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 crystalline brittle head ( 17, 67, 117 ), preferably made of pyrocarbon-coated graphite, is joined. The head interfaces with human bone, and its effective joinder to the stem element is achieved through a polymeric insert ( 15, 65, 115 ) of proportional shape and design 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. The method of joinder allows the construction of 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 reentrant region of reduced dimension and an implantable stem portion at the opposite end, said connector being a section of a sphere at least about 10% greater than a hemisphere,   an integral one-piece polymeric insert which has a central cavity that receives said connector,   a head formed of brittle crystalline material having an articular surface and an interior cavity being proportioned to receive said polymeric insert,   said head cavity having an entrance of a size such that at least a portion of said polymeric insert must elastically deform inward to enter said cavity,   said insert being formed with means for interengaging with said connector which requires radially outward deformation of at least a portion of said insert to permit entrance of said connector into said central cavity and lock said insert and said connector in engagement, and   means providing a circumferential relief region into which said portion of said polymeric insert can elastically deform to facilitate final assembly,   said integral polymeric insert being made of polymeric material having an elasticity, such that (a) it can deform radially inward sufficient to facilitate its entry into said head 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 insert has an exterior lateral cylindrical wall of circular cross-section and a tapered end region to facilitate its entry into said entrance of said head cavity, which entrance is defined by an inwardly protruding lip. 
     
     
         3 . The implant of  claim 2  wherein said connector is joined to the remainder of said metal stem element by a neck portion of reduced dimension. 
     
     
         4 . The implant of  claim 3  wherein said stem element has a radial flange of generally circular shape located between said neck portion and said implantable stem portion. 
     
     
         5 . The implant of  claim 4  wherein said polymeric insert central cavity has a spherical interior surface and wherein said polymeric insert has an entrance region leading to said central cavity that has a diameter between about 70% and about 85% of the diameter of said central cavity. 
     
     
         6 . The implant of  claim 5  wherein said neck portion of said stem element has a length such that a subassembly of said head and said polymeric insert can pivot on said connector in any direction through an arc of between about 5° and about 15°. 
     
     
         7 . The implant of  claim 2  wherein said head is made of isotropic crystalline graphite, all of the surfaces of which are coated with a continuous coating of pyrocarbon. 
     
     
         8 . The implant of  claim 7  wherein said head cavity and said insert have matching cylindrical surfaces, and the diameter of said cylindrical outer surface of said polymeric insert is such that assembly of said insert within said head creates an interference fit at said cylindrical surfaces. 
     
     
         9 . The implant of  claim 8  wherein said polymeric insert is made of ultrahigh molecular weight polyethylene which meets ASTM Standard F648. 
     
     
         10 . A prosthetic implant for implantation into a resected proximal end of the radius, which implant comprises:
 a metal stem element which has a connector at one end having a spherical surface with a reentrant region of reduced dimension between it and an opposite end that is implantable in the radius,   an integral one-piece polymeric insert which has a central cavity that receives said connector and a flange that circumscribes an entrance to said central cavity, and   a head having an exterior articular surface and an interior cavity of circular cross section proportioned to receive said insert, said head comprising 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 being of a size such that said polymeric insert must elastically deform radially inward to enter said interior cavity,   said polymeric insert central cavity having an entrance region of a size smaller than said connector and having an outer groove which receives said head entrance lip and is proportioned so that an annular relief region remains in said groove when said insert is assembled with said head, and said integral polymeric insert being made of polymeric material having an elasticity, such that (a) it can deform radially inward sufficient to facilitate its entry into said head cavity and return to shape, (b) its entrance region can deform radially outward to facilitate assembly with said connector, and (c) once assembled with both said head and said stem connector, disassembly cannot inadvertently occur.   
     
     
         11 . The implant of  claim 10  wherein said polymeric insert has an exterior lateral cylindrical wall of circular cross-section and a tapered end region to facilitate its entry into said entrance of said head cavity, which entrance is defined by an inwardly protruding lip. 
     
     
         12 . The implant of  claim 11  wherein said connector is joined to the remainder of said metal stem element by a neck portion of reduced dimension. 
     
     
         13 . The implant of  claim 12  wherein said stem element has a radial flange of generally circular shape located between said neck portion and said implantable stem portion. 
     
     
         14 . The implant of  claim 13  wherein said polymeric insert central cavity has a spherical interior surface and wherein said polymeric insert has an entrance region leading to said central cavity that has a diameter between about 70% and about 85% of the diameter of said central cavity. 
     
     
         15 . The implant of  claim 14  wherein said neck portion of said stem element has a length such that a subassembly of said head and said polymeric insert can pivot on said connector in any direction through an arc of between about 5° and about 15°. 
     
     
         16 . The implant of  claim 11  wherein said head is made of isotropic crystalline graphite, all of the surfaces of which are coated with a continuous coating of pyrocarbon. 
     
     
         17 . The implant of  claim 16  wherein said head cavity and said insert have matching cylindrical surfaces, and the diameter of said cylindrical outer surface of said polymeric insert is such that assembly of said insert within said head creates an interference fit at said cylindrical surfaces. 
     
     
         18 . The implant of  claim 17  wherein said polymeric insert is made of ultrahigh molecular weight polyethylene which meets ASTM Standard F648. 
     
     
         19 . A method for forming a prosthetic implant, which method comprises:
 providing a metal alloy stem element which has a connector at one end having a spherical surface that is shaped with a region of reduced diametric dimension intermediate of the other end,   providing a head of crystalline, brittle material having an interior cavity formed with an entrance of reduced diameter,   providing an integral polymeric insert that is proportioned to seat within the interior cavity in the head, which insert has a central spherical cavity that is proportioned to receive the connector at the end of the stem element,   forming a subassembly by mating the polymeric insert with the head by insertion of the polymeric insert through the entrance into the head interior cavity in a manner in which the polymeric material deforms elastically radially inward to facilitate its entry and returns to shape within said the interior cavity, and   then completing the prosthetic implant by mating said subassembly with the stem element by inserting the connector into the central cavity within the polymeric insert which causes the polymeric material to elastically deform radially outward which deformation is accommodated by a circumferential relief region in the polymeric insert, and then to return to a configuration having an interior dimension that thereafter prevents inadvertent disassembly of said subassembly from the stem element.