Flexible stent structure

Details Flexible stent structure Flexible stent structure

2000-05-04

2003-08-05

Milano, Michael J. (Department: 3731)

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

Arterial prosthesis

Stent structure

Reexamination Certificate

active

06602282

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to medical devices and methods. More particularly, the present invention relates to radially expansible luminal prostheses, such as vascular stents and grafts.
Luminal prostheses are provided for a variety of medical purposes. For example, luminal stents can be placed in various body lumens, such as blood vessels, the ureter, urethra, biliary tract, and gastrointestinal tract, for maintaining patency. Luminal stents are particularly useful for placement in atherosclerotic sites in blood vessels or fistula or bypass grafts. Luminal grafts can be placed in blood vessels to provide support in diseased regions, such as aortic abdominal, and other aneurysms.
Both stent and graft prostheses must meet certain mechanical criteria to function successfully. In particular, such prostheses should be at least partly flexible or articulated (i.e., adjacent expansible ring segments are connected by links that articulate relative to one another) over their lengths so that they may be advanced through tortuous body lumens, such as those of the coronary vasculature. In addition, the prostheses should have controllable length change properties, either to maintain their original length or to have the ability to elongate or foreshorten, as desired, when the prostheses assume an expanded configuration. Further such prostheses must have sufficient mechanical strength, particularly hoop strength after they are expanded, in order to mechanically augment the luminal wall strength and thus maintain lumen patency. The ability to meet these requirements is severely limited in the case of stents and grafts which are delivered in a radially constrained or collapsed configuration. Such prostheses must radially expand at a target site within the body lumen, so any adaptations which are intended to enhance flexibility must not interfere with the ability to radially expand or to maintain strength once expanded.
Prior luminal prostheses often have structures which present a risk of injury as they are endoluminally delivered (i.e., tracked) to and/or released at a target site within a patient's body lumen. In particular, many vascular stents comprise a plurality of circumferentially connected and spaced-apart ring segments which deform circumferentially as the stent is radially expanded. The Palmaz stent described in U.S. Pat. Nos. 5,102,417 and 4,776,337, is typical of such stents. Such stent designs can present challenges in both delivery and deployment. For example a phenomenon called “flaring” occurs when the longitudinal elements of the distal or proximal end of the prosthesis are bent outward to assume a crown-like configuration due to bending forces placed on these elements as the prosthesis passes through tortuous body passageways. Flaring can create the same deleterious effects as the previously described fish scaling phenomenon, injuring or traumatizing the blood vessel wall as the prosthesis is delivered or tracked within the blood vessel. In addition, flaring may increase a tendency for stent movement relative to a delivery balloon, thus causing an improperly deployed stent or, possibly, dislodging the undeployed stent completely from the catheter.
In addition to challenges during delivery, prior luminal prostheses can suffer problems during expansion, particularly during balloon expansion of malleable stents. For example, it has been found that balloon expansion of vascular stents often results in the ends of the stent expanding preferentially compared to the center of the stent. Such “dog-bone” expansion inhibits sufficient expansion of the center or ends of the stent, thus leaving a restricted luminal area in the fully deployed stent. Conversely, sometimes it will be desired to flare the ends of the stent in order to lock the stent in place and prevent the ends of the stent from collapsing after deployment. The ability to program stent expansion over the length of the stent has generally been lacking in prior stent designs.
A still further problem experienced by many prior stent designs is a lack of vessel coverage after expansion. It will be appreciated that the ability to support luminal patency and inhibit hyperplasia and other luminal in-growth can be enhanced if relative coverage of the luminal wall area by the expanded stent is increased. Thus, stent designs which afford a greater luminal wall coverage, or which minimize the free space between stent structures, while minimizing the amount of stent material used may be advantageous. Such increase of luminal wall coverage, however, should not be achieved at the expense of “crimpability.” Particularly for vascular applications, it is desirable that the diameter of the stent be reduced as much as possible during delivery, e.g., when crimped over a delivery balloon. By minimizing the crimped-stent diameter, both trackability and the ability to cross smaller lesions and access more distal lesions will be enhanced. In addition, a larger crimped-stent diameter may increase the risk of stent movement relative to the deployment balloon which, in turn, could cause an improperly deployed stent or even loss of the undeployed stent from the catheter. The ability to reduce the stent diameter is generally limited by the amount of material in the stent itself. Thus, designs which increase the ability of the stent to cover the luminal wall without significantly reducing the “crimpability” would be particularly desirable.
For these reasons, it would be desirable to provide improved stent, graft, and other luminal prostheses. In particular, it would be desirable to provide improved luminal prostheses which exhibit a high degree of flexibility with minimum losses of hoop strength and luminal wall coverage after the prostheses are expanded. For example, the design should be such that the expanded prostheses will conform to both curved and straight vessels with minimal or no straightening or other unintended deformation of the vessel wall. Such luminal prostheses should be trackable, preferably being both flexible and presenting minimum risk of injury to the luminal wall as they are being delivered. In particular, the prostheses should avoid “fish scaling” and should be highly “crimpable” so that the prostheses diameter during delivery can be reduced. The luminal prostheses will preferably further display superior expansion characteristics. In particular, the prostheses designs should permit selective programming of the expansion characteristics along the length of the prostheses. For example, the designs should permit preferential expansion over the central portion of the prosthesis, or alternatively at either or both ends of the prostheses depending on the particular application in which the prosthesis is to be used. Still further preferably, upon expansion the prostheses should display superior luminal wall coverage and adequate to superior hoop strength in order to best maintain patency of the body lumen being treated. At least some of these objectives will be met by the luminal prostheses described and claimed hereinafter.
2. Description of the Background Art
Stents having expansible ring segments joined by sigmoidal links and axial beams are described in WO 99/17680. Stents comprising expansible rings including struts and hinges where the hinges are configured to have different opening forces are described in U.S. Pat. No. 5,922,020. EP 662 307 describes an expansible stent having serpentine elements with varying degrees of curvature to provide controlled expansion characteristics. WO 00/003662 describes a stent delivery balloon which preferentially opens a center region of a stent as the balloon is expanded. U.S. Pat. No. 6,017,365, describes a stent with serpentine segments with non-linear struts and sigmoidal links. Other patents of interest include U.S. Pat. Nos. 4,776,337; 5,102,417; 6,017,362; 6,015,429; and 6,013,854.
SUMMARY OF THE INVENTION
The present invention provides improved luminal prostheses suitable for endoluminal placement within body lumens,

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