Flexible medical stent

Details Flexible medical stent Flexible medical stent

1999-10-18

2001-12-18

Truong, Kevin (Department: 3731)

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

Arterial prosthesis

Stent structure

Reexamination Certificate

active

06331189

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to implantable intravascular stents for maintaining vascular patency in humans and animals. More particularly, the present invention is directed to a balloon-expandable stent having a chevron or “maple leaf” design geometry characterized by improved scaffolding, hoop strength and longitudinal flexibility.
BACKGROUND OF THE INVENTION
This invention relates to intraluminal endovascular stenting, a method by which a prosthesis is inserted into a body tube and expanded so as to reopen a collapsed vessel wall and prevent the wall from recollapsing into the lumen. Endovascular stenting is particularly useful for arteries that are blocked or narrowed and is an alternative to surgical procedures that intend to bypass the occlusion.
Percutaneous transluminal coronary angioplasty (PTCA) is used to increase the lumen diameter of a coronary artery partially or totally obstructed by a build-up of cholesterol fats or atherosclerotic plaque. Typically a guidewire is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the guidewire and a balloon catheter advanced within the guiding catheter over the guidewire. 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. It has also been shown that the use of intravascular stents can measurably decrease the incidence of restenosis after angioplasty thereby reducing the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
An implanted prosthesis such as a stent can preclude additional procedures and maintain vascular patency by mechanically supporting dilated vessels to prevent vessel reclosure. Stents can also be used to repair aneurysms, to support artificial vessels as liners of vessels or to repair dissections. Stents are suited to the treatment of any body lumen, including the vas deferens, ducts of the gallbladder, prostate gland, trachea, bronchus and liver. The body lumens range in diameter from small coronary vessels of 3 mm or less to 28 mm in the aortic vessel. The invention applies to acute and chronic closure or reclosure of body lumens.
A stent typically is a cylindrically shaped device formed from wire(s) or a tube and intended to act as a permanent prosthesis. A typical stent ranges from 5 mm to 50 mm in length. A stent is deployed in a body lumen from a radially compressed configuration into a radially expanded configuration that allows it to contact and support a body lumen. Optionally, a balloon of appropriate size and pressure can be used to open the lesion prior to delivery of the stent to its intended location.
The stent can be radially self-expanding, or it can be expandable by the use of an expansion device. The self-expanding stent is made from a resilient springy material while the device-expandable stent is made from a material that is plastically deformable. A plastically deformable stent can be implanted during a single angioplasty procedure by using a balloon catheter bearing a stent that has been crimped onto the balloon. The stent expands radially as the balloon is inflated, forcing the stent into contact with the interior of the body lumen thereby forming a supporting relationship with the vessel walls.
Deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of a delivery catheter. In the case of a balloon expandable stent, the delivery catheter is a balloon catheter, and the stent is deployed when the balloon is inflated. 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.
Conventional angioplasty balloons fall into high, medium and low pressure ranges. Low pressure balloons are those which fall into rated burst pressures below 6 atmospheres. Medium pressure balloons are those which fall into rated burst pressures between 6 and 12 atmospheres. High pressure balloons are those which fall into rated burst pressures above 12 atmospheres. Burst pressure is determined by material selection, wall thickness and tensile strength.
Previous structures used as stents or intraluminal vascular grafts have included coiled stainless steel springs; helical wound spring coil made from shape memory alloy; expanding metal stents formed in a zig-zag pattern; 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., U.S. Pat. No. 5,421,955 issued to Lau et al., and U.S. Pat. Nos. 4,649,922, 4,886,062 and 4,969,458 issued Wiktor.
Problems to be overcome in stent design include (a) inadequate radial force to maintain expansion; (b) inadequate scaffolding of tissue to the wall; (c) pre-expansion longitudinal rigidity which negatively impacts on stent delivery; (d) inability to achieve a pre-expansion diameter small enough to pass through a narrow lumen; (e) in the case of balloon-expandable stents, unacceptable mechanical stress or strain levels in the expanded stent; (f) flaring of the ends of the stent during stent delivery; and (g) longitudinal shortening of the stent as a consequence of radial expansion.
Unfortunately, many of these problems result from or are exacerbated by the often conflicting goals of stent design. For example, it is desirable to have a high degree of scaffolding in the stent when the stent is expanded to its rated radial size so that the vessel wall will have uniform support. However, it is also desirable to have a small, relatively smooth delivered profile (sometimes referred to as “crimp diameter”)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 that reduce the amount of scaffolding available to support the vessel. Such as stent may also exhibit a reduction in the outward radial forces (hoop strength) generated by the stent after expansion within the lumen. The larger window area and, therefore, inferior body lumen scaffolding reduces the effectiveness against recurring restenosis. The reduced outward forces may also be problematic if the stent does not firmly engage the wall of the lumen.
Another example of the conflicting goals of stent design involves attempts to achieve improved scaffolding and longitudinal flexibility during catheter delivery, since proper scaffolding will not be accomplished if there are few supporting structural elements. Yet the inclusion of too many structural elements results in a loss of stent flexibility in both the crimped and expanded states.
One attempt at addressing the high bending stresses/strains in a radially expandable stent is described in U.S. Pat. No. 4,830,003 (Wolff et al.) in which the stent is made of a series of generally straight wire segments welded together at their ends to form a zigzag shaped stent when expanded. By using generally straight wires, the bending stresses/strains associated with bends in an integral wire-formed stent body can be avoided. Disadvantages associated with this approach include, h

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