Effect of torque loading on the pull-out response of a steel post inserted in a polyethylene cylinder

Abstract

Posts are used in various implant designs to contribute to the short- and long-term fixation stability of artificial joints. This study was undertaken to investigate the effect of torque loading on the pull-out response of a steel post inserted into high-density polyethylene (HDPE) material. An experimental apparatus was designed and fabricated to perform mechanical characterization of a steel post embedded in a polymer cylinder with initial interference fit under pull-out, torque and combined torque/pull-out loadings. To analyze the effect of preload applied torque to the load transfer at the post-fixation interface under pull-out loading, we have chosen HDPE material with uniform mechanical and tribological properties. Under pull-out loading, the micro-slip initiation and propagation at the post-HDPE interface was found to be progressive and assuming Coulomb friction at the interface, the friction coefficient was calculated from the measured pull-out force. In the torque loading condition it was found that the torque dropped suddenly from the maximum value to an initial dynamic sliding torque value. The interface behaves like a chemically bonded one, and static and dynamic friction coefficients were determined. It has been found under combined torque/pull-out conditions that in addition to the reduction of the maximum pull-out force, the preload applied torque generates two instabilities in the pull-out behavior. The first one happens once the maximum pull-out force is reached where the load falls to a level required for the post extraction from the HDPE cylinder. The second instability takes place during the extraction process for a residual (critical) implantation length which depends on the preload applied torque value. This latter instability was marked by a sudden rotation of the HDPE cylinder against the steel post.