articular cartilage

Biomechanics

· two distinct phases

· fluid phase (water with dissolved salts)

· solid phase (solid organic matrix)

viscoelasticity

· response to constant load or deformation varies with time

· due to

· viscous response of fluid

· elastic response of solid

· responses of viscoelastic material are creep and stress relaxation

· viscoelastic behaviour of articular cartilage due to

· flow of interstitial fluid (compressive)

· motion of long polymer chains (shear)

Biphasic creep in compression

· if constant load applied, response is

· rapid deformation

· slower deformation (creep phase)

· state of equilibrium with no further deformation

· caused by exudation of interstitial fluid

· exudation most rapid initially

· diminishes gradually and ceases

· load applied at surface balanced by

· compressive stress developed within collagen-PG solid matrix

· frictional drag generated by flow of interstitial fluid

· deformation ceases when compressive stress within matrix sufficient to balance applied load

· called creep equilibrium

· reached after 4-16 hrs

· up to 50% of fluid squeezed out

· exuded fluid fully recoverable when load removed

· rate of fluid exudation reflects tissue permeability

Biphasic stress relaxation in compression

· if constant deformation achieved, response is

· rapid increase in stress

· decrease in stress (stress relaxation phase)

· state of equilibrium with no further decrease in stress

· initial rise due to exudation of fluid and compaction of solid matrix near surface

· stress relaxation due to fluid redistribution and rebound of compaction region

· redistribution ceases when compressive stress developed within matrix reaches stress generated by applied load

· excessive stress difficult to achieve because of stress relaxation

· leads to rapid spread of contact area in joint during articulation

Permeability

Porosity

· porosity is ratio of fluid volume to total volume

· porosity is geometric concept

Permeability

· if pores interconnected, material is permeable

· permeability is measure of ease with which fluid flows through material

· permeability is inversely proportional to frictional drag of fluid

· permeability is physical concept

Cartilage

· is highly porous

· high water content

· has a low permeability

· high frictional resistive forces generated when fluid forced to flow through matrix

· increased water content found to cause increased permeability

· increased permeability causes decreased compressive strength of cartilage

response to tension

· in tension, is

· strongly anisotropic (wrt orientation)

· inhomogeneous (wrt layers)

· due to

· varying collagen and PG organisation

· layering structural arrangements

· demonstrates viscoelastic behaviour in tension

· stress-strain curve shows

· initial slowly rising region (toe region)

· subsequent constantly rising region (linear region)

· toe region due to collagen fibres aligning

· linear region due to stretching of aligned collagen fibres

· failure occurs when all fibres rupture

· disruption of collagen fibres decreases tensile properties

Wear

friction

· coefficient of friction very low in articular surfaces

· ranges from 0.003 to 0.03

· steel on steel is 0.8

· ice on ice is 0.1

Lubrication

· minimal wear of articular surfaces due to lubrication

Types

· two general types

Boundary lubrication

· in joints, lubricant is glycoprotein called lubricin

· lubricin adsorbed as macromolecule monolayer onto each articulating surface

Fluid film lubrication

· two types

· hydrodynamic

· squeeze film

· different form of lubrication occurs with deformable surfaces

· called elastohydrodynamic lubrication

Articular surfaces

Mixed lubrication

· articular surface not perfectly smooth

· boundary lubrication occurs where thickness of fluid film is of same order as surface roughness

· fluid film lubrication occurs in areas with more widely separated surfaces

· most of friction generated by boundary lubricated areas

· most of load carried by fluid film

Self-lubrication

· articular surfaces can exude and imbibe lubrication fluid

· as surfaces slide across each other

· fluid exuded in front of and under leading half of contact load

· fluid imbibed at trailing edge and behind contact load

· fluid movement may also aid in chondrocyte nutrition

Wear

· three mechanisms of wear

Interfacial wear

· low rate of interfacial wear in joint surfaces

· can occur when joint damaged

· articular cartilage more soft and permeable

· allows leaking away of lubricant from between surfaces

· increases likelihood of direct contact between surfaces

Fatigue wear

· may occur by one of two mechanisms

· disruption of matrix

· repetitive stress may cause tensile failure of collagen fibres

· PG washout

· repetitive exudation and imbibition of interstitial fluid may cause PG washout near joint surface

Rapid loading

· if loads applied rapidly, insufficient time for internal fluid redistribution to relieve compacted region

· high stress in matrix may cause damage