The natural hip joint in healthy people has a very low friction with very little (or no) wear. It works as a dynamically loaded bearing and is subjected to about 1-2 million cycles of loading per year. The applied load is the body weight which is tripled when walking and even higher during other activities such as running and jumping. Unfortunately these joints are not always healthy due to various causes such as fractures or disease leading to severe pain which necessitates joint replacement. Currently, the orthopaedic industries are working towards developing an ideal artificial hip joint with low wear, low friction, good lubrication, better fixation/stability and biocompatibility. Many different designs and materials have been investigated with some promising new implants which can be used depending on patients' individual need (large or small joint), activity and age. In this work, two types of artificial hip joints were tested for friction and lubrication studies: Metal-on-Metal (MoM) Biomet hip resurfacing ReCaps with large diameters (>35-60 mm) and different diametral clearances (~ 60-350 µm), and Zirconia Toughened Alumina (ZTA) heads against carbon-fibre-reinforced poly-ether-ether ketone (CFR PEEK) cups with different diameters (>35-60 mm) and diametral clearances (60-1860 µm). Seven serum-based lubricants with different viscosities were used with and without carboxy methyl cellulose (CMC) additions as gelling agent to increase viscosity depending on the CMC content. The maximum load applied was 2000 N for the stance phase with a minimum load of 100 N for the swing phase. A Pro-Sim friction hip simulator was used to investigate the frictional torque generated between the articulating surfaces so as the friction factor can be calculated. Stribeck analysis was then employed to assess the mode of lubrication. For the metal-on-metal hip resurfacing joints, the friction factors were in the range 0.03-0.151 and those for the ZTA ceramic heads versus CFR PEEK cups were in the range 0.006-0.32. Stribeck analyses showed mainly mixed lubrication for both MoM and ZTA ceramic-on-CFR PEEK joints. The experimental results were in agreement with most of the theoretical calculations suggesting mixed lubricating regimes at low viscosities and moving on to fluid film lubrication at higher viscosities. Joints with larger-diameters, lower clearances and lower surface roughness exhibited a higher lambda ratio suggesting improved lubrication. Viscosity flow curves for the serum-based lubricants having viscosity ≤ 0.00524 Pas showed non-linear relationship between viscosity and shear rate indicating non-Newtonian flow with pseudoplastic or shear-thinning characteristic, i.e. viscosity decreased as shear rate increased up to shear rates of ~ 1000 s⁻¹. However, at shear rates greater than 1000 s⁻¹ Newtonian flow became dominant with almost constant viscosity, i.e. a linear relationship between shear stress and shear rate. On the other hand, viscosity flow curves for the lubricants with viscosity ≥ 0.0128 Pas showed non-Newtonian behaviour up to a shear rate of 3000 s⁻¹ with shear-thinning characteristic.

The natural hip joint in healthy people has a very low friction with very little (or no) wear. It works as a dynamically loaded bearing and is subjected to about 1-2 million cycles of loading per year. The applied load is the body weight which is tripled when walking and even higher during other activities such as running and jumping. Unfortunately these joints are not always healthy due to various causes such as fractures or disease leading to severe pain which necessitates joint replacement. Currently, the orthopaedic industries are working towards developing an ideal artificial hip joint with low wear, low friction, good lubrication, better fixation/stability and biocompatibility. Many different designs and materials have been investigated with some promising new implants which can be used depending on patients¿ individual need (large or small joint), activity and age. In this work, two types of artificial hip joints were tested for friction and lubrication studies: Metal-on-Metal (MoM) Biomet hip resurfacing ReCaps with large diameters (>35-60 mm) and different diametral clearances (~ 60-350 µm), and Zirconia Toughened Alumina (ZTA) heads against carbon-fibre-reinforced poly-ether-ether ketone (CFR PEEK) cups with different diameters (>35-60 mm) and diametral clearances (60-1860 µm). Seven serum-based lubricants with different viscosities were used with and without carboxy methyl cellulose (CMC) additions as gelling agent to increase viscosity depending on the CMC content. The maximum load applied was 2000 N for the stance phase with a minimum load of 100 N for the swing phase. A Pro-Sim friction hip simulator was used to investigate the frictional torque generated between the articulating surfaces so as the friction factor can be calculated. Stribeck analysis was then employed to assess the mode of lubrication. For the metal-on-metal hip resurfacing joints, the friction factors were in the range 0.03-0.151 and those for the ZTA ceramic heads versus CFR PEEK cups were in the range 0.006-0.32. Stribeck analyses showed mainly mixed lubrication for both MoM and ZTA ceramic-on-CFR PEEK joints. The experimental results were in agreement with most of the theoretical calculations suggesting mixed lubricating regimes at low viscosities and moving on to fluid film lubrication at higher viscosities. Joints with larger-diameters, lower clearances and lower surface roughness exhibited a higher lambda ratio suggesting improved lubrication. Viscosity flow curves for the serum-based lubricants having viscosity ¿ 0.00524 Pas showed non-linear relationship between viscosity and shear rate indicating non-Newtonian flow with pseudoplastic or shear-thinning characteristic, i.e. viscosity decreased as shear rate increased up to shear rates of ~ 1000 s-1. However, at shear rates greater than 1000 s-1 Newtonian flow became dominant with almost constant viscosity, i.e. a linear relationship between shear stress and shear rate. On the other hand, viscosity flow curves for the lubricants with viscosity ¿ 0.0128 Pas showed non-Newtonian behaviour up to a shear rate of 3000 s-1 with shear-thinning characteristic. / Ministry of Higher Education, Libya