We studied the natural lubrication mechanism of synovial joints. We determined the effect of sodium hyaluronate (HA) on lubricating joints without the normal lubrication mechanism. The coefficient of friction (CF) of fresh pig hip joints was measured with the cartilage intact, washed, scoured with gauze and finally with sandpaper, to model cartilage degradation. Three formulas of HA (8x10(5) daltons 1%, 20x10(5) daltons 1%, 20x10(5) daltons 1.5%) and physiologic saline were used as lubricants. We observed the cartilage using light microscopy (LM) and scanning electron microscopy (SEM). The latter showed that the most superficial layer observed in the washed joint was disrupted after gauze scouring. Compared with intact cartilage the CF did not increase with washing. CF increased more after scouring with sandpaper than with gauze. Each formula of HA decreased the CF of joints scoured with gauze, but only the two more viscous HA formulas decreased the CF of sandpaper-scoured joints. A negative correlation was found between the CF of the sandpaper-scoured joints and the logHA viscosity (r=-0.733, P=0.0001), suggesting that RA with higher viscosity was more effective in lubricating the joints.