Endoluminal stent cross section for optimum biocompatibility

Details Endoluminal stent cross section for optimum biocompatibility Endoluminal stent cross section for optimum biocompatibility

2000-10-31

2004-02-03

Calvert, John J. (Department: 3765)

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

Arterial prosthesis

Stent structure

Reexamination Certificate

active

06685737

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient's body lumen, such as carotid arteries, coronary arteries, peripheral arteries, veins, or other vessels to maintain the patency of the lumen. More particularly, the invention relates to the design and configuration of the geometry of stent struts so as to minimize the disturbance to the blood flow in the vessel, and to minimize the trauma caused by the stent to the body lumen in which it is implanted.
Stents are frequently used in the treatment of atherosclerotic stenosis in blood vessels especially in conjunction with percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty (PTCA) procedures, with the intent to reduce the likelihood of restenosis of a vessel. Stents are also used to support a body lumen, tack-up a flap or dissection in a vessel, or in general where the lumen is weak to add support. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as a coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use in supporting and holding back a dissected arterial lining which can occlude the fluid passageway there through.
Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain their patency and are especially well-suited for the treatment of atherosclerotic stenosis in blood vessels. Intracoronary stents have become a standard adjunct to percutaneous coronary angioplasty in the treatment of arterial atherosclerotic disease. Although commercial stents vary in design and materials, they share similar structural features. Most stents in clinical use are metallic and are either self-expanding or are expanded by the force of an expandable member, such as an angioplasty dilatation balloon. These devices are typically implanted via a delivery catheter which is inserted at an easily accessible location on the patient and then advanced through the patient's vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen and into the stenosis. Once in position, the stent is deployed which, depending upon its construction, is achieved either automatically by the removal of a restraint, or actively by the inflation of a balloon about which the stent is carried on the delivery catheter.
The stent must be able to simultaneously satisfy a number of mechanical requirements. First and foremost, the stent must be capable of withstanding the structural loads that are imposed thereon as it supports the lumen walls. In addition to having adequate radial strength or more accurately, hoop strength, the stent should nonetheless be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material of which the stent is constructed must allow the stent to undergo expansion, which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear upon it, including the cyclic loading induced by the pulsatile character of arterial blood flow. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses. A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter, helically wound coiled springs manufactured from an expandable heat sensitive metal, and self-expanding stents inserted into a compressed state for deployment into a body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from, for example, shape memory metals or super-elastic nickel-titanum (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the body lumen. Such stents manufactured from expandable heat sensitive materials allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent.
Details of prior art expandable stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass, et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No. 5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat. No. 4,655,772 (Wallsten); U.S. Pat. No. 4,739,762 (Palmaz); and U.S. Pat. No. 5,569,295 (Lam). Further details of prior art self-expanding stents can be found in U.S. Pat. No. 4,580,568 (Gianturco); and U.S. Pat. No. 4,830,003 (Wolff, et al.).
Despite the widespread use of intracoronary stents, in-stent restenosis remains a major clinical problem; however, restenosis does not develop in all patients undergoing coronary angioplasty and stent implantation. The mechanism of restenosis after stent implantation is principally neointimal hyperplasia, as stents resist negative arterial remodeling. Relative to PTCA alone, stents improve the outcome by minimizing vessel recoil, reducing plaque prolapse, and affecting long term remodeling.
It has been shown that smooth muscle cell (SMC) proliferation is affected by signaling from the endothelium. In flowing blood, the endothelium seeks a surface shear stress of approximately fifteen dynes/cm
2
. This tendency is known as the “Glagov Effect.” If the shear stress is too high, the endothelial cells signal the smooth muscle cells to relax. When the detected shear stress is low, the endothelium signals for the vessel to constrict. If the shear stress remains consistently low, SMC proliferation occurs causing lumenal narrowing. Regions of flow reversal, or disturbed blood flow, also cause the endothelial cells to signal for lumenal narrowing in an effort to maintain the desired shear stress. This is one reason why atherosclerotic legions form first at vessel bifurcations and other regions of complex flow. Another reason is that in these regions there is an increased residence time of blood elements including atherogens such as lipids and cholesterol which increases the chance of deposition. It is known that stent struts alter the blood flow. Immediately upstream and downstream of the struts, the flow is disturbed, with flow reversals and eddies. Thus, the introduction of the stent into the vessel can cause lumenal narrowing and potentially sets the stage for further atherosclerotic disease.
Animal studies also have established a significant correlation between the degree of arterial injury caused by metallic stents and the resultant neointimal thickness and lumen stenosis at the stented site. Patients with restenosis may be those whose vessel incurred greater injury during revascularization. Indeed, breakage of the internal elastic lamina (IEL) has been correlated with a higher level of restenosis. Similarly, higher injury score has been implicated as a causal factor in neointimal formation, resulting in restenosis. In addition, inflammation caused by the implantation of the

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