Friction, lubrication and wear of total joint replacements

peer-reviewedThe work described in this report is a contribution to ongoing research to develop more effective orthopaedic joint systems though fundamental studies of the behaviour of various novel material combinations and component configurations. Compliant layer technology, incorporating a low elastic modulus polyurethane layer for the bearing surface, has been proposed to offer prolonged longevity of artificial joints by maintaining a fluid film between the articulating surfaces thus reducing friction and wear. To ensure a complaint layer joint retains fit, form and function over its lifetime, material characterisation techniques were employed. Dynamic mechanical thermal analysis (DMTA) highlighted the sensitivity of the modulus of Bionate® polyurethanes 80A and 75D to temperature; in particular for 75D it revealed the close proximity of its glass transition temperature to in vivo operating temperatures and this shows a strong need for accurate temperature control during in vitro testing. The ability of aqueous lubricants to plasticise Bionate® materials was also demonstrated through DMTA, with reductions in the moduli and damping behaviour for hydrated samples being seen. Through a series of creep experiments, the non-linear viscoelastic behaviour of Bionate® polyurethane materials was demonstrated. Creep rupture was not observed over the test periods, and sample deformation increased considerably with increasing stress as well as temperature and lubrication. Bionate® 75D displayed significantly lower creep strain and greater permanent deformation that 80A. The creep data set should essentially allow development of improved designs in compliant layer technology. The tribological performance of the compliant layer articulation as a function of material combinations, implant conformity and implant size was assessed. In order to do this it was necessary to measure the frictional torque and develop tribological functional plots to determine the mode of lubrication. In this study a variety of material combinations for both hip and knee prostheses were investigated using a friction simulator. Firstly, through correlation with other published studies a friction simulator was validated using conventional ultra- high molecular weight polyethylene. Subsequently, metal-on-compliant layer hip and knee prostheses were assessed both experimentally using Stribeck analysis and theoretically using the theory of Hamrock and Dowson. Overall, the performance of compliant layer bearings was promising of improved in vivo behaviour in that fluid film lubrication and relatively low friction factors were achieved with synthetic lubricants. A strong correlation between experimental results and theoretical predictions were observed. The wear performance of a newly developed compliant layer glenoid, designed specifically to utilise compliant layer technology, in total shoulder arthroplasty was investigated. Results suggest that this type of glenoid may be more robust compared to the conventional UHMWPE glenoid. Impingement wear on the compliant layer surface was a frequent finding and was caused from repeated contact between the rim of the hemispherical humeral component and the articular surface of the glenoid, under a constant load. The addition of Vitamin E to compliant layer acetabular components was shown experimentally to have no effect on the friction characteristics of the material. MPC (2-methacryloyloxethyl phosphorylcholine) grafting on the surface of highly crosslinked polyethylene acetabular components increased the friction and it was postulated that the phospholipids acted as boundary lubricants within a mixed lubrication regime. Larger joints are being developed to improve joint stability, propioception and functional longevity. The friction and lubrication properties of large diameter hip bearings of metal-on-metal and ceramic-on-reinforced polymer couplings were assessed. The frictional studies showed that the metal-on-metal joints worked under the mixed lubrication regime, producing similar friction factor values to each other. The ceramic-on-reinforced polymer samples were shown to operate with high friction factors and mixed lubrication. The study demonstrated that the component’s diameter had little or no influence on the lubrication and friction of the large bearing combinations tested