Combined chemical and mechanical effects on free radicals in UHMWPE joints during implantation

Abstract

An electron spin resonance (ESR) technique is employed to determine the free radical distribution in the articulating surfaces of retrieved acetabular cups and knee-joint plateaus (retrieved after more than 6 years of implantation). Similar measurements made on samples prepared from cyclically stressed and unstressed cups, and on samples following oxidations in nitric acid and intralipid solutions provided sufficient data to gain more knowledge about the combined chemical and mechanical effects on PE free radicals during implantation. In UHMWPE free radicals are primarily initiated by gamma-ray sterilization; however, during implantation, peroxy (scission type) free radicals are formed and reach a maximum concentration level (equilibrium state) due to oxidation by chemical (hemoglobin and/or synovial fluids) environment of the joints. Subsequently, due to frictional heating and stress in the loading zones, free radical reaction is accelerated and their number is reduced only in those areas. This is consistent with the observations of a temperature rise in acetabular cups during in vitro frictional wear stress tests and in vivo telemetry observations, as reported by others. Compared with the previously reported SEM micrographs the low-free-radical regions are correlated with high-wear areas and the high-free-radical regions with the low-wear areas.

Polyethylene wear in unicondylar knee prostheses

“…This implies that stresses alter the chemical nature of the polyethylene in these designs. A recent study has demonstrated higher levels of free radicals under the wear track of retrieved polyethylene knee components than in nonwom areas (Jahan et al 1991), which may account for the reduction in molecular weight seen in the St Georg design in this study.…”

PEEK biomaterials in trauma, orthopedic, and spinal implants

“…Radiation stability is another common concern for aliphatic polymers, including polyolefins such as UHMWPE, which are susceptible to bond cleavage during irradiation, leading to the generation of long-lived macroradicals (often referred to as "free radicals") [66]. In contrast, because of its distinctive aromatic chemical structure, PEEK displays remarkable resistance to gamma and electron beam radiation, with G values of radical formation about two orders of magnitude lower than aliphatic polymers, such as polyethylene [67].…”

Characterization of ultrahigh molecular weight polyethylene irradiated with ?-rays and electron beams to high doses

“…24 This radical transformation can be followed by the g-factor calculated from the ESR spectra. All the UHMWPE specimens just after irradiation had a g-factor between 2.0025 and 2.0030, independent of the irradiation conditions.…”