Potential errors in FTIR measurement of oxidation in ultrahigh molecular weight polyethylene implants

Potential sources of error in the use of FTIR to measure the level of oxidation in ultrahigh molecular weight polyethylene acetabular cups were evaluated using cups from a hip simulator wear study with and without artificial aging, as well as cups retrieved from clinically failed hip prostheses. Oxidation was measured as a function of depth below the bearing surface using transmission FTIR on microtomed sections of the cups. To account for the variation of the thickness of the microtomed sections, oxidation was plotted as the ratio of the absorbance of the carbonyl groups to the absorbance of a reference band at 2022 cm-1. Overnight soaking in hexane reduced the apparent levels of oxidation, presumably due to the extraction of absorbed contaminants. In cups with low to moderate levels of oxidation, the reference absorption was relatively independent of the level of oxidation and was linearly proportional to the thickness of the specimens, providing reproducible oxidation ratios. However, the scatter in the reference absorption and in the apparent oxidation ratio increased with increasing levels of oxidation and was greatest for the thickest (400 microm) microtomed sections. The profiles of the oxidation ratios for a given specimen that were plotted by the present study method could be numerically adjusted to coincide with the ratios plotted using the methods of two previous investigators, providing conversion factors that are useful for comparing results among the studies.