Effects of equine joint injury on boundary lubrication of articular cartilage by synovial fluid: Role of hyaluronan

Objective To compare equine synovial fluid (SF) from injured and control joints for cartilage boundary lubrication function; concentrations of the putative boundary lubricant molecules hyaluronan (HA), proteoglycan 4 (PRG4), and surface-active phospholipids (SAPLs); relationships between lubrication function and composition; and lubrication restoration by addition of HA. Methods Equine SF from normal joints, joints with acute injury, and joints with chronic injury were analyzed for boundary lubrication of normal articular cartilage (kinetic friction coefficient [μkinetic]). Equine SF samples were analyzed for HA, PRG4, and SAPL concentrations and HA molecular weight distribution. The effect of the addition of HA, of different concentrations and molecular weight, on the μkinetic of equine SF samples from normal joints and joints with acute injury was determined. Results The μkinetic of equine SF from joints with acute injury (0.036) was higher (+39%) than that of equine SF from normal joints (0.026). Compared to normal equine SF, SF from joints with acute injury had a lower HA concentration (−30%) of lower molecular weight forms, higher PRG4 concentration (+83%), and higher SAPL concentration (+144%). Equine SF from joints with chronic injury had μkinetic, PRG4, and SAPL characteristics intermediate to those of equine SF from joints with acute injury and normal equine SF. Regression analysis revealed that the μkinetic value decreased with increasing HA concentration in equine SF. The friction-reducing properties of HA alone improved with increasing concentration and molecular weight. The addition of high molecular weight HA (4,000 kd) to equine SF from joints with acute injury reduced the μkinetic to a value near that of normal equine SF. Conclusion In the acute postinjury stage, equine SF exhibits poor boundary lubrication properties, as indicated by a high μkinetic. HA of diminished concentration and molecular weight may be the basis for this, and adding HA to deficient equine SF restored lubrication function.