WO2023034303A1

WO2023034303A1 - Devices and systems for improving performance of a stent

Info

Publication number: WO2023034303A1
Application number: PCT/US2022/042044
Authority: WO
Inventors: Rodney Brenneman; Peter Kenneth Balmforth; Darren Spencer
Original assignee: DP Holding (U.K) Limited
Priority date: 2021-08-30
Filing date: 2022-08-30
Publication date: 2023-03-09

Abstract

A stent system is described that comprises a primary stent for location in a lumen of a target vessel, the primary stent defining an exterior surface that contacts a vessel wall and an interior surface that faces inwardly. The stent system further comprises at least one secondary stent element deployable wholly within the primary stent and configured to engage with the interior surface of the primary stent. The secondary stent element has a braided structure and is configured to apply a chronic outward radial force to the interior surface of the primary stent so as to effect modification an aspect ratio of the lumen of the target vessel at the location where the secondary stent element is deployed. Stent elements are also provided having the aforementioned braided structure. Methods of treating venous or arterial occlusions and compressions using the described systems and devices are also provided.

Description

DEVICES AND SYSTEMS FOR IMPROVING PERFORMANCE OF A STENT

TECHNICAL FIELD

The present invention relates to devices for implantation within the body for improving vessel or duct patency and stent performance as well as for delivery and/or deployment of such devices in the venous system and/or the arterial system.

BACKGROUND

Coronary stenting and most other stenting situations in the human body occur in relatively stable environments with limited flexure, allowing for stents to be designed to address the specific challenge posed by the environment without too much concern for the flexibility of the stent or its resistance to kinking.

Venous stenting for the treatment of external compressions has developed rapidly over the past few years, with multiple manufacturers looking to capitalize on a new market opportunity for an old well understood product - the stent. However not all stents are created equally, and the circumferential expansive force of each stent is very different, meaning that the amount of force a stent can overcome differs from stent to stent.

Each manufacturer’s stent has different properties derived their method of manufacturing (braided vs laser cut), the material it is developed from (Nitinol vs Elgiloy) and the pattern of the braid I laser cut to find the balance of radial expansive force, crush resistance and flexibility. So, with this stated, it is not surprising that the stents have different responses to testing on the benchtop, but often this does this does not correspond to predictable differences in-vivo. It has been reported that braided stents perform better under the inguinal ligament than laser cut stents and vice versa above the ligament. Laser cut stents have demonstrated more accurate deployment vs those of braided stents. However, there have been challenges that have appeared of late with two recent FDA notices for stents becoming dislodged and migrating from their implanted position or failing to deploy from the delivery system. It is an object of the invention to provide devices that address at least some of the disadvantages associated with the prior art, particularly in providing improved configurability, restoration and maintenance of vessel patency with minimal complexity.

SUMMARY

A first aspect of the invention provides for a stent system comprising: a primary stent for location in a lumen of a target vessel, the primary stent defining an exterior surface that contacts a vessel wall and an interior surface that faces inwardly; at least one secondary stent element deployable wholly within the primary stent and configured to engage with the interior surface of the primary stent, wherein the secondary stent element has a braided structure, wherein the at least one secondary stent element is configured to apply a chronic outward radial force to the interior surface of the primary stent so as to effect modification of or to resist change to an aspect ratio of the lumen of the target vessel at the location where the secondary stent element is deployed. The inclusion of the secondary stent element or elements may improve the crush resistance of the stent system when compared to a stent system without a secondary stent element.

The modification of the aspect ratio of the lumen of the target vessel may comprise modification of the aspect ratio to closer to unity. Preferably, the modification of the aspect ratio of the lumen of the target vessel may comprise modification of the aspect ratio to around unity, e.g. around 1 :1.

The primary stent may be comprised of a structure selected from the group consisting of: a wire mesh; a laser cut hypotube; a slotted tube; and a braided structure. Preferably, the primary stent comprises a braided structure. Both the primary stent and secondary stent may have a braided structure comprised of a series of interwoven coils.

The braid angle of the series of interwoven coils of the secondary stent relative to the primary stent may be at least 120 degrees when the secondary stent is deployed within the primary stent. Similarly, the braid angle of the series of interwoven coils of the secondary stent relative to the primary stent may be at most 160 degrees when the secondary stent is deployed within the primary stent. Preferably, the braid angle of the series of interwoven coils of the secondary stent relative to the primary stent may be between about 135 and about 150 degrees when the secondary stent is deployed within the primary stent.

When deployed within the primary stent, the secondary stent may exert an outward radial force of greater than 0.25 N/cm, suitably at least 0.5 N/cm, typically at least 1.0 N/cm and optionally at least 2 N/cm. When deployed within the primary stent, the secondary stent may exert an outward radial force of at most 25 N/cm, suitably at most 20 N/cm, optionally at most 15 N/cm.

The outward radial force may be determined at greater than 50% expansion of the secondary stent. Alternatively, the outward radial force may be determined at between about 10% and about 50% of the expansion of the secondary stent.

The secondary stent may be at least 1 mm, suitably 5 mm, optionally up to 10 mm in length. The secondary stent may be at most 30 mm, and typically not more than 20 mm in length.

The secondary stent may be comprised of a braided wire. The braided wire may have a cross sectional shape selected from one of the group consisting of: circular; elliptical; hexagonal; square; and rectangular.

The primary and/or secondary stent element may comprise one or more retention mechanisms to maintain the relative positions of the secondary stent within the primary stent upon deployment. The one or more retention mechanisms may be selected from the group consisting of: a hook; a tooth; a barb; and a spline.

The secondary stent may further comprise at least one anchor mechanism. The anchor mechanism may be selected from one or more of the group consisting of: a loop; a tap; a bend; one or more gripping members; or a surface modification. The anchor mechanism may be provided at either or both termini of the secondary stent. The secondary stent may comprise an outward taper at one or both end termini enabling interlocking with the primary stent upon deployment.

A plurality of radiopaque markers may be positioned along the length of the primary stent. Furthermore, the secondary stent may comprise a radiopaque material.

The primary stent and/or the secondary stent may be comprised of a surgically- and biocompatible metal or metal alloy, suitably comprising one or more of the group consisting of: stainless steel; nitinol®; cobalt chromium; tantalum; platinum; tungsten; iron; manganese; and molybdenum. The primary stent or portions of the primary stent and/or the secondary stent or portions thereof may be coated with a material selected from the group consisting of: PTFE; e-PTFE; polyurethane; silicone; papyrus; dacron®; goretex®; polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS- PCU); or biodegradable nanofiber material.

A second aspect of the invention provides for a stent system for restoring patency to a fully or partially occluded target vessel within the body of a subject, the system comprising: a primary stent for location in a lumen of the target vessel, the primary stent defining an exterior surface that contacts a vessel wall and an interior surface that faces inwardly; a plurality of secondary stent elements deployable wholly within the primary stent and configured to engage with the interior surface of the primary stent wherein the secondary stent elements have a braided structure, wherein the plurality of secondary stent elements are configured to apply a chronic outward radial force to the interior surface of the primary stent so as to effect modification of an aspect ratio of the lumen of the target vessel at the location where the secondary stent elements are deployed. The inclusion of the secondary stent element or elements may improve the crush resistance of the stent system when compared to a stent system without a secondary stent element. The modification of the aspect ratio of the lumen of the target vessel may comprise modification of the aspect ratio to closer to unity. Preferably, the modification of the aspect ratio of the lumen of the target vessel may comprise modification of the aspect ratio to around unity.

The primary stent may be comprised of a structure selected from the group consisting of: a wire mesh; a laser cut hypotube; a slotted tube; and a braided structure. Preferably, the primary stent comprises a braided structure. Both the primary stent and secondary stent element may have a braided structure comprised of a series of interwoven coils.

The secondary stent may be at least 1 mm, suitably 5 mm, optionally up to 10 mm in length. The secondary stent may be at most 30 mm, and typically not more than 20 mm in length. The secondary stent may be comprised of a braided wire. The braided wire may have a cross sectional shape selected from one of the group consisting of: circular; elliptical; hexagonal; square; and rectangular.

The primary and/or secondary stent may comprise one or more retention mechanisms to maintain the relative positions of the secondary stent within the primary stent upon deployment. The one or more retention mechanisms may be selected from the group consisting of: a hook; a tooth; a barb; and a spline.

The secondary stent may further comprise at least one anchor mechanism. The anchor mechanism may be selected from one or more of the group consisting of: a loop; a tap; a bend; one or more gripping members; or a surface modification. The anchor mechanism may be provided at either or both termini of the secondary stent.

The secondary stent may comprise an outward taper at one or both end termini enabling interlocking with the primary stent upon deployment.

A third aspect of the invention provides a stent element comprising: a series of interwoven coils that form a braided structure having a first end and a second end; and at least one of the first or second ends having an outward taper configured so as to engage with spaces between the wires of a previously placed stent in order to anchor the stent element to prevent longitudinal migration. A fourth aspect of the invention provides a method of treating an occlusion of a vessel or duct within the body of a subject, the method comprising:

(a) deploying a primary stent within the occluded vessel at a location that spans the occlusion;

(b) deploying at least one secondary stent element within the primary stent so that the secondary stent element applies a radial chronic outward force upon the primary stent thereby relieving the occlusion and restoring patency to the vessel or duct, wherein the stent element comprises: a series of interwoven coils that form a braided structure having a first end and a second end; and at least one of the first or second ends having an outward taper configured so as to engage with spaces between the wires of a previously placed stent in order to anchor the stent element to prevent longitudinal migration.

The vessel may be a vein. The vein may be comprised within the iliocaval region of the body of the subject. The method may be used to treat one or more of the conditions selected from: venous obstruction; venous stenosis; venous congestion; venous constriction; May-Thurner syndrome; deep vein thrombosis; intraluminal thickening; venous ulcers; Cockett’s syndrome; and chronic pelvic pain.

A fifth aspect of the invention provides a method of treating an occlusion of a vessel or duct within the body of a subject, the method comprising deploying either of the stent systems as described above within the vessel or duct within the body of the subject. The vessel may be a vein. The vein may be comprised within the iliocaval region of the body of the subject. The method may be used to treat one or more of the conditions selected from: venous obstruction; venous stenosis; venous congestion; venous constriction; May-Thurner syndrome; deep vein thrombosis; intraluminal thickening; venous ulcers; Cockett’s syndrome; and chronic pelvic pain.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a graph of compression force of single point load applied to a series of test stents;

Figure 2 shows a photograph of the benchtop set up apparatus for obtaining the data shown in Figure 1 ;

Figure 3 shows a side view of stent element according to one embodiment of the invention.

DETAILED DESCRIPTION

All references cited herein are incorporated by reference in their entirety. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

Prior to setting forth the invention, a number of definitions are provided that will assist in the understanding of the invention.

As used in this description, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, the term “a sensor” is intended to mean a single sensor or more than one sensor or to an array of sensors. For the purposes of this specification, terms such as “forward,” “rearward,” “front,” back, right, left, upwardly, downwardly, and the like are words of convenience and are not to be construed as limiting terms. Additionally, any reference referred to as being “incorporated herein” is to be understood as being incorporated in its entirety.

As used herein, the term "comprising" means any of the recited elements are necessarily included and other elements may optionally be included as well. "Consisting essentially of’ means any recited elements are necessarily included, elements that would materially affect the basic and novel characteristics of the listed elements are excluded, and other elements may optionally be included. "Consisting of’ means that all elements other than those listed are excluded. Embodiments defined by each of these terms are within the scope of this invention.

The term ‘braided’ refers to a metal or metal alloy stent or stent element that is produced using a plain weaving technique. The stent comprises a lumen capable of stretching in the longitudinal direction while circumferentially, the multiplicity of filament-like elements intersect a plane that is perpendicular to the longitudinal direction when in the expanded position. Braided stent elements typically comprise a essentially a series of interwoven coils including some spiralling in the opposite direction.

The term ‘kink resistance’ refers to a stent’s ability to withstand mechanical loads from the surroundings depending upon the position in the body. Usually, this is based upon the smallest radius of curvature a stent can withstand without the formation of a kink. In areas of high tortuosity within the body it is necessary for a stent to have increased kink resistance to prevent a reduction in lumen patency or even total occlusion.

The term ‘crush resistance’ refers to the ability of a stent experiencing external, focal or distributed loads to resist collapse. These loads ultimately lead to stent deformation and even full or partial occlusion which can result in adverse clinical consequences.

The term ‘venous ulcers’ refers to skin sores that form due to the persistent elevation of venous pressure. Often, they present in association with venous valve regurgitation. They are most commonly found on the lower limbs. It is thought that when venous valves become mechanically blocked or veins become engorged and the valve leaflets cannot co-opt to prevent regurgitation of blood, venous congestion worsens and the hydrostatic forces cause both extravasation of fluid from the veins into interstitium, and activation of inflammatory cytokines. This accumulation of fluid pressure and inflammatory cytokines contributes to skin break down, chronic ulceration and predisposes to local infections.

The term ‘venous obstruction’ refers to any occurrence whereby the diameter (or ‘caliber’) of a vein is reduced when compared to a normal, i.e. non-occluded, state. Venous obstruction can occur through the narrowing (stenosis) of the vein, through blockage or through externally applied pressure causing a localised compression of the vein. The term also includes venous occlusion, whereby the vein’s lumen is partially or totally obstructed to the flow of blood. Occlusion may result from thrombosis (e.g. deep vein thrombosis (DVT)) or may be due to tumour incursion.

The term ‘venous return’ is defined by the volume of blood returning to the heart via the venous system, and is driven by the pressure gradient between the mean systemic pressure in the peripheral venous system and the mean right atrial pressure of the heart. This venous return determines the degree of stretch of heart muscle during filling, preload and is a major determinant of cardiac stroke volume.

The term ‘venous compression’ refers to the external compression of the vein. The source of external compression may be caused by an adjacently located artery compressing the vein against another fixed anatomical structure, which can include the bony or ligamentous structures found in the pelvis, the spine itself, or overlapping arterial branches.

The term ‘May-Thurner syndrome’ (MTS) also known as iliac venous compression syndrome (which includes Cockett’s syndrome) is a form of ilio-caval venous compression wherein the left common iliac vein is compressed between the overlying right common iliac artery anteriorly and the lumbosacral spine posteriorly (fifth lumbar vertebra). Compression of the iliac vein may cause a myriad of adverse effects, including, but not limited to discomfort, swelling and pain. Other less common variations of May-Thurner syndrome have been described such as compression of the right common iliac vein by the right common iliac artery; this is known as Cockett’s syndrome. More recently, the definition of May-Thurner syndrome has been expanded to include an array of compression disorders associated with discomfort, leg swelling and pain, without the manifestation of a thrombus. Collectively, this has been termed non- thrombotic iliac vein lesions (NIVL). The term intraluminal thickening (also referred to as venous spurs or intraluminal spurs) is related to this external compression of the left common iliac vein by the right common iliac artery against the fifth lumbar vertebra. Venous spurs arise due to the chronic pulsation of the right common iliac artery, this ultimately results in an obstruction to venous outflow. Venous spurs are internal venous obstructions consequent to chronic external compression of veins by adjacent structures.

The term ‘Deep Vein Thrombosis’ (DVT) refers to the formation of blood clots or thrombus within the venous segment, and in itself is not life threatening. However, it may result in life threatening conditions (such as pulmonary embolism) if the thrombus were to be dislodged and embolize to the lungs. Additionally, DVT may lead to loss of venous valvular integrity, life long venous incompetence and deep venous syndrome which includes rest and exercise pain, leg swelling and recurrent risk of DVT and emboli. The following is a non-limiting list of factors that reflect a higher risk of developing DVT including prolonged inactivity, smoking, being dehydrated, being over 60, undergoing cancer treatment and having inflammatory conditions. Anticoagulation which prevents further coagulation but does not act directly on existing clots, is the standard treatment for deep vein thrombosis. Other potentially adjunct, therapies/treatments may include compression stocking, selective movement and/or stretching, inferior vena cava filters, thrombolysis and thrombectomy.

Stents were first designed for use in the cardiovascular space in the mid-1980s and have since undergone major refinements in design and composition. The indications for stenting and locations of their use in the human body has also developed; stenting of arterial and venous vessels is a regular occurrence in hospitals.

One of the original stents, the Wallstent (Schneider AG), was a self-expanding, stainless steel wire-mesh structure. This was superseded by the Palmaz-Schatz stent (Johnson & Johnson), which was the first FDA approved, balloon-expandable stainless steel slotted tube stent. Multiple stents and stent manufacturers followed shortly after with their own iterations that were designed to prevent elastic recoil and restenosis. These were far from optimal stent designs because they had a high metallic density that resulted in elevated rates of stent thrombosis, failed deployments, embolizations and in- stent restenosis (ISR). For example, restenosis occurred in 20% to 30% of all angioplasties.

Drug-eluting stents (DES) were developed to specifically address the issues of ISR. Seen as the next revolution in interventional cardiology, DES utilized a coating of various compounds to target proliferation of vascular smooth muscle cells, platelet activation, and thrombosis. Many compounds were tried with minimal response, including gold, carbon, heparin, and others such as oestrogen, glucocorticoids, and mineralocorticoids with modest effects. However, the greatest effect was seen in the use of anti-proliferative drugs. Drugs such as Sirolimus and Paclitaxel were the most effective in reducing ISR. This lead to a new generation of stents, stent design, and stent coating combinations. Early signals were very positive, indicating better efficacy when compared with bare metal stents. However in 2006, a safety signal began to emerge of an increased risk of stent thrombosis (ST) in first generation DES. A redesign of the first generation DES lead to a second generation of DES with novel antiplatelet agents and polymers.

While stents were being designed in an attempt to counteract ST, the physical implantation of a stent itself acted as the perfect recipe for thrombus formation, and required the use of complex anticoagulation regimens to combat ST. This caused further problems, leading to major bleeding and vascular complications in many cases. It wasn’t until the development of dual antiplatelet therapy (DAPT) that stents began to become safer to use in common practice.

Stent materials and designs have continued to be developed over the years. First generation stent materials such as stainless steel have been more recently superseded by cobalt-chromium alloys. Cobalt-chromium alloys allow for thinner stent strut designs without compromising radial strength or corrosion resistance of the stent. Other new alloys include platinum-chromium alloys, which are used for high conformability and radial strength and a thinner stent strut design.

In addition to drug coatings and drug developments, stents have also been covered with various synthetic or biological materials in order to cover perforations, aneurysms, or heavy thrombus. Bioresorbable stent scaffolds have also been designed to provide a vascular scaffold following a percutaneous coronary intervention (PCI). The bioresorbable scaffolds are gradually re-absorbed after placement, leaving the vessel in which the scaffold was placed free from any metallic caging and able to regain its normal function. A number of biodegradable compounds have been developed and utilized by various manufacturers for this purpose.

In today’s stent market there exist five different types of available stents:

• Dual therapy Stents (DTS)

• Bioresorbable Vascular Scaffolds (BVS)

• Bio-engineered Stents

• Drug Eluting Stents (DES)

• Bare Metal Stents (BMS)

Their overall main purpose is to keep narrowed blood vessels open to allow adequate flow of blood or other bodily fluid. A special group of stents, called stent grafts are used in the aorta to create a smaller conduit within which the blood can flow, as the original vessel has become enlarged and at risk of rupture. Various applications of stents in the body include:

• Coronary Stents

• Urinary stents

• Urethral and Prostatic stents

• Peripheral vascular stents

• Stent grafts

• Oesophageal stents

• Biliary stents

Hence, embodiments of the devices according to the invention can also be used during endoscopic and laparoscopic procedures where the vessel includes the bile duct, the intestine, the fallopian tubes, the ureter, the urethra, the oesophagus, bronchioles, or any other hollow vessel or duct within the body of an animal.

Venous stents require unique characteristics that differ from arterial stents. Veins are highly flexible and vary in diameter and luminal profile depending upon flow and surrounding structures that may impinge upon them. Veins operate at very low pressures, relative to arteries, therefore it is critical that they are able to expand to accommodate additional flow during exertion. Venous stents must likewise be selfexpanding, flexible and adapt to the changing nature of the veins in which they are placed. Venous walls are prone to deformation due to normal movements such as the overlying musculature, organ function (e.g. peristalsis), as well as the respiratory and cardiac cycle. At the same time, venous stents are placed because there is some obstruction or external compression to be resisted, so they must have appropriate strength to restore luminal flow diameter at the treatment site. Of course, once a stent is implanted the walls of the vein will react to the deformation inherently caused by the device. The interplay of the stent and the externally applied forces may vary along the length of the stent resulting in irregular mechanical interactions along the longitudinal axis. These irregularities can result in stent migration and associated complications.

Despite vast improvement in stent design since the origination of stenting, the stent options currently available on the market are plagued by a number of problems including foreshortening, device collapse, device failure, device wear and eventual perforation. Some of the main underlying factors contributing to the problems with these stents include a lack of flexibility or too much flexibility. Increased load on the deformation of the stent can cause early fatigue failure, and/or impedance of flow in the overlying iliac artery, potentially causing peripheral arterial disease.

• change in vessel angulation post implantation;

• complex lesions; or

• inappropriate stent locations.

In pursuing a more personalised or customized stent design, the inventors have identified that the design of stents itself can only be optimised to an individual patient’s anatomy to a certain extent, and that changes in the vasculature of patients may result in the off-the-shelf design of stent being unsuitable for the vasculature. It is also very difficult for physicians currently to assess the potential success for a given stent to adequately restore luminal diameter. It is only after placement of the chosen stent that a physician may realize that insufficient luminal diameter has been restored, resulting in an ovalized or high aspect ratio lumen, or insufficient force to resist the external compression with no good options for correction or adjustment. Accordingly, the inventors have provided a configurable stenting system having the primary stent and, within the primary stent at least one secondary stent, referred to hereafter as a stent element’, that provides a localised change in the physical and/or mechanical properties of the primary stent. In embodiments of the invention the stent element, when deployed, bears upon the inner luminal surface of the primary stent exerting a radial outward expansive force.

Typically, the stent elements are, as described herein, secondary stents for placement wholly within and encompassed by the primary stent. Therefore, the stent elements may be formed as a stent by any suitable means and in by any appropriate method. In a specific embodiment, both the stent element and the primary stent may be comprised of braided wire. The stent element may be formed of one or more wires, arranged to provide the optimal radial force and in the desired shape to restore the required aspect ratio of the stent and surrounding vessel. Hence, the wire may be formed of any appropriate material, in any cross-section, to provide the desired effect. Different shapes of wire in cross section (e.g. round, elliptical, hexagonal, square and/or rectangular) allows for different design characteristics in the stent element. For instance, a flat rectangular wire or ribbon, may allow for better engagement with the primary stent than a round wire which may slip. Oval wires may allow for added strength without increasing the overall thickness of the device. A stent element according to one embodiment of the invention is shown in Figure 3. In Figure 3, the stent element shows one end of the device that has modifications including an outward flare as well as a plurality of gripping members (circular appendages on the braided wire) that serve to improve anchoring/retention within the primary stent - see further description below.

In general, the radial force and crush resistance of the stent element is based on and controllable by varying the thickness and cross sectional shape of the wire, type of wire, construction of the element through twists or braids, angle of the braiding pattern relative to the primary stent and other properties of the wire forming the stent element. The properties of the stent element may be controlled by selecting specific braiding patterns or the specific number of coil turns or twists necessary to achieve a desired outward radial force. In embodiments of the invention, the braid angle of the stent element is selected to be between 120 and 160 degrees, more suitably between 135 and 150 degrees relative to the angle of opposing wire coils within the primary stent, stent elements of embodiments of the invention exert an outward radial force of greater than 0.25 N/cm, suitably at least 0.5 N/cm, typically at least 1.0 N/cm and optionally at least 2 N/cm. In further embodiments of the invention, the stent elements exert an outward radial force of at most 25 N/cm, suitably at most 20 N/cm, optionally at most 15 N/cm. In embodiments of the invention the outward radial force is determined at greater than 50% expansion of the stent element, or alternatively at between 10% and 50% of the expansion.

It is envisaged that in specific embodiments the individual stent elements will be at least 1 mm, suitably 5 mm, optionally 10 mm in length; and at most 30 mm, and typically not more than 20 mm in length.

In each of the above embodiments, the end termini of the wire or material forming the stent element may be free. In other examples, the free ends can be joined or locked together either prior to positioning within the primary stent or once deployment of the stent element within the primary stent is complete. Joining or locking the ends prevents the free ends from perforating the vessel walls or snagging the deployment device or guide wire, adds strength to the ends of the stent elements, adds strength to the entire length of the stent element, and improves the stability of the reinforcing stent element.

The stent elements and/or the primary stent may incorporate one or more physical mechanisms to maintain the relative positions of the stent elements and primary stent. In specific embodiments the retention mechanisms may incorporate one or more hooks, teeth, barbs or splines that engage with or bear upon the primary stent and prevent malposition or subsequent migration of the stent element. In general, it is expected that friction between the stent elements and the primary stent and the outward radial bias force exerted by the stent elements on the interior face of the primary stent will be sufficient to maintain the stent elements in position. However, a further mechanism may be particularly useful to ensure that the relative positions are maintained even through the continual movement and changes in vasculature that occur with everyday activity. The mechanism may anchor the stent element to the primary stent. In some embodiments, the stent element may incorporate the mechanism so that the stent element is attached to and grips the primary stent. In some embodiments, the primary stent may incorporate a mechanism so that the stent element is gripped or anchored by the primary stent. In some embodiments, both the stent element and the primary stent incorporate parts of the mechanism so that there is an interaction between two parts of the mechanism to anchor the stent element to the primary stent. The stent element may incorporate the anchoring mechanisms to prevent its migration relative to the primary stent and/or migration relative to the surrounding vessel. These mechanisms include but are not limited to loops, taps, bends, gripping members, or surface modifications of various shapes and designs. The anchor mechanisms may be provided at either or both termini of the stent element or anywhere along the length of the stent element. In a specific embodiment, the stent element may comprise an outward taper, or ‘flare’, at one or both end termini enabling improved outward bias and interlocking with the braids or laser cut sections of the outer primary stent.

In a particular embodiment, the stent elements may comprise flexible hooks arranged about the circumference of a coil. Each hook faces the same direction. The stent element can therefore be mounted to and affixed to the primary stent by positioning it and rotating in the direction of the hooks so that they hook onto the primary stent.

In some embodiments, the primary stent may incorporate an engaging mechanism for engaging the stent element and maintaining its position relative to it. Similarly to the anchoring mechanism of the stent element, the engaging mechanism may comprise loops, taps, bends, gripping elements of various shapes and designs. The engaging mechanism may interact with the anchoring mechanism of the stent element or may engage the stent element without a specific anchoring mechanism.

In one example, the engaging mechanism comprises have internally-extending hooks that all face in one direction longitudinally along the stent that are configured to hook onto the wire of the stent element. To engage the stent element, the stent element is maneuvered along the primary stent, and once the correct position for the stent element is reached, it is pulled backwards slightly to engage with the hooks, thereby connecting the stent element to the primary stent.

In one embodiment of the present invention, the primary stent may include one or more coupling elements to prevent migration of the primary stent within the vessel. The coupling elements may be provided at one or both termini of the primary stent.

It is also possible for the physical mechanisms of the stent elements described above to be used to join together two primary stent portions in order to prevent migration or separation of either of the primary stent portions. This beneficially allows for the accurate deployment of shorter primary stent sections to create a longer stented segment. It also helps avoid those primary stent portions being deployed inside each other, which creates stiffer, less flexible segments of the stent system. The deployment of the stent element therefore can help reduce the risk of stent fractures in such a scenario. Preferably, a short, lower pitch angled stent element may be deployed for this purpose.

The ends of the stent element may either be angled forwards, to interlock with the braid of both the primary stent portions, or may be angled backwards to give extra locking strength and migration prevention.

The stent element may also be placed at the end of a primary stent to provide additional expansive force or to further prevent migration of the primary stent. The ends, or termini, of the stent elements may have different designs. For example, one end of the stent element may have a high pitch angle to provide a greater outward force. Conversely, one end of the stent element may have a very low pitch angle and a very open weave in order to allow blood flow and to prevent clotting. One end of the stent element may be less braided to deliver a more open structure. This might usefully be applied at a confluence between two blood vessels so that the stent system is fixed in place without restricting or impacting blood flow.

To aid positioning of the stent elements, radiopaque markers may be provided along the length of the primary stent to indicate relative positions. The stent elements may also comprise a radiopaque material, or radiopaque zones.

The stent element and/or the primary stent may comprise of, either separately or in combination, stainless steel, nitinol, cobalt chromium, tantalum, platinum, tungsten, iron, manganese, molybdenum, or other surgically- and bio- compatible metal or metal alloy. The stent element and/or the primary stent may comprise non-metal material, including a polymer such as: a bioresorbable material such as poly (l-lactide) (PLLA), polyglycolic acid (PGA), polyglycolic-lactic acid (PLGA), polycaprolactone (PCL), polyorthoesters, polyanhydrides, or another aliphatic polyester fibre material; polypropylene; polyamide; carbon fibre; and glass fibre. In some embodiments, the stent element and/or the primary stent comprise both metal and non-metal portions. The stent element and/or the primary stent may comprise radiopaque markers to assist with optimal placement and orientation longitudinally and/or radially. Such radiopaque material may include titanium, tantalum, rhenium, bismuth, silver, gold, platinum, iridium, and/or tungsten.

The primary stent or portions of the primary stent may be covered. Such covering material may include: PTFE; e-PTFE; polyurethane; silicone; papyrus; dacron®; goretex®; other polymeric membrane; polyhedral oligomeric silsesquioxane and poly(carbonate-urea) urethane (POSS-PCU); other Biodegradable nanofibers. The stent element or portions of the stent element may be covered. Covering material may include any of the above-referenced covering materials.

In specific embodiments of the invention, the primary stent may contain a window or cell of increased size and identified by radiopaque markers to allow for the creation of an anastomosis shunting device with an adjacent vessel or duct, without requiring the perforation of the primary stent structure.

The primary stent and/or the stent element may comprise of a drug coating or combination of drug coating and graft covering to promote re-endothelization; improve endothelial function; reduce inflammatory reaction; inhibit neo-intimal hyperplasia (MM2A); prevent adverse events such as in-stent restenosis and stent thrombosis through antithrombotic action of heparin.

It will be appreciated that radial expansion mechanisms to deploy the stent element within a primary stent already located in situ may be implemented such as by introducing the stent element over a radially expandable bladder or balloon catheter device. In such an embodiment the bladder or balloon may be located appropriately in the location for deployment within a primary stent within a vessel and expanded to position the stent element appropriately. Upon deflation of the bladder or balloon the device may be withdrawn from the vessel leaving the stent element in place.

The above stent system including at least one primary stent and one or more stent elements may be particularly useful in the venous system. For example, the system may be particularly useful at locations of venous obstruction, which includes, at least, venous stenosis, venous congestion, and venous constriction. The stent system described herein may be used in the treatment of MTS, DVT, intraluminal thickening, venous ulcers, venous compression, and/or any other venous or arterial obstruction. Any of the aforementioned may contribute to chronic pelvic pain in a patient, which may be treated by the systems and methods of the invention. In an embodiment of the invention, the patient may be an adult female who suffers from chronic pelvic pain. Suitably the chronic pelvic pain may be a venous compression and/or any other venous or arterial obstruction that has arisen post-partum, or as a result of a gynaecological condition such as endometriosis.

According to one non-limiting example, an individual may have no apparent signs or symptoms of leg swelling but, nevertheless, an obstruction of the veins in the ilio-caval region may be suspected. Normal anatomy in this region sees the vein assume an upward sigmoidal curve from the femoral vein to the inferior-vena cava. It would be apparent to the skilled person that relieving the obstruction in this region by implanting a stent with low flexibility and high crush resistance would profoundly alter the local anatomy and may not be in the best interests of the body. For instance, application of a stent with fixed radial force I compression force, would most likely straighten out this region of the vein alleviating compression from the overriding artery. In the longer term this could induce restenosis and intimal hyperplasia resulting in stent failure and more severe venous occlusion. However, according to embodiments of the present invention described in more detail above a more promising solution would involve implanting a highly flexible primary stent to cover this region of the vein and, in so doing, maintain as normal vein positioning and orientation as possible (i.e. not moving or dislocating it).

Although particular embodiments of the invention have been disclosed herein in detail, this has been done by way of example and for the purposes of illustration only. The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims, which follow. It is contemplated by the inventors that various substitutions, alterations, and modifications may be made to the invention without departing from the spirit and scope of the invention as defined by the claims. In addition, the above described embodiments may be used in combination unless otherwise indicated.

The invention is further exemplified in the following non-limiting example. EXAMPLE

Bench top testing evaluation of predicate stents to evaluate the amount of force (grams) to overcome a single point load was performed using the apparatus shown in Figure 2. Analysis resulted in a wide variance between stent manufacturers and stent performance to overcome the single point load as shown in Figure 1 (crush resistance). In comparison to arteries veins lack a strong muscular wall for the stent to expand out against. As a result of this stents that fail to overcome the external compression, typically take on an ovalized shape as there is a distinct lack of wall strength to prevent the stent response. As detailed in our previous application this is failure to overcome the external compressive force occurs quite commonly in venous stenting in the range of 20 to 65% of cases. Sometimes the failed primary stent can be salvaged through simple ballooning techniques to force the stent into its natural pre-formed expanded diameter and fully open configuration, which likely allows some tissue ingress and support from collapsing. Some stents though, 20% on average, fail to achieve the desired venous flow and currently cannot be fixed, leaving the patient with no good options.

During this bench top evaluation it was determined that it was not necessarily that the primary stent was collapsing, it was that the vein was not being supported by the external structure to be able to support itself, under the load. This means there is no response from the venous wall to counter-act the force from the single point load. As such, to restore a failed primary stent, or to add more radial force to a venous stent, it is a counterintuitive fix. There is a need to generate a radial outward force at the point of the constriction rather than simply adding more strength to the side walls of the stent.

There are various options proposed to address this problem. The first is to provide a stent that has a sheath or covering along a section or sections of the stent where the force to be overcome is the greatest. In such a way, the stent will be supported by an outer covering that will prevent the stent from buckling and ovalizing.

Secondly, to recover a failed stent that has already been implanted, it may be necessary to implant a stent within a stent to “jack” the base existing stent radially outwardly. The challenge here is that placing a conventional expander device in the direct line of blood flow would hinder blood flow and become an impediment, with a significant potential for clot and thrombus formation. The current embodiment is a braided structure, which is essentially a series of interwoven coils including some spiraling in the opposite direction. In addition, the current embodiment uses nitinol wire as the preferred material as it can be shape-set with heat to form small bends at each wire crossing. This further locks in the braided structure and reduces wire shifting during crush stress. Finally, the braid angle of the supporting stent element is chosen to be high, preferably between 120 and 160 degrees, more preferably between 135 and 150 degrees when measuring the included angle relative to the opposing wire coils of the primary stent. The support stent element is sized to fit the intended final diameter of the primary stent so that the high braid angle at nearly full expansion is achieved. This provides maximum crush resistance. The current embodiment also preferably includes engagements at both ends of the stent element that engage the structure of the base stent to prevent migration and to lock in the braid angle. This also allows a balloon, or other expansion means to expand the stent element, forcing the support stent element to foreshorten and increase the braid angle. The end engagements then lock in the support stent to prevent elongation and lowering of braid angle. The end engagement feature maybe an outward flaring or belling of the ends. The flared ends would be used preferably with open ended braid wires, with or without attaching radiopaque marker sleeves.

The described balloon or other expansion of the braid to achieve foreshortening and braid angle increase may be performed on the support stent element as well as the primary (base) stent. The expansion may be performed along the entire length of the stents or it may be performed only locally, at the site of needed increased crush strength and radial expansion.

A thirdly, a system may be provided where the amount of force to overcome is known via a measured metric and with this information an appropriate primary base stent of appropriate length and diameter can be placed and then a bracing segment of known and appropriate crush resistance (the secondary stent element) can be placed within the base stent to completely overcome the compression.

The current standard of care for treating failed stents is to implant another stent covering the site of failure of the primary stent following balloon inflations.

There now follows a summary of core features of a stent element system according to embodiments of the present invention: • Altering braid angle of the stent to increase crush resistance o High braid angle heat-set in the device and adjustable after placement o Different braid angles may be heat-set into the support stent element or the primary base stent or other manufacturer stent to allow more flexible ends and more rigid mid-section o Can be specific to the amount of force being overcome - a selection of pitch angles for the braiding pattern of the stent element

■ Alternatively the stent element may be expanded or contracted longitudinally so that the pitch angle is naturally steeper or shallower to alter the amount of expansive/compressive force.

• Can be laser cut Nitinol or braided from any compatible metal

• Can be balloon mounted or can be deployed

• Can be self-expanding

• Has a flaring at one or both ends to lock into the base stent braid I structure to prevent migration.

• Can be post dilated with a balloon

• Can be round in end on shape I Design, but can also be a polygon of 6/8/12/14/16/18/20 facets. o Shape adds rigidity and also helps prevent migration by locking into the base braid

• Can be used for both arterial and venous applications

• Can be used to recover failed stents where the base stent cannot overcome the force of the external compression or internal obstruction to flow, or fails to expand normally during deployment and requires a further stent placement to repair.

• Can be used as part of a system where a focused amount of known force is required to overcome an external compression or internal obstruction to flow.

• Can be built in various lengths, diameters and degrees of crush resistance by altering the pitch angle where a lower pitch angle gives a lower crush resistance vs that a higher pitch angle. o This is important as increasing the stiffness of the system

• May have a covering of a biocompatible material o Which may or may not have a coating

■ Where the coating may be for drug delivery ■ Where the drug maybe anti-inflammatory

■ Where the drug maybe for some other purpose

• May have a coating of the individual wires within the braid structure to achieve any of the above benefits • May have a covering of biocompatible material such as Gore-Tex® etc o Gives support to the stent structure o This support is missing in veins as they are extremely thin walled, unlike arteries. o Addition of the covering prevents the stent from flattening and helps it maintain its pre-formed shape.

The invention is now followed by claims that define the scope of protection. However, it should be appreciated that the claims are not limiting upon the above description and that multiple embodiments described herein as well as combinations of those embodiments are also conceived of as being within the definition of the invention.

Claims

1 . A stent system comprising: a primary stent for location in a lumen of a target vessel, the primary stent defining an exterior surface that contacts a vessel wall and an interior surface that faces inwardly; and at least one secondary stent element deployable wholly within the primary stent and configured to engage with the interior surface of the primary stent, wherein the secondary stent element has a braided structure, wherein the at least one secondary stent element is configured to apply a chronic outward radial force to the interior surface of the primary stent so as to effect modification an aspect ratio of the lumen of the target vessel at the location where the secondary stent element is deployed.

2. The stent system of claim 1 , wherein the modification of the aspect ratio of the lumen of the target vessel comprises modification of the aspect ratio to closer to unity.

3. The stent system of claim 1 , wherein the modification of the aspect ratio of the lumen of the target vessel comprises modification of the aspect ratio to around unity.

4. The stent system of any one of claims 1 to 3, wherein the primary stent is comprised of a structure selected from the group consisting of: a wire mesh; a laser cut hypotube; a slotted tube; and a braided structure.

5. The stent system of claim 4, wherein the primary stent comprises a braided structure.

6. The stent system of any one of claims 1 to 5, wherein the primary stent and secondary stent element have a braided structure comprised of a series of interwoven coils.

7. The stent system of claim 6, wherein the braid angle of the series of interwoven coils of the secondary stent element relative to the primary stent is at least 120 degrees when the secondary stent is deployed within the primary stent.

8. The stent system of claim 6, wherein the braid angle of the series of interwoven coils of the secondary stent element relative to the primary stent is at most 160 degrees when the secondary stent element is deployed within the primary stent.

9. The stent system of claim 6, wherein the braid angle of the series of interwoven coils of the secondary stent element relative to the primary stent is between about 135 and about 150 degrees when the secondary stent element is deployed within the primary stent.

25 The stent system of any one of claims 1 to 9, wherein when deployed within the primary stent the secondary stent element exerts an outward radial force of greater than 0.25 N/cm, suitably at least 0.5 N/cm, typically at least 1 .0 N/cm and optionally at least 2 N/cm. The stent system of any one of claims 1 to 10, wherein when deployed within the primary stent the secondary stent element exerts an outward radial force of at most 25 N/cm, suitably at most 20 N/cm, optionally at most 15 N/cm. The stent system of either one of claims 10 or 11 , wherein the outward radial force is determined at greater than 50% expansion of the secondary stent element. The stent system of either one of claims 10 or 11 , wherein the outward radial force is determined at between about 10% and about 50% of the expansion of the secondary stent element. The stent system of any one of claims 1 to 13, wherein the secondary stent element is at least 1 mm, suitably 5 mm, optionally up to 10 mm in length. The stent system of any one of claims 1 to 14, wherein the secondary stent element is at most 30 mm, and typically not more than 20 mm in length. The stent system of any one of claims 1 to 15, wherein the secondary stent element is comprised of a braided wire. The stent system of claim 16, wherein the braided wire has a cross sectional shape selected from one of the group consisting of: circular; elliptical; hexagonal; square; and rectangular. The stent system of any one of claims 1 to 17, wherein the primary stent and/or secondary stent element comprises one or more retention mechanisms to maintain the relative positions of the secondary stent element within the primary stent upon deployment. The stent system of claim 18, wherein the secondary stent element comprises one or more retention mechanisms. The stent system of either one of claims 18 or 19, wherein the one or more retention mechanisms is selected from the group consisting of: a hook; a tooth; a barb; and a spline. The stent system of any one of claims 1 to 20, wherein the secondary stent element comprises at least one anchor mechanism. The stent system of claim 21 , wherein the anchor mechanism is selected from one or more of the group consisting of: a loop; a tap; a bend; one or more gripping members; or a surface modification. The stent system of claim 21 or 22, wherein the anchor mechanism is provided at either or both termini of the secondary stent element. The stent system of claim 21 , wherein the secondary stent element comprises an outward taper at one or both end termini enabling interlocking with the primary stent upon deployment. The stent system of any one of claims 1 to 24, wherein a plurality of radiopaque markers are positioned along the length of the primary stent. The stent system of any one of claims 1 to 25, wherein the secondary stent element comprises a radiopaque material. The stent system of any one of claims 1 to 26, wherein primary stent is comprised of a surgically- and bio- compatible metal or metal alloy, suitably comprising one or more of the group consisting of: stainless steel; nitinol®; cobalt chromium; tantalum; platinum; tungsten; iron; manganese; and molybdenum. The stent system of any one of claims 1 to 27, wherein the primary stent or portions of the primary stent are coated with a material selected from the group consisting of: PTFE; e-PTFE; polyurethane; silicone; papyrus; dacron®; goretex®; polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU); or biodegradable nanofiber material. The stent system of any one of claims 1 to 28, wherein secondary stent element is comprised of a surgically- and bio- compatible metal or metal alloy, suitably comprising one or more of the group consisting of: stainless steel; nitinol®; cobalt chromium; tantalum; platinum; tungsten; iron; manganese; and molybdenum. The stent system of any one of claims 1 to 29, wherein the secondary stent element or portions of the secondary stent element are coated with a material selected from the group consisting of: PTFE; e-PTFE; polyurethane; silicone; papyrus; dacron®; goretex®; polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU); or biodegradable nanofiber material. A stent system for restoring patency to a fully or partially occluded target vessel within the body of a subject, the system comprising: a primary stent for location in a lumen of the target vessel, the primary stent defining an exterior surface that contacts a vessel wall and an interior surface that faces inwardly; a plurality of secondary stent elements deployable wholly within the primary stent and configured to engage with the interior surface of the primary stent wherein the secondary stent elements have a braided structure, wherein the plurality of secondary stent elements are configured to apply a chronic outward radial force to the interior surface of the primary stent so as to effect modification of an aspect ratio of the lumen of the target vessel at the location where the secondary stent elements are deployed. The stent system of claim 31 , wherein the modification of the aspect ratio of the lumen of the target vessel comprises modification of the aspect ratio to closer to unity. The stent system of claim 31 , wherein the modification of the aspect ratio of the lumen of the target vessel comprises modification of the aspect ratio to around unity. The stent system of any one of claims 31 to 33, wherein the primary stent is comprised of a structure selected from the group consisting of: a wire mesh; a laser cut hypotube; a slotted tube; and a braided structure. The stent system of claim 34, wherein the primary stent comprises a braided structure. The stent system of any one of claims 31 to 35, wherein the primary stent and secondary stent elements have a braided structure comprised of a series of interwoven coils. The stent system of claim 36, wherein the braid angle of the series of interwoven coils of the secondary stent elements relative to the primary stent is at least 120 degrees when the secondary stent elements are deployed within the primary stent. The stent system of claim 36, wherein the braid angle of the series of interwoven coils of the secondary stent elements relative to the primary stent is at most 160 degrees when the secondary stent elements are deployed within the primary stent. The stent system of claim 36, wherein the braid angle of the series of interwoven coils of the secondary stent elements relative to the primary stent is between about 135 and about 150 degrees when the secondary stent elements are deployed within the primary stent. The stent system of any one of claims 31 to 39, wherein when deployed within the primary stent the secondary stent elements exert an outward radial force of greater than 0.25 N/cm, suitably at least 0.5 N/cm, typically at least 1 .0 N/cm and optionally at least 2 N/cm. The stent system of any one of claims 31 to 40, wherein when deployed within the primary stent the secondary stent elements exert an outward radial force of at most 25 N/cm, suitably at most 20 N/cm, optionally at most 15 N/cm. The stent system of either one of claims 40 or 41 , wherein the outward radial force is determined at greater than 50% expansion of the secondary stent elements.

28 The stent system of either one of claims 40 or 41 , wherein the outward radial force is determined at between about 10% and about 50% of the expansion of the secondary stent elements. The stent system of any one of claims 31 to 43, wherein the secondary stent elements are at least 1 mm, suitably 5 mm, optionally up to 10 mm in length. The stent system of any one of claims 31 to 44, wherein the secondary stent elements are at most 30 mm, and typically not more than 20 mm in length. The stent system of any one of claims 31 to 45, wherein the secondary stent elements are comprised of braided wire. The stent system of claim 46, wherein the braided wire has a cross sectional shape selected from one of the group consisting of: circular; elliptical; hexagonal; square; and rectangular. The stent system of any one of claims 31 to 47, wherein the primary and/or secondary stent elements comprises one or more retention mechanisms to maintain the relative positions of the secondary stents within the primary stent upon deployment. The stent system of claim 48, wherein the secondary stent elements each comprise one or more retention mechanisms. The stent system of either one of claims 48 or 49, wherein the one or more retention mechanisms is selected from the group consisting of: a hook; a tooth; a barb; and a spline. The stent system of any one of claims 31 to 50, wherein the secondary stent elements each comprise at least one anchor mechanisms. The stent system of claim 51 , wherein the anchor mechanism is selected from one or more of the group consisting of: a loop; a tap; a bend; one or more gripping members; or a surface modification. The stent system of claim 51 or 52, wherein the anchor mechanism is provided at either or both termini of each of the secondary stent elements. The stent system of claim 51 , wherein each of the secondary stent elements comprises an outward taper at one or both end termini enabling interlocking with the primary stent upon deployment. The stent system of any one of claims 31 to 54, wherein a plurality of radiopaque markers are positioned along the length of the primary stent.

29 The stent system of any one of claims 31 to 55, wherein the secondary stent elements comprise a radiopaque material. The stent system of any one of claims 31 to 56, wherein primary stent is comprised of a surgically- and bio- compatible metal or metal alloy, suitably comprising one or more of the group consisting of: stainless steel; nitinol®; cobalt chromium; tantalum; platinum; tungsten; iron; manganese; and molybdenum. The stent system of any one of claims 31 to 57, wherein the primary stent or portions of the primary stent are coated with a material selected from the group consisting of: PTFE; e-PTFE; polyurethane; silicone; papyrus; dacron®; goretex®; polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU); or biodegradable nanofiber material. The stent system of any one of claims 31 to 58, wherein secondary stent elements are comprised of a surgically- and bio- compatible metal or metal alloy, suitably comprising one or more of the group consisting of: stainless steel; nitinol®; cobalt chromium; tantalum; platinum; tungsten; iron; manganese; and molybdenum. The stent system of any one of claims 31 to 59, wherein the secondary stent elements or portions of the secondary stents are coated with a material selected from the group consisting of: PTFE; e-PTFE; polyurethane; silicone; papyrus; dacron®; goretex®; polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU); or biodegradable nanofiber material. A stent element comprising: a series of interwoven coils that form a braided structure having a first end and a second end; and at least one of the first or second ends having an outward taper configured so as to engage with spaces between the wires of a previously placed stent in order to anchor the stent element to prevent longitudinal migration. A method of treating an occlusion of a vessel or duct within the body of a subject, the method comprising:

(b) deploying at least one secondary stent element within the primary stent so that the secondary stent element applies a radial chronic outward force upon the primary stent thereby relieving the occlusion and restoring patency to the vessel or duct, wherein the stent element comprises: a series of interwoven coils that form a braided structure having a first end and a second end; and

30 at least one of the first or second ends having an outward taper configured so as to engage with spaces between the wires of a previously placed stent in order to anchor the stent element to prevent longitudinal migration. The method of claim 62, wherein the vessel is a vein. The method of claim 63, wherein the vein is comprised within the iliocaval region of the body of the subject. The method of claim 64, wherein the method is to treat one or more of the conditions selected from: venous obstruction; venous stenosis; venous congestion; venous constriction; May- Thurner syndrome; deep vein thrombosis; intraluminal thickening; venous ulcers; Cockett’s syndrome; and chronic pelvic pain. A method of treating an occlusion of a vessel or duct within the body of a subject, the method comprising deploying a stent system as claimed in claim 1 within the vessel or duct within the body of the subject. The method of claim 66, wherein the vessel is a vein. The method of claim 67, wherein the vein is comprised within the iliocaval region of the body of the subject. The method of claim 68, wherein the method is to treat one or more of the conditions selected from: venous obstruction; venous stenosis; venous congestion; venous constriction; May- Thurner syndrome; deep vein thrombosis; intraluminal thickening; venous ulcers; Cockett’s syndrome; and chronic pelvic pain. A method of treating an occlusion of a vessel or duct within the body of a subject, the method comprising deploying a stent system as claimed in claim 31 within the vessel or duct within the body of the subject. The method of claim 70, wherein the vessel is a vein. The method of claim 71 , wherein the vein is comprised within the iliocaval region ofthe body of the subject. The method of claim 72, wherein the method is to treat one or more of the conditions selected from: venous obstruction; venous stenosis; venous congestion; venous constriction; May- Thurner syndrome; deep vein thrombosis; intraluminal thickening; venous ulcers; Cockett’s syndrome; and chronic pelvic pain.

31