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

Abstract

Recent work suggests crack phenomena (eg, crack initiation and propagation) in UHMWPE do not depend on cyclic damage mechanisms. Materials for which crack phenomena occur in static (noncyclic) mode should exhibit similar crack propagation behavior under static and cyclic loading conditions. Do cracks in UHMWPE stably propagate from acute notches under static loading with a velocity dependent on crosslink density? Are material-ranking evaluations for crack propagation resistance similar under static and cyclic loading conditions? Does time to failure for a notched specimen under static loads yield the same material rankings as crack propagation data? Notched compact tension specimens were machined from UHMWPE gamma-irradiated with a 0-, 50-, 75-, or 100-kGy dose and subsequently remelted. Static loads were applied until failure occurred or 2weeks had elapsed. Crack propagation rates and time to failure were recorded and compared to data from cyclic experiments. Static and cyclic loading both produced stable crack propagation and similar performance rankings of material groups, except in the case of the unirradiated material, which did not fail under static loads. Normalized measures of crack propagation velocities generally showed quantitative agreement between the two methods. Normalized time to failure under static loading also agreed well with crack propagation velocity. Crack propagation under static loading produced qualitatively and quantitatively similar performance results as those under cyclic loading. Time to failure under static loads corresponded closely with the crack propagation velocity and may itself be a robust metric of crack propagation resistance. Total joint arthroplasties may experience superficial cracking in the UHMWPE bearing surface or catastrophic fracture. Quantifying resistance to crack phenomena in UHWMPE is important to design engineers and to clinicians using crosslinked UHMWPE materials under challenging mechanical conditions.