Abstract:
Medical devices, such as endoprostheses, are disclosed. In some embodiments, an endoprosthesis includes a circumferentially-oriented band including two curved portions and a first portion connecting the two curved portions, the first portion having a first width and at least one curved portion having a second width; and a connector extending from the band; wherein the second width is greater than the first width.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to medical devices, such as endoprostheses (e.g., stents).  
       BACKGROUND  
       [0002]     The body includes various passageways such as arteries, other blood vessels, and other body lumens. These passageways sometimes become occluded or weakened. For example, the passageways can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, the passageway can be reopened or reinforced, or even replaced, with a medical endoprosthesis. An endoprosthesis is typically a tubular member that is placed in a lumen in the body. Examples of endoprostheses include stents, covered stents, and stent-grafts.  
         [0003]     Endoprostheses can be delivered inside the body by a catheter that supports the endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. Upon reaching the site, the endoprosthesis is expanded, for example, so that it can contact the walls of the lumen.  
         [0004]     The expansion mechanism may include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries a balloon-expandable endoprosthesis. The balloon can be inflated to deform and to fix the expanded endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter withdrawn.  
         [0005]     In another delivery technique, the endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded, e.g., elastically or through a material phase transition. During introduction into the body, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.  
       SUMMARY  
       [0006]     The invention relates to medical devices, such as endoprostheses.  
         [0007]     In one aspect of the invention, an endoprothesis comprises: a circumferentially-oriented band including two curved portions and a first portion connecting the two curved portions, the first portion having a first width and at least one curved portion having a second width, wherein the second width is greater than the first width.  
         [0008]     Embodiments may include one or more of the following features.  
         [0009]     The endoprothesis can have a second width to first width ratio is greater than 1 and less than 3 (e.g. greater than 1.05 greater than 1.1, greater than 1.5, or greater than 1.75).  
         [0010]     The endoprothesis can have curved portions and first portion including a material with a yield strength of greater than 50 kilopounds per square inch and less than 150 kilopounds per square inch. (e.g. greater than 55 kilopounds per square inch, greater than 65 kilopounds per square inch, or greater than 70 kilopounds per square inch).  
         [0011]     The curved portions and first portions can comprise a material selected from the group consisting of stainless steel, stainless steel alloyed with a radiopaque element, nickel alloy, niobium alloy, and titanium alloy.  
         [0012]     The circumferentially-oriented band can include a plurality of curved portions and a plurality of first portions connecting adjacent curved portions to form a generally sinusoidal pattern. The endoprothesis can include a plurality of the circumferentially-oriented bands and a plurality of connectors, the connectors extending between adjacent bands.  
         [0013]     The circumferentially-oriented band can include a plurality of curved portions and a plurality of first portions connecting the curved portions, each first portion further having a length; wherein each band has an aggregate curved portion width to first portion length ratio, calculated by dividing an average curved portion width by a sum of lengths of the first portions, that is greater than 0.01 (e.g. greater than 0.015).  
         [0014]     In another aspect of the invention, an endoprothesis comprises a circumferentially-oriented band including a plurality of curved portions and a plurality of first portions connecting adjacent curved portions to form a generally sinusoidal pattern, the first portions having a first average width and the curved portions having a second average width, wherein a width ratio calculated by dividing the second average width by the first average width is greater than 1.05, and the curved portions and first portion comprise a material with a yield strength of greater than 55 kilopounds per square inch.  
         [0015]     Embodiments may include one or more of the following features.  
         [0016]     The width ratio can be greater than 1.1.  
         [0017]     The curved portions and first portion can comprise a material with a yield strength of greater than 70 kilopounds per square inch.  
         [0018]     The endoprothesis can include a plurality of the circumferentially-oriented bands and a plurality of connectors, the connectors extending between adjacent bands.  
         [0019]     In another aspect of the invention, an endoprothesis comprises: two first portions generally transverse to a longitudinal axis of the endoprothesis, the first portions having a first average width; and a second portion connecting the first portions, the second portion having a second average width, wherein the first average width is greater than the second average width. The second portion can be substantially parallel to the longitudinal axis. The first portions and the second portion can form angles of between about 80 and 110 degrees at their intersections.  
         [0020]     Embodiments may include one or more of the following advantages. Without wishing to be bound by theory, it is believed that certain stents, such as those including (e.g., made from) a high yield strength material, can experience relatively high recoil when the stents are expanded against a vessel wall or crimped onto a delivery catheter. As a result, the securement of the stents to the vessel wall or the delivery catheter may be less than desired. By forming the stents with curved portions with curved portions with widths that are greater than widths of adjoining straight portions, it is believed that the strain that occurs during crimping or expansion of the stents is transferred and concentrated to the relatively small volume of material of the straight portions, which can increase the strain more along the stress-strain curve and result in plastic, rather than elastic, deformation of the stent. As a result, recoil is limited and stent securement is enhanced. Furthermore, by increasing the width of only curved portion, it is believed that flexibility is enhanced, for example, relative to stents where the widths of both the curved and straight portions are increased.  
         [0021]     Other aspects, features, and advantages will be apparent from the description of the preferred embodiments thereof and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0022]      FIG. 1  is a perspective view of an embodiment of an expanded stent.  
         [0023]      FIG. 2A  is a detailed view of a section of the stent of  FIG. 1 .  
         [0024]      FIG. 2B  is a detailed view of section B of  FIG. 2A .  
         [0025]      FIG. 3  is a detailed view of an alternate embodiment of a stent.  
         [0026]      FIG. 4  is a detailed view of an alternate embodiment of a stent.  
         [0027]      FIG. 5  is a detailed view of an alternate embodiment of a stent.  
         [0028]      FIG. 6  is a detailed view of an alternate embodiment of a stent. 
     
    
     DETAILED DESCRIPTION  
       [0029]     Referring to  FIG. 1 , a stent  20  has the form of a tubular member defined by a plurality of bands  22  and a plurality of connectors  24  that extend between and connect adjacent bands. During use, bands  22  are expanded from an initial, small diameter to a larger diameter to contact stent  20  against a wall of a bodily vessel, thereby maintaining the patency of the vessel. Examples of stents are described in Burmeister et al., U.S. Pat. No. 6,451,052.  
         [0030]     As used herein, a band  22  refers to a portion of a stent that extends circumferentially about the stent. The band can extend completely about the circumference of a stent, for example, such that the ends of the band are joined, or the band can extend partially about the circumference. The band can extend substantially linearly or nonlinearly, for example, in an undulating pattern, a zigzag pattern (as shown in  FIG. 1 ), or a square-wave pattern. In some embodiments, bands  22  are connected together by integrally formed connectors  24  that extend between and transversely to the bands.  
         [0031]     As used herein, a connector  24  refers to a portion of a stent that extends from a band of the stent, for example, from a first band to an adjacent second band along the length of the stent. The connector can extend linearly (e.g., parallel to the longitudinal axis of the stent) or nonlinearly, for example, in an undulating patter or zigzag pattern.  
         [0032]     Referring to  FIGS. 2A and 2B , bands  22  and connectors.  24  have different shapes and dimensions. As shown, bands  22  have curved portions  26  connected by straight portions  28 . Curved portions  26  have a width (W bc ) that is greater than a width (W bs ) of straight portions  28 . Both of these widths (W bc , W bs ) are greater than a width (W c ) of connectors  24 .  
         [0033]     Without wishing to be bound by theory, it is believed that certain stents, such as those including (e.g., made from) a high yield strength material, can experience relatively high recoil when the stents are expanded against a vessel wall or crimped onto a delivery catheter. As a result, the securement of the stents to the vessel wall or the delivery catheter may be less than desired. By forming the stents with curved portions with curved portions  26  with widths (W bc ) that are greater than widths (W bs ) of adjoining straight portions  28 , it is believed that the strain that occurs during crimping or expansion of the stents is transferred and concentrated to the relatively small volume of material of the straight portions, which can increase the strain more along the stress-strain curve and result in plastic, rather than elastic, deformation of the stent. As a result, recoil is limited and stent securement is enhanced. Furthermore, by increasing the width of only curved portion  26 , it is believed that flexibility is enhanced, for example, relative to stents where the widths of both the curved and straight portions  26 ,  28  are increased.  
         [0034]     As shown, stent  20  has a width ratio of curved portion width (W bc ) to straight portion width (W bs ) (i.e. W bc /W bs ) is greater than one. The width ratio W bc /W bs  can be greater than or equal to about 1, about 1.25, about 1.5, about 1.75, about 2, about 2.25, about 2.5, about 2.75 and/or less than or equal to about 3, about 2.75, about 2.5, about 2.25, about 2, about 1.75, about, about 1.5, about 1.25. As shown, the width (W bc ) of the curved portions  26  and the width (W bs ) of straight portions  28  have substantially constant dimensions and thus the width ratio (W bc /W bs ) is consistent across the stent. In some stents, these widths can vary but it is anticipated, without wishing to be bound by theory, that each individual section comprising two curved portions connected by a straight section having an individual width ratio greater than 1 and less than about 3 can cause an incremental increase in securement performance.  
         [0035]     Bands  22  can have widths (W bc , W bs ) ranging from about 0.03 mm to about 0.20 mm. Particular widths of band  22  can be a function of, for example, the material(s) in stent  20 , the type of stent (e.g., balloon-expandable or self-expandable), and/or the desired performance. For example, a stent including 316L stainless steel can have band widths (W bc , W bs ) of from about 0.06 mm to about 0.25 mm; a stent including an alloy of 10-60 weight percent platinum and 316L stainless steel constituents (PERSS®) can have band widths (W bc , W bs ) of from about 0.04 mm to about 0.25 mm; and a stent including a Fe—Co—Cr—Ni alloy (such as Elgiloy, MP35N or L605) can have band widths (W bc , W bs ) of from about 0.03 mm to about 0.20 mm; and a stent including niobium alloyed with about 1-10 weight percent zirconium, about 1-70 weight percent tantalum, or about 1-10 weight percent tungsten can have band widths (W bc , W bs ) of from about 0.08 mm to about 0.30 mm. As shown, bands  22  include sinusoidal waves, but other embodiments, such as square waves, zigzag waves, or a plurality of connected polygons, can be used.  
         [0036]     Connector  24  can have a width (W c ) ranging from about 0.03 mm to about 0.20 mm. Particular widths of connector  24  can be a function of, for example, the material(s) in stent  20 , the type of stent (e.g., balloon-expandable or self-expandable), and/or the desired performance. For example, a stent including 316L stainless steel can have a connector width (W c ) of from about 0.05 mm to about 0.18 mm; a stent including a PERSS® alloy can have a connector width (W c ) of from about 0.03 mm to about 0.10 mm; a stent including an alloy having chromium and cobalt can have a connector width (W c ) of from about 0.02 mm to about 0.08 mm; a stent including a refractory metal can have a connector width (W c ) of from about 0.08 mm to about 0.20 mm; and a stent including an alloy having titanium can have a connector width (W c ) of from about 0.03 mm to about 0.15 mm. As shown, connectors  24  are straight, but other embodiments, such as connectors  24  that are bent, can be used. Additionally, a stent can be constructed without connectors, sharing material between adjacent bands.  
         [0037]     In stents  20  including a plurality of curved portions  26  connected by a plurality of straight portions  28 , straight portions  28  have a length (L) (see  FIG. 2A ). Some stents are configured with an aggregate curved portion width to straight portion length ratio, calculated by dividing an average curved portion width by a sum of lengths of the straight portions, that is greater than 0.01 (e.g., greater than 0.015.). For example, this ratio would be about W bc /12L (i.e. the average curved portion width W bc  divided by the sum of the lengths of the straight portions or the number straight portions, 12, times the average length L) for the stent illustrated in  FIG. 2A .  
         [0038]     As described above, other configurations of bands and connectors are possible. For example, referring to  FIG. 3 , a high yield strength stent  120  can include bands  122  configured as square waves joined by connectors  124  extending between adjacent bands  122 . Bands  122  include first portions  126  generally transverse to a longitudinal axis  127  of the stent and second portions  128  connecting the first portions  126 . Second portions  128  are transverse to first portions  126  and parallel to longitudinal axis  127 . First portions  126  and the second portions  128  can form angles of between about 80 and 110 degrees at their intersections. The first portions  126  have widths (W t ), the second portions have widths (W 1 ), and the connectors  124  have widths (W c ). As shown, the widths (W t ) of the first portions  126  are greater than the widths (W 1 ) of the second portions and both are greater than widths (W c ) of the connectors  124 . Some stents, such as high yield strength stents, with varying widths (W t , W 1 ) of the first  126  and second  128  portions have an average width of the first portions  126  that is greater than an average width of the second portions  128 . The ratios of W t  to W c  (W t /W c ) can range from about 1 to about 3 as described above for W bc /W bs .  
         [0039]     Referring to  FIGS. 4-6 , other configurations are possible such as bands  222 ,  322 ,  422  having curved portions  226 ,  326 ,  426  having a width W bc  connecting adjacent straight portions  228 ,  328 ,  428  with a width W bs  that is less than W bc . The bands can be joined by connectors (such as bands  222 ,  322  joined by connectors  224 ,  324 ) or the bands can be joined without connectors (such as bands  422 ) Bands  22  and connectors  24  can include (e.g., be manufactured from) one or more biocompatible materials with mechanical properties so that stent  20  can be compacted, and subsequently expanded to support a vessel. In some embodiments, stent  20  can have an ultimate tensile strength (UTS) of about 20-150 ksi, greater than about 15% elongation to failure, and a modulus of elasticity of about 10-60 msi. When stent  20  is expanded, the material can be stretched to strains on the order of about 0.4. In some embodiments, bands  122  and connectors  124  include material having yield strength of from about 55 kilopounds per square inch (ksi) to about 150 ksi. The yield strength can be greater than or equal to about 60 ksi, about 70 ksi, about 80 ksi, about 90 ksi, about 100 ksi, about 110 ksi, about 120 ksi, about 130 ksi, or about 140 ksi; and/or less than about 150 ksi, about 140 ksi, about 130 ksi, about 120 ksi, about 110 ksi, about 100 ksi, about 90 ksi, about 80 ksi, about 70 ksi, or about 60 ksi. Examples of high yield strength materials include alloys (e.g., PERSS®) including stainless steel and 5-60 weight percent of one or more radiopaque elements (e.g. Pt, Ir, Au, Ta, Pd), as described in U.S. Patent Publications US-2003-0018380-A1; US-2002-0144757-A1; and US-2003-0077200-A1. Other examples of materials that provide good mechanical properties and/or biocompatibility include, for example, stainless steel (e.g., 316L and 304L stainless steel, and PERSS®), Nitinol (a nickel-titanium alloy), Elgiloy, L605 alloys, MP35N, Ti-6Al-4V, Ti-50Ta, Ti-10Ir, Nb-1Zr, and Co-28Cr-6Mo. Other materials include elastic biocompatible metal such as a superelastic or pseudo-elastic metal alloy, as described, for example, in Schetsky, L. McDonald, “Shape Memory Alloys”, Encyclopedia of Chemical Technology (3rd ed.), John Wiley &amp; Sons, 1982, vol. 20. pp. 726-736; and commonly assigned U.S. patent application Ser. No. 10/346,487, filed Jan. 17, 2003.  
         [0040]     The material(s) can include one or more radiopaque materials to provide radiopacity. Examples of radiopaque materials include metallic elements having atomic numbers greater than 26, e.g., greater than 43. In some embodiments, the radiopaque materials have a density greater than about 9.9 g/cc. In certain embodiments, the radiopaque material is relatively absorptive of X-rays, e.g., having a linear attenuation coefficient of at least 25 cm −1 , e.g., at least 50 cm −1 , at 100 keV. Some radiopaque materials include tantalum, platinum, iridium, palladium, hafnium, tungsten, gold, ruthenium, osmium, and rhenium. The radiopaque material can include an alloy, such as a binary, a ternary or more complex alloy, containing one or more elements listed above with one or more other elements such as iron, nickel, cobalt, or titanium. Examples of alloys including one or more radiopaque materials are described in U.S. Patent Application Publication US-2003-0018380-A1; US-2002-0144757-A1; and US-2003-0077200-A1.  
         [0041]     In some embodiments, stent  20  includes one or more materials that enhance visibility by magnetic resonance imaging (MRI). Examples of MRI materials include non-ferrous metal-alloys containing paramagnetic elements (e.g., dysprosium or gadolinium) such as terbium-dysprosium, dysprosium, and gadolinium; non-ferrous metallic bands coated with an oxide or a carbide layer of dysprosium or gadolinium (e.g., Dy 2 O 3  or Gd 2 O 3 ); non-ferrous metals (e.g., copper, silver, platinum, or gold) coated with a layer of superparamagnetic material, such as nanocrystalline Fe 3 O 4 , CoFe 2 O 4 , MnFe 2 O 4 , or MgFe 2 O 4 ; and nanocrystalline particles of the transition metal oxides (e.g., oxides of Fe, Co, Ni). Alternatively or in addition, stent  20  can include one or more materials having low magnetic susceptibility to reduce magnetic susceptibility artifacts, which during imaging can interfere with imaging of tissue, e.g., adjacent to and/or surrounding the stent. Low magnetic susceptibility materials include tantalum, platinum, titanium, niobium, copper, and alloys containing these elements. The MRI visible materials can be incorporated into the structural material, can serve as the structural material, and/or be includes as a layer of stent  20 .  
         [0042]     One method of making stent  20  includes forming a tube that makes up the tubular member of stent  20 . The tube is subsequently cut to form bands  22  and connectors  24  to produce an unfinished stent. Areas of the unfinished stent affected by the cutting are subsequently removed. The unfinished stent is finished to form stent  20 .  
         [0043]     The tube that makes up the tubular member of stent  20  can be formed using metallurgical techniques, such as thermomechanical processes. For example, a hollow metallic member (e.g., a rod or a bar) can be drawn through a series of dies with progressively smaller circular openings to plastically deform the member to a targeted size and shape. In some embodiments, the plastic deformation strain hardens the member (and increases its yield strength) and elongates the grains along the longitudinal axis of the member. The deformed member can be heat treated (e.g., annealed above the recrystallization temperature and/or hot isostatically pressed) to transform the elongated grain structure into an initial grain structure, e.g., one including equiaxed grains. Small or fine grains can be formed by heating the member close to the recrystallization temperature for a short time. Large or coarse grains can be formed by heating the member at higher temperatures and/or for longer times to promote grain growth.  
         [0044]     Next, bands  22  and connectors  24  of stent  20  are formed by cutting the tube. Selected portions of the tube can be removed to form bands  22  and connectors  24  by laser cutting, as described in U.S. Pat. No. 5,780,807, hereby incorporated by reference in its entirety. In certain embodiments, during laser cutting, a liquid carrier, such as a solvent or an oil, is flowed through the lumen of the tube. The carrier can prevent dross formed on one portion of the tube from re-depositing on another portion, and/or reduce formation of recast material on the tube. Other methods of removing portions of the tube can be used, such as mechanical machining (e.g., micro-machining), electrical discharge machining (EDM), and photoetching (e.g., acid photoetching).  
         [0045]     In some embodiments, after bands  22  and connectors  24  are formed, areas of the tube affected by the cutting operation above can be removed. For example, laser machining of bands  22  and connectors  24  can leave a surface layer of melted and resolidified material and/or oxidized metal that can adversely affect the mechanical properties and performance of stent  20 . The affected areas can be removed mechanically (such as by grit blasting or honing) and/or chemically (such as by etching or electropolishing). In some embodiments, the tubular member can be near net shape configuration after this is performed. “Near-net size” means that the tube has a relatively thin envelope of material that is removed to provide a finished stent. In some embodiments, the tube is formed less than about 25% oversized, e.g., less than about 15%, 10%, or 5% oversized.  
         [0046]     The unfinished stent is then finished to form stent  20 . The unfinished stent can be finished, for example, by electropolishing to a smooth finish. Since the unfinished stent can be formed to near-net size, relatively little of the unfinished stent need to be removed to finish the stent. As a result, further processing (which can damage the stent) and costly materials can be reduced. In some embodiments, about 0.0001 inch of the stent material can be removed by chemical milling and/or electropolishing to yield a stent.  
         [0047]     Stent  20  can be of a desired shape and size (e.g., coronary stents, aortic stents, peripheral vascular stents, gastrointestinal stents, urology stents, and neurology stents). Depending on the application, stent  20  can have a diameter of between, for example, 1 mm to 46 mm. In certain embodiments, a coronary stent can have an expanded diameter of from about 2 mm to about 6 mm. In some embodiments, a peripheral stent can have an expanded diameter of from about 5 mm to about 24 mm. In certain embodiments, a gastrointestinal and/or urology stent can have an expanded diameter of from about 6 mm to about 30 mm. In some embodiments, a neurology stent can have an expanded diameter of from about 1 mm to about 12 mm. An abdominal aortic aneurysm (AAA) stent and a thoracic aortic aneurysm (TAA) stent can have a diameter from about 20 mm to about 46 mm. Stent  20  can be balloon-expandable, self-expandable, or a combination of both (e.g., U.S. Pat. No. 5,366,504).  
         [0048]     In use, stent  20  can be used, e.g., delivered and expanded, using a catheter delivery system. Catheter systems are described in, for example, Wang U.S. Pat. No. 5,195,969, Hamlin U.S. Pat. No. 5,270,086, and Raeder-Devens, U.S. Pat. No. 6,726,712. Stents and stent delivery are also exemplified by the Radius® or Symbiot® systems, available from Boston Scientific Scimed, Maple Grove, Minn.  
         [0049]     While a number of embodiments have been described above, the invention is not so limited.  
         [0050]     In some embodiments, bands  22  and connectors  24  can have different microstructures. For example, bands  22  and connectors  24  can have different grain sizes, with the grains in the bands being larger than the grains in the connectors. As a result, connectors  24  have a higher yield strength that the yield strength of bands  22 , since grain size is typically inversely related to yield strength. The high yield strength of connectors  24  allows them to have small cross-sectional sizes, which allows them to easily deform so that stent  20  can conform well to a vessel that is not straight. The yield strength and the section size are balanced to allow connectors  24  to easily deform while remaining resistant to fracture. In comparison, the low yield strength of bands  22  reduces elastic recoil when stent  20  is crimped to a delivery system and during in vivo expansion. The yield strength and the section size of bands  22  are balanced to provide good resistance to radial compression and to control elastic recoil. Stents having different grain sizes and methods of making the stents are described in commonly assigned U.S. patent application Ser. No. 10/961,289 filed on Oct. 8, 2004.  
         [0051]     Stent  20  can include more than one layer. For example, a stent can include a first “structural” layer, such as 316L stainless steel or PERSS®, and a second layer of a radiopaque element. The radiopaque layer can be formed after the heat treatment to prevent, e.g., separation due to thermal expansion differences. Either layer can be the inner or the outer layer, and either layer or both layers can include the microstructures as described above. A three-layered stent can include a layer including a radiopaque element formed between two structural layers.  
         [0052]     Stent  20  can also be a part of a covered stent or a stent-graft. In other embodiments, stent  20  can include and/or be attached to a biocompatible, non-porous or semi-porous polymer matrix made of polytetrafluoroethylene (PTFE), expanded PTFE, polyethylene, urethane, or polypropylene.  
         [0053]     Stent  20  can include a releasable therapeutic agent, drug, or a pharmaceutically active compound, such as described in U.S. Pat. No. 5,674,242, U.S. Ser. No. 09/895,415, filed Jul. 2, 2001, and U.S. Ser. No. 10/232,265, filed Aug. 30, 2002. The therapeutic agents, drugs, or pharmaceutically active compounds can include, for example, anti-thrombogenic agents, antioxidants, anti-inflammatory agents, anesthetic agents, anti-coagulants, and antibiotics.  
         [0054]     All publications, references, applications, and patents referred to herein are incorporated by reference in their entirety.  
         [0055]     Other embodiments are within the claims.