Near‐Surface Microstructural Reorganization of UHMWPE under Cyclic Load – A Pilot Study

Abstract

We report a depth‐resolved analysis of the microstructure of a bulge‐like failure in a medical grade ultra high molecular weight polyethylene (UHMWPE) implant material induced by a cyclic load. The depth‐dependent arrangement of the crystalline lamellae was analyzed by cross‐sectional transmission electron microscopy (TEM). The failure emerges at the top wear surface as an amorphous bulge with a maximum thickness of approximately 300 µm. A sharp interface below the bulge consists of an approximately 0.1 µm thick boundary layer with stacked lamellae with an outstanding degree of orientation perpendicular to the wear surface. The boundary layer is separated from the intact, unmodified base material with random lamellar orientation by an approximately 5 µm thick transition zone with a lamellar alignment perpendicular to the wear surface, which decreases toward the center of the UHMWPE test specimen. We further observed an overall decrease of lamellar thickness toward the wear surface within the influence zone. This indicates the strength and heterogeneity of the local dynamic stress field during cyclic wear and the ease for lamellar rearrangements presumably facilitated by a locally elevated temperature. We propose the bulge to originate from either wear material dragged along the wear surface or from a retransfer from the articulating counter surface, and discuss implications on the degree and relevance of adhesive wear. This study stresses the importance of the UHMWPE transfer material under such dynamic load conditions. Our approach seems suitable for investigating the mechanical aspect of failure mechanisms at the UHMWPE polymer biointerface under cyclic load.

Computational Analysis of Microstructure of Ultra High Molecular Weight Polyethylene for Total Joint Replacement

“…Although ultra high is a highly nonlinear material near failure, this first pass analysis is valid for small strains or over incremental sections of the stress-strain curve that can be approximated as lineM. This approach could also be applied to microstmcture imaged near failure or after alignment, as captured in other work [35,36]. Finally, the underlying constitutive relationship employed in the finite element model could be substantially expanded in future studies to represent complex nonlinear, viscoelastic, anisotropic constitutive behavior near yielding or failure and could be expanded to three dimensions, both of which have been done successfully with similar approaches in the field of bone biomechanics [37^0].…”

Damage due to rolling in total knee replacement—The influence of tractive force

“…In general, it is surprising how little scientific effort has been spent to disentangle the mechanistic interactions of sliding and rolling at the knee joint since the landmark study of Blunn et al [4]. Two studies which further contributed to the field are from Cornwall et al [20], who studied the three basic kinematic contact conditions (sliding, gliding, rolling) in a reciprocating wear tester, and from Keller et al [21], who studied the microstructural re-organization of polyethylene after sliding-rolling contact.…”