Abstract:
A flow diverter for implantation into a patient&#39;s vessel includes a distal annular support element and a proximal annular support element, the proximal and distal support elements supporting a longitudinally twisted diverter element. The flow diverter is designed to be disposed within a vessel and to impart a rotational or twisting motion to the flow of blood passing therethrough, thereby to reduce the pressure of blood at the center of the vessel. Such flow diversion can reduce the pressure of blood impinging upon an aneurysm at a bifurcation downstream of the vessel. The device can be particularly useful for the treatment of aneurysms occurring at the bifurcation between the basilar artery and the posterior cerebral arteries.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. §119(a) to Great Britain Patent Application No. 1308652.5, filed May 14, 2013, which is incorporated by reference here in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to an implantable medical device, in particular for diverting flow within the vessel of a patient. 
     BACKGROUND ART 
     Many factors contribute to the formation of saccular aneurysms, particularly neural aneurysms. One of the major contributors is wall shear stress (WSS), which in addition to hypertension leads to a reduction in the elastic tissue of the tunica media, thereby contributing to the formation of the aneurysm. The effect of wall shear stress is heightened at certain anatomical geometries such as bifurcations, and for instance at the point where the basilar artery (BA) divides into the posterior cerebral arteries (PCA). 
     It is known to try to treat aneurysms by filling the aneurysmal sac with a filler such as a prosthetic coil. The methodology behind the use of prosthetic coils is to establish a hard thrombus formation within the sac of the aneurysm as a means of isolating the aneurysm wall from the flow of blood. While this can be effective in the treatment of many types of aneurysm, it is less effective when the aneurysm occurs at a bifurcation or trifurcation, such as at the base of the basilar artery. 
     It is also known to close off the entrance to the aneurysmal sac or to divert the flow of fluid therefrom, but known devices are not general suitable for treatment of aneurysms at the base of the basilar artery. 
     Examples of devices for treating aneurysms can be found in US-2002/0179166, US-2002/0198591, US-2003/0100945, US-2010/0106180 and WO-2012/102919. 
     SUMMARY OF THE INVENTION 
     The present disclosure seeks to provide improved treatment of aneurysms, in particular an implantable flow diverter and method of diverting flow from an aneurysm. 
     According to an aspect of the present invention, there is provided an endoluminal flow diverter that comprises a central axis, a pair of supports, and a diverter element. The central axis runs through a diameter of the device and along a longitudinal direction. The device further comprises a pair of supports being a proximal and distal support spaced in the longitudinal direction along a length of the device. The device further comprises a diverter element being disposed between and supported by the proximal and distal supports. 
     The diverter element comprises a panel being twisted in the longitudinal direction and extending across the diameter, wherein the panel twists by an angle of about 90 degrees between the proximal and distal supports. 
     The flow diverter has a structure which enables it to divert the flow of blood within a vessel, in particular to reduce the flow at the central portion of the vessel by causing the pressure of blood to even out across substantially the entire width of the vessel. When disposed, for example, at the base of the basilar artery, that is by the posterior cerebral arteries, this has the effect of reducing the pressure of blood flow to the vessel wall at the bifurcation and opposite the basilar artery. This can reduce the pressure of blood into an aneurysmal sac and therefore assist in the treatment of such an aneurysm. 
     Advantageously, the panel is formed of a sheet of material. The panel is preferably substantially impermeable, that it has a structure which acts as a barrier to blood therethrough such that all or substantially all of any blood impinging on the panel is diverted by the panel. In some embodiments the panel may include perforations or slots therein. The slots or perforations can increase the ability of the panel to twist, or over twist, so as to compress the device radially for deployment purposes. The slots or perforations are, though, preferably small enough to retain the barrier effect of the panel, that is to prevent or substantially flow of fluid through the panel. The, panel provides a surface within which the perforations or slots are disposed, the surface providing a barrier to blood therethrough. 
     Preferably, the panel extends substantially across the diameter of the device. It may extend across the entirety of the diameter of the device or almost across its entirety, any shortfall being taken up in practice by the conformability of the vessel wall. 
     Preferably, the panel is of substantially uniform thickness. In practice, it is preferred that the panel is of thin construction, which will have little or no effect on the overall blood pressure, that it will not cause an increase in blood pressure as a result of vessel constriction, which is a characteristic of some implantable medical devices. In the preferred embodiment, the panel extends across the central axis of the device. 
     Advantageously, the panel twists uniformly in helical manner in the longitudinal direction of the device. Preferably, the panel twists in the longitudinal direction of the device by an angle of around 90 degrees between the proximal and distal supports. It has been found that this feature provides optimal flow diversion. In an embodiment, the panel is substantially rectangular. 
     Preferably, the proximal and distal supports are ring-shaped. They may be separate from one another, although in other embodiments they may be connected to one another. 
     Also described is a method of diverting fluid flow in a vessel including the following steps. A user deploys in a vessel an endoluminal flow diverter as described above. Deployment of the flow diverter causes blood flow in the vessel to be diverted by twisting the flow in the vessel. 
     Advantageously the panel extends across the central axis of the device, the method providing for flow in the center of the vessel to be diverted. In the preferred embodiment, the method reduces the flow of fluid in the center of the vessel and most preferably evens the flow of fluid across the width of the vessel. 
     Other features are disclosed in the description of the preferred embodiments of the invention which follows. It is to be understood that all such features are applicable to all embodiments disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an embodiment of flow diverter; 
         FIGS. 2 and 3  are side elevational views of the flow diverter; 
         FIG. 4  is a plane view of the flow diverter; 
         FIG. 5  is a perspective view of another embodiment of flow diverter; 
         FIG. 6  is a perspective view of another embodiment of flow diverter; 
         FIG. 7  is a perspective view of another embodiment of flow diverter; 
         FIG. 8  is a perspective view of another embodiment of flow diverter; 
         FIG. 9  is a schematic diagram of typical fluid flow in the basilar artery; 
         FIG. 10  is a view in which an aneurysm has formed at the bifurcation with the left and right posterior cerebral arteries; 
         FIG. 11  is a view in which the aneurysm sac has been filled with an embolization coil; and 
         FIG. 12  is a view in which a flow diverter has been deployed in the basilar artery. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Described below are various embodiments of flow diverter for diverting flow within a patient&#39;s vessel. The term flow diverter as used herein encompasses the guiding of the flow of blood within a vessel, in particular to alter the pressure profile across the diameter of the vessel and in the preferred embodiments to reduce the pressure flow at the center of the vessel and preferably so as to even out the flow pressure across the diameter of the vessel. 
     As will be appreciated from the disclosure of the preferred embodiments set out below and in the accompanying drawings, these provide a flow diverter which has the same or substantially the same cross-sectional area at the proximal and distal ends of the device, that is at the inlet and outlet of the device. As a result of this, there is no change in the overall volume of fluid passing through the flow diverter and no overall change in pressure of fluid passing through the diverter. Experimental data has shown that the preferred embodiments of flow diverter disclosed herein are able to effect a tenfold decrease in pressure within an aneurysm. As a result, there is no flow or virtually no flow of fluid within the aneurysm. 
     The preferred embodiments are described in connection with a flow diverter intended for implantation in the basilar artery and in particular adjacent to the bifurcation with the posterior cerebral arteries. Therefore, the various dimensions of the device disclosed below are chosen to suit the dimensions of the basilar artery. It is to be understood, however, that the teachings herein can be applied in the diversion of the flow in any vessel of a patient and not just the basilar artery and that it is also not restricted solely to application with aneurysms at bifurcations or elsewhere. It will be appreciated that the dimensions of the device will differ for different vessel sizes. 
     Referring first to  FIG. 1 , there is shown a preferred embodiment of endoluminal flow diverter  10  which in this particular example is sized to fit within the basilar artery of a patient, as described in further detail below. 
     The flow diverter  10  includes a distal support  12  which in this embodiment is in the form of a resilient ring of generally circular form. The distal support  12  is located at the distal extremity of the flow diverter  10 . A proximal support  14 , again being in this embodiment in the form of an annular ring of resilient material, is located at the proximal end of the flow diverter  10 . It is preferred that the distal and proximal supports  12 ,  14 , form the distal and proximal extremities, respectively, of the flow diverter  10 . 
     Supported by and extending between the proximal and distal supports  12 ,  14  is a diverter element  16  which in this example is a substantially rectangular panel made of impermeable or substantially impermeable material and which twists in the longitudinal direction of the flow diverter  10 . More specifically, the panel  16  is attached at diametrically opposite sides  20  of each support  12 ,  14 . The attachment may be by a solder joint, welding, use of bonding agent or any other suitable attachment method. In this embodiment, the panel  16  is attached to the supports  12 ,  14 , such that the proximal edge  22  of the panel  16  is aligned with the proximal edge  24  of the distal support  12 , while the distal edge  26  of the panel  16  is aligned with the distal edge  28  of the proximal support  14 . In other embodiments, the edges  22 ,  26  of the panel  16  may be attached at any location within the length, or depth, of the support rings  12 ,  14 . 
     The proximal and distal supports may be made of an elastically deformable material or a spring material such as spring steel. The supports are preferably made of a shape memory material, preferably a shape memory alloy such as nickel titanium alloy (Nitinol) or cobalt chromium. The panel  16  may be made of the same material as the proximal and distal supports  12 ,  14  but may be made of other materials. These materials and other materials preferably used for the device  10  are anti-thrombotic material. 
     In the preferred embodiments the device is made of radiopaque materials of or includes radiopaque markers. In the preferred embodiment, radiopaque markers or material are provided in the support rings  12 ,  14 . 
     It will be appreciated that when made of a spring or shape memory material, the device  10  will exhibit resilience, for example compressibility in the radial direction, yet will exert a force tending it to its rest shape, that is its shape when not subjected to an external force. The device can thus be radially compressed in an introducer assembly for delivery and will expand when released for the introducer assembly constraints, in practice until it abuts and presses against the vessel walls as described in detail below. When made of a shape memory material, the device can be manufactured to have a transition temperature around body temperature and thus to exhibit its elastic return force only once delivered into the patient&#39;s vessel. For delivery purposes, the device can be twisted along its longitudinal axis, which will cause twisting and radial compression of the device, specifically by twisting of the diverter element  16  and the supports  12 ,  14  on themselves. Once the device is freed to revert to its non-twisted shape, that is the device is released from the introducer assembly and hence from constraining elements holding it twisted, the supports  12 ,  14  and the diverter element  16  will untwist to adopt the configuration shown in  FIG. 1 . 
     The proximal and distal support rings  12 ,  14  may be continuous rings of strip material or wire. In other embodiments it may have other configurations, such as a split ring, or a conventional stent ring having, for example, a sinusoidal or zigzag shape for radial compressibility. Furthermore, each of the proximal and distal support rings  12 ,  14  may be made of a single element but could in other embodiments be a set of annular elements, such as turns of a coil, of a strip or the like.  FIG. 5  shows an embodiment of device  10  in which the proximal  12 ′ and distal  14 ′ support elements are in the form of split rings. This structure will enhance the radial compressibility of the support elements and will still enable them to expand radially outwardly on release from the introducer assembly, thereby to provide support against the vessel wall and support to the panel  16 . 
     Referring now to  FIG. 2 , there is shown a side elevational view of the flow diverter  10  of  FIG. 1 , from a view point perpendicular to the proximal edge  22  of the panel  16 . It can be seen that the edge  22  of the flow diverter is disposed in the plane of the sheet of the drawing, whereas the distal edge  26  of the panel is at substantially 90°, being substantially perpendicular to the plane of the sheet of the drawing. In other words, the diverter element  16  is in this embodiment twisted by an angle of about 90° between its proximal and distal extremities.  FIG. 2  also shows an example of the preferred lengths of the flow diverter  10  for deployment in the basilar artery. This preferred length is in the range of about 10 mm to about 15 mm. 
       FIG. 3  is a view similar to  FIG. 2  but in which the flow diverter  10  has been rotated by 90°, in which case the proximal edge  22  of the diverter element  16  is normal for the plane of the paper of the drawing, whereas the distal edge  26  of the diverter element  16  is parallel to the plane of the paper of the drawing.  FIG. 3  also shows an example of the dimensions of the proximal and distal support elements  12 ,  14  for a flow diverter  10  for deployment in the basilar artery. In this embodiment, the proximal and distal support elements  12 ,  14  preferably have a depth or length in the range of about 1 mm to about 2 mm. 
       FIG. 4  is a bottom plane view of the flow diverter  10 , in which the twist of the panel  16  can be seen through the 90° of the preferred embodiment.  FIG. 4  also shows the preferred diameter of the flow diverter  10 , again for a diverter sized to fit within the basilar artery. In this embodiment, the diverter  10  has a diameter in the range of about 2 mm to 5 mm. 
     The wall thickness of the support elements  12 ,  14  and of the diverter element  16  is between about 0.5 mm to about 2.0 mm for a flow diverter having the dimensions given in the described example. It is to be understood that the diverter element  16  could have a greater or lesser twist than 90°, although a twist of 90° is preferred. 
     With reference to  FIGS. 1 to 4 , it will be appreciated that the flow diverter  10  is sized such that the proximal and distal supports  12 ,  14  fit within a patient&#39;s vessel so as to press against the internal vessel walls to keep the flow diverter in position. The side edges  30 ,  32  of the diverter element or panel  16  are preferably shaped and sized so as to extend to the lateral periphery of the flow diverter  10  and in particular to or close to the deployed diameter of the proximal and distal support elements  12 ,  14 . In this regard, the thickness of the preferred support elements  12 ,  14  will generally be immaterial with respect to the contact of the side edges  30 ,  32  of the diverter element  16  with the vessel walls. 
     With such dimensions, the diverter element  16  will extend across the entire diameter of the vessel and will twist along the length of the flow diverter  10 , such that any blood passing into the flow diverter  10  (through the patient&#39;s vessel) will be subjected to the twisting flow diverter path produced by the diverter element or panel  16 . This is the preferred arrangement, namely that the entirety of fluid flow through the device  10  and through the vessel in which the device  10  is implanted is subjected to the twisting action produced by the twisted divert element or panel  16 . It is not excluded, however, in some embodiments that there may be a narrow gap between the side edges  30 ,  32  of the diverter element  16  and the vessel walls, possible primarily because of fluid stagnation or lamination at the vessel surfaces. This latter option is, however, not generally preferred. 
     The provision of a diverter element  16  in the form of uniform thickness along its length and which is fixed at diametrically opposite locations on the support elements  12 ,  14  ensures that there is the same volume of fluid passing that the inlet of the device than leaves the outlet of the device and also either side of the panel. This ensures that there is no overall change in pressure of fluid passing through the element  10 , solely a smoothing of that pressure across the cross-sectional area of the device  10  and thus of the vessel. 
     Referring now to  FIG. 6 , there is shown another embodiment of flow diverter  100  similar to the embodiment of  FIGS. 1 to 5  but in which the diverter panel  160  is formed with longitudinal slits  161  therein. The slits  161 , which in some embodiments may provide no gap in the surface of the panel  160 , make it easier for the panel  160  to twist on itself so as to compress the device  100  for delivery purposes. The panel  160  still provides a surface for guiding the flow of fluid through the device  100 , effectively the same as a whole panel  16  as in the previous embodiments. As will be apparent in  FIG. 6 , the slits are in a plurality of series along the length of the panel  160 , with the slits in adjacent series being laterally offset from one another. The skilled person will appreciate that the device  100  can have any of other characteristics and elements of the devices taught herein. 
       FIG. 7  shows another embodiment of device  110 , in which the proximal and distal support elements  112 ,  114  are stent rings of a stent  130  which extends for the whole length of the device  110 . The stent  130  provides longitudinal support to the device  110  and to the panel  16 . A similar embodiment of device  210  is shown in  FIG. 8 , in which the panel  116  is similar to the panel of the embodiment of  FIG. 6 . It will be apparent that in the embodiments of  FIGS. 7 and 8 , as with all the other embodiments taught herein, the supports  112 ,  114  may be in the form of zigzag stents, as well as taking any of the other forms taught herein. 
     Referring now to  FIG. 9 , there is shown by way of illustration only a schematic diagram of a cross-section of a part of a patient&#39;s cerebral vasculature and in particular of the basilar artery  40  and its bifurcation into the right posterior cerebral artery  42  and the left posterior cerebral artery  44 . In a healthy anatomy, the vessel wall opposite the bifurcation  46  exhibits an indentation  48  which assists in the guiding of blood into the posterior cerebral arteries  42 ,  44 . The arrows  50  in  FIG. 5  depict the flow of blood from the basilar artery into the right and left posterior cerebral arteries  42 ,  44 . The size and relative positions of the arrows  50  depict the speed and strength of the flow at the various positions within the basilar artery  40 . Specifically, adjacent the walls  52  of the basilar artery  40 , flow is reduced, whereas towards the center of the basilar artery  40  flow is as its highest. In other words, the strength of the flow and therefore flow pressure increases from the edges of the wall  52  of the basilar artery  40  towards the center point of the artery. This is typical of laminar flow. 
     Referring now to  FIG. 10 , this shows the same vessels as  FIG. 9 , that is the basilar artery  40  and right and left posterior cerebral arteries  42 ,  44 , but in a patient with a developed aneurysm  60  at the point of bifurcation. This is typically caused by a weakening of the vessel wall at the point of bifurcation, the weakening creating an aneurysm sack as the result of continued pressure of blood from the basilar artery  40  to the bifurcation. The skilled person will appreciate that in a condition such as that depicted in  FIG. 10 , the aneurysm  60  will be subjected to the highest pressure/flow part of the blood from the basilar artery  40 . This pressure will tend to cause the aneurysm sack  60  to grow in size, with consequential weakening of the vessel wall. Left untreated, this can result in the rupture of the vessel wall and of haemorrhaging. 
       FIG. 11  shows the vessel anatomy of  FIG. 10 , in which an embolization coil has been fitted into the aneurysm sack  60  in order to fill this. The purpose of the embolization coil  62  is to close off the volume of the sack  60  to prevent further flow of blood into the aneurysm sack and thereby to reduce the pressure on the walls of the vessel within the sack  60 . However, as depicted in  FIG. 11  there is continued flow of blood  50  towards the aneurysm sack  60 , with the part of greatest flow and pressure heading directly towards the neck or opening  64  of the aneurysm  60 . Thus, the flow of blood  50  will continue to apply pressure into the aneurysm. 
     Referring now to  FIG. 12 , this is a view similar to  FIG. 11 , but in which the flow diverter  10  taught herein is deployed within the basilar artery  40 . The flow diverter  10  has the effect of imparting a twist and slowing of the central portion of the flow  50  of blood in the artery  40 , as shown in schematic form by the arrows  70 . As a result, the flow diverter  10  causes a reduction in the flow and pressure of blood at the center point of the artery  40  and therefore towards the aneurysm  60 , with a consequential reduction in the force imparted to the vessel walls at the aneurysm  60 . In particular, the twist imparted to the flow  50  of blood will reduce lamination within the flow of blood and thus even out of the pressure across the diameter of vessel. Moreover, the twist or rotation imparted to the flow of blood can also reduce the flow of blood at the center of the vessel, in effect by the creation of a vortex void. This can substantially reduce pressure at the aneurysm  60  and assist in the healing thereof. 
       FIG. 12  shows an embolization coil  62  disposed within the aneurysm sac  60 , although it is to be understood that this may not be necessary in order to treat the aneurysm  60 . The diverter element or panel  16 ,  116  of the flow diverter preferably has an even twist along its length, although in other embodiments the diverter element  16  could have a varying twist, for instance an increasing twist from its distal end  12  to its proximal end  14 , in the orientation shown in  FIG. 12  and thus along the direction of fluid flow. 
     The diverter element  16 ,  116  need not be made of a spring material or shape memory material and in some embodiments could be made of a relatively soft material, or biomaterial which is absorbable or otherwise, supported in its twisted configuration by the proximal and distal support elements. In such an embodiment, the proximal and distal support elements could be coupled to one another, for example by appropriate struts or tethers. Struts or tethers of such a nature could be provided in all of the embodiments disclosed herein. 
     In other embodiments, the proximal and distal support elements could be connected together or a part of a singular support element. Such a support element could be a sleeve extending for the length of the flow diverter, in one example being in the form of a stent as shown in  FIGS. 7 and 8 . 
     It is to be appreciated that the embodiments of flow diverter taught herein may be provided with other features commonly found with implantable medical devices, for example anchoring elements in the form of barbs or the like, retrieval elements such as hooks and the like for withdrawing device from a patient&#39;s vessel after completion of a medical procedure. It is envisaged also that the device could be retained permanently within a patient&#39;s vessel, not just to treat a formed aneurysm but also in order to prevent the formation of aneurysms or further aneurysms over time. 
     All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the disclosure taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another. It will be understood that this invention is not limited to the disclosed embodiments, as those having skill in the art may make various modifications without departing from the scope of the following claims.