A Theoretical Background on Abrasive And Adhesive Wear

Adhesive wear refers to a type of wear generated by sliding one solid surface along another surface. When two surfaces are loaded against each other, the whole of the contact load is carried only by the very small area of asperity contacts. The real contact pressure over the asperity is very high, leading to adhesion between them. If one of the surfaces slides against the other, the adhesive junction may break. As sliding continues, fresh junctions will form and rupture in succession. The probability of adhesive wear may be minimized by the application of lubricants or by the application of hard coating with a low coefficient of friction. Abrasive wear is the removal of material from a surface by a harder material impinging on or moving along the surface under load. Abrasive and adhesive wear mechanisms usually prevail in retrieved acetabular liners, whereas fatigue-damaged surfaces are often observed in explanted tibial inserts. Therefore, wear resistance is of principal importance in acetabular components, whereas the typical lower congruency and higher contact pressures of the knee joint make fatigue and fracture properties more relevant. Nevertheless, acetabular component cracking and fractures at the rim of acetabular liners have been reported recently, typically associated with abnormal loading or impingement at the nonarticulating region. Furthermore, if the UHMWPE formulation has not been properly stabilized, in vivo oxidation may alter its initial properties, potentially influencing the wear and mechanical performance of prosthetic components in the long term. It can be thus concluded that the medical grade UHMWPE formulations should represent a balance between design considerations, wear resistance, oxidation stability, and fatigue and fracture properties. The aim of this Article is to provide a theoretical background on fatigue and fracture concepts, as well as the different philosophical approaches and experimental techniques available with special regard to UHMWPE. The main findings on fatigue and fracture properties of contemporary medical UHMWPEs are also outlined.