WO2025074253A1

WO2025074253A1 - Stents

Info

Publication number: WO2025074253A1
Application number: PCT/IB2024/059611
Authority: WO
Inventors: Kevin Heraty; Deirdre MORONEY; Hugh MCSWEENEY; Noel Boyhan
Original assignee: Otsuka Medical Devices Co Ltd
Current assignee: Otsuka Medical Devices Co Ltd
Priority date: 2023-10-02
Filing date: 2024-10-01
Publication date: 2025-04-10
Anticipated expiration: 2026-04-02
Also published as: GB2634709A; GB202315115D0

Abstract

Tubular stents are placed in series in a blood vessel, each stent having a body portion that extends along a majority of a length of the stent adjoining an end portion that extends along a minority of the length of the stent. The body portion and the end portion of at least one of the stents are pre-formed with diameters that differ from each other. The end portion of the first stent is a complementary inter-engaging fit with the end portion of the second stent, either receiving or received by the end portion of the second stent, for example in telescopic male/female relation.

Description

Stents

This invention relates to stents, especially stents for the treatment of occlusions in blood vessels.

Stents are typically inserted into a vessel in a narrowed initial state and are then expanded radially to support the surrounding wall of the vessel, hence restoring or maintaining its patency. Some stents self-expand elastically when released from within a sleeve or catheter. Others expand plastically either when activated, for example by heat acting on a shape-memory alloy, or by being pressed radially outwardly from within using an instrument such as a balloon catheter.

When a vein is narrowed or blocked, the consequent restriction of blood flow can lead to various symptoms such as swelling, pain and ulceration. Venous stents restore blood flow by providing internal support to the vein, preventing it from collapsing or narrowing further. In this way, venous stents can be used to treat conditions such as deep vein thrombosis (DVT), chronic venous insufficiency (CVI) and narrowing of a vein, otherwise known as venous stenosis.

In principle, braided or woven stents can be used in venous applications if they have sufficient radial strength or crush resistance. Currently, however, most self-expanding stents used to treat venous disease are laser-cut from tubes of nitinol. Laser-cut stents are cut in various patterns to create skeletal frame structures when expanded or opened out. Such structures typically comprise a longitudinal array or series of crowns, being circumferential or tubular segments or rings distributed along the length of the stent and extending parallel to its central longitudinal axis. Each crown comprises a continuous, circumferentially-extending zig-zag arrangement of struts that are the principal members of the skeletal frame. The crowns are spaced apart by circumferential gaps that are bridged by connectors extending longitudinally from one crown to the next in the series.

It is well known to employ venous stents to treat obstruction of venous outflow that is a characteristic of iliofemoral occlusive disease. Indeed, venous stenting has become the standard technique for treating occlusive blockages in the iliac and femoral veins. For example, venous stents are used to reduce pain and swelling and to promote healing of ulcers in patients suffering from post-thrombotic syndrome (PTS) following an episode of DVT. Venous stents can also be used to treat patients presenting with an acute DVT to prevent subsequent development of PTS.

In some cases, multiple stents are used in series to treat a lengthy segment of a vein where the affected portion of the vein is too long to be treated with a single stent or where multiple areas of narrowing or blockage require treatment. For example, narrowed or blocked iliofemoral veins may be treated using a series of stents that are interengaged in longitudinally overlapping succession to maintain patency and adequate blood flow.

While the use of multiple stents in series can be an effective treatment option, the procedure tends to be riskier and more complex than the use of a single stent. Potential complications include stent migration, stent fracture, and thrombosis or clotting. The invention addresses these disadvantages and especially the need to prevent disengagement or separation of inter-engaged stents from each other, thereby to mitigate a consequent risk of restenosis.

The invention has special benefits in the context of the iliofemoral veins and more generally in the context of venous applications. However, the inventive concept embraces other stent applications in which separation between longitudinally- successive stents could be a problem, including arterial applications.

Figure 1 shows the convergence of the iliofemoral veins 10 into the lower end of the inferior vena cava 12 in a human individual. The iliofemoral veins 10 drain blood from the lower limbs into the inferior vena cava 12 and from there up to the heart. More specifically, blood from each lower limb flows along the femoral vein 14 into the external iliac vein 16 and through the common iliac vein 18 into the inferior vena cava 12. The inguinal ligament 20 that defines the boundary between the femoral vein 14 and the external iliac vein 16 is also shown here schematically by a dashed line.

In patients with iliofemoral occlusive disease, it is common for the diseased segment or lesion to extend from the common iliac vein 18 into the external iliac vein 16 and from there beyond the inguinal ligament 20, hence into the femoral vein 14. The resulting length of the diseased segment requires multiple stents to be implanted in series and moreover for an end of each stent of the series to be overlapped telescopically, in male/female relation, with a cooperating end of each adjoining stent of the series.

Before a stent is placed across a venous lesion, the local diameter of the vein must be measured carefully to ensure that a stent of the appropriate diameter is chosen. However, vessel sizing is a difficult and imprecise art. Veins are challenging to measure accurately and the vein diameter can change due to various factors such as the patient’s level of hydration, their body position (supine vs standing), and a breathing manoeuvre or the Valsalva manoeuvre performed at the time of measurement. As a result, there is a risk that an undersized stent could be placed in a vein and if it migrates toward the vena cava as a result, the stent could eventually become lodged in the patient’s heart.

For safety, therefore, venous stents are usually oversized relative to the measured inner diameter of the vein in which they are to be placed. For example, laser-cut stents of nitinol are typically indicated as 1mm to 2mm oversized. So, say, a 14mm stent may be indicated for use in a vein with an inner diameter measured as 12mm.

It will be apparent from Figure 1 that the inner diameter or lumen of the iliofemoral veins typically increases from the femoral vein 14 to the external iliac vein 16 and from there to the common iliac vein 18. Consequently, as physicians choose the outer diameter of a stent to suit the inner diameter of a vessel in which that stent is to be placed, the stent diameter typically increases from stent to stent along the series in the direction of venous blood flow, that being a cranial or cephalic direction in this example. For example, a 16mm diameter stent may be placed in the common iliac vein 18 and overlapped telescopically with a 14mm diameter stent placed in the external iliac vein 16. In other words, an end portion of the wider stent surrounds and receives a cooperating end portion of the adjoining narrower stent.

It will be apparent that as the choice of stent diameter is determined by the local diameter of the surrounding vessel, maintaining a close telescopic fit between successive stents is barely even a secondary consideration. In particular, the choice of stent diameter is not determined by the diameter of the next stent of the series, with which it must mate. During the implant procedure, the larger- or largest-diameter stent is placed first, at or near to the bifurcation between the inferior vena cava 12 and the common iliac vein 18. One or more stents of successively smaller diameter are subsequently placed more caudally, for example into the external iliac vein 16 and the femoral vein 14. Telescopic overlap between the successive stents is typically achieved by inserting the cephalic end portion of a narrower stent into the caudal end portion of a wider stent implanted previously.

Reference is made to Figures 2a and 2b in this respect, which show a wider stent 22 in a cephalic position in the common iliac vein 18 in overlapping relation with a narrower stent 24 in a caudal position in the external iliac vein 16. A cephalic end portion of the narrower stent 24 is received telescopically within a caudal end portion of the wider stent 22, creating an overlap 26 between the stents 22, 24.

At the time of deployment as shown in Figure 2a, the end portions of the stents 22, 24 that are in overlapping relation are in close mutual contact at the overlap 26. This is because the inner diameter of the surrounding vessels 16, 18 initially limits radial expansion of the conjoined stents 22, 24. Consequently, the inner diameter of the wider stent 22 substantially corresponds to the outer diameter of the narrower stent 24 along the length of the overlap 26. The resulting intimate contact effects a direct structural or mechanical connection between the stents 22, 24 at the overlap 26, with the result that the overlapping stents 22, 24 behave as a single, unitary structure. There may also be mutual axial alignment between the stents 22, 24 at their common interface.

Over time, however, nitinol stents have a tendency to continue expanding as they approach their nominal diameter, at least until they reach whatever maximum external diameter may be permitted by the inward reaction of the surrounding walls of the vessels 16, 18. Consequently, in the case of two stents 22, 24 of different nominal diameters placed in a mutually overlapping configuration, both stents 22, 24 will expand radially over time but the outer, wider stent 22 will tend to expand radially away from the inner, narrower stent 24. As a result, the stents will no longer be in intimate mutual contact along and around the overlap 26, even if they were in such contact when initially deployed.

As radial separation between the overlapping stents 22, 24 undermines the direct structural or mechanical connection that previously existed between them, the stents 22, 24 will tend to behave more as individual stents over time. They will disengage from each other and may become misaligned as shown in Figure 2b. New tissue is likely to grow in the radial gap that opens in the overlap 26 between the stents 22, 24.

Stents 22, 26 implanted in the iliofemoral veins are subject to a complex combination of loads, which include bending, axial shortening and focal compression. These complex loads act on the entire stented segment. Consequently, if direct connection between the stents 22, 24 is lost, new tissue that grows in the overlap 26 between the stents 22, 24 will be subject to a complex stress field due to relative motion between the stents 22, 24. This could generate an adverse biological response in the new tissue in the region of the overlap 26.

US 2018/021155 and US 2012/296406 both disclose modular overlapping endovascular stent graft systems for treatment of aneurysms. These types of stents serve to deploy graft material at a location of an aneurysm to seal the aneurysm from circulating blood. These endovascular stent graft systems, or endografts for short, are generally composed of a metal frame covered with a woven polyester material that is similar to a standard surgical graft. These endografts are unsuitable for treatment of venous stenosis as they lack sufficient radial strength or crush resistance. As such, endografts do not face the same challenges as venous stents at least not to the same degree, for example, complex combinations of loads and continued expansion over time with potential radial separation at the stent overlap.

Other prior art resides in US 6264690, CN 217611585, US 8118861, KR 101651148, and WO 2017/100977.

It is against this background that the present invention has been devised. In one sense, the invention resides in a stent combination comprising a first stent and a second stent, the first stent having a first end portion, the second stent having a second end portion that is complementarily inter-engageable with the first end portion in a deployed state, and at least a first or body portion of the first stent having a diameter that is different from a diameter of the first end portion. The first stent may have a nominal diameter that is different from a nominal diameter of the second stent. The first end portion may be of lesser or greater diameter than the first portion but the diameter of the first end portion may be configured to be substantially the same as the diameter of the second end portion in the deployed state. The second end portion may be of greater diameter than a second or body portion of the second stent. The second end portion may be a complementary fit within the first end portion or the first end portion may be a complementary fit within the second end portion.

The first end portion may taper away from the first portion of the first stent whereas the second end portion may be flared away from the second portion of the second stent. The first and second portions of the first and second stents may each taper along their length from a wide end toward a narrow end. In that case, the first end portion of the first stent at its narrow end may be a complementary fit within the second end portion of the second stent at its wide end. The first and second portions of the first and second stents may taper in a common longitudinal direction; similarly, the first and second end portions of the first and second stents may each taper in that common longitudinal direction.

The first and second end portions of the first and second stents may each be of substantially constant cross section and may be a complementary fit in male/female relation.

The invention also resides in a stent combination comprising first and second tubular stents each with open distal and proximal ends, each stent having a body portion that extends along a majority of a length of the stent and an end portion that extends along a minority of the length of the stent, wherein the body portion and the end portion of the first stent are configured to be deployed to diameters that differ from each other and the end portion of the first stent is configured to be a complementary inter-engaging fit with the end portion of the second stent in a deployed state.

The invention also resides in a stent combination comprising first and second tubular stents each with open distal and proximal ends, each stent having a body portion that extends along a majority of a length of the stent adjoining an end portion that extends along a minority of the length of the stent. The body portion and the end portion of at least the first stent are pre-formed with diameters that differ from each other. The end portion of the first stent is a complementary inter-engaging fit with the end portion of the second stent. At least one of the stents may have at least one anchoring formation protruding radially from its body portion. The end portion of the first stent may be of lesser or greater diameter than the body portion of the first stent. The end portion of the second stent may be of greater diameter than the body portion of the second stent.

The end portion of the second stent may be a complementary fit within the end portion of the first stent, or the end portion of the first stent may be a complementary fit within the end portion of the second stent. In the latter case, the end portion of the first stent may taper away from the body portion of the first stent. Conversely, the end portion of the second stent may flare away from the body portion of the second stent.

The body portions of the first and second stents can each taper along their length from a wide end toward a narrow end. In that case, the end portion of the first stent at its narrow end may be a complementary fit within the end portion of the second stent at its wide end. The body portions of the first and second stents can taper in a common longitudinal direction, and the end portions of the first and second stents can each taper in that common longitudinal direction. Alternatively, the end portions of the first and second stents may each be of substantially constant cross section but may still be a complementary fit in male/female relation.

At least one of the stents of the combination could be a slit tube and at least one other of the stents of the combination could be a braided or woven tube. For example, at least one of the stents may comprise a longitudinal series of circumferential tubular segments that are separated by, and alternate with, gaps along the length of the stent. At least one segment of an end portion may be longer than a segment of a body portion of at least one of the stents.

Each segment of the series may comprises struts that are disposed in a circumferentially extending waveform arrangement and successive segments of the series may be interconnected by connectors that bridge the respective gaps. The connectors can extend between respective apices of the waveform arrangements of successive segments. For example, trough-to-trough connectors may extend between facing troughs of the waveform arrangements of successive segments, and could connect at least one segment of an end portion to at least one segment of a body portion of at least one of said stents. Conversely, peak-to-peak connectors may extend between facing peaks of the waveform arrangements of successive segments, and could connect successive segments of an end portion of at least one of said stents.

A stepped outer profile may be defined between the end portion and the body portion of at least one of said stents. Such a stepped outer profile could comprise a radially- extending circumferential shoulder.

The body portions of the stents of the combination may differ from each other in diameter. For example, when the stents are inter-engaged in series in situ within a blood vessel, the vessel may have a tapering lumen. In that case, the stent with the body portion of greater diameter may be disposed in a wider segment of the vessel than the stent with the body portion of lesser diameter.

One of the stents of the combination may have has greater columnar stiffness and/or greater resistance to radial compression than another of the stents of the combination.

The inventive concept embraces a corresponding method of deploying a series of stents in a blood vessel, each stent having a body portion that extends along a majority of a length of the stent adjoining an end portion that extends along a minority of the length of the stent. The method comprises: deploying a first stent of the series; and deploying a second stent of the series with its end portion inter-engaged with the end portion of the first stent as a complementary fit; wherein at least one of said stents is pre-formed before deployment such that the body portion and the end portion have diameters that differ from each other. The end portion of the first stent may receive, or be received by, the end portion of the second stent.

Where the stents are deployed in a vessel with a tapering lumen and the stents have body portions of different diameters, the stent with the body portion of greater diameter may be disposed in a wider segment of the vessel than the stent with the body portion of lesser diameter.

Where the stents are deployed in iliofemoral veins, one of the stents with greater resistance to radial compression may be deployed in a common iliac vein and another of the stents with lesser resistance to radial compression may be deployed in an external iliac vein. Conversely, a stent with lesser flexibility may be deployed in a common iliac vein and a stent with greater flexibility may be deployed in an external iliac vein. More generally, in a method of treating an occlusive condition of the iliofemoral veins, the veins may be stented with a series of two or more stents comprising a cephalic stent with greater resistance to radial compression and a caudal stent with greater flexibility to bend along its length.

An objective of the invention is to establish and to maintain a structural connection between overlapping stents of different diameters in a way that ensures a good connection is maintained between the stents over time. When used in the iliofemoral veins, a caudal end portion of a cephalically-placed stent may receive, or be received by, a cephalic end portion of a caudally-placed stent.

The objective of the invention can be achieved in various ways to ensure that overlapping end portions of adjoining stents remain in close contact over a substantial area and length of overlap, preferably with an interference fit between those portions. For example, a minor end portion of a stent can be expanded radially to a greater diameter than a major body portion of the stent or can be contracted radially to a lesser diameter than a major body portion of the stent. Alternatively, to the same effect as the latter case, the major body portion of the stent can be expanded radially to a greater diameter than the minor end portion of the stent.

An end portion of enlarged diameter can receive, or be received within, an end portion of an adjoining stent; conversely, an end portion of reduced diameter can surround and receive, or be received within, an end portion of an adjoining stent. In each case, the body portion of the receiving, or female, stent may have a greater diameter than the body portion of the received, or male, stent. Alternatively, the body portion of the receiving, or female, stent may have a lesser diameter than the body portion of the received, or male, stent.

In another approach, tapering stents with frusto-conical shapes can be placed in mutually overlapping relation with a common direction of taper but not necessarily a common angle of taper. In general, one stent is relatively wide and the other stent is relatively narrow. However, their overlapping end portions - at the narrow end of the wider stent and at the wide end of the narrower stent - are close to each other in diameter. Thus, the wide end of the narrower stent may be received in, or may receive, the narrow end of the wider stent, while maintaining a close fit between them. It is also possible for the overlapping end portions of frusto-conical stents to be defined by straight end sections of substantially constant diameter along their length to maintain efficient contact when those sections are overlapped with each other.

A laser-cut stent may have a major body portion terminating in a minor end portion that tapers away from the body portion or that is otherwise generally narrower than the body portion. The tapered or otherwise narrowed end portion may be received by a correspondingly flared or widened end portion of an adjoining stent, thus effecting an overlap between the stents. The adjoining stent with the flared or widened end portion could be a laser-cut stent or a woven or braided stent.

An additional objective of the invention is to reduce the risk of migration of a stent. This can be achieved by modifying the cross-section of a stent locally to create an anchor formation that can engage the surrounding vessel wall. The anchor formation is suitably provided in a major body portion of the stent. For example, an anchor formation could be defined by an expanded section of greater diameter than the nominal diameter of the stent. The expanded section protrudes radially from the surrounding outer surface of the body of the stent that has the lesser nominal diameter, in the manner of a step, shoulder or flange.

A laser-cut stent may comprise a longitudinal series of circumferential tubular segments that are separated by, and alternate with, gaps along the length of the stent between opposed end segments of the series. Each segment of the series may comprise struts that are disposed in a circumferentially extending waveform arrangement. Successive segments of the series can be interconnected by connectors that bridge the respective gaps.

Segments of the series disposed between the end segments may comprise at least one anchor segment that is pre-formed with a radially enlarged configuration relative to at least one interconnected adjacent body segment of lesser radius. The major body portion of the laser-cut stent may comprise a repeating pattern of crown pairs, each crown pair comprising two crowns joined by a set of the short connectors. Successive crown pairs of the repeating pattern may be joined by respective sets of the long connectors.

The anchor portion may comprise at least one of the crowns of the major body portion that is radially enlarged relative to at least one adjoining crown of the major body portion when the stent is in an expanded state. Similarly, at least one of the crown pairs of the major body portion may be radially enlarged relative to at least one of the adjoining crown pairs of the major body portion when the stent is in an expanded state. At least one of the crowns of the major body portion may be pre-formed with a radially enlarged configuration relative to at least one adjoining crown.

The or each radially enlarged crown may be at a position offset longitudinally closer to one of the end portions than to the other of the end portions. The or each radially enlarged crown may be of substantially constant diameter along its length in a longitudinal direction or may be flared along its length in a longitudinal direction. At least two crowns can be flared in opposed longitudinal directions. The or each radially enlarged crown can be joined to an adjoining crown by a set of the long connectors.

Where a laser-cut stent has a stepped profile to define an anchor formation, struts in crowns and/or connectors between crowns in or around an expanded section can be lengthened in comparison to the strut or connector lengths of adjoining stent portions of the nominal diameter. This reduces the outward radial force applied by the stent in and around the expanded section and so reduces a risk of injury to the surrounding vessel. For similar reasons, struts in crowns and/or connectors between crowns of the overlapping end portions of laser-cut stents could be lengthened relative to the strut or connector lengths of body portions of the stents. This compensates for the greater outward radial force applied by two stents in overlap.

The invention also contemplates a stent solution in which stents of a series have differing geometry or properties to accommodate different loads experienced at different locations in a vein. For example, caudal vessels in the iliofemoral vein system typically experience greater bending and axial loads than cephalic vessels of the system. Consequently, a caudal stent can be designed to have greater flexibility and additional kink resistance and to minimise foreshortening compared to a cephalic section of the stent. Conversely, a cephalic stent could be designed to have greater column stiffness to aid in stent deployment and/or additional crush resistance, for example to treat May-Thurner syndrome in which the right iliac artery compresses the left iliac vein.

In summary, the invention involves placing tubular stents in series in a blood vessel, each stent having a body portion that extends along a majority of a length of the stent adjoining an end portion that extends along a minority of the length of the stent. The body portion and the end portion of at least one of the stents are pre-formed with diameters that differ from each other.

The end portion of the first stent is a complementary inter-engaging fit with the end portion of the second stent, either receiving or being received by the end portion of the second stent, for example in telescopic male/female relation with a close initially sliding or interference fit between radially inner and radially outer parts. The end portion defining the inner part is inserted into and overlapped with the end portion defining the outer part and is then expanded. Relative longitudinal position between the inner and outer parts changes telescopically during insertion but remains fixed after expansion.

To put the invention into context, reference has already been made to Figures 1 to 4 of the accompanying drawings, in which:

Figure 1 is a schematic diagram showing the convergence of the iliofemoral veins into the inferior vena cava; and

Figures 2a and 2b are schematic diagrams showing a prior art series of two stents in situ within the left iliofemoral veins, showing the tendency of the stents to disengage from each other and to become misaligned after deployment.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings in which:

Figures 3a and 3b are schematic side views of a first embodiment of the invention;

Figures 4a and 4b are schematic side views of a second embodiment of the invention;

Figures 5a and 5b are schematic side views of a third embodiment of the invention;

Figures 6a and 6b are schematic side views of a fourth embodiment of the invention; Figures 7a and 7b are schematic side views of a fifth embodiment of the invention;

Figures 8a and 8b are schematic side views of a sixth embodiment of the invention; and

Figure 9 is a schematic detail side view of an implementation of the invention using laser-cut stents.

Figures 3a to 8b show various embodiments of the invention in which like numerals are used for like features. Thus, in each case, a nominally wider stent 22 is shown in combination with a nominally narrower stent 24, each having open distal and proximal ends. The stents 22, 24 are shown aligned on a mutual central longitudinal axis 28 to be brought together at an interface represented by an overlap 26. In this context, the nominal diameter of each stent 22, 24 is determined by its body portion 22’, 24’ that defines a major part of its length. The shorter cooperating end portions 22”, 24” of the stents 22, 24 may be wider or narrower than their associated body portions 22’, 24’. Indeed, the end portion 22”, 24” of one of the stents 22, 24 could be wider or narrower than the body portion 22’, 24’ of the other stent 22, 24.

By way of example, the end portions 22”, 24” defining an overlapped stepped section may have a length of 2% to 50%, for example 2% to 30%, of the overall length of the stents 22, 24.

In some cases, the end portion 24” of the narrower stent 24 is received by the end portion 22” of the wider stent 22, as is conventional. In other cases, however, the end portion 24” of the narrower stent receives the end portion 22” of the wider stent 22. Thus, it is possible for the end portion 22” of the wider stent 22 to be narrower than the end portion 24” of the narrower stent 24. The narrower stent 24 could be deployed first and the wider stent 22 could be deployed next, or the wider stent 22 could be deployed first and the narrower stent 24 could be deployed next.

As noted above, the wider stent 22 will typically be placed in a cephalic position in iliofemoral applications, for example in the common iliac vein, in overlapping relation with the narrower stent 24 placed in a caudal position, for example in the external iliac vein. In such applications, the overlap 26 is formed between a cephalic end portion 24” of the narrower stent 24 and a caudal end portion 22” of the wider stent 22. It is possible to deploy the narrower stent 24 in the caudal position first or to deploy the wider stent 22 in the cephalic position first.

The contours that define the shape and size of the end portions 22”, 24” relative to the body portions 22’, 24’ may, for example, be imparted by preforming each stent 22, 24 around a correspondingly-shaped mandrel inserted within. Such a preforming process is apt to be performed during heat treatment conventionally applied to each stent 22, 24. Preforming may be particularly apt for laser-cut stents. Braided or woven stents are not precluded from preforming but they could, in principle, be produced with the desired shape at the outset while they are being created by weaving or similar processes.

Figures 3a and 3b represent the wider stent 22 as a straight tube of constant crosssection along its length. Thus, the body portion 22’ and the end portion 22” are of substantially the same diameter. Conversely, the narrower stent 24 has an end portion 24” that is radially enlarged relative to its body portion 24’. The widened end portion 24” of the narrower stent 24 still fits within the end portion 22” of the wider stent 22 to create an overlap 26 when the narrower stent 24 is inserted into the wider stent 22 previously placed within a vein 10 as shown schematically in Figure 3b. However, by increasing the outer diameter of the end portion 24” of the narrower stent 24 to complement the inner diameter of the end portion 22” of the wider stent 22, a close fit between the stents 22, 24 is assured despite the greater disparity between the nominal diameters of their body portions 22’, 24’.

Figures 4a and 4b show another way of achieving a similar result, in this case by reducing the inner diameter of the end portion 22” of the wider stent 22 relative to its body portion 22’. Here, the narrower stent 24 is represented as a straight tube of constant cross-section along its length, hence with its body portion 24’ being of substantially the same diameter as its end portion 24”. Again, the end portion 22” of the wider stent 22 receives the end portion 24” of the narrower stent 24, with the inner diameter of the end portion 22” complementing the outer diameter of the end portion 24”. As before, this ensures a close fit at the overlap 26 when the stents 22, 24 are brought together as shown in Figure 4b. In a variant of the arrangement shown in Figures 4a and 4b, the end portion 22” of the wider stent 22 could be received within the end portion 24” of the narrower stent 24, with the outer diameter of the end portion 22” complementing the inner diameter of the end portion 24”. In this case, the wider stent 22 may need to be expanded after the narrower stent 24.

In the examples shown in Figures 3a, 3b, 4a and 4b, the diameter of the end portion of the received stent, when expanded, can be slightly (up to, say, 10%) greater than the diameter of the end portion of the receiving stent before expansion.

Figures 5a and 5b show an arrangement that is akin to that of Figures 4a and 4b, in that the inner diameter of the end portion 22” of the wider stent 22 is again reduced relative to its body portion 22’. Here, though, the end portion 24” of the narrower stent 24 received by the wider stent 22 is radially enlarged relative to its body portion 24’ in a manner akin to that shown in Figures 3a and 3b. In this example, a frusto-conical transition portion 24’” of the narrower stent 24 joins the end portion 24” to the body portion 24’. The transition portion 24’” reduces stress concentrations between the end portion 24” and the body portion 24’.

The narrower stent 24 is represented in Figures 5a and 5b as being of braided or woven construction in contrast to the wider stent 22, which may be laser-cut. This allows the conjoined stents 22, 24 to have markedly different properties to suit different segments of the iliofemoral veins. For example, a laser-cut wider stent 22 that is relatively stiff in the radial direction may be apt to be placed cephalically in the common iliac vein to correct constriction by an overlapping iliac artery, being a characteristic of May-Thurner syndrome. In contrast, a braided or woven narrower stent 24 may be apt to extend caudally into the external iliac vein or the femoral vein, where relative flexibility for the stent 24 to bend along its length could be beneficial and less radial stiffness is required.

A similar combination of a laser-cut wider stent 22 and a braided or woven narrower stent 24 is shown in Figures 6a and 6b. Again, the body portion 22’ of the wider stent 22 is a straight tube of constant cross-sectional diameter but in this case, the end portion 22” of the wider stent 22 is narrowed by imparting a frusto-conical taper to the end portion 22”. The end portion 24” of the narrower stent 24 has a correspondingly flared frusto-conical shape, narrower than the body portion 22’ of the wider stent 22, which complements and receives the tapered end portion 22” of the wider stent 22 as shown in Figure 6b. This exemplifies how the male/female relationship of the end portions 22”, 24” can be reversed, with the end portion 22” of the wider stent 22 being received within the end portion 24” of the narrower stent 24. In this case, the wider stent 22 may need to be expanded after the narrower stent 24.

Figures 7a and 7b show an embodiment in which the stents 22, 24, including their body portions 22’, 24’ and end portions 22”, 24”, taper in a common direction, that being the caudal direction in iliofemoral applications. Thus, the end portion 22” at the narrow end of the wider stent 22 is received by the end portion 24” at the wide end of the narrower stent 24 as shown in Figure 7b. The stents 22, 24 are each represented here as being frusto-conical but the angle of taper is not necessarily constant between the stents 22, 24 or indeed along each stent 22, 24. Again, in this case, the wider stent 22 may need to be expanded after the narrower stent 24.

Figures 8a and 8b show a variant of the embodiment shown in Figures 7a and 7b in which frusto-conical body portions 22’, 24’ of the stents 22, 24 again taper in a common direction. However, in this case, the end portions 22”, 24” do not taper; they are instead defined by complementary straight tubular sections of substantially constant diameter along their length. The end portion 24” of the narrower stent 24 is shown in Figure 8b as being received as a close fit within the end portion 22” of the wider stent 22 but this male/female relationship could be reversed, with the end portion 22” of the wider stent instead being received by the end portion 24” of the narrower stent.

Figures 8a and 8b also show an optional feature to reduce a risk of migration of the stents 22, 24, namely anchor formations 30 created by modifying the cross-section of the body portions 22’, 24’ to engage a surrounding vessel wall. In this example, the anchor formations 30 are defined by radially expanded circumferential sections of greater diameter than the nominal diameter of the respective stents 22, 24. The expanded section protrudes radially from the surrounding outer surface of the body portion 22’, 24’ in the manner of a step, shoulder or flange.

It will be apparent that the anchor formations 30 will improve mechanical and frictional engagement of the stents 22, 24 with a surrounding vessel in use and, in particular, will resist longitudinal movement and hence migration of the stents 22, 24. The protruding anchor formations 30 also focus the radial expansion force of the stents 22, 24 on a smaller portion of the overall outer surface area of the stents 22, 24. This increases the outward pressure acting against the interior of the surrounding vessel wall to enhance mechanical and frictional engagement of the stents 22, 24 with the vessel.

Similar anchor formations 30 could be applied to the body portions 22’, 24’ of any of the stents 22, 24 shown in the preceding embodiments. There could be more than one such anchor formation 30 on each stent 22, 24. Again, the anchor formations 30 are apt to be defined in a pre-forming process around a mandrel disposed in the lumen of each stent 22, 24.

Turning finally to Figure 9, this schematic drawing shows laser-cut stents 22, 24 that are akin to those of Figures 3a and 3b in that the narrower stent 24 has an end portion 24” that is radially enlarged relative to its body portion 24’ but fits within the end portion 22” of the wider stent 22. The body portions 22’, 24’ of the stents 22, 24 shown in Figure 9 also include radially expanded circumferential anchor formations 30 that may otherwise be referred to as a radially enlarged crown pair or a stepped portion.

Each stent 22, 24 comprises an open skeletal frame that is formed when an elongated tube is expanded radially into the state shown in Figure 9. The tube may, for example, be formed of nitinol laser-cut with a pattern of slits that define members of the frame between the slits when the tube is expanded.

Each stent 22, 24 comprises a longitudinal array or series of rings or circumferential or tubular segments referred to herein as crowns 32, distributed along its length and spaced apart by interstitial circumferential gaps 34. The length of the stent 22, 24 and hence the number of crowns 32 is indeterminate. Only a few of potentially several such crowns 32 are shown in Figure 9.

Each crown 32 is circumferentially continuous and, in end view, is rotationally symmetrical about the central longitudinal axis 28. Each crown 32 also extends parallel to the central longitudinal axis 28 and therefore defines a respective portion of the length of the stent 22, 24, in conjunction with the width of the gaps 34 between neighbouring crowns 32 along that axis 28.

Each crown 32 comprises a circumferentially-extending zig-zag arrangement of struts 36 that are the principal members of the skeletal frame. The zig-zag arrangement may also be described as a triangular waveform oscillating circumferentially around the crown 32. In this example, pairs of struts 36 of the crowns 32 that terminate the end portions 22”, 24” are in mutual opposition to define closed, diamond-shaped cells.

Each strut 36 is inclined relative to a line intersecting the circumference of the associated crown 32 and extending parallel to the central longitudinal axis 28. The inclination of each strut 36 opposes the inclination of the adjoining struts 36 of the same crown 32.

The apices 38 where the struts 36 of each crown 32 join in circumferential succession define peaks and troughs when viewed from the perspective of neighbouring crowns 32. The peaks are apices 38 that are relatively proximate to a neighbouring crown 32 whereas troughs are apices 38 that are relatively distant from a neighbouring crown 32. The peaks and troughs of each crown 32 alternate circumferentially around the crown 32.

The crowns 32 are joined to the, or each, adjacent crown 32 by sets of connectors 40, 42 that are distributed circumferentially around the central longitudinal axis 28. The connectors 40, 42 extend longitudinally to bridge the gaps 34 between successive crowns 32 and so serve as secondary members of the skeletal frame. More specifically, the stents 22, 24 shown in Figure 9 comprise two types of connectors 40, 42 between successive crowns 32, namely: long connectors 40 that extend from a trough of one crown 32 to a closest trough of an adjacent crown 32; and short connectors 42 that extend from a peak of one crown 32 to a closest peak of an adjacent crown 32.

The sets of long and short connectors 40, 42 alternate longitudinally, in that one gap 34 is bridged by a set of short connectors 42, the next gap 34 is bridged by a set of long connectors 40 and so on in sequence along the length of each stent 22, 24. The result is that adjacent crowns 32 are joined together as a pair by short connectors 42 and that pair of crowns 32 is joined to one or two adjacent crowns 32 by long connectors 40.

In this example, one pair of crowns 32 joined by short connectors 42 forms each anchor formation 30 and another pair of crowns 32 joined by short connectors 42 forms each end portion 22”, 24”. Thus, long connectors 40 join the anchor formations 30 and the end portions 22”, 24” to the body portions 22’, 24’ of the stents 22, 24. As the members of the skeletal frame are formed and defined between slits in a tubular workpiece, the waveform shape of each crown 32 complements the shape of the, or each, neighbouring crown 32. Thus, before longitudinal expansion of a slit tube to form the stent 22, 24, the successive crowns 32 are in nested relation, with the peaks of each crown 32 aligned with and received by the troughs of a neighbouring crown 32. Consequently, the peaks and troughs of a crown 32 are offset angularly or indexed about the central longitudinal axis 28 relative to the peaks and troughs of the, or each, neighbouring crown 32.

In view of the angular offset between the peaks and the troughs of successive crowns 32, the connectors 40, 42 allow for the resulting angular offset between their ends by being curved or kinked. Specifically, the short connectors 42 in these examples have an S-shape, being curved continuously through mutually-opposed inflections, whereas the long connectors 40 include a central chicane comprising bends in mutually- opposed directions.

The curved or kinked configurations of the connectors 40, 42 have the benefit of conferring flexibility on the stents 22, 24, including longitudinal extensibility to allow the stents 22, 24 to bend along their length. By virtue of their greater length making them easier to bend along their length, the long connectors 40 contribute flexibility additional to that provided by the short connectors 42.

Flexibility in the connectors 40, 42 from one crown 32 to the next is advantageous not only in use, so that the stents 22, 24 can more readily conform to internal contours of the vasculature, but also in manufacture so that the stents 22, 24 can be shaped in accordance with the invention without experiencing excessive local stress. The long connectors 40, especially, effectively decouple the radially-enlarged crowns 32 of the anchor formations 30 and of the end portions 22”, 24” from immediately adjacent crowns 32 of the body portions 22’, 24’ of lesser radius. In particular, the flexibility of the long connectors 40 to deflect or deform in response to axial and bending loads accommodates radial expansion of the crowns 32 of the anchor formations 30 and of the end portions 22”, 24” without experiencing excessive stress.

Figure 9 shows an embodiment of an anchor formation 30 and end portions 22”, 24”, comprising radially enlarged crowns 32 joined peak-to-peak by short connectors 42. The crowns 124 of anchor formation 30 and end portions 22”, 24” are expanded radially to a greater extent than neighbouring or adjoining crowns 32, disposed outboard of the anchor formation 30 to each side or inboard of the end portions 22”, 24”. The adjoining radially smaller crowns 32 are joined to respective radially enlarged crowns 32 by respective long connectors 40 extending trough-to-trough. The flexibility of the long connectors 40 to deflect or deform in response to axial and bending loads accommodates the radial expansion of the radially enlarged crowns 32 without experiencing excessive stress.

The long connectors 40 connecting the radially enlarged crowns 32 to the radially smaller crowns 32 effectively decouple the radially enlarged crowns 32 from the immediately adjacent radially smaller crowns 32. This allows the radially enlarged crowns 32 and the radially smaller crowns 32 to move and flex independently of each other to a helpful extent during manufacture, placement and in use.

The long connectors 40 allow the pairs of radially enlarged crowns 32 to remain of substantially uniform diameter along their length. In other words, the radially enlarged crowns 32 have radially outermost circumferential faces that are straight-sided and parallel to the central longitudinal axis 28. Nevertheless, by virtue of their radial enlargement, the radially enlarged crowns 32 protrude radially, like a flange, from the surrounding cylindrical outline of the stent defined by the radially smaller crowns 32 that adjoin them. It is also possible for either or both of the crowns 32 of an anchor formation 30 to be flared in a frusto-conical shape that tapers in either longitudinal direction.

The flange-like radial protrusion of the radially enlarged crowns 32 defines external formations being steps, edges or shoulders between the greater radius of the expanded radially enlarged crowns 32 and the lesser radius of the radially smaller crowns 32 that adjoin them. These circumferential external formations improve mechanical and frictional engagement of the stent with a surrounding vessel in use and, in particular, will resist longitudinal movement and hence migration of the stent 22, 24.

Advantageously, the presence of long connectors 40 between a radially enlarged crown 32 and an adjoining radially smaller crown 32 decouples the corresponding apices 38 of the radially enlarged crown 32 from those of the radially smaller crown 32. These unconstrained apices 38 on the outboard edges of the anchor formation 30 or end portions 22”, 24” allow for a beneficially sharp or sudden diametric step change along the length of the stent 22, 24, enhancing engagement of the stent 22, 24 with the surrounding wall of the vasculature to resist migration of the stent 22, 24.

The protruding radially enlarged crowns 32 also focus the radial expansion force of the stent 22, 24 on a smaller portion of the overall outer surface area of the stent 22, 24. This increases the outward pressure acting against the interior of the surrounding vessel wall to enhance mechanical and frictional engagement of the stent 22, 24 with the vessel.

To further benefit, the outboard apices 38 or peaks of the waveforms of the crowns 32 in the anchor formation 30 and/or end portions 22”, 24” lend a serrated profile to the external formations. When the stent 22, 24 is in situ within a vessel such as a common iliac vein, the serrated external formations of the radially enlarged crowns 32 dig into and engage mechanically with the surrounding wall of the vein.

It will also be noted from Figure 9 that the length of the crowns 32 can differ in a direction parallel to the central longitudinal axis 28. In other words, at least one crown 32 may be longer than other crowns 32 of the stent 22, 24. In particular, the length of a crown 32, hence the length of struts 36 making up a crown 32, can be varied to alter the outward radial force applied by the crown 32. For example, struts 36 can be lengthened to reduce radial force or can be shortened to increase radial force locally, expressed as radially outward pressure per unit area.

In this example, struts 36 defining the crowns 32 of the anchor formations 30 and of the end portions 22”, 24” are lengthened relative to struts 36 of the body portions 22’, 24’. This reduces the outward radial force applied locally by the stents 22, 24 to reduce any risk of injury to a surrounding vessel. In relation to the end portions 22”, 24” in particular, lengthening the struts 36 compensates for the greater outward radial force applied by two stents 22, 24 in overlap.

Many other variations are possible within the inventive concept. For example, whilst the stents exemplified in this specification are straight stents, it would also be possible for other stents of the invention to be formed helically so that they twist along their length about a centreline in three-dimensional space.

Claims

1. A stent combination comprising a first stent and a second stent, the first stent having a first end portion, the second stent having a second end portion that is complementarily inter-engageable with the first end portion in a deployed state, and at least a first portion of the first stent having a diameter that is different from a diameter of the first end portion.

2. The stent combination of Claim 1 , wherein the first stent has a nominal diameter that is different from a nominal diameter of the second stent.

3. The stent combination of Claim 1 or Claim 2, wherein the diameter of the first end portion is configured to be substantially the same as a diameter of the second end portion in the deployed state.

4. The stent combination of any preceding claim, wherein the first end portion is of lesser diameter than the first portion.

5. The stent combination of any of Claims 1 to 3, wherein the first end portion is of greater diameter than the first portion.

6. The stent combination of any preceding claim, wherein the second end portion is of greater diameter than a second portion of the second stent.

7. The stent combination of any preceding claim, wherein the second end portion is a complementary fit within the first end portion.

8. The stent combination of any of Claims 1 to 6, wherein the first end portion is a complementary fit within the second end portion.

9. The stent combination of Claim 8, wherein the first end portion tapers away from the first portion of the first stent.

10. The stent combination of Claim 9, wherein the second end portion is flared away from the second portion of the second stent.

11. The stent combination of any preceding claim, wherein the first and second portions of the first and second stents each taper along their length from a wide end toward a narrow end.

12. The stent combination of Claim 11 , wherein the first end portion of the first stent at its narrow end is a complementary fit within the second end portion of the second stent at its wide end.

13. The stent combination of Claim 11 or Claim 12, wherein the first and second portions of the first and second stents taper in a common longitudinal direction.

14. The stent combination of Claim 13, wherein the first and second end portions of the first and second stents each taper in the common longitudinal direction.

15. The stent combination of any of Claims 11 to 13, wherein the first and second end portions of the first and second stents are each of substantially constant cross section and are a complementary fit in male/female relation.

16. The stent combination of any preceding claim, wherein one of said stents is a slit tube and another of said stents is a braided or woven tube.

17. The stent combination of any preceding claim, wherein at least one of the stents has at least one anchoring formation protruding radially from its body portion.

18. The stent combination of any preceding claim, wherein at least one of the stents comprises a longitudinal series of circumferential tubular segments that are separated by, and alternate with, gaps along the length of the stent.

19. The stent combination of Claim 18, wherein each segment of the series comprises struts that are disposed in a circumferentially extending waveform arrangement and successive segments of the series are interconnected by connectors that bridge the respective gaps.

20. The stent combination of Claim 19, wherein the connectors extend between respective apices of the waveform arrangements of successive segments.

21. The stent combination of Claim 20, wherein trough-to-trough connectors extend between facing troughs of the waveform arrangements of successive segments.

22. The stent combination of Claim 21 , wherein the trough-to-trough connectors connect at least one segment of an end portion to at least one segment of a body portion of at least one of said stents.

23. The stent combination of any of Claims 20 to 22, wherein peak-to-peak connectors extend between facing peaks of the waveform arrangements of successive segments.

24. The stent combination of Claim 23, wherein the peak-to-peak connectors connect successive segments of an end portion of at least one of said stents.

25. The stent combination of any of Claims 18 to 24, wherein at least one segment of the first and/or second end portion is longer than a segment of the first and/or second portion of at least one of said stents.

26. The stent combination of any preceding claim, wherein a stepped outer profile is defined between the first and/or second end portion and the first and/or second portion of at least one of said stents.

27. The stent combination of Claim 26, wherein the stepped outer profile comprises a radially-extending circumferential shoulder.

28. The stent combination of any preceding claim, when inter-engaged in series in situ within a blood vessel.

29. The stent combination of Claim 28, where the vessel has a tapering lumen and one of said stents of greater diameter is disposed in a wider segment of the vessel than another of said stents of lesser diameter.

30. The stent combination of any preceding claim, wherein one of said stents has greater columnar stiffness and/or greater resistance to radial compression than another of said stents.

31. A method of deploying a series of stents in a blood vessel, each stent having a respective body portion adjoining a respective end portion, the method comprising: deploying a first stent of the series; and deploying a second stent of the series with its end portion inter-engaged with the end portion of the first stent as a complementary fit; wherein at least one of said stents is pre-formed before deployment such that its body portion and its end portion have diameters that differ from each other.

32. The method of Claim 31 , wherein the end portion of the first stent receives the end portion of the second stent.

33. The method of Claim 31 , wherein the end portion of the first stent is received by the end portion of the second stent.

34. The method of any of Claims 31 to 33, wherein said stents are deployed in a vessel with a tapering lumen and the stents have body portions of different diameters, the stent with the body portion of greater diameter being disposed in a wider segment of the vessel than the stent with the body portion of lesser diameter.

35. The method of any of Claims 31 to 34, wherein one of said stents has greater columnar stiffness and/or greater resistance to radial compression than the other of said stents.

36. The method of Claim 35, where the stents are deployed in iliofemoral veins with the stent with greater resistance to radial compression deployed in a common iliac vein and the stent with lesser resistance to radial compression deployed in an external iliac vein.

37. The method of any of Claims 31 to 36, wherein one of said stents has greater flexibility to bend along its length than the other of said stents.

38. The method of Claim 37, wherein the stents are deployed in iliofemoral veins with the stent with lesser flexibility deployed in a common iliac vein and the stent with greater flexibility deployed in an external iliac vein.

39. A method of treating an occlusive condition of the iliofemoral veins, the method comprising stenting the veins with a series of stents comprising a cephalic stent with greater resistance to radial compression and a caudal stent with greater flexibility to bend along its length.

40. A stent comprising: a body portion; and an end portion having a different diameter from the body portion when the stent is in a deployed state; wherein an outer or inner surface of the end portion is configured not to contact a body lumen directly but to contact an end portion of a second stent directly when the stent is deployed within the body lumen.