Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
2000-10-13
2002-11-26
Prebilic, Paul B. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
Reexamination Certificate
active
06485508
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to intraluminal endovascular stenting, and in particular, to a low profile stent.
BACKGROUND OF THE INVENTION
Endovascular stenting is particularly useful for arteries which are blocked or narrowed and is an alternative to surgical procedures that intend to bypass the occlusion. The procedure involves inserting a prosthesis into a body lumen and expanding it to prevent collapse of a vessel wall. While stenting has most commonly been used adjunctively, following an intervention such as angioplasty or atherectomy, there is increasing interest in primary, or direct stent placement.
Percutaneous transluminal angioplasty (PTCA) is used to open coronary arteries which have been occluded by a build-up of cholesterol fats or atherosclerotic plaque. Typically, a guide catheter is inserted into a major artery in the groin and is passed to the heart, providing a conduit to the ostia of the coronary arteries from outside the body. A balloon catheter and guidewire are advanced through the guiding catheter and steered through the coronary vasculature to the site of therapy. The balloon at the distal end of the catheter is inflated, causing the site of the stenosis to widen. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten re-closure of the dilated vessel or even perforations in the vessel wall. Implantation of a stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel or provide a patch repair for a perforated vessel wall until corrective surgery can be performed. Reducing the possibility of restenosis after angioplasty reduces the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
A stent is typically a cylindrically shaped device formed from wire(s) or a tube and is intended to act as a permanent prosthesis. A stent is deployed in a body lumen from a radially compressed configuration into a radially expanded configuration which allows it to contact and support a body lumen. The stent can be made to be radially self-expanding or expandable by the use of an expansion device. The self-expanding stent is made from a resilient springy material while the expandable stent is made from a material which is plastically deformable. A plastically deformable stent can be implanted during an angioplasty procedure by using a balloon catheter bearing a crimped, or compressed stent which has been loaded onto the balloon. The stent radially expands as the balloon is inflated, forcing the stent into contact with the body lumen, thereby forming a supporting relationship with the vessel wall. Deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of the balloon catheter.
A balloon of appropriate size and pressure is first used to open the lesion. The process can be repeated with a stent loaded onto a balloon. Direct stenting involves simultaneously performing angioplasty and stent implantation using a stent mounted on a dilatation balloon. The stent remains as a permanent scaffold after the balloon is withdrawn. A balloon capable of withstanding relatively high inflation pressures may be preferable for stent deployment because the stent must be forced against the artery's interior wall so that it will fully expand, thereby precluding the ends of the stent from hanging down into the channel, encouraging the formation of thrombus.
In adjunctive stenting, a stent delivery system with a small diameter profile is not required because the narrowing has already been enlarged by the preceding device. However, in direct stenting, the stent and delivery balloon catheter need to be inserted into a stenosis that has not been previously dilated. Thus, for direct stenting to be applicable to many patients, the stent and delivery system must have a very low profile. The primary advantage of direct stenting is the procedural efficiency gained by eliminating a primary angioplasty step. The resulting procedure can be shorter and less expensive.
Primary angioplasty followed by stent placement typically requires a catheter exchange, which is usually performed over a guidewire. Given the prevalence of this staged procedure, the most commonly used balloon catheters have been over-the-wire types, having either a full length guidewire lumen or a short, distal guidewire lumen as found in rapid exchange catheters. Fixed wire, or “balloon-on-a-wire” type balloon catheters have been seldom used for primary angioplasty in stenting procedures, and these catheters have not been used to deliver stents at all. With their small size and wire-like trackability, fixed wire catheters are able to provide relatively quick and simple balloon placement and access to lesions that cannot be reached with other types of catheters. The small size of fixed wire catheters also permits their use through very small guiding catheters. However, these balloon catheters lack the ability to maintain guidewire position across a lesion for exchange purposes and they may encounter problems re-crossing a dilated area. Another reason that fixed wire balloon catheters have not been used for stent delivery is that the very small deflated profile of the balloon on such a catheter may be too small to securely carry a compressed stent of conventional design.
Previous structures used as stents or intraluminal vascular grafts have included coiled stainless steel springs, helically wound spring coils made from a shape memory alloy, expanding metal stents formed in a zig-zag pattern, and diamond shaped, rectangular shaped, and other mesh and non-mesh designs. Exemplary stent devices are disclosed in U.S. Pat. No. 5,776,161 issued to Globerman, U.S. Pat. No. 5,449,373 issued to Pinchasik et al, U.S. Pat. No. 5,643,312 issued to Fischell et al and U.S. Pat. No. 5,421,955 issued to Lau et al.
Problems to be overcome in stent design include inadequate radial force to maintain stent expansion, inadequate scaffolding of tissue against the wall, predilatation longitudinal rigidity that negatively impacts on stent delivery, and shortening of the stent as a consequence of radial expansion. Predilatation longitudinal rigidity is a significant stent shortcoming that prevents the threading of the stent through long tortuous vessels and lesions. Shortening of the stent is also a problem, as it is important that the stent cover the entire lesion to minimize the risk of post-operative complications. Many of these problems result from the often conflicting goals of stent design. For example, to provide uniform support to the vessel wall, it is desirable to have a high degree of scaffolding in the stent when it is expanded to its nominal radial size. However, it is also desirable to have a small, relatively smooth delivered profile when the stent is mounted on the catheter to permit the stent and catheter to traverse small diameter lesions. The person skilled in the art will appreciate that, as a stent with a very small delivered profile expands radially, its structural elements become farther apart and create openings which reduce the amount of scaffolding available to support the vessel. A similar situation exists with respect to the conflicting goals of improved scaffolding and flexibility during catheter delivery since proper scaffolding will not be accomplished if there are too few supporting structural elements. However, a stent with too many structural elements may be difficult to crimp small enough to fit onto the balloon catheter such that the structural elements do not abut or interfere with each other during delivery through tortuous vessels. Also, in some stents, during plastic deformation of the stent (i.e. balloon expansion), the strain is concentrated at small zones. This limits the properties of the material that can be used as well as the radial force and the expansion rate.
Co-pending U.S. patent application Ser. No. 09/292,991, entitled Medical Device for Intraluminal Endovascular Stenting, add
Crittenden James F.
Maresh Catherine C.
Matthews William H
Prebilic Paul B.
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