Field of the Invention

The invention relates to an endovascular stent graft device preferably for treating descending thoracic aortic aneurysms.

BACKGROUND OF THE INVENTION

A stent graft is a medical device comprising a tubular column graft material supported by metallic wire frame that may be used in the treatment of vascular diseases including aneurysms. Blood pressure within the diseased portion of the blood vessel can cause the aneurysm to rupture and haemorrhage. Stent grafts can be used in the treatment of aneurysms, including thoracic aortic aneurysms, abdominal aortic aneurysms etc, by isolating the blood flow from the aneurysms. Typically, the stent graft device is delivered to the freatment site through the vascular system of the patient rather than by open surgery. The stent graft is reduced to a small diameter and placed into the leading or proximal end of a catheter delivery system. The stent graft devices of this kind are usually deployed percutaneously. Preferably, the delivery system is inserted into the vascular system of the patient such as through a femoral incision. After being deployed the stent graft expands and anchors onto a healthy portion of a blood vessel on both ends adjoining the aneurysm. Once installed, the blood flows through the tubular stent graft and the diseased portion of the blood vessel is isolated from the pressure of flowing blood.

A wide range of endovascular stent grafts have been developed for temporary or permanent implantation within a body lumen. For instance, US 7,232,459 discloses a thoracic stent graft having a tubular body with a proximal and distal end. Along the length of the body a number of self-expanding zigzag stents are provided, which are joined to the graft material by stitching using a monofilament or braided suture. At the outer surface of the tubular body, at the proximal end is smooth and with the assistance of an internal zigzag stent can engage and seal against the wall of the aorta when it expands and deployed. Also, affixed to some of the struts are barbs which extend distally fi"om the struts through the graft material for engaging and/or penetrating into the walls of the aorta and thus prevents distal movement of the stent graft caused by pulsating blood flow through the stent graft.

Other stent devices have angulated ends or hooks that achieve the same purpose of preventing distal movement of the stent graft upon deployment. Such use of barbs, hooks or angulated ends may result in tears in the endothelial wall thus producing trauma in the blood vessel at any time after deployment due to the constant pulsating blood flow in the vessel. It has also been studied that over time, and under influence of blood pressure, the unsupported region of the device, could bend and force the distal part of the stent graft to move into the aneurysm.

In other devices, there is a possibility of the rings overlapping each other when the device is deployed at the arch of the aorta. Overlapping of the rings would lead to fish scaling of the rings, which is not desirable.

SUMMARY OF THE INVENTION

In order to overcome the disadvantages associated with the stent graft devices of the prior art, the invention provides a stent graft device that includes a tubular body having a proximal end, a distal end and a middle region. The term distal end refers to the portion of the aorta that is fiirther away from the blood flow fi-om the heart and the term proximal end refers to the portion of the aorta that is closer to the heart. The proximal end of the tubular body, in particular, would require rings that would need to be placed in such a manner that it affixes firmly to the vessel wall as a good seal so as to prevent endoleaks. Thus, the stent graft according to the present invention avoids the tapered ends, or the presence of hooks, barbs etc. to ensure that proximal fixation is strong and in turn resists migration of the device with the blood flow.

A similar structure is also made in the distal end of the tubular body. The distal end provides anchoring to the stent graft and also prevents time related movement of the distal part of the stent graft into the aneurysm. Thus, the distal end anchoring prevents bending and forcing the distal part of the unsupported region of the stent graft to move into the aneurysm over time and influence of the blood pressure.
The invention relies on the number of struts present in a ring and the amplitude of the strut to provide enough radial force to achieve good fixation. The stent graft according to this invention provides good radial stability for sustaining the contraction forces of the heart. Also, a high plastic ductility is achieved for optimal dilatation/crimping. Since no barbs or projections are provided, a minimum shear stress is applied on the vascular walls.

Moreover, the stent graft device according to the invention provides superior anchorage because of the design of the proximal and distal end rings thereby preventing endoleaks. Also, the high radial force provided by the end rings prevent migration of the stent graft device. Fish scaling is avoided, according to the stent graft, according to this invention since the rings are being separated fi-om each other by a polymer. The rings are sandwiched within layers of the graft polymer, and the layers are thermally bonded to each other. This prevents the need for using

sutures to anchor the rings to the graft, thereby eliminating perforations in the graft, which could result in tears.

Prior to the percutaneous intervention, the stent graft device would be crimped to about 1/4* of the its diameter and subsequently inserted into, say, the descending thoracic aorta through the femoral artery. Upon deployment, the stent graft, made of smart alloy returns to its original diameter and due to the accurate and analysed number of struts in the ring and the amplitude of each strut, sufficient radial force is exerted on the walls of the vessel to achieve good fixation.

STATEMENT OF INVENTION

Accordingly, the invention relates to a stent graft device for treating aneurysms having: a tubular body including a proximal end portion for anchorage onto a healthy blood vessel upon deployment of the device, a distal end portion and a middle body portion; a plurality of rings embedded between layers of a polymer material and provided circumferentially throughout the length of the tubular body; wherein upon deployment of the device, the radial force necessary to resist migration of the device and/or to maintain endoleak seal is solely provided by the rings surrounding the circumference of the proximal end portion and/or the distal end portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein the showings are for the purpose of illustrating a possible embodiment of the invention only, and not for the purpose of limiting the same.

Figure 1 shows the stent graft device according to a preferred embodiment of the invention;

Figure 2 shows the front view of the proximal or distal end rings according to the preferred embodiment of the invention;

Figure 3 shows the front view of the middle rings of the stent graft device according to this invention;

Figure 4 shows the construction of a single ring;

Figure 5a shows the loading and unloading plot of the middle region rings based on various artery diameters;

Figure 5b shows the loading and unloading plot of the end rings based on various artery diameters;

Figure 6a shows the Haigh diagram for fatigue analysis of the middle region rings for a fatigue life of 400 million cycles;

Figure 6b shows the Haigh diagram for fatigue analysis of the end region rings for a fatigue life of 400 million cycles.

DETAILED DESCRIPTION OF THE INVENTION

Referring to figures 1-3, the front view of the stent graft device (20) according to the invention is illustrated. The stent graft device (20) includes a tubular body I having a proximal end portion (2) and a distal end portion (4) and a middle body portion (6). Along the entire length of the tubular body (20) a plurality of rings (3, 5, 7) are provided around the entire circumference of the tubular body (20).

The rings (3, 5, 7) are preferably made of any shape memory alloy, for instance, nitinol, (an alloy of nickel and titanium, known for its unique superelasticity, shape memory properties and radio-opacity) and have been shape set at a particular temperature to obtain a sinusoidal shape having crests and troughs. Each crest and trough on the ring forms a strut. The rings (3, 5, 7) would be embedded between layers of polymer, such that they are not exposed to the surrounding environment. Preferably, expanded polytetrafluoroethylene (ePTFE) is used as the polymer. The diameter of the device would vary depending on the patient and can be anywhere between 27 to 42mm.

According to the invention the proximal end rings (3) and the distal end rings (7) on the circumference sufficiently exerts radial force on the walls of the healthy blood vessel to resist migration and/or to help maintain a good seal thereby preventing endoleaks. Therefore, the present invention relies on the number of struts present in a ring and the amplitude of the struts to provide enough radial force to achieve good fixation.

For the sake of illustrating the invention one exemplary type of a stent graft device is chosen. The invention is not limited to the below example but can be extended to any other dimension of the stent graft based on the diameter of the artery and other factors.

For an exemplary stent graft device having a length of 150mm and an outer diameter of 36 mm the end rings (3, 7) is provided with rings having 8 struts, each strut having an amplitude of about 7.5mm. This small amplitude rings with increased number of struts results in the ring covering a larger surface area, thus supporting the graft much better. The reduced amplitude produces an increased radial force when the device is deployed, thereby resulting in a superior sealing action. Radial force is defined as the force exerted radially by the device on the

vessel, ensuring that the device is fixed firmly against the blood vessel, which is necessary to ensure that the device is aligned along the axis of the vessel and remins conformed to the vessel, preventing enodleaks.

The rings at the proximal region are also important to ensure that the device remains anchored in the landing zone of the vessel. The landing zone is the healthy part of the vessel preceding the aneurysm, a region where the interventional cardiologist would anchor the device. Normally, any device would aim at 20-3 0mm of the landing zone for anchorage. The stent graft device according to the present invention incorporates three rings (ERl, ER2, ER3), each having 8 struts with an amplitude of 7.5mm in the proximal end (2) and in the distal end (4), wherein even two rings being anchored in the landing zone would provide sufficient resistance to migration. The rings would preferably be parallel to each other as opposed to being staggered. Each of these proximal/distal region rings are arranged at a distance of around 5mm from each other. The device is manufactured such that during the crimping of the stent graft before being deployed percutaneously, the rings do not overlap each other even though there is only 5 mm distance from each other.

The middle body segment (6) includes at least 4 or 5 rings each having 6 struts depending on the length of the entire device (6). Figure 3 shows a middle body portion (6) having 4 rings (MRl, MR2, MRS, MR4). Each of these rings are arranged parallel to each other at a distance of about 10 mm from each other.

The construction of the end rings (3, 7) on the circumference of the tubular body (20) as mentioned above ensures that sufficient radial force is exerted on the walls of the healthy blood vessel to resist migration or distal movement of the stent graft caused by pulsating blood flow within the stent graft and helps in maintaining a good seal thereby preventing endoleaks, and of the middle body rings (5) such that fewer rings are more than sufficient to support the graft within the aneurysm, maintinaing its tubular shape even under high intrinsic forces.

Figure 4 shows a single end ring (Rl) according to an embodiment of the invention. The diameter of the stent wire constituting the end ring is about .35mm and is sinusoidally shaped with each strut having an amplitude of 7.5mm. Further, the thickness of the graft material which embeds the rings is about 0.15-0.2mm.

The forces acting on the artery of different diameter by a single ring which is .35mm in diamater is shovm in table 1 below:

TABLE 1

Radial Force for a single Radial force per unit length (N)

Artery Diameter

ring (N) (proximal/distal zone)

34 09 0.0831
32 L75 0.162
30 Is 0231

The charts illustrated in figures 5a and 5b show the loading and unloading plot of the device and from this, the radial force of the device acting on the vessel wall when deployed in artery of different diameters can be determined.

Radial strength is an indication for anchorage and proximal seal. The difference in the radial strength in the proximal and distal portion between an 8-strut ring and a 6-strut ring (0.35mm in diamter)is shown in table 2 below.

TABLE 2

Radial Force for a single Radial force per unit length (N)

Artery Diameter

ring (N) (proximal/distal zone)

8 strut 6 strut 8 strut 6 strut

34 0.9 06 0.0831 0.08
32 L75 U 0.1620 olOT
30 2l r6 0231 0A

The difference between an 8 strutted and a 6 strutted ring is that an 8 strutted ring is capable of providing a higher radial force, hence the need for it to be used at the proximal and distal end portions, which requires this high radial force to provide good anchorage. Overexpansion of the device is still maintained at 8-10% , with these ring configurations, thus preserving the vessels' elasticity to an extent. Since reducing the amplitude of a ring makes it capable of yielding a higher radial force, reducing the amplitude of a 6 strut ring could also yield similar radial forces as an 8 strutted ring, however, since a good seal is also of significant importance, in comparison, an eight strut ring gives good radial strength and increased metal surface area for better seal.

When a stent device configuration is chosen the device should be able to be crimped from a lower diameter to its actual diameter. In the case of the present invention the stress and strain values of the stent graft device fall within the threshold, indicating that the current construction holds good for crimping and is thus validated.

Also, from the strain table the strain amplitude is calculated for each element. The maximum strain amplitude is 0.000612 for a graft thickness of 0.15mm. When the device is deployed in the artery less than its diameter, the strain experienced by the device w^ould be the addition of strain amplitude and residual strain. From the reference SN curve the Haigh diagram is drawn with the calculated mean strain and strain amplitude to show whether the device withstands 400 million lifecycles. 5 Table 3 below shows the calculation of mean strain and strain amplitude for the body portion rings.

TABLE 3

10

Thick Residual Strain for Strain for Actual strain Actual strain for Mean strain Alternating

nessof Strain after 120mm of 80mm of for 120mm 80mmofHg(%) (%) strain (%)

the deployment Hg(%) Hg(%) of Hg(%)

Artery (%) (beam (beam (5) = (l) + (3) I(4)+(5)l/2 [(4)-(5)]/2
(mm) analysis) analysis) (4) = (l) + (2)
(1)

(2) (3)

34 Ol ^017 -0.112 0!33 0.388 0.359 0.029

32 i ^OIT -0.112 083 0.888 0.859 0.029

30 LS ^017 -0.112 r33 1388 1359 0i029

Table 4 below shows the calculation of mean strain and strain amplitude for the body portion rings.

TABLE 4

Thick Residual Strain for Strain for Actual strain Actual strain for Mean strain I Alternating

nessof Strain after 120mm of 80mm of for 120mm 80mmofHg(%) (%) strain (%)

the deployment Hg(%) Hg(%) of Hg(%)

Artery (%) (beam (beam (5) = (l) + (3) [(4)-K5)]/2 l(4H5)]/2

(mm) analysis) analysis) (4) = (l) + (2)

(1)
(2) (3)
34 07 ^^OAl -0.112 053 0.588 0.559 0.029
32 US ^OTT -0.112 U8 r238 1209 0.029
30 2A ^017 -0.112 L93 r988 r959 0.029

5 The results obtained from the Stress-Strain Analyses tabulated in tables 3 and 4 are used to obtain the threshold curve. The values plotted for strain for the stent graft device configuration of the present invention as plotted in figures 6b falls below the threshold indicating that the device would meet 400 million life cycles. Figure 6a shows the Haigh diagram for the middle body portion ring having 6 10 struts with 10mm amplitude with 0.15mm graft thickness and the Figure 6b shows the Haigh diagram for the proximal/end portion ring having 8 struts with 7.5mm amplitude and with 0.15mm graft thickness.

The radial force is good and sufficient in the proximal/distal rings when deployed in appropriate artery diameters and thus minimizing incidences of endoleaks or 15 device migration.

It is also observed that the device holds the full 400 million lifecycle v^^ithout any break or collapse.

The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof

WE CLAIM

1. A stent graft device for treating aneurysms having:

a tubular body including a proximal end portion for anchorage onto a healthy blood vessel upon deployment of the device, a distal end portion and a middle body portion;

a plurality of rings embedded between layers of a polymer material and provided circumferentially throughout the length of the tubular body;

wherein upon deployment of the device, the radial force necessary to resist migration of the device and/or to maintain endoleak seal is solely provided by the rings surrounding the circumference of the proximal end portion and/or the distal end portion.

2. The stent graft device as claimed in claim 1 wherein the rings have a predetermined number of struts in each of proximal end portion, distal end portion and the middle body portion and the radial force provided to resist migration of the device and/or maintain endoleak seal is solely based on the number of struts per ring and amplitude of each strut.

3. The stent graft device as claimed in claim 1, wherein, for a tubular body having a length of 150mm and diameter of 36mm, 3 rings are provided in the proximal end portion and the distal end portion of the tubular body.

4. The stent graft device as claimed in claim 3, wherein each ring surrounding the circumference of the proximal and distal end portions has 8 struts, each strut having an amplitude of 7.5mm.

5. The stent graft device as claimed in claim 3, wherein at least 4 rings are provided in the middle body portion of the tubular body

6. The stent graft device as claimed in claim 5, wherein each ring surrounding the circumference of the middle body portion has 6 struts.