Chemically crosslinked ultrahigh molecular weight polyethylene for artificial human joints

Abstract

The present invention discloses a method for enhancing the wear-resistance of polymers by crosslinking them, especially before irradiation sterilization. In particular, this invention presents the use of chemically crosslinked ultrahigh molecular weight polyethylene in in vivo implants.

We claim:  
     
         1 . A method for producing a crosslinked polymer with increased ability to withstand wear, comprising the steps of: 
 a) crosslinking a polymer to form a crosslinked polymer;    b) determining the degree of crystallinity of the crosslinked polymer; and    c) adjusting reaction conditions such that the degree of crystallinity of the polymer after crosslinking is reduced by 10% to 50%;     wherein the crosslinking is achieved by a method selected from the group consisting of: 
 i) irradiation crosslinking of the polymer when it is in a molten state;  
 ii) photocrosslinking of the polymer in the melt; and  
 iii) crosslinking of the polymer with a free radical generating chemical.  
   
     
     
         2 . The method of  claim 1 , further comprising the step of irradiating the crosslinked polymer in the solid state.  
     
     
         3 . The method of  claim 2 , further comprising the step of annealing the crosslinked polymer.  
     
     
         4 . A method for producing a crosslinked polymer with increased ability to withstand wear, comprising the steps of: 
 a) crosslinking a polymer to form a crosslinked polymer without irradiating the polymer in its solid state;    b) irradiating the crosslinked polymer in its solid state at a sterilization dose; and    c) selecting for the crosslinked polymer which, after the irradiation in step (b), possesses a degree of crystallinity of about 45% or less.    
     
     
         5 . The method of  claim 4 , wherein the crosslinking is achieved by a method selected from the group consisting of: 
 a) irradiation crosslinking of the polymer when it is in a molten state;    b) photocrosslinking of the polymer in the melt; and    c) crosslinking of the polymer with a free radical generating chemical.    
     
     
         6 . The method of  claim 5 , further comprising the step of annealing the crosslinked polymer.  
     
     
         7 . A method for producing a crosslinked polymer with increased ability to withstand wear, comprising the steps of: 
 a) crosslinking a polymer to form a crosslinked polymer without irradiating the polymer in its solid state;    b) annealing the crosslinked polymer;    c) irradiating the crosslinked polymer in its solid state at a sterilization dose; and    d) selecting for the crosslinked polymer which, after the irradiation in step (c) and the annealing in step (b), possesses a degree of crystallinity of about 45% or less.    
     
     
         8 . A method for making a crosslinked polymer suitable for use in vivo implant, said crosslinked polymer has an increased ability to withstand wear, said method comprising the steps of: 
 a) reducing crystallinity of the polymer by crosslinking the polymer; and    b) molding the resulting polymer into a shape suitable for in vivo implant;     wherein the crosslinking of step (a) does not include irradiating the polymer in a solid state.    
     
     
         9 . The method of  claim 8 , wherein the crosslinking is achieved by a method selected from the group consisting of: 
 a) irradiation of the polymer when it is in a molten state;    b) photocrosslinking of the polymer in the melt; and    c) crosslinking of the polymer with a free radical generating chemical.    
     
     
         10 . The method of  claim 9 , wherein the crosslinking is achieved with a free radical generating chemical.  
     
     
         11 . The method of  claim 10 , wherein the free radical generating chemical is selected from the group consisting of: peroxides, peresters, azo compounds, disulfides, dimethacrylates, tetrazenes, and divinyl benzene.  
     
     
         12 . The method of  claim 11 , further comprising the step of radiation sterilization of the in vivo implant.  
     
     
         13 . The method of  claim 12 , wherein the degree of crystallinity of the polymer is reduced by between 10 to 50% by crosslinking.  
     
     
         14 . The method of  claim 13 , further comprising the step of annealing the crosslinked polymer to stabilize its shrinkage.  
     
     
         15 . The method of  claim 14 , wherein the polymer is UHMW polyhydrocarbon.  
     
     
         16 . An in vivo implant made by a polymer produced by the method comprising the steps of: 
 a) reducing the crystallinity of the polymer to enable it to better withstand wear; and    b) molding the polymer into a shape suitable for in vivo implant;    wherein the reduction of crystallinity in step (a) does not include irradiating the polymer in a solid state.    
     
     
         17 . The in vivo implant of  claim 16 , wherein the step (a) is achieved by crosslinking the polymer using a method selected from the group consisting of: 
 a) irradiation crosslinking of the polymer when it is in a molten state;    b) photocrosslinking of the polymer in the melt; and    c) crosslinking of the polymer with a free radical generating chemical.    
     
     
         18 . The in vivo implant of  claim 17 , wherein the polymer is chemically crosslinked with a free radical generating chemical.  
     
     
         19 . The in vivo implant of  claim 18 , wherein the free radical generating chemical is selected from the group consisting of: peroxides, peresters, azo compounds, disulfides, dimethacrylates, tetrazenes, and divinyl benzene.  
     
     
         20 . The in vivo implant of  claim 19 , wherein the crosslinking reduces the crystallinity of the polymer by 10 to 50%.  
     
     
         21 . The in vivo implant of  claim 20 , wherein the polymer is a polyhydrocarbon.  
     
     
         22 . The in vivo implant of  claim 21 , wherein the polyhydrocarbon is an UHMW polyhydrocarbon.  
     
     
         23 . The in vivo implant of  claim 17 , wherein the in vivo implant is capable of possessing about 45% crystallinity or less if irradiated by gamma irradiation to an average dose of about 3.4 Mrad or less.  
     
     
         24 . The in vivo implant of  claim 23 , wherein the polymer is UHMW polyhydrocarbon.  
     
     
         25 . The in vivo implant of  claim 24 , wherein the in vivo implant increases in its degree of crystallinity by about 1% if annealed.  
     
     
         26 . The in vivo implant of  claim 23 , wherein the free radical generating chemical is selected from the group consisting of: peroxides, peresters, azo compounds, disulfides, dimethacrylates, tetrazenes, and divinyl benzene.  
     
     
         27 . The in vivo implant of  claim 26 , wherein the polymer is chemically crosslinked by a free radical generating chemical, and the polymer is UHMW polyhydrocarbon.  
     
     
         28 . The in vivo implant of  claim 17 , capable of suffering less than or equal to one-fifth of the wear suffered by another in vivo implant made from an uncrosslinked polymer.  
     
     
         29 . The in vivo implant of  claim 28 , wherein the polymer is chemically crosslinked by a free radical generating chemical, and the polymer is UHMW polyhydrocarbon.  
     
     
         30 . A polyhydrocarbon capable of maintaining a degree of crystallinity of about 42% or less after gamma irradiation to an average dose of about 3.4 Mrad or less.  
     
     
         31 . The polyhydrocarbon of  claim 30 , wherein the polyhydrocarbon has a crystallinity of about 39.8% or less before the gamma irradiation.  
     
     
         32 . The polyhydrocarbon of  claim 30 , wherein the polyhydrocarbon is UHMW polyhydrocarbon.  
     
     
         33 . A crosslinked in vivo implant comprising a component of an animal joint, said in vivo implant being capable of suffering about one-fifth or less of the wear suffered by an uncrosslinked in vivo implant, wherein the crosslinked in vivo implant and the uncrosslinked in vivo implant are made of UHMW polyethylene and have been sterilized by irradiation, and the crosslink is achieved by a method selected from the group consisting of: 
 a) irradiation crosslinking of the UHMW polyethylene when it is in a molten state;    b) photocrosslinking of the UHMW polyethylene in the melt; and    c) crosslinking of the UHMW polyethylene with a free radical generating chemical.    
     
     
         34 . The crosslinked in vivo implant of  claim 33 , wherein the in vivo implant is an acetabular cup and the crosslink is achieved by crosslinking of the UHMW polyethylene with a free radical generating chemical selected from the group consisting of: peroxides, peresters, azo compounds, disulfides, dimethacrylates, tetrazenes, and divinyl benzene.  
     
     
         35 . The crosslinked in vivo implant of  claim 34 , wherein the free radical generating chemical is a peroxide.  
     
     
         36 . An in vivo implant made from a polyhydrocarbon having a crystallinity of about 43% or less.  
     
     
         37 . The in vivo implant of  claim 36 , capable of maintaining a crystallinity of 45% or less after irradiation with gamma irradiation at a sterilization dose.  
     
     
         38 . The in vivo implant of  claim 37 , wherein said polyhydrocarbon is UHMW polyethylene.  
     
     
         39 . The in vivo implant of  claim 38 , wherein the UHMW polyethylene is chemically crosslinked.  
     
     
         40 . The in vivo implant of  claim 39 , wherein the UHMW polyethylene has a crystallinity of about 40% before irradiation in the solid state, and a crystallinity of about 42% after gamma irradiation to an average dose of about 3.4 Mrad.  
     
     
         41 . The in vivo implant of  claim 36 , wherein the polyhydrocarbon is chemically crosslinked.