Orthopedic Device For Repair Or Replacement Of Bone

Abstract

Prosthetic devices for repair of replacement of bone structure in a living body, and methods of orthopedic repair which employ such devices. The orthopedic devices comprise a substrate and a pyrolytic carbon coating on the substrate, which pyrolytic carbon coating is compatible with living tissue and which has a density of at least about 1.5 grams per cubic centimeter. Examples of suitable substrates are those which have a modulus of elasticity approximating that of natural bone such as polycrystalline carbon, and fiber aggregates such as carbon fiber aggregates and refractory wire metal screens. The pyrolytic carbon coating of the orthopedic devices may be polished to provide an effective wear surface, while the surface roughness of the as-deposited coating may be employed to achieve a bond with natural bone tissue. The pyrolytic carbon coating is preferably isotropic and may be doped with a suitable carbide-forming element, such as silicon, to provide additional structural strength and wear resistance.

Parent Case Text

This application is a continuation-in-part of copending application Ser. No. 649,811, filed June 29, 1967, now U.S. Pat. No. 3,526,005.

Description

The present invention relates to prosthetic devices designed for use in orthopedics. More particularly, the present invention relates to repairing orthopedic defects by using prosthetic devices designed for such purposes.

The use of prosthetic devices for repair or replacement of bone structure in a living body is well known. Conventional prosthetic devices have been constructed from metals, ceramic, and plastics, depending upon their intended application. Their development has been spurred by the intense human need peculiar to an adverse medical situation; such devices, however, are only partially or temporarily adequate and have various serious defects which limit their use to situations of such seriousness that their substantial disadvantages are outweighed.

One requirement for a satisfactory prosthetic device which is to be permanently implanted is that it be physiologically inert for indefinite periods of time, and one difficulty with conventional devices is that the materials from which they are constructed may be physiologically rejected, may cause inflammation of bodily tissue or may be degraded by bodily processes. For example, metals may corrode to cause structural weakness and pathology from the corrosion products. Plastics, even those extremely chemically inert, such as polytetrafluoroethylene may be degraded in the body over long periods of time, or cause inflammation or abnormal cell growth.

Furthermore, the stresses developed within the skeletal framework of a living body, which is naturally designed to serve a load-bearing function, provide considerable difficulties of a mechanical and structural nature. In addition, the functional interrelationship between the skeleton and the rest of the body is of such a biologically intricate nature that even more complex difficulties confront the development of suitable prosthesis. For example, joint prostheses are conventionally constructed from metals or plastics. Not only do plastics tend to be degraded in the body, but they also exhibit poor wear characteristics under the severe mechanical performance requirements for a joint prosthesis. Metal-metal joint prostheses tend to gall and wear, and the metallic dust created thereby may require re-operation. Metallic prostheses which replace only part of a functional joint may be so rigid that they cause damage to the remaining, natural portions of the joint.

These problems are particularly acute for skeletal regions which are normally subjected to considerable stress. For example, conventional hip joint ball prosthesis may appear to operate satisfactorily for up to several years; however, such prostheses generally tend to eventually cause thickening, inflammation, and pain in the synovial lining and capsule of the joint.

Replacement of one articulating portion of a joint with a rigid, metallic prosthesis may have a deleterious effect upon the tough, elastic cartilage which covers the remaining, articulating end of the natural portion of the joint. Contact and wearing, particularly under conditions of load-bearing or stress, of a hard metallic prosthesis against a portion of natural bone will usually result in damage to the living bone tissue. Furthermore, under the conditions of stress associated with skeletal functionality, conventional metallic, rigid prostheses exhibit considerable difficulty in remaining permanently implanted in a supportive function in living bone. Such metallic prostheses have so high a modulus of elasticity that they do not flex in harmony with the bone in which they are implanted, but rather, because they are more rigid than the bone, concentrate stress in portions of the remaining, lower modulus bone, particularly at the metal-bone interface. Thus, not only do conventional metallic prosthesis exhibit the undesirable characteristic of subjecting remaining portions of natural bone to unnatural, concentrated stress, but they also have the additional disability of being difficult to anchor in the natural bone because stress concentration at the interfaces between the bone and the prosthesis will cause the prosthesis to toggle or otherwise work loose from its fastening to the bone.

When a segment of bone is missing due to accident or surgery, there is at present no satisfactory method other than bone homograft to replace the missing bone segment and join the remaining portion of the bone. Attempts have been made to use porous ceramics to act as a skeleton or framework into which new bone might grow, so that the missing segment of bone would be replaced partially by natural processes. Such approaches employing porous ceramic prostheses have been generally unsuccessful, not only because such materials are often not accepted by the body but also because the very objective of fostering natural bone growth within the ceramic has not been successfully attained.

Metallic dental prostheses which abut or penetrate the jaw bones also suffer the disabilities and shortcomings associated with metal prostheses in general, and dental repair is still conventionally accomplished by extracorporeal support from remaining natural teeth or gums.

Prosthetic and orthopedic devices have been used for a number of years, and it is expected that usage of such devices will increase in the future as medical expertise continues to improve. The provision of improved prosthesis for repair or replacement of bone structure in a living body would greatly facilitate the development of that medical expertise and alleviate considerable human suffering.

It is an object of the present invention to provide improved prosthetic devices. Another object is to provide prosthetic devices which are physiologically compatible with body tissues although implanted in the body for long periods of time. A further object of the invention is to provide prosthetic devices which perform exceptionally well in the demanding mechanical environment of the skeletal structure of a living body, and which are capable of establishing a firm bond thereto. Still another object of this invention is to provide prosthetic devices which are eminently suited for extended functional performance as, for example, joint prostheses, dental prostheses, whole or partial bone replacement prostheses, and framework-like prosthesis into and throughout which natural, living bone tissue may grow so that repair or replacement of bone structure may be partially accomplished by natural processes. An additional object is to provide improved prostheses which have a modulus of elasticity approximating that of natural, living bone, and which do not concentrate substantial stress at the prosthesis-bone interface, but rather flex in harmony with the natural bone under applied stress. A still further object of this invention is to provide a method of repairing an orthopedic defect by using prosthetic devices through which the qualities of natural living bone may be closely approximated.

These and other objects of the invention are more particularly set forth in the following detailed description and in the accompanying drawings of which

FIG. 1 is a perspective view partially broken away, of a knee joint prosthesis;

FIG. 2 is a bottom view of the knee joint prosthesis of FIG. 1;

FIGS. 3 and 4 are perspective views of dental prostheses adapted to be implanted into a jaw bone of a living body;

FIG. 5 is a perspective view, partially broken away, of a hip joint prosthesis, and

FIG. 6 is a bone framework prosthesis into and throughout which living bone tissue may grow so that replacement of bone structure in a living body may occur partially through natural bone growth,

FIG. 7 is a side elevation, partially broken away, of the bone framework prosthesis of FIG. 6 in position between two segments of natural bone.

It has been found that prosthetic devices having improved characteristics can be made by coating suitable substrates of the desired shape and size with pyrolytic carbon. Pyrolytic carbon is capable not only of significantly increasing the strength of the substrate upon which it is coated, but it is also able to resist wear and deterioration even when implanted within a living body for long periods of time. While reference is hereinafter generally made to the use of prosthetic devices for repair or replacement of bone structure in a living human body, it should also be recognized that the improved prosthetic devices and orthopedic processes may also have veterinary applications in other living animals which have an internal skeletal structure. For example, it may be desirable to use prosthetic devices having the indicated pyrolytic carbon coatings for use in orthopedic repair or replacement of bone in horses, dogs, or other domestic animals.

In general, the pyrolytic carbon coating is applied to a suitable substrate material so that it covers at least a major portion of the surface thereof. The thickness of the pyrolytic carbon coating should be sufficient to provide the necessary strength for its intended use, and often it is desirable to employ the coating to impart additional strength to the particular substrate being coated. The coating will be at least 25 microns thick and usually at least about 50 microns thick. If a fairly weak substrate is being employed, for instance, one made of artificial graphite, it may be desirable to provide a thicker coating of pyrolytic carbon to strengthen the composite prosthetic device.

Moreover, although an outer coating which is substantially entirely fairly dense pyrolytic carbon has adequate structural strength, the codeposition of silicon or some other carbide-forming additive may be employed to improve the strength and wear resistance of the carbon coating. As described in more detail hereinafter, silicon in an amount up to about 20 weight percent can be dispersed as SiC throughout the pyrolytic carbon without detracting from the biologically compatible properties of the pyrolytic carbon.

For use on complex shapes and in order to obtain maximum structural strength, it is desirable that a pyrolytic carbon coating on the substrate be nearly isotropic. Anisotropic carbons tend to delaminate when complex shapes are cooled after depositing the pyrolytic coating at high temperatures. Thus, for coating complex shapes (i.e., those having a radius or radii of curvature less than 1/4 inch), the pyrolytic carbon should have a BAF (Bacon Anisotrophy Factor) of not more than about 1.3. For non-complex shapes, higher values of BAF up to about 2.0 may be used, and for flat shapes, pyrolytic carbon having a BAF as high as about 20 may be used. The BAF is an accepted measure of preferred orientation in the layer planes in the carbon crystalline structure. The technique of measurement and a complete explanation of the scale of measurement is set forth in an article by G.E. Bacon entitled "A Method for Determining the Degree of Orientation of Graphite" which appeared in the Journal of Applied Chemistry, Vol 6, p. 477, (1956). For purposes of explanation, it is noted that 1.0 (the lowest point on the Bacon scale) signifies perfectly isotropic carbon, while higher values indicate increasing degrees of anisotrophy.

The density of the pyrolytic carbon is considered to be an important feature in determining the additional strength which the pyrolytic carbon coating will provide the substrate. The density is further important in assuring tissue compatibility, and sufficient wear resistance of the prosthetic device where appropriate. It is considered that the pyrolytic carbon should at least have a density of about 1.5 grams per cubic centimeter, and preferably the pyrolytic carbon has a density between about 1.9 grams/cm.sup.3 and about 2.2 grams/cm.sup.3.

A further characteristic of the carbon which also affects its structural strength contribution is the crystallite height or apparent crystallite size. The apparent crystallite size is herein termed L.sub.c and can be obtained directly using an X-ray diffractometer. In this respect

L.sub.c = (0.89 .lambda.)/(.beta. cos .theta.)

wherein:

.lambda. is the wavelength in Angstroms

.beta. is the half-height (002) line width, and

.theta. is the Bragg angle.

Pyrolytic carbon coatings for use in prosthetic devices should have a crystallite size no greater than about 200 A. In general, the desirable characteristics for pyrolytic carbon for use in prosthetic devices are greater when the apparent crystallite size is small, and preferably the apparent crystallite size is between about 20 and about 50 A.

Since the substrate material for the prosthetic device will preferably be completely encased in pyrolytic carbon, choice of the material from which to form the substrate is not of utmost importance per se. However, the substrate material should have the requisite mechanical strength and structural properties for the particular prosthetic application for which it is going to be employed. However, if the prosthetic device in use will have portions of the substrate exposed to bodily tissues, for example, as might occur from machining the prosthetic device into final form after the basic shape has been coated with pyrolytic carbon, the substrate should be selected from materials which are relatively biologically inert, preferably artificial graphite.

It is very important that the substrate material be compatible with pyrolytic carbon, and more particularly that it be suitable for use in the process conditions for coating with pyrolytic carbon. Although it is desirable that the substrate material have sufficient structural strength to resist possible failure during its end use, materials which do not have sufficiently high structural strengths (by themselves) may be employed by using the pyrolytic carbon deposited thereupon to supply additional structural strength for the prosthetic device.

Because pyrolytic carbon is, by definition, deposited by the pyrolysis of a carbon-containing substance, the substrate will be subjected to the fairly high temperatures necessary for pyrolysis. Generally, hydrocarbons are employed as the carbon-containing substance to be pyrolyzed, and temperatures of at least about 1000.degree.C. are used. Some examples of the deposition of pyrolytic carbon to produce coated articles having increased stability under high temperature and neutron radiation conditions are set forth in U.S. Pat. No. 3,298,921. Processes illustrated and described in this U.S. patent employ methane as the source of carbon and utilize temperatures generally in the range from about 1500.degree. to 2300.degree.C. Although it is possible to deposit pyrolytic carbon having the desired properties with regard to the instant invention at somewhat lower temperatures by using other hydrocarbons, for example, propane or butane, generally it is considered that the substrate materials should remain substantially unaffected by temperatures of at least about 1000.degree.C., and preferably by even higher temperatures. The pyrolytic carbons deposited either with or without silicon at temperatures below about 1500.degree.C. are particularly suited for use in prosthetic devices because such pyrolytic carbons have exceptional tissue and bone compatibility, wear resistance, mechanical strength, and in combination with a suitable substrate will provide a prosthesis with a modulus of elasticity which is close to that of bone.

Because the substrate is coated at relatively high temperatures and the prosthetic device will be employed at temperatures usually very close to ambient, the coefficients of thermal expansion of the substrate and of the pyrolytic carbon deposited thereon should be relatively close to each other if the pyrolytic carbon is to be deposited directly upon the substrate and a firm bond between them is to be established. While the above-identified U.S. patent contains a description of the deposition of an intermediate, low density, pyrolytic carbon layer, the employment of which might provide greater leeway in matching the coefficients of thermal expansion, it is preferable to deposit the pyrolytic carbon directly upon the substrate and thereby avoid such an additional intermediate layer. Pyrolytic carbon having the desired characteristics can be deposited having a thermal coefficient of expansion in the range of between about 3 and about 6 .times. 10.sup.-.sup.6 /.degree.C. measured at 20.degree.C. Accordingly, substrate materials are chosen which have the aforementioned stability at high temperatures and which have thermal coefficients of expansion within or slightly above this general range, for example up to about 8 .times. 10.sup.-.sup.6 /.degree.C. Examples of suitable substrate materials include artificial graphite, boron carbide, silicon carbide, tantalum, molybdenum, tungsten, and various ceramics, such as mullite.

Prosthetic devices which are intended to replace a significant amount of bone tissue without partial replacement by means of natural bone growth, preferably employ artificial graphite as the substrate material because these materials have a modulus of elasticity of from about 2 to about 4 .times. 10.sup.6 psi., which is ordinarily in the range of that of natural living bone. For example, a particularly preferred form of graphite for use as a substrate material is polycrystalline graphite. An example of such a graphite is the polycrystalline graphite sold under the trade name POCO AXF Graphite, which has a density of about 1.9 grams per cubic centimeter, an average crystallite size (L.sub.c) of about 300 A, an isotropy of nearly 1.0 on the Bacon scale, and a modulus of elasticity of about 1.7 .times. 10.sup.6 psi.

When it is desired to provide a framework into and through which new bone tissue may grow, for example, in order to firmly attach a prosthetic device to a bone, or in order to replace or repair bone structure partially through replacement by new, natural bone tissue, as hereinafter described, substrates such as suitable metallic or carbon screens, wires or fibers may be employed as the substrate material. For example, a screen formed from an alloy of 50 percent molybdenum and 50 percent rhenium may be used. Screens of tantalum, tungsten, molybdenum or alloys thereof, which are preferably coated with tungsten to prevent embrittlement during the coating with pyrolytic carbon, may also be used. Fiber aggregates in addition to screens such as non-woven felted structures and filament-wound structures may also be used as substrate materials.

The pyrolytic carbon coating is applied to the substrate using a suitable apparatus for this purpose. Preferably, an apparatus is utilized which maintains a substrate in motion while the coating process is carried out to assure that the coating is uniformly distributed on the desired surfaces of the substrate. A rotating drum coater or a vibrating table coater may be employed. When the substrates to be coated are small enough to be levitated in an upwardly flowing gas stream, a fluidized bed coater is preferably used. When larger substrates are employed, or where it is desired to vary the thickness or other characteristics of the pyrolytic carbon coating over different portions of the substrate, different coating methods may be employed, such as supporting the substrate on a rotating or stationary mandrel within a large fluidized bed.

As discussed in detail in the above-identified United States patent, the characteristics of the carbon which is deposited may be varied by varying the conditions under which pyrolysis is carried out. For example, in a fluidized bed coating process wherein a mixture of a hydrocarbon gas, such as methane, and an inert gas, such as helium or argon, is used, variance in the volume percent of the hydrocarbon gas, the total flow rate of the fluidizing gas stream, and the temperature at which pyrolysis is carried out, all affect the characteristics of the pyrolytic carbon which is deposited. Control of these various operational parameters not only allows deposition of pyrolytic carbon having the desired density, apparent crystallite size, and isotropy, but it also permits regulation of the desired thermal coefficient of expansion of the deposited pyrolytic carbon. This control may also be used to "grade" a coating in order to provide a variety of exterior surfaces. One can deposit a strong base isotropic pyrocarbon coating, having a BAF of 1.3 or less, and near the end of the coating operation, the coating conditions can be gradually changed to obtain a highly oriented outer layer. Using this technique, suitable coatings having outer surfaces which are highly anisotropic and, for example, are about 25 microns thick, can be conveniently deposited.

Generally, when pyrolytic carbon is deposited directly upon the surface of the substrate material, the pyrolysis conditions are controlled so that the pyrolytic carbon which is deposited has a coefficient of expansion matched to within plus or minus 25 percent of the coefficient of expansion of the substrate material, and preferably to within about plus or minus 20 percent. Because pyrolytic carbon has greater strength when placed in compression than when placed in tension, the thermal coefficient of expansion of the pyrolytic carbon is most preferably about equal to or less than that of the substrate. Under these conditions, good adherence to the substrate is established and maintained during the life of the prosthetic devices, and upon cooling of the pyrolytic coating-substrate composite, the pyrolytic carbon coating is placed in compression under conditions of its intended use at about ambient temperature.

As previously indicated, the coating may be substantially entirely pyrolytic carbon, or it may contain a carbide-forming additive, such as silicon, which has been found to enhance the overall mechanical properties of the coating. Silicon in an amount of up to about 20 weight percent, based on the total weight of silicon plus pyrolytic carbon, may be included without detracting from the desirable physiological properties of the pyrolytic carbon, and when silicon is used as an additive, it is generally employed in an amount between about 10 and about 20 weight percent. Examples of other carbide-forming elements which might be used as additives in equivalent weight percents include boron, tungsten, tantalum, niobium, vanadium, molybdenum, aluminum, zirconium, titanium, and hafnium. Generally, such an element would not be used in an amount greater than 10 atom percent, based on the total atoms of pyrolytic carbon plus the element.

The carbide-forming additive is co-deposited with the pyrolytic carbon by selecting a volatile compound of the element in question and supplying this compound to the deposition region. Usually, the pyrolytic carbon is deposited from a mixture of an inert gas and a hydrocarbon or the like, and in such an instance, the inert gas is conveniently employed to carry the volatile compound to the deposition region. For example, in a fluidized bed coating process, all or a percentage of the fluidizing gas may be bubbled through a bath of methyltrichlorosilane or some other suitable volatile liquid compound. Under the temperature at which the pyrolysis and co-deposition occurs, the particular element employed is converted to the carbide form and appears dispersed as a carbide throughout the resultant product. As previously indicated, the presence of such a carbide-forming additive does not significantly change the crystalline structure of the pyrolytic carbon deposited from that which would be deposited under the same conditions in the absence of such an additive.

Pyrolytic carbon having the physical properties mentioned hereinbefore, is considered to be particularly advantageous for constituting the surface for a prosthetic device because it is inert to the metabolic processes, enzymes and other juices (physiological fluids) found within living bodies. In addition, such pyrolytic carbon is not injurious to natural bone growth. Particularly preferred is pyrolytic carbon which has a density between about 1.9 and about 2.2 grams per cubic centimeter, wherein the superficial porosity of such carbon facilitates the growth of bone thereto, which is particularly conducive to natural bone growth so that the creation of a good bond between natural bone and the prosthetic device may be established. In order to enhance attachment of either bone or tissue, the surface may be oxidized. This can be done to 1) enhance the superficial roughness and/or to 2) provide oxygen-bearing polar groups on the surface which may react with tissue or bone molecules.

The pyrolytic carbon surface of the prosthetic device may be fabricated with different physical properties at different surface locations, for example a dense, polished wear surface may be employed at the articulating end of a joint prosthesis while a surface having a rough, more porous surface may be employed at the surfaces at which the prosthesis is joined with natural bone to facilitate bone growth thereto.

In addition to mechanical modification of the pyrolytic carbon surface such as polishing, it may be desirable to utilize other physical or chemical modifications of the pyrolytic carbon surface. For example, chemisorbed gases, such as oxygen, may be removed to provide a more hydrophobic surface, or conversely, a surface having an adsorbed gas, such as oxygen, or which is chemically modified such as by forming surface hydroxyl groups, may be employed to provide a more hydrophilic surface. It is believed that pyrolytic carbon surfaces which are more hydrophilic, such as those having surface hydroxyl groups, are more conducive to the establishment of a firm bond with natural bone tissue.

In addition, the prosthetic device should be sterile before implantation in a living body. The device may be sterilized and chemisorbed oxygen removed by heating in a vacuum at an elevated temperature. For example, the pyrolytic carbon-coated prosthesis may be ultrasonically cleaned in benzene, and then again in distilled water, and then out-gassed at 900.degree.C. for two hours to effect complete sterilization and to remove absorbed gases and provide a hydrophobic surface. The device may be sterilized also by heating in a suitable vacuum for about 6 hours at 130.degree.C. or by steam autoclaving. The prosthetic device can also be sterilized chemically, as by treating with benzalkonium chloride, and optionally with a suitable anticoagulant which safeguards against the occurrence of thrombosis, such as heparin, for cases in which the application involves the intra-vascular system.

When the prosthetic device is ready for its intended use, for example, as a joint prosthesis or an internal replacement for another bone segment, or as a dental prosthesis, known surgical and dental procedures are employed. It is recognized that provision of the improved prosthesis of this invention will likely result in development of improved medical techniques of orthopedic repair. The prosthetic devices may be secured in the proper location within the body by suitable means and procedures, including those which are hereafter described for specific embodiments of the present invention.

Illustrated in FIG. 1 of the drawings is a knee joint prosthesis 10 for replacing damaged knee joints. The knee joint prosthesis 10 is formed from a substrate 12 upon which is deposited a pyrolytic carbon exterior coating 14. The prosthesis 10 has a pivotal wear surface 16 which is arcuate in one direction between a front surface 26 and a rear surface 28 and which is designed for pivoting against another such arcuate wear surface in the functioning of a knee joint. The wear surface 16 is only slightly curved in the direction between lateral edges 30, in order to restrict pivoting in that direction and thus stabilize the knee joint against sideward movement. The pivotal wear surface 16 is buff-polished with a diamond dust abrasive in order to reduce friction and wear.

As shown more clearly in FIG. 2, the face 18 which is opposite the wear surface 16 is provided with a series of projections or lugs 20 and indentations or grooves 22 to assure a strong, stable mechanical connection of the prosthesis 10 with the natural bone of the femur or tibia. The pyrolytic carbon coating on the bone-joining face 18 having the lugs 20 and grooves 22 is not polished, but rather is permitted to retain the degree of roughness associated with its deposition. In addition, it may be oxidized to enhance attachment. This roughness fosters the development of a strong mechanical joint between the prosthesis 10 and the natural bone through the growth of natural bone tissue thereto. The pyrolytic carbon deposited on the front and rear surfaces 26 and 28 and on the side surfaces 24 of the prosthesis 10 may also be polished in order to reduce the possibility of irritation of tissues which may come into sliding contact with these surfaces during movement of the joint.

The substrate 12 is preferably formed from a polycrystalline graphite, sold under the trade name POCO AXF graphite, which has a density of about 1.9 grams per cubic centimeter, and average crystallite size (L.sub.c) of about 300A, and an isotropy on the Bacon scale of nearly 1.0. The graphite has a Youngs modulus of elasticity of 1.7 .times. 10.sup. 6 psi. The substrate is formed in the shape of the prosthesis 10 and is coated with a layer of pyrolytic carbon about 500 microns thick which has a density of about 1.9 gm/cm.sup.3 and a modulus of elasticity of 4 .times. 10.sup.6 psi. The composite prosthesis 10 has an effective modulus of elasticity which is in the range of that of natural living bone, which has a modulus between about 2 .times. 10.sup. 6 and 4 .times. 10.sup. 6 psi.

For replacement of a damaged knee joint, the natural bone joint portion corresponding to the prosthesis 10 is surgically removed, and the remaining end is shaped to provide a mating face that will fit in interlocking relationship with the lug and groove containing face 18. The unpolished pyrolytic carbon coating 14 on the face 18 is conducive to natural bone tissue growth and adherence thereto, and its presence induces acceptance of the replacement prosthesis as a functionally permanent section of the natural bone. Because the prosthesis has a rigidity closely approximating that of the natural bone, an excellent anchor is provided, and the prosthesis will remain permanently and perform like natural bone. Thus, since the modulus of elasticity of the prosthesis 10 is very close to that of natural bone, the bone and the prosthesis which is attached to it deform elastically in harmony so that the tendency for the device to work loose is greatly reduced.

It is contemplated that the prosthesis 10 would be employed with a mating prosthesis; thus, for the design shown in FIGS. 1 and 2, the entire knee joint would be replaced by prostheses at the articulating ends of both the femur and the tibia. In use against a mating carbon piece as described above, the polished arcuate surface 16 of the pyrolytic carbon coating has extremely good wear resistance so that re-operation because of inflammation caused by dust resulting from wear is not necessitated. It is understood that if it is desired or necessary to only replace one portion of the knee joint, (i.e., only that of either the femur or tibia), that the pivotal wear surface 16 may be fabricated in a different shape to more beneficially function against the articulating end of the natural bone portion of the joint. In such a case, it is understood that the matching of physical properties of natural bone achieved through the use of the pyrolytic carbon-coated graphite substrate is superior to the mating of a natural bone end (or cartilage) into contact with, for example, a metal which has a modulus of elasticity which may be 10 times higher than that of the natural bone end. Where necessary to repair or replace a knee joint which requires additional replacement of other portions of natural bone, the illustrated prosthetic device may readily be increased in size as necessary.

Shown in FIG. 3 is a prosthesis 40 for implant dentistry. The dental prosthesis 40 is fabricated in the desired shape from a substrate 42 of polycrystalline POCO AXF graphite, such as described hereinabove, which has deposited thereon a pyrolytic carbon coating 44. The substrate 42 is fabricated so that it has a rectangular upper abutment 46 and a lower base section 54 having exterior spiral projections or threads 48 thereon so that the prosthesis may be screwed into a suitable hole drilled into the jawbone in order to provide an immediate anchoring for the prosthesis. The substrate is fabricated with an intermediate cylindrical portion 50 separating the upper abutment 46 and the base section 54 with threads 48, so that when the prosthesis is screwed into a hole drilled in the jawbone until the end 52 of base section 54 contacts the bottom of the hole, the upper abutment 46 will protrude the proper distance through the gum line.

The upper abutment 46 provides a stud-like surface to which a denture such as a porcelain tooth, can be readily mounted, for example, by standard cements. Subsequent growth of natural bone tissue will more firmly anchor the prosthesis into the jawbone. The porcelain tooth is preferably affixed only after sufficient bone tissue growth has occurred to functionally anchor the prosthesis.

FIG. 4 illustrates another dental prosthesis 60 which may also be fabricated from a polycrystalline carbon substrate 62 and a pyrolytic carbon exterior coating 64. The prosthesis 60 has an upper abutment 66 which is reinforced by a 10 percent tungsten-90 percent tantalum alloy wire 68 penetrating through the abutment 66 from a position slightly above the upper surface of the abutment, to a position extending into the intermediate portion 72 of the prosthesis. The wire 68 is inserted into and suitably bonded to the substrate 62 prior to deposition of pyrolytic carbon coating 64 thereon. After deposition of the pyrolytic carbon coating, longitudinal indentations or grooves 70 are machined into a base portion 74 of the prosthesis of sufficient depth to penetrate through the pyrolytic carbon coating 64 and expose the substrate 62. The prosthesis 60 has an intermediate portion 72 of sufficient length such that when the grooved base portion 74 is inserted into a suitably drilled hole in the jawbone, the pyrolytic carbon-coated upper abutment 66 and the wire 68 will protrude a sufficient distance so that a porcelain tooth may be adequately adhered thereto. The after-machined grooves 70 provide localized exposed regions of the porous substrate into which bone tissue may grow, which may be used in combination with the as-deposited surface roughness of the prosthesis to provide attachment to the jawbone through natural bone tissue growth.

The prosthesis has approximately the same modulus of elasticity as the natural bone into which it is inserted so that it performs in harmony with the natural bone without concentrating stress at the natural bone-prosthesis interface, which might cause the prosthesis to work loose. The wire 68, having pyrolytic carbon deposited thereon, provides structural reinforcement in the upper portion of the prosthesis without affecting the modulus of the base portion which is inserted into the natural bone, and is particularly adapted for longer, narrower artificial teeth which may be more difficult to anchor to the prosthesis, such as cuspids and incisors.

The pyrolytic carbon-coated dental prostheses have the outstanding strength which is required for satisfactory performance under the demanding conditions required of dental insert prostheses. While two possible configurations of prostheses have been illustrated, it is apparent that others may be used as well. For example, the root may have a multitude of grooves, holes or crevices that were originally machined into the substrate (a special case of this is the helical spiral threads illustrated in FIG. 3). A smooth-walled substrate might also possibly be used, in which case one might rely completely on the as-deposited surface roughness of the pyrolytic coating and the adherence of natural bone tissue growth thereto to provide attachment. The porosity of the pyrolytic carbon may be varied depending upon the degree of natural bone adhesion to the pyrolytic carbon which is desired in each case.

Alternately, the dental implant may be machined with the same profile as the extracted tooth and is implanted in the cavity left after the extraction. In practice, a tooth may be extracted and an impression made of the resulting cavity. The tooth may be replaced to retain the shape of the cavity until the carbon prosthesis is fabricated. Then the tooth is removed and the prosthesis implanted. The prosthesis is compatible with the natural tissue of the cavity.

Illustrated in FIG. 5 is a shock-absorbing hip joint prosthesis 80 which has a ball section 82 fabricated from a polycrystalline carbon substrate 84 having a coating 86 of pyrolytic carbon thereon and which is shaped so as to be an effective replacement for the natural bone portion of the joint. The exterior portion of the pyrolytic carbon coating 88 is polished to a high gloss by buffing with a diamond dust abrasive to provide an exceptional wear surface and to reduce friction. The ball 82 is fastened to a metal shank or strut 90 by means of a suitable elastomer interlayer 92 which is adhesively bonded to each.

One common problem with the metal-hip joint prostheses has been their lack of the shock-absorbing characteristics of natural bone; the use of the ball 82 which is a carbon substrate coated with pyrolytic carbon reduces this deficiency, but additional cushioning of the joint may be desirable. In this regard, the elastomer interlayer 92 not only provides bonding between the ball and the metal strut 90 but also serves a shock-absorbing function. The shock-absorbing layer 92 may be fabricated from polyethylene or other suitable rubbery material, and it may be protected from body juices by a suitable seal 98 which is held in slight compression against both the ball and the metal strut by biasing tension produced by the elastomer fastening.

At its end opposite the ball 82, the strut 90 angles to form a rod 100 which is inserted into and bonded to a tapered sleeve 102 fabricated from polycrystalline carbon, which may have an outer coating of pyrolytic carbon 104. A fastener 106 may be used on the end of the rod 100 to secure the sleeve thereto. The tapered sleeve 102 with the pyrolytic carbon coating 104 thereon is eminently suited for insertion into the femur to provide an excellent bond by means of adhesion with natural bone tissue.

Alternatively, the entire prosthesis 80 may be fabricated from a carbon substrate having deposited thereon a coating of pyrolytic carbon, either with or without the shock-absorbing feature of the illustrated embodiment.

In certain instances where a segment of bone is missing due to accident or surgery, there is at present no satisfactory method other than bone homograft which is available to join the remaining portions of the bone or replace the missing bone segment. The structure of natural bone is fibrous with channels running longitudinally therethrough. Illustrated in FIG. 6 is a tubular bone segment replacement framework 120. The framework 120 is formed by rolling a strip of woven metallic screen (such as 10 percent tungsten-90 percent tantalum alloy screen) into tubular form, followed by deposition thereon of a pyrolytic carbon coating 124. Thus, bone cells which are formed within the framework 120 have sufficient access to nutrients to become calcified (i.e., strong and supportive) via the longitudinal channels provided between and through the coils of the roller screen 120.

In the illustrated embodiment, the center of the framework is hollow, and the tightness of the roll, the mesh size of the screen 122 and the thickness of the pyrolytic coating 124 deposited thereon are variables that are determined by the particular application. The deposition of the pyrolytic carbon coating 124 upon the screen 122 is accomplished in such a manner that the physical characteristics of the screen are largely preserved, i.e., the regular geometry and porosity of the screen remains.

In use, the framework 120 abuts the bone segments to be joined (or the bone segment) and is fastened thereto by suitable means. For example, the framework 120 may be provided with a central elongated core portion 126 for insertion into the marrow portion of natural bone, as illustrated in FIG. 6. FIG. 7 illustrates the framework prosthesis 120 in position between two segments of natural bone 130.

Since the pyrolytic carbon coating 124 is conducive to natural bone growth, and since the regular geometry of the framework 120 is such that bone cells may grow throughout the framework while being provided with sufficient nutrients, the framework 120 provides a means of permitting natural bone tissue growth to replace or repair missing or diseased bone segments, partially through natural processes. Other more complicated shapes may be formed by laying up formed sheets of screening so that many layers are used, and fastening these together for coating. Filament winding procedures which employ the winding of single or multiple strands, or other suitable processes for forming structural networks of fibers may be used. The resulting shapes, when coated with pyrolytic carbon, should have sufficient porosity and oriented channels so that proper healthy bone can form within and around it. Suitable surgical procedures, such as "seeding" the framework with small portions of natural bone segments from the patient may be developed through the use of such devices to greatly speed up the natural replacement process.

Screening or fiber segments having pyrolytic carbon coatings deposited thereon may also be employed with other prosthetic devices such as those formed from a carbon substrate to aid in the formation of a strong bond between the prosthetic device and natural bone.

Pyrolytic carbon coated on a metal wire substrate may also have other prosthetic applications. For example, a prosthesis for insert dentistry may be provided which comprises a bridge constructed of a suitable refractory wire having an abutment thereon. The whole device is coated with pyrolytic carbon and placement is made, not into the jawbone, but rather so that the prosthesis straddles the jawbone. It is implanted under the skin so that only the small abutment protrudes through the skin; in this case also, the carbon coated framework grows to the bone to provide a strong support for the abutment. Subsequently, a denture such as a porcelain tooth or other dental prosthesis is fastened to the abutment, which provides firm support. Such implants are useful for supporting extracorporeal dental bridgework so that additional mechanical strain need not be placed on remaining teeth by fastening the bridgework to them.

Although the foregoing examples disclose the best modes presently contemplated by the inventors for carrying out their invention, it should be understood that these examples are only illustrative and do not constitute limitations upon the invention which is defined by the claims appearing at the end of the specification. The following Examples further illustrate the fabrication and use of specific embodiments of this invention.

EXAMPLE I

A knee-joint prosthesis such as illustrated in FIGS. 1 and 2 is machined from polycrystalline graphite (POCO AXF graphite). The machined substrate is designed to be of sufficient size for repair of the knee joint of an average, adult human male. The substrate is placed in a rotating wire cage within a reaction chamber that contains in addition a bed of particles. The substrate fits loosely in the wire cage so that, when it is rotated, the point or points of contact of the substrate with the wire cage will change as they rotate, so that the pyrolytic carbon coating will be evenly deposited.

The reaction chamber is heated to a temperature of about 1350.degree.C. When the temperature of the substrate reaches about 1350.degree.C., a flow of helium gas and propane, such that there is a partial pressure of propane of 0.4 (total pressure one atmosphere), is introduced into the reaction vessel so that it is directed against the surfaces of the substrate, which is being rotated in the wire cage. The propane decomposes under these conditions to deposit a dense isotropic pyrolytic carbon coating upon all of the surfaces of the polycrystalline graphite substrate. Under these coating conditions, the carbon deposition rate is about 5 microns per minute, and the propane gas flow is continued until an isotropic pyrolytic carbon coating about 500 microns thick is deposited on the substrate. After the desired thickness of pyrolytic carbon is deposited on the outside of the prosthesis substrate, the propane gas flow is terminated. The coated substrate is cooled fairly slowly in the helium gas stream, and it is then removed from the reaction chamber.

The wear surface and the front, rear, and side surfaces of the prosthesis are buffed to a high gloss by using a diamond dust abrasive. The as-deposited surface roughness of the pyrolytic coating is allowed to remain on the bone-joining face containing the lugs and grooves, and this facilitates the establishment of a strong bond between the prosthesis and the natural bone. Measurement shows that the pyrolytic carbon has a density of about 1.9 gm/cm.sup.3, an L.sub.c of about 30 A. and a BAF of about 1.1.

EXAMPLE II

Dental prosthesis substrates like those illustrated in FIG. 3 are machined from artificial polycrystalline graphite (POCO graphite). The prostheses have squared abutments about 4 mm. high and 2 mm. wide, an intermediate portion about 6 mm. long and 3 mm. in diameter, and a threaded base portion about 8 mm. long and 4 mm. in diameter at the outside of the threads.

The artificial graphite employed as a substrate has a coefficient of thermal expansion of about 8 .times. 10.sup.-.sup.6 /.degree.C. when measured at 50.degree.C. The dental prosthesis substrates are coated with pyrolytic carbon using a fluidized bed coating apparatus which includes a reaction tube having a diameter of about 3.8 centimeters that is heated to a temperature of about 1350.degree.C. A flow of helium gas sufficient to levitate a number of the relatively small prostheses along with the bed is maintained upwardly through the apparatus.

A number of prostheses are coated together with a charge of about 50 grams of zirconium dioxide particles which have diameters in the range of about 150 to 250 microns. The particles are added along with the prostheses to provide a deposition surface area of the desired amount, relative to the size of the region of the reaction tube wherein the pyrolysis occurs, inasmuch as the relative amount of available surface area is another factor which influences the physical characteristics of the resultant pyrolytic carbon. When the temperature of the particles which are levitated within the reaction tube reaches about 1350.degree.C., propane is admixed with the helium to provide an upwardly flowing gas stream having a total flow rate of about 10 l. per minute and having a partial pressure of propane of about 0.4 (total pressure 1 atmosphere). The propane decomposes under these conditions and deposits a dense isotropic pyrolytic carbon coating upon all of the articles in the fluidized bed. Under these coating conditions, the carbon deposition rate is about 5 microns per minute. The propane gas flow is continued until an isotropic pyrolytic carbon coating about 300 microns thick is deposited on the outside of the prostheses substrates. At this time, the propane gas flow is terminated and the coated articles are cooled fairly slowly in the helium gas and then removed from the reaction tube coating apparatus.

Photomicrographs taken of the pyrolytic carbon coating of the prostheses using an electron-scanning microscope show the as-deposited surfaces to have a coral-like appearance. The dental prosthesis may be screwed into suitably drilled holes in a living jawbone to provide an abutment to which an artificial tooth or a denture may be affixed. The pyrolytic carbon coating is conducive to bone growth, and the natural formation of bony tissue around and attached to the prosthesis in time will provide a very firm anchor for the prosthesis. In addition, the prosthesis matches the modulus of elasticity of natural bone, and there is little or no tendency for the prosthesis to work loose under severe loads after the prosthesis has been heated in place.

EXAMPLE III

A sheet of metal screen is selected which is composed of an alloy of 10 percent tungsten-90 percent tantalum, which is woven from wire having a diameter of 0.005 and a mesh of 20 such wires per inch. A single, T-shaped piece is cut from the screen having a cross piece dimension of 21/2 inches by 6 inches, and a base piece dimension of 11/2 inches wide, which is equal to the length of the missing bone segment. The wire screen is rolled up from the cross piece toward the base so that a tube such as that illustrated in FIGS. 6 and 7 is formed, which has an outside diameter equivalent to that of the bone to be replaced. The rolled tube has an outer layer of screen about 11/2 inches long, and a centrally located internal tubular portion of the rolled screen which is 21/2 inches long. The rolled screen is fastened in this position and structurally interlaced by means of fine wires of the same tungsten-tantalum alloy, and any sharp ends of wire from the screen or the fastening wires are bent inwardly and crimped. The rolled tube is placed loosely on a rotating mandrel in a reaction tube having a diameter of 4 inches through which a flow of helium is maintained and which is heated to a temperature of about 1300.degree.C.

In addition to the wire tube, a charge of about 100 grams of zirconium dioxide particles which have diameters in the range of about 150 to 250 microns is introduced to modify the relative amount of available surface area for deposition. When the screen reaches the temperature of the reaction tube, propane is added to the upward flow of helium past the screen on the rotating mandrel. The gas stream has a partial pressure of propane of 0.4 (total pressure one atmosphere). The propane decomposes under these conditions to deposit a dense isotropic pyrolytic carbon coating upon the screen. The propane gas flow is continued until an isotropic pyrolytic carbon coating about 200 microns thick is deposited on the outside of the wire screen substrate. This degree of deposition leaves intact the mesh nature of the screen, while strengthening it, and providing the physiologically beneficial pyrolytic carbon coating thereon.

The prosthesis may be used to replace a 11/2 inch section of damaged or diseased bone in a manner such as illustrated in FIG. 7. Thus, 1/2 inch lengths of the bone segment to be joined are bored out so that the inner tube portions may be slipped into the bone ends. The framework prosthesis may be affixed, for example by small pins or small pyrolytic carbon-coated screws.

EXAMPLE IV

A knee joint prosthesis is prepared exactly as Example I, except that different structure is provided for achieving a firm bond between the prosthesis and the natural bone to which it is attached. Instead of a series of lugs and grooves as in the prosthesis illustrated in FIGS. 1 and 2, the substrate is provided with only one lug located centrally in the bone joining face; however, a strip of metal screen (90 percent-tungsten-10 percent tantalum alloy is wrapped around and bonded to the periphery of the substrate so that one side of the screen does not extend quite up to the wear surface, but on the other side overlaps beyond the bone joining face a distance of about 1/2 inch. Deposition of the pyrolytic carbon coating as in Example I serves to further bond the screen to the substrate and to weld the entire prosthesis together as a unit. Following deposition of the pyrolytic carbon coating as in Example I, the wear surface is highly polished by buffing with a diamond dust abrasive.

The prosthesis may be implanted by machining the bone face it is to abut so that it receives the lug, and by slipping the pyrolytic carbon-coated screen over the outside of the bone end. The screen may be fastened by means of screws or pins if desired; subsequent growth of bone tissue to the prosthesis, including the bone joining face and the overlapping screen, provides a stable bond. The technique illustrated in this Example may also be employed in the joining of other types of prostheses to natural bone segments.

Although the figures and examples illustrate only certain specific embodiments of this invention, such as the specific types of prosthetic devices, specific pyrolytic carbon coating methods, fastening methods and substrate materials, it is contemplated that other embodiments may be employed also. Thus, in addition to knee and hip joints, other joints, such as finger, elbow and shoulder joints, may be repaired or replaced by pyrolytic carbon coated prostheses. In addition, long bones, others such as phlanges, vertebrae, etc., may be repaired or replaced in whole or in part.

Although the examples particularly describe the deposition of pyrolytic carbon through the use of propane, it is understood that other hydrocarbons, or mixtures of hydrocarbons, may be employed to deposit the pyrolytic carbon coating on the substrate. The variables of the deposition process itself may be employed to vary the properties of the pyrolytic carbon coating, or to accommodate the size, temperature-stable range, or structure of the substrate material.

A number of specific methods of fastening the prosthetic device to natural bone tissue have been demonstrated. The pyrolytic carbon coating is believed to have a particular degree of as-deposited surface roughness or surface porosity which aids in the adherence of growing bone tissue thereto. In support of this theory by which, however, it is not intended that the invention be limited, photomicrographs made through the use of an electron scanning microscope reveal that preferred pyrolytic carbon deposits have a surface which looks similar, under high magnification, to coral. The porosity is only superficial; the bulk of the coating is dense with a preferred range of density from about 1.9 g./cm.sup.3 to about 2.2 g./cm.sup.3.

Although a wide range of substrate materials may be employed, those which have a modulus of elasticity near that of natural bone are particularly preferred for use in prostheses for bulk replacement of bone. Artificial graphites are particularly preferred. For framework prostheses, various refractory fibers and wires which have been formed in woven, wound and nonwoven structure may be employed as substrates. Prostheses made from such substrates are also useful for providing support for mending shattered or diseased bones, which support does not corrode, weaken or inflame, as can prior art devices, and which may remain permanently as a structural part of the healed bone segment. Where it is desired that high modulus metallic fibers or screens not be used, lower modulus fibers, such as carbon fibers, cloth or screen, may be coated with pyrolytic carbon to provide a lower modulus framework which has high strength and the biological compatibility and other properties of pyrolytic carbon.

Various features of the invention are set forth in the following claims.

Claims

What is claimed is:

1. A composite orthopedic prosthetic device for repair or replacement of bone structure in a living body, comprising a refractory substrate of suitable predetermined shape and a vapor-deposited pyrolytic carbon coating on said substrate, said pyrolytic carbon coating being compatible with living tissue and having a bulk density of at least about 1.5 grams per cubic centimeter, a crystallite size no greater than about 200 A, a thickness of at least about 25 microns, and a Bacon Anisotrophy Factor of not more than about 2.0, and said pyrolytic carbon coating having an as-deposited surface porosity on at least a portion of its surface intended to be placed adjacent living bone tissue upon implantation.

2. A prosthetic device according to claim 1 wherein the prosthesis is a framework prosthesis adapted to provide sufficient access to nutrients for the growth of living bone tissue therethrough, and wherein said substrate is a fiber aggregate form.

3. A prosthetic device according to claim 2 wherein said substrate is made of refractory metal selected from the group consisting of tungsten, tantalum, molybdenum, and alloys thereof.

4. A prosthetic device according to claim 2 wherein said substrate is made of carbon fiber.

5. A prosthetic device according to claim 1 wherein said substrate has a thermal coefficient of expansion of from about 3 .times. 10.sup.-.sup.6 /.degree.C. to about 8 .times. 10.sup.-.sup.6 /.degree.C.

6. A prosthetic device according to claim 5 wherein the substrate has a modulus of elasticity of between about 1 .times. 10.sup.6 psi. to about 5 .times. 10.sup.6 psi.

7. A prosthetic device according to claim 6 wherein the substrate is artificial graphite having a modulus of elasticity between about 1.7 .times. 10.sup.6 psi. and about 4 .times. 10.sup.6 psi. such that the pyrolytic carbon coated substrate has a modulus of elasticity approximating that of natural bone.

8. A prosthetic device according to claim 7 wherein said substrate is isotropic polycrystalline graphite having an average crystallite size of about 300 A, a density of about 1.9 grams per cubic centimeter, and a modulus of elasticity of about 1.7 .times. 10.sup.6 psi.

9. A prosthesis according to claim 7 wherein said density of said pyrolytic carbon coating is between about 1.9 and about 2.2 grams per cm.sup.3.

10. A prosthetic device according to claim 7 wherein said device is a joint prosthesis having a bone-joining surface and an articulating wear surface, and wherein the pyrolytic carbon coating on the wear surface is polished to reduce friction and improve wear characteristics

11. A prosthetic device according to claim 10 wherein the bone-joining surface has at least one projection adapted to fit in interlocking relationship with a mating, natural bone surface.

12. A prosthetic device according to claim 7 wherein the prosthetic device is a dental prosthesis having a base portion adapted to be inserted into a jawbone, an abutment for attaching a tooth or other denture, and an intermediate portion between the abutment and base portion of sufficient length such that upon insertion the abutment protrudes the proper distance from the jawbone for attachment of the tooth or other denture.

13. A dental prosthesis according to claim 12 wherein the base portion of the prosthesis is formed with indentations penetrating through the pyrolytic carbon coating into the substrate.

14. A prosthetic device according to claim 1 wherein the surface of said pyrolytic carbon coating is provided with oxygen-bearing polar groups to enhance the attachment of bone or other tissue thereto.