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  Bioabsorbable marker having radiopaque constituents and method of using the sameRelated Patent Categories: Prosthesis (i.e., Artificial Body Members), Parts Thereof, Or Aids And Accessories Therefor, Arterial Prosthesis (i.e., Blood Vessel), Having Marker (e.g., Color, Radiopaque, Etc.)The Patent Description & Claims data below is from USPTO Patent Application 20060004440. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] This invention relates generally to a bioabsorbable marker having radiopaque constituents "bioabsorbable-radiopaque marker" for use on an implantable endoprosthesis such as a stent. The bioabsorbable marker includes dispersable radiopaque constituents which are not bioabsorbable or degradable, but are excreted from the body or stored in the body. [0002] Implantable endoprostheses including stents, stent-grafts, and grafts are used in percutaneous transluminal coronary angioplasty and in other medical procedures to repair and support diseased or damaged arteries and body lumens. Grafts are implanted to cover or bridge leaks or dissections in vessels. Stent-grafts are stents which generally have a porous coating attachment. Unsupported grafts are porous tubes which are typically implanted by surgical cut-down. [0003] In order to visualize the passage and placement of the implantable endoprosthesis in arteries and body lumens, many surgical procedures are performed with the aid of fluoroscopic angiography. The surgical delivery device and implantable endoprosthesis may be visualized if they are radiopaque and offer radiographic contrast relative to the body. For example, X-ray radiation may be used to visualize surgical delivery devices and deployment of the implant in the body. Also, radiographic contrast solution may be injected into the body lumen so that the lumen may be seen in the fluoroscopic image. [0004] In order for the Implantable endoprosthesis to be radiopaque, it must be made from a material possessing radiographic density higher than surrounding host tissue and have sufficient thickness to affect the transmission of x-rays to produce contrast in the image. Reference is made to the clad composite stent shown in U.S. Pat. No. 5,630,840. An implantable endoprosthesis may be made of metals including tantalum, or platinum having relatively high radiographic densities. Other metals such as stainless steel, superalloys, nitinol, and titanium having lower radiographic densities may also be used. Reference is made to implantable devices shown in U.S. Pat. Nos. 4,655,771; 4,954,126; and 5,061,275. [0005] An implantable polymeric endoprosthesis is generally radiolucent and does not possess sufficient radiographic density to be easily imaged by fluoroscopy. To improve the imaging of such polymeric materials, polymers may be mixed with radiopaque filler materials prior to molding or extruding in order to enhance the radiographic density. However, a disadvantage of using fillers with polymers is that changes in the properties of the polymer may occur. For example, the additions of fillers may reduce the strength or ductility of the polymer. [0006] There is a need for an improved bioabsorbable-radiopaque marker for use in medical devices, particularly in temporary medical devices having low radiopacity. The need to improve the radiopacity of a relatively low radiopaque implantable endoprosthesis or to improve imaging in low radiopaque conditions is particularly important for surgery, micro-surgery, neuro-surgery, and conventional angioplasty procedures performed under fluoroscopy. Physicians are constantly being challenged to place small implants at specific intraluminal locations. Various devices having radiopacity are known in the art such as shown in U.S. Pat. Nos. 4,447,239; 5,354,257; and 5,423,849. [0007] All documents cited herein, including the foregoing, are incorporated herein by reference in their entireties for all purposes. SUMMARY OF THE INVENTION [0008] Accordingly, there is a need for bioabsorbable-radiopaque markers for use on implantable endoprostheses in order to improve radiopacity and the locatability of an endoprosthesis during various medical procedures. Providing temporary radiopacity is especially advantageous for implantable endoprostheses having little or no radiopacity. The bioabsorbable-radiopaque markers allow radiographic identification of one or more locations of interest on an implantable endoprosthesis. Bioabsorbable-radiopaque markers in the fabric or covering materials of an implantable endoprosthesis are advantageous for indicating the location of the fabric or covering during implantation. [0009] Alternative uses include threading the markers: adjacent a helical strand in the implantable endoprosthesis; circumferentially around the implantable endoprosthesis; or in a straight line in the axial direction of the implantable endoprosthesis. One or more bioabsorbable-radiopaque markers may be used on the implantable endoprosthesis having little or no radiopacity. After implantation, the bioabsorbable-radiopaque marker may be absorbed, dissolved, or excreted from the body so as not to effect the function of the endoprosthesis. [0010] A disadvantage of certain permanent radiopaque markers is that they may compromise structural integrity, may not be biocompatible or biostable, and may be more thrombogenic than the implantable endoprosthesis. [0011] The bioabsorbable-radiopaque marker of the present invention advantageously allows most any implantable endoprosthesis to have temporary radiopacity over a predetermined portion of its structure, and advantageously assists with proper positioning and locatability of the implantable endoprosthesis in a body lumen. [0012] Use of the bioabsorbable-radiopaque marker is advantageous because the radiopaque property may be present only for a desired time period on an implantable endoprosthesis. For instance, once the implantable endoprosthesis is implanted, it may be more desirable to image with techniques such as ultrasound, magnetic resonance, and endoscopy and to avoid further radiation exposure to the patient. As the bioabsorbable polymer degrades, radiopaque material simultaneously or subsequently disperses into the body. The dispersion of the radiopaque material from the marker results in a loss of radiopacity in the marker. A predetermined rate of release of the radiopaque material may be designed into the bioabsorbable marker based on degradation of the polymer in the body or the design of the marker structure. [0013] The bioabsorbable material in the bioabsorbable-radiopaque markers may include polymers or copolymers such as polylactide [poly-L-lactide (PLLA), poly-D-lactide (PDLA)], polyglycolide, polydioxanone, polycaprolactone, polygluconate, polylactic acid-polyethylene oxide copolymers, modified cellulose, collagen, poly(hydroxybutyrate), polyanhydride, polyphosphoester, poly(amino acids), poly(alpha-hydroxy acid) or related copolymers materials, each of which have a characteristic degradation rate in the body. For example, polyglycolide and polydioaxanone are relatively fast-bioabsorbing materials (weeks to months) and PLA is a relatively slow-bioabsorbing material (months to years). For a PLA member, mass degradation is completed with total absorption of the polymer endoprosthesis in about 1.5 to 3 years after implantation. [0014] Bioabsorbable resins such as PLLA, PDLA, PGA and others are commerciallv available from several sources including PURAC America, Inc. of Lincolnshire, Ill. Radiopaque materials such as barium sulfate and bismuth trioxide are commerciallv available and compounded with the bioabsorbable resin by New England Urethane, Inc. of North Haven, Conn. The bioabsorbable resin or bioabsorbable-radiopaque resin may be extruded into filament by Albany International Research Co. of Mansfield, Mass. [0015] The bioabsorption rate of the marker may be designed to be fast for applications where acute radiopacity is desired such as during positioning and placement of the implant. Alternatively, the bioabsorption rate may be designed to be slower for applications where the implant must be radiographically imaged for at least a portion of its functional time, for example, in implants where healing may take months. Other bioabsorption rates are also possible. The bioabsorption rate of the marker may be tailored by controlling the type of bioabsorbable polymer; chemical composition of the bioabsorbable polymer; molecular weight of the bioabsorbable polymer; thickness and density of the bioabsorbable polymer; surface area of the marker, exit area for the radiopaque material, and design of the marker structure. [0016] The degradation products from the bioabsorbable marker and the dispersed radiopaque material are metabolized, excreted, or stored by the body. Metabolism is the chemical process in living cells by which energy is provided for vital processes and activities and new material is assimilated to repair the waste. It is the sum of the processes by which a particular substance is handled in the living body. Excretion is separation and elimination or discharge from the blood or tissues of useless, superfluous, or harmful material that is eliminated from the body. [0017] The biocompatibility of absorbable polymers during degradation depends upon the rate of accumulation and how well the surrounding tissue or fluid buffers or metabolizes the degradation products. If the products are metabolizable, the rate at which this will occur is dependent upon the blood circulation in the tissue. A well-vascularized lumen wall could buffer and metabolize degradation products as they are released from the implant. This biological process is important to minimize adverse tissue reaction to the degrading implant. [0018] The degradation products from PLLA and PGA are lactic and glycolic acid, respectively, which are normally present in the human body. The acids are metabolized by cells around the implant. The metabolization process is a citrate cycle which converts the acids to carbon dioxide which is respirated out of the body. [0019] The radiopaque agents added to the bioabsorbable marker are generally insoluble in the body and thus are not metabolizable. If these materials are trapped within tissue, the host generally reacts by encapsulation and acceptance of the biologically inactive particles. If the material is released from the implant into systemic circulation, it will migrate with fluid flow until being excreted or collected and stored by organs or tissue. The idea is to only have small amounts of the radiopaque substances in the implant by incorporating the discrete bioabsorbable-radiopaque marker rather than to load the entire implant with the radiopaque material. Minimization of the amount of radiopaque material which will be liberated from the marker upon absorption of the polymer must be considered when determining the loading percentage based on radiographic and mechanical properties. [0020] To be radiopaque, the markers should include material having atomic elements of sufficiently high atomic number and be of sufficient thickness to provide sufficient radiopacity for imaging. The bioabsorbable-radiopaque marker may have one or more hollow, cavity, or porous portions wherein radiopaque material may be disposed. [0021] Attenuation is the change in the number of photons in the incident x-ray beam due to the interaction with an absorber. To image an object implanted in the body, it would be desirable to have the object attenuate x-rays more than body tissue, bone, and fat so that the difference in contrast will be obvious in a radiograph. The difficulty in selecting a radiopaque material for surgical implants is that the material must have desirable radiographic characteristics and biocompatibility. [0022] In order to make an implant more radiopaque, a substance which absorbs more x-rays can be deposited on or mixed in with the implant material. If the implant absorbs more x-rays than the surrounding medium (for example tissue in the body), it will be visible as a sharp change in contrast on an x-ray film or fluoroscopy image. Continue reading... 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