BIOMECHANICS OF THE ARTICULAR CARTILAGE

COMPOSITION AND STRUCTURE OF ARTICULAR CARTILAGE 1. CHONDROCYTES, 10 % 2. COLLAGEN (fibrous ultrastructure, procollagen polypeptide), 10-30 % 3. PROTEOGLYCAN ( PG ) large protein polysaccharide molecules ( in form of monomers and aggregates) 3-10 % 4. WATER + inorganic salts, glycoprteins, lipids, 60-87 %

LOCATION OF THE COLLAGENE FIBERS

COMPRESSION

CARTILAGE AS VISCOELASTIC MATERIAL If a material is subjected to the action of a constant (time independent load or constant deformation and its response varies (time dependent) then the mechanical behavior of the material is said to be viscoelastic.

PERMIABILITY OF ARTICULAR CARTILAGE Porosity ( b ): 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 = b 2/ K [(m4/Ns

Basic responses 1. BIPHASIC CREEP 2. BIPHASIC STRESS RELAXATION

1. BIPHASIC CREEP The time taken to reach creep equilibrium varies inversely with the square of the thickness of the tissue

CREEP PHENOMENON CREEP EQUILIBRIUM 2-4 mm human and bovin articular cartilage > 4 - 16 hours FIG. 2-9 rabbit cartilage 1 mm > 1 hour above 1 Mpa > 50 % of total fluid is squeezed

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 s BOVIN CARTILAGE: 4.67 +/- 0.04 x 10 -15 m4 / N s

Equilibrium defomation can be used to measure the intrinsic compressive modulus INTRINSIC COMPRESSIVE MODULUS ( HA ) HUMAN CARTILAGE: 0.79 +/- 0.36 MPa BOVIN CARTILAGE: 0.85 +/- 0.21 MPa

“k” varies directly with water content “HA” varies inversely with water content

STRESS RELAXATION TISSUE PERMEABILITY (k) INTRINSIC COMPRESSIVE MODULUS ( HA ) similar to creep

UNIAXIAL TENSION

Tangent modulus, which denotes the stiffness of the material s / e Maximum strain : 3 - 100 MPa, Physiological strain: 15 % > 5 - 10 MPa compliance = e / s szigma, epszilon

PURE SHEAR FORCES No pressure gradiens or volumetric changes No interstitial fluid flow occures Thus , a steady dynamic pure shear experiment can be used to asses the intrinsic viscoelastic properties of the collagen - RG solid matrix.

PURE SHEAR storage modulus ( G` ), loss modulus ( G`` ) dynamic shear modulus ( G* )2 = ( G`)2 + ( G``)2 G* = ( G`)2 + ( G``)2 FIG. 2-15 phase shift angle ( d ) = tan -1 (G``/ G`) The magnitude of the dynamic shear modulus is a measure of the total resistance of the viscoelastic materials

for pure viscous fluid d is 90 degree The magnitude of the dynamic shear modulus is a measure of the total resistance of the viscoelastic materials d value is a measure of the total frictional energy dissipation within the material. FIG. 2-15, 16 In pure elastic material is no internal frictional dissipation: d is zero for pure viscous fluid d is 90 degree

Bovin articalar cartilage G* = 1 -3 Mpa d = 9 - 20 degrees

LUBRICATION

FLUID FILM LUBRICATION TYPES OF LUBRICATION BOUNDARY LUBRICATION FLUID FILM LUBRICATION

Absorbed boundary lubricant

glycoprotein, lubricin BOUNDARY LUBRICATION 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. FIG. 2-18 glycoprotein, lubricin lubricin is adsorbed as a macromolecule monolayer

FLUID FILM LUBRICATION HYDRODYNAMIC LUBRICATION SQUEEZE FILM 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 Elastohydrodynamic lubrication: the pressure generated in the fluid film substantially deforms the surface combinations

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