A fluid lubrication model for articular cartilage was put forward by Mc
Cutchen, in which a high percentage of the load is supported by fluid
pressurization in the interface region separating the two cartilage coated
surfaces as the cartilage is compressed under load. This reduces the
friction by reducing the percentage of the load which is carried by solid
material in the cartilage. For two bones which are in contact in a healthy
joint, which are each coated by a layer of cartilage whose thickness is much
smaller than its lateral dimensions, it will be argued that since the bone
is impervious to fluid flow in healthy joints, almost all of the fluid that
is expressed from the cartilage under load flows through the interface
region, where it supports part of the load. This is in contrast to previous
theoretical and in vitro experimental studies of this problem, in which most
of the fluid does not flow into the interface. It is shown that for mean
asperity height small compared to a length scale (which depends on the
cartilage or hydrogel permeability, the fluid viscosity and the dimensions
of the cartilage or hydrogel) a large percentage of the load is supported by
fluid pressurization.