The effects of degree of crosslinking on the fatigue crack initiation and propagation resistance of orthopedic‐grade polyethylene

Abstract

Crosslinked ultrahigh molecular weight polyethylene (UHMWPE) has been recently approved by the Food and Drug Administration for use in orthopedic implants. The majority of commercially available UHMWPE orthopedic components are crosslinked using e-beam or gamma radiation. The level of crosslinking is controlled with radiation dose and free radicals are eliminated through heat treatments to prevent long-term degradation associated with chain scission or oxidation mechanisms. Laboratory studies have demonstrated a substantial improvement in the wear resistance of crosslinked UHMWPE. However, a concern about the resistance to fatigue damage remains in the clinical community, especially for tibial components that sustain high cyclic contact stresses. The objective of this study was to investigate both the initiation and propagation aspects of fatigue cracks in radiation crosslinked medical-grade UHMWPE. This work evaluated three levels of radiation, which induced three crosslink densities, on the fatigue crack propagation and total fatigue life behavior. Both as-received UHMWPE, as well as those that underwent an identical thermal history as the crosslinked UHMWPE were used as controls. Fractured crack propagation specimens were examined using scanning electron microscopy to elucidate fatigue fracture mechanisms. The results of this work indicated that a low crosslink density may optimize the fatigue resistance from both a crack initiation and propagation standpoint.

Vitamin E-stabilized UHMWPE for Total Joint Implants: A Review

“…While conventional UHMWPE has high crystallinity and fatigue strength in its unaged form, its fatigue resistance is severely deteriorated due to oxidation [58,66]. Irradiated and melted UHMWPEs have low fatigue strength caused by crosslinking and the loss of crystallinity during melting [4,46,53,62]. The fatigue strength loss due to postirradiation melting has been recovered by vitamin E stabilization due to the maintenance of crystallinity.…”

Crack Propagation Resistance Is Similar Under Static and Cyclic Loading in Crosslinked UHMWPE: A Pilot Study

“…Crack propagation in UHMWPE is dominated by noncyclic phenomena [7], although it is typical to quantify the crack propagation resistance of the material under cyclic loading conditions [3,7,8,12]. We asked whether (1) cracks would initiate from acute notches and stably propagate under static loading with a velocity dependent on crosslinking radiation dose, (2) crack propagation under static loading would result in quantitatively similar material crack propagation resistance data to those obtained under cyclic loading conditions, and (3) specimen failure time under a static load would yield a similar evaluation of material crack propagation resistance as crack velocity under a static or cyclic applied load.…”

“…While cyclic tensile crack propagation experiments have been widely adopted as a screening method for the introduction of new UHMWPE formulations into clinical use [3,8,12], with considerable technical merit, there has been little analysis of the intrinsic validity or quantitative utility of such data. Our data suggest static load crack growth experiments may be used in place of the typical sinusoidally loaded fatigue crack propagation experiments and also the time to failure under a static load may be a sufficient metric of the relative crack propagation resistance of a given set of UHMWPE materials.…”

“…Currently, the fatigue performance of a UHMWPE formulation is evaluated chiefly by its resistance to fatigue crack propagation under cyclic loading conditions [3,8,12]. The stress intensity at the inception of crack growth K incep (often reported as DK incep for cyclic loading) has been used to establish the minimum conditions for detectable and stable crack growth (typically about 10 À6 mm/cycle) and is commonly compared between formulations to establish relative fatigue resistance [3]. A relative decrease in K incep generally shifts the stable crack propagation uniformly to lower values of stress intensity [3,8,12], and such a shift is known to accompany crosslinking in UHMWPE [3,8,12].…”

“…The stress intensity at the inception of crack growth K incep (often reported as DK incep for cyclic loading) has been used to establish the minimum conditions for detectable and stable crack growth (typically about 10 À6 mm/cycle) and is commonly compared between formulations to establish relative fatigue resistance [3]. A relative decrease in K incep generally shifts the stable crack propagation uniformly to lower values of stress intensity [3,8,12], and such a shift is known to accompany crosslinking in UHMWPE [3,8,12]. The reduced toughness and crack propagation resistance of crosslinked formulations of UHMWPE are of interest for clinical applications, particularly in light of recently reported cases of brittle damage or catastrophic fracture of highly crosslinked THA bearings [4][5][6]15].…”