Clearance

Clearance is the term used to describe the effective gap between the femoral head and acetabular cup in a Metal-on-Metal bearing.  It is calculated by subtracting the radius of the femoral head from the radius of the acetabular cup. This difference in radii is used to describe the gap at the equatorial position on the bearing when the femoral head is in contact with the acetabular cup in a polar orientation. Polar bearings operate with a large apparent contact surface area. However the real contact surface area is very small. It is at this point where the articular surfaces interact creating friction and wear.

What is Clearance?

Generation of fluid film

A fluid film is present when the two articulating surfaces are separated by the lubricant. It is the clearance (entrainment) angle and motion which generates the fluid film

Viscous entrainment