Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
2001-08-08
2002-11-05
McDermott, Corrine (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
C623S001180, C623S001200
Reexamination Certificate
active
06475237
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an expandable endoprosthesis that is placed within a tubular member of the human body to treat a region that is pathologically affected by supporting it or holding the tubular member outwards. More specifically the invention relates to an intravascular endoprosthesis placed within a blood vessel of the body generally at the site of a vessel lesion in order to provide a more widely open lumen and enhance patency of the vessel. The present invention further relates to a stent that can be used in blood vessels and tubular vessels of the body that have become stenotic or blocked by tissue or other material and require reestablishment of a lumen and maintenance of the lumen.
2. Description of Prior Art
Stents used to internally support tubular vessels of the body can generally be categorized into two groups, those that are mechanically expanded by an external device such as a balloon dilitation catheter, and those that are self-expandable. Advantages of the balloon expandable stents lies in part in the ability of these stents to be delivered accurately to the site of a stenotic lesion. The location of the stent prior to deployment or expansion can be visualized under flouroscopy and deployment of the stent is generally made by inflation of a dilitation balloon which expands the stent radially into contact with the inner surface of the vessel wall. After the dilitation balloon has been deflated and the dilitation catheter removed, the stent is left in place to balance radial forces applied by the vessel wall and ensure that the vessel lumen is maintained in a widely patent conformation.
Some difficulties associated with balloon expandable stents can be related to their lack of flexibility and their inability to withstand external forces that can lead to irreversible crushing of the stent. Several stents exhibit a structure that will not easily bend around tortuous pathways found in the human vasculature to reach the site of the lesion in their nondeployed state. Other stents have a structure that is made more flexible but are not appropriately capable of supporting or balancing the radial forces applied by the vessel acting to compress the stent. A stent with a low radial balancing force characteristics may be expected to undergo an irreversible crushing action if exposed to an externally applied force. Arteries of the neck and leg region can sometimes be exposed to such external forces resulting in permanent deformation of the stent and loss of vessel patency. This has been the case for some balloon expandable stents that have been placed in the carotid artery and exposed to digital or other external forces that have led to collapse of the stent.
In the balloon expandable stents currently found in the prior art one cannot adjust the amount of force required to expand the stent independently from the force required to crush the stent. As a result, a stent design that resists crushing action will generally be too stiff and require too much force to accomplish its deployment.
Self-expandable stents overcome some of the problems associated with the crushability of balloon expanded stents. These stents are typically made of Nitinol, a stainless steel with high yield strength, or some other material that can store energy elastically. Self-expandable stents can be delivered within a sheath to the site of the lesion. There the sheath can be removed and the stent can be deployed as it expands out to a larger diameter associated with its equilibrium diameter.
Some problems associated with self-expandable stents include the inability of the physician delivering the stent to define precisely the location of both ends of the stent. Oftentimes the stent can undergo significant changes in it axial length in going from an nondeployed state to a deployed state. Such length changes can result in inaccuracies in defining a precise placement for the stent. This disadvantage can be somewhat offset by the ability of some self-expandable stents to resist crushing deformation associated with an external force directed toward the side of the stent. Self-expandable stents also do not generally allow the radial expansion force to be adjusted independently from the stent forces that are directed to resist crushing forces. A self-expandable stent with an appropriate elastic balancing force to hold the vessel open may have a weak crush balancing force to resist crushing deformation due to externally applied crushing forces.
Palmaz discloses in U.S. Pat. No. 4,733,665 a balloon-expandable stent that is formed by machining slots into a metal tube forming a series of elongate members and bars. The stent is mounted in its nondeployed state onto the balloon portion of a balloon dilitation catheter and delivered to the site of a lesion that has been previously dilated to allow passage of the stent mounted balloon catheter. Dilation of the balloon causes the balloon-expandable stent to plastically deform at the junction of the elongate members and bars of the stent. For the stent to undergo an expansional deformation the balloon must supply an expansion applied force that exceeds the expansion yield force associated with the junction of the elongate members and the bars. Typically a balloon dilitation catheter used to dilate a coronary lesion found in a three millimeter diameter coronary artery can be dilated at a balloon pressure ranging from one to fifteen atmospheres. With the stent mounted on the balloon, the balloon must be capable of radially expanding the stent and holding the vessel in a widely patent conformation. Upon removal of the balloon catheter, the stent must continue to supply a compression balancing force to balance the compression applied force of the vessel acting inward on the stent. If an externally placed side force is imposed onto the side of the stent, the stent can deform into an oval or flattened shape representative of a crushing deformation. This deformation can involve an elongate member or it can occur at a junction of an elongate member with a bar. The elongate member can be formed such that it resists plastic deformation associated with crushing deformation. The junction of the elongate member with the bar cannot be adjusted to resist crushing deformation without also affecting the force required to expand the stent from its nondeployed state to its deployed state; additionally, the compression balancing force would also be affected. The Palmaz stent disclosed herein therefore can be susceptible to crush deformation in order to maintain appropriate characteristics for an expansion yield force during deployment and a compression yield force to hold the vessel in an open conformation.
A stent is required to have axial flexibility in order to negotiate the tortuous turns found in the coronary vasculature. Palmaz describes in U.S. Pat. No. 5,102,417 connector members that connect between small cylindrical stent segments. Although the connector members provide an enhanced axial flexibility, this stent is still subject to crush deformation. The compression yield force that is capable of holding the vessel outward with the stent in a deployed state is coupled to the crush yield force that prevents the stent from crush deformation.
Fischell describes in U.S. Pat. No. 5,695,516 a balloon-expandable stent formed from a metal tube and having circumferential arcs and diagonal struts. When this stent is expanded to a deployed state the junctions of the arcs and struts undergo plastic deformation and the deployed stent takes on a honeycomb shape. The expansion yield force of this stent describes the force required to plastically deform an arc with respect to a strut at a junction during stent expansion. The compression yield force describes the force required to plastically deform an arc with respect to a strut when exposed to a compression applied force by the blood vessel. Deformation due to crushing would also occur at the junction of the arc with the strut. The crush yield force of this stent is therefore directl
Drasler William J.
Thielen Joseph M.
McDermott Corrine
Phan Hieu
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