Composite and porous metallic members which can be used for bone prosthesis

Abstract

A prosthesis comprises a metal member of for example titanium or tantalum and includes a solid core or substrate and a covering of perforated metal of a porous nature. The perforations are preferably arranged in the foil so that when laminated the complete covering has pores of irregular shape. This irregularity assists in the keying of bone substance to the prosthesis. The minimum pore transverse dimension is 50 microns. A method is also described which involves dipping the member into a suspension of the hydride of the member of the core and subsequently heating the core at high temperature.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite and porous metallic members which can be used for bone prosthesis and also relates to methods of their manufacture. Such metallic members are termed implants.

2. Summary of the Prior Art

It has been proposed to use for certain types of implants, titanium, tantalum, and alloys with a cobalt base and in particular the alloy chrome-cobalt-molybdenum, known under the trade name "Vitallium" (According to Weisman, Annals of the New York Academy of Science, Vol. 146, article 1, pages 80-95, Jan. 8, 1968, Vitallium is an alloy, on a percent by weight basis, of: 25.5-30 Cr, 5-7 Mo, up to 0.35 C, up to 1.0 Mn, up to 1.0 Si, up to 2.0 Fe, up to 3.75 Ni, with the balance being Co.) One of the main problems to resolve was the bonding of the implants to the bone substance.

The following four solutions to this problem have already been proposed:

1. Force fit into the medullary canal of the bone an anchoring device or the prosthesis device itself.

This solution has several disadvantages which have caused it to be abandoned:

A. the effective contact surface between the bone and the implant is small. The joint is thus insecure;

B. the radial constraints exerted on the bone are dangerous or damaging to the integrity of the latter. They are concentrated at a small number of contact points because of the difficulty which arises in adjusting exactly the metallic part to the canal in which it must be implanted;

C. the bone, when submitted to a permanent stress, reacts in such a manner that it tends to reduce this stress. This reaction is the result of the rheological properties of the bone material which undergoes an extrusion and an actual biological change. The result is, in the more or less long term, the loosening of the implant.

2. Securing of the implant in the bone with the aid of screws or pins. Clinical experience has shown that in the long term this mode of fixing loses its rigidity probably as a result of similar changes to those which have been referred to above.

3. Bonding of the implant by a plastic methacrylic resin which hardens by polymerization. The metallic part is provided with extensions or keying means which are introduced into the natural or into artificial cavities of the bone. These cavities are filled with the plastic resin. After the hardening it ensures the required fixing by bonding.

This technique is used to a large extent. It has enabled substantial progress in the repair of necks of femurs which have been broken and hip joints which have been damaged by arthritis. The methacrylic resin is not however always tolerated completely by the organism and may give rise to detachment which cannot be repaired.

4. A recent method consists in using porous metallic implants or composite metallic implants including a nucleus, core, or substrate of solid metal and a porous coating or layer which adheres to the surface of the core or substrate and which covers it at least partially. The pores of the coating are for the most part open pores, that is to say communicating with the exterior. The implant being placed in contact with the bone substance, the bone substance itself grows into the pores. There is thus produced in a few weeks a true biological anchorage of the implant.

The porous body or the porous layer is formed by small discrete particles of metal welded together and for a proportion of them to the substrate or core. These welded connections only occupy limited space so that between the particles free spaces are left which provide the desired porosity. These particles may be cylindrical or spherical.

The cylindrical particles are formed by fragments of extruded wire of small diameter. The spherical particles may be made by the procedure known in the industry by the term atomization. Now, titanium like tantalum is an extremely reactive metal at high temperature, so much so that its atomization presents problems which can only be resolved by the use of exceptional means in costly installations with handling and maintenance which requires great care. The extrusion of Vitallium into fine wire gives rise to problems which are almost insurmountable.

Moreover, the contact between the spherical or cylindrical particles before the welding operation is reduced to a point or to a line. It is thus necessary that the operation of welding should itself give rise to the growth of a contact surface of larger size when filaments or atomized powders are sued. This welding can only be effected by roasting, that is to say, by thermal treatment at elevated temperature, about 1200.degree.C in the case of Vitallium, 1100.degree.C in the case of titanium. At these temperatures diffusion phenomena in the solid phase give rise to the joining of the elements in contact by the growth of joining surfaces from the contact points. These phenomenon are slow and growth of the contact areas slows up when the connecting or joining surfaces increase. It is necessary therefore to rely on excessive thermal treatment times in practice if it is desired that the connecting surfaces of the spheres to the core should be equal for example to a large circle or proportion of these spheres. As micro-photographs reveal, sections of the porous body or of the composite body connecting surfaces between particles or between particles and the core are very small and these impair the integrity of the bonds between the bone and the metal.

SUMMARY OF THE INVENTION

According to the present invention there is provided a composite metallic member comprising a core, and a porous covering welded to the core, said covering having a thickness between 5 and 50% of the maximum transverse dimension of the core and said covering comprising a plurality of layers of foil, the foil having a thickness between 0.05 and 0.5 mm. and having perforations, the covering being formed by welding the foil layers so that the perforations together form passages of which the minimum transverse dimension is at least 50 microns.

In order properly to explain the basis of the invention as well as the advantages which it provides in relation to known methods, it is desirable first of all to list the qualities which should be possessed by a metallic porous prosthesis or a prosthesis having a porous outer layer.

It is essential first of all that the metal of the particles should be the same as that of the core in the case of composite implants. Without this, corrosion of electrolytic origin may be produced. With biological liquids which contain in solution ionized elements, the two different metals form an electrical cell.

The pores must be open and form passages of microscopic size into which the bone substance can grow up to the compact core. Closed pores, that is to say pores which do not open to an accessible surface at the bone reduce the integrity of the bond between the bone and the metal and are thus to be avoided as far as possible.

So that the bone cells can invade the pores, it is necessary that the latter should have an opening of at least 50 microns across. Only considerations of mechanical rigidity can impose an upper limit to the size of the opening of the pores. Depending on conditions it may reach or even exceed 400 microns.

From what has been stated above, the length of the passages perpendicular to the surface of the core is equal to the thickness of the porous layer. It is larger in general if the passages are tortuous or otherwise irregular.

The bond must be integral and it is this requirement for integrity to be maintained over very long periods that it has been necessary to define the optimum values for the thickness of the porous layer, of the shape, the dimensions, the number and the distribution of the pores.

During their use a prostheses, the implants according to the invention are subjected to two kinds of forces of which the line of action are perpendicular; shearing forces and tension forces.

The shearing forces acting parallel to the direction of the longitudinal axis of the core create complementary compressive stresses. It is necessary that these stresses, exerted on the material of the bone, should not give rise in the interior of this material to a deleterious change comparable to that which has limited the fixing methods for the implant by force fit in the medullary canal or by the intermediary of pins and screws. It is thus necessary that the surfaces on which the compressive forces are exerted under the action of a given imposed force should be sufficiently large so that the complementary stresses should remain low. As a result, it is necessary that the connecting surface between the metal and bone should be adequate and that the thickness of the porous layer should also be adequate. Each implant constitutes a specific case. The dimensions of the bone and as a result of the prosthesis being adapted to the forces to be resisted and, in practice, the thickness of the porous layer should be between 5 and 50% of the diameter of the core or substrate, and preferably between 10 and 25%.

The bond of the bone and metal may be subjected to tension forces in a direction perpendicular to the longitudinal axis of the core. In this case, the porous covering in accordance with the invention will not generate any resistance to the separation of the two elements, bone and metal, where the passages present in the interior of the porous covering are straight and perpendicular to the longitudinal axis of the core. It follows that if the passages are tortuous, the porous covering ensures an improved connection between the bone and the implant.

The porous covering is constituted by several layers of metallic foil, preferably of very light gauge, pierced by numerous apertures and welded to one another as well as to the core or substrate. The metallic foils are pierced by perforation method with raising of the material and/or stamping, or by expanding to form a mesh. It is also possible to use mesh formed by weaving wire. The perforations occupy a sufficient fraction of the surface of each foil so that by superposing said foils the whole portions of one will not block the apertures of the other. On the contrary, the perforations by their superimposition constitute capillary passages or ducts of tortuous or labyrinth nature which reach to the surface of the core as well as to the free surface of the composite body. The perforated metallic foils must be sufficiently supple and malleable so that they can take-up the shape of the surface of the core or substrate.

In order to achieve the required flexibility, the laminated sheets used as starting material must have a thickness less than about 0.5 millimetres. In the case of titanium, it is difficult, by laminating, to go below 0.05 millimeters. Metallic foils of 0.1 to 0.25 millimeter thickness are therefore preferably selected.

Similarly in order to achieve the desired flexibility and moreover so that the metal should be readily deformable, metal in the annealed state is preferably used.

The perforations are defined by their shape, their dimensions and their distribution. In general, manufacturers provide these data in the form of drawings. They add to this the fraction of the space cut away by the apertures which they denote by the term "transparence."

The resistance to fracture should be mainly considered at the level of the junction between the core and the perforated covering. This resistance is proportional to the surface welded. The highest forces which can be applied to a human bone are those which may be applied to the neck of a femur. These forces are estimated to be six times the weight of the individual up to a maximum value of 600 kg; these forces cause in the narrowest zone of the neck of the femur a maximum stress of the order of 3 kg/mm.sup.2. The fracture load of the metals or the alloys used is substantially in excess of this value (in the case of titanium it is of the order of 30 kg/mm.sup.2) and it will be apparent that the fracture will not arise in this part of the prosthesis.

It is moreover, necessary to avoid all permanent deformation of the metal, in other words, it is necessary to avoid applying to the metal a stress which reaches its elastic limit. In the case of titanium, the elastic limit is 20 kg/mm.sup.2. The surface of welding should thus be in excess of 600/20 = 30mm.sup.2. If the metal employed initially has a transparence of 50% and if, the welding having been poorly carried out, the welded proportion represents only 20% of the metallic surface, it will be apparent that it is sufficient to cover with welded expanded metal, a surface of the compact core of ##EQU1##

In example 2, given below, it will be apparent that this value is in fact very substantially exceeded.

Other advantages of implants according to the invention in comparison with prosthesis previously known are, in particular, as follows:

The perforated or expanded sheets are current industrial products fabricated in a large variety of shapes and dimensions and sizes of perforations, of length, spaces which separate two successive perforations, of thicknesses of the initial sheet which it is possible to change after perforation, if it is required, by operations such as laminating.

The techniques of powder metallurgy do not enable the use of particles all of the same size and to orientate them with perfect regularity. The dimensions of the pores which can be obtained are always distributed at random; at best one can determine only certain limits. The perforations in the sheets used in accordance with the invention are all substantially identical and are distributed with exact regularity. The perforated or expanded sheets thus enable the complete avoidance of closed pores and enable the production of pores of relatively high uniformity.

The techniques of powder metallurgy necessitate the employment of compression molds, of presses, of vacuum ovens or ovens operating at atmospheric pressure controlled for treatments of long duration. In contrast, the operation of welding sheets may be carried out with equipment which is little different from that used with dental prosthesis. The perforated sheets may, moreover, be readily shaped by conventional brazing operations, by stamping or swaging likewise similarly to the prostheses used in dental work.

The surfaces of the weld joint may have sizes as large as is required.

The invention relates also to another type of improved implant characterized in that the external surface of the porous covering of an implant carries fine particles of the same metal as the implant, of a diameter less than 10 microns and welded to this surface.

These particles provide anchorages and supplementary contact points along the external surface of the porous covering and thus reduce the shear stresses exerted on the said surface.

The invention also relates to a method of manufacturing the improved implants characterized in that an implant in accordance with the invention is plunged into a suspension constituted by a mixture of ethyl cellulose, ethyl glycol and particles of metallic hydride of a diameter of less than 10 microns, the implant is withdrawn from the suspension and the excess of particles removed and the implant is treated for about 2 hours at high temperature under a pressure of approximately 15mn (millimicrons), viz, 15.times. 10.sup..sup.-6 mm of mercury.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic section of an implant in accordance with the invention;

FIGS. 2a, 2b, 2c, 2d, 2e and 2f illustrate Example 1 which relates to an implant in accordance with the invention for use in an animal experiment;

FIGS. 3a and 3b illustrate a second example as used in the repair of the neck of a human femur by an implant in accordance with the invention; and

FIGS. 4a and 4b illustrate the third example relating to the repair of the acetabulum of a human hip.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 an implant 1 is illustrated formed from a core 2 and a porous covering 3 constituted by metallic foils 4 of a light gauge and which are superposed on one another. The bone 5 grows into the pores of this porous covering 3. The implant is subjected to shear forces F.sub.c and to tension forces F.sub.t.

EXAMPLE 1

This example relates to the manufacture of an implant of titanium for use in an experiment on an animal.

FIG. 2a is a sketch of the implant to be manufactured, and the implant includes a drawn rod 7 partially covered with a porous covering 8. FIG. 2b is a view to a much enlarged scale of expanded metal forming the porous covering 8. FIG. 2c is a section on the line x-y of expanded metal used to build up the covering.

The method is commenced by taking a drawn rod of titanium of 3.17mm diameter and a sheet of expanded titanium. Before being expanded this sheet has a thickness of 0.1mm. After expansion the mesh has the form of lozenges of which the larger diagonal measurement is 1.45mm. The width of the remaining strips is 0.12mm. The optical transparency is 52%. The thickness overall of the sheet, after the expansion which causes a certain distortion of the strips, is 0.2mm.

To construct the member, the drawn rod and the expanded metal are first of all carefully degreased. The expanded titanium is cut into a strip of 12.7mm width. The drawn rod 7 is clamped in the chuck 6 of a lathe as shown in FIG. 2d, then welding is initiated at one end of the strip of expanded titanium 10 following a generating line of the cylindrical envelope of the screw. The lather is then turned slowly by hand, while exerting a substantial tension on the strip 9 of expanded metal which is rolled around the drawn rod 7. Welds 11 are effected on lines as designated in FIG. 2e. When the thickness required is obtained and the last line of welds is effected (FIG. 2f), the member is withdrawn from the lathe and the part formed by the rolled on mesh which has been secured to the rod, is plunged into a suspension of titanium hydride powder having the following composition:

Titanium hydride 30 parts by weight Ethyl cellulose in a solution of 4% in terpinol 30 parts by weight Ethyl glycol 10 parts by weight.

The member is withdrawn from suspension tapped to remove the excess of the latter, dried and then treated for 2 hours at 1,100.degree.C under a pressure of 15.times. 10.sup..sup.-6 mm of mercury. The completed rod is then cut to the required size.

EXAMPLE 2

This example relates to the repair of the neck of a human femur.

FIG. 3b is a section on a line A--A of FIG. 3a.

In these two figures, it is seen that the core 12 of forged titanium is covered with a porous part 13. The porous part is produced by cutting up elements of expanded titanium to give the following dimensions:

large diagonal of the pattern of the base 0.75mm.

width of the joining threads 0.12mm.

initial thickness of the sheet 0.10mm.

overall thickness of the expanded metal 0.20mm.

transparence 40%.

Strips of this titanium are cut which are welded to the core or substrate. Eight layers are used which give rise to a thickness of about 1.5mm. and the surface of the substrate of the porous layer is about 40 cm.sup.2

EXAMPLE 3

This example relates to the repair of the acetabulum of a human hip.

FIG. 4a is a perspective view of the prosthesis and FIG. 4b is a section on the line A--A of FIG. 4a.

In these two figures a hemispherical core 14 is covered with a porous covering 15. Broken lines in FIG. 4a indicate a prosthesis for the neck of the femur. The same expanded titanium is used as in the preceding Example. Discs are cut and made hemispherical by pressing. These part-spheres are laid on the convex part of the core to which they are spot welded. The covering is then impregnated with a suspension of titanium hydride of the same composition as that of Example 2, but has been crushed or pulverized until all the particles of titanium hydride have a diameter at the most equal to 10 microns.

After drying the prosthesis is treated in a vacuum of 15.times. 10.sup..sup.-6 mm at 1,100.degree.C.

It is possible to provide the concavity with a porous layer in order to key a plastics material having lubricant properties.

Claims

I claim:

1. A composite metallic member comprising

a core, and

a porous covering welded to the core, said covering having a thickness between 5 and 50% of the maximum transverse dimension of the core and said covering comprising

a plurality of layers of foil, the foil having a thickness between 0.05 and 0.5mm. and having perforations, the covering being formed by welding the foil layers so that the perforations together form passages of which the minimum transverse dimension is at least 50 microns.

2. A member according to claim 1, wherein the metal of the core and covering is selected from the group titanium, tantalum, alloys of titanium, alloys of tantalum and Vitallium, the latter being an alloy, on a percent by weight basis, of: 25.5-30 Cr, 5-7 Mo, up to 0.35 C, up to 1.0 Mn, up to 1.0 Si, up to 2.0 Fe, up to 3.75 Ni, with the balance being Co.

3. A member according to claim 1 wherein the said passages are irregular.

4. A member according to claim 1 wherein the foil takes the form of expanded metal.

5. A member according to claim 1, wherein the covering is in the form of a sheet of annealed foil formed into a laminate.

6. A member according to claim 1, comprising fine particles of the same metal as the core and covering welded to the surface of the covering and having a diameter less than 10 microns.

7. A member according to claim 1 wherein the thickness of said covering is between 10 and 25% of the maximum transverse distance of the core.

8. A member according to claim 1 wherein the foil is wire mesh.

9. A member according to claim 1 wherein each layer of foil has a thickness of 0.1 to 0.25 millimeters.

10. A member according to claim 1 wherein the metal of the core and covering is titanium.