Objective To compare equine synovial fluid (eSF) from post-injury and control joints for (1) cartilage boundary lubrication function, (2) putative boundary lubricant molecules hyaluronan (HA), proteoglycan-4 (PRG4), and surface-active phospholipids (SAPL), (3) relationships between lubrication function and composition, and (4) lubrication restoration by addition of HA. Methods eSF from normal (NL), acute injury (AI), and chronic injury (CI) joints were analyzed for boundary lubrication of normal articular cartilage as kinetic friction coefficient (μkinetic). eSF were also analyzed for HA, PRG4, and SAPL concentrations and HA molecular weight (MW) distribution. The effect of addition of HA, of different concentrations and MW, to AI- and NL-eSF samples on μkinetic was determined. Results The μkinetic of AI-eSF (0.036) was higher (+39%) than that of NL-eSF (0.026). Compared to NL-eSF, AI-eSF had a lower HA concentration (−30%) of lower MW forms, higher PRG4 concentration (+83%), and higher SAPL concentration (+144%). CI-eSF had μkinetic, HA, PRG4, and SAPL characteristics intermediate to that of AI-eSF and NL-eSF. Regression analysis revealed that μkinetic decreased with increasing HA concentration in eSF. The friction-reducing properties of HA alone improved with increasing concentration and MW. Addition of high-MW HA (4,000kDa) to AI-eSF reduced μkinetic to a value near that of NL-eSF. Conclusion In the acute post-injury stage, eSF exhibits poor boundary lubrication properties as indicated by a high μkinetic. HA of diminished concentration and MW may be the basis for this, and adding HA to deficient eSF restored lubrication function.

Knees afflicted with a tibial plateau fracture have synovial fluid with decreased lubrication properties in association with a decreased concentration of hyaluronan.

These results indicate that mild wear (i.e., wear-line formation) at the articular surface has deleterious functional effects on articular cartilage and represent an early aging-associated degenerative change. The identification and recognition of functional biomechanical consequences of wear-lines are useful for planning and interpreting tensile biomechanical tests in human articular cartilage.

Objective To determine the effects of acute injury and tribosupplementation by hyaluronan (HA) on synovial fluid (SF) modulation of cartilage shear during tibio-femoral articulation. Methods Human osteochondral blocks from the lateral femoral condyle (LFC) and tibial plateau (LTP) were apposed, compressed 13%, and subjected to sliding under video microscopy. Tests were conducted with equine SF from normal joints (NL-SF), SF from acutely injured joints (AI-SF), and AI-SF to which HA was added (AI-SF+HA). Local and overall shear strain (Exz) and the lateral displacement (Δx) at which Exz reached 50% of peak values (Δx1/2) were determined. Results During articulation, LFC and LTP cartilage Exz increased with Δx and peaked when surfaces slid, with peak Exz being maintained during sliding. With AI-SF as lubricant, surface and overall Δx1/2 were ~40% and ~20% higher, respectively than values with NL-SF and AI-SF+HA as lubricant. Also, peak Exz was markedly higher with AI-SF as lubricant than with NL-SF as lubricant, both near the surface (~80%) and overall (50–200%). Following HA supplementation to AI-SF, Exz was reduced from values with AI-SF alone by 30–50% near the surface and 20–30% overall. Magnitudes of surface and overall Exz were markedly (~50–80%) higher in LTP cartilage than LFC cartilage for all lubricants. Conclusion Acute injury impairs SF function, elevating cartilage Exz markedly during tibio-femoral articulation; such elevated Exz may contribute to post-injury associated cartilage degeneration. Since HA partially restores the function of AI-SF, as indicated by Exz, tribosupplements may be beneficial in restoring cartilage mechanobiology.