Metallic stent which is degradable in vivo

ES 2 200 368 T3

DESCRIPTION

In vivo degradable metal implant.

The present invention relates to implants of metallic materials for use in the human or animal body.

Such implants are basically known for a long time. The first implants were developed for orthopedic purposes, for example screws and nails to fix broken bones. These were primarily composed of relatively simple iron alloys, which under in vivo conditions tended to corrode. Corrosion resulted in metals being released as ions in the immediate vicinity of the bone, which gave an unwanted stimulus for the growth of bone tissue. The bone grew more intensively than was actually desired and necessary. In this way the healthy material of the bones was damaged.

For this reason, there have been efforts to manufacture metal implants basically from materials that are as resistant as possible to corrosion. Corrosion resistant alloy steels, tantalum and titanium are mainly used here today. These implants remain present after implantation as foreign bodies and are recognized as such by the body. They can only be removed by a second operation.

Furthermore, metallic implants are known in the field of vascular surgery and cardiology, angiology and radiology. These implants include for example endoluminals and vessel supports (extensors) for treating injuries. These supports serve, for example, to widen and maintain the lumen in narrowed vessels, such that starting from the vessel lumen and by means of a balloon catheter (expandable balloon) or self-expanding (self-expanding) they maintain the vessel lumen with an internal diameter. correspondingly optimal. The implant is necessary only until the diseased vessel, due to biological repair processes, by its own strength can sustainably maintain the necessary diameter. This is generally the case about 4 weeks after implantation.

However, the long-term permanence of a metal implant involves some drawbacks. The implant gives rise, as a foreign body, to local and possibly systemic reactions. Additionally, self-regulation of the affected vessel segment is hampered. Permanent (pulsating) loading of the metal can lead to fatigue breaks, which in large lumen implants (eg closure systems such as shields) lead to new medical problems. Small lumen vessel supports (2.5 - 6 mm) generate a new stenosis in approximately 20% of cases (the so-called extensor stenosis) which, due to the high number of accumulated implants, leads to a load additional important both medical and economic. In some regions of the vessels (for example, extracranial vessels, arteries of the legs) the metallic structure can be permanently deformed due to the action of external forces, with the consequences of a new obstruction of the vessels or an induced closure of the vessels. cups. Each permanent implant additionally involves problems, particularly in young patients, since its permanence for decades is inevitable.

Fully biologically degradable implants have hitherto only been known from plastic materials, for example from DE 2502884 C2. There is disclosed a coating of an orthopedic implant with polymethyl methacrylate which is biodegradable. Other plastic materials include polyglycol and polyactidic acid esters. Furthermore, EP 0006544 B1 discloses a biodegradable ceramic material based on calcium phosphate, which is also used to coat metal implants.

Finally, from WO 81/02668 an orthopedic implant is known which has a corrosion-resistant metal base body, as well as a biologically degradable metal intermediate layer for the bone contact area. Together with the base body, this intermediate layer forms an electrochemical cell and generates an electrical voltage that promotes bone growth. Simultaneously, the surface layer decomposes, which can be composed for example of silver alloys. This results in the intended effect that bone growth is positively influenced as long as this is necessary and then, after complete breakdown of the surface coating, the electrical stimulation is lowered.

Hitherto known biodegradable substances based on polymers are used in vascular surgery. Their mechanical properties, on the one hand, and the subsequent reaction to foreign bodies during biodegradation, on the other, mean that as the only material they are unsuitable for an implant. Metallic materials / alloys have favorable mechanical properties (elasticity, deformability, stability) when their mass is small, which is an important premise for the application by means of thin lumen guide systems in transcutaneous procedures.

It is therefore the task of the present invention to make available implants made of biodegradable material which at the same time have advantageous mechanical properties.

This task is solved by implants with the particularities of claims 1, 2 and 3.

Because the medical implant is manufactured from a metallic material, which by corrosion can degrade in vivo, the mechanical advantages of metallic materials are paramount. The corrosive decomposition of the implant within a time scale adjustable by the choice of material, on the other hand, avoids the long-term negative effects of metallic foreign bodies. In this connection, it is biologically advantageous if the material is pure iron, optionally containing up to 7% carbon or an alloy of which iron is the main component. Iron is currently preferred as the main component.

The biological, mechanical and chemical properties of materials can be positively influenced when manganese, cobalt, nickel, chromium, copper, cadmium, lead, tin, thorium, zirconium, silver, gold, palladium, platinum, rhenium are foreseen as secondary component, silicon, calcium, lithium, aluminum, zinc, iron, carbon, or sulfur. Currently preferred material as a whole is an iron alloy with a low proportion of aluminum, magnesium, nickel and / or zinc.

The medical implant is made in several variants

ES 2 200 368 T3 basic. For vascular support, a tubular structure with additional treatment is provided as the base body. As closure systems (eg ductus botalli, innate and acquired septum defects, arteriovenous shunt junctions), passively and / or actively deploying forms of protection, coils or complex bodies are advantageous. The invention can also be used in occluders as closure systems for joint cavities, vessels or gait systems.

It is furthermore advantageous to provide the implant as a fixation device or support for the temporary fixation of tissue parts in the form of implants or transplants.

To adjust the corrosion rate of the material, it is advantageous if the thickness of the material is chosen according to the composition of the material in such a way that the decomposition or corrosion process in vivo is completed in practically between 5 days and 6 months, in particular between 2 weeks and 8 weeks.

It is then achieved that after the growth of the tissue implant, the fixation device that is no longer needed disappears.

Various exemplary embodiments of the present invention are listed below.

Example 1

Vascular support

A stent according to the invention is manufactured from a tubular base body of metal and subsequent processing. From the point of view of their mechanical structure, such extenders are known, for example, from EP 0221570 B1, the material nevertheless being a corrosion-resistant noble steel.

In the extender according to the invention of this example, the material is an alloy with the main component iron and the secondary components chromium and nickel, as well as, if necessary, traces of other additives. The percentage composition of the iron alloy should be approximately in the range of 88 - 89% iron, 0.5 - 7% chromium and 0.5 - 3.5% nickel, as well as less than 5% other metals. The wall thickness of the stent holder should be between 50 and 100 μm after processing.

In practice, the stent according to the invention is introduced in the known manner with a balloon catheter into a diseased narrowed blood vessel and there it is dilated or released as a self-expanding stent, keeping the blood vessel in the desired diameter. A restenosis (setback) that remains without the implantation of the extensor and / or a tear of the tissue induced by the dilation, are treated in a totally effective way. Within 2-4 weeks, the extender is covered by internal tissue and retains its supporting function. The blood vessel obtains, due to the growth of the tissue due to the self-healing processes in the area of ​​the implanted extensor, a new stability of its own. The vessel lumen stabilizes at an optimal level. The choice of the alloy material together with the chosen wall thickness, on the other hand, results in the stent gradually decomposing in the wall of the blood vessel and after about 4 - 12 weeks it only exists as indications. The disadvantages reflected on page 2 of a permanent implant disappear.

Example 2

Locking system

A closure system according to the invention (protection) is manufactured from a metal skeleton to which a plastic protection is attached. Such protections are known for example from the alloy MP35N or nitinol. Such closure systems are used for the closure of defects in the separating walls of the heart. The wall thickness of the metal framework is around 500 mm. In practice, the protection is folded in the known way and released in the defect to be closed. Within 3-4 weeks, the protection is covered by the body's own tissue and through this tissue growth it gains a new self-stability. The choice of the alloy material together with the wall thickness of the fabric, causes the metallic framework to decompose within about 4 weeks to a few months and after a year it only exists as indications. The plastic proportion of the shield is preserved, which is not critical due to the flexibility of the material. The decomposition of the metal part has the advantage over known protections that even when there are unforeseen loads, for example in traffic accidents, there is no danger of the vessel walls breaking. The advantage corresponding to the invention is then already achieved as mechanical instability of the framework is achieved due to the degradation.

Example 3

Spirals for closing vessels (occluders)

A coil according to the invention is manufactured from a metallic material wound in the shape of a helix and the coil is pre-bent. The diameter of the primary coil is 0.1 - 1 mm, depending on the vessel to be closed. Such coils are known for example from nitinol, platinum alloys or tungsten alloys.

In the present constructive form corresponding to the invention, the material is an alloy with the main component iron, the secondary components nickel and / or chromium, as well as traces of magnesium and zinc.

In practice, the closing coil (coil) is inserted in the known manner in a stretched manner into a heart catheter and through this it is slid into the vessel to be closed. When it is released from the heart catheter, the coil assumes its previous shape and closes by means of its volume and its thrombogenicity, which can be increased by means of dacron or other fibers, the vessel to be closed. Following vessel thrombocyte and attachment tissue growth, the closure mechanism achieves a new stability of its own. The applied coils progressively break down, so that after about a year the implanted material only exists as traces.

The exemplary embodiments cited so far can be manufactured from iron alloys. There are no known toxic effects of the materials at the concentrations that are to be expected.

Iron alloys are advantageous in terms of their mechanical stability, which is reflected in the small wall thicknesses possible for implants. The material for the alloy can therefore be chosen depending on the application.