shown in the embodiment depicted in FIGS. 2(a)-2(c), resemble 
those seen in the wall of a Palmaz stent, U.S. Pat. No. 4,739,762; the 
difference being that while the present invention is a flat ribbon, the 
Palmaz stent is a cylinder. 
It will be appreciated by one skilled in the art that many alternative 
configurations are possible and can differ from the precise lattice 
configuration depicted in FIGS. 2(a)-2(c), depending on the actual shapes 
and spacing of the slots that are etched into the ribbon. 
As described above, the rigidity of the ribbon depends on the limitation of 
differential elongation between its two edges. Since any change in strut 
22 orientation produces more elongation when the ribbon is at the 
collapsed end of its compression range, and less when it is at the 
extended end of its compression range, the potential for differential 
elongation, diminishes as the ribbon reaches full extension. 
An important factor in ribbon stability is the separation between the 
chains of struts that comprise the structural elements of its two edges. 
The wider the separation, the greater the differential elongation for any 
given degree of bending. Therefore, the wider the ribbon, the less it is 
likely to bend. With reference to FIGS. 3(a) to 3(c), which depict an 
alternative preferred embodiment of the lattice structure of the ribbon 
element comprising the coiled ribbon stent of the present invention, and 
presents a more stable version of the ribbon which is produced by 
introducing spacers 26 along the line of connections 24 between the chains 
20 of struts 22. This allows the ribbon to be extended to the point where 
the struts 22 are almost parallel to the long axis of the ribbon, while 
retaining some separation between the chains 20 that form the edges of the 
ribbon. 
Once the ribbon reaches the configuration shown in FIG. 3c, it is very 
rigid, and the stent is very stable. However, the expanding (FIGS. 3a and 
3b) stent is less stable. This characteristic is not a problem with 
versions of the stent intended for balloon-expansion, because the balloon 
bulges between the turns of the coil and maintains their relative 
positions. However, self-expanding versions need to be constructed in a 
way that maximizes stability. One way to do this is to widen the ribbon, 
as discussed above. Another alternative way of maximizing stability is to 
add one or more layers of longer, more rigid struts 28 to the ribbon 
lattice structure as depicted in FIGS. 4(a)-4c). The layer of longer, more 
rigid struts extends from one edge of the ribbon to the other, and ensures 
that the changes in strut orientation, and accompanying changes in length, 
are equal on both edges of the ribbon. FIG. 4(c) shows that one set of 
long struts 28 can be created by changing the etch pattern of the ribbon 
element to create a thicker strut that is unaffected by the deformation of 
stent expansion. The other set of struts has to be created separately and 
bonded to the lattice. 
The orientation of the lattice has an important effect on the length of the 
stent. As seen in FIG. 1, the angle 30 between the long axis 32 of the 
ribbon 12 and the long axis 34 of the stent 10 needs to fall, if the stent 
is to remain at the same overall length throughout expansion. A lattice, 
formed by slots perpendicular to the long axis of the ribbon, will produce 
extension of the ribbon directly along its long axis. This would mean that 
the long axis of the ribbon would remain at a constant angle to the long 
axis of the stent, and the stent would elongate as it expanded, which may 
be undesirable. To maintain constant stent length, the plane of ribbon 
elongation needs to be perpendicular to the long axis of the stent. 
A preferred embodiment of a lattice designed to produce purely trans-axial 
expansion is illustrated in FIG. 5. The narrowing of the ribbon, 
associated with stent expansion, has no effect on overall stent length so 
long as the axial orientation of the ribbon remains constant. The space 
between adjacent turns of the coil just increases, while maintaining a 
constant relative position between adjacent coil turns. 
The advantages of the extending ribbon coil stent design, which are 
flexibility and a stable length, are present whether the stent is 
self-expanding or balloon expanded, and the same basic construction can be 
used for both types of stent; the main difference residing the mechanical 
properties of the metal. If the stent changes configuration by elastic 
deformation, it is self-expanding, and if it changes configuration by 
plastic deformation, it is balloon-expanded. 
The balloon-expanded version differs in one other regard; the terminal loop 
must be closed, rather than open, as it is in the self-expanding stent. If 
the coil in a balloon-expanded stent were left open, it would tend to bend 
outward and unwind slightly, instead of elongating. Following balloon 
deflation, the coil end would tend to return to its former position, 
resulting in incomplete expansion of both ends of the stent. Therefore, in 
order to ensure full expansion of the coiled stent in such a situation, 
the last turn of the coil must be closed, as depicted in FIG. 6, by 
joining the end of the ribbon to the adjacent ribbon segment. This 
prevents unwinding and ensures full ribbon elongation. 
The stent coil can be formed either by wrapping an extendible ribbon around 
a cylindrical form or, alternatively, for example, by etching one or more 
spiral slots into a single cylinder of metal. The spiral slots divide 
adjacent turns of the stent creating a coil. This method of construction 
is particularly applicable to the construction of balloon-expanded 
versions of the stent, in which the last turns of the coil must be closed. 
In this case, the spiral slots end before they exit the end of the metal 
cylinder, thereby closing the last turn of the coil, see FIGS. 7(a)-7(b) 
which depict a preferred embodiment of one type of spiral coil in the 
compressed and expanded configuration, respectively. 
Other turns of the coil can also be linked in this way to increase 
stability. The resultant loss of flexibility can be minimized by 
offsetting and overlapping the spiral slots slightly as depicted in FIGS. 
7(a)-7(b). The resulting oblique connectors 36 allow limited separation of 
adjacent turns of the coil (A-A', or B-B'), hence the flexibility. In the 
embodiment of the present invention depicted in FIG.7(a)-7(b), there are 
two sets of connectors between any pair of adjacent turns of the coil; in 
opposite directions and on opposite sides of the stent. In another 
configuration, the connectors 36 would all be parallel and all on the same 
side of the stent, with one connector 36 per turn. Variations in the 
width, length, direction, and number of these oblique connectors 36 can be 
used to adjust the flexibility and longitudinal stability of the stent. 
It will be further apparent to one skilled in this art that the 
improvements provided for in the present invention, while described with 
relation to certain specific physical embodiments also lend themselves to 
being applied in other physical arrangements not specifically provided for 
herein, which are nonetheless within the spirit and scope of the invention 
taught here.