Abstract
Summary
Both endogenous lubricin and injectable hyaluronic acid reduced cartilage friction coefficients, but by distinct mechanisms. Lubricin operated in boundary mode and hyaluronic acid shifted lubrication to mixed or hydrodynamic mode.
Introduction
Intra-articular injections of viscous agents and boundary lubricants have been presented as options to mitigate the progression of articular cartilage damage after the onset of osteoarthritis1,2. Mechanically, these injections are predicted to lower the friction coefficient within a load bearing joint and consequently slow the propagation of damage at the articular surface. Tribologically, boundary lubricants and viscous agents are hypothesised to be effective through different mechanisms affecting boundary-mode lubrication and transition to mixed-mode lubrication, respectively. By normalizing sliding speeds on a Stribeck curve, this study evaluated the efficacy of injectable hyaluronic acid (HA) supplements and endogenous lubricin to alter tribological properties.
Methods
Cartilage samples were extracted from the patellofemoral groove of neonatal bovine. A custom-built tribometer was used to measure friction coefficients of cartilage sliding against polished glass while in a lubricant solution. Cartilage samples were compressed to 20% strain and the normal load was allowed to reach a steady-state value before sliding at speeds from 0.1 to 10mm/s. For some samples, endogenous lubricin was removed from the surface as described previously3 via incubation in 1.5M NaCl in PBS for 20 minutes followed by re-equilibration in PBS for 1hr. Samples were tested in bathing solutions of PBS (control), equine synovial fluid (ESF), 10mg/ml HA, and a hydrophobic HA derivative (HYADD).
Results
Friction coefficients as a function of sliding speed for some lubricants are presented. Comparisons show that lubricin removal from the tissue surface increases friction coefficients when PBS is used as a lubricant (p<0.05). At slow sliding speeds there was no significant difference between PBS and 10mg/ml HA, but at higher speeds HA transitions to a reduced friction coefficient. The hydrophobic HA (HYADD) provides significantly reduced friction coefficients compared to regular HA for speeds up to 7mm/s. These trends can be explained mechanically by normalizing data to a Hersey number (sliding speed∗viscosity/normal load). The data curves are similar to Stribeck curves which are characterised by different lubrication modes: boundary, mixed, and hydrodynamic. The PBS data appear to be in boundary mode (characteristic of highest friction coefficients), and a transition to mixed mode lubrication appears to occur within the ESF and HA solutions. The most viscous forms of HA (HYADD) lowered the friction coefficient to ≤ 0.05, with an apparent minimum at a Hersey number of ∼10−6m, suggesting that this lubricant may have enabled hydrodynamic lubrication, a phenomenon not noted previously in this system. Lubricin removal increased friction coefficient from 0.16 to 0.25, occurring at slow sliding speeds and Hersey numbers of 10−12m
Conclusions
Endogenous lubricin and injectable hyaluronan both effectively lower friction coefficients, but do so by distinct mechanism. At the same operating conditions (normal loads and sliding speed), lubricin lowers boundary mode friction coefficient, while hyaluronan shifts behavior to mixed mode (HA) or hydrodynamic mode (HYADD). The combination of the presence of lubricin on cartilage surface in the most viscous formulation of HA (HYADD) lowered the friction coefficient of articular cartilage from 0.26 to 0.05.
