United States Patent
`Porter
`Date of Patent:
`[451
`Nov, 12, 1991
`
`[11]
`
`Patent Number:
`
`5,064,435
`
`[19]
`
`“When Hope is All In Vein”, Sweden Now, Mar., 1988.
`“Transluminally-Placed Coilspring Endarterial Tube
`Grafts", Charles T. Dotter, MD, pp. 239-332, Investiga-
`tive Radiology, Sep.—Oct. 1969, vol. 4.
`Technical Developments and Instrumentation, “Tran-
`sluminal Expandable Nitinol Coil Stent Grafting: Pre-
`liminary Report”, Dotter et al, pp. 259-260, Radiology
`147, Apr. 1983.
`“Atherosclerotic Rabbit Aortas: Expandable Intralu-
`minal Grafting”, Radiology, 1986 (Sep.), pp. 723-726,
`Palmaz et al.
`
`Primary Examiner—Randy Citrin Shay
`Attorney, Agent, or Firm—l-Iaugen and Nikolai
`[57]
`ABSTRACT
`
`A body implantable stent consists of two or more gener-
`ally tubular, coaxial and slidably connected stent seg-
`ments. Each of the stent segments is of open weave
`construction,
`formed of multiple braided, helically
`wound strands of resilient material. The stent is elasti-
`cally deformed to a reduced radius when deployed.
`When released after positioning, the stent self-expands
`radially into contact with a tissue wall segment defining
`a blood vessel or other body cavity. As each stent seg-
`ment expands radially, it contracts in the axial direction.
`To preserve a consistent length of the stent in spite of
`axial contraction of the segments, the axially outward
`and non-overlapping portions of the stent can be de-
`signed for secure fixation to the tissue wall segment, for
`example as radially outward flares. Accordingly, axial
`contraction occurs as a reduction in the length of the
`medial regions where adjacent stent segments overlap.
`Alternative approaches to maintain axial length include
`the addition of reinforcing filaments near the stent op-
`posite ends to increase the restoring force, the provision
`of fixation hooks at opposite ends of the stent, and se-
`curing an elongate, axially directed, flexible and inex-
`tensible wire to the opposite ends of the stem.
`
`26 Claims, 2 Drawing Sheets
`
`[54] SELF-EXPANDING PROSTHESIS HAVING
`STABLE AXIAL LENGTH
`
`[75]
`
`Inventor:
`
`Christopher H. Porter, Woodenville,
`Wash.
`
`[73] Assigneei Schneider (USA) Inc., Minneapolis,
`Minn.
`
`[21] Appl. No.: 544,923
`
`[22] Filed:
`
`Jun. 28, 1990
`
`Int. Cl.5 .............................................. .. A61F 2/04
`[51]
`[52] U.S. C1. .................................... .. 623/12; 606/151;
`606/198
`
`[58] Field of Search ....................... .. 623/1, 11, 12, 13;
`604/96, 104; 606/151, 153, 155, 158, 191, 198
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.
`3/1975 Alfidi et al.
`3,868,956
`........................ 604/104
`4,553,545 11/1985 Maass et al.
`.
`604/104
`4,572,186
`2/1986 Gould et al.
`4,649,922
`3/1987 Wiktor ....... ..
`623/1
`4,655,771
`4/1987 Wallsten ................................. 623/1
`4,681,110
`7/1987 Wiktor .
`4,699,611 10/1987 Bowden ............................ .. 604/105
`4,732,152
`3/1988 Wallsten et al.
`....... ..
`4,733,665
`3/1988 Palmaz .................................... 623/l
`4,793,348 12/1988 Palmaz .
`4,800,882
`1/1989 Gianturco ............................. 623/13
`4,830,003
`5/1989 Wolff et al.
`. . ... .
`. . . .. ... 623/1
`4,848,343
`7/1989 Wallsten et al.
`.
`604/271
`4,856,516
`8/1989 Hillstead .... ..
`623/1
`4,886,062 12/1989 Wiktor .................................. .. 623/1
`
`
`
`
`
`OTHER PUBLICATIONS
`
`“Self—Expanding Metallic
`Interventional Radiology,
`Stents for Small Vessels: An Experimental Evaluation”,
`Duprat et al, pp. 469-472, vol. 162, Feb. 1987.
`“Self-ExpandingEndovascular Prosthesis: An Experi-
`mental Study”, Radiology, 1987 (Sep.), pp. 709-714,
`Rousseau et al.
`
`
`
`
`
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`ENDOLOGIX, INC
`
`EX. 1006
`
`001
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`

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`U.S. Patent
`
`Nov. 12, 1991
`
`Sheet 1 of 2
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`5,064,435
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`U.S. Patent
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`Nqv. 12, 1991
`
`Sheet 2 of 2
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`5,064,435
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`5,064,435
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`i 2
`case, the spring preferably is elastic, with a memory
`favoring the radially expanded configuration.
`A self-expanding stent or prosthesis often is preferred
`over a plastically deformed device. Resilient stents can
`be deployed without dilatation balloons or other stent
`expanding means. A self-expanding stent can be prese-
`lected in accordance with the diameter of the blood
`
`vessel or other fixation site. While deployment requires
`skill
`in positioning the prosthesis,
`the added skill of
`properly dilating the balloon to plastically expand a
`prosthesis to a selected diameter is not required. Also,
`the self-expanding device remains at least slightly com-
`pressed after fixation, and thus has a restoring force
`which facilitates acute fixation. By contrast, the plasti-
`cally expanded stent must rely on the restoring force of
`deformed tissue, or on hooks, barbs or other indepen-
`dent fixation means.
`
`Further advantages arise from constructing the pros-
`thesis of multiple, braided and helically wound strands
`or filaments as in the aforementioned Wallsten patent.
`The filaments themselves have a restoring force which
`causes the filaments to bear against tissue walls of the
`body cavity in which the stent is fixed, thus maintaining
`the cavity open. At the same time there is sufficient
`space between adjacent filaments to promote embed-
`ding of the stent into the tissue, and fibrotic growth to
`enhance long-term fixation. A further advantage of this
`construction is that it enables a substantial radial con-
`
`traction of the prosthesis during deployment, for exam-
`ple to as little as about one-fourth of the normal diame-
`ter (the diameter in the relaxed state, i.e. when subject
`to no external forces). This facilitates deployment of the
`prosthesis through narrow vessels or other constrictions
`on the way to the point of fixation.
`At the same time, a substantial axial elongation ac-
`companies the radial contraction. There is a substantial
`axial contraction or shortening as the stent self expands,
`once free of its radial constraint. Thus, there is a rubbing
`or scraping action axially along tissue as the radially
`expanding stent also axially shortens. Should tissue at
`the fixation area further yield to radial prosthesis expan-
`sion in the longer term, such expansion causes further
`axial shortening and wiping action, and presents further
`risk of injury to tissue. A further drawback is that a stent
`during its fixation may radially expand more than ex-
`pected, retaining less than the intended or minimum
`necessary axial length. Likewise, a plastically deform-
`able stent may require more than the anticipated radial
`expansion and axial shortening.
`Therefore, it is an object of the present invention to
`provide a prosthesis of open weave, helical and braided
`construction capable of _ substantially maintaining its
`axial length as it radially self-expands.
`Another object
`is to provide a radially expanding
`tubular stent comprised of at least two stent segments,
`with an area of overlap of the sections variable in axial
`length to maintain a consistent axial separation between
`non-overlapping ends of the stent.
`Yet another object is to provide a stent with a medial
`portion variable in axial
`length,
`in combination with
`means at the opposite end portions of the stent for fixing
`the stent to bodily tissue, such that the bodily tissue
`maintains a substantially constant axial separation of the
`two end portions during any radial expansion or con-
`traction of the stent.
`
`SELF-EXPANDING PROSTHESIS HAVING
`STABLE AXIAL LENGTH
`
`BACKGROUND OF THE INVENTION
`
`5
`
`The present invention relates to body implantable
`devices, and more particularly to prostheses and grafts
`intended for long-term or permanent fixation in body
`cavities.
`'
`
`A wide variety of patient treatment and diagnostic
`procedures involve the use of devices inserted into the
`body of the patient, with some of these devices being
`permanently implanted. Among these devices are pros- _
`theses or grafts for transluminal implantation, for exam-
`l5
`ple as disclosed in U.S. Pat. No. 4,655,771 (Wallsten).
`The prosthesis described in Wallsten is a flexible tubular
`braided structure formed of helically wound thread
`elements. Gripping members at opposite ends of the
`prosthesis initially secure it to a catheter, with the proxi-
`mal gripping member being movable distally to give the
`prosthesis the shape of a balloon. In deployment, the
`gripping members and catheter are removed, leaving
`the prosthesis to assume a substantially cylindrical
`shape as it slightly expands and substantially conforms
`to a blood vessel wall or other tissue. Another prosthe-
`sis is disclosed in U.S. Pat. No. 4,681,110 (Wiktor). A
`flexible tubular liner, constructed of braided strands of a
`flexible plastic, is insertable into the aorta, whereupon it
`self-expands against an aneurysm to direct blood flow
`past the aneurysm. The braided stents of Wallsten and
`Wiktor axially contract as they radially expand.
`Another elastic stent
`is shown in U.S. Pat. No.
`4,830,003 (Wolff et al). The stent includes a series of
`generally longitudinal wires welded together in pairs,
`with the wires in each pair then bent into a “V” shape.
`Like the braided stents, this stent shortens axially as it
`radially expands.
`Prostheses also have been constructed of plastically
`deformable materials. U.S. Pat. No. 4,733,665 (Palmaz)
`discloses intraluminal vascular grafts radially expanded
`using angioplasty balloons. The grafts are wire mesh
`tubes, and axially shorten as they radially expand. U.S.
`Pat. No. 4,800,882 (Gianturco) features a stent formed
`of wire,
`including a plurality of serpentine bends to
`form opposed loops. A balloon is inflated to radially
`expand the stent, without substantial axial shortening.
`Yet another approach to prosthesis design is shown in
`U.S. Pat. No. 3,868,956 (Alfidi et al). Alfidi et al dis-
`closes a strainer or screen with a plurality of generally
`longitudinal wires, bound together by a cylindrical
`sleeve. The wires are deformable into a longitudinal,
`straight-line configuration for implantation. Once im-
`planted, the device is heated. Due to the recovery prop-
`erty of the metal forming the wires (e.g. nitinol alloy),
`heating causes the wires to flare radially outward at the
`opposite ends, thus to secure the device at the desired
`location.
`
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`A stent including means for maintaining a constant
`axial length in spite of radial expansion or contraction, is
`disclosed in U.S. Pat. No. 4,553,545 (Maass et al), as a
`prosthesis in the form of a helical coil spring. In one
`embodiment, a constant axial length of the spring is
`_maintained, with opposite ends of the spring rotated
`relative to one another to change the spring pitch and
`radius. An alternative approach involves maintaining a
`constant pitch over a given section of a spring, by pro-
`viding spring material to a “constant length” section
`from a more compressed section of the spring. In each
`
`65
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`004
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`3
`
`‘SUMMARY OF THE INVENTION
`
`5,064,435
`
`To achieve these and other objects, there is provided
`a body implantable device, including coaxial first and
`second open weave stent segments slidably engaged to
`form a stent. The stent segments are engaged along
`respective concentric first and second axially inward
`portions overlapping one another to form a medial re-
`gion of the stent. Further,
`the stent segments include
`opposite non-overlapping first and second axially out-
`ward regions with respective and opposite first and
`second ends of the stent. The stent segments, at least
`along the axially inward portions, have a predetermined
`first diameter and a predetermined first axial
`length.
`The stent segments are radially compressible to a sec-
`ond diameter less than the first diameter and to a second
`axial length longer than the first axial length, to facili-
`tate an axial insertion of the stent into a body cavity for
`delivery to a selected location along the body cavity
`and subsequent fixation of the stent to a cavity wall
`segment defining the body cavity. During its fixation,
`the stent radially expands. The first and second axially
`inward portions slide relative to one another to reduce
`the axial length of the medial region during the radial
`expansion. Thus the stent maintains a substantially con-
`stant axial length during radial expansion.
`A preferred approach uses means for fixing the out-
`ward ends of a self-expanding stent, e.g. respective first
`and second flared outer end portions along the axially
`outward regions of the stent. The first and second ends
`have diameters greater than the first diameter when the
`stent is in the relaxed state, and when compressed tend
`to have a greater restoring force against the cavity wall
`segment, as compared to the remainder of the stent. The
`end diameters should be greater than the medial region
`diameter by five percent or more, ensuring a substantial
`difference in restoring force for a relatively constant
`diameter of the cavity along the tissue wall segment.
`Alternatively,
`the outer end portion of each stent
`segment can have the same diameter as the medial re-
`gion, but be composed of larger diameter filaments,
`added windings of filaments or otherwise have in-
`creased stiffness or resistance to radial contraction as
`compared to the medial region Yet another alternative
`is to provide fixation elements, for example hooks, at
`the opposite ends of the stent.
`In combination with positive fixation of the stent
`ends, a substantial medial overlapping region is pro-
`vided when the stent segments are in a radially com-
`pressed or delivery configuration. For example,
`the
`overlapping region may comprise three-fourths or more
`of the axial length of the compressed stent. Then, upon
`deployment of the stent, both stent segments radially
`expand and axially shorten. With the outer ends of the
`stent fixed, the axial shortening occurs only along the
`medial region, substantially shortening the region of
`overlap but maintaining the desired axial separation of
`the opposite stent ends.
`An open weave of braided, helically wound strands
`or filaments is the preferred structure of the tubular
`stent. The open weave structure enables substantial
`self-expansion in the stent, for example to a fixation
`diameter at least three times the diameter during deliv-
`ery. This of course results in a substantial corresponding
`axial shortening in each of the stent segments, but due to
`the overlapping medial region of the stent, the overall
`axial length remains virtually constant.
`
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`4
`A pliable catheter is a suitable apparatus for delivery
`and deployment of the stent. More particularly, a pli-
`able sheath can surround at least the distal end portion
`of the catheter and extend beyond the distal tip to sur-
`round the stent segments as well, maintaining them in a
`radially compressed delivery configuration. The cathe-
`ter can be provided with a lumen,
`through which a
`guide wire may be inserted to facilitate travel of the
`catheter and compressed stent through blood vessels or
`other body cavities to the fixation area. Once the cathe-
`ter is inserted properly to position the stent at the de-
`sired fixation point, the outer sheath is withdrawn prox-
`imally, with the stent abutting the catheter and thus
`secured against proximal travel with the sheath. The
`distal portion of the stent self-expands first. and in ex-
`panding against tissue, secures the stent segment against
`proximal travel. With one end of the stent constrained
`by tissue and the opposite end constrained by a station-
`ary catheter, the axial length of the stent remains sub-
`stantially constant. Axial shortening of the stent seg-
`ments, which accompanies their radial expansion, tends
`to diminish the length of the medial region and leave the
`overall axial length unaffected.
`Following fixation, further yielding of the tissue seg-
`ment can result in further radial expansion of the stent.
`However, with the opposite ends of the stent secure,
`any axial shortening of the stent segments again affects
`only the medial overlapping region. Thus, the advan-
`tages of the open weave construction are retained, with-
`out an undesirable shortening of the stent as it radially
`self-expands.
`
`IN THE DRAWINGS
`
`For a further understanding of the above and other
`features and advantages, reference is made to the fol-
`lowing detailed description and the drawings, in which:
`FIG. 1 is a side elevation of a body implantable de-
`vice constructed in accordance with the present inven-
`tion;
`FIG. 2 is a side sectional view of a catheter and
`sheath retaining the implantable device in a radially
`compressed condition;
`FIG. 3 is an end view of the device, catheter and
`sheath;
`
`FIG. 4 is a side sectional view showing deployment
`of the device within a body cavity;
`‘
`FIG. 5 is a side view of the device fixated within the
`cavity;
`FIG. 6 is a side elevation of an alternative embodi-
`ment device in the relaxed or fully radially expanded
`condition;
`FIG. 7 is a side elevation showing yet another alter-
`native device in the expanded or relaxed condition;
`FIG. 8 is a side elevation illustrating a further alterna-
`tive device in a radially compressed state;
`FIG. 9 is a side elevation of the device of FIG. 8 in
`the expanded condition;
`FIG. 10 is a side elevation showing yet another alter-
`native device, in a radially compressed condition;
`FIG. 11 is a side elevation of the device of FIG. 10 in
`the radially expanded condition;
`FIG. 12 is a side elevation of another alternative
`device, in a radially expanded condition;
`FIG. 13 is a side elevation of a further alternative
`device, in a radially expanded condition;
`FIG. 14 is a side elevation of another alternative
`device, in a radially expanded condition; and
`
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`5
`FIG. 15 is a side elevation of yet another alternative
`device, in a radially expanded condition.
`
`5,064,435
`
`6
`nary bypass operations, acute closure and recurrence of
`stenosis are significant problems in up to about thirty
`percent of constricted or blocked passages opened by
`balloon angioplasty. The fixation of stent 16 within a
`blood vessel along a previously occluded region tends
`to keep this region permanently open.
`'
`Fixation of stent 16, within a blood vessel 46 having
`a tissue wall segment 48, begins with intravascular in-
`sertion of the stent, catheter and sheath in the delivery
`configuration shown in FIGS. 2 and 3. The reduced
`radius facilitates insertion of this assembly through
`blood vessel 46 until stent 16 reaches a predetermined
`fixation location along the blood vessel. Once the
`proper positioning of the stent
`is confirmed, e.g.
`through use of one or more radiopaque markings on the
`stent, sheath or catheter, sheath 38 is moved proximally
`with respect to catheter 40.
`With distal tip 42 abutting stent 16, the catheter pre-
`vents the stent from traveling proximally with sheath 38
`as the sheath is withdrawn. Thus, as seen from FIG. 4,
`stent 16 becomes free of sheath 38 over an increasing
`distal portion of its axial length. As each of stent seg-
`ments 20 and 22 becomes free, it radially self-expands
`until contacting tissue wall segment 48, then undergoes
`slightly further radial expansion until the tendency to
`radially expand is counterbalanced by the restoring
`force exerted radially inward by the tissue wall seg-
`ment. At the equilibrium condition, shown in FIG. 5,
`stent is not fully radially expanded to the relaxed config-
`uration shown in FIG. 1, and thus applies a restoring
`force which tends to maintain the stent at the fixation
`position within vessel 46.
`A salient feature of the present invention is the con-
`centric and slidable mounting of stent segments 20 and
`22 in combination with the fixation provided by flared
`ends 34 and 36. During initial withdrawal of sheath 38,
`the distal flared end 36 is the first to encounter tissue
`wall segment 48. Due to its larger nominal (relaxed
`state) diameter, flared end 36 tends to radially expand
`somewhat more than the remainder of axially outward
`portion 32 of this segment, and applies comparatively
`greater restoring force in the radially outward direction
`against the tissue wall segment. Accordingly, the axial
`shortening of distal stent segment 22 which accompa-
`nies radial expansion, e.g. from a length of 100 mm
`when delivered to a fixation length of 50 mm, occurs
`almost entirely by travel of axially inward portion 28,
`distally or rightwardly as viewed in FIG. 4. The slid-
`able engagement of segments 20 and 22 permits such
`distal travel while proximal segment 20 remains sub-
`stantially fixed relative to catheter 40.
`As sheath 38 is further withdrawn, proximal segment
`20 likewise radially expands and axially shortens As
`illustrated in FIG. 4, much of axially outward portion
`30 of segment 20 remains radially compressed within
`sheath 38, and thus is held fixed with respect to the
`catheter. Consequently, the axial contraction of proxi-
`mal stent segment 20 during radial expansion occurs
`almost entirely by virtue of proximal travel of its axially
`inward portion. This of course involves further sliding
`of the stent segments relative to one another, and fur-
`ther reduces the axial
`length of medial overlapping
`region 24.
`As seen from FIGS. 2 and 5, the total axial length of
`stent 16, designated “L“,
`is substantially the same
`whether the stent is in the deployment state, or the
`radially expanded to equilibrium or fixation. Proximal
`stent segment 20 and distal stent segment 22 are each
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`Turning now to the drawings, there is shown in FIG.
`1 a body implantable prosthesis or stent 16. Stent 16 has
`an open mesh or weave construction, formed of heli-
`cally wound and braided strands or filaments 18 of a
`resilient material, for example a body compatible stain-
`less steel or an elastomer, e.g. polypropylene, polyure-
`thane, polysulfone or a polyester.
`Stent 16 includes coaxial proximal and distal stent
`segments 20 and 22. A medial region 24 is formed by the
`overlapping of respective axially inward portions of
`stent segments 20 and 22. Axially outward, non-over-
`lapping portions of the stent segments are indicated at
`30 and 32, respectively. At opposite ends of the stent are
`flared ends 34 and 36, each having a greater radius than
`the nominal radius over the majority of the stent length.
`As is later explained, flared ends 34 and 36 provide a
`fixation feature useful to maintain a constant overall
`axial length in stent 16, even while stent segments 20
`and 22 radially self-expand and axially contract during
`fixation.
`_
`
`In FIG. 1, stent 16 is shown in its relaxed condition,
`with no external forces applied to radially contract the
`stent. Stent 16 is self-expanding in the sense that when
`not subject
`to external forces,
`it assumes a diameter
`much larger than the diameter illustrated in FIGS. 2
`and 3. In these figures, the stent is elastically deformed
`and maintained in a radially reduced configuration by a
`pliable, dielectric sheath 38 surrounding the stent.
`An elongate and pliable catheter 40, of which just the
`distal end region is shown in FIG. 2, includes a distal tip
`42 which abuts the proximal end of the stent. The proxi-
`mal portion of sheath 38 surrounds the distal end region
`of the catheter. Catheter 40 has a central lumen 44 open
`to tip 42 and running the length of the catheter, to per-
`mit delivery of a drug, in liquid form, to the catheter
`distal tip from a supply at the proximal end of the cathe-
`ter. Lumen 44 further enables the use of a guide wire
`(not shown) which can be intravenously inserted, by its
`distal end to the desired point of fixation for stent 16.
`With the guide wire in place, catheter 40, stent 16 and
`sheath 38 are positioned to surround the proximal end
`of the guide wire with the guide wire contained within
`lumen 44. Then,
`the catheter, sheath and stent are
`moved distally or advanced, directed by the guide wire
`to the fixation location, whereupon the guide wire can
`be withdrawn.
`_
`Sheath 38 preferably isconstructed of silicone rubber
`or other suitable biocompatible material, and surrounds
`the stent and catheter at least along the catheter distal
`end region’, or along the full length of the catheter if
`desired. Sheath 38 preferably is thin to facilitate intra-
`vascular insertion of the catheter, sheath and stent, yet
`is sufficiently thick to maintain stent 16 in a reduced
`radius or delivery configuration against the restoring
`force of strands 18. The outside diameter of the assem-
`bly including the catheter, stent and sheath is approxi-
`mately 2.3 millimeters.
`Stent 16 is particularly well suited for use as a pros-
`thesis or graft in a blood vessel or other body cavity.
`One advantageous use of the stent occurs in connection
`with percutaneous transluminal coronary angioplasty
`(PTCA) procedures. While such procedures afford
`significantly reduced cost and risk as compared to coro-
`
`10
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`15
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`20
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`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`006
`
`006
`
`

`
`7
`
`5,064,435
`
`8
`
`substantially shorter in equilibrium. However, virtually
`all of the reduction in axial length is reflected in the
`substantially reduced length of medial overlapping re-
`gion 24, which accounts for more than three-fourths of
`the total stent length in FIG. 2, and only about one-fifth
`of the overall stent length in FIG. 5.
`Eventually, fixation of stent 16 becomes permanent
`by virtue of the embedding of strands 18 into tissue wall
`segment 48, and flbrotic growth of tissue between and
`around strands to anchor the stent. This type of fixation
`occurs over a period of weeks, and in the intervening
`time, tissue wall segment 48 may yield to allow further
`radial expansion of a stent,-and further axial shortening
`of stent segments 20 and 22. The axial length “L” re-
`mains substantially constant nonetheless, as this further
`axial contraction is again reflected in a further shorten-
`ing of the medial overlapping region. Axial contraction
`occurs along the medial region, since flared ends 34 and
`36 continue to exert a comparatively greater restoring
`force against the tissue, thus more securely anchoring
`the ends as compared to the central portions of the
`stent. Thus, the overall length of the stent is maintained
`not only during and immediately after fixation, but in
`the interim until fibrosis permanently secures the stent.
`FIG. 6 shows an alternative embodiment stent 52,
`again with concentric and slidably connected proximal
`and distal stent segments as indicated at 54 and 56. Axi-
`ally inward portions of the stent segments overlap to
`form a medial region 58. Stem 52 has an open mesh or
`weave construction, formed of helically wound and
`braided filaments 60.
`
`5
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`10
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`15
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`20
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`25
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`30
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`body tissue. with axial contraction occurring as substan-
`tial reduction in the length of medial region 58.
`FIG. 7 illustrates yet another approach to preserving
`the axial length of the stent, in this case, a plurality of
`fixation hooks 70 at the opposite ends of a stent 72 hav-
`ing a slidably interconnected and coaxial proximal and
`distal stent segments 74 and 76. Fixation hooks 70 pres-
`ent some risk of injury and thus are more limited in their
`application than the fixation alternatives previously
`discussed. Nonetheless, hooks 70 provide a positive and
`immediate fixation of stent 72 within a cavity at the
`opposite stent ends. Subsequent radial expansion and
`axial contraction of stent segments 74 and 76 serves to
`reduce the length of a medial region 78, preserving the
`overall length of the stent.
`FIGS. 8 and 9 illustrate a further embodiment stent or
`prosthesis 80 including a proximal segment 82, a distal
`segment 84 and a center segment 86 slidably engaged
`with the proximal and distal segments. All three seg-
`ments of prosthesis 80 have the previously described
`open mesh or weave construction of braided filaments.
`Stent 80 thus includes two overlapping regions interme-
`diate its proximal and distal ends 88 and 90, namely a
`proximal intermediate region 92 and a distal intermedi-
`ate region 94. While center segment 86 is shown with a
`smaller radius than the other segments for convenience
`of illustration, all segments preferably have substan-
`tially the same radius.
`FIG. 9 illustrates stent 80 in the relaxed or radially
`expanded state. Each of segments 82, 84 and 86 has a
`reduced axial dimension as well as a larger radius.
`Nonetheless, the axial distance between proximal end 88
`and distal end 90 remains about the same, with virtually
`all of the axial contraction reflected in the substantially
`reduced axial dimensions of intermediate overlapping
`regions 92 and 94.
`Prosthesis 80 can be deployed in the manner de-
`scribed above in connection with other embodiments.
`Following the desired positioning of the prosthesis
`within a blood vessel or other body cavity, a surround-
`ing sheath is withdrawn slidably or folded back from a
`surrounding relation to the prosthesis, permitting it to
`radially self-expand into contact with a tissue wall seg-
`ment forming the cavity (not shown). Of course,
`the
`diameter of the cavity should be less than the normal or
`radially expanded diameter of the prosthesis. Prosthesis
`80 does not utilize any special end fixation structure
`such as the earlier described hooks, reinforced ends or
`flared ends. Rather, the prosthesis is positioned by vir-
`tue of the self-expansion and restoring force of the seg-
`ments, to maintain their relative positions, particularly
`during their deployment and release from a sheath or
`the like, but also after fixation. It should be noted that
`this approach is suitable for the two-segment stents
`earlier described, although some type of end fixation
`means facilitates maintaining a constant axial length of
`the stent. If desired, a fixation structure can be provided
`at ends 88 and 90.
`
`35
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`40
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`45
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`50
`
`55
`
`Stent 52, illustrated in its relaxed or unstressed state,
`does not include radially outward flares at its opposite
`ends. In lieu of flared ends, each of stent segments 54
`and 56 includes at its axially outward end a plurality of
`reinforcing strands 62 connected to the braided fila-
`ments 60, thus to create respective proximal and distal
`reinforced end regions 64 and 66. The reinforcing
`strands 62 can, but need not, be of the same construction
`as the base filaments. In either event, the reinforcement
`strands lend further elastic resistance to radial compres-
`sion, such that a given elastic radial compression of
`stent 52 requires a greater force at reinforced end re-
`gions 64 and 66 as compared to the force required be-
`tween these regions.
`Stent 52 can be deployed in the manner described
`above in connection with stent 16. Following proper
`positioning of the stent within a blood vessel or other
`body cavity, a surrounding sheath similar to sheath 38 is
`withdrawn proximally from its surrounding relation
`with stent 52, allowing the stent to radially self-expand
`into contact with the tissue forming the cavity. Again,
`stent 52 is selected to have a nominal diameter (in the
`relaxed state) greater than the diameter of the body
`cavity, so that base filaments 60 and reinforcement
`strands 62 engage the tissue before full expansion, and
`are contained short of full expansion by body tissue, for
`an equilibrium of the restoring force in the stent and the
`oppositely directed restoring force in the body tissue.
`With the stent in equilibrium (as shown in FIG. 5 in
`connection with stent 16), reinforced end regions 64 and
`66 may or may not flare slightly radially outward from
`the remainder of the stent. In either event, the restoring
`force at the reinforced end regions is greater than the
`restoring force along the remainder of the stent length.
`Accordingly, the opposite ends of stent 52 tend to re-
`main secure in their axial positioning relative to the
`
`FIGS. 10 and 11 illustrate yet another embodiment
`stent 96 including proximal and distal segments 98 and
`100, slidably engaged and overlapping along a medial
`region 102. A strand or wire 104 runs parallel to stent 96
`and is secured at points 106 and 108 near proximal and
`distal ends 110 and