  Several factors, such as the material, geometry, roughness, relative angular
movement and loading conditions, influence the hip implant's lubrication
regime, thereby affecting the friction and wear performance including
longevity. In this study, a ball-on-plane model is considered to predict the
lubrication regime across the entire gait cycle for the metal-on-metal and
ceramic-on-ceramic hip implants. According to ISO 14242-1, a normal walking
gait cycle is considered for the loads and rotations in the hip implant for
this study. A comprehensive analysis is done to estimate the lubrication regime
by considering different material combinations, body weights, femoral head
sizes, clearances, and roughness across the entire gait cycle. The correlation
coefficients show that the geometrical parameters are dominant in affecting the
lubrication film thickness compared to the operational and material parameters.
The femoral head size and body weight are found to be the most dominant and
least dominant parameter respectively. Among the hard-on-hard tribo-pair,
ceramics operate predominantly in the full-film lubrication regime compared to
metals due to its fine surface finish. The present research suggests that in
order to maximise the longevity of a hip implant, an orthopaedic surgeon should
choose one with a larger femoral head diameter, less clearance, and an
ultra-fine surface finish, regardless of any tribo-pair.
