CA1186107A - Compound material for prosthetic devices - Google Patents

Abstract

A B S T R A C T

A compound material, suitable for making prosthe-tic 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 metal but preferably by pressure-sintering, sometimes preceded by a pre-compacting step. Sintering temperatures between 600°C and 1000°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.

Description

COMPOUN3 MATERIAL FOR PROSTHETIC DEVICES.
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This invention relates to a compound material comprising a porous upper layer of metal wires on a metal substrate, and to a met~od of producing th.e same. Such a compound material may potentially be used in ïm~lantation ~rosthetic devices, where the porous layer is coming into direct contact with. the surrounding bone tissue and allows the bone tissue to grow into th.e porous upper layer so as to achieve.a firm anchoring of the prosthetic devices.
lQ 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 implantation into the remaining part of th.e skeleton. This may lead to various problems, includin~ those per~aining to excessi.ve 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 dura~le fixation, such as ~echanical impactation of a prosthetic stem into the 2Q - medullary cavity of the bone; mechanical fixat~.on by means of screws; and fixation by means of bone cement polymeri-zing in situ. The latter method in parti.c~lar is used frequently, with methyl methacrylate as cement. The pro~lem 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 achievin~ dura~le ~.ixation which make use o the bondin~ and adh.erin~ ability of th.e surroun-ding bone tissue have been suggested recently~ Examples of these methods are: the use of indentations, ~.ib~ed or rough areas on the surface of the prosthetic device to cause bone tissue to grow over them; the use of biolo~ically active materials that may react with the bone tissue, thus forming a solid bond; and the use of porous layers or porous 35 elements into wh.ich. the bone tissue may penetrate and which it may grow together so as to provï.de a stable anchoring.
The invention is concerned in particula.r with the last-mentioned method.
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2--For prosthetic de~ices ha~ing a porous layer or element, various embodiments ha-~e already ~een proposed.
It has turned out that the use of a porous upper la~.er on a substrate is preferable to an entirely porous ohject in view of the mechanical requirements, such.as tensile stren~th and fatique strength, that should satisfied in the prosthetic device. It has ~een outlined moreover, that metals in view of their bette.r mechanical properties such as tensile strength.and ductility, are more satisfactory . 10. than ceramic materials or polymers for constituting the material of the porous layer ~f 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 havin~ a hi~her avera~e pore size and considerably better stren~th. and ductility with the same porosity, may be obtained with metal wires.
U.S. patent No. 3,~06,~5Q to Rostoker and Galante discloses a prosthetic device of the.implantation type which. in an embodiment comprises a me.tal stem as a 2~ load-bearing element surrounded by a porous layer of th.in metal wires wh.ich.have been sintered to~ether. The porous layer has an open pore structure .and is intended to allow the ~one tissue to grow into the pores in the manner described above;, in order to ach.ieve anchox$ng.
In this known prosth.etic.device, the porous layer is made hy fold1ng 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 fihers and sintering th.is network in order to create metallurgic ~onds between t~e fi~ers at th.eir points of contact.
The porous layer thus obtained may be fastened to th.e metal stem of the prosthetic device hy means o sintering or some other method.
As a consequence of the manuf~cturing method usea herein, the metal wires .in the porous layer of the known prosthetic device are rather randomly ori.entated. The result is an open pore struct~re indeed, ~ut the s.h.a~e. and dimen-sions of the pores, as well as tlle de~ree: o~ porosity~

may vary ~uite 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 consia-erably the liberation of metal ions ln vivo. Finally, the fastening o~ 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 simpler than the known method and which allows large scale production.
The invention provides a prosthetic device which is useful for implantation and comprises a porous upper layer of metal wires on a metal substrate, which material is characteri~ed in that the porous layer is huilt up from at least one layer of metal wire mesh in which the wires are joined by metallurgical bonds to each other and to the substrate at their points of contact, said porous layer of metal wires being suitable for bone tissue ingrowth.
In addition, the invention provides a method of producing such material, which is characterized by applying at least one la~er of metal wire mesh onto a metal suhstrate and by metallurgically bonding the wires of the mesh to each other and to the substrate at the points of contact, said bonding being -~3 created ~y pressure sintering at a temperature between 600 and 1000C, a pressure between 1 and 150 MPa, for a period of between 5 minutes and 8 hours.
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 adjusta~le pore size as well as uniform porosity may be obtained.
In addition, it becomes possible to obtain any pore size and porosity desired, and, in particular, to make larger-size pores without any harm to the mechanical strength or integrity of the porous layer. Furthermore, a smaller ratio of specific surface to volume of the material to be implanted is ensured, which reduces - 3a -, t~

the li~eration of metal ions in ~iYo. ~oreover, another advantage of the pro~uction method of the i~nvention is th~t the formation of -the porous layer, the fastening of the metal wires to each other and the attachment of the S metal wires to the substrate may now ~e effected practically in one single operation, which ~rings ab.out a considerable simplifi.cation and which allows large scale production.
In order to give a better understandin~ of the in~ention, reference is made to the drawing, ~hich shows .10 in perspective ~iew and by way of example, a compound material according to the invention. Th.is mat~rial com~rises a metal substrate 1 carrying a layer of metal wire mesh. 2.
The mesh 2 is a woven fabric of flat weave, ~aviny warp wires 3 and weft wires 4. It is c].early visi~le that the 15 wires 3 and 4 are joined to each.other and to the substrate S by metallurgic bonds at their points of contact, in such a way that a strong bond is achieved. Between the various wires 3 and 4 their remain pores 5 and 6, which are uniform in shape and size thanks to the type of wea~e used. Although 20 only one layer of wire mesh has been shown, it will he clear that a numb.er of layers of wire mesh. may be present in superimposed condition, the wires th.ereof ~eing ~oined to each other a~d to the wires of a ~revious layer by means of metallurgic bonds.
The invention will now be descrihed in more detail.
Su~strate 1 may consist oflany metal which. is compati~le with. livi.n~ tissue, such.as, for instance, stainless steel, titanium, titanium alloys, Co-Cr-alloys, tantalum, tantalum alloys, niob.ium~ niobium alloy5, or 30 ~irconium. A metal is preferred aboYe other materials such.
as ceramics or polymers, since the su~strate when used in prosthetic devices will normally be the load-bearing element;
therefore, it must have good mech.anical strength and a good ductility. The substrate may have any appropriate 35 shape, for instance the shape of a flat plate, a round rod or the like. It is not absolutely necessary th~t th.e suh-strate has already the shape desired for a prosth.etic de~ice, since it may be rendered i.nto that shape lateron.

The wires of wire mesh. 2 may also be of any metal which.is compatible with. li~in~ tissue. In principle, the same metals can be used for the wires as for the sub-strate. Althou~h the porous layer of wire mesh. is not intended to serve directly as a load-carrylin~ ele~ent, it must nevertheless assure a strong ~ond wit~.the bone tissue and a still stronger internal coh.e.rence of the com~ound material. Therefore, metals are'pre:Eerred above'other materials here, too.
lQ hTire mesh 2 ensures a porous layer of almost uniform mechanical properties, compris-in~ pores 5 of uni-form size. This mesh. can be eith.er woven or knitted, al thou~h.a wo~en mesh. is prefera~le. Furth.er it is important that the wires of the mesh cross each oth.er at ~any points, thus creatin~ the important pores 6', wh.ich.are su~table ~or ingrows of bone tissue and thereby .~or 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 ~etween wide limits, 2Q depending on the pore:size and ~orosity required. Thus, ~etal wires havin~ a diameter ~etween ~Ø25 and.l.25 mm may be used and the number of wires per inch. in any direc-tion may vary between 400 and 6. When th.ese ranges are . observed, it is ensured that the mesh.~i.dth. or pore si.ze in the mesh.is always lar~er than the desired minimum size for in~rows of. the intended tissue i.nto the pores.' It i.s not necessary that th.e wire mesh.'has already th.e desired shape and dimensions for a prosthetic device at this stage, but shape and sizes sh.ould be adapted to

3~ s.o~e extend to those of the substrate used.
One or more layers of wire ~esh.may be used at will, dependin~ 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 sta~ered fashion, 3S in ordèr to obtain a better bond of the vari.ous layers amon~ each.other. However, the possi~i.li.ty of a number o~
layers of equir mesh. width in unsta~e.red superim~osed condi.tion is not excl~ded.

3~ ?7 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 metallur~ically bonded to each.oth.er and to the sub-strate at their points of contact.
The metallurgical bond is preferably obtainedby sintering and still more preferably ~y pressure sintering~
Alth.ough, in principle, such.a bond could also ~e obtained by spotw.elding, this method i5 not preferred because in lO. practice it appears to ~e nearly impossïble to weld a lar~e num~er of ~etal wires simultaneously to each.other and to a substrate at their points of contact by mea.ns of s~ot-welding, especially when the substrate surfaces are curved.
A good bond between the various metal wires and a ~ood bond between the wires. and the substrate may b.e o~tained by sintering and in particular pressure-sintering and there-fore, th.is method is preferable ~y far.
It is remarked, that a compound material for bone prostheses, said material comprising a compact metal ~0 core and a porous metal coating, has b.een disclosed in French. patent ~o. 2,215,927. In that k.nown material, the metal coating comprises a number of layers of perforated~
metal ~oil which.have been joined to each other and to th.e core ~y means of spotwelding. In contrast thereto, the 25 present invention makes use of metal wire mesh and prefera-bly, of a pressure-sintering method.
A first advantage of pressure~sintering ab.ove normal sintering is that the wires- of t~e wïre mesh. are pressed against the sub.strate during the sinteriny process, .3Q so that a good contact is achieved for causing sinternec~
formation. Moreover, pressure-sintering has the a~vantage of allowing the sintering temperature to be lower.
The metals which. are pre~era~ly used ~or ~ire mesh.and substrate, can normally he sintered at temperatures above llQOC. However, when such.hi.gh temperatures are applied, the mechanical properties of the substrate (tensile strength., ductility, fatigue strength.l are greatly reduced and this constitutes a problem. By sinterin~ under pressure, th.e sinLering temperatures can be selected about 200-50QC
lower, and thereby his problem is el;~inated or at least consïderably reduc~d.
It is pointed out that the use of pressure-sinte-ring for o~taining 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 h.avin~ th.e greates-t possi~.le tensity. E~en th.e theoretical density o~ the material may lQ often ~e attained. Further, application of pressure-sinte-riny to a mass of randomly oriented metal wires would also cause a considerable increase in~den$.ity of th.e material.
In the present invention, howe~er, thanks to the use of wire mass, a pressure sinterin~ method may be used that will not cause such h.igh density (and a loss of porosity).
The pressure-sintering process may ~e performed with.e~uipment that is commercially available and that will provide for the required pressure as well as for thR requi-red te~perature and atmosphere.
The temperature and duration of th.e sintering process and .he pressure applied therein are strongly depen-dent from the type of metal used. In addition, these fac-tors are mutually interdependent, since an increase in sintering pressure or duration automatically implies a decrease in the required sinterin~ temperature. In general, it can be stated that pressure.si.ntering has good results at temperatures of 60Q-10.00C, pressures of :l-lSQ MPa and durations of from 5 minutes to 8 hours. Withln these general limits, however, more detailed specificati.ons are 3Q possible according to the type of material used. For instance, stainless steel may be sintered well under pressure at 85Q-95QC, whereas for titanium the temperature is around 650-700C and for titanium alloys around 850-95~C. ~he pressures and durations corresponding thereto may be easily dete.rmined by means of experiment.
The effect of pressure-si.nterin~ may be imp ro~ed if th.e wire mesh is pressed against th.e su~strate in a separate operational step prior to sintePi.n~. A contact suface rather than a point or line of contact is already present t~.en at th.e start o the sinterin~ operation and th.is may ~.e most advantages for o~taining a good metallur-gic ~.ond. Pressures of 10-600 MPa may be used in th.is pre-compacting step, depending on th.e metals of wire mesh and su~.strate and on the mechanical propertïes and weaving characteristîcs of the mesh.
There is no change in processin~ ï.f more than one layer of wïre mesh is used. ~he vari`o~s layers are superimposed on each.other and on the su~strate in a desired position and thexeafter, th.e product as- a whole is su~jected to pressure-sinterin~, sometimes preceded by a pre-compacting step. Althou~h. the. phenomena that will now appear are more complecated sïnce metallur~ic bonds have now to be produced hetween the crossing wi.res of each layer, between th.e crossing wires of two consecutive layers and also between the wires of th.e lowest layer and the su~strate, i~t is still possible to perform these phenomena simultaneously in one single sintering operation.
.Thanks to this simple method, an econïrnic manu-facture of the.product and even mass production ~lill he possi~le.
The product of th.e invention as obtained by means of th.e described method, is a compound material comprisin~
a dense metal su~.strate carrying a porous upper layer of sintered wire mesh. Thanks to the-use of relati.vely low sinterin~ temperatures, the substrate has optimal mechanical chacteristi.cs. Thanks to pressure-sintering in one single operation, a good bond between the wire mesh. and the substrate and between al crossing wires in the wire mesh amongst each. other is obtained. Th.e porous layer has pores of uniform size which are evenly distri~uted over the layer.
- Any desired pore size, and in particular also a relati~ely lar~e pore size, may thereby be o~tained.
The compound material according to th.e invention is especially suitable for application in implantation prosthetic devices since the porous layer on th.e substrate constitutes an exceIlent medium for propagating the in~rows of bone tissue and there~y achieving a durable fixation of the prosthetic device to the ~one. 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 de~ices in~o the metal substrate carrying the porous upper layer. Further, the compound material may ~e used ïn dental prostheses, e.g. prosthetic devices ~or br;!d~ing defects in the jaws. In al these applications~ excelle.nt results lQ may ~e obtained with respect to th.e in~rows of ~one tissue into t~e porous layer and a dura~.le fixation of the prosthe-tic de~ice to the skeleton.
Some non-limitative em~odiments o.f th.e 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 th.e same material was laid.
The wire mesh.had a flat weave. with.20 w.ires per inch. in each direction and a wire diameter of a . 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 suhstrate by pressure-sinterin~ with. a normal pressure~sintering press. A sintering temperature of 875C
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 adhere'd well to the substrat 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 900C and a pressure of 10 ~Pa. The result was a compound material having similar characteristics as in example 1.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A prosthetic device useful for implantation comprising a porous layer of metal wires on a metal substrate, 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 metallurgical bonds to each other and to the substrate at the points of contact, said porous layer of metal wires being suitable for bone tissue ingrowth.

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

4. The device 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 device of claim 1, characterized in that more than one layer of wire mesh is present on the substrate.

6. A method of producing a prosthetic device suitable for implantation 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 metallurgical bonds at their points of contact, said bonds being created by pressure sintering at a temperature between 600° and 1000°C, a pressure between 1 and 150 MPa, for a period of between 5 minutes and 8 hours.

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 alloys or zirconium.

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 metal-lurgical bonds are formed by means of pressure-sintering.

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

13. The method of claim 12, characterized in that the pre-compacting step is effected at a pressure of 10-600 MPa.