Cartilage Biomechanics Flashcards
What are the functions articular cartilage?
➢ Lines the articulating ends of all diarthrodial
joints
➢ Cartilage layer minimizes contact stresses
generated during loading and contributes to
lubrication mechanisms in the joint.
➢ Can maintain this function for seven to eight decades.
➢ A complete understanding of cartilage
mechanics remains elusive due to its
remarkable versatility and complexity
What are the physiological loading conditions of cartilage?
Peak mags of load within joints are multiple factors of BW: 4.9 in hip, 3.4 in knee
Cyclical/intermittent
mean contact stresses of 2-6 MPa in joints
cartilage deformation: 6% to 20%
What are the mechanical properties of cartilage?
response vastly diff for comp,tensile and shearing stresses
viscoelastic:Drag force of the interstitial fluid flow through the permeable solid matrix (flow-dependent)
Time dependent deformation of the solid macromolecules (flow independent)
Biphasic material: interstitial fluid phase
porous permeable solid phase
Describe the process of creep in cartilage
the load applied at the surface is balanced by the compressive stress developed within
the collagen-PG matrix and the
frictional drag generated by the flow of
the interstitial fluid during exudation.
2-4mm takes 4-16hrs to reach creep equilibrium
Describe the affect of hydrostatic fluid pressure in cartilage
support up to 95% or more of the load
within tissue - pressurization occurs because the interstitial water attempts to
squeeze out of the loaded region, but is impeded by the extremely low
permeability of the collagen matrix.
shown to subside over time in vitro, however
this is unlikely to occur in vivo, therefore cartilage loading is always
accompanied by significant interstitial fluid pressurization - Shield ECM and cells within it from crushing stresses and deformations
Describe compression in cartilage
Confined compression
➢ Compressive aggregate modulus (HA) ranges from 0.1-2.0 MPa
Equilibrium confined compressive aggregrate modulus (Ha)
Modulus of tissue after fluid has stopped moving
Ha = E(1-V)/ (1+v)(1-2v)
Unconfined compression
Young’s modulus (E) ranges from 0.41-0.85 Mpa
Poissons ratio = 0.06-0.18
Describe the role of tension in cartilage
small deformations - a non-linear toe region
is seen due to collagen network re-alignment.
Equilbrium tensile modulus depends on:
➢ Density collagen fibers,
➢ Fiber diameter,
➢ Collagen cross-linking,
➢ Interactions between collagen and PG
Describe the role of permeability in cartilage
Darcys Law - relates the relative fluid flux to the pressure gradient with a proportionality constant known as the tissue hydraulic permeability
Permeability coefficient is related to the
extracellular matrix pore structure, apparent size and connectivity
permeability of the tissue is also related to the
amount of compaction the tissue experience
Describe anisotropy in cartilage
Split lines are presumed indicators of collagen
fiber directions in articular cartilage.
0% strain in the toe region, equilibrium tensile
modulus is significantly greater in samples
orientated parallel to the local split-line direction.
Describe the inhomogenity of articular cartilage
Tensile modulus of cartilage in STZ is significantly greater than cartilage in deep zone and the other way around for compression
What are the mechano-electrochemical properties of cartilage
ability of PGs to resist compression arises from:
Donnan Osmotic pressure
Each proteoglycan associated with negative charge requires a mobile counter ion (Na++) to maintain electroneutrality
Swelling occurs from the presence of a high density of neg charge PGs - swelling pressure balanced by tensile forced in collagen
Increased hydration = damaged collagen network (hydration is earliest sign of degradation during OA)
Swelling pressure associated with (-) charged PGs
Stiffness of the Pg molecule itself
What is boundary lubrication?
generate a thin protective film over one or both of the contacting surfaces in order to reduce adhesion minimising the friction between contacting surfaces
independent of lubricant or bearing material properties (i.e. viscosity and stiffness respectively) but is dependent on chemical properties of lubricant
What is fluid film lubrication?
Utilizes thin film that separates bearing
surfaces
➢ Load is supported by pressure developed in
the fluid.
hydrodynamic and squeeze-film
Explain the three types of fluid film lubrication?
Hydrodynamic: As one surface moves across the other, friction between adjacent fluid molecules drags a layer of fluid into the space between them creating a hydrostatic pressure in the fluid, generating lift, forcing the two surfaces apart
Squeeze film: Because a viscous fluid cannot be instantaneously forced from a bearing gap, it becomes pressurised as it is been squeezed out. Only works for high loads over short durations.
Elasto-hydrodynamic lubrication: when the
bearing surfaces are sufficiently compliant to deform under the pressures produced in the lubricating fluid.
