Claims:We claim,

1. A self-expanding stent, with high radial strength and optimum flexibility for the treatment of venous vasculature defects, comprising a plurality of coaxially aligned radially expandable rings consisting of individuals cells, said rings being extended from a proximal to a distal end around a longitudinal axis of the stent and are interconnected by plurality of S-links; wherein said cells being arranged in a manner to form a close, open or hybrid structure.

2. The stent as claimed in clam 1, wherein the said cells are diamond, hexagonal, elliptical or rectangular shaped.

3. The stent of claim 1, wherein the first connection point of the said S-links in one ring is circumferentially offset from the second connection point in the next ring resulting in the link being transverse to the longitudinal axis of the stent between its proximal and distal end.

4. The stent of claim 1, wherein the number of S-link is two.

5. The stent as claimed in claim 1, wherein the S-links interconnecting the rings are planar, non-planar or spiral.

6. The stent of claim 1, wherein the said rings at the proximal and distal end of the stent are configured to provide a flared structure.

7. The stent of claim 6, wherein the said proximal and distal end rings are diamond shaped.

8. The stent of claim 6, wherein the inflection of said flared rings is in the range of 160° to 180°, preferably around 162° to 172° in respect to the longitudinal axis of the stent.

9. The stent as claimed in claim 1, wherein the stent has a closed cell structure with spiral S- links and wherein such S-links having at least one bulging in its center.

10. The stent as claimed in claim 9, wherein the cells are hexagonal.

11. The stent as claimed in claim 9 or 10, wherein each of the hexagonal cells in two consecutive rings in longitudinal direction are connected by S-link of spiral shape forming a stent with consecutive rows of hexagonal and rectangular closed cell structure.

12. The stent as claimed in claim 9, wherein the bulging is elliptical or oval.

13. The stent as claimed in claim 9, wherein bulging width length ranges from 200µm to 600µm and preferably from 350µm to 450µm and the length of bulging ranges from 500µm to 1000µm and preferably from 600µm to 900µm.

14. The stent as claimed in any preceding claim, wherein the wall thickness of the strut is in the range of 150µm-400µm, and preferably between 250µm-350µm.

15. The stent as claimed in any preceding claim, wherein the width thickness of the strut is in the range of 170µm-230µm.

16. The stent as claimed in any preceding claim, wherein the individual cell length is 0.5mm to 15mm.

17. The stent as claimed in any preceding claim, wherein the diameter of the stent in expanded state is in the range of 10mm-18mm.

18. The stent as claimed in claim 1 adapted to be delivered at the required body part by a delivery catheter of 8F to 12F.

19. A process of manufacturing a stent as claimed in claim 1, comprising the steps of
-laser cutting of suitable nitinol tube;
-grinding the internal and outside diameter;
-shape setting process using mandrel or mold;
-adjusting angles between cells and struts;
-electropolishing; and
-marker welding.
, Description:SELF-EXPANDING VENOUS STENT AND METHOD OF MANUFACTURE THEREOF

Field of Invention:
The present invention relates to a self-expanding stent capable of being permanently implanted in venous vasculature and the method of manufacture thereof. Particularly the invention relates to a venous stent system with sufficient radial strength and optimal flexibility for ease of deployment in the curved venous system.

Background of the Invention:
Stenting in veins is performed for maintaining patency of veins such as Inferior vena cava, iliac veins, femoral veins, and popliteal vein. Nitinol stents are widely used for venous stenting application due to its elasticity, flexibility, biocompatibility and long history of safety.

The blood vessel walls like arteries and veins are made up of tissues that may get damaged due to various causes leading to constriction and perforation of the vessels causing clinically relevant diseases like vessel perforation, ischemia, aneurysms, etc. Stents are generally used to keep the vessel lumen intact, open and facilitate normal blood flow. Stent may be made up of stainless steel, cobalt chromium alloy, nitinol alloy, etc. Stent made up of nitinol alloy is widely use for making self-expanding stents, especially for peripheral vascular applications which include stenting in iliac, femoral popliteal and infra-popliteal arteries and veins. Stents implanted in the peripheral arteries and veins are exposed to high mechanical stresses from the surrounding environment, for example bending of the knee, twisting and compression during walking or running. Nitinol stents are able to handle these external forces better than other shape memory materials due to its characteristic properties of super elasticity, extreme stresses and kink resistance. Nitinol stents are well suited for the tortuous vessel pathways of peripheral arteries and veins.

Based on the mechanical attributes, anatomy and the physiology of the vessel, its corresponding stent has to meet requirements for safe performance. The biological and mechanical response of a vessel to its stent is another fact that should be considered for long-term performance of the stent. Venous stents are designed for venous disease treatment, and in comparison to arterial stents, the venous stent has not been sufficiently investigated. The mechanical properties for venous stents are not necessarily same as arterial stent, thus, it is not always effective to treat a venous disease by arterial stent. The requirement of venous vasculature comprises of high radial force, crush resistance and excellent flexibility to sustain in the venous environment.
Most of the presently available venous stent have thicker strut with good radial strength and flexibility leading to more profile.

US2012/0078341 discloses a stent having expandable rings formed from a plurality of interconnected struts. A plurality of bridges couple adjacent rings together. The bridges are connected to adjacent rings at first and second connection points, and a first brace element is disposed there between. The first connection point is circumferentially offset relative to the second connection point so that the bridge is transverse to the longitudinal axis of the stent. The first brace element of one bridge engages an adjacent bridge or a brace element of the adjacent bridge when the corresponding adjacent rings are in the contracted configuration thereby providing additional support and rigidity to the stent to lessen buckling of the stent during loading onto a delivery catheter or during deployment therefrom.

However, the above type stents are bulky and complex to manufacture and need exists for new design of stents with improved radial strength and flexibility which can withstand high compressive force at venous vasculature.

Objects of the Invention:

It is the principal object of the present invention is to provide a stent for venous implantation having optimal radial strength and flexibility.

It is another object of the invention to provide a stent with reduced profile but enhanced strength.

It is a further object of the invention to provide a modular intra-luminal tubular stent system.
It is another object of the invention to provide a stent for venous implantation having improved ease of use in venous vasculature.

It is yet another object of the invention to provide a stent with reduced chance of kinking.

It is another object of the invention is to provide a stent with proximal and distal flaring for enhanced resistance to migration and enhanced holding capacity.

It is a further object of the invention is to provide a stent where the rows of cells are connected with S-links.

Summary of the Invention:

Accordingly the present invention provides a self-expanding stent, with high radial strength and optimum flexibility for the treatment of venous vasculature defects, comprising a plurality of coaxially aligned radially expandable rings consisting of individuals cells, said rings being extended from a proximal to a distal end around a longitudinal axis of the stent and are interconnected by plurality of S-links; wherein said cells being arranged in a manner to form a close, open or hybrid structure.

The said cells are diamond, hexagonal, elliptical or rectangular shaped

Preferably, the first connection point of the said S-links in one ring is circumferentially offset from the second connection point in the next ring resulting in the link being transverse to the longitudinal axis of the stent between its proximal and distal end. The number of S-link is two or more depending on diameter of stent.

The S-links interconnecting the rings are planar, non-planar or spiral.

Preferably, the said rings at the proximal and distal end of the stent are configured to provide a flared structure. The said proximal and distal end rings may be diamond shaped.

The inflection of said flared rings is in the range of 160° to 180°, preferably around 162° to 172° in respect to the longitudinal axis of the stent.

In one embodiment, the stent has a closed cell structure with spiral S- links and wherein such S-links having at least one bulging in its center. The cells may be hexagonal. Each of the hexagonal cells in two consecutive rings in longitudinal direction is connected by S-link of spiral shape forming a stent with consecutive rows of hexagonal and rectangular closed cell structure. The bulging is elliptical or oval.

The bulging width length ranges from 200µm to 600µm and preferably from 350µm to 450µm and the length of bulging ranges from 500µm to 1000µm and preferably from 600µm to 900µm. The wall thickness of the strut is in the range of 150µm-400µm, and preferably between 250µm-350µm. The width thickness of the strut is in the range of 170µm-230µm. The individual cell length is 0.5 mm to 15mm.

The diameter of the stent in expanded state is in the range of 10mm-18mm.

The stent is adapted to be delivered at the required body part by a delivery catheter of 8F and 12F.

The invention also provides a process of manufacturing a stent as claimed in claim 1, comprising the steps of laser cutting of suitable nitinol tube; grinding the internal and outside diameter; shape setting process using mandrel or mold; adjusting angles between cells and struts; electropolishing; and marker welding.

Brief Description of the Drawings:
Figure 1 illustrates a typical open cell and closed cell stent design.
Figure 2 illustrates a laser cut nitinol stent with hybrid design.
Figure 3 shows a laser cut expanded close cell nitinol stent with straight link.
Figure 4 shows the laser cut nitinol stent closed cell with S-link of the present invention.
Figure 5 shows a representation of one embodiment of the expanded stent of the present invention.
Figure 6 illustrates the uniformly expanded stent with hexagonal cell of the present invention
Figure 7 illustrates S-link of stent between two rings/crowns of the present invention.
Figure 8 shows flared diamond shape proximal or distal crown in expanded stage and welded tantalum markers and laser cut nitinol marker, in an embodiment of the present invention.
Figure 9 illustrates complete closed cell design with spiral like shape S-link design of the present invention.
Figure 10 shows a preferred inflexion angle of the flared rings of an embodiment of the present invention.
Figure 11 shows the preferred bulging shape and dimension of the S-link according an embodiment of the present invention.
Figure 12 is a process flow chart for manufacturing the stent of the present invention.

Detailed Description of the Invention:
The correct balance between the radial strength and flexibility of a venous stent is of much research. It is observed that the desired optimal strength of a stent can range from 0.5 N/mm to 1.50 N/mm, more preferably from 0.75 N/mm to 1.0 N/mm. With this strength, the stent is having optimum flexibility as well as its strength for the defined purpose.

Nitinol is widely used in the medical devices or medical field due to its shape memory and bio-compatibility properties. Due to its unique properties it has seen a large demand for use in less invasive medical devices. Compared to other alloys tubing technologies, nitinol tubing technology is fairly recent and has many advantages over stainless steel hypo tube like kink resistance, crush resistance, flexibility, high amount of recoverable deformation. The stenting application, the stent must be able to insert easily into catheter in compressed position. Secondary it should have good bond strength and enough flexibility for the easy deployment from the tube. Therefore, choice of design is very critical for the versatile application of stent.

Accordingly the present invention aims to fulfill the above objects. The basic design of a stent includes an array of repeating element known as strut. Struts are disposed around the circumference of a stent, and joined together by links. A series of struts and cell that spans one complete circumference is termed as a crown or ring. Adjacent rings of struts are joined by connectors forming a particular pattern. The long-term stability of the stent is needed for supporting the lumen wall and promoting the re-institution of a healthy lumen.

Based on the strut patterns, the stents are classified into open-cell design stents and closed-cell design stents as shown in Fig.1. Beside strut pattern, the number and position of connectors between the cells/rings differentiate the two designs. Closed-cell designs describe a ring where bridge elements connect all internal inflection points. Closed cell design provides uniform stenting and support to lumen walls but have issue related to flexibility of stent. Open-cell designs have fewer bridge connectors allowing more flexibility due to a reduced amount of connection points between adjacent sections. They also have large intra-strut areas as compared to the closed-cell designs. Open cell design offers better conformability to vessel curvature. Apart from design, choice of material also depends on the type of application. Various materials are used for stenting application in vascular and non-vascular applications. Stent materials are chosen based on their properties such as strength, corrosion resistance or radio-opacity. Biocompatible materials such as stainless steel, nitinol, titanium and cobalt-chromium are commonly used for stenting application i.e. cardiovascular, neurovascular, peripheral, etc.

The vein and artery are two major categories of blood vessels which have different functions and mechanical properties (Table 1). These differences arise due to their physiological functions and histological structure. For instance, the artery has more flow rate and its function is to carry the oxygenated blood to the organs, on the other hand, vein plays the role of a reservoir for blood circulatory system. Therefore, arterial stent is not recommended for vein stenting. Different characteristics of artery and vein are shown in Table 1 below.

Table 1

Characteristic Vein Artery
Blood Pressure (mmHg) 5-10 80-120
Type of Blood Flow Steady Pulsatile
Structure Compliant Stiff
The venous stent can be used in various venous stenting such as, to treat iliac vein, femoral vein, Inferior vena cava, superior vena cava, sub-clavian vein, any other veins, etc. For stenting in vein, nitinol based stents are widely used. Nitinol stent offers the best compromise between engineered plastics and traditional metals. Nitinol is a highly elastic material that can be processed to maintain a desired geometry. Further, nitinol has high fatigue resistance, better biocompatibility and better anti-migration property which make it ideal for use in numerous medical implants and devices. Nitinol tubing is also commonly used in catheters, stents, super elastic needles and other medical applications.

In the present invention, implantable grade nitinol tube is used for the manufacturing of stent. Nitinol tube with varying diameter and wall thickness can be used for manufacturing stent depending on the indication of stent and characteristic desired such radial strength, flexibility, holding capacity. In present invention, variation of nitinol tube, design, width thickness and wall thickness is made to achieve sufficient radial strength and flexibility for venous stenting. The diameter of nitinol tube used for stent cutting ranges from 1.5mm to 4.0mm, most preferably from 2.5mm to 3.5mm. Wall thickness of nitinol tube ranges from 150µm to 400µm and most preferably around 250µm to 350µm.

Normally, the design of the conventional stent is hybrid containing open cell between proximal and distal ring and close cell at both end as shown in Fig. 2. In this design cells are interlinked by straight link which gives good flexibility but lower radial strength that ranges from around 15N to 25N for stent size of 14mm diameter and 50mm length.
As shown in fig. 3 another type of conventional stent has close diamond shape cell throughout the length and cells are interlinked with 2 or 3 straight link. The radial strength of said design ranges from around 25N to 40N for stent size of 14mm diameter and 50mm length which is superior to previous design but result in reduced flexibility which may be due to its straight link.

In order to overcome the reduced flexibility problem of the prior art stent, the invention provides a self-expanding stent, with high radial strength and optimum flexibility for the treatment of venous vasculature defects. The stent (1) of the present invention as shown in fig. 4 and 5, has laser cut hybrid design, comprises a plurality of coaxially aligned radially expandable rings (2) consisting of individuals cells 3, said rings being extended from a proximal (P) to a distal (D) end on a longitudinal axis of the stent and are interconnected by plurality of S-links (4). The cells are being arranged in a manner to form a close, open or hybrid structure.

In a preferred embodiment, the said cells may be diamond, hexagonal, elliptical or rectangular shaped.

The S-links connect the distal end of one ring to the proximal end of the next ring along the longitudinal axis of the stent.

In one embodiment S-link connects to every third to fifth peak to every third to fifth valley of an adjacent hexagonal ring depending on number of crowns cell and diameter of stent. This requires fewer S-links and that provides the stent with optimal flexibility between adjacent hexagonal rings and which reduce the amount of metal that is implanted in a patient, while still providing the necessary support and rigidity to the stent during loading or deployment.

The first connection point of the said S-links in one ring at the distal end is circumferentially offset from the second connection point at the proximal in the next ring resulting in the links being transverse to the longitudinal axis of the stent between its proximal and distal end.

Fig. 6 also shows a hybrid design of the present invention wherein the link between two crown/ring is S-link. The number of S-link is two and pattern of S-link is alternative. However, it may be possible to have more than two S-links in order to increase the radial strength of the stent without affecting its flexibility.

The modifications made in interlinks enhances the flexibility of the stent, reduce the chances of perforation and kink resistance during bending of stent. The crown or ring may have odd pattern of connection, continuous pattern of connection, continuous on both or either proximal or distal end. An exploded view of the s-link of the present invention is shown in Fig. 7.

Fig. 5 and 7 shows another preferred embodiment of the laser cut hybrid design of the stent of the present invention. In this embodiment, the distal (7) and proximal crown/ ring (6) are transformed into flared shape for increasing holding capacity and reducing migration chances of stent. In this embodiment, preferably proximal and distal end rings are diamond shaped as shown. The inflection angle of said flared rings is in the range of 160 to 180 degrees and preferably around 162° to 172° in respect to the longitudinal axis of the stent. This is shown in Fig. 10.

Flared proximal and distal ends of stent provide better resistance to migration, and improved radial strength. Radial strength of stent ranges from around 40N to 60N for stent size of 14mm diameter and 50mm length.

In order to make such a stent structure, the diameter of the tube ranges from 1.5mm to 5.0mm, more preferably 2.5mm to 4.0mm.

The strut wall thickness ranges from 150µm to 400µm and more preferably from 250µm to 350µm and strut width thickness ranges from 170µm to 230µm.

Preferred designs of the present invention stent are illustrated in Fig. 6 and 8. In these embodiments the cell length ranges from 0.5mm to 15mm and most preferably ranges from 2mm to 9mm. The stent may consist of 12 to 18 crown or may increase or decrease as per diameter of stent. The length of the stent varies from 40mm to 160mm as per the application and lesion length.

In another embodiment as shown in Fig. 9, the stent comprises of completely closed cell with hexagonal cells (8) connected by S- link which is spiral like shape with at least one bulging at the center to enhance strength and flexibility to device suitable for venous vessel morphology. In this embodiment, each independent hexagonal cell ring is connected with spiral like shape S-link as shown in Fig. 9 and thus increasing radial strength suitable for May-Thurner syndrome and deep venous thrombosis. In view of use of spiral S-link connection between each of the hexagonal rings in two consecutive rings in longitudinal direction, it will form a stent with consecutive rings having hexagonal (8) and rectangular (9) closed ring structure. Here also the first connection point of the said S-links in one ring at the distal end is circumferentially offset from the second connection point at the proximal in the next ring resulting in the links being transverse to the longitudinal axis of the stent between its proximal and distal end.

In yet another embodiment, one or two additional bulging can be incorporated, in order to cater to specific radial strength/flexibility requirements.

The shape of the bulging(s) may be elliptical or oval. Preferably the dimension of bulging width length ranges from 200µm to 600µm and more preferably ranges from 350µm to 450µm. Preferably, the length of bulging ranges from 500 µm to 1000 µm and more preferably ranges from 600µm to 900µm. Fig. 11 shows an exemplary bulging shape and dimension.

It may also be possible in this embodiment to have proximal and distal flared hexagonal or diamond shaped rings to further reduce the displacement possibility of the stent from its implanted position.

In one embodiment as shown in Fig. 8, radiopaque markers are attached or integrated into the design of the stent. Tantalum markers of riveted, coined, ellipses, rectangular shape marker are attached into stent by laser welding process. Nitinol markers may also be attached to the crown of the stent.

The process of manufacturing stent is shown in the flow chart of Fig. 12. Laser cutting uses a beam of laser to cut the materials. It works by emerging the output of a high-power laser most commonly through the optics. As the lasing material is stimulated, the beam is reflected internally by means of a partial mirror, until it achieves sufficient energy to escape as a stream of monochromatic coherent light. Mirrors or fiber optics are typically used to direct the coherent light to a lens, which focuses the light at the work zone. Nitinol tube after laser cutting of present invention has close cell hybrid design.

In an embodiment, laser cutting is done in two numbers of passes for smooth and effective edges of cutting path. The laser power setting ranges from 15watt to 55watt preferably ranges from 20watt to 50watt and more preferably ranges from 30watt to 40watt. The frequency during the process ranges from 5000Hz to 15,000Hz preferably ranges from 7000Hz to 13,000Hz and more preferably ranges from 9000Hz to 11,000Hz. The argon gas pressure during the process ranges from 9bar to 24bar preferably ranges from 11bar to 21bar and more preferably ranges from 13bar to 18bar. The argon gas is heavy and is useful to cut higher wall thickness and penetrates deeper into nitinol tube and avoid altering properties of stent structure during laser cutting. The pulse width during the process ranges from 0.01ms to 0.09ms preferably ranges from 0.02ms to 0.08ms and more preferably ranges from 0.04ms to 0.06ms.

Grinding and honing process is used for the production of smooth surface finishes inside bores, or to hold precise tolerances on bore diameters. Honing produces the required finished surface by utilizing an abrasive stone which turns while being moved in and out of the work piece. Fluids like glycol and Alconox® detergent powder i.e. a mixture of Sodium tripolyphosphate, Sodium Alkylbenzene Sulfonate and Tetrasodium Pyrophosphate which is anionic detergent use for manual and ultrasonic cleaning and is excellent replacement for corrosive acids and nasty solvents. Therefore, in an embodiment said detergent powder is used to provide smooth surface and to remove the remaining material from the bore. During honing the material is abrasively removed by the shearing action of the grains leading to improvement in roundness, straightness, diameter of stent. Moreover, multiple grits and stages must be used in sizing if the rough diameter is not within certain limits of the finished requirements.

After the grinding & honing process, desired shape of stent is achieved by shape setting process using mandrel or mold in high temperature oven or high temperature furnace. Further, the angles between the cell and struts can be adjusted by help of needle during shape setting process. Shape setting is done by constraining the element on a mandrel or fixture of the desired shape and applying an appropriate heat treatment. During heat treatment temperatures ranges from 480°C to 600°C and more preferably ranges from 500°C to 530°C temperature. The required time ranges from about 01 minutes to 20 minutes and more preferably ranges from 5 minutes to 15 minutes. Rapid cooling is preferred via a water, oil or inert gas quench. In present invention, cooling/quenching is done by water. Stent is dipped into Aluminum white sand at specific temperature for specified time for effective shape setting. Shape setting process enhances internal grains hardness and relieves stress. On the other hand, inappropriate temperature leads to the destruction of shape memory of stent.

Sand blasting is used to provide the highly smooth surface. Devices such as stents, shunts, cages, etc. are commonly blasted to deburr them and to remove the oxide layers from surfaces and also to remove pulse marks and striations left by laser machining, cut or remove laser slag, decrease the propensity for micro cracking and lightly texture the surface to improve adhesion characteristics during sand blasting. Aluminum oxide power is strike on the device surface at certain velocity. The pressure exertion of the powder ranges from 20 psi to 90 psi more preferably from 30 psi to 60 psi. Due to abrasion, the micro cracks and burrs are removed and provide the highly smooth surface with the increase in surface texture of stent after sand blasting. After sandblasting, electro-polishing is done.

Electrochemical process removes oxide layer and highly improves the surface finish of metallic work piece. It is used to polish, passivate, and deburr metal parts. The work-piece is immersed in a temperature-controlled bath of electrolyte which serves as the anode and further wire is connected to the positive terminal of a DC power supply, the negative terminal being attached to the cathode. A current passes from the anode, where metal on the surface is oxidized and dissolved in the electrolyte. Electrolytes used for electro-polishing are most often concentrated acid solutions having a high viscosity, such as mixtures of concentrated sulphuric acid and phosphoric acid. Other electro-polishing electrolytes reported in the literature include mixtures of perchlorates with acetic anhydride and methanolic solutions of sulfuric acid. In present invention mixture of perchloric acid and acetic acid is used during process. To achieve electro-polishing of a rough surface, the protruding parts of a surface must dissolve faster than the recesses. The volume of solution used is from 800 ml to 1600 ml more preferably from 900 ml to 1100 ml for the process. Anodic dissolution during electro-polishing conditions de-burr metal objects due to increased current density on corners and burrs. Most importantly, successful electro-polishing should operate under diffusion limited constant current plateau that is achieved by following current dependence on voltage, under constant temperature and stirring condition. In present invention, voltage ranges from 1V to 20V more preferably from 5V to 15V and the current ranges from 0.1A to 2.0A, more preferably ranges from 0.6A to 1.2A. The time for the electro-polishing ranges from 1 minute to 30 minutes more preferably ranges from 5 minutes to 20 minutes.

Radiopaque markers are attached or integrated into the design of the stent. Tantalum markers of riveted, coined, ellipses, rectangular shape marker are attached into stent by laser welding process. Laser welding is done with 0.15mm to 0.3mm laser spot diameter and 0.4ms to 0.6ms pulse width. The number of tantalum marker or laser cut nitinol marker ranges from 1 to 10; more preferably ranges from 4 to 8. The presence of laser cut nitinol marker in addition to tantalum marker offer enhancement in radiopacity.

LOADING AND DEPLOYMENT PROCESS OF VENOUS STENT
Loading of the stent can be accomplished by manual crimping or crimping machine. During loading process, stent is crimped and loaded into delivery sheath. Further, the inner diameter of sheath depends on the expanded diameter of the stent. For example, stent having expanded diameter 10mm and 18mm, the inner diameter of delivery sheath is 8F to 12F. Loading parameters include quill speed, strut position, final position and hold diameter range from 0.02mm to 0.8mm/sec, 100mm to 200mm, 200mm to 300mm and 1.5mm to 5.0mm more preferable ranges 0.05mm to 0.5mm/sec, 120mm to 180mm, 250mm to 260mm and 2.5mm to 4.0mm, respectively. The stent loading parameters are critical for undamaged and uniform loading of stent as well for deployment of stent. During loading process, the stent crimped by any crimping mechanism and sequentially loaded into delivery sheath through force applied from other end.

For deployment of venous stent, as per the standard practice access site is prepared. Then, guide wire of 0.035” (0.89mm) is advanced to the location of lesion. If necessary, pre-dilate the treatment site. After pre-dilation, keep the guide wire for stent system advancement. Place delivery system over the guide wire and advance the delivery system as a unit through the hemostatic valve of the introducer sheath. Conform that the introducer sheath is secure and remains stable during deployment. Place the stent at treatment site by advancement of delivery system till stent reach the center region of the lesion by inspecting stent marker position. Support the delivery system with second hand and hold the stability sheath straight under tension throughout the procedure. Avoid touching braided Inner tubing while stent releasing. Hold the handle in fixed position and pull back the outer sheath till distal and proximal marker of the stent placed to target lesion. Complete stent deployment by rotating thumb wheel on delivery system. During fluoroscopy, ensure the position of distal and proximal stent radiopaque markers relative to the targeted site. Once the distal stent radiopaque markers are starts separating, the stent is start getting deployed. Keep on turning the thumbwheel until the distal end of the stent obtains full wall apposition. After the distal end is well apposed to vessel wall, final deployment can be further proceed by rotating thumbwheel.

Examples of Venous Stent Design:
Example 1
The diameter of nitinol tube used for stent cutting ranges from 1.5mm to 4.0mm, most preferably from 2.5mm to 3.5mm. Wall thickness of nitinol tube ranges from 150µm to 400µm and most preferably around 250µm to 350µm. The design of stent is hybrid containing non-hexagonal cell i.e. open cell between proximal and distal ring and close cell at both end. In this design, cells are interlinked by straight link which gives good flexibility but lower radial strength that ranges from around 15(N) to 25(N) for stent size of 16mm diameter and 50mm length. Further, the design of stent modified with hexagonal shape closed cell throughout the stent which leads to the further improvement in radial strength 40(N) to 60(N) but leads to reduced in stent flexibility.

Example 2A
Further, modification is made in the linking pattern of stent and remaining stent design is kept same as describe in example 1 i.e. hybrid design containing non-hexagonal cell with open cell between proximal and distal ring and close cell at both ends. S-shape link is kept instead of straight link. The radial strength observed with non-hexagonal type with S-link pattern ranges around 25(N) to 40(N).

Example 2B
Further, design combination containing the hexagonal cells are interconnected with S-link instead of straight link, results in better strength ranges from 45(N) to 65(N) and have enhance flexibility of stent. Use of S type interlink/connecting link enhances the flexibility of the stent, reduces the chances of perforation and kink resistance during bending of stent. It is well known that crown or ring may have odd pattern of connection, continuous pattern of connection, continuous on both or either proximal or distal end. The incorporation of S-link leads to improvement in the flexibility of the stent than straight link stent. Due to presence of S-shape link, considerable increase in the stent flexibility was observed without compromising radial strength.

Example 3
Further, modification of stent design is made by changes in shape of proximal and distal end of the stent. Diamond shape cell at proximal and distal end are modified from cylindrical shape to flared shape. The degree of bending of both ends range from 160° to 180°, most preferably around 162° to 172° for improved holding capacity and resistance to migration of stent.

Example 4
In another example, the stent comprises of completely close cell with hexagonal cell connecting by S or spiral like shape with bulging at centre as shown in Fig 9. The completely close cell design provides improved radial strength and S-shape or spiral link allows optimal flexibility. Moreover, each independent hexagonal cell ring is connected with S or spiral like shape with bulging at centre to withstand mechanical compression to keep vein patent. The shape of bulging can be elliptical or oval which helps to distribute stress during bending in venous vasculature. The optimal dimension of bulging width thickness ranges from 200µm to 600µm and preferably ranges from 350µm to 450µm. The length of bulging ranges from 500µm to 1000µm and more preferably ranges from 600µm to 900µm.

The present invention has been described with the help of some preferred embodiments which is not anyway limiting and various modifications and improvements may be achieved without departing from the scope of the invention as described in the preceding description and the appended claims.