Lowering of pain threshold in rat knee joint induced by sodium hyaluronate (HA) injection

The change in pain threshold level by injection of sodium hyaluronate (HA average molecular weight 600, 000 to 1, 200, 000) into the knee joint cavity was evaluated in rats using supported hind limb paw pressure measurement. Paw pressure ratio (ppr) of treated to non-treated hind limb was monitored by pressure transducers with computer analysis. HA of 0.25 to 1.0 mg/rat in constant volume (0.1 ml) was injected into the joint cavity resulting in a significant decrease in ppr with dose dependent manner. The nociceptive effect was transient and restored 10 minutes after HA was administrated. Monosodium urate crystals (MSU) injected directly into the hind limb joint cavity produced severe pain with the threshold being restored to normal after 7-day MSU-treatment. Long lasting and drastic nociceptive effect was induced with HA injected into the 7-day MSU-treated joint cavity. These results suggest that HA injected into the inflamed joint cavity produces a lowering effect of the pain threshold.