Microstructure changes of extruded ultra high molecular weight polyethylene after gamma irradiation and shelf-aging

Ultra high molecular weight polyethylene (UHMWPE) components in joint prostheses may undergo oxidative degradation in the long term. This leads to material embrittlement, associated with a subsurface defect named “white band” that favours surface delamination and premature implant failure. The polymer microstructure, which affects the mechanical properties of the material, is altered by oxidative degradation. In this study, the microstructure of bar-extruded UHMWPE (GUR 1050) was compared before irradiation, one week after 25 kGy gamma irradiation in air (GUR 1050), and 7 years after commercial (25-40 kGy) gamma irradiation in air and shelf-aging (GUR 415). This was performed using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) which help to characterize the changes in material microstructure and crystallinity during degradation. Quantitative (lamellar thickness), semiquantitative (lamellar twisting, orientation, lamellar density), and qualitative (lamellar border, stacking) parameters were recorded in the surface, subsurface and inner material. There was an evolution in the degradation in the bar-extruded UHMWPE microstructure during irradiation and shelf-aging. Changes in the in-depth microstructure indicate the progression of an oxidation front. The stacking or high lamellar concentration in the subsurface in shelf-aged samples was coherent with higher crystallinity and with a degradation process controlled by oxygen diffusion and free radical distribution after gamma irradiation.