HK1036208A - Low pressure stent - Google Patents

Description

Low pressure stent fixing mould

The present application is a copending application, co-pending U.S. patent application No. 08/835,015 entitled "Method for manufacturing a Stent" filed on 8/4 1997. The content of the application mentioned in this paragraph is hereby incorporated by reference into the present application.

The present invention relates generally to percutaneous transluminal devices and methods for treating vascular lumen obstructions (sclerosis) in a human body. More particularly, the present invention relates to an improved stent which requires a lower expansion pressure during deployment and which improves stent insertion into the vessel wall.

Cardiovascular disease is generally considered one of the most dangerous diseases facing today's society that severely affect physical health. Diseased and occluded coronary arteries can restrict blood flow and cause tissue ischemia and necrosis. Although the cause of the presence of cardiovascular sclerosis is not well understood, the treatment of coronary artery stenosis is well established. When a partially diseased vessel segment is present in one or more arteries, a surgical procedure such as Coronary Artery Bypass Grafting (CABG) is often selected. Of course, in such procedures, conventional open heart procedures are obviously very damaging or injurious to the patient. In many cases, alternative methods of less traumatic treatment would be beneficial for transdermal treatment of cardiovascular disease. These procedures typically employ various types of balloons (angioplasty) or ablation devices (atherectomy) to remodel or ablate the diseased vessel segment. Another treatment is percutaneous transluminal insertion of one or more expandable tubular stents or prostheses into the site of the sclerotically damaged condition. Transluminal endovascular prosthesis implantation is one method of replacing traditional vascular surgery. Transluminal endovascular implantation a tubular prosthetic graft is inserted percutaneously into a blood vessel and inserted by means of a catheter to a desired location within the vascular system. Another method of percutaneous angioplasty is to surgically place a vein, artery or other bypass from the aorta over the coronary arteries, which requires open heart surgery and is associated with high morbidity and mortality. Advantages of percutaneous angioplasty procedures over traditional vascular procedures include the elimination of the need for surgical exposure, removal, replacement or diversion of the problematic blood vessels (including cardio-pulmonary diversion, thoracotomy, and general anesthesia).

Stents or prostheses are known as implants which function to keep the lumen of the human body from closing, and in particular such implants for blood vessels. They are generally constructed of a cylindrical metal mesh that expands when internal pressure is applied. Alternatively, they may be wound from wire into a cylindrical shape. The present invention relates to an improved stent design that facilitates placement and insertion of the stent into a blood vessel by a specially configured stent. Formed during the stent fabrication process, which provides a controlled and high pressure yield point and high ultimate tensile properties.

Stents or prostheses may be used in various tubular structures of the human body, including, but not limited to, arteries and veins, ureters, common bile ducts, and the like. Stents are used to expand the lumen of a blood vessel or, after angioplasty or atherectomy, to cover the aorta from which the aneurysm is removed, to tack the anatomical product to the vessel wall to keep the lumen open, to eliminate the obstruction caused by flaps resulting from intimal tears during the initial insertion, or to prevent elastic recoil of the vessel.

Stents may be used after atherectomy (for cutting out plaque), cutting balloon angioplasty (recording arterial wall conditions prior to dilatation), or standard balloon angioplasty to maintain vessel patency acutely and chronically.

Stents may also be used in shunt grafts to keep blood vessels unclosed. Stents may also be used to enhance collapsed structures at the respiratory, biliary, urinary and other tracts.

A more detailed description of existing stents can be found in the following patents: U.S. Pat. No. 3,868,956(Alfidi et al); U.S. Pat. No. 4,739,762 (Palmaz); U.S. patent 4,512,338(Balko et al); U.S. Pat. No. 4,553,545(Maass et al); U.S. Pat. No. 4,733,665 (Palmaz); us patent 4,762,128 (Rosenbluth); us patent 4,800,882 (Gianturco); us patent 4,856,516 (Hillstead); U.S. Pat. No. 4,886,062 (Wiktor); us patent 5,102,417 (Palmaz); us patent 5,104,404 (Wolff); U.S. Pat. No. 5,192,307 (Wall); us patent 5,195,984 (Schatz); U.S. Pat. No. 5,282,823(Schwartz et al); U.S. patent 5,354,308(Simon et al); U.S. Pat. No. 5,395,390(Simon et al); U.S. Pat. No. 5,421,955(Lau et al); U.S. Pat. No. 5,443,496(Schwartz et al); U.S. Pat. No. 5,449,373(Pinchasik et al); us patent 5,102,417 (Palmaz); us patent 5,514,154(Lau et al) and us patent 5,591,226(Trerotola et al).

In general, it is an object of the present invention to provide a stent or prosthesis that can be easily expanded and embedded into an occlusion mass or vessel wall at low inflation pressures, thereby minimizing trauma and injury to the vessel wall during stent placement.

It is a further object of the present invention to provide a stent having a specially designed outer stent surface to facilitate insertion of the stent structure into an obstruction or vessel wall at low inflation pressures.

It is another object of the present invention to employ a manufacturing process that optimizes the stress-strain curve characteristics, thereby increasing the yield strength and ultimate tensile strength as compared to other non-metallic linear prior art stents.

The present invention relates to an expandable stent which is relatively flexible along its longitudinal axis to facilitate its passage through curved body lumens, but which also has sufficient rigidity and stability in the radial direction in the expanded state to maintain the lumens unclosed when the stent is implanted in a body lumen, such as an artery. In addition, the stent of the present invention has a particular trapezoidal, triangular or reduced radius configuration which tends to bulge radially outward and which serves to reduce the force required by the stent to pierce the vessel wall, thereby minimizing injury or damage to the vessel wall during deployment.

The present invention generally comprises a plurality of radially expandable annular members that are relatively independent in expansion capability and flexible relative to one another. The individual radially expandable members of the stent (the cross-section of the stent) are sized such that the height to width ratio is designed to minimize the likelihood of the stent coiling or rotating during expansion. Inner connecting members or backbones extend between adjacent loop members to enhance stability and preferably each loop member is positioned to resist bending of the stent during expansion. The resulting stent structure is a series of radially expandable annular members longitudinally spaced apart a distance sufficient to permit obstructions, vessel walls and any other small anatomical products located at the treatment site of the body lumen to be stretched or compressed into position against the lumen wall. The outer convex stent surface converges toward the terminal end and is configured in a trapezoidal, triangular or circular shape, thereby facilitating the insertion of the stent into the vessel wall with low expansion pressure. The individual ring members can flex relative to adjacent ring members without significant deformation, and this flexing can accumulate to render the stent flexible along its length about its longitudinal axis, but still sufficiently stiff in the radial direction to resist collapse.

Presently preferred configurations of the expandable ring members that comprise the stents of the present invention are generally annular undulating or alternating ring configurations that comprise one of the radially expandable cylindrical members. The width to height ratio of the cross-section of the undulating component of the annular member is preferably about 1: 1 (width to height ratio) so as to minimize the tendency of the stent to coil during expansion. The open, reticulated structure of the stent exposes a significant portion of the vessel wall to the blood, which promotes healing and repair of the various damaged vascular linings.

Radial expansion of the expandable cylinder deforms the wave or alternating annular structure, similar to the waveform changes caused by decreasing the amplitude and frequency of the waveform. Preferably the waves or alternating structures of the single annular structure are in phase with each other to produce uniform spreading and to suppress any curling along their length. The expandable cylindrical structure of the stent is plastically deformed during expansion so that the stent will be maintained in the expanded state, and therefore, the structure of the stent needs to have sufficient rigidity during expansion so as to prevent the structure from collapsing partially or entirely due to the stent being compressed after deployment. The optimized stress-strain curve characteristics are used in the manufacture of the stents of the present invention to improve the mechanical properties of the overall stent over that of other non-metallic linear stents. Optimizing the stress-strain curve increases both the yield strength and ultimate tensile strength of the expanded stent, increasing the resistance to structural damage (crushing) or stent collapse. As the stent is expanded, the radially protruding trapezoids, triangles or reduced radius structures of the stent's outer surface will penetrate into the obstruction or vessel wall. Due to the reduced area of the outer surface, the stent can penetrate relatively easily into obstructions or vessel walls, thereby minimizing damage or injury to the vessel walls. In addition, this structural feature of the present invention helps to anchor the expanded stent so that it does not move once implanted and minimally protrudes into the blood stream.

The cross-section of the elongated member connected within the adjacent radially expandable member should be similar to the transverse dimension of the undulating or alternating annular elements on the radially expandable member. The inner connecting member is preferably not a unitary structure but is alternatively segmented at various angles along its length around the circumference of the stent. In another embodiment, the inner connecting member is a unitary structure similar to a skeleton connecting the expandable ring members.

In the presently preferred embodiment of the invention, the stent is simply and easily formed by the following steps: first heat treating the mechanically hardened tubular member to obtain optimized stress-strain characteristics, such as yield strength, elongation, and ultimate tensile strength; the tubular member, which may be composed of stainless steel, platinum, a gold alloy or a gold/platinum alloy, is then electro-cleaned with a suitable solution. After the contamination of the tubular member is removed, a photosensitive protective layer is uniformly coated on the outer surface thereof. Alternatively, a coupling agent may be used to facilitate bonding of the photosensitive protective layer to the tubular member. The coupling agent is not required, and some tubular members may be directly combined with the photosensitive protective layer solution without the coupling agent.

The coated tubular member is then placed in an apparatus configured to rotate the tubular member while the coated tubular member is exposed to a prescribed pattern of Ultraviolet (UV) light. The apparatus controls the exposure of the coated tubular member by illuminating the coated tubular member with UV light in a specific pattern using photographic film and a specific computer that produces the configuration of the print. The UV light activates the photosensitive protective layer so that the areas where the UV light is present can be exposed (cross-linked) to the photosensitive protective layer. The photosensitive protective layer, upon exposure to UV light, forms crosslinks, thereby forming a hardened and cured polymer pattern that mimics the particular stent structure surrounded by uncured polymer. The sizing sheet is in fact suitable for many complex stent designs. The apparatus is manufactured such that the tubular member has a discontinuous pattern of exposed photosensitive material and the remaining areas have an unexposed photosensitive protective layer.

The exposed tubular member is immersed in a negative resist developer for a specified period of time. The developer removes the relatively soft, uncured photoresist polymer and leaves the cured photoresist patterned to mimic the stent. Thereafter, excess developer is removed with a suitable solvent. At this point, the entire tubular member is allowed to warm for a specified period of time, allowing the remaining photopolymer resist to fully cure and bond to the treated tubular member surface.

The treated tubular member is then subjected to an electrochemical etching process which removes the uncovered metal of the tubular member to obtain the final tubular member or stent configuration. Since the tubular member has not undergone any process such as additional heat treatment, welding/brazing or laser cutting, the final stent will maintain the optimized stress-strain characteristics obtained during the initial heat treatment.

The stent having features of the present invention is readily delivered to the desired luminal site by attaching the stent to the expandable member of a delivery catheter, such as to a balloon or mechanical expansion device, and passing the catheter/stent assembly through the body lumen to the site of use.

Other features and advantages of the present invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying exemplary drawings.

FIG. 1 is an elevational view of a stent incorporating features of the present invention mounted on a delivery catheter and within a segment of an artery;

FIG. 2 is a plan view of a piece of film on which the stent configuration of the present invention is imprinted;

FIG. 3 is a plan view of a piece of film with a stent configuration of another embodiment of the present invention imprinted thereon, showing a single skeletal connection member;

FIG. 4 is a perspective view of an entire stent incorporating features of the present invention in an unexpanded condition;

FIG. 5 is a perspective view of a stent structure showing the position and relationship of the loops or stent to the connecting members;

FIG. 6A is a cross-sectional view of one configuration of the outer surface of a stent taken along line 4-4 of FIG. 4 showing a trapezoidal projecting structure projecting radially from the longitudinal axis of the stent;

FIG. 6B is a cross-sectional view of an alternative configuration of the outer surface of the stent taken along line 4-4 of FIG. 4 showing a triangular projection projecting radially from the longitudinal axis of the stent;

FIG. 6C is a cross-sectional view of an alternative configuration of the outer surface of the stent taken along line 4-4 of FIG. 4 showing the radius-reducing protrusion radially projecting directly from the longitudinal axis of the stent;

FIG. 7A is an enlarged view of a portion of one loop or stent of the stent of FIG. 4 showing the trapezoidal shaped stent structure projecting radially directly from the longitudinal axis of the stent;

FIG. 7B is an enlarged view of a portion of one of the loops or legs of the stent of FIG. 4 showing the triangular projections projecting radially from the longitudinal axis of the stent;

FIG. 7C is an enlarged view of a portion of one of the loops or legs of the stent of FIG. 4 showing the reduced radius protrusions projecting radially from the longitudinal axis of the stent;

FIG. 8A is a cross-sectional view showing a stent of trapezoidal configuration scoring and penetrating an occlusion in an artery wall;

FIG. 8B is a cross-sectional view showing the triangular-shaped structural stent scoring and penetrating an occlusion in an artery wall;

FIG. 8C is a cross-sectional view of the reduced radius structural stent scoring and penetrating an occlusion in an artery wall;

FIG. 9A is a fragmentary cross-sectional view similar to FIG. 1, wherein the stent is collapsed and just proximal to a vessel occlusion when the delivery catheter is within the arterial segment;

FIG. 9B is a fragmentary cross-sectional view similar to FIG. 1, with the stent in a flattened configuration within a vascular occlusion;

FIG. 9C is a fragmentary cross-sectional view similar to FIG. 1, wherein the stent is expandable within the vessel segment with the stent of a particular configuration against and embedded in the artery wall;

FIG. 9D is a fragmentary cross-sectional view similar to FIG. 1, with the delivery catheter withdrawn and the stent anchored fully functional within the vessel segment;

FIG. 10 shows a single stent or loop in an unexpanded configuration and an expanded configuration, illustrating the values of optimized stress-strain characteristics obtained when using a prior art non-metallic linear stent;

FIG. 11 illustrates a single stent or loop in an unexpanded configuration and an expanded configuration, showing values for optimized stress-strain characteristics obtained when using a stent of the present invention;

fig. 12 shows a stress-strain curve showing a comparison of the relative ultimate tensile values of a prior art stent and a stent of the present invention.

Fig. 1 shows a stent 10 incorporating features of the invention mounted on a delivery catheter 11 over a guidewire 20. The delivery catheter 11 has an expandable portion or balloon 14 for expanding the stent 10 within the artery 21. The delivery catheter 11 with the stent 10 mounted thereon is substantially the same as a conventional balloon dilatation catheter used in angioplasty, or may be constituted by a mechanical expansion device. The bladder 14 may be made of a suitable material, such as polyethylene, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and the like. In order to maintain the stent 10 secured to the balloon 14 during delivery to an obstruction within the artery 21, the stent 10 is typically crimped onto the balloon.

Fig. 2 is a preferred stent configuration imprinted on a transparent photographic film. The structural pattern is generated on a computer program, reduced and printed on a transparent film. The computer output data can be further utilized, for example, using a mechanical mapping or stress analysis program. The output data is then sent to a film processing apparatus which simplifies the output data and produces a negative film of precise dimensions. As will be described in greater detail below, the negative dimension must be calibrated to replicate a particular stent configuration. Since the principle of the drawings of the patent document is that a large area of a black region is not allowed, it is necessary to explain the drawings showing a photographic film. In fig. 2, the open (transparent) space 38 that allows UV light to pass through the film is represented by a solid black line (with a white core) that includes a series of alternating rings 15. The alternating loops of film 32 form a plurality of stents 50, as shown in fig. 5, which circumferentially comprise the expandable columnar elements 12 of stent 10. Similarly, the line segments 36 associated with the alternating or undulating configuration comprise the inner connecting elements 32 of the stent 10 (FIG. 5). The white areas 40 of fig. 2 represent exposed (black) areas of the film that will block UV light from passing through the film, thereby preventing UV exposure in the areas under the film. One example of a suitable film that may be used in the present invention is Kodak ALI-4 Accumax film manufactured by Kodak Industries. The stent footprint length 30 is directly equal (1: 1) to the circumferential length of the stent of the present invention. The width 35 is equal to the working length of the stent being treated.

Fig. 3 shows another embodiment of the stent of the present invention wherein the segments 36 (herein referred to as inner connecting elements) are located between alternating rings 32 (herein referred to as radially expandable cylindrical elements) of different configurations. In fig. 3, the stent configuration ultimately formed from the imprinted film will have a separate connecting backbone 52. The separate inner connecting element represents the backbone 52 associated with the radially expandable cylindrical element 12 of the stent 10. Although not shown, the present invention contemplates that the inner connecting element 34 can be distributed about 120 degrees around the circumference of the stent 10. The provision of three or more internal connecting elements 34 between adjacent radially expandable columnar elements 12 is generally due to the same considerations as previously discussed with one and two internal connecting element designs.

FIGS. 4 and 5 illustrate a preferred stent design 10 made in accordance with the photographic and etching method and embodiment shown in FIG. 2. Stents generally comprise a plurality of radially expandable cylindrical elements 12 which are substantially coaxial and interconnected by elements 34 located between adjacent expandable elements. The photoresist-covered metal portions exposed to UV radiation and changing physical properties are retained during the electrochemical treatment and remain intact as a stent or ring 50 of stent 10. In the development stage, the portions of the photoresist not exposed to UV radiation are removed. The exposed metal is then chemically dissolved using an electrochemical process to provide the open space 39 of the stent 10. The structure resulting from the pattern of loops or stents 50 and open spaces comprises the desired stent configuration. As described herein, and as shown in FIGS. 4 and 5, the radially expandable cylindrical element 12 is configured as a plurality of alternating or undulating rings 23 of the stent, which resemble a serpentine pattern. FIG. 4 also shows the stent structure wherein its radially expandable cylindrical elements 12 are in an undulating configuration, but adjacent expandable elements are out of phase.

Fig. 5 is an enlarged perspective view of the stent 10 of fig. 4, with one end of the stent shown in a partially exploded view to show in greater detail the arrangement of the inner connecting elements 12 between adjacent radially expandable elements 12. Each pair of inner link elements 34 on one side of the expansible member 12 is preferably arranged to maximize the flexibility of the stent. In the embodiment shown in FIG. 5, the stent 10 has two inner connecting elements 34 between adjacent radially expandable cylindrical elements 12 that are substantially 180 degrees apart. The next pair of inner connecting elements 13 on one side of columnar element 12 is offset 90 degrees from the adjacent pair. The alternating arrangement of the inner connecting elements makes the stent axially flexible in substantially all directions. The inner connecting elements may be arranged in various configurations, another example of which is shown in FIG. 3. However, all of the inner connecting members of a single stent should be anchored at the peaks or valleys of the alternating ring elements to prevent shortening of the stent during expansion, and all radially oriented stents will have one of the configurations of the particular structure.

The structural figures of fig. 4 and 5 may have any dimensions: the preferred dimension of the stent 10, as formed and in the constrained configuration, is between 0.035 to 0.100 thousandths of a diameter. The expanded or deployed diameter of the stent 10 is in the range of 2.0mm to 8.0mm, with a preferred range for coronary arteries being 2.5mm to 6.0 mm. The length of the stent 10 is virtually constant as it is initially formed and expanded, ranging from 2mm to 50mm, with a preferred length for coronary arteries ranging from 5mm to 20 mm.

Each radially expandable cylindrical element 12 of the stent 10 is independently expandable. Thus, the balloon 14 can be provided in an expanded shape such as a tapered shape in addition to a cylindrical expanded shape, thereby facilitating implantation of the stent 10 into various shapes of body lumens.

The particular pattern and how many waves per unit length encircle the circumference of the radially expandable cylindrical element 12, or the amplitude of the rings, are selected to meet the particular mechanical requirements of the stent being fabricated, e.g., expansion size and radial stiffness. The number of waves may also be varied to accommodate the placement of the inner connecting member 34 at the crest or along the sides of the waves (not shown). As previously described, each radially expandable columnar element 12 is connected by an inner connecting element 34. The wave pattern 23 is formed by a plurality of alternating U-shaped loops. In addition, the wave pattern may be formed of W-shaped elements or Y-shaped elements having different radii, so that the spreading force is more evenly distributed over the individual elements.

The stent 10 is used to keep the artery 21 open after the catheter 11 is withdrawn, as shown in fig. 9D. The undulating portions of the radially expandable section 12 have good positioning characteristics that prevent the stent from moving within the artery. In addition, the dense radially expandable columnar elements 12 at regular intervals are able to uniformly support the wall 22 of the artery 21, and are thus better suited to hold and retain small skin or anatomical products in place within the wall 22 of the artery 21.

The preferred stent structure 10 made according to the method of manufacture of the present invention has a specially configured stent 50. FIGS. 6A, 6B and 6C are cross-sectional views showing the stent structure of three typical stents. As shown in FIG. 6A, the preferred stent structure has stent outer portions projecting in a trapezoidal configuration 54, which points radially outward from the longitudinal axis of the stent. The stent structure preferably takes the cross-sectional view of fig. 6A, with a series of U-shaped loops 50 and alternating connecting elements 34 extending along the length of the stent, thereby forming the basic skeleton of the stent structure.

Another embodiment of the stent 10 configuration is similar to that shown in FIGS. 4,5 and 6A, but the outer portion of the stent is not the same, having a triangular configuration (FIG. 6B) with the apex of the triangle pointing radially outward from the longitudinal axis of the stent. In another embodiment, the stent 10 has a configuration similar to that shown in FIGS. 4,5 and 6A, but the stent does not have an outer portion which comprises an extended base and which tapers outwardly at a radius 58 (FIG. 6C), the reduced radius portion pointing radially outwardly from the longitudinal axis of the stent.

The final loop segment of stent 10 is shown in FIGS. 7A, 7B and 7C. As can be seen in the cross-sectional views shown in the figures, the stent in FIG. 7A has a trapezoidal configuration 53, the stent in FIG. 7B has a triangular configuration 55, and the stent in FIG. 7C has an outside radius reducing configuration. Each stent structure may be used in combination with any combination of alternating rings or stents 50 and inner connecting elements 34. In addition, it can be seen that the height to width ratio should minimize the potential for winding or spinning during expansion.

As shown in fig. 8A, 8B and 8C, the specific configuration of the radially facing stent surface is designed to facilitate the insertion of the expanded stent into the arterial wall or obstruction. By providing a trapezoidal 54 (FIG. 8A), triangular 56 (FIG. 8B) or reduced radius 58 (FIG. 8C) configuration, stent insertion is relatively less damaging to the vessel wall because less stent area is required to penetrate the vessel wall. The expansion and eventual insertion of the stent of the present invention is accomplished in such a way that the gas pressure of the blood vessel is overcome in a controlled and relatively manageable manner. Thereby reducing vascular injury or damage leading to subsequent reduction of intimal or smooth muscle proliferation. In contrast, prior non-metallic wire linear stents have a relatively flat surface for piercing the vessel wall and thus do not have the above-described advantages of the stents of the present invention.

In a preferred embodiment, the delivery of the stent 10 is performed in the following manner. The stent 10 is first mounted to an inflatable balloon 14 or mechanical delivery system (not shown) at the distal end of a delivery catheter 11. The stent 10 is crimped or crimped onto the outer surface of the balloon 14. The stent/catheter assembly is then introduced into the patient's vascular system through the guide catheter using conventional Seldinger techniques. The guidewire 20 is positioned through the obstruction within the vessel segment and the stent/catheter assembly is advanced over the guidewire 20 to the obstruction (see fig. 9A). The stent/catheter assembly is then advanced until the stent 10 is centered within the obstruction 25 (see FIG. 9B). The balloon 14 of the catheter is then inflated, expanding the stent 10 against the obstruction 25 and possibly against the arterial wall 22, as shown in fig. 9C.

As shown in FIG. 9D, the artery 21 is preferably slightly expanded by expanding the stent 10, thereby obtaining an expanded lumen of a certain volume. Due to this embedding, the stent has minimal disturbance to the blood flow, and this further prevents the stent from moving. The radially expandable element 12 (or stent 50) of the stent 10 that is pressed into the wall of the artery 21 will eventually be covered by growing endothelial cells, which further reduces the disturbance to blood flow.

Fig. 10 illustrates the limited amount of stiffening (increased tensile loop or strength) that occurs when a prior art non-metallic linear stent is plastically deformed during placement. When the prior art non-metallic wire-type stent 50 expands, only a relatively small area 62 stiffens when deformed, and thus is less resistant to crushing or further deformation.

Fig. 11 shows a large amount of stiffening (increased tensile strength) when the stent of the present invention is plastically deformed during placement. In accordance with the manufacturing process of the present invention, a modest increase in cross-sectional area tensile strength is achieved, and is substantially greater than prior art non-metallic linear stents. As can be seen from a comparison of the above and below, as the ring or stent 61 expands, the center of the ring stiffens. Once the center hardens, the adjacent areas on either side of the hardened center harden as plastic deformation continues to progress. Due to the optimized stress-strain characteristics during manufacture, the stiffened total area of the present invention is significantly greater than prior art non-metallic linear stents when the ring is expanded to full extension. The larger stiffened portion 66 corresponds to an increased resistance to collapse and further deformation of the stent. In contrast, prior art non-metallic linear stents have limited stiffened portions and thus have significantly less resistance to buckling and further deformation. This feature is clinically important because any tendency of the stent to collapse under the harsher conditions of placement or post-placement would limit blood flow or increase the potential for stenosis.

FIG. 12 is a graph showing a standard stress-strain curve for a prior art non-metallic linear stent compared to a stent of the present invention. As shown in the graph, the prior art non-metallic linear stent has a yield strength of about 30,000psi at which additional stress causes it to plastically deform. The present invention can produce yield strengths in the range of 35,000 to 70,000 psi. Any yield point within the above ranges may be selected during manufacture to achieve the desired result. The high end of the range is significantly larger than prior art non-metallic linear stents. These properties indicate an increase in crush resistance of the present invention.

In addition, FIG. 12 also shows that the ultimate tensile strength of the prior art non-metallic linear stent is about 60,000 psi. Any additional stress beyond this point will result in material failure. The present invention can produce ultimate tensile strengths in the range of 65,000 to 120,000 psi. Any ultimate tensile strength can be selected within the ranges described during manufacture to achieve the desired results. The high end of the range is significantly larger than prior art non-metallic linear stents. These properties also indicate an increase in crush resistance of the present invention.

While the invention has been shown and described with respect to the use of an intravascular stent, it will be apparent to those skilled in the art that the stent may be used in other applications, such as for expanding the prostatic urethra in the case of prostate hyperplasia. Other modifications and improvements may be made to the invention without departing from the scope thereof.

Other modifications and improvements may be made to the invention without departing from the scope as described below.

Claims (28)

1. A low pressure stent for implantation into a blood vessel, comprising:

a plurality of substantially cylindrical ring-shaped elements independently radially expandable and connected to one another so that the cylindrical ring-shaped elements are substantially aligned on a common longitudinal axis;

one or more connecting elements for internally connecting said cylindrical annular elements; and

an outer surface on the cylindrical ring element, the outer surface including a converging structure projecting radially outward from the longitudinal axis prior to expansion of the stent, the converging structure remaining radially outward when the stent is expanded radially outward from a first straight diameter to a second increased diameter.

2. The low pressure stent of claim 1, wherein said converging configuration of said outer surface of the annular element comprises a trapezoidal configuration.

3. The low pressure stent of claim 1, wherein said converging structure of said outer surface of the annular element comprises a triangular structure.

4. The low pressure stent of claim 1, wherein said converging configuration of said outer surface of the annular element comprises a reduced radius configuration.

5. The low pressure stent of claim 1, wherein said annular element comprises a wavy, alternating ring, or serpentine pattern.

6. The low pressure stent of claim 1, wherein said outer surface of the annular member is embedded within the vessel wall of the body lumen so that said stent is more securely attached to the vessel wall.

7. The low pressure stent of claim 1, wherein said loop element is capable of maintaining its expanded state after its expansion.

8. The low pressure stent of claim 1, wherein said stent is made of a material selected from the group consisting of: stainless steel, platinum, gold alloy or gold/platinum alloy.

9. The low pressure stent of claim 1, wherein said stent is comprised of a single tubular member.

10. The low pressure stent of claim 1, further comprising coating said stent with a biocompatible coating.

11. The low pressure stent of claim 1, wherein said loop element has a yield strength greater than 35,000 psi.

12. The low pressure stent of claim 1, wherein said loop element has an ultimate tensile strength greater than 65,000 psi.

13. A low pressure stent for implantation into a blood vessel, comprising:

a plurality of substantially cylindrical ring-shaped elements independently radially expandable and connected to each other so that the cylindrical ring-shaped elements are concentrically aligned on a common longitudinal axis;

one or more connecting elements for interconnecting said cylindrical ring elements, said stent maintaining its entire length without being significantly shortened when expanded radially outwardly; and

an outer surface on the cylindrical ring element, the outer surface including a converging structure projecting radially outward from the longitudinal axis prior to expansion of the stent, the converging structure remaining radially outward when the stent is expanded radially outward from a first straight diameter to a second increased diameter.

14. The low pressure stent of claim 13, wherein said converging configuration of said outer surface of the annular element comprises a trapezoidal configuration.

15. The low pressure stent of claim 13, wherein said converging configuration of said outer surface of the annular element comprises a triangular configuration.

16. The low pressure stent of claim 13, wherein said converging configuration of said outer surface of the annular element comprises a reduced radius configuration.

17. The low pressure stent of claim 13, wherein said annular element comprises a wavy, alternating ring or serpentine pattern.

18. The low pressure stent of claim 13, wherein said outer surface of the annular member is embedded within the vessel wall of the body lumen so that said stent is more securely attached to the vessel wall.

19. The low pressure stent of claim 13, wherein said loop element is capable of maintaining its expanded state after its expansion.

20. The low pressure stent of claim 13, wherein said stent is made of a material selected from the group consisting of: stainless steel, platinum, gold alloy or gold/platinum alloy.

21. The low pressure stent of claim 13, wherein said stent is comprised of a single tubular member.

22. The low pressure stent of claim 13, further comprising coating the stent with a biocompatible coating.

23. The low pressure stent of claim 13, wherein said loop element has a yield strength greater than 35,000 psi.

24. The low pressure stent of claim 13, wherein said loop element has an ultimate tensile strength greater than 65,000 psi.

25. A low pressure stent for implantation into a blood vessel, comprising:

a plurality of substantially cylindrical ring-shaped elements independently radially expandable and connected to each other so that the cylindrical ring-shaped elements are concentrically aligned on a common longitudinal axis;

one or more connecting elements for interconnecting said cylindrical ring elements such that said stent maintains its entire length without becoming significantly shorter when expanded radially outwardly; and

the annular member has a yield strength of at least 35,000 psi.

26. A low pressure stent for implantation into a blood vessel, comprising:

a plurality of substantially cylindrical ring-shaped elements independently radially expandable and connected to each other so that the cylindrical ring-shaped elements are concentrically aligned on a common longitudinal axis;

one or more connecting elements for interconnecting said cylindrical ring elements such that said stent maintains its entire length without becoming significantly shorter when expanded radially outwardly; and

the loop member has an ultimate tensile strength of at least 65,000 psi.

27. A method of placing a stent, the stent comprising: a plurality of substantially cylindrical ring-shaped elements independently radially expandable and connected internally and substantially aligned on a common longitudinal axis with said cylindrical ring-shaped elements; one or more connecting elements for internally connecting the ring-like elements; an outer surface on said annular element, said outer surface including a converging configuration or a reduced radius configuration projecting radially outward from said longitudinal axis prior to expansion of said stent, said converging configuration or reduced radius configuration maintaining its radially projecting configuration as said stent expands radially outward from a first half to a second increased radius;

providing a catheter having an expandable member at its distal end and coaxially positioning the stent over the expandable member;

positioning said expandable member with said stent at a selected graft site within a patient's blood vessel;

radially expanding an expandable member to expand the stent within the lumen of the blood vessel;

collapsing the expandable member; and

removing the catheter from the patient.

28. The method for placing a stent of claim 27, further comprising the step of implanting the stent into a vessel wall after expanding the expandable member within the lumen of the vessel.