Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
1999-06-25
2001-09-11
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
C623S001120, C623S001380, C623S001490
Reexamination Certificate
active
06287332
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an implantable, bioresorbable vessel wall support, in particular a coronary stent.
2. Background Art
As for the background of the invention, so-called “vessel wall supports” or “stents”—as they are called in the technical language—are used in the therapy of stenoses, i.e. pathologically constricted passages of a coronary vessel. To this end, such a coronary stent is introduced transvenously into the human body by means of a catheter and delivered through the coronary system to the treatment site in the heart. In this condition of introduction and delivery, the stent must have an outside diameter of not more than approximately 1 mm for sufficient mobility. Having reached the stenotic passage of the coronary vessel, the stent is durably expanded for the stenosis to be removed. By means of the catheter on which it has been introduced, the stent is radially expanded to a diameter of approximately 4 mm, which is accompanied with plastic deformation. To this end, the catheter is a balloon catheter, in which the lengthwise section which supports the stent is dilated in a manner similar to a balloon by the application of superpressure by means of salt solution.
Conventional implanted stents consist of a metal material suitable for the medical use and which may be provided with an anticoagulant layer for the avoidance of thrombosis problems. A drawback of these durably implanted stents resides in the detectable permanent irritation of the tissue surrounding the stent, since the stent, because of its rigidity, does not perform the flexions, caused by heartbeat, of the coronary vessel it supports.
Furthermore, attention is drawn to the fact that as a rule the support by a stent for the expansion of a stenosis is required only for a period of some months. Afterwards, the part of the vessel affected by stenosis would remain open even without any support.
For the elimination of the afore-mentioned problems, stents have been proposed to be manufactured from bioresorbable materials which decompose in the human body in the course of few months. A method for the manufacture of such bioresorbable coronary stents is known for instance from U.S. patent application Ser. No. 08/733,172. In this case, a stent blank of homogeneous polymeric structure is built up from a viscous solution of poly-&bgr;-hydroxybutanoic acid as a bioresorbable polymeric material in a solvent by successively coating a male mold core with layers of the polymer solution in several steps by precipitation of the polymeric material by the solvent being evaporated and by the layer previously precipitated being dissolved at least partially. This stent blank is then drawn off the male mold core and subsequently treated to finish the shaping of the stent.
It is true that bioresorbable coronary stents of polymeric materials have the desired biological resorbability and biocompatibility. However, problems are posed by the often insufficient mechanical properties such as a lack of plastic deformability of this stent. Upon dilatation to as much as four times the diameter, this will lead to fissuring with the consequence of reduced mechanical stability and a high degree of re-deformation. This means that for a final diameter of 4 mm, the maximum expansion must be clearly higher, which again leads to a further increase of fissuring accompanied with corresponding destabilization of the stent.
SUMMARY OF THE INVENTION
To solve these problems, the invention provides to manufacture the vessel wall support of a combination of metal materials which decomposes in the human body without any harmful effects on the person who wears the implant. The combination of metal materials is to be designed such that the material of the vessel wall support dissolves at a certain decomposition rate and without the production of bio-incompatible decomposition products. A vessel wall support of this type combines the advantageous mechanical properties of metal stents with the bioresorbability of polymer based stents.
In the first fundamental embodiment of the invention, the combination of metal materials is a metal alloy, the selection of the alloy constituents—as explained in detail below—serving to attain the prerequisite of biocompatible decomposition. Consequently, the metal alloy has to consist of a combination of material that will decompose in the body comparatively rapidly—within a period of some months—forming harmless constituents, which can be defined by the palpable term of “biocompatible corrosion”.
For correspondingly uniform corrosion to be obtained, such an alloy comprises a component A which covers itself with a protective oxide coat. This component A is selected from one or several metals of the group of magnesium, titanium, zirconium, niobium, tantalum, zinc or silicon. For uniform dissolution of the mentioned oxide coat to be attained, a component B is added to the alloy, possessing sufficient solubility in blood or interstitial fluid, such as lithium sodium, potassium, calcium, iron or manganese.
The mentioned elements are suitable because they are present in the human body anyway—such as magnesium, zinc, sodium, potassium, calcium, iron and manganese—or are know to be nontoxic—such as titanium, zirconium, niobium, tantalum, silicon and lithium. The combination of a passivating and a soluble component ensures a timely and uniform decomposition into biocompatible breakdown products. The corrosion rate can be regulated through the ratio of the two components.
In an especially preferred manner, the alloy is to be composed so that the corrosion products are soluble salts, such as sodium, potassium, calcium, iron or zinc salts, or that non-soluble corrosion products, such as titanium, tantalum or niobium oxide originate as colloidal particles. The corrosion rate is adjusted by way of the composition so that gases, such as hydrogen which evolves during the corrosion of lithium, sodium, potassium, magnesium, calcium or zinc, dissolve physically, not forming any macroscopic gas bubbles.
Furthermore, for instance a so-called super light alloy of lithium and magnesium known from aeronautics can be used as a possible alloy, which is however optimized with a view to increased fatigue durability and reduced avidity for the field of application mentioned above. The magnesium-lithium ratio is in the range of 60:40, fatigue durability being increased by the addition of further components such as zinc or by gassing by hydrogen. Also, special melting and forging methods are used to increase the fatigue durability.
For putting the present invention into practice, lithium-magnesium alloys can be used, which have a lower fatigue durability during conventional treatment and in the body sphere. Lithium hydroxide and magnesium hydroxide are to be expected as decomposition products, which can both be considered non-toxic and biocompatible.
Problems posed by the mentioned lithium-magnesium alloy reside in that the decomposition products lithium hydroxide and magnesium hydroxide are poorly soluble and with the absorption of carbon dioxide convert to the carbonates which are also poorly soluble. In particular lithium hydroxide is very voluminous in this case. The corrosion products encrust on the stent, these crusts being able to occupy a multiple of the volume of the stent.
In this regard, other combinations of alloys are more suitable, for example a sodium-magnesium alloy. Since sodium hydroxide as a corrosion product possesses a high solubility, this alloy decomposes without voluminous encrusting. Sodium dissolves and magnesium hydroxide forms a fine precipitate which may deposit without risk in the developing vascular skin, the so-called intima.
Another preferred embodiment of a decomposable combination of metal materials is a zinc-titanium alloy, the percentage by weight of which is in the range of 0.1% to 1%. This combination precludes the comparatively strong crystalline growth of zinc as a material used, which would cause a comparatively brittle and fragile behavior of the vessel wa
Bolz Armin
Popp Thomas
Biotronik Mess -und Therapiegeraete GmbH & Co. Ingenieurbuero Be
Browdy & Neimark
Koh Choon
Willse David H.
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