Curved nitinol stent for extremely tortuous anatomy

[0002] The present invention generally relates to self-expanding endoprosthetic devices. In particular, the present invention relates to self-expanding, intraluminal, vascular grafts, generally called stents, adapted to be implanted in a body lumen, such as carotid arteries, coronary arteries, peripheral arteries, veins, or other vessels to maintain the patency of the lumen.

[0003] These devices are frequently used in the treatment of atherosclerotic stenosis in blood vessels especially after percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty (PTCA) procedures, with the intent to reduce the likelihood of restenosis of a vessel. Stents are also used to support a body lumen, tack-up a flap or dissection in a vessel, or in general where the lumen is weak to add support.

[0004] For example, during PTCA procedures, it is common to use a dilation catheter to expand a diseased and partially occluded coronary artery so that blood freely flows. Despite the beneficial aspects of PTCA procedures and its widespread and accepted use, it has several drawbacks, including the possible development of restenosis and perhaps acute thrombosis and sub-acute closure. This recurrent stenosis has been estimated to occur in seventeen to fifty percent of patients despite the initial PTCA procedure being successful. Restenosis is a complex and not fully understood biological response to injury of a vessel which results in chronic hyperplasia of the neointima. This neonintimal hyperplasia is activated by growth factors which are released in response to injury. Acute thrombosis is also a result of vascular injury and requires systemic antithrombotic drugs and possibly thrombolytics as well. This therapy can increase bleeding complications at the catheter insertion site and may result in a longer hospital stay. Sub-acute closure is a result of thrombosis, elastic recoil, and/or vessel dissection.

[0005] Several procedures have been developed to combat restenosis and sub-acute or abrupt closure, one of which is the delivery and implanting of an intravascular stent. Stents are widely used throughout the United States and in Europe and other countries. Generally speaking, the stents can take numerous forms, however, most common is a generally cylindrical hollow tube that holds open the vascular wall at the area that has been dilated by a dilation catheter. One highly regarded stent used and sold in the United States is sold under the tradename ACS Multi-Link Stent, which is made by Advanced Cardiovascular Systems, Inc., Santa Clara, Calif.

[0006] For expandable stents that are delivered with expandable catheters, such as balloon catheters, the stents are positioned over the balloon portion of the catheter and are expanded from a reduced delivery diameter to an enlarged deployment diameter greater than or equal to the inner diameter of the arterial wall by inflating the balloon. Stents of this type are expanded to an enlarged diameter through deformation of the stent, which then engages the vessel wall. Eventual endothelial growth of the vessel wall covers over the stent.

[0007] Other stents are self-expanding where the expansion occurs through the properties of the material constituting the stent. Examples of intravascular stents can be found in U.S. Pat. No. 5,292,331 (Boneau); U.S. Pat. No. 4,580,568 (Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No. 5,092,877 (Pinchuk); and U.S. Pat. No. 5,514,154 (Lau et al.).

[0008] One problem with some prior art stents, especially those of the expandable type, is that they are often stiff and inflexible. Often, the expandable type stents are formed from stainless steel alloys and are constructed so that they are expanded beyond their elastic limit. Such stents are permanently deformed beyond their elastic limits in order to hold open a body lumen and to maintain the patency of the body lumen. By the same token, since the material is stressed beyond its elastic limit into the plastic region, the material becomes stiff and inflexible.

[0009] There are several commercially available stents that are widely used and generally implanted in the coronary arteries after a PTCA procedure. Another class of stents is implanted in vessels that are closer to the surface of the body, such as in the carotid arteries in the neck or in peripheral arteries and veins in the leg. Because these stents are so close to the surface of the body, they are particularly vulnerable to impact forces that can partially or completely collapse the stent and thereby block fluid flow in the vessel. Since these prior art stents are plastically deformed, once collapsed or crushed, they remain collapsed, permanently blocking the vessel. Thus, the prior art stents can pose an undesirable condition to the patient.

[0010] Other forces can impact the prior art stents and cause similar partial or total vessel occlusion. Under certain conditions, muscle contractions might cause the prior art stents to partially or totally collapse and to restrict blood flow in the vessel in which they are implanted.

[0011] Such important applications as mentioned above have prompted stent designers to use superelastic or shape memory alloys in their stent to exploit the materials' properties. An example of such shape memory alloy stents is disclosed in, for example, European Patent Application Publication No. EP0873734A2, entitled “Shape Memory Alloy Stent.” This publication suggests a stent for use in a lumen in a human or animal body having a generally tubular body formed from a shape memory alloy which has been treated so that it exhibits enhanced elastic properties.

[0012] The evolution of superelastic and shape memory alloy stents progressed to use of ternary elements in combination with nickel-titanium alloys to obtain specific material properties. Use of a ternary element in a superelastic stent is shown in, for example, U.S. Pat. No. 5,907,893 to Zadno-Azizi et al. As a general proposition, there have been attempts at adding a ternary element to nickel-titanium alloys as disclosed in, for instance, U.S. Pat. No. 5,885,381 to Mitose et al.

[0013] Another goal has been to design stents that are capable of easy passage through tortuous anatomies such as those found in a coronary artery. One design entails a nitinol stent having a multiplicity of undulating longitudinal struts that can readily change their lengths in the longitudinal direction so as to provide increased longitudinal flexibility for the stent. An example of such a construction is shown in U.S. Pat. No. 5,879,370 to Fischell et al.

[0014] Designing stents for extremely curved and highly tortuous anatomies requires a stent that can bend sufficiently without the struts kinking. To address this kinking problem, one concept is to construct a tubular stent with helically-arranged undulating members having a plurality of helical turns. Linking members formed by rings are laced or interwoven between the undulations in adjacent turns of the helical undulating members. U.S. Pat. No. 6,042,605 to Martin et al. discloses such a construction. The linked undulating elements facilitate bending of the stent.

[0015] The foregoing stent designs address the problems with delivering a straight length stent into a tortuous anatomy. These designs, do not, however, address the problems with deploying a straight length stent in an extremely curved vessel. Indeed, when a straight length stent is deployed in a curved vessel, the stent tends to straighten the curved vessel to follow the form of the stent. It is believed that the straightening forces of the stent is damaging to the health of the vessel, may creating emboli, and may generate intimal flaps that promote restenosis.

[0016] One possible solution suggests assembling a composite stent piecemeal at the curved vessel delivery site by using short modular sections. This approach is disclosed in U.S. Pat. No. 5,824,037 to Fogarty et al. In this design, modular sections of the prosthesis may be selectively combined to form a composite prosthesis having characteristics that are tailored to the specific requirements of the patient. Each prosthetic module includes one or more standard interface ends for engaging another module, the module interface typically having ends that overlap and/or lock within a predetermined axial range. Selection of the appropriate prosthetic modules and the flexibility of the interface overlap range provide a custom fit intraluminal prosthesis tailored to the individual patient's needs. The module sections may include bends, although the modules are individually introduced into a lumen system of a patient body so that the composite prosthesis is assembled in situ. Generally, the prosthetic body modules have a variety of selectable body links, bends, and taper characteristics.

[0017] Although the foregoing conventional stent designs begin to address the problems with deploying a straight stent in an extremely tortuous or curved anatomy, there is however still a need for a superelastic stent that is specifically intended for use in tortuous anatomies. The present invention satisfies this need.

[0018] The present invention is directed to a stent for use in a curved body lumen, comprising a cylindrically-shaped stent including a superelastic alloy, wherein the stent has a unitary construction, and has a length that is greater than a diameter. The superelastic alloy has a low temperature phase that induces a first shape to the stent, and a high temperature phase that induces a second shape with a bend along the length of the stent, and wherein the bend substantially conforms to the curved body lumen.

[0019] In a preferred embodiment, the high temperature phase corresponds to an austenitic phase and the low temperature phase corresponds to a martensitic phase. Also, preferably, the superelastic alloy is a nickel titanium composite that may optionally include a ternary element selected from the group of elements consisting of palladium, platinum, chromium, iron, cobalt, vanadium, manganese, boron, copper, aluminum, tungsten, tantalum, or zirconium.

[0020] In a preferred embodiment, a nickel titanium or nitinol self-expanding stent can be heat set with various degrees of arch or curvature along its length to accommodate a curved or tortuous vessel anatomy. Therefore, when the stent is deployed in a patient's body at above the superelastic alloys phase transformation temperature, the stent reverts to its austenitic phase. In this state, the present invention stent assumes its shape with a bend, wherein the bend was heat set to match the curvature of the curved vessel.

[0021] Prior art self-expanding nitinol stents that have a straight length, when deployed, exert a continuous radial force on the vessel wall at the deployment site. These prior art stents have a tendency to straighten the lumen regardless of the lumen's natural curvature. In contrast, the present invention with a heat set curve along its length does not have the same tendency to straighten when inside the curved vessel. Accordingly, trauma to the vessel is minimized and damage to the intima is diminished. Furthermore, the longitudinal bend or bends that are heat set into the present invention stent can vary in both angle and radius of curvature. In various alternative embodiments, the present invention when in the high temperature state may include a curved length that bends in two dimensions, or may have a bend of greater than 90 degrees, or may have compound curves, or any combination of the foregoing.

[0022] In the preferred embodiment, the present invention stent is unitary, being fashioned from a single piece of material. The present invention stent is also preferably of sufficient length to have an aspect ratio in which the length is greater than its diameter. This ensures that the stent does not tip within the lumen, and minimizes the chance that the stent may migrate and causing an embolism.

[0023] The present invention may optionally include radiopaque markers that assist the physician in proper orientation of the curved stent at the deployment site. In particular, the radiopaque marker may include directional indicia that can be seen in a fluoroscope or by X-ray that help the physician recognize the orientation of the stent. Moreover, the present invention may be delivered by any delivery system and method presently known in the art.

[0024] 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.

[0025] FIG. 1 is a partial cross-sectional view of a stent delivery system for use with the present invention.

[0026] FIG. 2 shows, in a cross-sectional view, the stent delivery system of FIG. 1 with an optional expandable balloon.

[0027] FIG. 3 is a cross-sectional view of a curved vessel exposing a deployed stent of the present invention.

[0028] FIGS. 4a-4fare schematic drawings of exemplary embodiments of curves and bends of the present invention stent.

[0029] The present invention in a preferred embodiment is directed to a stent for use in a curved body lumen. The stent is preferably made of a superelastic alloy in which the material properties are exploited to achieve a specific curved shape for the stent when deployed.

[0030] The stents of the present invention can have virtually any configuration that is compatible with the body lumen in which they are implanted. The radial force of the stent should be configured so that there is a substantial amount of open area once the stent has been deployed. Preferably the open area to metal ratio is at least 80 percent. The stent also should be configured so that dissections or flaps in the body lumen wall are covered and tacked up by the stent.

[0031] Referring to FIGS. 1, 2 and 3, in a preferred embodiment, a stent 10 of the present invention is formed partially or completely of alloys such as nitinol (NiTi) which have superelastic (SE) characteristics. The stent 10 is somewhat similar to the stent disclosed in, for example, U.S. Pat. No. 5,569,295, “Expandable Stents and Method for Making Same,” issued to Lam on Oct. 29, 1996, which is incorporated herein by reference in its entirety.

[0032] The strut configuration of the stent 10 shown in FIG. 3 is just one example of many strut configurations that are contemplated by the present invention. In this embodiment, the stent 10 has a plurality of radially expandable cylindrical elements 24 disposed generally coaxially to achieve a tubular form. This imaginary tube has a flexible but straight length prior to deployment as shown in profile in FIGS. 1 and 2. In the deployed state in FIG. 3, the tubular form is curved along its length to follow the curvature of the artery or vessel 28.

[0033] The radially expandable cylindrical elements 24 are interconnected by spines 26 disposed between adjacent cylindrical elements 24 and that generally extend the length of the stent 10. The shape of the struts are designed so they can preferably be tightly packed. This means that the serpentine shaped struts have extended portions in one cylindrical element 24 that intrude into a complementary space within the circumference of an adjacent cylindrical element 24.

[0034] Furthermore, the stent 10 is unitary, meaning that it is fashioned from a single piece of material. For example, the stent 10 is cut to length from stock nitinol or like material tubing. The tubing is then laser cut to form the strut pattern. Through this process, the present invention unitary stent construction avoids the process variances and deficiencies of welded or mechanically linked together stents. Certainly engineering properties and dimensional tolerances can be more easily controlled with a one-piece stent as compared to a stent pieced together from component parts.

[0035] As mentioned above, an exemplary stent of the present invention includes a superelastic material. The term “superelastic” refers to alloys having superelastic properties that include at least two phases: a martensitic phase, which has a relatively low tensile strength and which is stable at relatively low temperatures; and an austenitic phase, which has a relatively high tensile strength and which is stable at temperatures higher than the martensitic phase. Superelastic characteristics generally allow the metal stent to be deformed by collapsing and deforming the stent and creating stress which causes the NiTi to change to the martensitic phase. The stent is restrained in the deformed condition within a delivery system to facilitate the insertion into a patient's body, with such deformation causing the phase transformation. Once within the body lumen, the restraint on the stent is removed, thereby reducing the stress therein so that the superelastic stent can return to its original undeformed shape by the transformation back to the austenitic phase.

[0036] More precisely, when stress is applied to a specimen of a metal such as nitinol exhibiting superelastic characteristics at a temperature at or above that which the transformation of the martensitic phase to the austenitic phase is complete, the specimen deforms elastically until it reaches a particular stress level where the alloy then undergoes a stress-induced phase transformation from the austenitic phase to the martensitic phase. As the phase transformation progresses, the alloy undergoes significant increases in strain with little or no corresponding increases in stress. The strain increases while the stress remains essentially constant until the transformation of the austenitic phase to the martensitic phase is complete. Thereafter, further increase in stress is necessary to cause further deformation. The martensitic metal first yields elastically upon the application of additional stress and then plastically with permanent residual deformation.

[0037] If the load on the specimen is removed before any permanent deformation has occurred, the martensite specimen elastically recovers and transforms back to the austenitic phase. The reduction in stress first causes a decrease in strain. As stress reduction reaches the level at which the martensitic phase transforms back into the austenitic phase, the stress level in the specimen remains essentially constant (but less than the constant stress level at which the austenitic crystalline structure transforms to the martensitic crystalline structure until the transformation back to the austenitic phase is complete); i.e., there is significant recovery in strain with only negligible corresponding stress reduction. After the transformation back to austenite is complete, further stress reduction results in elastic strain reduction. This ability to incur significant strain at relatively constant stress upon the application of a load and to recover from the deformation upon the removal of the load is commonly referred to as superelasticity.

[0038] The prior art makes reference to the use of metal alloys having superelastic characteristics in medical devices which are intended to be inserted or otherwise used within a patient's body. See, for example, U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamoto et al.).

[0039] Returning to FIG. 1, the graphic illustrates, in a partial cross-sectional view, a rapid exchange stent delivery system that includes a manipulating device 12, a guide wire 14, a delivery sheath 16, and an intravascular catheter 18. The stent 10 is usually held underneath the delivery sheath 16, but FIG. 1 illustrates the system with the sheath 16 retracted. This delivery system is just one example of a delivery system that may be used with the present invention. More details of this type of delivery system may be found in, for example, U.S. Pat. No. 5,458,615, “Stent Delivery System,” issued to Klemm et al. on Oct. 17, 1995, which is incorporated herein by reference in its entirety. Other delivery systems such as an over-the-wire delivery system may be used without departing from the scope of the instant invention.

[0040] FIG. 2 depicts in a partial cross-sectional view a variation of the delivery system of FIG. 1, which variation includes an optional expandable balloon 20 and an optional balloon inflation lumen 22. The stent 10 is disposed over the expandable balloon 20, and the entire assembly is kept underneath the delivery sheath 16 until the moment the stent 10 is deployed. Again, this illustration shows the delivery sheath 16 in the retracted condition.

[0041] The present invention stent 10 is preferably formed from a superelastic material such as NiTi and undergoes an isothermal transformation when stressed. The stent is first compressed to a delivery diameter, thereby creating stress in the NiTi alloy so that the NiTi is in a martensitic state having relatively low tensile strength. Alternatively, the present invention stent 10 may be chilled to its low temperature martensitic state and deformed into its small delivery diameter. In either case, while still in the martensitic phase, the stent 10 is mounted onto a catheter by known methods such as adhesives, or other restraining means. By virtue of the superelastic properties, the stent 10 when mounted within the delivery sheath 16 tends to spring back to a larger diameter, and pushes radially outward against the inside diameter of the sheath 16 when held therein.

[0042] In its delivery diameter, the overall diameter of the stent and catheter are less than the inside diameter of an artery 28 or the vessel in which they are inserted. After the stent 10 is inserted into the artery or other vessel, the stress exerted by the stent 10 may be released by withdrawing the delivery sheath 16 in a proximal direction, whereupon the stent 10 immediately expands and returns to its original, undeformed shape by transforming back to the more stable austenitic phase. If the expandable balloon 20 of FIG. 2 is used, the stent 10 may be further expanded by inflation of the expandable balloon 20 via the balloon inflation lumen 22 by known methods.

[0043] To exploit the properties of superelastic alloys, the present invention stent 10 can be heat set with various degrees of curvature along its length to conform to the curvatures of any number of patients' vasculatures. A conventional, superelastic stent that is heat set straight along its length, when deployed, exerts a continuous radial force and attempts to straighten the lumen regardless of the lumen's original anatomy or curvature. Therefore, in some cases, it may be better to deploy a self-expanding stent that more closely matches the original curvature of the lumen to avoid these stresses upon the vessel. The arch shape that is heat set into the present invention superelastic stent 10 can vary in both total angle and radius of curvature.

[0044] For example, FIG. 3 is a partial cross-sectional view of a vessel 28 having a particular curvature or bend 30. The superelastic stent 10 has been deployed and is illustrated in its high temperature, expanded state. In other words, the stent 10 is in its austenitic phase in which the stent 10 assumes a shape having a curvature or bend along its length that conforms and matches the curvature 30 of the vessel 28. In contrast, prior to reaching the deployment site as depicted in FIG. 3, the stent 10 remains in its low temperature or martensitic state. In this state, the stent 10 is held inside delivery sheath 16 and has a shape that does not include the bend shown in FIG. 3. In this martensitic state, the present invention stent 10 is fairly pliable and follows the flexures of the delivery system along the tortuous anatomy of the patient.

[0045] As seen in FIG. 3, the length of the stent 10 can be as long as needed to perform its scaffolding function along the diseased portion of the vessel 28. In the exemplary embodiment, the diameter of stent 10 is preferably smaller than the length of the stent 10 because this aspect ratio ensures that the stent does not tip and migrate downstream causing an embolism. In its austenitic phase, the heat set curve along the length of the stent 10 conforms closely to the natural bend 30 of the vessel 28. Ideally, this bend should follow the curvature 30 of the vessel 28 fairly closely. Therefore, the present invention contemplates a variety of curvatures that may be heat set into the length of the superelastic stent 10 to match the unique vasculatures in different patients.

[0046] FIGS. 4a-4fprovide schematic views of the present invention stent in its high temperature phase with a variety of induced bends and curves along the length of the stent. More precisely, FIG. 4aillustrates a stent 32 having a 180 degree bend 34 along its length. In FIG. 4b, stent 36 has a 90 degree bend 38 along its length. FIGS. 4cand 4dillustrate stents 40 and 42 having multiple bends 44. FIG. 4eillustrates a stent 46 having a compound curve 48 in which the compound curve has at least two radii of curvature, r and R. FIG. 4fillustrates yet another alternative embodiment stent 50 having a bend 52 that extends into three dimensions. In particular, the bend 52 occurs along plane x-y, along plane x-z and along plane y-z. Any and all combinations of the foregoing and other bends and curves known in the art are also contemplated.

[0047] The delivery system for the present invention curved, self-expanding stent 10 can be as straight and as flexible as a conventional delivery system for a straight, self-expanding stent. Upon deployment, the superelastic stent assumes its heat set curved and expanded shape shown in FIG. 3. The delivery system optionally includes some type of radiopaque marking to indicate the orientation of the stent 10 prior to its deployment, thus allowing the physician to rotationally position the stent 10 such that its curvature or bend matches the curvature or bend of the vessel. The delivery system at its proximal end, perhaps on the handle, may display markings to assist in physician stent orientation and placement.

[0048] Optional radiopaque bands or markers 54 may be included on the stent 10 to assist the physician in orienting and positioning the stent within the curved vessel 28. The radiopaque markers 54 may be in the form of bands as shown in FIGS. 1 and 2, they may be coatings, they may be embedded within the stent, or they may be simply thicker or wider struts that appear more distinctly on a fluoroscope. Also, selecting more radiopaque materials to alloy with the base material is another solution. Preferably, the radiopaque markers 54 have some unique directional indicia to assist the physician in recognizing the orientation of the stent when viewed in a two-dimensional screen such as a fluoroscope often found in a cath lab.

[0049] To illustrate, in an alternative embodiment shown in FIG. 4f, curved stent 50 has directional radiopaque markers 56 in the form of chevrons that assist the physician in recognizing the orientation of this curved stent inside the patient's vasculature. Although as shown in FIG. 4fthe stent 50 is in its expanded, deployed state, the chevron markers 56 nevertheless are effective in the compressed delivery diameter to help with placement and orientation.

[0050] The present invention superelastic alloy is preferably formed from a nickel titanium composition known in the art. The process to heat set the curved shape of the present invention stent is performed by mounting the stent on a mandrel having the desired curvature and raising the temperature of the stent to a point above the austenitic finish (A_f) of the material. The process details for heat setting a nickel titanium alloy is well known in the art. After the heat set procedure, the stent 10 is removed from the mandrel and returned to room temperature. In the exemplary embodiment, the nitinol alloy has a transition temperature that is below room temperature.

[0051] The superelastic alloy of the present invention may alternatively be formed from a composition consisting essentially of about 30 to about 52 percent titanium and the balance nickel and up to 10 percent of one or more additional ternary alloying elements. Such ternary alloying elements may be selected from the group consisting of palladium, platinum, chromium, iron, cobalt, vanadium, manganese, boron, copper, aluminum, tungsten, tantalum, or zirconium. In particular, the ternary element may optionally be up to 3 percent each of iron, cobalt, platinum, palladium, and chromium, and up to about 10 percent copper and vanadium. As used herein, all references to percent composition are atomic percent unless otherwise noted.

[0052] In another preferred embodiment, a NiTi stent with SME (shape memory effect) is heat-treated at approximately 500 degrees C. The stent is mechanically deformed into a first, smaller diameter for mounting on a catheter delivery system, such as the delivery system of FIG. 2, that includes the expandable balloon 20 and the balloon inflation lumen 22. After the stent has been expanded by the balloon and deployed against arterial wall 29 of artery 28, 45 degrees C. heat is applied causing the stent to return to its fully expanded, curved length and to contact the curved arterial wall of the artery. The application of 45 degrees C. of heat is compatible with most applications in the human body, but it is not to be limited to this temperature as higher or lower temperatures are contemplated without departing from the invention. The 45 degrees C. temperature can be achieved in a conventional manner well known in the art such as by warm saline injected into the delivery catheter and balloon.

[0053] The shape memory characteristics allow the devices to be deformed to facilitate their insertion into a body lumen or cavity and then to be heated within the body so that the device returns to its original, curved-length shape. Again, alloys having shape memory characteristics generally have at least two phases: a martensitic phase, which has a relatively low tensile strength and which is stable at relatively low temperatures, and an austenitic phase, which has a relatively high tensile strength and which is stable at temperatures higher than the martensitic phase.

[0054] Shape memory characteristics are imparted to the alloy by heating the metal to a temperature above which the transformation from the martensitic phase to the austenitic phase is complete; i.e., a temperature above which the austenitic phase is stable (A_f). The curved shape of the metal during this heat treatment is the curved shape “remembered.” The heat-treated metal is cooled to a temperature at which the martensitic phase is stable, causing the austenitic phase to transform to the martensitic phase. The metal in the martensitic phase is then plastically deformed, e.g., to facilitate the entry thereof into a patient's body via a delivery system. Subsequent heating of the deformed martensitic phase to a temperature above the martensite to austenite transformation temperature causes the deformed martensitic phase to transform to the austenitic phase. During this phase transformation the metal reverts back to its original curved shape.

[0055] The recovery or transition temperature may be altered by making minor variations in the composition of the metal and in processing the material. In developing the correct composition, biological temperature compatibility must be determined in order to select the correct transition temperature. In other words, when the stent is heated, it must not be so hot that it is incompatible with the surrounding body tissue. Other shape memory materials may also be utilized, such as, but not limited to, irradiated memory polymers such as autocrosslinkable high density polyethylene (HDPEX). Shape memory alloys are known in the art and are discussed in, for example, “Shape Memory Alloys,” Scientific American, Vol. 281, pp. 74-82 (November 1979).

[0056] Shape memory alloys undergo a transition between an austenitic state and a martensitic state at certain temperatures. When they are deformed while in the martensitic state they retain this deformation as long as they are retained in this state, but revert to their original configuration when they are heated to a transition temperature, at which time they transform to their austenitic state. The temperatures at which these transitions occur are affected by the nature of the alloy and the condition of the material. Nickel-titanium-based alloys (NiTi), wherein the transition temperature is slightly lower than body temperature, are preferred for this embodiment of the present invention. It is desirable to have the transition temperature set at just below body temperature to enable a rapid transition from the martensitic state to the austenitic state when the stent is implanted in a body lumen.

[0057] Turning again to FIGS. 2 and 3, the stent 10 is formed from a shape memory alloy, such as NiTi discussed above. When positioned at the delivery site within the vessel, the protective sheath 16 is retracted to expose the stent 10. The stent then immediately expands due to contact with the higher temperature within artery 28 as described above for devices made from shape memory alloys. The stent 10 assumes its curved shape in the austenitic state and its curved shape closely conforms to the curvature 30 of the artery 28. After the stent 10 is deployed at the curved artery 28, the expandable balloon 20 is inflated via the balloon inflation lumen 22 by conventional means so that the stent 10 expands radially outward. The balloon 20 may optionally be curved (not shown) when inflated to follow the natural curvature 30 of the artery 28. The bend in the inflated balloon also avoids straightening out the curved, expanded stent.

[0058] While the present invention has been illustrated and described herein in terms of a superelastic stent wherein the stent assumes a curved shape in its high temperature state, it is apparent to those skilled in the art that the present invention can be used in other instances. Other modifications and improvements may be made without departing from the scope of the present invention.