Prediction of lubrication regimes in wrist implants with spherical bearing surfaces.

The wrist joint is frequently affected by rheumatoid arthritis, resulting in wrist pain, deformity and ultimately loss of function. Artificial wrist implants have been introduced to treat the rheumatoid wrist, to attempt to alleviate pain and restore some function to the joint. The aim of this study was to predict the likely lubrication regimes that occur in wrist implants with spherical bearing surfaces. The implant was modelled as an equivalent ball-on-plane. Elastohydrodynamic lubrication theory was used to determine the minimum film thickness for the implant under different load, entraining velocity, lubricant viscosity, size of implant and material combinations. The results show that the highest film thickness is found in large implants, with high viscosity, high entraining velocity and low load. Hard-on-soft material combinations will operate with a boundary lubrication regime. Material combinations involving ceramic bearing surfaces have the potential to operate with a mixed lubrication regime.