BIOMECHANICS OF THE ARTICULAR CARTILAGE

Dr Susanta kumar khuntiaDr Susanta kumar khuntiaMKCG medical college MKCG medical college

Berhampur,OdishaBerhampur,Odisha

COMPOSITION AND STRUCTURE OF ARTICULAR CARTILAGE

2. COLLAGEN (fibrous ultrastructure, procollagen 2. COLLAGEN (fibrous ultrastructure, procollagen polypeptide), 10-30 %polypeptide), 10-30 %

3. PROTEOGLYCAN ( PG ) large protein 3. PROTEOGLYCAN ( PG ) large protein polysaccharide molecules ( in form of monomers and polysaccharide molecules ( in form of monomers and aggregates) 3-10 %aggregates) 3-10 %

4. WATER + inorganic salts, glycoprteins, lipids, 60-4. WATER + inorganic salts, glycoprteins, lipids, 60-87 %87 %

1. CHONDROCYTES, 10 %

LOCATION OF THE COLLAGENE FIBERS

COMPRESSION

CARTILAGE AS VISCOELASTIC CARTILAGE AS VISCOELASTIC MATERIALMATERIAL

If a material is subjected to the action of a If a material is subjected to the action of a constant (time independent load or constant constant (time independent load or constant

deformation and its response varies (time deformation and its response varies (time dependent) then the mechanical behavior of dependent) then the mechanical behavior of

the material is said to be viscoelastic.the material is said to be viscoelastic.

PERMIABILITY OF ARTICULAR CARTILAGE

Porosity ( ): ratio of the fluid volume (m3 )to the total amount (m) of the porous material

Permeability ( k ): a measure of the ease with which fluid can flow through a porous permeable material and it is inversely proporsional to the frictional drag ( K )

k = 2/ K [(m4/Ns

Basic responses

1. BIPHASIC CREEP 1. BIPHASIC CREEP

2. BIPHASIC STRESS RELAXATION2. BIPHASIC STRESS RELAXATION

1. BIPHASIC CREEP 1. BIPHASIC CREEP

The time taken to reach creep The time taken to reach creep equilibrium varies inversely with the equilibrium varies inversely with the square of the thickness of the tissuesquare of the thickness of the tissue

CREEP PHENOMENON

CREEP EQUILIBRIUM

• 2-4 mm human and bovin articular cartilage > 4 - 16 hours

• rabbit cartilage 1 mm > 1 hour

above 1 Mpa > 50 % of total fluid is squeezed

FIG. 2-9

The rate of fluid exudation governs the creep rate, so it can be used to determine

the permeability coefficient

TISSUE PERMEABILITY COEFFICIENT(k)

HUMAN CARTILAGE: 4.7 +/- 0.04 x 10 -15 m4 / N sBOVIN CARTILAGE: 4.67 +/- 0.04 x 10 -15 m4 / N s

INTRINSIC COMPRESSIVE MODULUS ( HINTRINSIC COMPRESSIVE MODULUS ( HAA ) )

HUMAN CARTILAGE: 0.79 +/- 0.36 MPaHUMAN CARTILAGE: 0.79 +/- 0.36 MPa

BOVIN CARTILAGE: 0.85 +/- 0.21 MPaBOVIN CARTILAGE: 0.85 +/- 0.21 MPa

Equilibrium defomation can be used Equilibrium defomation can be used to measure the intrinsic compressive to measure the intrinsic compressive

modulusmodulus

““k” varies directly with water k” varies directly with water contentcontent

““HHAA” varies inversely with water ” varies inversely with water contentcontent

STRESS RELAXATION

TISSUE PERMEABILITY (k)

INTRINSIC COMPRESSIVE MODULUS ( HA )

similar to creep

UNIAXIAL TENSION

Tangent modulus, which denotes the stiffness of the material

/

Maximum strain : 3 - 100 MPa,

Physiological strain: 15 % > 5 - 10 MPa

compliancecompliance =

szigma, epszilon

PURE SHEAR FORCES

No interstitial fluid flow occures

No pressure gradiens or volumetric changes

Thus , a steady dynamic pure shear experiment can be used to asses the intrinsic viscoelastic properties of the collagen - RG solid matrix.

PURE SHEARstorage modulus ( G` ), loss modulus ( G`` )

dynamic shear modulus ( G* )2 = ( G`)2 + ( G``)2

phase shift angle ( phase shift angle ( tantanG``/ G`)G``/ G`)FIG. 2-15

The magnitude of the dynamic shear The magnitude of the dynamic shear modulus is a measure of the total resistance modulus is a measure of the total resistance

of the viscoelastic materialsof the viscoelastic materials

G* = ( G`)2 + ( G``)2

The magnitude of the dynamic shear modulus is a measure of the total resistance

of the viscoelastic materials

value is a measure of the total frictional energy dissipation within the material.

In pure elastic material is no internal frictional dissipation: is zero

for pure viscous fluid for pure viscous fluid is 90 degree is 90 degree

FIG. 2-15, 16

Bovin articalar cartilage

G* = 1 -3 Mpa

= 9 - 20 degrees

LUBRICATION

BOUNDARY LUBRICATION

FLUID FILM LUBRICATION

TYPES OF LUBRICATION

Absorbed boundary lubricant

BOUNDARY LUBRICATION

FIG. 2-18

independent of the physical properties of either lubricant (eg. its viscosity) or the bearing material (eg. its stiffness), but instead depends almost entirely on the chemical properties of the lubricant.

glycoprotein, lubricin

lubricin is adsorbed as a macromolecule monolayerlubricin is adsorbed as a macromolecule monolayer

FLUID FILM LUBRICATION

SQUEEZE FILM LUBRICATION

HYDRODYNAMIC LUBRICATION

FLUID FILM LUBRICATION

Utilizes a thin film of lubricant that causes greater bearing surface separation

> 20 um

HYDRODYNAMIC LUBRICATION

Occurs when nonparallel rigid bearing surfaces lubricated by a fluid film move tangentially with respect

to each other (i.e. slide on each other), forming a covering wedge of fluid.

A lifting pressure is generated in this wedge by the fluid viscosity as the bearing motion drags

the fluid into the gap between the surfaces.

SQUEEZE FILM LUBRICATION

Occurs when the rigid bearing surfaces move perpendicularly towards each other. In the gap between the two surfaces, the fluid viscosity generates pressure,

which is required to force the fluid lubricant out.

The squeeze film mechanizm is sufficient to carry high loads for short duration

In the hydrodynamic and squeeze film lubrication, the thickness and extent of fluid film, as well as its load-bearing capacity, are characteristics independent of rigid bearing

material properties.

Determined by

• reologic properties (viscosity)

• the film geometry ( the shape of the gap)

• speed of the relative surface motion

Elastodynamic lubrication

(surface of the bearing material is not smooth)

Self-lubrication

The effective mode of lubrication depends on the applied loads and on the velocity (speed and direction) of the bearing surfaces.

Boundary lubrication: high loads, low speed, long periods

Fluid film lubrication: low loads, high speed

combinations

Elastohydrodynamic lubrication: the pressure generated in the fluid film substantially deforms the surface

Summary

1. Elastohydrodynamic fluid film of both sliding (hydrodynamic)and the squeeze type probably play an important role in lubricating the joints

2. With high load and low speed of relative motion, such as during standing, the fluid film will decrease in thickness as the fluid is squeezed out from between the surface.

3. Under extreme loading conditions, such as during extended period of standing following impact, the fluid film may be eliminated, allowing surface-to- surfabe contact.

WEAR OF ARTICULAR CARTILAGE

1. INTERFACIAL (ABRESIVE) WEAR

interaction of bearing surfaces

2. FATIGUE WEAR

accumulation of microscopic damage (disruption of the collagen-PG matrix) within the bearing materials under repetitive stressing

erosion

BIOMECHANICS OF CARTILAGE DEGENERATION

Failure progression relates• magnitude of the imposed stresses• total number of sustained stress peaks• changes in the intrinsic molecular and microscopic structure of the collagen-PG matrix• changes in the intrinsic mechanical property of the tissue