GB2059267A

GB2059267A - Compound material for prosthetic devices

Info

Publication number: GB2059267A
Application number: GB8030611A
Authority: GB
Original assignee: Katholieke Universiteit Leuven
Current assignee: Katholieke Universiteit Leuven
Priority date: 1979-09-28
Filing date: 1980-09-23
Publication date: 1981-04-23
Also published as: AU6271180A; CH646597A5; DE3036520A1; FR2466241B1; BE885384A; NL7907231A; JPS5656795A; AU539092B2; FR2466241A1; GB2059267B

Abstract

A compound material, suitable for making prosthetic devices, comprises at least one layer of metal wire mesh on a metal substrate. The wires are joined to each other and to the substrate by means of metallurgic bonds at their point of contact. These metallurgic bonds may be formed by any method but preferably by pressure-sintering, sometimes preceded by a pre-compacting step. Sintering temperatures between 600 DEG C and 1000 DEG C, at pressures between 1 and 150 MPa and durations between 5 minutes and 8 hours have given good results. The pre-compacting step may be effected at pressures of 10-600 MPa. <IMAGE>

Description

SPECIFICATION Compound material for prosthetic devices This invention relates to a compound material comprising a porous upper layer of metal wires on a metal substrate, and to a method of producing the same. Such a compound material may potent;ally be used in implantation prosthetic devices, where the porous layer is coming into direct contact with the surrounding bone tissue and allows the bone tissue to grow into the porous upper layer so as to achieve a firm anchoring of the prosthetic devices.

It is well known that prosthetic devices are used for complete or partial replacement of joints or bone segments in the skeleton of man and animal. These prosthetic devices are mostly intended for impiantation into the remaining part of the skeleton. This may lead to various problems, including those pertaining to excessive wear and a consequently short life, and also those pertaining to durable fixation to the remaining hard bone parts. Various methods are used at present for such durable fixation, such as mechanical impactation of a prosthetic stem into the medullary cavity of the bone; mechanical fixation by means of screws; and fixation by means of bone cement polymerizing in situ. The latter method in particular is used frequently, with methyl methacrylate as cement.The problem with this and other methods is, however, that failure of the fixation is often accompanied by destruction or resorption of the surrounding bone tissue.

Other methods for achieving durable fixation which make use of the bonding and adhering ability of the surrounding bone tissue have been suggested recently. Examples of these methods are: the use of indentations, ribbed or rough areas on the surface of the prosthetic device to cause bone tissue to grow overthem; the use of biologically active materials that may react with the bone tissue, thus forming a solid bond; and the use of porous layers or porous elements into which the bone tissue may penetrate and which it may grow together so as to provide a stable anchoring.

The invention is concerned in particular with the last-mentioned method.

For prosthetic devices having a porous layer or element, various embodiments have already been proposed. It has turned out that the use of a porous upper layer on a substrate is preferable to an entirely porous object in view of the mechanical requirements, such as tensile strength and fatigue strength, that should satisfied in the prosthetic device. It has been outlined moreover, that metals in view of their better mechanical properties such as tensile strength and ductility, are more satisfactory than ceramic materials or polymers for constituting the material of the porous layer of heavily loaded prosthetic parts.

Further, a porous upper layer of metal wires is more satisfactory than a porous upper layer of metal powder, since an open pore structure having a higher average pore size and considerably better strength and ductility With the same porosity, may be obtained with metal wires.

U.S. patent No. 3,906,550 to Rostoker and Galante discloses a prosthetic device of the implantation type which in an embodiment comprises a metal stem as a load-bearing element surrounded by a porous layerofthin metalwireswhich have been sintered together. The porous layer has an open pore structure and is intended to allow the bone tissue to grow into the pores in the manner described above, in order to achieve anchoring.

In this known prosthetic device, the porous layer is made by folding very thin wires of stainless steel or a similar metal, cutting them into pieces, stamping the pieces in a die to form a three-dimensional network of mechanically interlocking fibers and sintering this network in order to create metallurgic bonds between the fibers at their points of contact.

The porous layer thus obtained may be fastened to the metal stem of the prosthetic device by means of sintering or some other method.

As a consequence of the manufadturing method used herein, the metal wires in the porous layer of the known prosthetic device are rather randomly orientated. The result is an open pore structure indeed, but the shape and dimensions of the pores, as well as the degree of porosity, may vary quite considerably from point to point. Moreover, the pores should not be too large in size, since this would reduce the mechanical strength of the material too much. Furthermore, the porous layer has a relatively large specific surface with respect to the volume of the prosthetic device and this increases considerablythe liberation of metal ions in vivo.

Finally, the fastening of the porous layer to the metal stem, which must be effected in a separate manufacturing step, presents a significant problem.

It is an object of the invention to eliminate these disadvantages and to provide a compound material having a porous upper layer and a metal substrate which satisfies high requirements and is suitable for use in prosthetic devices of the type described herein above. Another object is to provide a manufacturing method for such a material which is sim plerthanthe known method and which allows large scale production.

The invention provides a compound material comprising a porous upper layer of metal wires on a metal substrate, which material is characterized in that the porous layer is built up from at least one layer of metal wire mesh in which the wires are joined by metallurgic bonds to each other and to the substrate at their points of contact.

In addition, the invention provides a method of producing such material, which is characterized by applying at least one layer of metal wire mesh onto a metal substrate and by metallurgically bonding the wires of the mesh to each other and to the substrate at the points of contact.

In this way, the objects of the invention may be reached well. Thanks to the use of a metal wire mesh, a uniform and easily adjustable pore size as well as uniform porosity may be obtained. In addition, it because possible to obtain any pore size and porosity desired, and, in particular, to make largersize pores without any harm to the mechanical strength of integrity of the porous layer. Furthermore, a smaller ratio of specific surface to volume of the material to be implanted is ensured, which reduces the liberation of metal ions in vivo.

Moreover, another advantage of the production method of the invention is that the formation of the porous layer, the fastening of the metal wires to each other and the attachment of the metal wires to the substrate may now be effected practically in one single operation, which brings about a considerable simplification and which allows large scale production.

In order to give a better understanding of the invention, reference is made to the drawing, which shows in perspective view and by way of example, a compound material according to the invention. This material comprises a metal substrate 1 carrying a layer of metal wire mesh 2. The mesh 2 is a woven fabric of flat weave, having warp wires 3 and weft wires 4. It is clearly visible that the wires 3 and 4 are joined to each other and to the substrate 5 by metallurgic bonds at their points of contact, in such a way that a strong bond is achieved. Between the various wires 3 and 4their remain pores 5 and 6, which are uniform in shape and size thanks to the type of weave used.Although only one layer of wire mesh has been shown, itwill be clear that a number of layers of wire mesh may be present in superimposed condition, the wires thereof being joined to each other and to the wires of a previous layer by means of metallurgic bonds.

The invention will now be described in more detail.

Substrate 1 may consist of any metal which is compatible with living tissue, such as, for instance, stainless steel, titanium, titanium alloys, Co-Cralloys, tantalum, tantalum alloys, niobium, niobium alloys, or zirconium. A metal is preferred above other materials such as ceramics or polymers, since the substrate when used in prosthetic devices will normally be the load-bearing element; therefore, it must have good mechanical strength and a good ductility. The substrate may have any appropriate shape, for instance the shape of a flat plate, a round rod or the like. It is not absolutely necessary that the substrate has already the shape desired for a prosthetic device, since it may be rendered into that shape lateron.

The wires of wire mesh 2 may also be of any metal which is compatible with living tissue. In principle, the same metals can be used for the wires as for the substrate. Although the porous layer of wire mesh is not intended to serve directly as a load-carrying element, it must nevertheless assure a strong bond with the bone tissue and a still stronger internal coherence of the compound material. Therefore, metals are preferred above other materials here, too.

Wire mesh 2 ensures a porous layer of almost uniform mechanical properties, comprising pores 5 of uniform size. This mesh can be either woven or knitted, although a woven mesh is preferable.

Further it is importantthatthe wires of the mesh cross each other at many points, thus creating the important pores 6, which are suitable for ingrows of bone tissue and thereby for interlocking of porous surface and bone.

The diameter of the wires and the number of wires per unit of length in the mesh may vary between wide limits, depending on the pore size and porosity required. Thus, metal wires having a diameter between 0.025 and 1.25 mm may be used and the number of wires per inch in any direction may vary between 400 and 6. When these ranges are observed, it is ensured that the mesh width or pore size in the mesh is always largerthan the desired minimum size for ingrows of the intended tissue into the pores.

It is not necessary that the wire mesh has already the desired shape and dimensions for a prosthetic device at this stage, but shape and sizes should be adapted to some extend to those of the substrate used.

One or more layers of wire mesh may be used at will, depending on the desired thickness of the porous layer on the substrate. In the case of several layers, these are mostly attached to each other in a staggered fashion, in order to obtain a better bond of the various layers among each other. However, the possibility of a number of layers of equal mesh width in unstaggered superimposed condition is not excluded.

During production of the compound material of the invention, one or more layers of wire mesh are applied to the metal substrate and thereafter the wires of the mesh are metallurgically bonded to each other and to the substrate at their points of contact.

The metallurgical bond is preferably obtained by sintering and still more preferably by pressure sintering. Although, in principle, such a bond could also be obtained by spotwelding, this method is not preferred because in practice it appears to be nearly impossible to weld a large number of metal wires simultaneouslyto each other and to a substrate at their points of contact by means of spotwelding, especially when the substrate surfaces are curved. A good bond between the various metal wires and a good bond between the wires and the substrate may be obtained by sintering and in particular pressuresintering and therefore, this method is preferable by far.

It is remarked, that a compound material for bone prostheses, said material comprising a compact metal core and a porous metal coating, has been disclosed in French patent No. 2,215,927. In that known material, the metal coating comprises a number of layers of perforated metal foil which have been joined to each other and to the core by means of spotwelding. In contrast thereto, the present invention makes use of metal wire mesh and preferably, of a pressure-sintering method.

Afirst advantage of pressure-sintering above normal sintering is thatthe wires of the wire mesh are pressed against the substrate during the sintering process, so that a good contact is achieved for causing sinterneckformation. Moreover, pressuresintering has the advantage of allowing the sintering temperature to be lower.

The metals which are preferably used for wire mesh and substrate, can normally be sintered at temperatures above 1100 C. However, when such high temperatures are applied, the mechanical properties of the substrate (tensile strength, ductil ity, fatigue strength) are greatly reduced and this constitutes a problem. By sintering under pressure, the sintering temperatures can be selected about 200-500 C lower, and thereby this problem is eliminated or at least considerably reduced.

It is pointed outthatthe use of pressure-sintering for obtaining a layer of porous structure, as in the present invention, is in sharp contrast to the present state of the art. Pressure-sintering is normally used to obtain compact sintered objects having the greatest possible tensity. Even the theoretical density of the material may often be attained. Further, appiication of pressure-sintering to a mass of randomly oriented metal wires would also cause a considerable increase in density of the material. In the present invention, however, thanks to the use of wire mass, a pressure sintering method may be used that will not cause such high density (and a loss of porosity).

The pressure-sintering process may be performed with equipment that is commercially available and that will provide for the required pressure as well as for the required temperature and atmosphere.

The temperature and duration of the sintering process and the pressure applied therein are strongly dependent from the type of metal used. In addition, these factors are mutually interdependent, since an increase in sintering pressure or duration automatically implies a decrease in the required sintering temperature. In general, it can be stated that pressure sintering has good results at temperatures of 600-1000 C, pressures of 1-150 MPa and durations of from 5 minutes to 8 hours. Within these general limits, however, more detailed specifications are possible according to the type of material used.For instance, stainless steel may be sintered well under pressure at 850-950 C, whereas for titanium the temperature is around 650-700 C and for titanium alloys around 850-950 C. The pressures and durations corresponding thereto may be easily determined by means of experiment.

The effect of pressure-sintering may be improved if the wire mesh is pressed against the substrate in a separate operational step prior to sintering. A contact surface rather than a point or line of contact is already present then at the start of the sintering operation and this may be most advantages for obtaining a good metallurgic bond. Pressures of 10-600 MPa may be used in this pre-compacting step, depending on the metals of wire mesh and substrate and on the mechanical properties and weaving characteristics of the mesh.

There is no change in processing if more than one layer of wire mesh is used. The various layers are superimposed on each other and on the substrate in a desired position and thereafter, the product as a whole is subjected to pressure-sintering, sometimes preceded by a pre-compacting step. Although the phenomena that will now appear are more complicated since metallurgic bonds have now to be produced between the crossing wires of each layer, between the crossing wires of two consecutive layers and alsd between the wires of the lowest layer and the substrate, it is still possible to perform these phenomena simultaneously in one single sintering operation.

Thanks to this simple method, an economic manufacture of the product and even mass production will be possible.

The product of the invention as obtained by means of the described method, is a compound material comprising a dense metal substrate carrying a porous upper layer of sintered wire mesh. Thanks to the use of relatively low sintering temperatures, the substrate has optimal mechanical characteristics.

Thanks to pressure-sintering in one single operation, a good bond between the wire mesh and the substrate and between all crossing wires in the wire mesh amongst each other is obtained. The porous layer has pores of uniform size which are evenly distributed over the layer. Any desired pore size, and in particular also a relatively large pore size, may thereby be obtained.

The compound material according to the invention is especially suitable for application in implantation prosthetic devices since the porous layer on the substrate constitutes an excellent medium for propagating the ingrows of bone tissue and thereby achieving a durable fixation of the prosthetic device to the bone. The material may be used in many orthopedic prostheses, such as prosthetic devices for hips, knees and other joints. It is possible thereby to encase non-metallic parts of these prosthetic devices into the metal substrate carrying the porous upper layer. Further, the compound material may be used in dental prostheses, e.g. prosthetic devices for bridging defects in the jaws. In all these applications, excellent results may be obtained with respect to the ingrows of bone tissue into the porous layer and a durable fixation of the prosthetic device to the skeleton.

Some non-limitative embodiments of the method according to the invention will follow now.

Example 1 Upon a flat metal substrate of stainless steel AISI 316 L, a layer of wire mesh of the same material was laid.

The wire mesh had a flat weave with 20 wires per inch in each direction and a wire diameter of 0.375 mm.

The wire mesh was pressed against the substrate at room temperature and at a pre-compacting pressure of 150 MPa. Thereafter, the wires were fastened to each other and to the substrate by pressuresintering with a normal pressure-sintering press. A sintering temperature of 875 C was maintained during one hour at a pressure of 10 MPa. The result was a compound material comprising a porous upper layer of metal wire mesh on a metal substrate in which the upper layer adhered well to the substrate and showed a uniform porosity and pore size.

Example 2 The method of example 1 was repeated with the exception that sintering now lasted for 2 hours at a temperature of 900 C and a pressure of 10 MPa. The result was a compound material having similar characteristics as in example 1.

Claims

1. A compound material comprising a porous layer of metal wires on a metal substrate, character ized in that the porous layer is built up from at least one layer of metal wire mesh in which the wires are joined by metallurgic bonds to each other and to the substrate at the points of contact.

2. The material of claim 1, characterized in that the metal wires of the wire mesh are composed of stainless steel, titanium, titanium alloys, cobalt alloys, tantalum, tantalum alloys, niobium, niobium alloys or zirkonium.

3. The material of claim 1, characterized in that the wire mesh is a woven fabric.

4. The material of claim 1, characterized in that the wire mesh has a wire diameter of 0.025-1.25 mm and comprises 400 to 6 wires per inch in each direction.

5. The material of claim 1, characterized in that more than 1 layer of wire mesh is present on the substrate.

6. A method of producing a compound material comprising a porous layer of metal wires on a metal substrate, characterized by applying at least one layer of metal wire mesh onto a metal substrate and joining the wires of the mesh to each other and to the substrate by metallurgic bonds at their points of contact.

7. The method of claim 6, characterized in that the wires of the wire mesh are composed of stainless steel, titanium, titanium alloys, cobalt alloy, tantalum, tantalum alloy, niobium, niobium alloy or zirkonium.

8. The method of claim 6, characterized in that the wire mesh is a woven fabric.

9. The method of claim 6, characterized in that the wire mesh has a wire diameter of 0.025-1.25 mm and comprises 400 to 6 wires per inch in each direction.

10. The method of claim 6, characterized in that more than one layer of metal wire mesh is applied onto the substrate and connected therewith.

11. The method of claim 6, characterized in that the metallurgic bonds are formed by means of pressure-sintering.

12. The method of claim 7, characterized by a sintering temperature between 600 and 1000 C, a sintering pressure between 1 and 150 MPa and a sintering time between 5 minutes and 8 hours.

13. The method of claim 6, characterized by a pre-compacting stap prior to pressure-sintering.

14. The method of claim 13, characterized in that pre-compacting is effected at a pressure of 10-600 MPa.

15. A compound material resulting from the method of claims 6-14.

16. A prosthetic device made from the compound material of claims 1-5 or 15.

17. Acompound material according to claims 1-5, substantially as hereinbefore described with reference to the accompanying drawing.

18. A method according to claims 6-14 substantially as hereinbefore described with reference to the accompanying drawing.