Animal joints contain two systems which require lubrication: a soft-tissue system, involving the sliding of synovial membrane on itself or on other tissues, and a cartilage-on-cartilage system.

The resistance to motion caused by soft-tissue stretching and friction is far greater than that due to cartilage rubbing on cartilage; in fact, soft-tissue resistance may be primarily responsible for clinical joint stiffness. Lubrication of the softtissue system depends on molecules of hyaluronate in the synovial fluid which stick to the synovium in a layer and keep moving surfaces apart.

Cartilage-on-cartilage lubrication, by contrast, is independent of hyaluronic acid. A hyaluronate-free glycoprotein fraction which has been isolated from synovial fluid confers on a buffer solution a lubricating advantage equal to that of whole synovial fluid. Removal of this fraction from synovial fluid deprives the fluid of any lubricating advantage over buffer. The action of this glycoprotein on cartilage is a boundary phenomenon similar to that of hyaluronate on synovium. In addition to this boundary effect, cartilage surfaces are kept apart by a fluid squeeze film made up of joint fluid and interstitial fluid which weeps from the articular cartilage itself. The squeeze-film effect is probably potentiated by the undulations of the surface and the elasticity of the cartilage, which may lower frictional resistance by elastohydrodynamic effects.