3 COMPOSITION AND STRUCTURE OF ARTICULAR CARTILAGE 1. CHONDROCYTES, 10 %2. COLLAGEN (fibrous ultrastructure, procollagen polypeptide), %3. PROTEOGLYCAN ( PG ) large protein polysaccharide molecules ( in form of monomers and aggregates) 3-10 %4. WATER + inorganic salts, glycoprteins, lipids, %
8 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.
9 PERMIABILITY OF ARTICULAR CARTILAGE Porosity ( b ): ratio of the fluid volume (m3 )to the total amount (m) of the porous materialPermeability ( 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
14 The rate of fluid exudation governs the creep rate, so it can be used to determine the permeability coefficientTISSUE PERMEABILITY COEFFICIENT(k)HUMAN CARTILAGE: / x m4 / N sBOVIN CARTILAGE: / x m4 / N s
15 Equilibrium defomation can be used to measure the intrinsic compressive modulus INTRINSIC COMPRESSIVE MODULUS ( HA )HUMAN CARTILAGE: / MPaBOVIN CARTILAGE: / MPa
16 “k” varies directly with water content “HA” varies inversely with water content
17 STRESS RELAXATION TISSUE PERMEABILITY (k) INTRINSIC COMPRESSIVE MODULUS ( HA )similar to creep
22 Tangent modulus, which denotes the stiffness of the material s / eMaximum strain : MPa,Physiological strain: 15 % > MPacompliance = e / sszigma, epszilon
23 PURE SHEAR FORCES No pressure gradiens or volumetric changes No interstitial fluid flow occuresThus , a steady dynamic pure shear experiment can be used to asses the intrinsic viscoelastic properties of the collagen - RG solid matrix.
24 PURE SHEAR storage modulus ( G` ), loss modulus ( G`` ) dynamic shear modulus ( G* )2 = ( G`)2 + ( G``)2G* = ( G`)2 + ( G``)2FIG. 2-15phase 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
25 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 materialsd value is a measure of the total frictional energy dissipation within the material.FIG. 2-15, 16In pure elastic material is no internal frictional dissipation: d is zerofor pure viscous fluid d is 90 degree
30 glycoprotein, lubricin BOUNDARY LUBRICATIONindependent 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. 2-18glycoprotein, lubricinlubricin is adsorbed as a macromolecule monolayer
31 FLUID FILM LUBRICATION HYDRODYNAMIC LUBRICATIONSQUEEZE FILM LUBRICATION
32 FLUID FILM LUBRICATION Utilizes a thin film of lubricant that causes greater bearing surface separation> 20 um
33 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.
34 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
36 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 byreologic properties (viscosity)the film geometry ( the shape of the gap)speed of the relative surface motion
40 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 periodsFluid film lubrication: low loads, high speedElastohydrodynamic lubrication: the pressure generated in the fluid film substantially deforms the surfacecombinations
41 Summary1. Elastohydrodynamic fluid film of both sliding (hydrodynamic)and the squeeze type probably play an important role in lubricating the joints2. 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.
42 WEAR OF ARTICULAR CARTILAGE 1. INTERFACIAL (ABRESIVE) WEARinteraction of bearing surfaces2. FATIGUE WEARaccumulation of microscopic damage (disruption of the collagen-PG matrix) within the bearing materials under repetitive stressingerosion
43 BIOMECHANICS OF CARTILAGE DEGENERATION Failure progression relatesmagnitude of the imposed stressestotal number of sustained stress peakschanges in the intrinsic molecular and microscopic structure of the collagen-PG matrixchanges in the intrinsic mechanical property of the tissue