Solid state deformation processing of crosslinked high molecular weight polymeric materials
Solid-state deformation processing of crosslinked high molecular weight polymers such as UHMWPE, for example by extrusion below the melt transition, produces materials with a combination of high tensile strength and high oxidative stability. The materials are especially suitable for use as bearing components in artificial hip and other implants. Treated bulk materials are anisotropic, with enhanced strength oriented along the axial direction. The material is oxidatively stable even after four weeks of accelerated aging in a pressure vessel containing five atmospheres of oxygen (ASTM F2003). Because of its oxidative stability, the deformation processed material is a suitable candidate for air-permeable packaging and gas sterilization, which has thus far been reserved for remelted crosslinked UHMWPE.
1. A method of making a bearing component made of UHMWPE, having enhanced strength in a load bearing direction suitable for use in a medical implant comprising
radiation crosslinking a UHMWPE;
preheating the crosslinked UHMWPE to a temperature above 80° C. and below its melting point;
solid-state-extruding the UHMWPE at a draw ratio of greater than 1;
cooling the extruded UHMWPE to a solidification temperature;
annealing the cooled UHMWPE at a temperature above 100° C. and below the melting point; and
machining the component from the annealed UHMWPE, wherein the component has a load bearing axis parallel to an axial direction of the annealed UHMWPE.
2. A method according to claim 1 , wherein the UHMWPE is in the form of a cylinder having a diameter of from about 2 to about 4 inches.
3. A method according to claim 1 , wherein the UHMWPE is in the form of a cylinder having a diameter of about 3 inches.
4. A method according to claim 1 , wherein the radiation crosslinking is to a dose of 0.1 to 10 Mrad.
5. A method according to claim 1 , wherein the radiation crosslinking is to a dose of 2 to 10 Mrad.
6. A method according to claim 1 , wherein the radiation crosslinking is to a dose of 5 Mrad.
7. A method according to claim 1 , wherein preheating is to a temperature of from 100° C. to 130° C.
8. A method according to claim 1 , wherein the draw ratio is less than 3.0.
9. A method according to claim 1 , wherein the draw ratio is less than 2.0.
10. A method according to claim 1 , wherein the draw ratio is about 1.5.
11. A method according to claim 1 , wherein annealing comprises heating at from 90° C. to 135° C.
12. A method according to claim 1 , further comprising sterilizing the UHMWPE prior to machining the component.
13. A method according to 2 , wherein the sterilizing is non-irradiative.
14. A method according to 1 , further comprising sterilizing the component after machining.
15. An artificial joint comprising a bearing component made according to claim 1 .
16. An implant comprising a bearing component made according to the method of claim 1 .