Class: BE-210
Group: W4
Members: Kevin Justice, Joanna Law, Seungtaek Lee, Emily Rothman
Date: April 30, 1997
Full Text

Abstract:
Two molded samples of ultra high molecular weight polyethylene used as bearing materials in total knee replacements were obtained from Zimmer (Warsaw, Indiana). Each sample had been subjected to a different technique of gamma irradiation sterilization: one packaged and irradiated in an inert atmosphere, the other in air. It was sought to determine both qualitatively and quantitatively the effects of the two procedures on the physical and mechanical properties of the UHMWPE. The samples were tested with the aid of the Perkin-Elmer DSC-7 to determine the heat of fusion and the percent crystallinity of the two samples. Additionally, an Instron testing machine Model 1331 was used to conduct mechanical testing, which included three point bending and a pseudo-Brinell hardness test. The value of heat of fusion of the air-irradiated sample was determined to be statistically constant at an average value of 117.31 J/g + 3.93%. The inert-irradiated samples showed a trend for the heat of fusion to increase from an initial value of 104.47 J/g + 3.0% to a value measured in the third week as 125.12 J/g + 10.6%. It was further found that the modulus of elasticity as determined from the three point bending testing was not significantly different between the two sterilization techniques. Within experimental error, the mean Young’s Modulus was calculated as 1142.78 MPa + 153.22 MPa. Further, the hardness testing showed that the air-irradiated sample was harder by an average 59.46% with a deviation 11.88% over a range of loading of 10 N through 190 N. Our findings, while limited by the brevity of the study, show fairly conclusively that the irradiation of UHMWPE in air leads to oxidative degradation, as evidenced by changes in heat of fusion, percent crystallinity and hardness (britttleness).