The theories of lubrication were reviewed and it was concluded that neither the hydrodynamic theory nor the boundary theory permits an adequate description and explanation of lubrication in synovial joints.

It is suggested, in principle, that joint lubrication depends on the existence of a thixotropic and elastic fluid between the articular surfaces, that the area of the load-carrying film depends on the elasticity of cartilage, and that the velocity gradient existing in the gap between articular surfaces depends also on the lateral deformation of these surfaces.

Synovial fluid has thixotropic and elastic (instantaneous dilatant) properties. Its viscosity decreases with an increase in the rate of movement, but it is pressure-resistant under sudded impact. Owing to its viscoelastic properties (that is, normal stress effect) and to its affinity to the cartilage surfaces, it cannot be squeezed out from between the opposing surfaces. If, however, synovial fluid undergoes any change and acquires Newtonian characteristics, it may be squeezed out and it is no longer capable of carrying a constant load at all rates of movement. Such changes in the synovial fluid result in damage to the articular surfaces since the elasticity of the cartilage and the slippery characteristics of the cartilage would not be sufficient to prevent increased friction.

On the other hand, synovial fluid per se is not sufficient to maintain efficient lubrication. The resiliency and the time-dependent and stress-dependent elasticity of cartilage play an important role. When the cartilage degenerates or disappears, the synovial fluid undergoes mechanochemical degradation.

The proposed outline of lubrication in synovial joints explains the interdependence of synovial fluid and articular surfaces and may serve as a basis for a more fundamental study of joint lubrication in health and disease.