Abstract:
Medical implants are formed at least in part from porous metal wherein the pores of the metal are filled with a hardenable material. The hardenable material can be used to assist in the manufacture of the implant, permitting detailed machining of the implant. Further, the hardenable material can be a resorbable material that strengthens the implant during implantation but is resorbed, once implanted, to permit bone growth and infiltration. Further, in an alternate embodiment, the hardenable material can be a thermoplastic material that can be heated by ultrasonic energy to create an adhesive bond between a fastener and bone structure.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to medical implants and particularly to medical implants having a porous metal portion. 
       BACKGROUND OF THE INVENTION 
       [0002]    Porous metals have been used extensively in medical implants. As disclosed in U.S. Pat. No. 5,282,861, these porous metal members permit bone growth into the pores, which, in turn, binds the implant to the bone. 
         [0003]    The general nature of the porous metal provides some difficulties. The porous metal can be easily bent or distorted while being implanted, especially if the implant is hollow or very thin. For example, twisting forces developed during insertion may shear a screw in half even though the screw may have adequate strength for in situ loads. Further, the nature of the metal makes it difficult to machine certain fine parts from the porous metal. In particular, threaded fasteners are difficult to machine from the porous metal. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is premised on the realization that porous metal members that are part of medical implants can be strengthened by incorporating into the pores of the porous metal a hardening material. The hardening material strengthens the implant and allows it to be easily processed, e.g., machined. 
         [0005]    In one embodiment, the hardening material is a resorbable material, which, once implanted, is resorbed allowing bone growth within the metal. In another embodiment, the hardening material is a resorbable material, which, once implanted, is resorbed and incorporated into the mineral matrix of ingrowth bony tissue. 
         [0006]    The hardening material can further be various polymers that can be removed prior to implantation, or, alternatively, in situ, to form an adhesive bond once implanted. 
         [0007]    The objects and advantages of the present invention will be further appreciated in light of the following detailed description and drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]      FIG. 1  is a cross section of an exemplary implant incorporating porous metal; 
           [0009]      FIG. 2  is an enlarged cross sectional segment taken at circle  2  of  FIG. 1 ; 
           [0010]      FIG. 3  is a cross section of a fastener formed from porous metal; 
           [0011]      FIG. 4  is an enlarged cross sectional segment taken at circle  4  of  FIG. 3 ; 
           [0012]      FIG. 5  is a diagrammatic cross section of an implanted fastener; and 
           [0013]      FIG. 6  is an enlarged cross sectional segment taken at circle  6  of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    According to the present invention, a medical implant such as a spinal implant or an orthopedic implant is formed at least in part from a porous metal member wherein the pores of the porous metal member are filled with a hardening material, which facilitates fabrication and/or implantation of the medical implant. 
         [0015]    For use in the present invention, any biologically acceptable porous metal can be used. These can be formed from tantalum, titanium, zirconium, cobalt, chrome and stainless steel, as well as alloys of these. Preferably the pores of the porous metal will have a pore size of about 150 microns to about 500 microns, or more, depending upon the particular application. However, in certain applications a smaller pore size may be preferred. 
         [0016]    One preferred porous metal is Trabecular Metal™ material, which is a porous tantalum material marketed by Zimmer Spine, Inc., of Edina, Minn. This material is also described in several U.S. patents, including, for example, U.S. Pat. Nos. 5,282,861; 5,443,515; and 6,063,442, the disclosures of which are incorporated herein by reference. These patents describe formation of a tantalum porous structure by chemical vapor deposition of tantalum onto a foam carbon structure. 
         [0017]    The porous metal member can be part of any medical implant.  FIG. 1  shows an exemplary embodiment of a medical implant  10  made according to the present invention. Implant  10  includes a thin plate  11  and a plurality of fasteners, such as screws  12 , with the plate  11  and fasteners  12  all formed from porous metal, preferably the porous tantalum previously discussed. The porous metal member can also be a nonthreaded fastener such as the fastener  14  shown in  FIG. 3 , or virtually any other implanted structure. 
         [0018]    The hardenable material, which fills the pores at least on the surface of the porous metal member, can be ceramic, or a polymer, or a composite of a polymer and a ceramic, a composite of ceramics or a blend of polymers. In one preferred embodiment, the hardenable material is a resorbable material, which, once implanted within the body, is resorbed allowing bone growth into the pores. 
         [0019]    There are many well known resorbable polymers. Particularly polymers formed from lactic acid and glycolic acid are suitable for use in the present invention. Specific resorbable materials include polylactic acid (PLA), polyglycolic acid (PGA), poly-L-lactide (PLLA), poly-DL-lactide (PDLLA), copolymers of PLA and PGA (PLGA), copolymers of PLLA and PDLLA. Other resorbable materials include resorbable hydrogels of poly(ethylene oxide), as well as methyl cellulose, hyaluronic acid, chitosan, collagen, gelatin, fibrin, dextran, agarose and carboxy methyl cellulose. The resorbable material can also be a composite of resorbable polymers with calcium sulfate, calcium phosphate, calcium phosphate cements, hydroxyapatite, beta-tricalcium phosphate and bioactive glass and glass ceramics. The resorbable materials can also be a ceramic including dicalcium phosphate dihydrate, calcium phosphate cements, hydroxyapatite, beta tricalcium phosphate, and bioactive glass and glass ceramics. Preferred polymers include poly (L-lactide), polyglycolic acid, polylactic acid and poly (L-lactide-co-glycolide). 
         [0020]    The different resorbable materials have different rates of resorption, allowing the implant to be designed for the particular application requirements. The surgeon can select the bioresorbable material in the porous implant based on the resorption characteristics of the bioresorbable material. Quick resorption may be required when implanting forces are greater than physiological loads. Slower resorption is preferred if implant strength must be maintained during osteointegration. The filler can also be doped with osteoinductive or osteoconductive agents, antibiotics, or agents to enhance integration. 
         [0021]    Alternately, nonresorbable polymers can be used such as thermoplastic or thermoset polymers, particularly polymers that can be softened subsequent to implantation using application of ultrasonic energy, light or heat. It may also be preferred to infiltrate the pores of the porous member with other harder polymers, such as the polyarylether ketone family of polymers, to provide strengthening for various portions of the porous metal member as discussed hereinafter. Other polymers may be infused into the porous metal to facilitate fabrication. These can be removed through solvent leaching prior to implantation. 
         [0022]    The hardenable material can be infused into a porous metal member using a variety of different methods, either before or after forming the porous metal member. Thermoplastic polymers can be injection molded into the porous metal part by placing the porous metal into a mold and injecting a flowable material into the mold (i.e., insert molding). The porous metal part can be prepared by curing thermoset monomers with the assistance of initiators. The porous metal part can also be immersed in a molten solution or slurry of the hardening material. Many of the hardening materials are pastes. These can be pressed into the pores and allowed to set. 
         [0023]    To form fasteners, such as screws  12  shown in  FIG. 1 , a cylinder of porous metal is formed and infiltrated with a hardening material  16  (as shown in  FIG. 2 ). In this embodiment the hardening material can be any material that can saturate the pores, provide strength to the overall porous metal composite, and can subsequently be removed either through resorption or through other processes. After the voids in the porous metal cylinder are filled with the desired hardening material  16 , the threads  18  are machined in the cylinder to form screw  12 . If the hardening material is a resorbable material, it will be left in the screw. If the hardening material is not resorbable, it may be removed prior to implantation. 
         [0024]    Other machining and forming processes, such as grinding, shaping, bending, milling, and the like, can also be used to form an implant or part from a porous metal member filled with a hardened filler. 
         [0025]    The plate  11  of implant  10  is also formed from a porous metal material. The plate  11  is first cut to shape and subsequently is filled with a resorbable hardening material such as the PLA, PLLA, PGA, a calcium phosphate material, or a blend or composite of them. Again, the pores are simply infused with the hardenable material, which is allowed to harden either through cooling or evaporation, and the implant  10  is then ready for implantation. Subsequent to implantation, the hardening material will resorb into the body and bone growth will replace the hardening material. 
         [0026]    The pores in the plate  11  may be filled with a different resorbable material than the pores in the screws  12 . For example, the plate  11  may be filled with a rapidly resorbable material, whereas the screws  12  are filled with a filler that is resorbed more gradually. Different portions of plate  10  may also be filled with different fillers. For example, a first side  20  of plate  10  may be filled with a first filer such as calcium phosphate and the second side  22  can be filled with PLA or even a thermoset polymer that is not resorbed. 
         [0027]      FIGS. 3 and 4  show a further alternate embodiment of the present invention. In this embodiment, fastener  14 , such as one that might be used in a spinal implant fixation assembly to attach a rod to a spinal column, as disclosed in Boschert published US application US2005/0277928, the disclosure of which is hereby incorporated by reference, is formed from a porous metal. This is merely an exemplary fastener. 
         [0028]    In this embodiment, the fastener  14  is a tapered cylinder formed from the porous metal. Alternately, the fastener  14  can have a nontapered shape such as a cylinder or rectangle. As shown in  FIG. 4 , two separate materials are used to fill the pores of the porous metal. The interior portion  26  of the porous metal is formed with a very hard polymer  28 , which is intended to remain in place and provide strength to the fastener  14 . Any biologically acceptable polymer having the desired physical characteristics can be employed. One particular polymeric material suitable for use in the present invention is polyetheretherketone. 
         [0029]    The pores in the outer portion  30  of the fastener  14  are filled with a polymeric material  32  which can be applied by localized application of energy such as ultrasonic, light, or thermal energy. Any biologically acceptable thermoplastic adhesive can be used. One suitable polymeric material is polyethylene. 
         [0030]    This fastener  14  can be filled with the hardening materials  28  and  32  in a variety of different manners. The inner, harder, polymeric material  28  can be injected through a central shaft (not shown) drilled through the fastener, or, alternately, the entire structure can be impregnated with the material  28  and, subsequently, the outer portion of the material removed by immersing fastener  20  for a period of time in a suitable solvent to dissolve the polymer  28  from the outer portion  30  of the fastener  14 . 
         [0031]    The softer polymer  32  on the outer surface  30  would then simply be applied by immersing the fastener  14  in the softer polymer either as a solution or as a melt. The polymer can be one which changes its physical state upon application of energy. 
         [0032]    Referring to  FIGS. 5 and 6 , the fastener  14  can be implanted in a unique manner. In particular, the fastener would be inserted in a hole which extends through the cortical bone  40  and into the cancellous bone  42 . Fastener  14  would then be subjected to energy, for example, to ultraviolet, ultrasonic, or thermal energy to cause the outer polymeric material  32  in pores of fastener  14  to melt. The polymeric material would flow as indicated by arrows  43 , forming a bond by causing at least a portion of the polymeric material  32  to flow into pores  44  in cancellous bone  42 . This would leave open pores  46  remaining within the fastener  14 , which may be subsequently filled with new bone growth. 
         [0033]    This structure will provide a stronger fastener because of the inner polymeric material or filler  28  in inner portion  26 , and will provide a stronger bond to the bone because of the coupling activity of the softer polymeric material  32  which flows from the outer peripheral portion  30  of the porous metal into the porous cancellous bone  42 . 
         [0034]    In another embodiment, the inner portion of the fastener can be solid, non-porous material, such as a metal, and the porous material can be coupled to the outer surface of the solid core through a coating/adhesion process. The porous outer portion of the fastener can then be infused with the bioresorbable polymer. 
         [0035]    Alternatively, two different resorbable materials could be used in the fastener  14 . The inner material may be one that is resorbed at a different rate than the outer material. 
         [0036]    Further, porous metal fastener  14  can be formed with a lower pointed portion  38  filled with a first filler and an upper portion  40  filled with a second filler. 
         [0037]    The first filler can be an ultrasonically meltable polymer or a rapidly resorbed filler. The second filler may be a thermoset polymer or a resorbable filler. If the first and second fillers are resorbable, it may be desirable for the first filler to resorb quickly to promote rapid bone infusion. The second filler may be a slowly resorbed filler that mains the strength of the fasteners. 
         [0038]    Using multiple hardening fillers allows the porous metal part, in this case a fastener, to be specifically engineered to suit a particular application. Different hardenable materials can be employed as needed to provide for machining prior to implantation, short term or long term strength during implantation, or to provide adhesion or a combination of these. 
         [0039]    These different embodiments have all been described, as well as the preferred method of practicing the present invention. However, the invention itself should only be defined by the appended claims.