Pitting and delamination remain causative factors of
polyethylene failure in
total knee replacement. Gamma irradiation induces cross linking in
ultra-high-molecular-weight polyethylene, which has been shown to improve wear resistance. Irradiation may reduce fracture toughness and
fatigue strength, however, and the effects of irradiation are dependent upon the resin, processing technique, and radiation dose. The effects of varying levels of gamma irradiation (0, 33, 66, and 100 kGy) on the fracture toughness and
fatigue-crack resistance of
UHMWPE, isostatically molded from 1900H and
GUR 1050 resins, were examined. Paris law regressions were performed to quantify
fatigue-crack propagation rates as functions of change in stress intensity, and J-integral methods were used to quantify the elastic-
plastic fracture toughness. The results indicated that gamma irradiation reduced the resistance of both materials to
fatigue-crack growth, and that the reductions were radiation dosage and resin dependent. Irradiation at any level was detrimental to the fracture toughness of the 1900H specimens. Irradiation at 33 kGy increased fracture toughness for the
GUR 1050 specimens, and substantial reductions were observed only at the highest irradiation level. Scanning electron microscopy of the fracture surface revealed
diamond-like fracture patterns of the nonirradiated specimens indicative of ductile, multilevel fracture. Pronounced striations were apparent on these fracture surfaces, oriented perpendicular to the direction of crack growth. The striations appeared as folds in surface layers of the
GUR 1050 specimens. At the highest irradiation levels, the striations were nearly eliminated on the fracture surfaces of the 1900H specimens, and were markedly less severe for the
GUR 1050. These results demonstrated that at higher irradiation levels the materials became more brittle in
fatigue, with less ductile folding and tearing of the fracture surfaces.