Helical stent

This application claims the benefit of U.S. Provisional Patent Application No. 60/731,724 filed Oct. 31, 2005, the entirety of which is hereby incorporated by reference into this application. This application is related to U.S. patent application Ser. No. 11/250,226, filed Oct. 14, 2005, the entirety of which is incorporated by reference into this application.

1. Field of the Invention

The invention relates to a stent-like structure which in one embodiment can be used for treating neurovascular or brain aneurysms.

2. Description of Related Art

A stent is a tubular structure that in a radially compressed state (crimped state) may be inserted into a vessel in the body. Once located in the vessel, the stent may be expanded radially to a predefined size (expanded state). Stents are typically characterized as balloon expanding or self expanding. A balloon expanding stent requires a balloon, which is part of the delivery system, to expand the stent from the crimped state to an expanded state. A self expanding stent is designed, through choice of material, geometry, or manufacturing techniques, to expand from a crimped state to an expanded state once the stent is released into the intended vessel.

A stent is commonly thought of as a device that opens a narrowed artery. A stent-like structure has similar characteristics to a stent, but may not open or support an artery. Unless otherwise indicated, as used hereafter, the term “stent” will be understood to encompass both a stent and stent-like structure.

An aneurysm is a weakened area in a vessel that has expanded or bulged. Untreated aneurysms are susceptible to rupture. Stenting of an aneurysm could strengthen the weakened vessel by cutting off significant blood flow to the aneurysm. The reduced blood flow could result in thrombosis in the aneurysm and promote healing regardless of the aneurysm form or shape. Stents have been used to treat aneurysms. Conventional stent grafts have been used to treat Thoracic and Abdominal Aortic Aneurysms. Stent grafts have a metallic stent structure covered by graft material. Stents have also been used experimentally to repair neurovascular aneurysms which were first treated with platinum coils that have migrated over time into the parent vessel. See Pride, Jr. et al., Endovascular problem solving with intravascular stents, AJNR Am J Neuroradiol. 2000; 21:532-540.

One system, Boston Scientific's Neuroform Microdelivery Stent System, is described for use with embolic coils. Conventional treatment includes aneurysm clipping, which requires a craniotomy, or aneurysm coiling, placing small platinum or polymer coils in the aneurysm to facilitate thrombosis. The above-described conventional methods for treatment of neurovascular aneurysms have certain drawbacks. Coiling is effective when the aneurysm has a well defined neck (transition from the bulging portion of the vessel to the nominal vessel diameter). If this neck transition is wide or long, coiling is not as effective as the coils tend to migrate and cause other complications, as described in Lownie et al., Endovascular therapy of a large vertebral artery aneurysm using stent and coils, Can J Neurol Sci 2000; 27:162-165 and Wanke et al., Treatment of wide-necked intracranial aneurysms with a self-expanding stent system: initial clinical experience, AJNR Am J Neuroradiol. 2003; 24:1192-1199. Clipping is very invasive, risky and expensive.

Current coronary stents typically have an expandable “Z” strut scaffolding structure that allows efficient radial expansion and effective vessel dilation. This type of structure requiring a large open area is not expected to be effective for treating neurovascular aneurysms because the large open areas would not inhibit blood flow. In coronary stents, radial expansion is achieved by opening the scaffolding structure to produce a structure that is mostly open area. Coronary stents typically have 75% to 90% open area (10 to 25% vessel coverage).

It is believed that in order for stenting to be effective for treating brain aneurysms, the percent of the vessel covered would need to be at least about 35%. It is desirable to provide a stent for treating neurovascular or brain aneurysms having the percent of the vessel covered of at least about 35%.

The present invention relates to a stent providing a high percentage of vessel coverage, preferably at least about 50% of the portion of the vessel covered by the helical elements of the stent. The stent comprises helical elements interposed between strut members in which the helical elements are connected to the strut members by linking elements. The strut members provide radial structure of the stent and an anchoring mechanism. The portion of the stent having helical elements provides a high percentage of covered area for example, in an aneurysm area. The linking elements provide part of a mechanism that allows vessel coverage to be maintained as the stent is deployed from a crimped state to an expanded state. In the stent of the present invention, vessel coverage in the expanded state is primarily achieved by shortening the pitch (distance per turn) of the helical elements during radial expansion. The linking element between the strut members and helical elements is part of a mechanism allowing the helical elements to change its helix angle (the angle at which the helical elements progresses around the circumference and along the length of the stent) and thereby the pitch of the helical elements as the stent is expanded.

The invention will be more fully described by reference to the following drawings.

FIG. 1A is a plan view of a first embodiment of the stent of the present invention including helical elements interposed between strut members and connected to the strut members by flexible linking elements.

FIG. 1B is a plan view of an alternate embodiment of the strut members and flexible linking elements including helical strut ring portions.

FIG. 1C is a plan view of an alternate embodiment of the strut members and flexible linking elements including a single strut ring portion.

FIG. 1D is a plan view of an alternate embodiment of the strut members and flexible linking elements including multiple strut ring portions.

FIG. 1E is a schematic profile diagram of an alternative embodiment of the stent of the present invention in which strut members have a flared configuration.

FIG. 1F is a schematic profile diagram of an alternative embodiment of the stent of the present invention in which strut members have a larger diameter than the helical member.

FIG. 1G is a schematic profile diagram of an alternative embodiment of the stent of the present invention in which a helical member has a larger diameter than the stent members.

FIG. 1H is a schematic profile diagram of an alternative embodiment of the stent of the present invention in which strut members include barbs at either end thereof.

FIG. 1I is a schematic profile diagram of an end view of the stent shown in FIG. 1H .

FIG. 2A is a plan view of an alternate embodiment of the present invention having split helical elements interposed between strut members and connected to the strut members by flexible linking elements.

FIG. 2B is a detailed view of FIG. 2A showing a portion of the split helical elements.

FIG. 2C is a detailed view of an alternative embodiment of the helical element comprising a plurality of segments.

FIG. 3 is a plan view of an alternate embodiment of the present invention having helical elements with thin sections interposed between strut members and connected to the strut members by flexible linking elements.

FIG. 4 is a plan view of an alternate embodiment of the present invention comprising helical elements with thin diagonal sections with portions of the thin diagonal sections having a reduced width and curved flexible linking elements connecting the helical elements to the strut members.

FIG. 5A is a plan view of an alternate embodiment of the present invention having a variable pitch of helical elements including curved sections having a reduced width and curved flexible linking elements connecting the helical elements to the strut members.

FIG. 5B is a detail view of FIG. 5A showing the helical elements including curved sections having a reduced width

FIG. 6A is a schematic diagram of a balloon expanding stent delivery system before inflation of the balloon with a stent on the balloon.

FIG. 6B is a schematic diagram of a balloon expanding stent delivery system before inflation of the balloon without a stent on the balloon.

FIG. 6C is a schematic diagram of a balloon expanding stent delivery system after partial inflation of the balloon without a stent on the balloon.

FIG. 6D is an end view of FIG. 6C showing the rotation of the balloon expanding stent delivery system.

FIG. 7A is an end cross section schematic representation of a self-expanding stent delivery system of the present invention prior to stent deployment.

FIG. 7B is an end cross section schematic representation of a self-expanding stent delivery system of the present invention during stent deployment.

Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts.

FIG. 1A is a plan view of a first embodiment of stent 10 . As used herein, the term plan view will be understood to be a 2 dimensional (2-D) view where the stent has been cut along the axis and laid out flat, such that the bottom edge could be wrapped around a cylinder and connected to the top edge. Unless otherwise indicated all plan views are plan views of the stent in an expanded state. Stent 10 comprises strut members 12 , helical elements 13 and flexible linking elements 14 . In this representation one of helical elements 13 has been shaded. Strut members 12 comprise one or more strut ring portions 15 and 16 . Strut ring portions 15 and 16 comprise respectively a plurality of strut elements 17 and 18 . In this representation, one of strut elements 17 and 18 has been shaded. The plurality of strut elements 17 can be integral to one another. The plurality of strut elements 18 can be integral to one another. Connecting elements 19 connect strut elements 17 and 18 together at peaks 20 and 21 . Connecting elements 19 can be rigid, flexible or of various predetermined lengths, for example, a helix configuration. Helical elements 13 are arranged in a side-by-side configuration in which each of helical elements 13 is helically wound about an axis of stent 10 . Helical elements 13 are expandable radially upon deployment. In this embodiment, each helical element 13 makes more than one rotation about the surface of stent 10 . However, they can make a partial rotation or one complete rotation. Strut member 12 generally provides an anchoring mechanism for stent 10 and comprises one or more strut ring portions 15 and 16 depending on the requirements of the stent. Strut ring portions 15 and 16 of strut member 12 provide radial structure and a mechanism for radial expansion of stent 10 . In a preferred embodiment, strut member 12 is positioned at either end of stent 10 . Strut member 12 being radially expandable upon deployment. Strut ring portions 15 and 16 can be formed of a variety of shapes and sizes to provide desired strength and stiffness upon expansion. For example, strut ring portions 15 and 16 can have a circumferential configuration in which strut ring portions 15 and 16 wrap around a circumference of stent 10 and each connects to itself.

In an alternate embodiment, stent 10 b includes strut ring portions 15 b and 16 b having a helical configuration in which strut ring portions 15 b and 16 b wrap in a spiral around the circumference of stent 10 b and along the length of stent 10 b , as shown in FIG. 1B . Flexible linking element 14 b can include a curved section for connecting helical element 13 b to strut member 12 b . Connecting elements 19 b connect strut elements 17 b and 18 b together diagonally at peaks 20 b and 21 b . Accordingly, adjacent peaks 20 b and 21 b are offset diagonally from one another.

Any number of strut ring portions 15 and 16 can be used in strut member 12 . For example, strut member 12 c can comprise a single strut ring portion 15 c , as shown in FIG. 1C , a pair of strut ring portions 15 and 16 , as shown in FIG. 1A , or multiple strut ring portions 15 d connected together with connecting elements 19 d , as shown in FIG. 1D . Connecting elements 19 d connect adjacent strut ring portions 15 d together diagonally at peaks 20 d and 21 d in either diagonal direction D 1 and/or diagonal direction D 2 .

Helical elements 13 can be connected with linking element 14 to each respective strut element 17 , as shown in FIG. 1A . Linking element 14 can be flexible. Alternatively, helical elements 13 can connect to some of strut elements 17 as shown in FIG. 1B . FIG. 1B illustrates an embodiment in which every other strut member 17 is connected with linking element 14 to helical element 13 . It will be appreciated that alternate combinations of connections of helical elements 13 to some of strut elements 17 can be used in accordance with the teachings of the present invention.

In an alternate embodiment, strut members 12 e can be flared to provide improved anchoring of stent 10 e , as shown in FIG. 1E . In an alternate embodiment helical elements 13 g have a larger diameter than strut member 12 , as shown in FIG. 1G . In an alternate embodiment, strut members 12 f can have a bulged configuration in which strut member 12 f has a larger diameter than helical elements 13 to provide improved anchoring, as shown in FIG. 1F .

Strut member 12 can have additional features to provide improved anchoring in the vessel. For example, strut member 12 g can include at least one barb 24 on distal end 22 and/or proximal end 23 of stent 10 g , as shown in FIGS. 1H and 1I .

Referring to FIG. 1A , linking elements 14 connect strut members 12 to helical elements 13 . Linking elements 14 allow the helix angle of helical elements 13 to change as the diameter of stent 10 increases during expansion. Linking elements 14 can be straight or curved.

Pitch of helical elements 13 is related to the diameter of stent 10 and helix angle of helical elements 13 by the following formula:

Pitch=π D Tan(θ)  (1)

wherein θ is the helix angle, as shown in FIG. 1A , of the helical elements and D is the diameter of the stent.

The Diameter of stent 10 and Helix angle of helical elements 13 in an expanded and a crimped state are related by the following formula:

De /Cos(θ e )= Dc /Cos(θ c )  (2)

wherein De is the expanded stent diameter, θe is the expanded helix angle, Dc is the crimped diameter and θc is the crimped helix angle.

As stent 10 is expanded the diameter of stent 10 gets larger and the helix angle of helical elements 13 decreases. The crimped diameter is limited by the helix angle, the width of helical elements 13 and number of helical elements 13 .

During use, stent 10 comprised of helical elements 13 and linking elements 14 provides a high percentage of vessel covered by the stent, for example, such as a vessel near an aneurysm. For example, the percentage of the portion of the vessel covered by the helical elements of the stent can be in the range of about 35% to about 90%. Preferably, the percentage of the portion of the vessel covered by the helical elements of the stent is at least about 50%. Stent 10 provides an open area of about 10% to about 65%.

Linking element 14 can have a length and shape to minimize the strain and optimize the expansion behavior of stent 10 . Linking element 14 can have a thickening which varies along the length of linking element 14 . This can include relatively straight and long links having a length of about 25% to about 50% of the circumference of stent 10 . Linking element 14 can also comprise relatively straight and short links having a length in the range of about 10% to about 25% of the circumference of stent 10 . Alternatively, linking elements 14 can comprise curved links as described below.

FIG. 2A is a plan view of an alternate embodiment of stent 30 . Stent 30 comprises helical element 33 . Helical element 33 is split into two segments 34 and 35 . FIG. 2B is a detailed view showing the two segments 34 and 35 comprising helical element 33 . Splitting helical element 33 into a pair of segments improves crimping and expansion characteristics of stent 30 , allowing helical element 33 to more easily change helix angle as the diameter of stent 30 expands from a crimped state to an expanded state. In an alternative embodiment, helical elements 33 c can be split into a plurality of segments 34 c , 35 c , and 36 c to provide additional improvement in the crimping and expansion characteristics, as shown in FIG. 2C .

FIG. 3 is a plan view of an alternate embodiment of stent 40 . Stent 40 comprises helical elements 43 having notches 44 at points along its length. Notches 44 create thin sections 45 of helical element 43 . For example, thin sections 45 can have a thickness of about 10% to about 80% of the adjacent thicker portion 46 of helical element 43 . Thin sections 45 created by notches 44 can act as hinge points and offers improved crimping and expansion characteristics allowing the helix angle of helical element 43 to more easily change helix angle as the diameter of stent 40 expands from a crimped state to an expanded state. In one embodiment, notches 44 can be rectangular. Alternatively, thin sections 45 can be curved, circular, square, rectangular with fillets, and the like, and still provide improved crimping and expansion characteristics with different strain profiles.

FIG. 4 shows an embodiment in which thin sections 45 are diagonal, for connecting adjacent portions 46 a and 46 b of helical element 43 . Sections 45 have a thickness of about 10% to about 80% of portions 46 a and 46 b . Curved linking elements 51 connect helical elements 43 to strut member 12 .

FIG. 5A shows an embodiment in which thin sections 45 are curved and have a thickness of about 10% to about 80% adjacent portions 46 . In this embodiment, the pitch of helical element 43 changes along the length of stent 50 where the pitch is longer on left hand side 47 of stent 50 and shorter on right hand side 48 of stent 50 . Varying the pitch in this manner changes the percent of vessel coverage along the length of the stent. This may be advantageous in an embodiment for treating an aneurysm, in which right hand side 48 of stent 50 including a short pitch section could be placed at an up-stream side of the aneurysm for effectively cutting off blood flow to the aneurysm, and left hand side 47 of stent 50 can be used in the down stream side for not entirely covering the aneurysm or vessel wall with stent material. Stent 50 also has curved flexible linking elements 52 connecting helical element 43 to strut member 12 . FIG. 5B is a detailed view of thin section 45 of stent 50 .

The helical stents of the present invention may be placed within vessels using procedures well known in the art. The helical stents may be loaded into the proximal end of a catheter and advanced through the catheter and released at the desired site. Alternatively, the stent may be carried about the distal end of the catheter in a compressed state and released at the desired site. The stent may either be self-expanding or expanded by means such as an inflatable balloon segment of the catheter. After the stent(s) has been deposited at the desired intralumenal site, the catheter is withdrawn.

The self expanding embodiment of the stent would not necessarily require additional expansion methods. However, the balloon expanding embodiment can use additional expansion methods. Expansion methods for a balloon expanding embodiment of the present invention are described below.

In a balloon expanding embodiment of the present invention, a balloon expands the diameter of the stent and also rotates the helical elements such that the helix angle decreases, the pitch between helical elements shortens and the stent provides the desired vessel coverage of at least about 35% to about 90%, preferably about 50% of the portion of the vessel covered by the helical elements of the stent. FIG. 6A illustrates a balloon expanding delivery system 70 with a stent 40 mounted on the system. FIG. 6B illustrates balloon expanding delivery system 70 in a pre-inflated state, stent 40 has been omitted for clarity. Balloon 71 includes twists 72 which rotate in an opposite direction of helical elements 43 . Twisted balloon 71 is placed within stent 40 , as shown in FIG. 6A . Balloon 71 is inflated to an inflated state, as shown in FIG. 6C . As balloon 70 is inflated it rotates, as shown in FIG. 6D , imparting to stent 40 a torque so as to rotate helical elements 43 to a smaller helix angle, thereby shortening the helical pitch so as to provide the desired vessel coverage.

A conventional self expanding stent delivery system utilizes an outer sheath to hold the stent in a crimped (radially compact) state and a pusher positioned at the proximal end of the stent which react the forces developed as the outer sheath is retracted during stent deployment. A conventional delivery system often has an inner tube which goes over a guide wire, also used during a typical procedure.

FIGS. 7A and 7B are cross sectional schematic representations illustrating self-expanding stent delivery system 90 . FIG. 7A shows stent 10 crimped and positioned within outer sheath 91 . Pusher balloon 93 is positioned within stent 10 in a deflated state. Pusher balloon 93 is adjacent inner tube 94 . Inner tube 94 can be a tube for a guide wire. Pusher balloon 93 is sized such that it can replace or augment the pusher positioned at the proximal end of the stent in a conventional delivery system as described above.

In FIG. 7B pusher balloon 93 is inflated to a diameter about the same as the inside diameter of stent 10 in a crimped state. Pusher balloon 93 in its inflated state can distribute the reaction force developed during retraction of outer sheath 91 over the length of stent 10 instead of only at the proximal end. Accordingly, stent 10 does not significantly deform even though the axial stiffness of stent 10 is relatively low.

In an alternative embodiment, fluid used to inflate pusher balloon 93 can be chilled to a temperature in the range of about −10° C. to 10° C., thereby lowering the temperature of stent 10 , reducing the outward force that stent 10 exerts on outer sheath 91 , and lowering the reaction force developed to retract outer sheath 91 .

The stent of the present invention may be placed within body lumen such as neurovascular vessels of non-neurovascular vessels of any mammal species including humans, without damaging the lumenal wall. For example, the stent can be placed within a lesion or an aneurysm for treating the aneurysm. In one embodiment, the stent can be placed within a brain aneurysm for treating the brain aneurysm. In one embodiment, the stent can be placed in a super femoral artery. Upon insertion into the vessel, the stent provides coverage of at least about 35% to about 90% of the portion of the vessel covered by the helical elements of the stent, preferably at least about 50% of the portion of the vessel covered by the helical elements of the stent.

The stent of the present invention can be formed of any biocompatible material, such as stainless steel, titanium, gold, nickel-titanium (often referred to as shape-memory metal or “nitinol”) alloys, plastics and combinations thereof.

It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.