Gradient crosslinking of UHMWPE using irradiation in molten state for total joint arthroplasty

Increased crosslink density increases the wear resistance of ultra-high molecular weight polyethylene (UHMWPE) acetabular components used in total hip reconstructions. However, increasing crosslink density can reduce the mechanical properties of UHMWPE. Other researchers have tried to limit the crosslinking to a superficial layer on the articulating surfaces to retain the properties of the virgin polymer. We present here a method of producing a gradient of crosslink density across the acetabular component. Acetabular liners with 26 mm inner diameter were machined from the UHMWPE. The liners were then irradiated at 140 degrees C in the molten state of the polymer using a 2 MeV electron beam with limited penetration of the effects of radiation into polyethylene. The liners were then machined to an inner diameter of 32 mm to remove the radiation induced oxidized surface layer. The limited penetration of the e-beam resulted in a gradient of crosslink density with a crosslink density of 0.15 +/- 0.01 mol/dm3 near the articulating surfaces and 0.12 +/- 0.01 mol/dm3 near the backside. The concentration of the trans-vinylene unsaturations decreased gradually as a function of depth from the articulating surfaces to the backside of the liners. The wear resistance of the melt-irradiated liners was contrasted with those of conventional liners using the Boston hip simulator. The gravimetric wear rate was 27 +/- 5 mg million cycles with the conventional liners, while the melt-irradiated acetabular liners did not show any weight loss.