Biomedical Engineering Reference
In-Depth Information
in the cartilage, aligned parallel to the surface in the superficial zone and
perpendicular to the surface in the deep zone, with random orientation in
the middle zone . The proteoglycan content and chondrocyte distribution
also vary between the different zones.
Because of the complex microstructure of cartilage, and the key roles
played by both the collagen II and proteoglycan molecules in load
support in the joint, it is desirable to measure mechanical properties in
cartilage without disrupting the cartilage structure. This means that,
ideally, mechanical testing should occur
in situ
, with the cartilage layer
still attached to the bone, so that water can only flow from the cartilage
along the free surface and the proteoglycan matrix will remain trapped
within the collagen network.
2.2
. Mechanical characterization of cartilage
Macroscale and microscale indentation tests are the most common
method for characterizing the mechanical properties of cartilage because
they allow for
in situ
testing without disruption of the cartilage
microstructure.
10,11
In a typical indentation test, the entire joint is
mounted in the testing apparatus, and a porous cylindrical tip with
diameter on the order of 1 mm is used to compress different regions of
the sample, mapping mechanical properties across the joint surface.
Load-relaxation (or stress-relaxation) tests are frequently used to
characterize the mechanical properties of cartilage samples using
indentation.
10,11
In a load-relaxation test, a displacement is applied to the
cartilage surface using the indenter tip. While maintaining the same
displacement, the force (or load) on the tip is monitored. In viscoelastic
materials, the load tends to decrease with time under a constant
displacement, a phenomenon referred to as relaxation. Eventually,
usually after minutes or hours, equilibrium is reached and the load
stabilizes. Depending on the assumed material model used to interpret
the load-relaxation results, elastic, viscoelastic or biphasic poroelatic
properties can be measured from this indentation test.
10,12-15
Indentation and microindentation tests using a variety of tip
geometries and material models have been used to characterize cartilage
in animal models
in situ
for the past 30 years.
10,12-15
The correlations
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