Joints - Tribology

Question 1

What are the requirements for Fluid-film lubrication

Answer 1

Thick fluid film leading to complete separation of surfaces (lambda >3)
High viscosity lubricant
High speed motions

Question 2

How does fluid-film lubrication work?

Answer 2

The fluid film is trapped between the 2 surfaces
Hydrodynamic lubrication: when the joint is loaded, the separated surfaces slide over each other and cause a LIFTING PRESSURE

Question 3

How is low surface separation overcome in boundary lubrication (3)

Answer 3

Phospholipids and lubricin adsorbed to surfaces help lubrication
Boosted lubrication
Biphasic lubrication (seen in cartilage)

Question 4

What mode of lubrication is exhibited by articular cartilage

Answer 4

Boosted lubrication

Biphasic lubrication

Question 5

Explain biphasic lubrication

Answer 5

• Due to biphasic & porous nature of at least one articular surface, water diffuses out of cartilage into synovial upon loading
• This increases pressure in the synovial fluid can support a fraction of the load
• The frictional force is proportional to the load (less load = less friction)

Question 6

Explain boosted lubrication

Answer 6

• Under load water diffuses into cartilage from synovial fluid
• This concentrates hyaluronic acid in the synovial fluid
• This is rich in negatively charged GAGs which exert repulsion between surfaces