Bioabsorbable self-expanding stent

Details Bioabsorbable self-expanding stent Bioabsorbable self-expanding stent

1997-08-01

2001-06-12

Willse, David H. (Department: 3738)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Arterial prosthesis

Stent structure

C623S001380, C606S200000

Reexamination Certificate

active

06245103

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to implantable, radially expandable medical prostheses which are frequently referred to as stents. In particular, the present invention is a bioabsorbable self-expanding stent.
Self-expanding medical prostheses frequently referred to as stents are well known and commercially available. They are, for example, disclosed generally in the Wallsten U.S. Pat. No. 4,655,771, the Walisten et al. U.S. Pat. No. 5,061,275 and in Hachtmann et al., U.S. Pat. No. 5,645,559. Devices are used within body vessels of humans for a variety of medical applications. Examples include intravascular stents for treating stenoses, stents for maintaining openings in the urinary, biliary, tracheobronchial, esophageal, and renal tracts, and vena cava filters.
A delivery device which retains the stent in its compressed state is used to deliver the stent to a treatment site through vessels in the body. The flexible nature and reduced radius of the compressed stent enables it to be delivered through relatively small and curved vessels. In percutaneous transluminal angioplasty, an implantable endoprosthesis is introduced through a small percutaneous puncture site, airway, or port and is passed through various body vessels to the treatment site. After the stent is positioned at the treatment site, the delivery device is actuated to release the stent, thereby allowing the stent to self-expand within the body vessel. The delivery device is then detached from the stent and removed from the patient. The stent remains in the vessel at the treatment site as an implant
Stents must exhibit a relatively high degree of biocompatibility since they are implanted in the body. An endoprosthesis may be delivered into a body lumen on or within a surgical delivery system such as delivery devices shown in U.S. Pat. Nos. 4,954,126 and 5,026,377. Preferred delivery devices for the present invention include U.S. Pat. Nos. 4,954,126; 5,026,377. Suitable materials for use with such delivery devices are described in U.S. patent application Ser. No. 08/833,639, filed Apr. 8, 1997.
Commonly used materials for known stent filaments include Elgiloy® and Phynox® metal spring alloys. Other metallic materials than can be used for self-expanding stent filaments are 316 stainless steel, MP35N alloy, and superelastic Nitinol nickel-titanium. Another self-expanding stent, available from Schneider (USA) Inc. of Minneapolis, Minn., has a radiopaque clad composite structure such as shown in U.S. Pat. No. 5,630,840 to Mayer. Self-expanding stents can be made of a Titanium Alloy as described in United States patent application Ser. No. 08/598,751, filed Feb. 8, 1996.
The strength and modulus of elasticity of the filaments forming the stents are also important characteristics. Elgiloy®, Phynox®, MP35N and stainless steel are all high strength and high modulus metals. Nitinol has relatively low strength and modulus.
The implantation of an intraluminal stent will preferably cause a generally reduced amount of acute and chronic trauma to the luminal wall while performing its function. A stent that applies a gentle radial force against the wall and that is compliant and flexible with lumen movements is preferred for use in diseased, weakened, or brittle lumens. The stent will preferably be capable of withstanding radially occlusive pressure from tumors, plaque, and luminal recoil and remodeling.
There remains a continuing need for self-expanding stents with particular characteristics for use in various medical indications. Stents are needed for implantation in an ever growing list of vessels in the body. Different physiological environments are encountered and it is recognized that there is no universally acceptable set of stent characteristics.
A need exists for a stent which has self expanding characteristics, but which is bioabsorbable. A surgical implant such as a stent endoprosthesis must be made of a non-toxic, biocompatible material in order to minimize the foreign-body response of the host tissue. The implant must also have sufficient structural strength, biostability, size, and durability to withstand the conditions and confinement in a body lumen.
All documents cited herein, including the foregoing, are incorporated herein by reference in their entireties for all purposes.
SUMMARY OF THE INVENTION
The present invention is an improved implantable medical device comprised of a tubular, radially compressible, axially flexible and radially self-expandable structure including elongate filaments formed in a braid-like configuration. The filaments consist of a bioabsorbable polymer which exhibits a relatively high degree of biocompatibility.
Briefly, self-expanding stents of the present invention are formed from a number of resilient filaments which are helically wound and interwoven in a braided configuration. The stents assume a substantially tubular form in their unloaded or expanded state when they are not subjected to external forces. When subjected to inwardly directed radial forces the stents are forced into a reduced-radius and extended-length loaded or compressed state. The stents are generally characterized by a longitudinal shortening upon radial expansion.
In one preferred embodiment, the device is a stent which substantially consists of a plurality of elongate polylactide bioabsorbable polymer filaments, helically wound and interwoven in a braided configuration to form a tube. Bioabsorbable implantable endoprostheses such as stents, stent-grafts, grafts, filters, occlusive devices, and valves may be made of poly(alpha-hydroxy acid) such as poly-L-lactide (PLLA), poly-D-lactide (PDLA), polyglycolide (PGA), polydioxanone, polycaprolactone, polygluconate, polylactic acid-polyethylene oxide copolymers, modified cellulose, collagen, poly(hydroxybutyrate), polyanhydride, polyphosphoester, poly(amino acids), or related copolymers materials, each of which have a characteristic degradation rate in the body. For example, PGA and polydioxanone are relatively fast-bioabsorbing materials (weeks to months) and PLA and polycaprolactone are a relatively slow-bioabsorbing material (months to years).
A stent constructed of a bioabsorbable polymer provides certain advantages relative to metal stents such as natural decomposition into non-toxic chemical species over a period of time. Also, bioabsorbable polymeric stents may be manufactured at relatively low manufacturing costs since vacuum heat treatment and chemical cleaning commonly used in metal stent manufacturing are not required.
The present invention includes a method of designing and manufacturing an improved braided bioabsorbable stent which is different from practices used to make braided metal wire stents. The method involves selecting a specific bioabsorbable polymer based on a desired stent functional absorption time and stent radial force. The stent functional absorption time is the time period within which the stent retains at least 80% of its original radial strength. The stent is made by first selecting a braid design from the invention and making two different annealed stents. Radial force and dimensional test results from the two stents are used to develop a nearly linear mathematical equation to determine the parameters to meet the design goals. This method advantageously limits costly and time consuming trial and error to arrive at the optimum design.
Bioabsorbable polymer stents are radiolucent and the mechanical properties of the polymers are generally lower than structural metal alloys. Bioabsorbable stents may require radiopaque markers and may have a larger profile on a delivery catheter and in a body lumen to compensate for the lower material properties.
Bioabsorbable PLLA and PGA material are degraded in vivo through hydrolytic chain scission to lactic acid and glycolic acid, respectively, which in turn is converted to CO
2
and then eliminated from the body by respiration. Heterogeneous degradation of semicrystalline polymers occurs due to the fact that such materials have amorphous and crystalline regions. Degra

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