No abstract

An international oxidation index standard would greatly benefit the orthopedic community by providing a universal scale for reporting oxidation data of ultra-high molecular weight polyethylene (UHMWPE). We investigated whether severe oxidation associated with long-term shelf aging affects the repeatability and reproducibility of area-based oxidation index measurement techniques based on normalization with the use of 1370- or 2022-cm(-1) infrared (IR) absorption reference peaks. Because an oxidation index is expected to be independent of sample thickness, subsurface oxidation was examined with the use of both 100- and 200-microm-thick sections from tibial components (compression-molded GUR 1120, gamma irradiated in air) that were shelf aged for up to 11.5 years. Eight institutions in the United States and Europe participated in the present study, which was administered in accordance with ASTM E691. On average, the 100-microm-thick samples were associated with significantly greater interlaboratory relative standard uncertainty (40.3%) when compared with the 200-microm samples (21.8%, p = 0.002). In contrast, the intralaboratory relative standard uncertainty was not significantly affected by the sample thickness (p = 0.21). The oxidation index method did not significantly influence either the interlaboratory or intralaboratory relative standard uncertainty (p = 0.32 or 0.75, respectively). Our interlaboratory data suggest that with the suitable choice of specimen thickness (e.g., 200 microm) and either of the two optimal oxidation index methods, interlaboratory reproducibility of the most heavily oxidized regions in long-term shelf-aged components can be quantified with a relative standard uncertainty of 21% or less. Therefore, both the 1370-cm(-1) and the 2022-cm(-1) reference peaks appear equally suitable for use in defining a standard method for calculating an oxidation index for UHMWPE.

No abstract

Nitrogen ion implantation was investigated as a means of improving the wear resistance of Ti-6Al-4V alloy and the performance of the Ti-6Al-4V/ultra high molecular weight polyethylene (UHMWPE) wear couple. Included in the pin-on-disc wear study were cobalt-chromium and stainless steel controls as well as non-ion-implanted Ti-6Al-4V. Ion implanted Ti-6Al-4V and the control alloys exhibited minor scratching whereas non-ion-implanted Ti-6AI-4V exhibited definite wear scars. Gravimetric analysis of the UHMWPE discs revealed no significant difference between those worn against ion implanted Ti-6Al-4V and the control alloys. Discs worn against non-ionimplanted Ti-6Al-4V, however, exhibited significantly more weight loss than the other couples. The results of this testing indicate that nitrogen ion implantation improves the wear resistance of Ti-6Al-4V with an attendant decrease in the amount of polyethylene abrasive wear.

An AMTI knee simulator was used to evaluate the wear of UHMWPE tibial inserts which were gamma sterilized in air and displayed a high or low level of oxidation. After 5 million cycles, four out of five samples from the high oxidation group displayed subsurface cracking and/or delamination. The five specimens in the low oxidation group experienced burnishing only. These results indicate that gamma irradiation in air together with high oxidation due to shelf aging can increase susceptibility to wear. The appearance of the wear scars indicates that knee simulator testing is a valid method of producing burnishing, cracking, and delamination similar to that seen in-vivo. [S0742-4787(00)04301-0]