MXPA00002599A - Crush resistant stent - Google Patents

Abstract

The present invention is directed to a method for maintaining the patency of a vessel via a stent while minimizing both the risk of permanent vessel collapse and the risk of dislodgment of the stent from the implant site if the stent is temporarily deformed due to external forces. The method utilizes a crush-resistant stent having either shape memory or superelastic properties.

Description

ENDOPRÓTESIS RESISTANT TO CRUSHING BACKGROUND OF THE INVENTION The present invention is generally related to self-expanding stent devices, in particular I to self-expanding intraluminal vascular grafts, generally referred to as stent grafts, adapted to be implanted in a body lumen, such as carotid arteries, coronary arteries, peripheral arteries, veins, or other vessels to maintain the opening of the lumen. These devices are frequently used in the treatment of atherosclerotic stenosis in blood I vessels, especially after percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty (PTCA) procedures. of reducing the likelihood of restenosis of a vessel. Stents are also used to support a body lumen, attach a flap or dissection to a vessel, or in general when the lumen is weak to add support. The present invention also relates to an intraluminal vascular graft which can be used in essentially any body lumen. In expandable stents that are provided with expandable catheters such as balloon catheters, stents are placed on the balloon portion of the catheter and expand from a reduced diameter to an enlarged diameter greater than or equal to the inner diameter of the arterial wall at Inflate the balloon. Stents of this type can expand to an enlarged diameter by deforming the stent by coupling the walls of the stent together and by coupling a stent wall path in conjunction with endothelial growth above and above the stent. Other endoprostheses are self-expanding, tgh the properties of the material that constitutes the stent or by design. Examples of intravascular stents can be found in U.S. Pat. Do not. ,292,331 (Boneau); the U.S. patent No. 4,580,568 (Gianturco); the U.S. patent No. 4,856,516 (Hillstead); the U.S. patent No. 5,092,877 (Pinchuk); and the US patent. No. 5,514,154 (Lau et al.), Which is hereby incorporated by reference in its entirety. The problems with some prior art stents especially those of the expandable type is that they are often rigid and inflexible. Often, stents of the expandable type are formed from stainless steel alloys and the stents are constructed in such a way that they expand beyond their elastic limit. These stents are permanently deformed beyond their elastic limits and are capable of keeping a body lumen open and maintaining the opening of the body lumen. There are several commercially available stents that are widely used and are generally implanted in the coronary arteries after a PTCA procedure. Stents are also 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 crush the endoprosthesis and thus block fluid flow in the vessel. Because the prior art stents are deformed plastically, once collapsed or crushed they will remain so, permanently blocking the vessel. In this way, prior art stents may present an undesirable condition for the patient. Other forces may impact the stents of the prior art and cause similar or total partial vessel blockage. Under certain conditions, muscle contractions may cause prior art stents to partially or totally collapse and restrict blood flow in the vessel in which they are implanted. Attempts have been made to fabricate stents from shape memory alloys (see discussion below) but the shedding of the implant site can result if these stents of the prior art are temporarily crushed due to external forces. What has been required to date is not available in the prior art stents is a stent that is formed from a metal alloy that has recovery and crushing resistance properties and will not significantly detach from the site of implant if it is temporarily crushed due to external forces. The present invention satisfies these needs. COMPENDIUM OF THE INVENTION The present invention is directed to a method for maintaining the opening of a body lumen, including providing a stent of substantially cylindrical shape having super elastic properties resistant to crushing; implant the endoprosthesis in the body lumen; and providing projections that form an outer wall surface of the stent and thus at least partially penetrate the inner wall surface of the body lumen, thereby facilitating the connection of the stent to the inner wall surface of the body lumen, such that as an external force is applied to the body lumen, the stent temporarily at least partially collapses, and the stent returns to the substantially cylindrical shape to thereby maintain the opening of the body lumen when the external force is withdrawn. An object of the present invention is to provide a method for maintaining the opening of a vessel through a stent while minimizing both the risk of permanent vessel crushing and the risk of stent detachment from the implant site, if the stent is crushed. temporarily due to external or internal forces. In a preferred method, a super elastic crush-resistant stent can be made from a material including nickel-titanium alloy. The stent of the present invention is placed under stress by crushing it to a supply diameter. Once a stent is placed in a body lumen, it will expand in the radial direction before a reduction in stress applied to the stent. During stent expansion, projections are formed on the exterior wall surface of the stent. These projections at least partially penetrate the inner wall surface of the body lumen. If an external force is then applied to the body lumen, the stent at least partially collapses or collapses. However, due to the projections, the inner surface of the body lumen remains held to the outer wall surface of the stent. Therefore, the body lumen is temporarily crushed with the stent and the stent will not migrate into the body lumen. The stent then quickly regains its previous expanded form due to its super elastic qualities. In another preferred method, the stent may be constructed of a shape memory alloy such as a nickel-titanium alloy. The stent is designed to expand in the radial direction, when it is inserted into a body lumen and upon reaching a transition temperature that is below the body's normal temperature. The endoprosthesis also exhibits super elastic qualities resistant to crushing. Again, during stent expansion, projections are formed on the outer wall surface of the stent. These projections at least partially penetrate the inner wall surface of the body lumen. If an external force is then applied to the body lumen, the stent temporarily at least partially collapses. However, due to the projections, the inner surface of the body lumen remains held to the outer wall surface of the stent. Therefore, the body lumen is temporarily crushed with the stent and the stent will not migrate into the body lumen. The stent then quickly recovers its expanded anterior shape due to its super elastic qualities. 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. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partial cross-sectional view of a stent delivery system. Figure 2 shows a stent delivery system of Figure 1 with an optional expandable balloon. Figure 3 is an elevation view, partly in section, illustrating a stent mounted on a delivery catheter and expanding within a damaged vessel, pressing the lining of a damaged vessel against the vessel wall. Figure 4 is a view in elevation, partially in section illustrating the expanded stent within the vessel after removal of the delivery catheter. Figure 5 is a view of a portion of the stent in the expanded condition. Figure 6 is a schematic graphic illustration of a typical stress-strain relationship of super elastic material. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES During PTCA procedures, it is common to use a dilatation catheter to expand a diseased area to open the patient's lumen, so that the blood flows freely. Despite the beneficial aspects of PTCA procedures and their widespread and accepted use, it has several disadvantages, including the possible development of restenosis and possibly acute thrombosis and sub-acute closure. Recurrent stenosis has been estimated to occur in 17 to 50% of patients, despite the fact that the initial PTCA procedure is successful. Restenosis is a complex biological response and not completely understood to a lesion of a vessel that results in chronic hyperplasia of the neointima. This neointima hyperplasia is activated by growth factors that are released in response to injury. Acute thrombosis is also a result of vascular injury and requires systemic antithrombotic drugs and possibly thrombolytics alike. 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 re-winding and / or vessel dissection. Several procedures have been developed to combat restenosis and sub-acute and abrupt closure, one of which is the delivery and implantation of an intravascular stent. Stents are widely used throughout the United States and in Europe and other countries. Generally speaking, stents can take many forms, however the most common is a hollow cylindrical tube in general, which keeps the vascular wall open in the area that has been dilated by a dilatation catheter. A highly esteemed endoprosthesis employed and sold in the U.S.A. it is sold under the ACS Multi-Link Stent brand, which is manufactured by Advanced Cardiovascular Systems, Inc., Santa Clara, California. The stents of the present invention can have virtually any configuration that is compatible with the body lumen in which they are implanted. The stent should be configured such that a substantial amount of open area and preferably the ratio of open area to metal is at least 80%. The stent will also be configured in such a way that dissections or flaps in the wall of the body lumen are covered and joined by the stent. With reference to Figure 1, in a preferred embodiment, the stent 10 of the present invention is partially or completely formed of alloys such as nitinol (NiTi) having super elastic characteristics (HE) . The stent is modeled according to the stent described in the U.S. patent. No. 5,569,295, "Expandable Stents and Method for Making Same" (Expandable Endoprosthesis and Method to Produce the Same) granted to Lam on October 29, 1996, which is hereby incorporated by reference in its entirety, with the main difference that the stent of the present invention is constructed of a super elastic material. The configuration of the stent is only one example of many stent configurations that can be employed in the method of the present invention. The super elastic refers to an isothermal reaction; that is, stress induced martensite (SEM = Stress Induced Martensite). Alloys having super elastic properties generally have at least two phases: a martensite phase, which has a relatively low tensile strength and which is stable at relatively low temperatures and an austenite phase, which has a relatively high tensile strength and which is stable at higher temperatures than in the martensite phase. Super elastic characteristics generally allow the metal stent to deform by crushing and deforming the stent and creating stress that causes NiTi to change to the martensite phase. The endoprosthesis is restricted in the deformed condition to facilitate insertion into the patient's body, with this deformation causing the phase transformation. Once inside the body lumen, the restriction in the endoprosthesis is removed, thereby reducing the stress, so that the super elastic stent can return to its original undeformed shape by transforming back to the austenite phase. Figure 1 illustrates a rapid exchange stent delivery system, including a manipulation device 12, a guidewire 14, a delivery liner 16 and an intravascular catheter 18. This delivery system is only one example of a delivery system. supply that can be employed with the present invention. More details of this type of delivery system can be found in the US patent. No. 5,458,615, "Stent Delivery System" granted to Klemm et al. On October 17, 1995, which is hereby incorporated by reference in its entirety.

Other delivery systems such as the well-known on-the-wire delivery systems can be employed without departing from the scope of the invention. Figure 2 illustrates a variation of the delivery system of Figure 1, and includes an expandable balloon 20 and a balloon inflation lumen 22. Following the invention, and moving on to Figure 3, the stent 10 preferably includes a plurality of radially expandable cylindrical elements 24 disposed generally coaxially and interconnected by members 26, disposed between adjacent cylindrical elements 24. The stent is formed from a super elastic material such as NiTi and subjected to an isothermal reaction when tensioned. The stent is first compressed to a supply diameter, thereby creating stress in the NiTi alloy, such that NiTi is in a martensitic state having a relatively low tensile strength. In the martensitic phase, the stent is mounted on a catheter by known methods such as adhesives or other means of restriction. Alternatively, the stent may be mounted within the delivery liner 16, such that the stent, which is radially outwardly directed, is pushed radially outwardly against the liner. In its delivery diameter, the total diameter of the stent and catheter is less than the inside diameter of artery 28 or vessel in which they are inserted. After the stent is inserted into the artery or other vessel, the stress on the stent can be removed by removing the supply liner in a proximal direction, whereby the stent immediately expands and returns to its original undeformed shape by the stent. transformation back to the more stable austenite phase. If the expandable balloon 20 of Figure 2 is implemented, the endoprosthesis can also be expanded by inflating the expandable balloon by the balloon inflation lumen 22 by known methods. Figure 4 illustrates a stent 10 in the expanded condition after the delivery system has been removed. As the stent expands from the supply diameter to the expanded diameter, the peaks 32 are simultaneously formed in the projections 34 (see Figure 5) on the exterior wall surface of the stent and at least partially penetrate the interior wall surface of the stent. artery 28 or other body lumen. If an external force is then applied to the artery, the stent is temporarily at least crushed or partially deformed. As the stent becomes deformed, the stress in the NiTi alloy causes a phase transformation, from the austenite phase to the martensite phase. Due to the projections on the outer wall surface of the stent that are embedded in the arterial wall 29, the artery is temporarily crushed with the stent and the stent will not migrate into the artery. When the external force is removed, the effort in the stent is removed, such that the stent rapidly transforms again from the martensite phase to the austenite such that the stent is fully expanded and the artery remains open. In this way, the super elastic stent resistant to crushing is implanted in an artery, thus maintaining the opening of the artery while minimizing both the risk of permanent arterial collapse and the risk of detachment of the stent from the implant site. , if the endoprosthesis is temporarily deformed due to external forces. When stress is applied to a specimen of a metal such as nitinol, which exhibits super elastic characteristics at a temperature at or above the temperature at which the transformation of the martensite phase into the austenite phase is completed, the specimen is elastically deformed until reaches a particular stress level, where the alloy is then subjected to a phase transformation induced by tension, from the austenite phase to the martensite phase. As the phase transformation proceeds, the alloy undergoes significant increases in effort, but with little or no corresponding increase in effort. The deformation increases while the stress remains essentially constant until the transformation of the austenite phase to the martensite phase is complete. Subsequently, a greater increase in effort is necessary to cause greater deformation. The martensitic metal first yields elastically before the application of additional stress and then plastically with permanent residual deformation. If the load on the specimen is removed before any permanent deformation has occurred, the martensitic specimen will recover elastically and transform back to the austenite phase. The reduction in stress first causes a decrease in deformation. As the reduction in effort reaches the level at which the martensite phase is transformed back into the austenite phase, the level of stress in the specimen will remain essentially constant (although substantially less than the level of constant stress at which the austenite is transformed). in martensite) until the transformation back to the austenite phase is complete; that is, there is a significant recovery in deformation with only negligible reduction in corresponding effort. After the transformation is completed back to austenite, greater reduction in effort results in reduction of elastic deformation. This ability to incur significant deformation at relatively constant stress before the application of a load and recovery of the deformation before the removal of the load is commonly referred to as super elasticity or pseudo-elasticity. The prior art makes reference to the use of metal alloys having super elastic characteristics in medical devices, which are intended to be inserted or otherwise employed within the body of a patient. See, for example, US Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamoto et al.) Which are incorporated herein by reference in its entirety. Figure 6 illustrates an example of the preferred strain-strain relationship of an alloy specimen, such as a stent 10 having super elastic properties as would be exhibited on specimen tensile testing. The line from point A to point B represents the elastic deformation of the specimen. After point B, the deformation is no longer proportional to the applied stress and it is in the region between point B and point C that the stress induced transformation of the austenite phase to the martensite phase begins to occur. There may be a developed intermediate phase, sometimes called the rhombohedral phase depending on the composition of the alloy. At point C, the material enters a relatively constant stress region with significant deformation. It is in this region that the transformation of austenite into martensite occurs. At point D, the transformation to the martensite phase due to the application of tensile stress to the specimen is substantially complete. Beyond point D, the martensite phase begins to deform, elastically at the beginning, but beyond point E, the deformation is plastic or permanent. When the stress applied to the super elastic metal is removed, the metal will recover to its original shape, as long as there is no permanent deformation to the martensite phase. At point F in the recovery process, the metal begins to transform from the unstable martensite phase induced by stress back to the more stable austenite phase. In the region from point G to point H, which is also a region of essentially constant stress, the phase transformation of the martensite back to austenite is essentially complete. The line from point I to starting point A represents the elastic recovery of the metal to its original shape. The super elastic alloy is preferably formed of an alloy that essentially consists of about 30% to about 52% titanium and the nickel moiety and up to 10% of one or more additional alloying elements. These other alloying elements can be selected from the group consisting of up to 3% each of iron, cobalt, platinum, palladium and chromium and up to about 10% in copper and vanadium. As used herein, all references to the composition in percent are atomic percent unless otherwise noted. In another preferred embodiment, a NiTi stent with shape memory effect (SME = Shape Memory Effect) is heat-treated at approximately 500 ° C. The stent is mechanically deformed into a smaller first diameter for mounting in a catheter delivery system, such as the delivery system of Figure 2, which includes an expandable balloon 20 and a balloon inflation lumen 22. After the endoprosthesis has expanded across the balloon and deployed against arterial wall 29 of artery 28, heat is applied at 45 ° C, causing the stent to return to its largest fully expanded diameter and is in contact with the arterial wall. The application of heat at 45 ° C is compatible with most applications in the human body, but is not limited to this temperature since higher or lower temperatures are contemplated without departing from the invention. The temperature of 45 ° C can be achieved in a conventional manner well known in the art such as by hot saline injected into the delivery and balloon catheter. The shape memory features allow the devices to be deformed to facilitate their insertion into a n or body cavity and then heated inside the body in such a way that the device returns to its original shape. Again, alloys having shape memory characteristics generally have at least two phases: a martensite phase, which has a relatively low tensile strength and which is stable at relatively low temperatures and an austenite phase, which has a resistance to relatively high traction and which is stable at higher temperatures than the martensite phase. Shape memory characteristics are imparted to the alloy by heating the metal to a temperature at which the transformation of the martensite phase to the austenite phase is complete, i.e. a temperature at which the austenite phase is stable. The shape of the metal during this heat treatment is the "remembered" shape. The heat-treated metal is cooled to a temperature at which the martensite phase is stable, causing the austenite phase to transform into the martensite phase. The metal in the martensite phase is then plastically deformed, for example to facilitate its entry into the body of a patient. Subsequent heating of the deformed martensite phase at a temperature above the transformation temperature of martensite into austenite causes the deformed martensite phase to transform to the austenite phase. During this phase transformation, the metal returns back to its original shape. The transition or recovery temperature can be altered by making minor variations in the composition of the metal and by processing the material. When developing the correct composition, compatibility with the biological temperature must be determined in order to select the correct transition temperature. In other words, when the stent is heated, it should not be so hot that it is incompatible with the surrounding body tissue. Other shape memory materials may also be used such as, but not limited to, irradiated memory polymers such as self-latching high density polyethylene (HDPEX = Autocrosslinkable High Density Polyethylene). Shape memory alloys are known in the art and are discussed in Shape Memory Alloys (Structured Memory Alloys), Scientific American vol. 281, pages 74 to 82 (November 1979) incorporated herein by reference.

The shape memory alloys undergo a transition between an austenitic state and a martensitic state at certain temperatures. When deformed while in the martensitic state, they will maintain this deformation as long as they are retained in this state, but will return to their original configuration when heated to a transition temperature, at which time they are transformed 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 (NiTi) -based alloys wherein the transition temperature is slightly lower than body temperature, are preferred for the present invention. It is convenient to have the transition temperature adjusted just below the body temperature to ensure a rapid transition from the martensitic state to the austenitic state when the stent is implanted in a body lumen. Turning back to Figure 3, the stent 10 is formed of a shape memory alloy such as NiTi discussed above. After the stent is inserted into artery 28 or other vessel, the expandable balloon 20 is inflated by the balloon inflation lumen 22 by conventional means such that the stent is radially outwardly expanded. The stent then expands immediately due to contact with the higher temperature within artery 28 as described for devices made from shape memory alloys. Again, peaks 32 are formed simultaneously in projections (see Figure 5) on the exterior wall surface of the stent and at least partially penetrate the arterial wall 29 of the artery or other body lumen. Again, if an external force is then applied to the artery, the stent is temporarily at least partially crushed. However, due to the projections, the inner surface of the artery remains attached to the outer wall surface of the stent. Therefore, the artery is temporarily crushed with the stent and the stent will not migrate into the artery. The stent then quickly recovers its previous expanded form due to its shape memory qualities. In this way, the crush-resistant endoprosthesis, which has shape memory characteristics, is implanted in a vessel, thereby maintaining the opening of a vessel while minimizing both the risk of permanent vessel crushing and the risk of detachment from the vessel. stent from the implant site if the stent becomes temporarily deformed due to external forces.

While the invention has been illustrated and described herein in terms of its use as a method for maintaining the opening of a vessel through a stent, while minimizing both the risk of permanent vessel crush and the risk of stent detachment from the vessel. If the stent is temporarily crushed due to external forces, it will be apparent to those skilled in the art that the invention can be used in other cases. Other modifications and improvements can be made without departing from the scope of the invention.

Claims (13)

1. CLAIMS 1. - A method for mounting a stent in a delivery catheter, characterized in that it comprises: providing a stent of substantially cylindrical shape having super elastic properties resistant to crushing and including a plurality of cylindrical elements arranged generally coaxially and connected together by interconnection members disposed between adjacent cylindrical elements; temporarily fixing the endoprosthesis in a substantially cylindrical fashion with respect to the delivery catheter; placing the stent substantially cylindrically under stress and compressing the stent to a supply diameter; providing a retractable supply l over the stent to maintain the effort in the stent.
2. 2. The method according to claim 1, characterized in that during the step of placing the stent substantially cylindrically under stress and compressing the stent to a delivery diameter, the stent transits from an austenitic phase to a martensitic phase.
3. 3. - The method according to claim 1, characterized in that the stent is made from a super elastic alloy which essentially consists of about 40 to about 49% titanium and the rest of nickel and up to 10% of other elements of alloy.
4. 4. - The method according to claim 3, characterized in that the other alloying elements are chosen from the group consisting of iron, cobalt, vanadium and copper.
5. 5. - The method according to claim 4, characterized in that the alloy contains vanadium or copper in amounts up to approximately 10% and up to about 3% of iron, cobalt, vanadium and copper.
6. 6. - The method according to claim 2, characterized in that the deformation of the super elastic material during the transformation of the austenitic phase to the martensitic phase is within the range of about 2% to about 8%.
7. 7. The method according to claim 6, characterized in that the austenitic-to-martensitic transformation occurs at a relatively constant yield stress of about 50 ksi.
8. 8. The method according to claim 6, characterized in that the austenitic-to-martensitic transformation occurs at a relatively constant yield stress of about 70 ksi.
9. 9. - The method according to claim 6, characterized in that the austenitic-to-martensitic transformation occurs at a relatively constant yield stress of about 90 ksi.
10. 10. - The method according to claim 1, characterized in that the endoprosthesis is made from a shape memory alloy that includes a nickel-titanium alloy, the method further comprising providing a transition temperature for the nickel alloy -titanium that is below the normal body temperature.
11. 11. The method according to claim 1, characterized in that the endoprosthesis is formed from a metal alloy selected from the group consisting of metal alloys consisting of stainless steel, nickel-titanium, nickel-copper-titanium, nickel-titanium-vanadium, titanium-nickel-cobalt, copper-zinc, copper-zinc-aluminum, copper-zinc-gallium, copper-zinc-tin, copper-zinc-silicon, copper-aluminum-nickel, copper-aluminum- zinc, copper-tin, gold-cadmium, nickel-aluminum, iron-platinum and nickel-titanium-x, where x is a tertiary element, the method further comprises a transition temperature for the alloy that is below the temperature of the normal body.
12. 12. - The method according to claim 1, characterized in that an outer wall surface of the stent is adapted to form projections at the time the stent is deployed.
13. 13. - The method according to claim 12, wherein the projections are formed on the exterior wall surface of the stent, when the stent reaches a transition temperature.