This invention relates to a surgical implant adapted for secure attachment to living bone. More particularly this invention relates to an improved orthopedic or dental implant formed to have bone contacting surfaces comprised of biocompatible organic polymers substituted with oxyacid groups or salts thereof. Applicant has found that the presence of such oxyacid substituted polymers, at least at the bone contacting surface, promotes interfacial osteogenesis and provides sites to which bone can chemically bond, thus fostering direct chemical bonding of the implant surface with the biological polymers present in developing bone tissue.
In severe cases of arthritis or other bone and joint degenerative diseases, surgical replacement of the affected joint and bone tissue is a commonly used procedure. In cases of dentition lost to disease or trauma, endosseous dental implants have been used to restore function. Research and development efforts have been highly successful in identifying materials for use alone or as composite structures for prosthetic implants --materials which meet both the basic physical (biomechanical) demands and the chemical biocompatibility requirements dictated by their use in implanted devices. Due to the stringent mechanical demands placed on load bearing bone prostheses, metals have been the material of choice for the most severely loaded parts of such implants. The metals generally used in load bearing components of orthopedic devices are limited to the cobalt, chromium, molybdenum alloys, titanium and surgical stainless steel. More recently, high strength ceramics and reinforced polymers have been introduced for such applications.
Notwithstanding the major advances which have been made in implant materials development, patients still face the trauma and expense of implant failure. The most common point of bone implant failure is not breakage or failure of the prosthetic implant itself; the more common failure of implants is at the living bone-implant interface. In other words, the implant simply works loose from its implanted position. Early total joint replacements were fixed to the bone of a recipient through a press fit of the prosthesis into a carefully prepared surgical bed. This method often resulted in loosening of the implant in the long term.
A review of the recent literature reveals significant research and development efforts directed to improving implant-bone fixation. A major advance in joint replacement surgery was the introduction of the use of poly(methyl methacrylate) [PMMA] bone cement for fixing the components of a joint prosthesis to bone. PMMA is not a glue or adhesive but a true cement which works mechanically. It is applied in a dough-like state as a grouting agent between the bone and the implant so that it can flow around the contours of the bone and the implant and into the interstices of cancellous bone. Upon hardening it forms a mechanically interlocked attachment between the bone and the implant. While PMMA bone cement provides a secure fixation of the prosthesis with living bone in the short term, the long term loss of implant fixation has proven to be a significant problem. The degeneration in implant fixation begins with a resorption of the bony tissue immediately adjacent to the bone cement and the replacement of that tissue with a soft fibrous tissue capsule. Since the fibrous tissue is far more compliant than bone, the thicker the capsule the looser the implant becomes. Since the thickness of the capsule tends to increase with motion the loosening process is self-reinforcing.
In an alternative method of implant fixation enjoying widespread use, the implant is provided with a highly porous surface coating that provides interstices into which bone can grow. Materials which provide pore size distributions of 50 to 500 microns have shown considerable promise and have become more widely used especially in young, active patients. For bone to interlock with the pore structure of the implant the implant must be firmly fixed at the time of surgery and load application must be minimized during the in growth period. Immediate surgical fixation is usually accomplished by the mechanical impaction of the implant into a slightly sub-sized surgical bed. The problems related to both of these fixation methods arise from the lack of affinity demonstrated by healing bone for the heretofore known metallic alloys and polymeric materials used in reconstructive orthopedic and oral surgery. When these materials are placed into bony defects, bone does not deposit directly onto the implant surfaces but, significantly, remains separated from them by at least a thin layer of soft tissue. This precludes the possibility of any chemical bond between the bone and the implant and limits the fixation modes to those based on mechanical interlock. Materials of this type are defined as "osteophobic" for the purposes of this disclosure.
Due to the problem of obtaining fixation to living bone by purely mechanical means, more recent research efforts have been directed at finding more bone-tissue-compatible implant materials, particularly materials having some demonstrable chemical affinity for regenerating bone tissue. It has been found that when certain minerals are implarted into osseous defects, newly forming bone will deposit directly onto the mineral surface without any intervening layer of soft tissue. Furthermore not only does bone deposit directly on the surface of these minerals, but it has been observed that bone will adhere even to a smooth surface of said minerals through the formation of chemical bonds across the bone-mineral implant surface.
For the purposes of this disclosure materials onto which bone deposits and chemically bonds are defined as "osteophilic", and the process whereby bone deposits directly onto a free surface is defined as "interfacial osteogenesis." The first materials recognized to have osteophilic properties were hydroxyapatite [Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2 ] and tricalcium phospate [Ca.sub.3 (PO.sub.4).sub.2 ]. Their osteophilicity has been rationalized by their close similarity in chemical structure with the mineral phase of mammalian bone. More recently, this inventor has made the surprising discovery that chemical similarity with bone structure is not necessary for a mineral substance to exhibit osteophilic properties. Indeed, it has been found that calcite [CaCO.sub.3 ], dolomite [CaMg(CO.sub.3).sub.2 ] (See my copending U.S. patent application Ser. No. 251,225), and more recently the minerals fluorapatite, aragonite, magnesite, witherite and barite have been found to exhibit osteophilic properties. A number of researchers have sought to take advantage of in vivo chemical bonding of bone to bioactive mineral surfaces. See, for example, U.S. Pat. Nos. 3,787,900; 3,919,723; 4,168,326; 4,320,514; 4,366,188; 4,373,217; and 4,437,192.
The present invention is based on the discovery that certain organic polymers, that is, organic polymers bearing salt-forming oxyacid functional groups, exhibit an affinity toward developing bone tissue similar to that which has been observed for the above-mentioned inorganic mineral materials. The brittle nature of osteophilic minerals limits their usefulness to low stress applications. One such application is as a coating for porous-surfaced dental and orthopedic implants. It is known that pore sizes on the order of about 100-200 microns diameter are required for bony in growth into osteophobic materials. However, bone will grow into cracks and pores as small as 2 microns in diameter in osteophilic minerals. The pores of osteophobic metals, for example, have been coated with a thin layer of an osteophilic mineral to promote the rate of bony in growth. One serious problem related to the application of thin mineral coatings to porous metal surfaces is the lack of adhesion of such coatings to the metal substrate and the propensity for the minerals to dissolve in vivo when applied as thin films.
The present invention is based on the discovery that bone will deposit onto and chemically bond with organic polymeric materials having, at least on their bone contacting surfaces, covalently attached salt forming oxyacid functional groups. That discovery coupled with the inherent versatility of organic polymers for implant fabrication represents a significant advance in the art. Polymeric materials exhibit superior processability and mechanical properties. They can be extruded, molded, shaped and machined, or they can be applied, as a melt or dissolved in solution, to coat the surfaces of prosthetic devices constructed of other materials or material composites. Many biocompatible polymers in bulk form exhibit weight/strength properties (especially when reinforced with high tensile strength fibers) unmatched by the metal or metal/ceramic materials which have been used for construction of prosthetic devices. Still a further advantage of the use of biocompatible organic polymers for construction of orthopedic protheses is their chemical versatility. That is, implant devices constructed of or coated with biocompatible organic polymers can be surface modified by chemical treatment using a wide variety of reaction conditions to bear the desired surface-active oxyacid groups in accordance with this invention.
It is therefore an object of this invention to provide a method for promoting interfacial osteogenesis on an implant surface.
It is another object of this invention to provide a method for promoting bone in growth and bone adhesion to bone contacting surfaces of implanted prostheses.
Another object of this invention is to provide orthopedic prostheses having improved bone contacting surfaces comprised of a biocompatible organic polymer substituted with salt-forming oxyacid groups selected from the group consisting of the oxyacids of carbon, sulfur and phosphorus.