MXPA97003231A

MXPA97003231A - Implantable and grafted protestism of allocation protosisimplantable cobalt-cromo-molibd

Info

Publication number: MXPA97003231A
Application number: MXPA/A/1997/003231A
Authority: MX
Inventors: S Stinson Jonathan
Original assignee: Schneider (Usa) Inc
Priority date: 1996-04-30
Filing date: 1997-04-29
Publication date: 1998-04-01

Abstract

The present invention relates to an implantable medical device comprised of a tubular and radially expandable structure that includes at least one elongate element formed of cobalt, chromium and molybdenum alloy (Co-Cr-Mo) containing less than about 5 percent in weight of níqu

Description

IMPLANTABLE AND IMPARTABLE PROSTHESIS OF IMPLANTOBLE PROSTHESIS OF ALLOY COBOLTO-CROMO-MO IBDENO FIELD OF THE INVENTION The present invention relates generally to radially expandable medical and plantable prostheses that include implantable prosthetic graft. In particular, the present invention is an implantable prosthesis and graft of implantable prosthesis of cobalt-chromium-rnolibdene alloy.

BACKGROUND OF THE INVENTION Medical prostheses are well known and commercially available. One type of implantable prosthesis, known as a self-expanding implantable prosthesis, is generally described in U.S. Patent No. 4,655,771, for Uallsten, U.S. Patent 5,061,275, for U listen et al., International Application Publication No. UO 94/24961, and International Application Publication No. Uo 94/16646. These devices are used within body vessels of humans and other animals for a variety of medical applications including stenosis treatment, maintenance of openings in the urinary, biliary, esophageal and renal tracts, and vena cava filters to combat embolism.

Briefly, self-expandable implantable prostheses of the type described in the patent documents identified above are formed from a number of filaments or elastic elements that are helically wound and woven into a braid-like configuration. The implantable prostheses assume a substantially tubular shape in their unloaded state, or expanded when they are not subjected to external forces. When subjected to internally directed radial forces, the plantable prostheses are forced into a reduced radius and loaded or compressed in extended length. A releasing device that retains the implantable prosthesis in its compressed state is used to release the implantable prosthesis to a treatment site through the vessels in the body. The flexible nature and reduced radius of the compressed implantable prosthesis allow it to be delivered through relatively small and curved vessels. After an implantable prosthesis is positioned at the treatment site, the delivery device acts to release the implant prosthesis, thereby allowing the prosthesis to self-expand within the body vessel. Then, the delivery device is separated from the implant prosthesis and removed from the patient. The implantable prosthesis remains in the vessel at the treatment site. Commonly used materials for self-expandable implantable prosthetic filaments include Elgiloy® and PhynoxR flexible alloys. The Elg? LoyR alloy is available from Carpenter Technology Corporation of Reading Pennsylvania. The PhynoxR alloy is available from Metal Irnphy of I phy, France. Both of these metals are cobalt-based alloys that also include chromium, iron, nickel and rnolibdene. Other materials used for self-expandable implantable prosthetic filaments are 316 stainless steel and MP35N alloy, which is available from Carpenter Technology Corporation and Latrobe Steel. Campany from Latrobe, Pennsylvania, and nickel-titanium superelastic alloy Nitinol, which is available from Shape Mernory Applications of Santa Clara, California. The resistance to deformation and the modulus of elasticity of the filaments that form the self-expanding implantable prosthesis are important characteristics. The flexibility characteristics of an alloy and the implantable prostheses formed therefrom are determined to a large extent by the modulus of elasticity of the alloy. In general, the modulus of elasticity must be high enough to allow the prosthesis to spring back towards its discharge state of the compressed state, with sufficient radial force to meet the requirements of the application for which the prosthesis was designed. The material must also have sufficient strength so that it can be compressed for release without being plastically deformed or permanently bent. ElgiloyR, PhynoxR, MP35N and stainless steel are all high strength and high modulus metals. Nititol has a relatively low strength and modulus. The alloys of Elgiloy "*, PhynoxR, MP35N and stainless steel all contain approximately 10% to 20% nickel." Nickel increases the ductility of the alloys, improving their ability to be stretched or mechanically molded (ie, reduced in size). cross-sectional area) in wire of relatively thin diameters required for implantable prostheses (between approximately 0.025 mm and 0.500 mm) by a procedure known as cold-working, cold-working is also convenient because it increases the strength of the material. However, the deformation resistance that can be obtained by cold working of Elg? loyR, PhynoxR, MP35N, Nit ol and stainless steel alloys (e.g., approximately 1738 Mpa for Elg? loyR alloy) is generally not sufficiently high for many applications of implantable prostheses.As a result, impiantable prostheses manufactured from cold-worked Elg? loyR and PhynoxR (also known as set) are typically treated with heat after being cold-worked, a process that significantly increases their resistance to deformation and thereby allows the fabrication of implantable prostheses with filaments of relatively smaller diameter. By way of example, the deformation resistance of the Elg? LoyR alloy can be increased by heat treatment to about 2861 MPa. The strength of stainless steel and Nitol alloys can not be significantly increased by means of heat treatment, so that these materials are not typically used in the construction of expandable and expandable prostheses with high radial strength. Cold working is a method by which the metal is plastically deformed to a particular shape and hardened (deformation) to increase the strength of the material. The processes that can be carried out to carry out cold working are stretching, rolling, extrusion, forging, setting and the like. Is the starting material introduced into the cold-working process in the form of ingots, rods, bars, small ingots, discs? other appropriate forms. The workpieces are forced to pass through a given die, fill a die cavity, or conform to the given shape. The output of the cold-working process is typically material with a new shape and with superior strength and hardness of the hardening of metallurgical deformation that occurs with the plastic deformation. In the cold-working process, described in International Publication No. UO 94/16646, ingots, rods, rods or wire are stretched or extruded through a series of round dice and an incremental reduction in material diameter is achieved. that the final desired wire size is obtained to braid the implantable prosthesis. The filaments of the implantable prostheses described above can form a lattice structure that includes large amounts of open area. However, in some cases this large open area allows the tissue to grow through the implantible prosthesis and obstructs portions of the tract that were opened by the implantable prosthesis. The use of covered implantable prostheses is generally known for applications where tissue growth of this type is inconvenient, as well as for applications in which portions of the tract undergoing treatment (aneurysms) are weak or have spaces. Implantable prostheses and implantable prosthetic grafts can be covered, for example, by means of porous membranes, interwoven organic filaments or the like. Implantable prostheses of this type are sometimes known as prostheses or grafts of coated prostheses and are described, for example in Experimental Assessment of Newly Devised Trans-Catheter Stent-Graft for Atopic Dissection, Annual of Thoracic Surgery, M. Kato et al., 59: 908-915 (1995). The membranes incorporated in prosthetic grafts are typically formed from polymeric materials. However, many of these polymeric materials can degrade when exposed to temperatures used for hot-treated alloys of the type described above. Therefore, the need to heat treat the lattice structure of the metal alloy and the temperature sensitivities of the polymers used to form the membranes thereof, restrict the designs of the prosthetic grafts and their application.

In addition to stretching elongated filaments for implants implants of interwoven elements of the type described above and in the UA North American Patent No. 4,655,771, metal alloy materials are stretched or extruded into other forms for manufacturing implantable prostheses. U.S. Patent No. 4,733,655 to Palrnaz, refers to a plantable prosthesis i made from a stretched or extruded stainless steel tube. US Pat. No. 4,800,882 to Giant? Rco, refers to a plantable prosthesis i assembled from drawn stainless steel wire. Other known implantable prostheses are made of stretched, extruded or rolled nickel-titanium alloy tape. Alloys of cobalt, chromium, molybdenum (Co-Cr-Mo) have been used in medical implant applications. In ASTM Standard Designations F 75 and F 799, chemical, mechanical and metallurgical requirements are published for alloys of these types used for surgical implant applications. One such alloy known as BioDur Carpenter CCMR is commercially available from Carpenter Tecnhology Corporation. These chromium-cobalt-molybdenum alloys are highly biocornpatible. However, since they have a relatively low nickel content (approximately 1% at most), the cobalt-o-rnolibdene alloys have relatively low ductilities and high work hardening rates that limit their moldability. For this reason, conventional wisdom has been that these alloys can not be cold drawn to reach the fine wire diameters required for implantable prostheses and implantable prosthetic grafts. There is a continuing need for implantable prostheses and improved implantable prosthetic grafts. In particular, there is a need for implantable prostheses and implants of implantable prostheses made from highly biocompatible alloys that have high resistance to deformation and high modulus of elasticity. There is also a need for implantable prostheses and implantable prosthetic grafts that do not require heat treatment.

BRIEF DESCRIPTION OF THE INVENTION The present invention relates to an improved implantable medical device comprised of a tubular and radially expandable structure that includes at least one elongate element formed from cobalt o-chrome-rnolibdene alloy (Co-Cr-Mo) containing less than about 5% nickel. The CO-CR-Mo alloy is highly biocompatible and has a relatively high resistance to deformation and modulus of elasticity. One embodiment of the invention is a radially expandable self-expanding stent that includes a plurality of elongated Co-CR-Mo alloy filaments that are interwoven in a braid configuration. The alloy contains at least 50% by weight of cobalt, between about 26 and 31% by weight of chromium, between about 4 to 8% by weight of molybdenum and less than about.% By weight of nickel.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an isometric view of an embodiment of the present invention, illustrating an implantable prosthesis having a braided configuration of filaments. Figure 2 is a partially longitudinal cross-sectional view of the implantable prosthesis shown in Figure 1. Figure 3 is a cross-sectional view of one of the filaments of the implantable prosthesis shown in Figure 1. Figure 4 is a cross sectional view of a mixed filament according to another embodiment of the invention. Figure 5 is a photograph of an implantable prosthesis graft in accordance with the present invention. Figure 6 is a schematic illustration of different discrete layers that can be formed by the three-dimensional braiding of multiple chains and incorporated in the implantable prosthesis graft shown in Figure 5. Figures 7-9 schematically illustrate a procedure for manufacturing the graft. of unplantable prosthesis shown in Figure 5. Figure 10 schematically illustrates an alternative procedure for manufacturing the implantable prosthesis graft shown in Figure 5.

DETAIL DESCRIPTION »OF THE PREFERRED MODALITIES In FIGS. 1 and 2, an implantable prosthesis 10 according to the present invention is generally illustrated. The implantable prosthesis 10 is a tubular device formed of two groups of elongated elements or filaments 12, oppositely directed, parallel, spaced apart and helically wound. The groups of filaments 12 are interwoven in a braided configuration up and down, and are interconnected at points such as 14 to form an open, open structure. The invention is based on the discovery that, contrary to conventional wisdom, certain cobalt-chromium-rnolibdene (Co-Cr-Mo) alloys containing less than about 5% by weight of nickel can be stretched or molded in some other way by cold-working in forged elements such as filaments 12 suitable for implantable prostheses 10. Non-limiting examples of cold-working methods that can be used to form said Co-Cr-Mo alloying elements include wire drawing, tube stretching and Similar. At least one, and in a preferred embodiment all, the filaments 12 are formed from commercially available Co-Cr-Mo alloy, which contains less than 2% by weight of nickel. The methods for manufacturing plantable prostheses 10 are generally known and described, for example, in US Pat. No. 4,655,771, Uallsten, North American Patent Nurn. 5,061,275, from Ualleten et al., And International Application Publications Nos. UO 94/24961 and UO 94/16646. In figures 1 and 2 the implantable prosthesis 10 is shown in its expanded or relaxed state, that is, in the configuration it assumes when it is not subjected to external loads or stresses. The filaments 12 are elastic, allowing radial understanding, of the implantable prosthesis 10 up to a configuration or reduced radius state of extended length suitable for delivery to the desired location or treatment site through a body vessel (i.e. transluminally). The implantable prosthesis 10 is also self-expandable from the compressed state and is axially flexible. Stated another way, the implantable prosthesis 10 is a radially and axially flexible tubular body having a predetermined diameter that is variable under axial movement of the ends of the body in relation to each other. The implantable prosthesis 10 is composed of a plurality of individually rigid but flexible elastic twisted filaments or elements 12, each of which extends in a helical configuration along a longitudinal centerline of the body as a common axis. At least one, and in a preferred modality, all the filaments 12 are formed of Co-Cr-Mo alloy containing less than about 5% by weight of nickel. The filaments 12 define a body that radially self-expands. The body is provided by a first number of filaments 12 having a common winding direction but displaced axially in relation to each other, and crossing a second number of filaments 12 also displaced axially in relation to each other but having an opposite direction of winding. The tubular and self-expanding body or structure formed by the interwoven filaments 12 is a prosthetically functional primary structure of implantable prosthesis 10, and for this reason, the device can be considered to consist substantially of this structure for the exclusion of other structures. However, it is known that other structures and characteristics can be included in the implantable prostheses, and in particular, characteristics that increase or cooperate with the tubular and self-expanding structure or that facilitate the implantation of the structure. One example is the inclusion of radiopaque markers on the structure that are used to visualize the position of the implantable prosthesis by fluoroscopy during implantation. Other examples include crushable threads? other structures to facilitate replacement and removal of the implantable prosthesis. Implantable prostheses of these types nevertheless still consist substantially of the tubular and self-expanding structure formed by interwoven filaments 12 and shown in Figures 1 and 2., many of the desirable properties and characteristics of the implantable prosthesis 10 will be present if some, but not all, of the filaments 12 consist of the Co-Cr ~ Mo alloy. Figure 3 is a cross-sectional view of one embodiment of the filaments 12 of the Co-Cr-Mo alloy. As shown, the filaments 12 are substantially homogeneous in cross section. Commercially available alloys may have minor fluctuations in concentration of components while remaining substantially homogeneous. The composition of the filaments 12 can also be homogeneous in the longitudinal direction. Figure 4 is a cross-sectional illustration of a mixed filament 22 including a central core 24 and a cover 26 surrounding the core. The filaments 22 can be used to fabricate implantable prostheses such as 12, and are described in greater detail in International Application UO 94/16646. The core 24 or the cover 26 can be formed from the Co-Ci- shaped alloy or described herein. A preferred embodiment of an implantable prosthesis such as 12 is formed from mixed filaments 24 having frames 26 of the Co-Cr-Mo set alloy. The filaments 12 can be formed from a wide variety of Co-Cr-Mo alloys containing less than about 5% by weight of nickel, preferably containing less than about 2% nickel, and preferably containing no more than about 1% of nickel. The alloys may include nitrogen (N) in an amount of between 0.00% by weight and approximately 0.25% by weight, and carbon (O in an amount of between about 0.00% by weight and about 0.35% by weight). the alloy can vary up to a maximum of about 31.0% by weight, and is preferably contained in an amount of between about 26.0% by weight and 30.0% by weight.The amount of Mo in the alloy can vary up to a maximum of about 8.0 % by weight, and is preferably contained in a blend of between about 5.0 wt% and about 7.0 wt% Other elements that may be contained in the Co-Cr-Mo alloy, preferably are in amounts not greater than about 1.0. % by weight, and are iron (Fe), silicon (Si), manganese (Mn), copper (Cu), phosphorus (P), sulfur (S), and tungsten (U) .The rest of the composition of the alloy can be Co, which is preferably contained in a antity of at least 60.0% by weight. Any known or otherwise conventional cold-working method can be used to form the elements 12 and 22. Non-limiting examples include drawing, rolling, extruding, forging, setting, and the like. The Co-Cr-Mo alloy can be introduced into the cold-working process in the form of ingots, rods, bars, small ingots, discs? other appropriate configurations. The sample filaments 12 were cold drawn from BioDur Carpenter CCMR alloy, which is available commercially from Carpenter Technology Corporation of Reading, Pennsylvania. The published composition for this alloy is Co, 26 Cr, 6 Mo, 1 Si, 1 Fe, 1 Mn, 1 Ni, 0.5 U, 0.5 C ?, 0.18 N, 0.05 C, 0.015 P, 0.015 S. The filaments 12 to The wire alloys of this alloy have a nominal diameter of approximately 0.1 millimeter which was cold stretched to an end of approximately 50% - 80% reduction in its area by Fort Uayne Metals Research Products Corporation of Fort Uayne, Indiana. The ultimate tensile strength of this was measured as drawn wire and was found to be approximately 2889 MPa. The measured deformation resistance of the stretched wire samples was 2489 MPa. The measured elongation of stretched wire samples was 2.4%. The measured modulus of elasticity of the samples was 168.238 MPa. The mean modulus of flexure determined in the stretched sample was 157.896 MPa. The average cutting modulus determined in the stretched samples was 85, 884 MPa. A number of CCMR alloy samples were also heat treated in argon. A heat-treated wire sample was tested for approximately 13 minutes at 500 ° C and found to have a final tensile strength of about 3185 MPa, a resistance to deformation of about 3068 MPa, an elongation of about 2%, and a modulus of elasticity of approximately 193,750 MPa. A heat-treated sample was tested for approximately 13 minutes at 600 ° C and found to have a final tensile strength of about 3172 MPa, a deformation strength of about 2992 MPa, an elongation of about 2%, a modulus of elasticity of approximately 204,092 MPa, an average flexural modulus of approximately 170,609 MPa and an average shear modulus of approximately 96,627 MPa. Another sample of heat treated wire was tested for approximately 13 minutes at 700 ° C and found to have a final tensile strength of about 2965 MPa, a resistance to deformation of about 2710 MPa, an elongation of about 2% and a modulus of elasticity of approximately 207.540 MPa. The deformation resistance and modulus of elasticity of the sample as drawn wire of CCM alloy are generally similar to those of heat treated wire of Elgiloy alloy of a similar diameter. The implantable prostheses made with CCMR alloy wire can therefore have spring, radial pressure and wire strength and tension (ie, properties) properties similar to those of similarly sized plantable prostheses i manufactured from ElgiloyR alloy wire . With this, equivalent physical characteristics of plantable prosthesis can be obtained from an implantable prosthesis having a relatively low nickel content. In addition, relatively high levels of strength are achieved only by cold-working the alloy. Implantable prostheses made of CCMR alloy wire do not therefore need to be heat treated to achieve the required strength levels for certain applications. Another desirable feature of the CCMR alloy wire is that it has a high surface hardness and a smooth surface finish. In the stretched state, the measured hardness values of the CCMR alloy wire samples were between about 46.2 and about 48.7 in the Rockwell C scale and averaged about 47.3 on the Rockwell C scale. The heat treated samples of the CCMR alloy wire had measured hardness values between about 55.2 and 57.8 on the Rockwell C scale, and averaged approximately 56.6 on the Rockwell C scale. These hardness values are relatively high compared to stainless steel (approximately 34 to 40 on the Rockwell C scale when stretched) and ElgiloyR alloy (approximately 42.2-44 on the Rockwell C scale when stretched, and approximately 53.7-55.4 on the Rockwell C scale, when it is aged). These relatively high surface hardness characteristics are advantageous in self-expanding implantable prostheses since they improve the resistance to use of the filaments 12 and reduce friction at the points 14 in which the filament intersects with others in the implant-able prosthesis 10. 5 is an illustration of a prosthesis graft 30 including filaments or structural chains 32 of Co-Cr-Mo alloy, of the type incorporated in the implantable prosthesis 10 and described above (for example filaments 12). As shown, the structural chains 32 of Co-Cr-Mo alloy are braided with layers of more closely woven textile chains 42 that reduce permeability. The structural chains 32 are selectively configured prior to their braiding with the textile chains 42, either by a permanent thermal deformation or by selective plastic deformation, and in any case they are configured without adversely affecting the textile chains. The plastic deformation of structural chains 32 by cold working is advantageous, since it allows a continuous cold-working process followed by braiding. The result is a braided prosthesis that incorporates the strength, elasticity and scale of radii associated with self-expandable implantable prostheses without the need for heat treatment for aging hardening and the impermeability associated with vascular grafts. Figure 6 schematically illustrates the manner in which multiple structural chains 32 and multiple textile chains 42 are braided together to form several discrete layers of prosthesis 30. These include an inner (radially inward) layer 44 consisting primarily of textile chains. 42, an outer layer 46 which also consists mainly of the textile chains, and a middle layer 48 that incorporates the structural chains 32. The layers 44-48 are formed simultaneously in a single braiding operation that also interlaces the layers, in the that at least one of the chains of each of the layers is braided in one of the other layers. In a preferred approach, the inner layer 44 and the outer layer 46 are formed substantially completely from textile chains 42, while the middle layer 48 is a braided combination of textile chains 42 and structural chains 32, for example in a one-to-one relationship. one, or two to one relationship in favor of textile chains. The inner layer 44 includes a first group of its textile chains that extend into the middle layer, and a second group of its textile chains that extend through the middle layer into the outer layer, and then back into the inner layer. . These groups together can comprise a relatively small percentage of the textile chains of the layer 44. The middle layer 48 and the outer layer 46 similarly have groups of textile chains extending to the other layers. In this way, there is a substantial intermixing between chains of the different layers for effective interleaving, although the layers remain different from each other in character. The textile chains 42 are preferably multi-filament yarns, although they may be monofilaments. In any case, the textile chains are much thinner than the structural chains 32, ranging from about 10 to 400 denier. The individual filaments of the multifilament yarns can vary from about 0.25 to about 10 denier. Multi-filament yarns generally have a high degree of deformation, which may or may not include elasticity. Suitable materials include PET, polypropylene, polyurethane, urethane polycarbonate, HDPE, polyethylene, silicone, PTFE, ePTFE and polyolefin. A suitable high molecular weight polyethylene is sold under the brand name "Spectra". The fine textile chains are closely woven into layers 44, 46 and 48, and can be considered to form a textile liner or fabric in each layer. Due to the fineness of the textile chains 42 and a narrow or tight fabric, the textile linings can be multi-porous, still substantially impervious to body fluids. Also, the textile lining layers are highly deformable, adjusting to changes in the lattice shape formed by the structural chains 32 to the extent that the prosthesis 30 radially self-expands or radially compresses. The shape of the reticle determines in this way the shape of the prosthesis 30. A particularly favorable structure for the prosthesis 30 has a middle layer 48 formed by braided metal structural chains 32 with multi-strand strands of dacron (polyester) as the textile chains. 42. Metal structural chains exhibit high strength in terms of elastic modulus. By contrast, for example, polyethylene has an elastic modulus in the range of approximately 1.4-3.85 x 10a kg / cmz, and other polymeric materials have elastic moduli in this order of magnitude. Therefore, for a given diameter of chain, helical diameter and helical tilt, a lattice of metal chains is considerably more resistant to radial compression, and provides a greater residual force for sharp attachment. Dacron polyester filament yarn has a high elastic recovery and elongation (up to 36% for polyester fiber) and a low elastic pattern, which ensures that the textile liner 40 fits the reticle. To achieve favorable characteristics of implantable prostheses and implantable prosthetic grafts, the prostheses 30 can be manufactured in accordance with several steps as illustrated in Figures 7-9. Figure 7 shows two structural chains (metal monofilaments) 32a and 32b, one of each group of oppositely directed structural chains, wound around a mandrel 60 and supported by respective coils 62 and 64. Although only the chains 32a and 32b are illustrated for convenience, it is appreciated that all structural chains are wound around the mandrel and held together for formation. However, only structural chains are present, as formation occurs before braiding with textile chains. Hardening by aging is carried out inside a furnace 66 in a vacuum or protective atmosphere. The temperatures are within the range of approximately 350-1000 ° C, with the specific temperature depending on the structural material. The filaments extend one over the other to form multiple intersections, one of which is indicated at 68. The coils, including 62 and 64, are placed to apply tension to their respective chains during aging hardening. The proper duration of aging hardening varies with materials and dimensions, but can be as short as 30 seconds, up to about 5 hours. After hardening by aging, the structural chains are allowed to cool, whereby each structural chain retains the helical shape as its nominal form. In the context of elastic materials, "nominal form" refers to the form in a relaxed state, that is when there is no external effort. Metal monofilaments hardened by aging are highly elastic, that is, deformable under external stress, but return elastically to the nominal form when they are free from external stress. Braiding occurs of the structural chains 32 and the textile chains 42 after selective formation. Figure 8 illustrates schematically a braiding apparatus 70 including a cylindrical carrier assembly 72 including several annular arrangements of coils, two of the coils are indicated at 80a and SOb. The apparatus further includes a mandrel 78, centered within the cylindrical assembly and movable longitudinally relative to the assembly, as indicated by the arrow. Figure 9 illustrates part of the carrier assembly 72 in greater detail, to reveal five annular arrangements or groups of carrier coils indicated at 80, 82, 84, 86 and 88. These groups are separated coaxially and axially, each includes forty-eight coils , twenty-four coils for respective dextrorotatory and levorotatory windings around the mandrel 78. Although those skilled in the art are familiar with the use of braiding machinery, it is emphasized herein that the braiding apparatus 70 is configured as or described in the patent publication. International previously mentioned No. UO91 / 10766. Suitable braiding machinery is available from Albany International Research Company of Mansfield Massachusette. Figure 10 illustrates schematically an alternative three dimensional braiding apparatus 92 in which the structural chains are selectively formed by cold treatment. In particular, a cylindrical carrier assembly 94 is mounted concentrically on a longitudinally movable mandrel 96. As before, the carrier assembly supports multiple coils in arrangements that include several groups of concentric circular coils, two of the coils are indicated at 98 and 100. A structural chain 32 has been wound on the bobbin 98, while the bobbin 100 carries a textile chain 42. The structural chain is not thermally formed before braiding, and thus, at the beginning, it has a linear nominal shape. The structural chain 32 is plastically deformed by cold treatment when it is moved from the coil 98 to the mandrel. Along the path traveled by the chain 32, there is arranged a small diagonal forming pulley 102 and a larger diameter idle pulley 104. Although the pulleys 102 and 104 are shown in lateral elevation in FIGS. 10, it should be understood that in the actual braiding device, the pulley 102 is orthogonal to the pulley 104 to effect the selected formation of the chain 32. The forming pulley 102 it exerts a bending stress on the movable structural chain transported around this pulley, particularly on radially outer portions of the chain. The coil 98 is supported on a carrier including a clutch (not shown) adjustable to adjust the tension applied to the chain, thereby adjusting the amount of bending stress. The tension is controlled so that the bending stress, at least along the radially outer portions of the chain along the pulley 102, exceeds the deformation resistance of the material. The appropriate level of tension is on the scale of approximately 200-1000 gms, depending on factors such as the material, the diameter of the monofilament and the radius of flexure around the pulley 102. The result is a plastic deformation of cold working. The plastic movement is continuous, and changes the nominal shape of the structural chain, from linear to helecoidal. Furthermore, in this regard, it is observed that the pulley 102 imparts a nominal curved shape to the structural chain in any case, and that the nominal helecoidal shape with the desired inclination is obtained by suitable orientation of the pulley with respect to the carrier assembly, maintaining at the same time the desired tension in the chain. No heat treatment is necessary for aging hardening after braiding when using structural metal filaments with sufficiently high deformation resistance and modulus, such as the filament of the Co-Cr-Mo alloy described herein. Although the present invention has been described in reference to preferred embodiments, those skilled in the art will recognize that changes in form and detail can be made without departing from the spirit and scope of the invention. In particular, expandable balloon implantable prostheses and other implantable prostheses made in accordance with the present invention will also offer important advantages with the elements formed of Co-Cr-Mo alloy containing compounds of about five percent by weight of nickel.

Claims

NOVELTY OF THE INVENTION CLAIMS

1. An implantable medical device comprised of a tubular and radially expandable structure including at least one elongate element formed of cobalt, chromium and molybdenum alloy (Co-Cr-Mo) containing less than about 5 weight percent nickel.

The medical device according to claim 1, further characterized in that the device is comprised of a lattice structure that includes at least one elongate element formed of Co-Cr-Mo alloy.

3. The medical device according to claims 1 and 2, further characterized in that the device is comprised of a radially self-expanding structure that includes at least one elongate element formed of Co-Cr-Mo alloy.

The medical device according to claims 1-3, further characterized in that the device comprises an axially flexible structure including a plurality of the elongate elements formed of Co-Cr-Mo alloy which are woven in a configuration similar to braid.

5. The medical device according to claims 1-4, further characterized in that the Co-Cr-Mo alloy contains less than about 2 weight percent in nickel.

6. The medical device according to claims 1-4, further characterized in that the Co-Cr-Mo alloy contains a maximum of about 1 percent by weight of nickel.

7. The medical device according to claims 1-6, further characterized in that the Co-Cr-Mo alloy contains between about 0 and about 0.25 percent by weight of nitrogen (N), and between about 0 and about 0.35 percent by weight. weight of carbon (C).

8. The medical device according to claims 1-6, further characterized in that the Co-Cr-Mo alloy contains between about 0.15 and about 0.20 weight percent nitrogen (N) and between about 0.01 and about 0.10 weight percent. of carbon.

9. The medical device according to claims 1-8, further characterized in that the structure consists substantially of at least one filament of Co-Cr-Mo alloy.

10. The medical device according to claims 1-9, further characterized in that each elongated element of Co-C.r-Mo is formed of a filament of diameter at least 50% reduced.

11. The medical device according to claims 1-10, further characterized in that each Co-Cr-Mo element has a stretching hardness of at least 45.5 on the Rockwell C scale.

12. The medical device according to claims 1-11, further characterized in that each Co-Cr-Mo filament is free from heat treatment after stretching.

The medical device according to claims 1-6 and 9-12, further characterized in that the Co-Cr-Mo alloy contains at least about 50 percent by weight of cobalt, between about 25 to 31 percent by weight of chrome, between about 4 to 8 weight percent of rnolibdene, between about 0.15 to 0.20 weight percent nitrogen and between about 0.01 to 0.10 weight percent carbon.

14. The medical device of claims 1-13, further characterized in that the device includes a membrane of coextensive porous material with at least a portion of the length of the expandable structure.

15. The medical device according to claim 14, further characterized in that the membrane is formed of polymeric material. RFfUMEN OF THE INVENTION A self-expanding implantable prosthesis (10) formed of helically wound and braided filaments (14) of cobalt, chromium and molybdenum-set alloy containing less than about five percent by weight of nickel; The composition of the alloy from which a modality of the implantable prosthesis is formed is Co-26Cr-6Mo-lSi-lFe-lMn-lNi. EA / avc- * mmrn * fac- * apm P97 / 324