Expandable tube for deployment within a blood vessel

An expandable tube for deployment within a blood vessel is disclosed. In one arrangement, the tube comprises an elongate frame that is reversibly switchable from a radially expanded and longitudinally contracted state to a radially contracted and longitudinally expanded state. The frame comprises a plurality of longitudinally deformable elements for providing longitudinal expansion and contraction of the frame and a plurality of circumferentially deformable elements for providing radial expansion and contraction of the frame. The longitudinally deformable elements can be expanded or contracted longitudinally substantially without any change in the shape of the circumferentially deformable elements. The plurality of circumferentially deformable elements comprises a plurality of sets of circumferentially deformable elements. Each set of circumferentially deformable elements forms a closed ring around an axis of elongation of the frame. Each closed ring consisting exclusively of the circumferentially deformable elements. At least two of the closed rings occupy overlapping ranges of longitudinal positions when the frame is in the radially expanded and longitudinally contracted state and occupy non-overlapping ranges of longitudinal positions when the frame is in the radially contracted and longitudinally expanded state.

BACKGROUND OF THE INVENTION

The present invention relates to an expandable tube for deployment within a blood vessel, particularly for use in redirecting blood flow away from an aneurismal sac.

An intracranial aneurysm is a weak region in the wall of an artery in the brain, where dilation or ballooning of the arterial wall may occur. Histologically, decreases in the tunica media, the middle muscular layer of the artery, and the internal elastic lamina cause structural defects. These defects, combined with hemodynamic factors, lead to aneurismal out-pouchings. Intracranial aneurysms are quite common diseases with a prevalence ranging from one to five percent among adult population according to autopsy studies. In the US alone, ten to twelve million people may have intracranial aneurysms.

Current methods for treating intracranial aneurysms include surgical clipping and endovascular coiling. In the surgical clipping method, the skull of the patient is opened, and a surgical clip is placed across the neck of the aneurysm to stop blood from flowing into the aneurysm sac. The risk of this method is relatively high, especially for elderly or medically complicated patients. Endovascular coiling is a less invasive method involving placement of one or more coils, delivered through a catheter, into the aneurysm until the sac of the aneurysm is completely packed with coils. It helps to trigger a thrombus inside the aneurysm. Although endovascular coiling is deemed to be safer than surgical clipping, it has its own limitations. First, after the aneurysm is filled with the coils, it will remain its original size. As a result, the pressure on the surrounding tissue exerted by the aneurysm will not be removed. Second, this procedure is not very effective for wide-necked aneurysms, where the coil is likely to protrude into the parent vessels. This problem may be mitigated by using a stent in combination with coiling embolization, but the procedure is difficult and time-consuming.

BRIEF SUMMARY OF THE INVENTION

Using an expandable tube, sometimes referred to as a stent, alone to treat the aneurysm is a promising way to avoid the problems stated above. In this method, a tube with an area of relatively low porosity is placed across the aneurysm neck in such a way as to redirect blood flow away from the sac and trigger formation of a thrombus within the aneurysm. Because the aneurysm solidifies naturally on itself, there is less danger of its rupture. Furthermore, because no coil is involved in this method, the aneurysm will gradually shrink as the thrombus is absorbed. Consequently, the pressure applied on the surrounding tissue can be removed. It is difficult, however, to manufacture a tube having optimal characteristics for this application. The tube has to be flexible enough to pass through and adapt to the shape of the very tortuous blood vessels in the brain while at the same time providing sufficient coverage (low porosity) to redirect blood flow away from the aneurysm to an adequate extent.

It is an object of the invention to provide an expandable tube for deployment within a blood vessel that has improved properties, particularly in the context of redirecting blood flow away from an aneurysm.

According to an aspect of the invention, there is provided an expandable tube for deployment within a blood vessel, comprising: an elongate frame that is reversibly switchable from a radially expanded and longitudinally contracted state to a radially contracted and longitudinally expanded state, wherein: the frame comprises a plurality of longitudinally deformable elements for providing longitudinal expansion and contraction of the frame and a plurality of circumferentially deformable elements for providing radial expansion and contraction of the frame; the longitudinally deformable elements can be expanded or contracted longitudinally substantially without any change in the shape of the circumferentially deformable elements; the plurality of circumferentially deformable elements comprises a plurality of sets of circumferentially deformable elements, each set of circumferentially deformable elements forming a closed ring around an axis of elongation of the frame, each closed ring consisting exclusively of the circumferentially deformable elements; and at least two of the closed rings occupy overlapping ranges of longitudinal positions when the frame is in the radially expanded and longitudinally contracted state and occupy non-overlapping ranges of longitudinal positions when the frame is in the radially contracted and longitudinally expanded state.

As compared to surgical clipping, the presently disclosed tube can be configured to redirect blood flow away from an aneurismal sac in a minimally invasive method that is much safer, has lower morbidity and mortality rates, requires less hospital stay and reduces the overall treatment cost. As compared to other minimally invasive methods, e.g. coiling embolization or stent-assisted coiling, the presently disclosed tube does not involve coils, which leads to several advantages, e.g. the mass effect of the aneurysm is reduced, and the tube is suitable for treating both saccular and fusiform aneurysms. As compared to current flow-diverters (i.e. stents configured to divert flow away from an aneurismal sac), the presently disclosed tube can provide higher radial strength, more controlled deployment and tailored surface coverage which is useful to prevent the blockage of branch blood vessels.

The provision of a frame that elongates as part of the radial contraction allows a high degree of radial contraction even when the frame is configured to present a low porosity in the radially expanded state. It is therefore possible to provide a frame that can be inserted into delivery catheters of very small diameter, for example less than 3 mm diameter, or more preferably less than 1 mm diameter. This property expands the range of clinical uses that are available.

The use of longitudinally overlapping closed rings of circumferentially deformable elements facilitates provision of low porosities in the radially expanded and longitudinally contracted state, which favours high radial strength and/or good flow redirection properties. This feature also allows the switching from the radially expanded and longitudinally contracted state to the radially contracted and longitudinally expanded state to be achieved without excessive strains in any portion of the frame.

In an embodiment, the closed rings form an alternating sequence of first type closed rings and second type closed rings, wherein: each of one or more of the circumferentially deformable elements on each first type closed ring is aligned in a direction parallel to the axis of elongation of the elongate frame with a corresponding identical one of the circumferentially deformable elements on each other first type closed ring and is not aligned with a corresponding identical one of the circumferentially deformable elements on any of the second type closed rings, when the frame is in the radially expanded and longitudinally contracted state.

This configuration reduces or avoids twisting of the tube during the switching from the radially expanded and longitudinally contracted state to the radially contracted and longitudinally expanded state. In an embodiment, twisting is further prevented by arranging for the longitudinally deformable elements to comprise sets of identical first type longitudinally deformable elements and sets of identical second type longitudinally deformable elements, wherein the sets of first type longitudinally deformable elements and the sets of second type longitudinally deformable elements are arranged in an alternating sequence such that each first type closed ring is connected to the next second type closed ring in a given direction parallel to the axis of elongation exclusively by first type longitudinally deformable elements and each second type closed ring is connected to the next first type closed ring in the same given direction parallel to the axis of elongation exclusively by second type longitudinally deformable elements, wherein the first type longitudinally deformable elements have a different shape and/or orientation from the second type longitudinally deformable elements, for example mirror images of each other when the frame is viewed in an unfolded planar state.

In an embodiment, each of one or more of the longitudinally deformable elements is curved along at least 20% of the length of the longitudinally deformable element. This helps to spread the strain over the longitudinally deformable element during the switching from the radially expanded and longitudinally contracted state to the radially contracted and longitudinally expanded state, allowing large overall deformations to be achieved without excessively stressing the longitudinally deformable elements.

In an embodiment, each of one or more of the longitudinally deformable elements is connected to one of the closed rings at a junction and configured such that an angle between the longitudinally deformable element and a circumferentially deformable element at the junction changes by less than 30 degrees during switching from the radially expanded and longitudinally contracted state to the radially contracted and longitudinally expanded state. This feature reduces undesirable stresses at the junction.

Embodiments of the invention provide a tube suitable for deployment within a blood vessel. The tube comprises an elongate frame2.FIG.1depicts the outer geometry of the frame2in a radially expanded and longitudinally contracted state.FIG.2depicts the outer geometry of the frame2in a radially contracted and longitudinally expanded state. The frame2can be switched reversibly from the radially expanded and longitudinally contracted state shown inFIG.1to the radially contracted and longitudinally expanded state shown inFIG.2.

The frame2is expandable, optionally self-expanding. The frame2may comprise a shape memory alloy, for example, such as nitinol. Alternatively, the frame2may comprise a stainless steel, polymer or other biocompatible material.

The frame2is elongate relative to an axis of elongation4. The frame2may be cylindrical for example. When the frame2is cylindrical, the maximum lateral dimension is the same at all positions and angles (i.e. it is equal to the diameter). When the frame2is not cylindrical the maximum lateral dimension may be different at different positions and/or angles. The maximum lateral dimension defines the minimum interior diameter of a cylindrical tube (e.g. a delivery catheter) that the frame could be inserted into.

In the radially contracted state the frame2is substantially narrower than in the radially expanded state. Preferably the maximum lateral dimension is 30% smaller in the radially contracted state, more preferably 50% smaller. Radially contracting the frame2allows the frame2to be inserted into a narrower delivery catheter for deployment at the site of interest. It is generally desirable for the delivery catheter to be as narrow as possible. This is particularly the case where access to a deployment site requires navigation of tortuous regions of vasculature. This may often be the case, for example, when treating a cerebral aneurysm.

DETAILED DESCRIPTION OF THE INVENTION

In the discussion below it is understood that the term porosity, p, refers to the ratio of the surface area of open regions to the total external surface area occupied by the frame or portion of frame that is being described. The total external surface area is the sum of the surface area of the open regions and the surface area of the regions occupied by the material of the frame. When the frame is cylindrical, the total external surface area is simply 2π·R·L, where R is the radius of the cylinder and L is the length of the cylinder.

Consider a stent with a porosity ρ in the fully radially expanded state. If the radius and length of the frame in the fully radially expanded state are R0and L0, respectively, the minimum radius Rminthat the frame2can achieve in the radially contracted state, defined by the state in which the porosity becomes zero, is governed by

Rmin=(1-ρ)⁢L0L1·R0
where L1is the length of the frame in the radially contracted state. This relationship assumes that elements of the frame are not allowed to overlap with each other in the radial direction.

This relationship illustrates that if the length of the frame is not allowed to change to any significant extent, the radius can only reduce by a factor of ρ. As ρ needs to be quite low (e.g. less than 80%, at least in a low porosity region, such as a region intended for positioning in use over the opening to an aneurismal sac), this represents a significant limitation to the extent to which the stent can be narrowed for insertion into a delivery catheter. For example, if the porosity ρ of the frame is 20% and the length of the frame is not allowed to change during radial contraction, i.e. L1=L0, the frame can achieve only a maximum 20% reduction in radius. The provision of a frame that can expand longitudinally when adopting the radially contracted state is based on this understanding and allows much greater reductions in radius to be achieved. For example, if the length is allowed to double, i.e. L1=2·L0, the frame can achieve a 60% reduction in radius for a porosity of 20%.

Preferably, the frame2is configured so that it can be elongated by at least 25%, more preferably by at least 50%, even more preferably by 100% or 150%. Optionally, the elongation can be even longer, for example, 400%, 600%, 800%, or more.

FIG.3shows an example frame2notionally unfolded so that it is flat (rather than cylindrical). The longitudinal axis runs horizontally in the plane of the page and the circumferential direction runs vertically in the plane of the page. The longitudinal length of the frame2is marked L0and the circumferential length of the frame is marked C0.

FIG.4is a magnified view of a portion of the frame2ofFIG.3. The frame2comprises a network of interconnecting arms. The interconnecting arms form a plurality of circumferentially deformable elements6for providing radial expansion and contraction of the frame2. The frame2further comprises a plurality of longitudinally deformable elements8, distinct from the circumferentially deformable elements6, for providing longitudinal expansion and contraction of the frame2. The circumferentially deformable elements6and the longitudinally deformable elements8may be connected together to form an integrally interconnected network, such that there are no material interfaces between any of the elements. The frame2may be formed for example by laser cutting a hollow tube, by3D printing, or by other techniques known in the art for manufacturing such structures. All of the circumferentially deformable elements6and longitudinally deformable elements8may be provided at the same radius and, without any overlaps in the radial direction.

The plurality of circumferentially deformable elements6comprises a plurality of sets of circumferentially deformable elements6. Each set of circumferentially deformable elements6forms a closed ring around the axis of elongation4of the frame2. Each closed ring consists exclusively of the circumferentially deformable elements6. In the example ofFIGS.3-8, each circumferentially deformable element6is substantially V-shaped. Each closed ring thus consists of a plurality of Vs connected together at the outer ends of the arms of each V.

Each of the closed rings occupies a range of longitudinal positions. InFIG.4, three such ranges of positions are marked11-13for three different closed rings. It can be seen that the three ranges of longitudinal positions overlap with each other. This is a general characteristic of the frame2according to embodiments of the present disclosure: at least two of the closed rings should occupy overlapping ranges of longitudinal positions when the frame2is in the radially expanded and longitudinally contracted state. In the particular embodiment shown, each closed ring overlaps with four other closed rings. In general, each closed ring should overlap with at least one other closed ring, optionally at least two other closed rings.

Arranging for the closed rings to overlap in the radially expanded and longitudinally contracted state allows the frame to achieve a low porosity in this state. The low porosity may be suitable for example for redirecting blood flow away from an aneurismal sac and thereby promoting thrombus formation in the aneurismal sac. Preferably, the porosity is less than 90%, optionally less than 80%, optionally less than 70%, optionally less than 60%, optionally less than 50%.

Arranging for the closed rings to overlap in the radially expanded and longitudinally contracted state also helps to provide high radial stiffness by providing a high density of the circumferentially deformable elements per unit length. This may be useful when the tube is used to treat aneurysms and in other applications.

The frame2is further configured such that the closed rings that occupy overlapping longitudinal positions when the frame2is in the radially expanded and longitudinally contracted state occupy non-overlapping ranges of longitudinal positions when the frame2is in the radially contracted and longitudinally expanded state. Thus, the closed rings effectively move out of the way of each other and allow the frame to contract radially to a greater extent. This process is illustrated schematically inFIGS.5and6.

FIG.5depicts a portion of a frame2of the type depicted inFIG.4after longitudinal expansion of the frame2. The longitudinal expansion can be achieved, at least partially, by longitudinal expansion of the longitudinal deformable elements8A and8B substantially without any change in the shape of the circumferential deformable elements6(forming closed rings21and22). The longitudinal expansion results in the closed rings no longer overlapping with each other. The ranges of longitudinal positions11-13no longer overlap.

FIG.6depicts the portion of the frame2ofFIG.5after a subsequent radial contraction. The radial contraction is achieved principally or substantially entirely by deformation of the circumferentially deformable elements. In an embodiment, the deformation of the circumferentially deformable elements6occurs substantially without any deformation of the longitudinally deformable elements for at least a portion of the deformation.

The longitudinal expansion ofFIG.5and the radial contraction ofFIG.6may be achieved in separate stages as depicted inFIGS.5and6or may be implemented at the same time or during overlapping time periods.

In an embodiment, each of one or more of the circumferentially deformable elements6on one of the closed rings is aligned in a direction parallel to the axis of elongation of the frame2with a corresponding identical one of the circumferentially deformable elements6on another of the closed rings when the frame2is in the radially expanded and longitudinally contracted state. This facilitates efficient interlocking of different closed rings in the radially expanded and longitudinally contracted state, promoting low porosity and/or high radial stiffness. In such an embodiment, the aligned circumferentially deformable elements6will also have the same orientation as each other. For example, in the case where each circumferentially deformable element comprises a V-shaped element, the aligned circumferentially deformable elements6will comprise V-shaped elements pointing in the same direction.

In an embodiment, directly adjacent closed rings will comprise circumferentially deformable elements6that are aligned with each other. However, the inventors have found that this configuration can lead to undesirable twisting of the frame2during switching from the radially expanded and longitudinally contracted state to the radially contracted and longitudinally expanded state. Twisting can be reduced by arranging for the aligned circumferentially deformable elements6to be separated from each other by at least one closed ring having circumferentially deformable elements6that are not aligned.

In an embodiment of this type, of which the embodiment ofFIGS.2-8is an example, the closed rings form an alternating sequence of first type closed rings21and second type closed rings22. The first type closed rings21and second type closed rings22are labelled inFIGS.5-7. InFIG.7, example circumferentially deformable elements6have been overlaid with thick lines to indicate the different types of closed ring. Thick broken lines indicate example circumferentially deformable elements6in first type closed rings21. Thick solid lines indicate example circumferentially deformable elements6in second type closed rings22.

Each of one or more of the circumferentially deformable elements6(e.g. V-shaped elements) on each first type closed ring21is aligned in a direction parallel to the axis of elongation of the frame2with a corresponding identical one of the circumferentially deformable elements6on each other first type closed ring21and is not aligned with a corresponding identical one of the circumferentially deformable elements on any of the second type closed rings, when the frame is in the radially expanded and longitudinally contracted state. Alternatively or additionally, each of one or more of the circumferentially deformable elements6(e.g. V-shaped elements) on each second type closed ring22is aligned in a direction parallel to the axis of elongation of the frame2with a corresponding identical one of the circumferentially deformable elements on each other second type closed ring22and is not aligned with a corresponding identical one of the circumferentially deformable elements on any of the first type closed rings21, when the frame is in the radially expanded and longitudinally contracted state. In the particular example shown the first type closed rings21and the second type closed rings22are offset from each other circumferentially when the frame2is in the radially expanded and longitudinally contracted state by distance30.

Alternatively or additionally, and also or further contributing to the reduction of twisting, the longitudinally deformable elements comprise sets of first type longitudinally deformable elements8A and sets of identical second type longitudinally deformable elements8B. The sets of first type longitudinally deformable elements8A and the sets of second type longitudinally deformable elements8B are arranged in an alternating sequence such that each first type closed ring21is connected to the next second type closed ring22in a given direction parallel to the axis of elongation exclusively by first type longitudinally deformable elements8A and each second type closed ring22is connected to the next first type closed ring21in the same given direction parallel to the axis of elongation exclusively by second type longitudinally deformable elements8B. An example of such an arrangement can be seen most clearly inFIG.5. The first type longitudinally deformable elements8A have a different shape and/or orientation from the second type longitudinally deformable elements8B. Optionally, the first type longitudinally deformable elements8A are mirror images of the second type longitudinally deformable elements8B when the frame2is viewed in an unfolded planar state (as inFIGS.3-8). In the example shown, the first type longitudinally deformable elements8A curve downwards to the right and the second type longitudinally deformable elements are mirror images and curve upwards to the right.

In various embodiments, the embodiment ofFIGS.3-8being an example, at least two of the (optionally overlapping) closed rings are identical to each other. The identical closed rings may be aligned with each other in the longitudinal direction or may be offset with respect to each other in the circumferential direction, at least in the radially expanded and longitudinally contracted state. The embodiment ofFIGS.3-8comprises closed rings of both types: first type closed rings21are aligned with first type closed rings21, second type closed rings22are aligned with second type closed rings22, and first type closed rings21are circumferentially offset relative to second type closed rings22.

In various embodiments, the embodiment ofFIGS.3-8being an example, at least two of the (optionally overlapping) closed rings each consists of a plurality of identical circumferentially deformable elements connected together in the same orientation. In the particular example ofFIGS.3-8each of the identical circumferentially deformable elements comprises a V-shaped element. More generally, the closed rings may be formed from plural straight elements, such that at least 50%, optionally at least 75%, optionally at least 85%, optionally at least 90%, optionally at least 95%, of a path along each of the at least two closed rings is formed from elements that are substantially straight. Forming the closed rings in this way helps to provide high radial stiffness.

In various embodiments, the embodiment ofFIGS.3-8being an example, two of the closed rings21,22are connected to each other exclusively by a plurality of longitudinally deformable elements8,8A,8B that can be expanded or contracted longitudinally substantially without any change in the shape of the circumferentially deformable elements forming the two closed rings21,22. Optionally, none of the longitudinally deformable elements8,8A,8B is connected directly to any other longitudinally deformable element. In order to spread the strain during the switching from the radially expanded and longitudinally contracted state to a radially contracted and longitudinally expanded state (or vice versa), each of one or more of the longitudinally deformable elements8,8A,8B is curved along at least 20%, optionally along at least 50%, optionally along at least 75%, optionally along at least 90%, optionally along substantially all of, the length of the longitudinally deformable element8,8A,8B.

In various embodiments, the embodiment ofFIGS.3-8being an example, each of one or more of the longitudinally deformable elements8,8A,8B is connected to one of the closed rings21,22at a junction32(depicted inFIG.8) and configured such that an angle34between the longitudinally deformable element8,8A,8B and a circumferentially deformable element6at the junction32changes by less than 30 degrees, optionally less than 20 degrees, optionally less than 10 degrees, during switching from the radially expanded and longitudinally contracted state to the radially contracted and longitudinally expanded state.

In the example ofFIGS.3-8, each circumferentially deformable element6is substantially V-shaped. Each closed ring thus consists of a plurality of Vs connected together at the outer ends of the arms of each V. More generally, the closed rings may be formed from plural straight elements. In such embodiments, the junctions at which the longitudinally deformable elements are connected to the closed rings of circumferentially deformable elements may be located at a point other than at the centre of a joining region where the two arms of the V join (i.e. away from all such joining regions), and at a point other than at a centre of a joining region where each straight element is joined to a neighbouring straight element (i.e. away from all such joining regions). The junction may be located away from the centre of the nearest joining region by 2% or more, optionally 5% or more, optionally 10% or more, optionally 20% or more, optionally 40% or more, of the length of at least one of the arms of the V (e.g. the length of a straight element of the V).

Locating the junction away from the centre of the joining region reduces the amount of material around the junction, increasing the flexibility of the longitudinally deformable elements near the junction and allowing for greater longitudinal contraction and expansion of the tube. Locating the junction away from the centre of the joining region also makes it possible to lengthen the longitudinally deformable element, thereby spreading out (and thereby reducing) the bending stresses associated with deformation of the longitudinally deformable element more. Thus, in some embodiments, the two junctions at the respective ends of each of one or more of the longitudinal deformable elements are arranged to be on the outermost sides of the centres of the respective nearest joining regions between arms of the circumferentially deformable elements. Referring toFIG.8for example, a first end of a longitudinal deformable element may be connected above the centre of a joining region (as is the case for the leftmost junction32inFIG.8) and a second end of the longitudinal deformable element may be connected below the centre of the joining region (as is the case for the rightmost junction32inFIG.8, although the particular junction shown belongs to a different longitudinal deformable element of course).

In an embodiment, a joining point (i.e. a point at the outer ends of the arms of each V where two adjacent V-shaped elements on the same closed ring join) may be separated from an adjacent joining point on the same closed ring of circumferentially-deformable elements6by a separation distance. This separation distance will increase as the tube moves from the radially contracted and longitudinally expanded state to the radially expanded and longitudinally contracted state, as seen by comparingFIGS.5and6. The separation distance in the radially expanded and longitudinally contracted state may be sufficiently large that one of the longitudinally deformable elements8and/or a V-shaped element from an adjacent closed ring of circumferentially deformable elements6can fit into the space between the two adjacent joining points, as seen inFIGS.4and8. Thus, in an embodiment, the separation distance in the radially expanded and longitudinally contracted state is such as to allow a vertex of a V-shaped element in an adjacent closed ring located at a circumferential position lying between the circumferential positions of the two joining points (but at a different longitudinal position), and at least a portion of a longitudinally deformable element connected closest to the vertex, to move, during a transition from the frame being in the radially contracted and longitudinally expanded state to the frame being in the radially expanded and longitudinally contracted state, through a notional line joining the two joining points (i.e. so at to move from a longitudinally non-overlapping state to a longitudinally overlapping state).

To facilitate the separation distance being sufficiently large while allowing the longitudinally deformable element to be relatively long, which reduces stress concentrations, the smallest angle between the two arms of the V-shaped elements in the radially expanded and longitudinally contracted state may be larger than 60 degrees, optionally larger than 80 degrees, optionally larger than 100 degrees, optionally larger than 120 degrees.

Having a sufficiently large separation distance allows adjacent rings of circumferentially-deformable elements to move closer together in the radially expanded and longitudinally contracted state, which in turn decreases the porosity of the tube in this state and improves the performance and effectiveness of the device. Providing a relatively large separation distance makes it possible for the longitudinally deformable element to be relatively long, which advantageously spreads out stress along the longitudinally deformable element.

The tube of any of the above embodiments may be used in a method of treating an aneurysm, comprising deploying the tube over an opening to the aneurismal sac and thereby redirecting blood flow away from the aneurismal sac to promote thrombus formation in the aneurismal sac.