Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
1999-08-16
2002-08-13
Thaler, Michael H. (Department: 3731)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
C623S901000, C140S07100R
Reexamination Certificate
active
06432134
ABSTRACT:
This invention relates to surgical implants which are commonly known as stents and more particularly to stents for occluding the human ductus arteriosus, cardiac septal defects and other blood vessels needing occlusion. This invention also relates to delivery systems for such stents and to methods of manufacturing such stents.
In the fetus, the ductus arteriosus is a patent vessel connecting the pulmonary artery to the aorta. At birth, a variety of physiological changes cause the ductus to constrict and it becomes permanently closed after one or two days. In a few cases, the ductus fails to occlude completely (patent ductus), allowing blood to flow from the aorta to the pulmonary artery, compromising respiration and circulation. Serious consequences may result.
Conversely, in some other circulatory birth defects in which the ductus has closed naturally, the patient benefits when the ductus is artificially re-opened.
Surgical interventions to alleviate a patent ductus arteriosus consist of either surgically ligating the leak path or implantation of a “single umbrella”, such as the C R Bard PDA Umbrella. The delivery system for this device is large and it cannot be used in very small children. The clinical approaches currently in use are problematic; the surgery is major, requires significant exposure and has an associated morbidity. Technically, considerable difficulties can be experienced in accurately siting and deploying the umbrella device because of its large delivery system.
As an alternative to surgery, pharmacological agents can be used to constrict the ductus but this technique can only be used in babies and is not always successful. Also such agents have side effects.
There are many designs of stents currently in commercial use, the Gianturco, (EP0282175) Palmatz-Schatz, Wall stent and others are well known in the art. These stents are designed for use in vessels which are several times longer than their diameter and the stents share this aspect ratio. Most designs exhibit spring resistance to radial compression but nevertheless are intended to be approximately parallel sided cylindrical tubes, once implanted. The ductus arteriosus is short with respect to its diameter and as a result conventional stent designs are the wrong shape and tend to migrate. It is quite usual for surgeons to modify existing devices to make them more appropriate for implantation in this procedure. But this is not an ideal solution to the problem.
Existing stents are intended to maintain the patency of diseased blood vessels and to minimise the risk of creating further local disease or trauma. For instance, introduction of a stent can cause intimal hyperplasia by damaging plaques on the vessel wall. Blood flow which impinges on the stent may become turbulent and create further neo intimal proliferation. For these reasons, stents are usually designed with the minimum of material to reduce their surface area.
EP-A0666065 discloses stents for maintaining the patency of passages in the biliary, urinary or vascular system, wherein the stent is formed of a shape memory alloy (SMA) wire or mesh and can be changed thermally from a first configuration in which it has distal and proximal coiled or mesh cylindrical portions which are spaced apart, into a second configuration in which the proximal portion flares into a funnel shape which increases in diameter away from the distal portion which expands in diameter. However, such stents require a relatively large delivery system.
Other examples of stents based on SMA wire for maintaining the patency of vessels are disclosed in U.S. Pat. No. 3,868,956 and U.S. Pat. No. 4,512,338.
U.S. Pat. No. 5,192,301 and DD 233303 disclose a number of occluding stents for the ductus arteriosus based on SMA materials. However, none of these is capable of adopting a small cross-sectional area for delivery to the required site.
It is an object of a first aspect of the present invention to provide an occluding stent which is capable of being delivered and placed relatively easily.
According to said first aspect of the present invention, there is provided a stent for surgical implantation into a patient, said stent including a wire which is expandable from a relatively straightened state for introduction into the patient, to an occluding state wherein the wire defines an occluding anchor part in which the wire has adopted a series of turns extending over the cross-sectional area of the occluding anchor part.
The stent of the present invention, despite occupying a small cross-sectional area in its relatively straightened state, presents a significant surface area to blood flow in its expanded occluding state, both to increase the probability of clotting and to provide a significant support for mechanical, biological or pharmacological coatings or surface treatments. In one embodiment of the design, the stent can be used as a drug delivery vehicle by means of slow release pharmacological coatings. In another embodiment, the surface of the stent is roughened to provide an attractive substrate for cell adhesion so that the mechanical action of the device alone will fuse the ductus. Combinations of these characteristics are possible and the coatings or treatments can be applied over all or part of the surface of the stent.
Conveniently, in its expanded occluding state, the stent has three distinct regions which can be manufactured as separate components or as a monobloc construction and which comprise the occluding anchor part, another anchor part which may also have an occluding function, and a linking part between the anchor parts. The anchor parts are typically circular in section and their diameters are greater than the minimum diameter of the vessel within which the stent is placed. The stent may be placed such that the anchor parts are positioned either side of the narrowest part of the vessel. In this way it will be impossible for either anchor part to pass through the entire length of the vessel, preventing its migration. This construction is particularly suitable for short vessels which are to be occluded.
The link part may be a simple wire rod, or it may be a tube or mesh. Its principal function is to maintain the separation of the anchor parts to their designed value. It has an outer diameter which is less than the maximum outer diameters of either anchor part.
At least one of the anchor parts may be designed to lie within the vessel and have a conical form in the occluding state, expanding distally and applying a radially expansive force to secure it in position. Alternatively, at least one of the anchor parts is of substantially planar spiral form in its occluding state, and is sufficiently flexible that, upon expansion, it distorts so as to adopt the cross-sectional profile of the wall of the passage in which it is located in use.
If the stent is deployed close to the junction of the vessel with another, larger, vessel then one anchor part can have such a large diameter that it can only sit within the larger vessel. In this case, the form of the anchor part is conveniently planar or saddle shaped so that it conforms to the inner wall of the larger vessel.
The stent can be manufactured using established techniques such as helically wound wire, (single-stranded or twisted or braided multi-stranded) and welded wires, from metal or plastics including biodegradable plastics such as polylactic acid. Specifically included are shape memory effect materials, superelastic materials and polymers.
When a shape-memory effect material is employed, its transition temperature may be in the range of 0° C. to 50° C., permitting designs of stent which will trigger at blood temperature and therefore need to be actively cooled to keep them from deforming prematurely, or devices which trigger above blood temperature and so need to be actively heated to make them adopt their final shape. A number of means for heating can be employed, including perfusion with warm liquid, or electrical or induction heating.
The stent can be used on a wide range of ages of child from pre-term babies and neon
Anson Anthony Walter
Phillips Peter William
Qureshi Shakeel
Reidy John
Anson Medical Limited
Cesari and McKenna LLP
Thaler Michael H.
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