Orientation Softening as a Mechanism of Ultra-High Molecular Weight Polyethylene (UHMWPE) Wear in Artificial Hip and Knee Joints

Abstract

The wear of ultra-high molecular weight polyethylene is one of the most important factors in the longevity of total joint replacement prostheses. This paper investigates the morphological changes in UHMWPE induced by plastic deformation and wear. A plasma etching technique was used to reveal the lamellar structure and its crystalline orientation in UHMWPE. Specimens under examination included unconsolidated UHMWPE powder, freeze fractured surfaces, microtomed slices, surfaces of tensile stretched specimens, shear fracture surfaces, and worn surfaces of acetabular and tibial components that had been tested on hip and knee joint simulators. For the unconsolidated powder and the freeze fracture surfaces, the orientation of the lamellae on the surface was random. For the surfaces of microtomed slices and tension specimens, the orientation of the lamellae was perpendicular to the direction of microtoming and tensile deformation. X-ray diffraction of the tension specimen indicated preferential orientation of the c-axis of the orthorhombic structure in the stretching direction. A similar orientation phenomenon was observed on the worn surfaces of acetabular and tibial components. A sequential biaxial tension test was conducted to study the effect of molecular orientation on the longitudinal and transverse strength of UHMWPE. An orientation-induced softening phenomenon was observed when the UHMWPE was stretched in a direction perpendicular to the molecular chains. Based on these observations, an orientation softening wear model was proposed. The model states that molecular orientation induced by plastic deformation during joint articulation is undesirable for the wear resistance of UHMWPE acetabular and tibial components.

Molecular Deformation Mechanisms in UHMWPE During Tribological Loading in Artificial Joints

“…7. This orientation of the molecules in the zone close to the surface had been observed earlier [47,48]. To generate such a microfibrillar structure, high stresses have to occur close to the surface.…”

A novel cross-shear metric for application in computer simulation of ultra-high molecular weight polyethylene wear

“…Wear volume was expressed as a function of the amount of frictional work done during sliding perpendicular to the principal direction. The premise for this theory was derived from earlier experiments in which UHMWPE samples exhibited molecular orientation under uniaxial tension with a measurable increase in rupture strength along the stretch direction (Wang et al 1998); rupture strength was also significantly diminished perpendicular to the stretch direction. In the context of wear, molecular orientation is believed to strengthen the material on the surface of the UHMWPE along the principal direction of sliding, with a corresponding decrease in strength in the transverse direction.…”