Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
2001-07-06
2004-03-02
McDermott, Corrine (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
C623S001340
Reexamination Certificate
active
06699278
ABSTRACT:
BACKGROUND OF THE INVENTION
Stents are well known medical devices that are used for maintaining the patency of a large variety of vessels of the human body. A more frequent use is for implantation into the coronary vasculature. Although stents have been used for this purpose for more than ten years, and some current stent designs such as the CORDIS BX Velocity® stent, Cordis Corporation, Miami Lakes, Fla., have the required flexibility and radial rigidity to provide an excellent clinical result, they are not always clearly seen under standard fluoroscopy.
Many current tubular stents use a multiplicity of circumferential sets of strut members connected by either straight longitudinal connecting links or undulating longitudinal connecting links. The circumferential sets of strut members are typically formed from a series of diagonal sections connected to curved sections forming a closed-ring, zig-zag structure. This structure opens up as the stent expands to form the element in the stent that provides structural support for the arterial wall. A single strut member can be thought of as a diagonal section connected to a curved section within one of the circumferential sets of strut members. In current stent designs such as the BX Velocity® stent, these sets of strut members are formed from a single piece of metal having a uniform wall thickness and generally uniform strut width. Although a stent with uniform width of the strut members will function, if the width is increased to add strength or radiopacity, the sets of strut members will experience increased strain upon expansion. High strain can cause cracking of the metal and potential fatigue failure of the stent under the cyclic stress of a beating heart.
Existing highly radiopaque stents, such as the gold plated NIROYAL stent sold by Boston Scientific, Inc., Natick Mass., can obscure the inside of the vessel due to the high radiopacity over the entire length of the stent. The BeStent sold by Medtronic, Inc., Minneapolis Minn., has small gold markers at the ends of the stent. Those markers only mark an end point without allowing visualization of the entire end set of strut members.
Fischell et al, in U.S. Pat. No. 6,086,604, discloses a stent with the end sets of strut members being gold plated. Such a stent would have ideal radiopacity but may be subject to the corrosive effects incurred through placement of dissimilar metals in an electrolytic solution such as blood. There has also been significant evidence that gold is a poor surface material for stents because it may increase the risk of subacute thrombosis or restenosis. Further, Fischell et al, in U.S. Pat. No. 5,697,971 discloses in its
FIG. 7
, a stainless steel stent with increased width diagonal sections in all the circumferential sets of strut members.
SUMMARY OF THE INVENTION
An ideally radiopaque stent would have end sets of strut members that are highly radiopaque so that they can be readily seen, even using low power fluoroscopy, and would further contain a central section that is visible but not too bright so as to obscure the lumen when high power cine film angiograms are taken. The stent should also have only one material on its outside surface to avoid potential corrosion; that material should not promote subacute thrombosis or restenosis.
The present invention is a stent that is designed to have optimal strength and radiopacity with good biocompatibility. Unfortunately, the choices of appropriate biocompatible metals available as thin wall tubing for stent construction are somewhat limited. To achieve optimal radiopacity, the stent design of the present invention is adjusted to the specific radiopacity and strength characteristics of the metal from which the stent is fabricated. What is more, coatings such as parylene may be needed to avoid corrosion from stents with less biocompatible materials and/or dissimilar metals on the stent's outer surface. Of extreme importance to the present invention is the achievement of optimal radiopacity in a stent that ideally is only 0.004 inches wall thickness or less. Such a stent would have a pre-deployment outer diameter (profile) that would be at least 0.003 inches less than currently marketed stents. Ideally, the stent described herein would have a wall thickness between 0.0025 inches and 0.004 inches.
Described herein are the novel design elements for stents formed from the following materials:
1. A highly radiopaque metal such as tantalum;
2. Metals somewhat more radiopaque than stainless steel, such as the cobalt based alloy L605;
3. Stents coated or plated with highly radiopaque materials like gold; and
4. Layered materials such as alternative layers of tantalum and stainless steel.
5. The novel design elements that are described herein include:
1. Tapered Strut Width for Stents Formed from Highly Radiopaque Metals.
Although reducing the width of the longitudinally diagonal section alone will reduce radiopacity without significantly affecting radial strength, by having a taper on the curved sections of the circumferential sets of strut members, a greatly reduced level of strain upon stent expansion can be achieved without sacrificing radial strength. This is extremely important, as it allows a stent to be made much stronger than a stent with uniform width of the strut members while staying within the same strain limit for the material.
Tantalum is a metal that has been used in stents; which metal is highly radiopaque. The optimal radiopacity for a stent design using tantalum could have uniform width for the circumferential sets of strut members and a wall thickness of about 0.0025 inches. To provide more radial strength and to reduce the probability of the stent ends flaring out during deployment, a wall thickness of about 0.003 inches to 0.035 inches would be highly desirable. With uniform width sets of strut members, a 0.035 inches wall thickness tantalum stent would be too bright under cine angiography. To reduce the radiopacity of the design without significantly impacting the radial strength of the deployed stent, the present invention envisions curved sections and diagonal sections, either or both of which could have a variable or tapered width. The curved sections should be tapered (wider at the center compared to the ends) to reduce strain as previously described. The longitudinally diagonal sections can be thinner in the center than at the ends, to reduce radiopacity for the central sets of strut members.
It is envisioned that the novel stent described herein might have wider diagonal sections for the end sets of strut members as compared to the central sets of strut members. This feature would enhance the radiopacity of the end sets of strut members while retaining a moderate level of radiopacity for the central sets of strut members. It is also envisioned to have both reduced width diagonals and/or reduced wall thickness for the central sets of strut members. It should be remembered that it is fluoroscopic visualization of the end sets of strut members that is most important for visualizing stents placed inside a coronary artery.
2. Thicker Diagonal Sections for Metals with Radiopacity Slightly Better than Stainless Steel.
The cobalt/tungsten alloy L605 is a stronger and more radiopaque metal compared to stainless steel. To achieve optimal radiopacity using L605 with uniform width sets of strut members, the wall thickness is optimally equal to or greater than 0.0045 inches. To provide optimal radiopacity with such a metal in stents of wall thickness 0.004 inches or less, the present invention envisions wider diagonal sections in the sets of strut members. Thus, the tapered diagonal sections would be wider than the curved sections. The tapered curved section design for reduced strain may also be highly desirable for stents made from the L605 alloy.
3. End Sets of Strut Members with Thinner Curved Sections.
Stent deliverability into curved coronary arteries is improved when the diagonal sections of the end sets of strut members have a decreased length as compared to the length of the diagonal secti
Burpee Janet
Fischell David R.
Fischell Robert E.
Cordis Corporation
Landrem Kamrin
McDermott Corrine
Rosenberg , Klein & Lee
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