JOINT LUBRICATION

JOINT LUBRICATION

• difference of human joints from bearings in engineering
• operate in low sliding speed
• expected to last a life time

• 

  joint lubrication

  
  
• Tribology : science of rubbing surfaces
• as on surface slides over another the movement is restricted by frictional force – lubrication decreases the frictional force and produce smooth movement – reduces wear and tear of bearing surfaces by interposing a substance that keeps them apart

Types of lubrication

• Boundary lubrication
• Fluid film lubrication

Boundary lubrication

• depends on chemical property of lubricant
• synovial fluid – special type of protein – lubrin – adsorbed to each articular surface – prevent direct surface to surface contact

Fluid film lubrication

• depends on viscosity of fluid – fluid film completely separates the surfaces
• two types of mechanisms occur when joint is loaded
  • hydrodynamic
  • squeeze film
• hydrodynamic
  • occurs when two non parallel surfaces move tangentially on each other
  • fluid forms into converging wedge of fluid – lifts the bearing surfaces apart
  • motion of surfaces drags the fluid into the gap between surfaces
• squeeze film
  • occurs when surfaces move perpendicular to each other
  • tendency to squeeze liquid film out – resisted by viscosity of fluid
  • very high fluid pressure generated – supports the heavy load transiently – but eventually the film becomes thin – contact between surface occurs
• Enhancements of fluid film lubrication
  • elasto-hydro-dynamic
  • boosted
• elasto – hydro-dynamic
  • occur in both hydro-dynamic and squeeze film mechanisms
  • occurs when surfaces are not rigid – surfaces get deformed under load
  • deformation increases the surface contact area – also prevent the escape of lubricating fluid
• boosted
  • occurs in squeeze film mechanism and sliding loads
  • depends on ability of the solvent of synovial fluid to pass into articular cartilage in perpendicular motion
  • decrease in solvent component increases the hyaluronic acid protein complex concentration – increases viscosity – boosts the lubrication
  • as load increases – fluid gets trapped between the asperities on surface – progressively becomes more viscous by boosting mechanism

Normal joint motion

• swing phase ( slight loading)
  • fluid drawn between the surfaces
• stance phase ( and in case of severe loads)
  • boundary lubrication
  • fluid film maintained by squeeze film
• motion starts
  • combination of boundary and fluid film lubrication
  • as the speed increases film enhanced by elastohydrodynamic mechanism
  • as slows down – squeeze film mechanism operates
• cycle
  • boundary lubrication – squeeze film – elasto hydro-dynamic – squeeze film – boundary lubrication
• exudation of fluid from cartilage surface when gets deformed – Weeping lubrication
• trapping of fluid in the undulations of cartilage to enhance lubrication – Trapped pool lubrication
• increasing the concentration of fluid – boosted lubrication

Replaced joints

• ineffective squeeze film mechanism
  • hard material of surfaces
  • limited surface area
• boundary lubrication seen more – after total replacement synovial fluid quantity would be small

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