WO2011158045A1

WO2011158045A1 - Stent-graft

Info

Publication number: WO2011158045A1
Application number: PCT/GB2011/051148
Authority: WO
Inventors: Peter William Phillips
Original assignee: Lombard Medical Limited
Priority date: 2010-06-18
Filing date: 2011-06-20
Publication date: 2011-12-22
Also published as: GB201010246D0

Abstract

A tubular medical implant is provided comprising a first longitudinal section having a supported section which is crescent-shaped in cross section. The supported section may be defined by a stent, and the implant may also comprise graft material with a circular cross section. The implant can be implanted in an artery alongside a tubular implant having a circular cross-section.

Description

Stent-Graft

The present application describes a reinforced vascular graft, sometimes known as a stent-graft which is used in vascular surgery. Particularly, this disclosure describes a stent graft which is designed to be used singly or multiply in conjunction with a primary stent graft such that the present invention stent graft lies in the same blood vessel (the main vessel) alongside said primary stent graft for part of its length, is easily deformed to a non circular cross-section for at least the part of its length that lies alongside the primary stent graft and is sufficiently flexible that it can be deflected away from the second stent graft to lie within a vessel branching from the main vessel that is covered by the main stent graft. Of essence to this disclosure is the ability of the said stent graft to have a cross section that will conform and seal both within the branch vessel and in the space it occupies between the primary stent graft and the wall of the main vessel. The construct formed by the present invention and the primary stent graft provides the ability to place stent grafts in vessels that branch without requiring the use of a branched implant.

The use of stent-grafts to treat aortic aneurysms has been established for many years and a number of methods for preserving blood flow into smaller branch arteries, such as renal, superior mesenteric, celiac, subclavian, carotid or brachiocephalic, have been explored. Cook Inc have developed a product which has holes cut into the wall of the stent graft, said holes being aligned with the entrances to branch arteries. Once aligned, a stent is passed from the lumen of the stent graft through the hole in the stent-graft' s wall and into the branch artery but ensuring that several millimeters of the stent remain within the lumen of the stent graft. An over- sized high pressure balloon is used to expand the stent in the branch artery and to dilate the portion of the stent remaining in the lumen of the stent graft. Usually, the projecting part of the stent is over-dilated so that the stent is made to act as a rivet, attaching the wall of the stent graft directly to the entrance to the branch artery. This technique is effective but involves precise measurement of the patient' s anatomy, exact cutting and placing of the holes in the wall of the stent graft, a stent graft that is custom manufactured for a particular patient and a complex delivery process involving multiple guide wires that are arranged to pass into different lumens. The procedure has been said to require the surgeon to have 2 brains, 4 eyes and 4 hands to complete.

More recently, Dr Chuter of UCSF has developed a 'snorkel' technique in which he places a flexible stent partially in the entrance to a branch vessel (as shown in Figures 1 and 2). The stent graft is then deployed over the pre-placed stent such that the stent is bent in to the blood flow and creates a deliberate leakage path down the side of the stent graft to allow continued perfusion of the branch vessel. This technique overcomes many of the issues of the fenestration method noted above but requires significant lengths of overlap between the stent- graft, the artery and the 'snorkel' stent to avoid unwanted leaks.

A third method of handling branch arteries, also pioneered by Dr Chuter and Cook Inc involves attaching small, highly flexible grafts or stent-grafts to the wall of the main stent graft and guiding these branches into the branch vessels during deployment. This technique still requires a skill level during deployment of a similar order to placing a fenestrated graft, however, the flexibility of the branched graft avoids the need for accurate measurement and custom manufacture required in a fenestrated device.

In a first aspect of the present invention, there is provided a tubular medical implant comprising a first longitudinal section having a supported section which is crescent- shaped in cross section. The supported section is preferably defined by filamentary reinforcing material. In a preferred embodiment, the supported section is defined by a stent and the first longitudinal section additionally comprises flexible graft material which is substantially circular in cross-section, the outer curve of said crescent having a radius which is substantially the same as the radius of said circle.

A crescent (also called a "lune" in planar geometry) is a convexo-concave shape formed from two intersecting circles. The realization of the present invention is that a medical implant having a crescent-shaped supported section can be interposed between the concave inner wall of an artery and the convex outer wall of an aligned primary implant (e.g. stent graft) and that the resulting arrangement allows blood flow into a branch artery to be preserved whilst minimizing leakage in the principal artery.

It will be appreciated that the implant in question can comprise graft material having a circular cross-section which can be compressed against the convex wall of the main stent graft until the crescent-shaped supported stent section contacts the convex wall.

US patent publication no. US 2008/0015672 (Binford) discloses a graft (not a stent-graft) which has a D-shaped internal cross-section in order to reduced turbulent blood flow and thereby reduce blood clots. The graft also has a D-shaped external cross-section. As such, it could not be employed between an implant with a circular cross-section and a concave artery wall, as this would result in blood leakage in the principal artery.

US patent no. 6,214,037 discloses a stent for decompressing an obstructed passage within the body to facilitate stone passage therethrough. One of the embodiments is crescent- shaped in cross-section but is not tubular and could not be employed in the field of the present invention.

In a second aspect of the present invention, there is provided a tubular medical implant comprising flexible material in the form of a tube and filamentary reinforcing material for the tube, wherein on at least a section of the tube the reinforcing material has a crescent shape when viewed in cross-section along the longitudinal axis of the tube.

In a third aspect of the present invention, there is provided a tubular medical implant comprising a first longitudinal section with a first peripheral section having a first resistance to radial compression and a second peripheral section having a second resistance to radial compression, the first resistance being greater than the second resistance, wherein the first peripheral section occupies more than 50% of the periphery of said longitudinal section.

In a particularly preferred embodiment, the internal cross-section of the implant in its uncompressed configuration is substantially circular.

In a fourth aspect of the present invention, there is provided a method for using an implant as defined above in combination with a second tubular medical implant having a circular cross-section, wherein both implants are implanted in a body lumen so that the first implant is interposed between the second implant and the lumen, with the outer curve of the crescent contacting the lumen and the inner curve of the crescent contacting the second implant.

The subject of this invention enhances the 'snorkel' technique described above by creating a replacement for the flexible stent (the snorkel) that is designed specifically to lie alongside a stent graft and to provide an effective seal without the need for excessive lengths of overlap between the stent graft, the vessel and the snorkel.

The snorkel here described is preferably of tubular construction with a wall made of a conventional material for vascular grafting, such as woven polyester or expanded PTFE. In common with existing stent grafts, the wall of the snorkel preferably carries re- enforcements that are designed to urge the wall of the snorkel outwards, said reinforcements usually being metallic tubular shaped springs with a radial spring action. Wire rings and helices have been used successfully in stent grafts by Lombard Medical pic (see for example the subject matter of WO 99/37242), as have 'Z-stents' such as the Gianturco (Cook Inc) or expanded mesh designs such as the Palmaz (Cordis Inc) or the Aneurx (Medtronic Inc). These existing designs provide a uniform radial force around the circumference of the stent, yielding a circular cross section to the stent graft.

The snorkel of the present invention preferably requires that the radial force of the stent structure is not uniform around the circumference of at least a length of the snorkel. The design is arranged so that the aspect of the snorkel that lies against the stent graft has a lower radial force than the rest of the wall at that axial position, allowing the wall to become concave where it lies against the stent graft but to remain convex where it lies against the internal wall of the blood vessel. In this way, the cross section of the snorkel, where it lies against the stent graft will be crescent shaped and will provide improved apposition between the snorkel and the stent graft to minimize leakage. The cross sectional area of the snorkel can be arranged to be high without the need to lift the stent graft a large distance from the native wall of the vessel.

Advantageously, the snorkel is arranged to have different radial force characteristics along its length so that, for example, a first section of the snorkel can be designed to have uniform radial force around its circumference so as to fit and seal uniformly within the lumen of a branch vessel.

A second section of the snorkel may be required to be especially flexible in the region of the snorkel where it emerges from the branch vessel and bends sharply into the blood flow. A third section of the snorkel will be required to lie alongside the stent graft and will have non-uniform radial force around its circumference as described above. This section is present in all embodiments of the snorkel. A fourth section of the snorkel may revert to a uniform radial force to provide a circular open orifice at the entrance to the snorkel. Advantageously, in embodiments in which this fourth section is used, it is placed to project slightly above the proximal end of the stent graft i.e. the blood flow will encounter this entrance slightly before it encounters the entrance to the stent graft.

Save for the third section of the snorkel described above, the other sections described are not necessarily included in all embodiments of the design but will be selected depending upon the type of branch vessel to be treated, the type of stent graft with which the snorkel must interface and the sizes of the vessels involved.

A number of preferred embodiments of the invention will now be described with reference to the drawings, in which:

Figure 1 is a cut-away illustration of a prior art stent graft;

Figure 2 is a cross-sectional plan view of the stent graft of Figure 1;

Figure 3 is a cut-away illustration of the wall of a stent graft in accordance with the invention;

Figure 4 shows in cross-section a stent-graft in accordance with the invention in situ in a body vessel; and

Figure 5 is a schematic illustration of the reinforcing wire of a stent-graft in accordance with the invention.

Turning first to Figure 1, primary stent graft 15 is shown implanted in main artery 5 proximate and bridging branch artery 10. Prior art secondary flexible stent graft 20 is shown inserted into branch artery 10 and curved into main artery 5 so as to lie in between primary stent graft 15 and the interior wall of main artery 5. Because secondary stent graft 20 is more rigid that primary stent graft 15, the wall of primary stent graft 15 is held away from the wall of main artery 5 at region 25 of primary stent graft 15.

The disadvantage of this prior art system can be seen from Figure 2, where it is apparent that there is a relatively large leakage of 30 in the space between the interior wall of main artery 5 and the exterior wall of primary stent graft 15. Preferred embodiments of the present invention are illustrated in Figures 3 and 4. Figure 3 shows the basic construction of the non-circular secondary stent graft, including graft membrane 50 supported by reinforcing ribs 55 in order to form supported graft wall 60. Unsupported graft wall 65 is formed from graft membrane 50 in a region without reinforcing ribs 55.

Figure 4 shows non-circular secondary stent graft 75 in situ in main artery 5 alongside primary stent graft 15. It can be seen that the supported graft wall 60 contacts the concave inner wall of main artery 5 and the unsupported graft wall 65 rests against the outer wall of primary stent graft 15, which is held away from the wall of the main artery 5 by reinforcing ribs 55 of non-circular stent graft 75 at region 70.

It will be appreciated that the non-circular stent graft 75 is sufficiently flexible to be bent into the branch artery 10 of main artery 5 (not shown in Figure 4). Conveniently, the stent used to support the snorkel in the section where the radial force is not uniform around the circumference can be 'C shaped, the open part of the 'C providing the region of low force and the rest of the 'C providing high force. The 'C stent should approximate to the form of a segment of a circle and can be formed from individual lengths of wire, strip or tubing, or by using an existing stent structure which has been radially compressed and the resulting flattened stent is formed into a 'C shape. A simple wire or strip 'C stent can be constructed with thickened or fine tips so that its stiffness varies progressively around its circumference. Advantageously, fine tips are used so that the transition from high radial force to low radial force is not abrupt and does not cause high stress concentrations in the design.

It is necessary that the closed part of the 'C stent cover more than half the circumference of the graft and less than the complete circumference to ensure that the unsupported part of the graft is tensioned, ensuring that the crescent- shaped lumen will remain patent.

Stents can be attached to the snorkel by a variety of means including weaving, threading, sewing, lamination, gluing or insert molding. Stents can be placed on the lumenal (inner) surface of the snorkel, within the wall of the snorkel or on the outer wall. The snorkel can be constructed using a continuous wire to produce a series of stents and it is possible to construct the complete snorkel using a continuous wire to form all the stents in the device. In this case, in the section of the snorkel which has a non-uniform radial force, the wire runs approximately circumferentially for part of the circumference of the snorkel, the wire turns through approximately 180° and returns along a path that is approximately parallel to its previous path but separated from it by a distance SI (see Figure 5). The distance S 1 is dependent upon the diameter of the snorkel and will lie in the range bounded by half the diameter of the snorkel at the largest end and one twentieth the diameter at the smallest. Optimally, SI lies in the range 1/3 to 1/6 the diameter of the snorkel.

Other sections of the snorkel with a uniform radial force can be made using one of two wire paths. One wire path uses a similar pattern to that described above except that the wire covers at least the complete circumference of the snorkel before turning to take its return path (as in WO 99/37242 in the name of the present applicant, the contects of which are incorporated by reference). Advantageously, the wire should run alongside itself for a short distance before reversing direction.

An alternative wire path involves wrapping the wire in a helix along the wall of the snorkel where each turn is separated from the last by S2. S2 has the same constraints as S 1 described above but does not need to have the same value.

In some embodiments, it is advantageous to provide at least one end of the snorkel with a stent structure having a higher radial force than the rest of the device. These higher radial force zones provide sealing and patent openings to the snorkel.

Claims

1. A tubular medical implant comprising a first longitudinal section having a supported section which is crescent-shaped in cross section.

2. An implant as claimed in claim 1, wherein the supported section is defined by filamentary reinforcing material.

3. An implant as claimed in claim 1 or 2, wherein the supported section is defined by a stent and wherein the first longitudinal section additionally comprises flexible graft material which is substantially circular in cross-section, the outer curve of said crescent having a radius which is substantially the same as the radius of said circle.

4. An implant as claimed in any preceding claim, wherein the first longitudinal section comprises a first peripheral section having a first resistance to radial compression and a second peripheral section having a second resistance to radial compression, the first resistance being greater than the second resistance, wherein the first peripheral section occupies more than 50% of the periphery of said longitudinal section.

5. An implant as claimed in claim 4, wherein the first peripheral section occupies from 51 to 75% of the periphery of the first longitudinal section.

6. An implant as claimed in claim 4 or 5, wherein the first peripheral section has at least one reinforcing element to provide said first resistance to radial compression.

7. An implant as claimed in claim 6, wherein the path followed by the reinforcing element comprises a series of substantially parallel runs lying on a part of the

circumference of the implant, which runs are connected by a series of turns through about 180°.

8. An implant as claimed in claim 7, wherein the longitudinal distance between said parallel runs is from 1/3 to 1/6 of the diameter of the implant.

9. An implant as claimed in any preceding claim, wherein the length of the first longitudinal section is from 2 to 5 times the diameter of the tubular implant.

10. An implant as claimed in any preceding claim, further comprising a second longitudinal section having a supported section which is substantially circular in section.

11. An implant as claimed in claim 10, wherein the second longitudinal section has a substantially uniform resistance to radial compression about its circumference.

12. An implant as claimed in claim 11, wherein the second longitudinal section has at least one reinforcing element to provide said uniform resistance to radial compression.

13. An implant as claimed in claim 12, wherein the path followed by the reinforcing element comprises a series of substantially parallel runs lying on the circumference of the implant, which runs are connected by a series of turns through about 180°.

14. An implant as claimed in claim 12, wherein the path followed by the reinforcing element comprises a helix wound around the longitudinal axis of the implant.

15. An implant as claimed in any of claims 10 to 14, wherein the length of the second longitudinal section is from 1 to 4 times the diameter of the tubular implant.

16. An implant as claimed in any of claims 10 to 14, wherein the length of the second longitudinal section is from 1.5 to 3 times the diameter of the tubular implant.

17. An implant as claimed in any of claims 10 to 16, additionally comprising a third longitudinal section which has a uniform resistance to radial compression about its circumference.

18. An implant as claimed in any of claims 10 to 17, further comprising a fourth longitudinal section which is more flexible to bending about a transverse axis than the other longitudinal sections.

19. An implant as claimed in claim 18, wherein the fourth longitudinal section is interposed between the first and the second longitudinal sections.

20. A method for using an implant as claimed in any preceding claim in combination with a second tubular medical implant having a circular cross-section, wherein both implants are implanted in a body lumen so that the first implant is interposed between the second implant and the lumen, with the outer curve of the crescent contacting the lumen and the inner curve of the crescent contacting the second implant.