BACKGROUND OF THE INVENTION
A stent is used to provide a prosthetic intraluminal wall, e.g., in the case of a stenosis to provide an unobstructed conduit for blood through the area of the stenosis. An endoluminal prosthesis comprises a stent which carries a prosthetic graft layer of fabric or other material and is used in the instant invention to treat an aneurysm by removing the pressure on a weakened part of an artery so as to reduce the risk of distal embolization or rupture of the distended wall of the aneurysm.
Typically, a stent or endoluminal prosthesis is implanted in a blood vessel at the site of the aneurysm by minimally invasive techniques in which the stent is compressed along its long axis and is delivered by catheter to the site where it is required. A cut down technique in which the blood vessel to be used as conduit is exposed by minor surgical means. Various catheters or sheaths are inserted through small openings made in the conduit blood vessel. The stent is directed into the correct position and the catheter is withdrawn. The catheter's withdrawal allows the stent to re-expand to a predetermined diameter in the vessel.
Aneurysms in vessel walls at or near bifurcations pose additional problems because the intraluminal support provided by the stent must extend from the main vessel into both branches of the bifurcation. This requirement complicates the procedures for introducing the catheters for positioning the stent(s), as well as insertion and placement of the stent components to conform to the configuration of the bifurcation. Stent branches must be joined with the main stent in the correct configuration and without leakage at any juncture. It is also desirable to conduct additional procedures to thrombose the material in the aneurysm cavity and to ensure re-endotheliazation of the stent walls.
A number of prostheses are known for treating the formation of aneurysms in bifurcated body lumens. A typical previously practiced bifurcated prosthesis comprises a hollow tubular graft having a main section and first and second legs or extensions. For example, in open surgery to repair an abdominal, aortic aneurysm, the main section of the prosthesis is sutured to the aorta below the renal arteries, and the first and second branches of the graft are sutured to the corresponding iliac arteries. Surgical implantation of known prostheses poses a major risk of mortality and morbidity, and open surgical implantation frequently cannot be performed on patients in poor health.
Even if surgery is deemed of acceptable risk, the cumulative mortality of patients with spontaneous rupture of abdominal aortic aneurysms is approximately seventy-five percent (75%), despite emergency surgical treatment. The likelihood of rupture is under five percent in aneurysms less than 5 cm in diameter; however, in aneurysms larger than this, surgery is required to reduce the risk of spontaneous rupture and hemorrhage. The mortality of such surgery ranges between four and eight percent (4-8%). The highest risk occurs in patients greater than sixty-nine (69) years of age with chronic obstructive pulmonary disease, left ventricular hypertrophy or a history of cerebral vascular disease. In an attempt to lower the mortality of aneurysm repair, percutaneous treatment of aortic aneurysms has been attempted. Attempts using covered stent grafts to treat the aneurysm have been approximately eighty-five percent (85%) successful. Some patients undergoing this procedure require conversion in open surgical operations. Other serious complications occur in approximately ten percent (10%) of the patients so treated. Many of these patients develop a post implantation syndrome consisting of leukocytosis and elevated C-reactive protein levels. Other difficulties with covered stent grafts include difficulty in placement of the grafts, potential for obliteration of branch vessel ostia with the stent graft which in turn might compromise flow to lumbar or visceral arteries.
In attempts to overcome the drawbacks associated with surgically implanted grafts, a number of methods and devices have been developed to implant such grafts percutaneously. Kornberg, U.S. Pat. No. 4,562,596 describes a bifurcated graft for intraluminal, percutaneous implantation. The graft comprises a hollow tubular main portion connected to a first leg and a shorter second leg. The main portion includes a plurality of barbs that impale the healthy tissue of the aorta to maintain the graft in position. Such grafts have a drawback in their inability to provide a fluid tight seal at the ends of the graft. Without adequate sealing, bypass flow paths (endo leaks) may develop between the graft and the aneurysm wall, which may eventually cause rupture of the aneurysm.
Chuter, U.S. Pat. No. 5,387,235 describes a bifurcated graft having a main portion connected to first and second legs. The main portion, first and second legs each include a barbed self-expanding anchor ring that engages healthy tissue in either the aorta or the iliac arteries to retain the graft in position. These types of devices have a drawback that the diameter of the delivery system must be large enough to accommodate the combined diameters of each of the anchor deployment mechanisms. Thus, it may not be possible to use the device taught in the Chuter '235 patent in patients having small diameter femoral or iliac arteries.
Yet another drawback of previously known bifurcated grafts is the difficulty encountered in pulling the legs of the graft into the branch vessels. While a number of methods have been described in the prior art for accomplishing this task, these methods generally involve snaring a guide wire, either in the iliac artery or in the abdominal aorta, to place a guide wire for deploying the leg of the graft in the contralateral branch.
Shmulewitz, U.S. Pat. No. 5,961,548 describes a bifurcated graft, and methods of implantation that provide positive sealing between the graft and healthy tissue proximal and distal of the graft site. Shmulewitz further describes methods of implantation of a bifurcated graft that enable smaller diameter delivery systems to be employed than are taught in the prior art as well as enhancement of the ease with which the legs of the graft may be deployed in the branches of a bifurcated body lumen.
The drawback presented in the graft and methods taught in Shmulewitz '548 is that the stent graft is covered, which has four major problems:
(1) Covered stent grafts are 22 F or greater and must be inserted by surgical cutdown due to the obligate nature of increased bulkiness of a stent covering combined with the metallic substrate being incompressible and, therefore, unable to be compressed below 22 F.
(2) Covering the stent graft completely eliminates blood flow to the side branches, such as the lumbars from the central lumen. This elimination of blood flow has produced paraplegia in animal models. However, retrograde collateral flow from the lumbars is not prevented. This leads to endo leaks with the potential for aneurysm expansion in humans who have been implanted with similar devices.
(3) In patients with short cuffs of normal aorta below the renal arteries, completely covered stent grafts are not able to achieve secure placement and are therefore prone to dislodgment and axial migration with subsequent endo leaking and aneurysm expansion and rupture.
(4) The second covered stent graft is designed to dock minimally within the first. There are several other similar designs, which have a very high incidence of left iliac side branch graft dissociation in time due to healing and/or expansion of the aneurysm. This disassociation of the left iliac side branch portion from the main graft results in leakage of blood into the aneurysm cavity, further expansion of the aneurysm and late, often fatal, aneurysm rupture.
Rhodes, U.S. Pat. No. 5,843,160, describes expandable, intraluminal prosthesthes for the treatment of aneurysmal or occlusive disease at a bifurcation of a vessel, duct or lumen. Each prosthesis taught by Rhodes includes three expandable sleeve sections which are arranged to be located in respective portions of the vessel, duct or lumen contiguous with the bifurcation and secured together in situ. The Rhodes prosthesis for aorto-iliac aneurysmal disease includes a common, stent supported sleeve section having an expandable outer balloon for engaging and trapping the thrombus in the aneurysmal space in the abdominal aorta, and a pair of stent-supported sleeve limb sections. The limb sections are connected to the common section to provide a passage for blood to flow through the prostheses. One or more of the limb sections can also include an outer balloon for engaging the thrombus within its associated aneurysmal space. Sealing mesh is provided on the prosthesis to prevent the egress of emboli and to permanently secure it in place. The Rhodes prosthesis for aorto-iliac occlusive disease is similar to the prosthesis for aneurysmal disease, except that the sections of the prosthesis do not include an outer balloon. The invention described by Rhodes presents several problems:
(1) The device is bulky, including at least four superimposed layers with inner stent rings, an outside fabric covering, balloon material external to this fabric covering, and subsequently mesh or other thrombogenic material covering the balloon. This degree of bulkiness may preclude percutaneous delivery and requires surgical interventions for insertion.
(2) The external balloon feature of this design is particularly problematic. In other applications with permanently implanted balloons (such as embolization balloons for treating large arterial venous malformations) the balloon material degrades and ruptures after several months to years post implant. Such an occurrence in this situation would be disastrous and would likely lead to further aneurysm, expansion, and rupture.
(3) Simply placing mesh or fibers in a potential space has not produced thrombosis and fibrosis of the aneurysm cavity in humans. This space must be almost completely filled with thrombogenic material for the progressive thrombosis and fibrosis to occur.
(4) This covered stent design precludes placement in patients with a short cuff of normal aorta below the renal ostia.
(5) This stent graft will lack axial stability due to the fact that the stent segments are interspersed throughout the fabric graft. The fabric will then be prone to collapsing upon itself axially without support of a continuous stent within it to form a backbone.
(6) In human applications of similar devices, axial dislodgment also produces a significant incidence of endo leaks, further aneurysm expansion, and rupture.
Fischell, U.S. Pat. No. 5,749,825, discloses a system designed for treatment of coronary artery stenoses, not aortic aneurysms. It has no provision for exclusion of the aneurysm cavity. Treatment of bifurcation coronary disease is already readily accomplished using conventional balloon expandable stent systems.
Dorros, U.S. Pat. No. 5,720,735, discloses a device designed for treatment of stenotic or occlusive vascular disease in bifurcated segments. There are no implications for its use in aneurysmal disease as taught by the instant invention, particularly with regard to occlusion of the aneurysm cavity.
Goicoechea et al., U.S. Pat. No. 5,718,724, describes a covered stent graft for treatment primarily of aortic aneurysm and bifurcation aortic aneurysm disease. Problems with this device include:
(1) This device is designed to be deployed just below the renal arteries without retention hooks, thus allowing for axial dislodgment in the case of short necks of normal aorta immediately below the renal artery ostia. Without a direct application mechanism, and without adequate axial stability, this graft will have a tendency to suffer axial dislodgment with subsequent endo leaks, aneurysm expansion, and rupture.
(2) The links in the stent material are not constructed from a solid tube and thus are prone to late fracture with subsequent penetration of the graft material which would permit leakage of blood into the aneurysm cavity, aneurysm expansion, and late rupture.
(3) The short overlap segment of the bifurcation segments. Lack of positioning of the bifurcation segments at the aortic bifurcation leads to significant axial instability of the bifurcation segments and late disruption of the junction with leakage of blood into the aneurysm cavity, aneurysm expansion, and late aneurysm rupture. Similar devices have suffered significant incidences of late aneurysm rupture due to the above mentioned design issues.
Parodi, U.S. Pat. No. 5,693,087, discloses a device designed as a composite of a proximal stent bonded to traditional tubular vascular grafting material such as DACRON®.
Problems with this device include:
(1) There is no internal stent structure, which allows the potential for axial migration.
(2) The proximal stent is not designed to cover the renal ostia; therefore, positioning this stent in patients with short necks below the renal arteries can be problematic and prone to axial migration and endoluminal leakage.
(3) The bulkiness of the device precludes percutaneous delivery and it must be inserted using surgical cutdown techniques.
(4) The commercial version most commonly utilized is not bifurcated and therefore the contralateral iliac is excluded from blood flow within the stent. This exclusion necessitates coil embolization of the contralateral vessel, followed by a surgical femoral-femoral bypass to the contralateral iliac, which is decidedly invasive.
(5) Further difficulty with deployment of this device is the necessity for stopping the heart during proximal stent expansion to prevent axial migration. While in most cases this can be done with intravenous adenosine without sequelae, significant potential risk does exist with this maneuver.
Marin, U.S. Pat. No. 5,695,517, discloses a bifurcation stent graft utilizing separate bifurcation limbs. The graft material is sewn to stents, which are subsequently manually formed into a “D” type configuration. Problems with this device include:
(1) The device is bulky and cannot be delivered percutaneously. It must be delivered by insertion by surgical incision and repair.
(2) Since the graft material does not have continuous stent support, there is a considerable degree of axial instability. The short stent segment requires a long segment of normal aorta prior the aneurysm, below the renal arteries, for deployment and there is no provision for above renal artery deployment.
(3) Since the grafts are independent, slight slippage of either limb of the graft produces a major disruption of the mechanical connection between them, which is largely frictionally based. This disruption potentially leads to a major endoluminal leak, aneurysm expansion, and rupture.
(4) The “D” type configuration is achieved by use of a mechanical linkage device rather than self-expansion of preformed “D” segments. Furthermore, the “D” segment is just below the renal arteries rather than at the bifurcation and it is not constrained by an external main stent. Therefore, mechanically, it is significantly unstable.
Marin et al., U.S. Pat. No. 5,507,769. This patent is similar to the previous Marin U.S. Pat. No. 5,695,517 and presents similar limitations and/or disadvantages including:
(1) The short stent segment proximally precludes axial stability since the renal vessels cannot be crossed with this device.
(2) The lack of continuous stent material, proximally to distal, leads to further axial instability. Axial displacement of the graft would lead to massive endo leak, aneurysm expansion, and rupture.
(3) The covered, non-permeable nature of the graft material could exclude side branches, providing further pressurization of the aneurysm cavity from the side branches, but, in addition, limiting antegrade blood flow through the side branches, which may lead to lumbar ischemia.
(4) The semi-circular proximal segment is by definition in a tapered area of the aorta and, therefore, cannot achieve sufficient axial stability to maintain its position. It also has no longitudinal tapering which would help in preventing axial displacement.
Edoga, U.S. Pat. No. 5,591,228, teaches a device requiring insertion through a surgical incision, including left subclavian artery and bilateral femoral arteries. The device entails graft material adherent to loose, large cell stents inside the proximal distal portions of the graft material. Problems include:
(1) This device is bulky and cannot be inserted percutaneously.
(2) It has poor axial stability since the proximal and distal limbs are fixed by stents, but there is loose graft material between the stents.
(3) It will not be applicable for patients with small aneurysm necks below the renal arteries, or short aneurysm necks below the renal arteries.
(4) The risk of graft material perforation secondary to fatigue fracture of the loose stent struts is excessive. This perforation could subsequently result in aneurysm expansion and late rupture.
Vorwerk et al., U.S. Pat. No. 5,562,724, discloses a design consisting of a porous bag attached to a proximal ring stent. The bag is non-penetrable to blood and, therefore, cannot be crossed to embolize the surrounding aneurysm cavity. Problems with this design include:
(1) This design allows for poor axial stability given the short stent segment and the covered design therefore precluding placement above the renal arteries. Therefore, aneurysms with short necks will have a very short space for stabilization of the proximal portion. Axial migration will be prone to occur with subsequent severe endo leak, aneurysm expansion, and late rupture.
(2) The legs are covered stents. The wires that are used to insert these legs are not angled and, therefore, they have potential for kinking within the main proximal stent. The non-permeable nature of this device, as described in the '724 patent, predisposes it to significant endo leaks from the lumbars as well as potentially from the iliac vessels.
Shaknovich, U.S. Pat. No. 5,669,924, describes a Y-stent/shuttle delivery system designed for use in treatment of atherosclerotic bifurcation disease in relatively small vessels. To be applicable for aortic aneurysm applications it would have to be delivered from a superior approach. The size necessary for such a device would exceed the available access size for superior delivery, such as the brachial artery. Therefore, this device would be limited to treating atherosclerotic obstructions in small-to-medium size vessels, significantly smaller than the aortic bifurcation, and would have no utility for treatment of aortic aneurysms. Similarly, there is no provision for occluding blood flow into aneurysm cavities should it be modified for treatment of aortic aneurysms.
SUMMARY OF THE INVENTION
To overcome the disadvantages and limitations of the prior art, the present invention provides a main wire mesh stent or scaffold for placement in the lumen of a blood vessel with an aneurysm at or near a bifurcation in the vessel. A particular example of the invention may be used in the abdominal aorta in patients with an abdominal aortic aneurysm. DACRON® tufts, bio-compatible foam or gel are bonded to the external surface of the stent to stimulate thrombosis of the aneurysm cavity and endothelialization of the graft lumen. Radio-opaque markers delineate the margins of the partially covered regions on the stent and assist in placement of the elements of the stent in the appropriate position in the vessel proximate to the location of the aneurysm. The present invention also comprises two bifurcation segments, or stents, comprised of a memory retentive material that is compressible to a first shape for insertion into the vessel and subsequent expansion in a position that substantially overlaps with and fits tightly within the main stent, with a larger profile above the bifurcation, acting like a “cork” to resist displacement and/or subsequent disassociation of the bifurcation stents. Each of the bifurcation stents that comprise the present invention are configured in a semi-circular, half-cork type of configuration that is tapered at the level of the bifurcation to give the stent a frustroconical shape with the larger, proximal end being the end that expands into contact with the inside wall of the main stent, thereby preventing axial displacement.
Variable cell sizes (openings in the stent wall) are utilized to minimize obstruction to side branch flow proximally (at the level of the renal arteries) and to maximize internal laminar flow distal to the renal arteries. Tapered, semi-circular, proximal orifices on the bifurcation stent elements optimize in-flow proximally and facilitate proximation and juncture with the main stent. Distally, the stents expand to a tubular conformation, thus maximizing contact with the iliac lumens.
Angled guide-wires with marker bands are used to position the bifurcation stents and direct their deployment. Radio-opaque markers on the bifurcation deployment sheath are used to insure proper alignment of the bifurcation stent elements. Trans-stent placement of vascular occlusion coils, polymeric foams, gels or glues stimulates closure of the aneurysm cavity external to the stent assembly. The small sheath size allows percutaneous closure of the vascular access sites in most patients, further reducing risks for bleeding complications, reducing both morbidity and time to ambulation.
In a preferred method in accordance with the present invention, the delivery of coils, or other thrombogenic material such as polymers, gels or glues, is guided angiographically or with intravascular ultrasound to insure obliteration of the aneurysm cavity external to the stent lumen. Gradual thrombosis and fibrosis of the aneurysm cavity lead to shrinkage of the aneurysm cavity around the wire/steel, NITENOL™ reinforced aortic wall, which in turn will prevent later rupture of the aneurysm. This safe and effective technique for percutaneous treatment of aortic aneurysms presents very low risk and enables preventive placement of the device in patients not previously considered good candidates for surgery, e.g., patients with small (less than four centimeter) aortic aneurysms or those with significant co-morbidities.
It is therefore an object of the instant invention to provide a device specifically designed for treatment of an aneurysm of the descending aorta or other blood vessels with naturally occurring or traumatic (pseudo aneurysm) arterial bulging with the potential for either rupture or distal embolization. The invention may be used in linear, curved or bifurcated vessels. The invention presents a unique device and method for treating aneurysms particularly when an aneurysm occurs proximal to a bifurcation in a vessel.
It is a further object of the instant invention to provide a device featuring an open or semi-permeable stent design, allowing for secure placement with stent overlap of normal aorta above the renal ostia. The bifurcation segments of the instant invention are deployed across the bifurcation with significant increase in bifurcation segment size and significant overlap with the main stent above the bifurcation allowing for axial stability, which resists axial migration and late disruption. Covered stent grafts with non-permeable coverings predispose to perigraft leakage from branch vessels, which can lead to aneurysm expansion and late rupture, as well as the potential, at least in animal models, of lack of blood flow in the side branches of lumbars with subsequent paraplegia. The open stent design with a semi-permeable covering of the instant invention allows for continued blood flow into the side branches that are immediately opposed to the stent, such as in the lumbar location. Coil, gel or foam embolization of the remaining side branches can be utilized to prevent perigraft leakage such as at the level of the inferior mesenteric artery.
It is a further object of the instant invention to provide a device with a continuous stent backbone that effectively prevents axial migration. The instant device and method are designed to facilitate deployment above the renal arteries without impeding flow into the renal arteries in the case of patients with a short segment of normal aorta below the renal arteries and in whom both iliacs are included in the bifurcation, precluding the need for concomitant vascular surgery.
It is a further object of the instant invention to provide a device utilizing tapered, semi-circular segments to be situated in the main stent and extending into the branch vessels proceeding from the point of bifurcation. The taper of each segment narrows proceeding from the proximal end which is placed in the main stent. The tapered segments thus form a stable union by virtue of their “cork-like” semi-conical configuration since when the segments are deployed in the main stent their wide semi-circular ends contact each other with the flat end of the semi-circles in contiguity and the outward pressure of the segmental stent expansion maintains the rounded portion of the semi-circles against the walls of the main stent. This pressure, combined with the taper of each segment matching the taper of the distal aorta and exceeding the diameter of the iliac vessels prevents the segments from slipping distally in the stent and/or becoming dislodged. The main stent configuration of the instant invention allows for placement of the initial, large-cell uncovered segment above the renal arteries for axial stability and a continuous stented segment to the bifurcation for further axial stability, which obviates the possibility of independent motion of the iliac branches.
It is a further object of the instant invention to provide an aneurysmal repair device with a long stent segment, a portion of which, with large cell size, permits placement above the origins of the renal arteries for additional axial stability. The entire stent device is intact, therefore, there should be no possibility of axial migration. Additionally, the device of the instant invention is semi-permeable for both side branch continuity (i.e. lumbars), as well as providing access for embolization of the potential space in the aneurysm sac surrounding the main stent. The instant invention described herein includes use of angled guide-wires which will minimize kinking of the legs during deployment.
It is the further object of the instant invention to:
(1) Allow percutaneous insertion and percutaneous suture closure of the arteriotomy due to the lack of a bulky covering of the stent.
(2) Utilize partial covering of the stent which permits blood flow into important side branches, such as the lumbars, which then seal off around their ostia, forming an incorporation into the stent lumen, effectively forming a biologic connection between the stent lumen and the side branch blood vessel(s) without allowing blood into the extra stent aneurysm cavity.
(3) Further utilize the uncovered portion of the stent with large cell size which permits anchoring of the graft above the renals without compromising the blood flow into the renals due to the large cell size of that portion of the stent.
(4) Provide minimal covering, allowing percutaneous placement.
(5) Provide an open space at the superior margin that allows for anchoring above the renals.
(6) Utilize the continuous axial skeleton to prevent axial dislodgment after implant.
The methods of the instant invention utilize a smaller percutaneous entry site, avoidance of side branch occlusion and slow, controlled fibrosis of the aneurysm cavity, reducing the risk for potential complications such as compromise of blood flow to the mesentery and spinal chord, late perigraft leaks and subsequent aneurysm expansion and late rupture.
This safe and effective technique for percutaneous treatment of aortic aneurysms presents very low risk and enables preventive placement of the device in patients not previously considered good candidates for surgery; e.g., patients with small (less than four centimeter) aortic aneurysms or those with significant co-morbidities.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention including novel stent constructions and methods of use hereof. The embodiment that is illustrated in these figures does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.