The present disclosure relates to expandable devices for use in supporting a passageway. While not limited to medical applications only, this disclosure specifically contemplates medical uses such as a vascular prosthesis, commonly referred to as s stent.
Stents are now widely used in interventional cardiovascular procedures for treating narrowed regions within coronary arteries, and other vessels. Such stent devices generally have a tubular shape and are deployed in a vessel to restore and maintain the patency of a segment of a vessel which has become partially occluded by plaque, and is deployed into the vessel after the occluded region has been re-opened by use of a catheter having an expandable dilatation balloon.
Until recently, previously-known vascular stent prostheses have been either “self-expanding” or “plastically-deformable” devices. While stents are frequently deployed after performing a percutaneous transluminal coronary angioplasty (PTCA) procedure to dilate occluded coronary arteries, efforts have also been made to use such stent devices for treatment of occlusive peripheral vascular disease, such as for carotid arteries, renal arteries and superficial femoral arteries. However, stents used for such peripheral applications frequently require a different set of structural characteristics than those typically used in cardiac stenting. The present disclosure offers several improvements over self-expanding and plastically-deformable stents, not only with respect to the flexibility of the stent implant over known devices, but also as to improved ease of delivery and deployment.
U.S. Pat. No. 4,733,665 to Palmaz discloses two types of plastically-deformable stents (e.g., stents typically formed of relatively inelastic metals, such as stainless steel, cobalt chromium, etc.), which are delivered within the vasculature via a balloon catheter, onto which the stent is mounted for deployment by balloon expansion. The stents described in Palmaz are constructed from a wire mesh tube or a slotted metal tube. These stents are crimped around the deflated balloon of a delivery catheter to prevent premature release from the catheter while being introduced into the vessel location being treated. Deployment of these stents is accomplished by inflating the balloon at high pressure to expand the tubular device to a predetermined diameter through plastic deformation until it approximates the desired dimension of the patent vascular lumen being treated. Since plastically-deformable stents are typically formed of relatively inelastic metal alloys (such as stainless steel or cobalt chromium), they tend to provide less flexibility than self-expanding stents formed of more elastic materials (such as nitinol). Consequently, plastically-deformable stents are considered generally inappropriate for deployment into blood vessels that are subject to recurring forces associated with compression and elongation, as well as torsional forces, or other forms of dynamic loading. While plastically-deformable stents generally provide adequate radial strength, such stents typically also have a high degree of axial rigidity. Thus, plastically-deformable stents should not be employed in vessels that routinely experience longitudinal shape changes, because the stents lack flexibility to conform to the vessel, and may fracture, deform or cause dissection of the vessel.
Over the past several years, much effort has been expended attempting to design plastically-deformable stents having strut arrangements with flexible axial connectors or links which permit adjacent circumferential rings of a plastically-deformable stent to provide improved longitudinal flexibility, so that the stent will more readily bend and articulate to conform to the particular shape of a vessel during delivery and upon implant. Examples of various plastically-deformable stents with improved articulating properties are found in U.S. Pat. No. 5,195,984 to Schatz. U.S. Pat. No. 5,514,154 to Lau et al., and U.S. Pat. No. 6,723,119 to Pinchasik et al. However, reliance upon such articulating links does not entirely solve the issues with respect to fatigue and fracture. In other words, plastically-deformable stents typically incorporate metal alloys which are inherently limiting with respect to the amount of bending tolerated before the material work-hardens and fractures. Plastically-deformable stents not only suffer from the above limitations, but also present an expanded structure with very limited resilience. Consequently, such stents are not deemed to be suitable for use in vessels that may be subject to high radially-compressive forces, such as the carotid arteries, which might abruptly collapse due to a sudden blow or other pressure to the neck.
Another design approach which has been practiced with plastically-deformable stents, with limited success to improve the longitudinal flexibility of the stent, is the use of expandable stent cells which are open, rather than closed. However, there is always been a difficult tradeoff between cell size and cell density, such that the relative ratio of metal struts in contact with the artery wall being supported cannot be reduced to the point that the stent either fails to provide adequate radial strength, or sufficient vascular wall coverage to prevent ingress of vascular tissue between the adjacent struts and into the vascular lumen (i.e., tissue prolapse).
For this reason, among others, self-expanding stents have been a primary focus for stent development for vascular applications with dynamic loading. Examples of such self-expanding stents include various mesh-like tubes (such as described in U.S. Pat. No. 4,655,771 to Wallsten), tubes formed of resilient materials (such as zig-zag stainless steel struts described in U.S. Pat. No. 4,580,568 to Gianturco), and tubes formed of superelastic shape memory materials such as nitinol (such as described in U.S. Pat. No. 6,306,141 to Jervis). However, self-expanding stents also suffers certain shortcomings. Mesh-like stents, as well as coiled and zig-zag stents described above, generally fail to provide a high degree of crush resistance or radial strength, and may tend to migrate from their initial deployment site. Additionally, the catheter delivery systems required for most self-expanding stents usually require a proximally-retractable constraining sheath, which tends to make the delivery systems larger in diameter and less flexible, thus limiting access to smaller vasculature and preventing treatment of more distal vascular blockages or lesions.
Significantly, a new type of stent prosthesis has been recently introduced, based upon the concept of a “bistable cell.” The bistable cell is described in U.S. Pat. No. 6,488,702 to Besselink. The bistable cell comprises a first strut respectively joined at each of its ends to adjacent ends of a second strut which is relatively more flexible than the first, relatively rigid strut (e.g., the first strut can have a greater width than that of the second strut), thereby forming a closed cell which is defined by the enclosed area bounded by the first and second struts. The bistable cell design is such that it will operate as a spring between only two stable configurations, namely, a stable collapsed (unexpanded) configuration and a stable expanded configuration. A stent which is formed with conventional bistable cells will thus comprise a plurality of interconnected, closed bistable cells. When subjected to a radial force applied outwardly upon the interior surface of such a stent, the relatively flexible second strut will gradually deflect outwardly away from the more rigid first strut, until reaching a transition point where it will abruptly move in a spring-like fashion to a stable expanded position. Consequently, the bistable cell is unstable at any position intermediate the stable collapsed and expanded configuration.
Since this bistable cell is also collapsible by application of a force onto the outer surface of such a stent, directed in an inward radial direction, the stable expanded configuration can be reversibly moved into a stable collapsed configuration. Consequently, it is possible to practice this bistable cell design with much less regard to material properties, and generally permits the use of several metal alloys with varied properties of elasticity, elongation and tensile strength, such as stainless steel, cobalt alloys, nitinol alloys, and even polymers. Interestingly, this bistable cell design permits, for the first time, the ability of stents formed out of shape memory alloys such as nitinol to be crimped onto a balloon for retention until delivery to the vascular site. Since it is no longer necessary to use a delivery catheter provided with a retractable constraining sheath with retraction mechanisms for delivery of nitinol stents, it is now possible to deploy such bistable nitinol stents using catheters having a reduced profile and increased flexibility, and thus treat more tortuous anatomy and more distal lesions.
However, it has been generally been believed that the bistable cell design would optimally require a closed cell structure. Surprisingly, however, the present disclosure has proven that the full benefits of a bistable cell design can still be practiced with an open cell design, namely a cell which consist of at least two lobes within the cell boundary, wherein the multi-lobed open cell comprises more than a single pair of relatively rigid and relatively flexible struts. Consequently, this unique open-cell embodiment of the bistable cell provides significant improvement in the overall flexibility of the stent, thereby facilitation greater ease of catheter delivery into tortuous anatomy, as well as treatment of more difficult vascular conditions throughout the entire cardiovascular/peripheral system.
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