Taylor Castagna
MANE 6963 – Friction and Wear of Materials
Articular Cartilage Notes Biomechanics and Biology of Movement (Nigg, Macintosh)
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Seems reasonable to ignore the time dependency and approximate cartilage response using
short time properties
An assumption is cartilage behaves elastically when subjected to fast load application
o Ideally this is only true after a short time period of load application before the fluid in
the cartilage has had time to flow or at equilibrium when movement of interstitial fluid
ceases
o Normal gait cycle loading occurs within an average of 0.5 seconds, in impact, load times
are much less (milliseconds).
Under other loading conditions with a longer timescale, cartilage has been shown to have
mechanical behavior governed by its multiphasic structure (mixed solid and fluid phases) and its
fluid-bathed environment.
Cartilage deformation can be linked to fluid flow found in soil consolidation theory
o Biot (1941) formulated a 3 dimensional theory on consolidation in which he considered
three phases in soil settling.
 First phase was due to water flowing out of soil directly under load
 Second phase involved water flowing from loaded to unloaded regions in the
soil
 Third phase was linked to the restraining effect of the unloaded region upon the
loaded region
o Zarek and Edwards (1963) considered cartilage a two-phase (biphasic) structure
consisting of a porous elastic solid matrix made up of collagen fibers, ground substance,
and cells saturated by a pore fluid.
 Load application was assumed to be carried partly by pressure in the fluid and
partly by the compressive stresses in the solid matrix.
o Zarek and Edwards (1967) loaded cartilage in compression using a porous piston. This
allowed negligible flow in the parallel direction to the loaded surface while allowing
unrestrained flow in the normal direction. The results showed that articular cartilage
could not be modeled accurately as a linearly elastic material (VISCOELASTIC)
o Mow and colleagues (1980 and 84) suggested a mechanical model for cartilage should
include the influence of mobile electrolytes (triphasic) and consider the solid phase as a
fiber reinforced composite porous matrix
 This biphasic model of articular cartilage represents a soft porous elastic solid
permeated by water. The theory accounts for time-dependent aspects of
cartilage and associated load-deformation behavior in terms of resistance to
interstitial fluid flow.
 Limited permeability of the solid matrix is responsible for flow
resistance associated with drag and with ionic and electrostatic forces.
 Theory depends on 3 material coefficients:
o Permeability
o Equilibrium modulus
Taylor Castagna
MANE 6963 – Friction and Wear of Materials
Articular Cartilage Notes Biomechanics and Biology of Movement (Nigg, Macintosh)
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o Solid phase poisson’s ratio
 Linear biphasic theory assumes that the solid phase is isotropic and
linearly elastic and the fluid phase is a linearly viscous fluid.
Based on the poroelastic behavior of articular cartilage both a creep response and a stress
relaxation response will occur in the cartilage
o Creep response
 Initially fluid flow occurs rapidly with high deformation and diminishes gradually
as permeability decreases with increasing strain until flow stops.
 During creep, load applied at the surface is balanced by the compressive stress
developed within the solid matrix and by frictional drag generated by interstitial
fluid flow .
 Creep stops when the compressive stress in the matrix balances the applied
stress. (4 to 16 hours to reach creep equilibrium; varies inversely with the
square of the tissue thickness)
 At equilibrium when fluid flow has stopped the deformation can be used to
determine the intrinsic compressive aggregate modulus of the solid matrix.
o Stress relaxation
 During compression stress rises due to fluid pressurization since exuding the
fluid is constrained by the matrix permeability
 Stress relaxation is associated with a redistribution of fluid within the solid
matrix.
 The stress relaxation stops when the compressive stress developed equals the
equilibrium stress level determined by the compressive modulus of the solid
matrix.
 Under loading with low strain, high stress levels are difficult to maintain because
stress relaxation rapidly occurs. Leads to rapid spreading of the contact area in
the joint during articulation.
Biphasic theory has been shown to be sufficiently accurate to explain much of the observed
mechanical behavior of articular cartilage in compression. Does not accurately predict behavior
in short time periods following abrupt loading.
Taylor Castagna
MANE 6963 – Friction and Wear of Materials
Articular Cartilage Notes Biomechanics and Biology of Movement (Nigg, Macintosh)