Surgical Prosthetic Device With Porous Metal Coating

Abstract

A novel surgical prosthetic device having many useful surgical applications comprises a composite structure. The composite consists of a solid metallic material substrate and a porous coating adhered to and extending at least partially over the surface of the substrate. The porous coating has certain critical characteristics, and the individual values depend on the end use to which the device is put. There are described a number of surgical and dental applications.

Parent Case Text

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No. 148,316 filed June 1, 1971, now abandoned.

Description

FIELD OF INVENTION

This invention relates to surgical prosthetic devices.

BACKGROUND TO THE INVENTION

The use of surgical prosthetic devices, otherwise known as implants, is well known in various surgical applications, such as reconstructive surgery, for example, in the replacement of hip joints or the like. These applications generally involve the use of an implant constructed of metal or alloy which substantially is not corroded or otherwise degraded by body fluids. These prior art implants, however, suffer from a number of defects.

Typically, in the setting of broken bones metal plates have been used which are secured to either side of the bone fracture. The plates are commonly secured to the bone by screws. While the plate in time becomes encapsulated in bone and body tissue, no bond is formed between the implant and the tissue. If one of the screws comes loose, the patient may have to undergo additional corrective surgery.

Suggestions have been made in the prior art to provide surgical prosthetic devices which are capable of permanent incorporation into the body, usually the bone with bonding between the implant and the tissues.

In one prior art suggestion, there is described a prosthetic device consisting of a metal substrate or base having a thin porous coating of metal overlying and bonded to the surface. The presence of the pores allows the soft or hard tissue to grow into the porous coating of the device and hence achieve incorporation into the body.

The only method of forming the coating which is described in this prior art suggestion is the technique of plasma or flame spraying onto the metal substrate. The result of this process is a densely adherent layer of the sprayed metal on the substrate metal with no porosity or practically no porosity at the interface between the coating and the substrate and with gradually increasing porosity, including increasing pore size and decreasing density, from the interface to the surface of the coating.

While this technique may be effective in providing a porous coating on a metal substrate, nevertheless the technique results in a very serious defect in the finished prosthetic device. In tests designed to show the in-growth of tissue into the coated surface of the device a pin, having the coating thereon, after embedding in a bone for a period of time was subjected to a pull-out test. This pull-out test resulted in shearing at the interface between the coating and the base metal. This result indicates that the overall strength of the device is less than the bone. Quite clearly, the provision of a device weaker than the bone to which it is attached could result in failure of the device due to shearing at the interface with harmful and painful consequences for a patient who is treated using such a device.

This defect of this device is a direct result of its method of manufacture. Plasma spraying is a well known technique and generally is employed where it is desired to achieve a low porosity coating, often entirely pore free. Very thin plasma coatings therefore tend to be very dense and a progressive increase in pore size and decrease in density is a commonly-known result. If plasma spraying is continued eventually a uniform pore size of the coating is achieved, but the thickness of the coating required is at least ten thousands of an inch.

Another result of plasma or flame spraying is that a very hot molten mass impinges onto a relatively cold substrate surface causing the setting up of considerable interfacial thermal stresses which result in an inherent weakness which manifests itself in the interfacial shearing action observed in tests.

Another prior art suggestion involves the provision of a prosthetic device constructed of porous ceramic material. This material is structurally weak and attempts to overcome this defect involve filling the bulk of the device with resin material, leaving a porous surface area. Although the presence of the resin may increase the strength of the central portion of the device, the surface region remains weak. Further, the presence of resin material degradable by body fluids would lead to unsatisfactory use in the human body. In addition, the maximum pore size for the ceramic is indicated to be 50 microns, and much smaller sizes are preferred. If the pore size were greater than 50 microns, then the structure would become too weak for effective use. A consequence of this pore size limitation will become apparent hereinafter in the discussion of the present invention

SUMMARY OF INVENTION

The surgical prosthetic device of the present invention has a unique construction which overcomes the weakness problems associated with the prior art devices, as disscused above. The surgical prosthetic device of the invention comprises a composite structure consisting of a solid metallic material substrate and a porous coating adhered to and extending at least partially over the surface of the substrate. The porous coating on the surface of the substrate has several parameters, described in detail below, which are essential to the provision of a satisfactory device free from the defects of the prior art devices.

The porous coating consists of a plurality of small discrete particles of metallic material bonded together at their points of contact with each other to define a plurality of connected interstitial pores in the coating. The particles are of the same metallic material as the metallic material from which the substrate is formed. It is essential that this be the case otherwise corrosion at the substrate-coating interface may occur due to a cell action with body fluids.

The metallic material from which the substrate and coating are formed is one which is not corroded or otherwise degraded by the body fluids of the patient. Examples of suitable materials include austenitic stainless steel, titanium, titanium alloys and cobalt alloys. The cobalt alloy VITALLIUM (Trademark) has been found to be especially useful.

BRIEF DESCRIPTION OF DRAWING

The accompanying drawing is a photomicrograph of the surface structure of an implant in accordance with one embodiment of the invention after four months implantation in a dog femur.

DETAILED DESCRIPTION OF INVENTION

In the surgical prosthetic device of the invention, the metal particles are bonded to one another and to the substrate in such a manner that particle-to-particle separation and particle-to-substrate separation would require a shear stress greater than the transverse shear stress required for bone fracture. Thus, in the present invention, the composite structure is stronger than bone and hence is free from the interfacial shearing encountered by the prior art and discussed in detail above and also is free from intracoating failure.

In the present invention, therefore, when tissue ingrowth and tissue ossification is complete, break away of the coated part of the prosthetic device will occur by a bone fracture rather than a fracture within the coating itself or at the coating-substrate interface.

Another essential parameter of the coating which assists in the provision of a superior product is the pore and pore size distribution through the depth of the coating. In the present invention, both the pore and pore size distribution are substantially uniform from the coating-substrate interface to the surface of the coating. This uniformity of pore and pore size distribution through the coating results in a substantial uniformity of strength through the thickness of the coating and ensures ingrowth of tissue and its calcification through the entire thickness of the layer to the interface with the substrate.

These latter conditions are in contrast to the structure of the prior art where there is a progressive increase in pore size and porosity from the coating-substrate interface to the surface. The prior art therefore lacks uniformity of strength throughout the thickness of the layer and further does not allow the ingrowth of tissue through the whole depth of the layer, resulting in a less-satisfactory incorporation into the body.

It also is essential to control the interstitial pore size and the coating porosity within critical limits, although variations between the limits may be made depending on the individual requirements. The critical limits depend on the application to which the implant is to be put.

In order for the porous adherent coating to be able to sustain bone, or other hard tissue growth, it is essential that the interstitial pore size exceed about 50 microns. Generally, the interstitial pore size is between about 50 and 100 microns, although larger pore sizes up to about 200 microns may be employed. Since the particles in the powder from which the coating is formed are not usually of uniform size, the pore sizes vary somewhat throughout the coating, although their distribution is substantially uniform.

Where the ingrowth of soft tissue only is to be sustained, the interstitial pore size may be less than 50 microns, down to about 20 microns.

This finding of essential pore sizes for ingrowth of tissue contrasts markedly with one of the prior art suggestions mentioned above wherein the maximum pore size indicated is 50 microns, with the preferred range being considerably less. This prior art device therefore is capable only of sustaining ingrowth of soft tissue and in its preferred aspects is incapable of sustaining even soft tissue ingrowth.

Further, it is essential that the porosity of the surface coating not exceed about 40 percent and be at least about 10 percent. It is only be controlling the porosity within this range at the pore sizes recited above that it is possible to provide a surgical prosthetic device that has an overall strength greater than the shear strength of bone and at the same allow for ingrowth of tissue. At porosities below about 10 percent, there are insufficient pores in the surface to provide sufficient ingrowth whereby upon later calcification a strong mechanical fixation is achieved. At porosities above about 40 percent the overall mechanical strength falls below the required level. The pore size and porosity values are achieved by controlling the manner of formation of the coating and the particle size of the material used in the coating formation.

The depth of the porous coating on the surface of the substrate and the ratio of depth of coating to depth of substrate may vary over a wide range between essential limits. The lower limit of thickness is about 100 microns, which is the thickness of surface coating required to sustain bone tissue ingrowth with good mechanical interlocking in the pores, generally equivalent to about 2 to 3 monolayers of particles, while the upper limit of thickness is about 1,000 microns which is dictated by the strength considerations discussed above. Typically, a depth of about 500 microns is used on about a 1/4 inch round substrate, using from +325 to -100 mesh particle coatings.

Referring now to the accompanying photomicrograph there is shown a 250 times magnification of an elongated substrate 10 of circular cross-section and a porous adherent coating 12. The substrate 10 and the coating 12 each are formed of VITALLIUM. The coating 12 is formed of from +325 to -100 mesh powder and a plurality of interconnected interstitial pores filled with bone tissue is provided. The pores and pore size distribution are substantially uniform through the depth of the coating 12.

A dog's femur 14 is situation adjacent the coating 12 and the ingrowth of bone tissue 16 can be seen. The growth of bone tissue is throughout the depth of the coating to the substrate surface. The ingrowth of the bone tissue was woven and lamullar together and had the architectural configuration of adult compact bone with osteone formation. Since the appearance of tissue elements may be deceptive by reflection under the optical microscope, further testing by way of electron microprobe scan analyses of the porous coating 12 was carried out. Probing from the bone 14 across the coating 12 to the substrate 10 showed the calcium and phosphorus contents of the tissue ingrowth 16 to be the same as that of the femur 14. These observations confirm that the coating 12 was not only penetrated by living tissue but it was sufficiently porous to allow the infiltration of bone growth.

The micrograph shows no untoward reaction of the bone tissue 16 to the metal of the coating 12. This observation is significant in view of the large surface area of metal exposed to possible reaction in an open pored structure.

The surgical prosthetic device of the invention has a number of uses. For example, the implant may be used to bridge the gap between bone ends caused by removal of a portion of the bone. The removal of the portion may be due to irreparable shattering, a cancerous growth or the like.

The implant, generally in the form of a cylindrical rod, is positioned within the bone ends. At least those areas of the implant in contact with and adjacent the bone ends are provided with the porous adherent coating. The presence of the porous adherent coating allows bone or hard tissue to grow into the surface of the implant, so that the implant is incorporated into the bone and the implant thereby is secured to the bone ends.

The implant of the invention by the presence of the porous adherent coating allows bone or hard tissue to grow into the surface of the implant, so that the implant is incorporated into the bone and a much stronger joint is provided.

When the implant is used in this bridging role, in those areas between the bone ends, a porous adherent coating also may be provided on the surface of the implant, so that body (or soft) tissue may grow into the surface. Therefore, the implant is not only encapsulated in the body, but is incorporated into the bone and soft tissue of the patient. In this way, a very rigid structure is obtained.

The process of growth of bone tissue into the porous adherent coating takes some time and it is necessary to provide an initial affixing of the bone ends and the plate to position the implant for ingrowth of the bone tissue. In the case of bone setting, the implant, in the form of a plate, may be secured by screws, either side of the break. In the case where the implant is used as a bridge between bone ends, the implant, in the form of an elongated cylindrical rod, may be secured within the bone ends.

A further use of the surgical prosthetic device of the invention lies in artifical joints, for example, a hip prosthesis. When artifical joints are included in the body, it is necessary that they be affixed in the joint socket and this has been achieved using cements. In some joints, such as the hip joint, the stresses at certain positions are greater than at others and it may be desired to utilize adhesion achieved other than by the use of cements.

The artifical joints may be constructed in accordance with the present invention. The coating may be provided on the substrates, if desired, only at those positions where the joint will be subjected to high stress. As in the case of the bridging of bone, the bone tissue of the socket grows into the surface coating and thereby more tightly binds the prosthesis. Cement is used to position the prosthesis in the socket and cement may be employed additionally at the areas of the porous coating. The entire prosthesis may be formed as a composite and it has been found that the use of a cement, such as methyl methacrylate, together with the porous surface gives rise to a much enhanced adhesion of the prosthesis to the socket as compared to use of a cement in the absence of a porous surface.

In this case, the increased surface area and morphology of the implant occasioned by the presence of the porous coating provides the enhanced adhesion. A further application of the implant of the invention is in a McIntosh arthroplasty of the knee.

An additional use of the implant of the invention is in the affixing of artificial limbs, etc. to amputees. In this embodiment, the implant, usually as an elongated rod, is secured in the bone of the stump. In the area of the implant adjacent the bone, an adherent porous coating is provided in accordance with this invention. The presence of the coating allows bone tissue to grow into the surface of the implant and rigidly secure the implant therein. The rod projects from the stump and after the implant is securely affixed to the bone, the artificial limb then may be secured to the projecting portion.

At least at the portion of the rod adjacent the body surface, the implant is provided with a porous surface. The soft body tissue at the surface of the stump, therefore, may grow into the surface of the implant at the point where it projects from the stump and thereby the surface of the skin is sealed to the implant. In the absence of the porous coating of the invention, such a seal does not form and infection of the stump may occur.

In common with the embodiments discussed above, certain parameters are necessary for the coating in the area of the fixture to the bone in order to sustain bone tissue growth into the coating and the discussion above with reference to the implant used in the setting of bone applies equally here. In particular, the pore size of the porous coating in at least the area of the implant adjacent the bone stump should exceed 50 microns.

The implant of the invention may be used in brain surgery, to replace bone removed from the cranium during a brain operation. The surgical prosthetic device of the invention in this embodiment assumes the form of a circular disc, or other shape, conforming in size to the hole formed in the skull, with a porous adherent coating formed around the peripheral areas where the device engages the skull bone. Bone tissue grows from the skull into the porous surface, and the disc thereby is incorporated into the skull.

The implant of the invention in the form of a staple may be used in the rejoining of tendon, muscle or other soft tissue to bone, for example, in a shoulder. The staple consists of a substrate and a coating, the coating being formed to sustain the growth of bone and soft tissue therein. The implant may be formed with different forms of coating, one area sustaining bone tissue growth and the other soft tissue growth. It is possible to employ a smaller interstitial pore size for soft tissue growth, as discussed above, down to about 20 microns, as compared to the pore size for bone tissue growth where an interstitial pore size exceeding about 50 microns is required.

Another area of use of the present invention is in dentistry. The implant may be anchored to the mandible and at the point of projection of the implant through the gingival, a porous coating is provided.

Gum tissue grows into the coating and thereby seals the gum surface. In this way, the collection of foreign substances, causing infection, at the implant-gingival interface is prevented.

The area of the implant adjacent the mandible may be provided with a coating, in accordance with the present invention, in order to more securely anchor the implant to the jaw bone by bone tissue ingrowth.

The implant of the present invention also may be employed to provide a quick-release valve secured to the body surface of a patient for connecting internal parts of the body to external treatment devices. The implant generally consists of a ring of metal having a peripheral porous coating adjacent the areas of attachment to the patient's body surface. Since the growth of body tissue into the surface takes time, temporary securement of the implant may be achieved in any desired manner, such as by the use of sutures.

The ring is formed so that efficient connection between internal parts of the body and devices external of the body may be provided, the particular form depending on the end use.

It will be seen that a novel surgical prosthetic device is provided which has many useful surgical applications and which has many advantages over prior art methods. The enhanced strength of implant to bone or soft tissue provided by the porous adherent surface represents a significant advance in surgery.

The surgical prosthetic device of the invention may be formed in any convenient manner. For example, the porous coating may be formed by diffusion bonding of the particles and the surface. Typically, it is possible to provide a porous adherent coating having the desired characteristics by using metallic particles, such as VITALLIUM, having particle sizes from +325 to -100 mesh in this technique. It is not possible to use -325 mesh powder when bone tissue ingrowth and calcification is to be sustained since the interstitial pore sizes of coatings formed from such powder drop below 50 microns. Implants including coatings formed from such -325 mesh powder are useful, however, where soft tissue ingrowth is to be sustained, provided that the interstitial pore size does not fall below 20 microns.

One convenient powder metallurgy technique for forming the coating in the device of the present invention utilizes a slurry of metallic powder suspended in aqueous solution with organic binders. The slurry may be held in a mold around the area of the substrate to which it is desired to impart the porous coating. Alternatively, the slurry may be of a consistency to be self-supporting on the surface.

The slurry is heated to remove the water and finally sintered in an inert or reducing atmosphere, such as hydrogen, to burn off the organic binder and fuse the particles together and to the substrate.

The particle size of the metallic powder and the conditions of formation of the porous coating are controlled to provide the desired interstitial pore size, porosity, strength and depth of coating. For a typical implant of the invention, employing a VITALLIUM substrate, VITALLIUM powder of from +325 to -100 mesh particle size requires to be heated at about 2,200.degree.F for at least 2 hours in a dry hydrogen atmosphere to provide a satisfactory product. Longer sintering times produces a stronger product.

A study of the effect of firing time on the properties of VITALLIUM powder coating (+325 mesh) at a firing temperature of about 2,200.degree.F has indicated a relationship between the density of the coating and the sintering time. The density is related to the porosity and the shear strength of the final coating, and the shear strength therefore varies with sintering time. The results are indicated in the following Table I:

TABLE I ______________________________________ Sintering Density Shear Porosity Time g/cc Strength % (hrs.) (psi) ______________________________________ 2 4.8 2,500 40 5 5.2 4,000 35 ______________________________________

(Average values for samples formed by coating and slurry methods.)

Under certain circumstances, it may be desired to machine the coating to a particular shape and this may be achieved by firing the coating to a strength at which the coating is machinable, machining the coating to the desired shape and then firing the machined coating to the final forms.

The coating may be formed in any other desired manner, for example, from the metallic powder. A depression may be formed in the surface of the substrate, free-flowing metallic powder, in the absence of organic binder, may be poured into the depression and the powder fired to produce the coating.

In a typical two-stage operation, a VITALLIUM coating may be fired at 2,100.degree. to 2,150.degree.F for about one hour to give the strength. The coating then is machined and fired again at about 2,200.degree.F for at least 2 hours, sometimes for as long as 8 hours, depending on the strength requirements of the final coating.

Where a slurry is used, the mold may be subjected to pressure during whole or part of the firing operation.

It is possible to provide the same pore size at two different positions of tissue ingrowth but in certain circumstances it may be desirable to provide two different types of coating on the same implant.

After formation of the composite structure, if desired, the porous surface may be treated with a variety of materials prior to implant in the body. These materials may include materials to promote the growth of hard or soft tissues or with antibiotics.

In the present invention, both the substrate and the powder are sintered to achieve diffusion bonding between the metal particles and between the metal particles and the substrate, and hence the thermal stresses encountered by the prior art by the use of flame spraying are avoided.

The product of the invention is superior to the prior art in terms of overall strength and ability to sustain satisfactory interlocking ingrowth of both bone and soft tissues.

EXAMPLES

The invention is illustrated by the following Examples:

EXAMPLE 1

This example illustrates the formation of a surgical prosthetic device of the invention.

A VITALLIUM rod of 1/4 inch diameter was degreased and cleaned. An aqueous VITALLIUM powder slurry consisting of 74 parts by weight of +325 mesh atomized VITALLIUM powder, 25 parts by weight of an aqueous solution of 1 percent methylcellulose, 1 part by weight of a 21/2 percent aqueous solution of dioctyl sodium sulfosuccinate and 0.25 parts by weight of ammonium hydroxide was made up. This slurry was applied to the degreased and cleaned VITALLIUM rod to a depth of 1/32 inch.

After drying, the coated rod was sintered at 2,200.degree.F in a dry hydrogen atmosphere of 99.99 percent purity for approximately two hours. The product was cooled in the hydrogen atmosphere. Examination of the product indicated that the spherical powder particles had fused at each contact point between themselves and the rod and an interior communicating substantially uniform pore structure with pore sizes ranging from 50 to 100 micron was evident.

EXAMPLE II

Cylindrical rods cut from the product of Example 1 were implanted into the tibia of adult mongrel dogs. Drill holes were made at right angles to the longitudinal axes of the tibial shafts and the implants were introduced so as to penetrate only the medial cortex, the inner portion of the implant being free in the medullary cavity. The effective area of contact between the VITALLIUM coating and the bone was relatively small, being equal to the cortical surface exposed by the drillhole.

A push-out test designed to measure the force required to dislodge the implant from the cortical bone was carried out in the following way. An Instron tester with a compression cell and specially designed adaptor was used to apply compressive loading directly to the implant. The specimen of bone was held rigidly in a vice with care being taken to ensure that the line of action of the loading force was directly perpendicular to that of the implanted rods. In each case, a compression load was applied at a rate of 0.2 inches per minute and the force recorded graphically. The force required to dislodge the implant was taken as that required to produce the first movement of the rod. The force per unit area in p.s.i. then was calculated, affording an expression of the shear strength of the interface between the bone and the implant material.

The push-out force was tested on various samples after first inserting into the bone and then after four months' implantation. Parallel studies were carried out using the rods having porous coatings formed from -325 mesh VITALLIUM powder. By way of comparison, test results on smooth VITALLIUM rods also were obtained. The results are reproduced in the following Table II:

TABLE II __________________________________________________________________________ +325 mesh VITALLIUM -325 mesh VITALLIUM No coating - psi psi VITALLIUM rod psi 0 time 4 months 0 time 4 months 0 time 4 months __________________________________________________________________________ 148 1520 49 850 210 220 164 1670 66 930 240 220 180 1690 74 1060 270 300 230 1740 90 1060 300 330 278 1780 98 1140 315 340 __________________________________________________________________________

It will be seen from the results of this Table II that implants of the +325 mesh VITALLIUM coated specimens showed a markedly enhanced bonding characteristic, with the force required to dislodge those being in the range of 1,500 to 1,800 pounds per square inch. The -325 mesh VITALLIUM coated specimens also bonded well, but the degree of enhancement of fixation (850 to 1,150 psi) obtained was less. In complete contrast to these results obtained with products of the invention, the implants formed only of VITALLIUM rod exhibited little or no change in bonding characteristics over a four-month period.

EXAMPLE III

Rods of metal were embedded in polymethylmethacrylate cement. The metals involved were VITALLIUM having a coating of +325 mesh VITALLIUM powder, formed as outlined in Example 1, buffed VITALLIUM and stainless steel.

After 24 hours' immersion, the specimens were trimmed so that there was no cement adherent to the inferior pole of the rods and the samples were mounted in the Instron machine of Example II for compression testing. The flat undersurface of the acrylic provided an ideal base for conducting the push-out test, care having been taken in embedding the metal to ensure that the compressive forces would be applied directly along the long axis of the rod.

The results are reproduced in the following Table III:

TABLE III ______________________________________ VITALLIUM coating Buffed VITALLIUM Stainless Steel +325 mesh psi psi psi ______________________________________ 1740 530 580 2180 545 595 2470 580 610 2470 610 633 2700 610 660 ______________________________________

It will be seen from these results that the implants formed in accordance with prior art methods may be dislodged from the acrylic with a force of 500 to 600 psi. In contrast, with the implant of the present invention, the implant could not be dislodged at all; instead the acrylic cement shattered, breaking through the line of the implant without movement of the metal rod. It is apparent, therefore, that the porous surface provides a greatly increased surface area for fixation and mechanical interlocking, with consequentially increased adhesive power. The cement adheres much more strongly to the porous surface and this fact has considerable practical application in providing greater stability for the components of a total hip prosthesis or McIntosh arthroplasty of the knee.

Modifications are possible within the scope of the invention.

Claims

What I claim is:

1. A surgical prosthetic device comprising a composite structure consisting of a solid metallic material substrate and a porous coating of said metallic material adhered to and extending at least partially over the surface of said substrate to a thickness of about 100 microns to about 1,000 microns, said metallic material being substantially non-corrodable and non-degradable by body fluids,

said porous coating consisting of a plurality of small discrete generally ball-shaped particles of said metallic material bonded together at their points of contact with each other and said substrate to define a plurality of connected, interstitial pores uniformly distributed throughout said coating,

said particles being of a size and being spaced from each other to establish an average interstitial pore size of from about 20 microns to about 200 microns substantially uniformly distributed throughout said coating and a coating porosity of between about 10 and about 40 percent.

2. The device of claim 1 wherein said average interstitial pore size is from about 50 to about 200 microns.

3. The device of claim 1 wherein said average interstitial pore size is from about 50 to about 100 microns.

4. The device of claim 1 wherein said metallic material is selected from austenitic stainless steel, titanium, titanium alloys and cobalt alloys.

5. The device of claim 1 wherein said metallic material is VITALLIUM.

6. The device of claim 1 wherein said metallic material is VITALLIUM and said particles are of +325 mesh size.

7. The device of claim 6 wherein said particle sizes are from +325 mesh to -100 mesh.

8. The device of claim 2 wherein said coating is impregnated with bone-binding cement.

9. The device of claim 7 wherein said coating is provided to a depth of about 500 microns.

10. The device of claim 1 wherein said solid substrate has two of said porous coatings of said metallic material adhered to and each extending partially over the surface of said substrate to said thickness, one of said porous coatings having an average interstitial pore size below about 50 microns for the ingrowth of soft tissue and the other of said porous coatings having an average interstitial pore size above about 50 microns for the ingrowth of hard tissue.