Rotational Wear and Friction of Ti-6Al-4V and CoCrMo against Polyethylene and Polycarbonate Urethane

Total joint replacement (TJR) is a successful procedure for millions of patients each year. Optimizing mechanical properties of bearing couples is important to increase implant longevity and improve patient outcomes. Softer viscoelastic materials offer a potential solution by more closely replicating the mechanical properties and lubrication regime of a native joint, but their wear properties are relatively unknown compared to the wealth of knowledge about polyethylene. In this study, the utility of an experimental set-up not widely used in wear testing was investigated through the evaluation of the mechanical characteristics of four bearing couples often used in TJR. A flat-on-flat rotational test evaluating wear through a change in height of the upper sample introduced several variables that are thought to alter the mechanical properties of compliant bearing materials. The wear properties and coefficient of friction (COF) of two polymer surfaces, ultra-high molecular weight polyethylene (UHMWPE) and polycarbonate urethane (PCU) were directly compared as they articulated against both CoCrMo and Ti-6Al-4V at contact stresses of 3.46, 2.60, and 1.73 MPa. Wear rate was influenced by both polymer surface and normal force while independent of metal counter bearing, with increased wear of couples containing PCU, and at higher forces. Increased COF was seen with PCU, but was independent of other variables. This study elucidated several factors present with this experimental set-up that may contribute to an inadequate lubrication regime and subsequently increased wear and friction of PCU. These are important considerations to maximize the mechanical properties and longevity of implants.