Sequentially cross-linked polyethylene

Abstract

A method of producing an improved polyethylene, especially an ultra-high molecular weight polyethylene utilizes a sequential irradiation and annealing process to form a highly cross-linked polyethylene material. The use of sequential irradiation followed by sequential annealing after each irradiation allows each dose of irradiation in the series of doses to be relatively low while achieving a total dose which is sufficiently high to cross-link the material. The process may either be applied to a preformed material such as a rod or bar or sheet made from polyethylene resin or may be applied to a finished polyethylene part. If applied to a finished polyethylene part, the irradiation and annealing must be accomplished with the polyethylene material not in contact with oxygen at a concentration greater than 1% oxygen volume by volume. When applied to a preform, such as a rod, the annealing of the bulk polymer part of the rod from which the finished part is made must take place on the rod before the implant is machined therefrom and exposed to oxygen.

1 . A preformed material for subsequent production of a medical implant with improved wear resistance comprising a polyethylene cross-linked at least twice by irradiation and thermally treated by annealing after each irradiation.  
     
     
         2 . The preformed material as set forth in  claim 1 , wherein the material is cross-linked by a total radiation dose from about 2 to about 100 MRad.  
     
     
         3 . The preformed material as set forth in  claim 2 , wherein the total radiation dose is between about 5 to about 10 MRad.  
     
     
         4 . The preformed material as set forth in  claim 3 , wherein three radiation doses are applied with an incremental dose for each irradiation is between about 2 and about 5 MRad.  
     
     
         5 . The preformed material as set forth in  claim 1 , wherein three radiation doses are applied with an incremental dose for each irradiation being between about 2 and about 5 MRad.  
     
     
         6 . The preformed material as set forth in  claim 5 , wherein the total radiation dose is between about 5 to about 10 MRad.  
     
     
         7 . The orthopedic preformed material as set forth in  claim 1 , wherein the polyethylene has a weight average molecular weight of greater than 400,000.  
     
     
         8 . The preformed material as set forth in  claim 1 , wherein the annealing takes place in air at a temperature greater than 25° C.  
     
     
         9 . The preformed material as set forth in  claim 8 , wherein the annealing takes place for a time and temperature selected to be at least equivalent to heating said irradiated material at 50° C. for 144 hours as defined by Arrhennius equation (14).  
     
     
         10 . The preformed material as set forth in  claim 9 , wherein said material is heated for at least about four hours.  
     
     
         11 . The orthopedic preformed material as set forth in  claim 1 , wherein the polyethylene is at room temperature for each irradiation.  
     
     
         12 . The preformed material as set forth in  claim 1 , wherein the polyethylene is cross-linked three times by irradiation and thermally treated by annealing after each irradiation at a temperature between 25° C. and 135° C. for at least 4 hours.  
     
     
         13 . A method for increasing the wear resistance of a preformed polyethylene comprising: 
 irradiating the preformed polyethylene in the solid state at least two times; and    annealing the preformed polyethylene after each irradiation.    
     
     
         14 . The method for increasing the wear resistance as set forth in  claim 13 , wherein the material is cross-linked by a total radiation dose from about 1 to about 100 MRad.  
     
     
         15 . The method for increasing the wear resistance as set forth in  claim 14 , wherein the total radiation dose is between about 5 to about 10 MRad.  
     
     
         16 . The method for increasing the wear resistance as set forth in  claim 15 , wherein an incremental dose for each irradiation is between about 2 and about 5 MRad.  
     
     
         17 . The method for increasing the wear resistance as set forth in  claim 16 , wherein an incremental dose for each irradiation is between about 2 and about 5 MRad.  
     
     
         18 . The method for increasing the wear resistance as set forth in  claim 17 , wherein the total radiation dose is between about 4 to about 10.5 MRad.  
     
     
         19 . The method for increasing the wear resistance as set forth in  claim 18 , wherein the weight average molecular weight of the polyethylene is greater than 400,000.  
     
     
         20 . The method for increasing the wear resistance as set forth in  claim 19 , wherein the annealing takes place at a temperature greater than 25° C.  
     
     
         21 . The method as set forth in claim  29  wherein the annealing takes place between 110° C. and 135° C.  
     
     
         22 . The method for increasing the wear resistance as set forth in  claim 20 , wherein the annealing takes place for a time and temperature selected to be at least equivalent to heating said irradiated material at 50° C. for 144 hours as defined by Arrhennius equation (14).  
     
     
         23 . The method for increasing the wear resistance as set forth in  claim 23 , wherein said material is heated for at least about 4 hours.  
     
     
         24 . The method as set forth in  claim 13  further including the step of machining the preformed polyethylene into a medical implant.  
     
     
         25 . The method as set forth in  claim 13 , wherein the material polyethylene is cross-linked three times by irradiation and thermally treated by annealing after each irradiation at a temperature between 25° C. and 135° C. for at least about 4 hours.  
     
     
         26 . A medical device comprising a polyethylene material irradiated at least two times and annealed at a temperature lower than the melting point of the material after each irradiation.