1. BIOMECHANICS OF THE ARTICULAR CARTILAGE

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, %

5. LOCATION OF THE COLLAGENE FIBERS

7. COMPRESSION

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 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

10. Basic responses
1. BIPHASIC CREEP
2. BIPHASIC STRESS RELAXATION

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

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

14. 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: / x m4 / N s
BOVIN CARTILAGE: / x m4 / N s

15. Equilibrium defomation can be used to measure the intrinsic compressive modulus
INTRINSIC COMPRESSIVE MODULUS ( HA )
HUMAN CARTILAGE: / MPa
BOVIN 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

19. UNIAXIAL TENSION

22. Tangent modulus, which denotes the stiffness of the material
s / e
Maximum strain : MPa,
Physiological strain: 15 % > MPa
compliance = e / s
szigma, epszilon

23. 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.

24. 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

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 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

26. Bovin articalar cartilage
G* = 1 -3 Mpa
d = degrees

27. LUBRICATION

28. FLUID FILM LUBRICATION
TYPES OF LUBRICATION
BOUNDARY LUBRICATION
FLUID FILM LUBRICATION

29. Absorbed boundary lubricant

30. 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

31. FLUID FILM LUBRICATION
HYDRODYNAMIC LUBRICATION
SQUEEZE 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 by
reologic properties (viscosity)
the film geometry ( the shape of the gap)
speed of the relative surface motion

37. Elastodynamic lubrication

38. (surface of the bearing material is not smooth)

39. Self-lubrication

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 periods
Fluid film lubrication: low loads, high speed
Elastohydrodynamic lubrication: the pressure generated in the fluid film substantially deforms the surface
combinations

41. 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.

42. 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

43. 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