Compositional and rheological determinants of friction and wear in osteoarthritic synovial fluid

This study examined how osteoarthritis-induced compositional changes in synovial fluid (SF) relate to its rheological, interfacial and tribological behaviour, and whether friction and wear respond to the same fluid properties. SF from 121 patients (aged 39–90 years) was characterised per patient by rotational rheometry, ATR-FTIR spectroscopy and contact-angle goniometry. Friction and wear were measured by reciprocating tribometry with confocal profilometry on a CoCrMo/Al 2 O 3 hard contact, chosen to isolate the lubricant contribution from cartilage poroelasticity. Rheological properties declined with patient age across all tested regimes, from steady flow to large-strain post-yield deformation (Spearman ρ = −0.37 for low-shear viscosity). The lipid accumulation index was the principal compositional correlate of rheological decline, correlating negatively with the loss modulus ( ρ = −0.41) and, as trends, with age and high-shear viscosity. Fluids with higher macromolecular content produced higher contact angles on the metallic surface, consistent with the formation of an adsorbed boundary film. Friction was uniform (CV = 4.6 %) and insensitive to differences in bulk SF properties. Wear correlated with the post-yield storage modulus ( ρ = −0.30) and with macromolecular content indices ( ρ = +0.38), but not with small-strain viscoelastic properties. The rigid, impermeable couple confines the contact to boundary lubrication, so these findings characterise the fluid rather than the whole joint. Within that regime, friction and wear followed different fluid properties and appear to be independent: friction set at the adsorbed boundary film, wear driven by abrasive particulates.