Patent Application
20060229709
This invention relates to vascular and endovascular grafts, such as for abdominal aortic aneurysms (AAA) or any other vascular disease, such as stenois or blocked arteries, or for the airways of the lung.
An aneurysm is an abnormal localised sac or an irreversible dilation caused by a weakness (decreased elastin) of the arterial wall. The arterial wall comprises three layers: the intima (inner wall), the media (middle wall) and the adventitia (outer wall). Damage to the media gives rise to AAA. Aneurysms are classified as either fusiform or saccular. In the fusiform case the entire circumference is affected, while one side is affected in the saccular form. Aneurysms can result from accidents, arteriosclerosis, high blood pressure, or a congenital disease. Over time the vessel wall loses its elasticity and the normal blood pressure in the aneurysm sac can lead to rupture of the vessel wall, which causes internal bleeding and eventual death in many cases. Even if the vessel wall does not rupture, a large aneurysm can impede circulation and promote unwanted blood-clot formation.
Most patients do not indicate any specific symptoms that they have an abdominal aortic aneurysm. The problem is normally diagnosed during routine medical examination or when diagnostic imaging such as x-ray is performed for other reasons.
There are currently two surgical treatments for acute AAA: open surgery or minimally invasive repair also known as the endovascular repair procedure. The objective of both methods is to isolate the aneurysm sac from systemic blood pressure and flow so as to minimize the risk of arterial wall rupture. Clinical success is defined by the “total exclusion” of the aneurysm. As a result most AAAs should stabilize or shrink. Traditional surgical repair involves opening the chest or abdomen, gaining temporary vascular control of the aorta, and below the lesion, opening the aneurysmal sac and suturing a prosthetic synthetic graft to the healthy aorta within the aneurysm itself. The outcome of standard surgical Abdominal Aortic Aneurysm (AAA) repair has proven to be excellent, with mortality rates in the range of 3% to 5%. However, standard AAA repair is not perfect, and the quality of life after this repair is impaired by postoperative pain, sexual dysfunction, and a lengthy hospital stay resulting in high health costs. These negative effects are related to the large incision and extensive tissue dissection. Mortality and morbidity increase with the presence of associated diseases and a mortality rate of 60% has been reported for high-risk patients. The standard repair is also extremely difficult in patients with a prior history of abdominal operations where extensive scarring and infection may be present. Endovascular grafting is an alternative treatment to standard open aneurysm repair. This treatment involves a surgical exposure of the common femoral arteries where the endovascular graft can be inserted by an over-the-wire technique. This is where the endovascular graft is positioned onto a catheter (tubing based delivery system) over a guidewire. Using x-ray imaging this tubing based delivery system containing the endovascular graft is introduced via the femoral artery and positioned inside the aneurysm as shown in Fig. A.
The graft itself is a synthetic material often supported with a metal (typically nitinol or 316L stainless steel) endoskeleton. Graft fixation is often achieved by the stent which creates a fixation at the proximal end by barbs or by a stent portion that is uncovered by graft material. Distal end fixation is attained by friction within the branch or iliac arteries. Such endovascular treatments offer the economic advantages of short hospital stays or even treatment as an outpatient, as well as elimination of the need for postoperative intensive care and are, therefore, extremely attractive to both patients and physicians.
Fig. B shows a typical 3D line drawing of a prior art bifurcated stent graft device comprising a stent mesh integrated into the graft. Referring to Fig. C, the Ancure® stent graft is a bifurcated, non supported stent-graft with proximal and distal “hook like” fixation devices made of Elgiloy™. The Zenith™ stent graft consists of a main body and is comprised of an aortic section, one short iliac limb (contralateral limb) and one long iliac limb (ipisalateral limb), as shown in Fig. D. The main graft component consists of woven polyester and is fully stented with self-expanding stainless steel z-stents. It also contains an uncovered suprarenal stent with hooks, which aids in fixation. The AneuRx® AAA stent graft system is a modular design with self-expanding stents with a thin wall polyester graft material, as shown in Fig. E.
Referring to Fig. F, U.S. Pat. No. 6,685,738 describes a bifurcated stent graft device comprising a proximal end, which bifurcates into a first frustoconical leg transition with a dependant iliac leg. There is also a second frustoconical leg transition, which joins up to a dependant iliac leg. For modular design stent grafts the second iliac leg is connected separately via the frustoconical leg transition, which may have barbs to help firmly connect second leg to leg transition. The proximal stent is typically implanted within the vasculature below the renal arteries in the aorta such that the main body and leg transitions are positioned within the aorta main portion and with dependant first and second leg each positioned within respective iliac arteries.
Other grafts are described in U.S. Pat. No. 6,695,875, U.S. Pat. No. 6,576,009, U.S. Pat. No. 6,224,609, U.S. Pat. No. 6,773,454 and WO99/40875. The first endovascular repair of abdominal aortic aneurysms was performed more than a decade ago. Preliminary results have been promising with short-term results comparable with conventional surgical repair. Long-term results are not so encouraging with stent graft migration, endoleaks, material failure and aneurysm rupture all being reported.
Secure proximal fixation of stents for AAA is pivotal to the long-term success of the endovascular procedure. Problems due to stent graft fixation can lead to endoleaks and stent graft migration, leaving the aneurysm exposed to systemic blood pressure. A well-known complication with this endovascular procedure is the late migration of the graft in which most of the devices are diagnosed after the first 12 months after the procedure. The effect of the migration is to expose the aneurysm sac to systemic blood pressure and flow, which if left untreated has serious consequences for the patient. Endoleaks lead to the total volume increase of the aneurysm due to the direct arterial flow into the aneurysm. This generates systemic pressurization of the aneurysm sac that eventually leads to expansion and rupture. There are five types of endoleaks: Type I—originating at the attachment sites in the aneurysm neck or iliac arteries; Type II—retrograde flow into the aneurysm sac through the lumbar arteries or inferior mesenteric artery (IMA); Type III—modular disassociation such as fabric tears or an inadequate seal for modular devices; Type IV—graft material porosity and Type V—Endotension.
Gradual enlargement of the proximal neck has been reported after stent graft repair with an enlargement rate of approximately 1 mm/year. Usually the proximal end of an endograft is oversized by 2 to 4 mm and the significance of this dilation is that the attachment mechanism loses its radial force and therefore starts to migrate.
Endovascular stent graft fatigue failures have been recognized in devices after aortic implantation. This fatigue failure leads to delayed hook fractures, metallic stent fractures, suture disruptions, fabric erosion (caused by abrasion of the polyester woven fabric with the underlying stent) and late failure of aortic neck attachments.
Stent graft failures are known to occur at the bifurcation points. Stent graft thrombosis and micro-embolism are two complications associated with endovascular repair of AAA. Stent graft occlusion in the iliac legs has also been shown. Several cases of fatal multi-organ failures have been linked to micro-embolism.
Fig. G shows the geometry of various AAA configurations and the suitability of vascular and endovascular surgery. Depending on the location and extent of the aneurysm, Types A, B and C are generally suitable to both the endovascular and surgical procedure while Types D and E can only be treated surgically.
Fig. H shows the typical internal dimensions of AAA as determined pre-operatively by the Eurostar Data Registry System. Generally, for a population base there can be quite a wide range of dimensional variation. Symmetric and unsymmetric iliac artery set ups were found with the bifurcation angle θ varying considerably from 5° to 90°.
This invention is directed towards providing an improved vascular graft.
According to the invention there is provided a vascular graft comprising:
In one embodiment of the invention the graft comprises a blending section between the proximal section and the distal legs. Preferably the blending section defines a first lumen for fluid flow from the proximal section to the first distal leg. Ideally the blending section defines a second lumen for fluid flow from the proximal section to the second distal leg. Most preferably the first lumen is separate from the second lumen. The longitudinal axis of the first lumen may be substantially parallel to the longitudinal axis of the second lumen.
In one case the first distal leg and the second distal leg are connected to the blending section at a bifurcation region. Preferably the graft is substantially “Y”-shaped.
At least one of the distal legs may be formed integrally with the blending section. The blending section may be formed integrally with the proximal section. The graft may be of integral construction.
In one case the blending section comprises a gradual flow separator to separate flow from the proximal section into the first lumen and into the second lumen. An apex section may be incorporated in the blending section. The gradual flow separator may take the form of a parabolic, hyperbolic, elliptical, circular, Bezier or B-Spline shape or a combination of these curves. The apex section may take the form of a parabolic, hyperbolic, elliptical, circular, Bezier or B-Spline shape or a combination of these curves. The apex section and gradual flow separator may be connected as one and take the form of a parabolic, hyperbolic, elliptical, circular, Bezier or B-Spline shape or a combination of these curves.
In one embodiment the blending section provides for a small difference in cross-sectional area between the proximal section and the sum of the cross-sectional areas of the distal legs. The blending section may be between the proximal section and an apex of the graft. The blending section may be incorporated in either one or both of the distal legs.
In another case the cross-section of the proximal section, and/or of the gradual flow separator, and/or of the apex section, and/or of the distal leg is circular, elliptical, parabolic, hyperbolic, Bezier, B-spline shape or a combination of these curves.
In one embodiment of the invention the blending section is shaped to minimise pressure wave reflection back to the proximal section. The blending section may be shaped to minimise flow recirculation. The blending section may be shaped to minimise skewing of flow and secondary flow profiles throughout the graft.
In one case at least part of the graft tapers distally inwardly. At least part of the proximal section may taper distally inwardly. At least part of the distal leg may taper distally inwardly.
In another case at least part of the graft tapers distally outwardly. At least part of the distal leg may taper distally outwardly. The proximal section may be tapered. The distal legs may be bell-shaped.
In one case the first distal leg and the second distal leg are substantially symmetrical. In another case the first distal leg and the second distal leg are substantially asymmetrical. Eccentricity may be included.
In another embodiment the angle subtended between:
The graft may be of a material having elasticity properties matching those of a host vessel. The elasticity properties of the graft may vary from 0.1 MPa to 500 MPa. The elasticity characteristics may have viscoelastic or non-linear stress/strain properties.
In one case the graft is of a mono or multi-filament yarn material. The graft material may be a combination of polyester knit and polyurethane or silicone or any other biocompatible rubber or polymer material. The graft may be at least partially of a stretchable material.
In another case the stent material is a shape memory alloy, such as Nitinol, stainless steel or any other biocompatible metal or polymer. The graft may have a stented structure. The graft may have a partially stented structure with stents at the proximal and distal legs.
In one case the graft comprises struts from the proximal section to the distal legs. The graft may comprise a tissue based structure.
In one embodiment the graft is configured for treatment of Abdominal Aortic Aneurysms or any other vascular disease, such as stenois or blocked arteries, or for treatment of blockages in the airways of the trachea entering the lung. The graft may be configured for implantation by vascular surgery. The graft may bes configured for implantation by endovascular surgery. The graft may be modular, having different sized sections for the proximal and both distal legs exist for general and patient-specific anatomy sizes.
In a further aspect of the invention there is provided vascular graft comprising:
According to the invention, there is provided a vascular graft comprising a proximal section and at least two distal legs, wherein the graft further comprises a blending section between the proximal section and the distal legs, the blending section being shaped to minimize one or more of:
In one embodiment, the graft configuration is suitable for the treatment of Abdominal Aortic Aneurysms or any other vascular disease such as stenois or blocked arteries or for treatment of blockages in the airways of the trachea entering the lung.
In another embodiment, the graft is suitable for vascular surgery.
In a further embodiment, the graft is suitable for endovascular surgery.
In one embodiment, the total cross-sectional area of the distal legs is equal to or greater than that of the proximal section thus resulting in an area ratio (ratio of proximal to distal leg areas) of less than or equal to 1.
In another embodiment, the blending section provides for a small difference in cross-sectional area between the proximal section and the total cross-sectional area of the distal legs.
In a further embodiment, a bifurcation begins at the proximal end.
In one embodiment, the graft comprises a gradual flow separator.
In another embodiment, an apex section is incorporated in the blending section.
In a further embodiment, the gradual flow separator takes the form of a parabolic, hyperbolic, elliptical, circular, Bezier or B-Spline shape or a combination of these curves.
In one embodiment, the apex section takes the form of a parabolic, hyperbolic, elliptical, circular, Bezier or B-Spline shape or a combination of these curves.
In another embodiment, the apex section and gradual flow separator are connected as one and take the form of a parabolic, hyperbolic, elliptical, circular, Bezier or B-Spline shape or a combination of these curves.
In a further embodiment, eccentricity is included.
In one embodiment, the blending section is between the proximal end and an apex of the graft.
In another embodiment, the blending section is incorporated in either one or both of the distal legs.
In a further embodiment, the cross-sections of the proximal section, gradual flow separator section, apex section and distal legs may be circular, elliptical, parabolic, hyperbolic, beizer, B-spline in shape or a combination of these curve details.
In one embodiment, the proximal section is tapered.
In another embodiment, the distal legs are bell-shaped.
In a further embodiment, the graft is of a material having elasticity properties matching those of the host vessel.
In one embodiment, the elasticity properties of the graft varies from 0.1 MPa to 500 MPa.
In another embodiment, the elasticity characteristics have viscoelastic or non-linear stress/strain properties.
In a further embodiment, the graft is of a mono or multi-filament yam material.
In one embodiment, the graft material is a combination of polyester knit and polyurethane or silicone or any other biocompatible rubber or polymer material.
In another embodiment, the stent material is a shape memory alloy, stainless steel or any other biocompatible metal or polymer.
In a further embodiment, the graft has a stented structure.
In one embodiment, the graft has a partially stented structure with stents at the proximal and distal legs.
In another embodiment, the graft comprises struts from the proximal to distal legs.
In a further embodiment, the graft comprises a tissue based structure.
In one embodiment, the graft is of integral construction.
In another embodiment, the graft is modular, having different sized sections for the proximal and both distal legs exist for general and patient-specific anatomy sizes.
The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which:
FIGS. 4(a), 4(b) and 4(c) are line diagrams showing the sections associated with the lateral view of a vascular graft according to the invention along the proximal end;
FIGS. 5(a) and 5(b) are line diagrams showing the curve details for a blended bifurcation lateral section;
FIGS. 9(a) and 9(b) are diagrams showing a vascular graft according to the invention with a tapered section at the proximal end and a bell shaped configuration at the distal ends;
FIGS. 14(a) and 14(b) are diagrams showing wall shear stress for grafts of the prior art and of the invention respectively; and
FIGS. 15(a) and 15(b) are plots of pressure along the wall of a prior art graft and of the vascular graft according to the invention respectively.
Referring to
The characteristics of the graft 1 are such that the cross-sectional area of the proximal section (Area 1) at the bifurcation point is less than or equal to the sum of the two cross sectional areas of both iliac legs 3, 4 (Area 2 and Area 3), i.e. Area1≦Area 2+Area 3. The area ratio of (Area2+Area 3)/Area1 should be as close to unity or greater as possible. This gives a total transmission of forward incident pressure wave (PI) with no reflection at the junction. The blending section 5 minimizes wave reflections (PR). This is very different from prior grafts which incorporate a shape at the bifurcation, which introduces a sudden cross sectional area change, at the bifurcation point from the proximal section to both iliac legs.
The blending section 5 generates a smooth transition from the proximal leg 2 to both iliac legs 3, 4 which minimizes wave reflections by ensuring that the area ratio at the bifurcated junction 5 is as close to unity or greater than unity as possible. This reduces the adverse effects subsequent to endovascular treatment of AAAs.
Based on the area ratio criteria, a blending section 7 was devised as shown in FIGS. 2(a), 2(b) and 2(c). This blending section 7 incorporates the following; a bifurcation which starts further upstream from the apex just below or at the proximal end 5. This creates a gradual flow separator, which aids in splitting the flow before the apex of the bifurcation, which is shown in FIGS. 2(b) and 2(c). This flow separator feature does not occur in prior art grafts.
The blending section 7 defines a first lumen for fluid flow from the proximal section 5 into the first distal leg, and a separate second lumen for fluid flow from the proximal section 5 into the second distal leg. The two lumen are separated by means of the gradual flow separator which separates the fluid flow from the proximal section 5 into each lumen. As illustrated in
As illustrated in
Depending on the type of AAA, and the dimensions of AAA at the bifurcation and iliac legs as given in Fig. G and H respectively, non-blended or blended distal legs may be used as shown in FIGS. 3(a) and 3(b). Ideally a blended distal leg is preferred as this gives the smoothest transition for the blended section.
The distal legs may be symmetrical or asymmetrical.
FIGS. 8(a) and 8(b) shows a 3D view of the blended bifurcation for both large and small distal leg configurations. As illustrated in
FIGS. 9(a) and 9(b) shows a top view of the blended bifurcation section of a graft with
Another variable which minimises the effects of wave reflections is the Young's modulus of the chosen material for the stent and graft. The Young's modulus varies according to the material type and the weaving method chosen i.e. either mono or multi-filament fabric. This implies that there is always a wave reflection due to a change in the elastic properties of a graft or a mismatch in compliance between the host artery and the stent graft. The wave reflection here cannot be totally eliminated, but is minimised by the choice of graft material and stent material that would reduce the difference in mismatch.
The graft is manufactured from biocompatible materials. Monofilament yarn has very high stiffness. The preferred choice of fabric covering is a multi-filament yam or combination of polyester knit/polyurethane material. This fabric reduces the difference in arterial compliance of the diseased artery. The fabric at each attachment site stretches and pulsates with the arterial wall, thus eliminating the need to oversize the fabric. This aids the use of smaller delivery systems. A total pulsating graft in combination with the blending section would minimize the effects of wave reflections being generated.
A preferred stent material is shape memory alloy Nitinol (Nickel/Titanium). This material is one of the most conforming stent materials for attachment against the arterial wall. The lower the Young's modulus of the material the lower the reflection wave will be. A pulsatile fabric or polymer is the preferred option.
For the endovascular procedure the blended graft is positioned below the renal arteries and the right and left common iliac artery as shown in
The advantages associated with a blending section with the optimized relationship for the area ratios at the bifurcated junction are:
To test the effects of the graft of the invention over a typical prior device, two rapid prototype parts made from ABS plastic were manufactured. The first part was made to typical commercial shaped geometry while the other incorporated a blending section. A pressure pulse was generated in both models with the same resistance downstream.
To examine the compliance mismatch effect, two prior stent graft devices were tested in vitro under pulsatile flow conditions and the resulting dynamic displacement was measured by the ME-46 Full Image Video Extensometer (Messphysik GmbH). The Ancure® and Zenith™ stent graft devices were tested experimentally under physiological flow conditions in an idealised and realistic silicone AAA models based on computed tomography scans. There was a considerable reduction in compliance for both stent graft devices which resulted in an increased pulse wave velocity (PWV) and a significant amount of the forward pressure wave being reflected. A reduction in dynamic compliance of 45 and 54% for both the Ancure® and Zenith™ stent graft devices was found respectively. This generated a reflected pressure wave at the proximal stent interface which resulted in 16 and 21% of the forward pulse wave being reflected for the Ancure® and Zenith™ stent graft devices respectively. The blending section reduces the need for high stiffness proximal stents.
A preliminary Computational Fluid Dynamics (CFD) study was conducted to determine the flow patterns associated with a commercial stent graft and a graft of the invention. FIGS. 12(a) and 12(b) show the axial velocity flow across the centre for both grafts.
As can be seen from
Due to both the blended section and gradual flow separator incorporated in the graft of the invention, there are reduced secondary flows for the blended section graft when compared to the prior art grafts. This is shown by the cross-sectional axial and secondary flow velocities as given in
The wall shear stress (WSS) for a prior device as can be seen from
This high WSS and recirculation region is the main reason for the reported cases of stent graft occlusions and failure of stent graft devices at bifurcation points.
There are two steep decreases in pressure for the commercial stent graft as can be seen from
The invention is not limited to the embodiments hereinbefore described, with reference to the accompanying drawings, which may be varied in construction and detail.
| Number | Date | Country | |
|---|---|---|---|
| 60666193 | Mar 2005 | US |
