Prosthetic implant and assembly method

Abstract

An orthopedic prosthetic implant comprises a metal alloy stem element ( 13, 63, 113 ) which has one end portion ( 19, 69, 119 ) constructed to reside in the medullary cavity of a bone and an integral connector ( 23, 73, 123 ) at the opposite end, to which a crystalline brittle head ( 17, 67, 117 ) preferably made of pyrocarbon-coated graphite is joined. The effective joinder of the head to the stem element is achieved through a polymeric insert ( 15, 65, 115 ) which has selected elastic properties. The design and material of the polymeric insert allow it to be securely received within an interior cavity ( 35, 77, 131 ) of the pyrocarbon-coated graphite head and mated to the stem connector in an either rigidly or bi-polar arrangement. Such joinder allows the composite implants that utilize the most desirable properties of metallic and brittle crystalline materials.

1 . A prosthetic implant for implantation into a resected bone, which implant comprises:
 a metal stem element which has a connector at one end that is shaped with a surface region of reduced dimension,   an integral one-piece polymeric sleeve which defines a central cavity that receives said connector, and   a head formed of brittle crystalline material having an articular exterior surface and an interior cavity that is proportioned to receive said polymeric sleeve,   said head interior cavity having an entrance of a size such that at least a portion of said polymeric sleeve must elastically deform inward to enter said interior cavity,   said sleeve being formed with means for interengaging with said connector which requires radially outward deformation of at least a portion of said sleeve to lock said sleeve and said connector in engagement, and with means providing a relief region into which said portion of said polymeric sleeve can elastically deform to facilitate final assembly, and   said integral polymeric sleeve being made of polymeric material having an elasticity, such that (a) it can deform radially inward sufficient to facilitate its entry into said head interior cavity and then return to shape, (b) it can deform radially outward to facilitate assembly with said connector and then return to shape, and (c) once assembled with both said head and said connector, disassembly cannot inadvertently occur.   
     
     
         2 . The implant of  claim 1  wherein said polymeric sleeve has an exterior lateral wall with a cylindrical wall region of circular cross-section and a tapered end region to facilitate its entry into said entrance to said head interior cavity, which entrance is defined by an inwardly protruding lip. 
     
     
         3 . The implant of  claim 2  wherein said stem element has a radial flange of generally circular shape spaced from said connector. 
     
     
         4 . The implant of  claim 3  wherein said connector is joined to the remainder of said metal stem element by a neck portion of reduced dimension which extends to said connector from said radial flange. 
     
     
         5 . The implant of  claim 2  wherein said interengaging means comprises a grooved region formed in the exterior lateral wall of said connector and an annular locking flange protruding radially inward from an interior wall of said polymeric sleeve, which locking flange is received in said grooved region, and wherein said relief region is provided by a circumferential hollow in the exterior lateral wall of said polymeric sleeve that is located at about the same axial location on said sleeve as said inwardly protruding locking flange. 
     
     
         6 . The implant of  claim 5  wherein the cross-sectional volume of said circumferential hollow is at least about 90% of the volume of said protruding locking flange. 
     
     
         7 . The implant of  claim 5  wherein said connector grooved region has a pair of spaced apart radial surfaces that are essentially parallel to each other and essentially perpendicular to the axis of said stem element, and wherein said protruding locking flange has an arcuate shape when cut by an axially aligned plane, which flange becomes seated between said parallel radial surfaces in the assembled implant. 
     
     
         8 . The implant of  claim 5  wherein said polymeric sleeve exterior lateral wall terminates in a pair of spaced apart transitional distal and proximal surfaces, one of which provides said tapered end region, said transitional surfaces lying juxtaposed with complementary surfaces within said interior cavity of said head. 
     
     
         9 . The implant of  claim 2  wherein said polymeric sleeve is made of ultrahigh molecular weight polyethylene which meets ASTM Standard F648. 
     
     
         10 . The implant of  claim 2  wherein said head is made of isotropic crystalline graphite substrate surfaces of which are coated with a continuous coating of pyrocarbon, wherein said head interior cavity and said sleeve have matching lateral cylindrical surface regions, and wherein the diameter of said cylindrical lateral surface region of said polymeric sleeve is such that assembly of said sleeve within said head creates an interference fit at said cylindrical surface regions. 
     
     
         11 . A prosthetic implant for implantation into a resected bone at a joint, which implant comprises:
 a metal stem element which has a connector at one end with a cylindrical lateral surface and an implantable stem portion at the opposite end,   an integral one-piece polymeric sleeve which has a central cavity that receives said connector and an annular flange that extends into said central cavity, and   a head having an exterior articular surface and an interior cavity of circular cross section proportioned to receive said sleeve, said head being formed from an isotropic crystalline graphite substrate having interior and exterior pyrocarbon surfaces,   said head interior cavity having an entrance formed by a reentrant entrance lip of a lesser inner diameter than said head interior cavity and a size such that said polymeric sleeve must elastically deform radially inward to enter said interior cavity,   said polymeric sleeve annular flange having an inner diameter smaller than the diameter of the lateral surface of said connector and having an outer annular hollow located radially outward of said annular flange, which hollow provides an annular relief region when said connector is assembled with said sleeve, and   said integral polymeric sleeve being made of polymeric material having an elasticity, such that (a) it can deform radially inward sufficient to facilitate its entry into said head interior cavity and return to shape, (b) it can deform radially outward to facilitate assembly with said connector and (c) once assembled with both said head and said connector, disassembly cannot inadvertently occur.   
     
     
         12 . The implant of  claim 11  wherein said polymeric sleeve has an exterior lateral wall with a cylindrical wall region of circular cross-section and a tapered end region to facilitate its entry into said entrance to said head interior cavity. 
     
     
         13 . The implant of  claim 12  wherein said stem element has a radial flange of generally circular shape spaced from said connector. 
     
     
         14 . The implant of  claim 13  wherein said connector is joined to the remainder of said metal stem element by a neck portion of reduced dimension which extends to said connector from said radial flange. 
     
     
         15 . The implant of  claim 12  wherein said connector has a grooved region formed in the exterior lateral wall into which said annular locking flange is received. 
     
     
         16 . The implant of  claim 15  wherein the cross-sectional volume of said annular hollow is at least about 90% of the volume of said annular locking flange. 
     
     
         17 . The implant of  claim 15  wherein said connector grooved region has a pair of spaced apart radial surfaces that are essentially parallel to each other and essentially perpendicular to the axis of said stem element, and wherein said annular locking flange has an arcuate shape when cut by an axially aligned plane, which flange becomes seated between said parallel radial surfaces in the assembled implant. 
     
     
         18 . The implant of  claim 15  wherein said polymeric sleeve exterior lateral wall terminates in a pair of spaced apart transitional distal and proximal surfaces, one of which provides said tapered end region, said transitional surfaces lying juxtaposed with complementary surfaces within said interior cavity of said head. 
     
     
         19 . The implant of  claim 15  wherein said polymeric sleeve is made of ultrahigh molecular weight polyethylene which meets ASTM Standard F648. 
     
     
         20 . A method for forming a prosthetic implant, which method comprises:
 providing a metal alloy stem element which has a connector at one end that is shaped with a region of reduced diametric dimension,   providing a head of crystalline, brittle material having an articular outer surface section and an interior cavity formed with an entrance of reduced diameter,   providing an integral polymeric sleeve that is proportioned to seat within the interior cavity in the head, which sleeve has a central cavity that is proportioned to receive the connector and has means to interengage with the connector region of reduced diametric dimension,   forming a subassembly by mating the polymeric sleeve with the head by insertion of the sleeve through the entrance into the head interior cavity in a manner in which the polymeric material elastically deforms radially inward to facilitate its entry and returns to shape within the head interior cavity, and   then completing assembly of the prosthetic implant by mating said subassembly with the stem element by inserting the connector into the central cavity within the polymeric sleeve which causes the polymeric material to elastically deform radially outward at the interengaging means, said deformation being accommodated by a circumferential relief region, and then to return to a configuration having an interior dimension that thereafter prevents inadvertent disassembly of said subassembly from the stem element.