Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
2000-04-07
2002-06-04
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
Reexamination Certificate
active
06398805
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to stents which are implantable or deployable in a vessel or duct within the body of a patient to maintain the lumen of the duct or vessel open, and more particularly to an improved highly flexible, low friction stent structure.
Stents are implantable by deployment in a vessel or duct within the body of a patient to maintain the patency (unblocked or unclogged characteristic) of the lumen of the duct or vessel; i.e., to keep the vessel open. The stent itself is a tubular, perforated wall, open-ended, expandable vascular or endoluminal prosthesis. Although it has enjoyed widespread use principally to keep a designated target site of the lumen in a blood vessel open and unoccluded, especially in the coronary and femoral arteries following angioplasty, the device has found increasing use for the same or similar purpose in other places in the human body. Examples include maintaining the lumen open and unobstructed at a preselected target site in the tracheobronchial system, the biliary hepatic system, the esophageal bowel system, or the urinary tract system.
A vascular stent, in particular, must be of sufficient dimensional stability to keep the lumen of the vessel open while resisting recoil of its elastic wall that naturally occurs when the target site within the vessel or luminal structure has been subjected to outwardly directed forces necessary to expand the elastic fibers during deployment of the stent. It was found that a large percentage of patients who had undergone an angioplasty procedure were returning with blockage of the same coronary artery within three to six months after the angioplasty was performed. It was subsequently discovered that the new blockage was attributable to a different mechanism—the trauma to the artery wall during the angioplasty procedure had caused a proliferation of smooth muscle cells (hyperplasia) which constituted restenosis—in this case somewhat akin to scarring. Implantation of a coronary stent can serve not only to reduce acute complications following an angioplasty intervention, but also improve the long term outcome, such as to inhibit restenosis and suppress or limit recoil by the stent's scaffolding and support of the vessel wall.
Among the different stent designs are a wire mesh type, a coil type with a helical wire configuration, a slotted tube type, and a multicellular type which is a modification of the slotted tube type with less surface coverage and smaller openings. Typically, the stent is implanted from a delivery system which includes a catheter, a balloon generally at or near the distal end of the catheter, and an inflation lumen in the catheter for selectively inflating and deflating the balloon with a suitable biologically compatible fluid, the stent being crimped onto the balloon. The balloon catheter with stent must, of course, have a diameter smaller than the diameter of the vessel in which the stent is to be implanted. A coronary artery may have a diameter of only about 3 millimeters. The catheter is inserted from its proximal end into the vessel and advanced until the stent is aligned (as viewed under fluoroscope by the implanting physician) at the target site, such as a section of a coronary artery which has just been treated with an angioplasty procedure, and the stent is then deployed by inflating the balloon to expand the stent diameter, whereby the stent engages and at least slightly expands the lumen diameter of the vessel wall.
In addition to adequate support strength in the deployed state—sometimes referred to as mechanical scaffolding, hoop strength or radial strength—to resist vessel wall recoil and to maintain the vessel patency, the stent also must be sufficiently flexible to be advanced through the lumen of an often narrow and tortuous vessel on its delivery system catheter without injuring or distending the vessel wall. Indeed, the stent must have a capacity to resist and yet flex with the repetitive pressures exerted on it by the coronary artery wall according to the systole and diastole of a beating heart. It is therefore necessary that some compromise be reached between these two conflicting requirements. The '600 patent discloses a composite stent design pattern of interconnected struts and openings therebetween in the stent's tubular wall which is different along its mid-section from either of its end segments, giving the stent greater rigidity at its mid-section and greater flexibility at its ends. The more rigid mid-section can better withstand recoil and repetitive pressure of the vessel wall when the stent is deployed. The more flexible ends allow the undeployed (crimped, or compressed) stent to better traverse tortuous paths encountered during advancement through the lumen of the vessel, and the deployed stent to accommodate repetitive wall flexation. Also, the more flexible ends provide a smooth transition between the native vessel wall and the more rigid mid-section, so as to match the biomechanics of the vessel itself
A coronary (vascular) stent must be implanted rapidly, to avoid the possibility of subjecting the patient to risk of myocardial infarction owing to the reduction or even complete blockage of blood flow through the coronary artery while the stent delivery system is being navigated through the vascular system and ultimately deployed at the target site. This imposes even greater importance on axial or longitudinal flexibility of the stent, as well as the skill of the implanting physician. Additionally, it is crucial that the stent exhibit low surface friction. Many of the body vessels, tracts or ducts through which a stent must be advanced to reach the target site exhibit a surface which is not smooth, but rather, uneven, calcified or stenosed.
Therefore, an ideal stent must be structured to minimize the impact of surface friction along the path it must traverse, as well as possess features of longitudinal flexibility and mechanical scaffolding. Low surface friction is especially important in the compressed state or condition of the stent when it is mounted on the balloon catheter of the delivery system, for it is in this condition that the stent must be guided through the vessel. It is unacceptable for the stent structure itself to exacerbate the problem of friction along the path, by presenting a compressed state whose surface friction characteristics, coupled with longitudinal bending of the stent that must take place during advancement through a curved vessel, creates hook-like anomalies at the outer surface of the stent.
It is a principal aim of the present invention to provide a stent having structural characteristics of high longitudinal flexibility, strong mechanical scaffolding, and low surface friction, compared to previous stent designs.
In heretofore available stent designs, whether of the mesh, coil, slotted or multicellular type, it has been customary to provide transversely or laterally oriented structural elements (relative to the longitudinal axis of the stent) that interconnect longitudinally oriented elements in the stent structure. An extreme example is the coil stent, in which a single element (the coil itself) provides both longitudinal and transverse orientation relative to the direction of advancement of the stent through the vessel or duct. Transverse elements or portions of a stent structure tend to exacerbate surface friction during advancement (or withdrawal) of the stent through the vessel, particularly if the stent undergoes longitudinal bending as it traverses a tortuous vessel. In general, bending becomes more pronounced as stent length increases.
Therefore, another aim of the present invention is to provide a stent structure of the slotted tube or multicellular type in which the structural elements are oriented or aligned in a predominantly (i.e., virtually entirely) longitudinal direction relative to the axis of the stent.
An additional factor which makes stent structural elements of predominantly longitudinal orientation a functionally desirable design is that the
Inflow Dynamics Inc.
Jackson Suzette J.
Willse David H.
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