10 CHAPTER 1. THE TEMPOROMANDIBULAR JOINT
progression of TMJ osteoarthrosis. These loading conditions could arise from abnormal/unstable
occlusion (how the teeth fit together for bite) and internal derangement of the TMJ disc.
1.6 GUIDELINES FOR TESTING AND MODELING OF
TISSUE MECHANICS
A brief summary of mechanical testing modalities and methods for modeling tissue behavior is
presented here. Note that this is meant to be a general overview; a more thorough introduction
to modeling biomechanical behavior can be found in a book entitled “Introduction to Continuum
Biomechanics” authored by Athanasiou and Natoli [58].
Biological tissues have unique characteristics that require more complicated testing methods
than those developed for testing of traditional engineering materials. One marked characteristic is
the presence of a large water component. Because of this, tests should be performed in a hydrated
and osmotically balanced environment similar to what the tissue experiences in situ. Most biological
tissues also display anisotropic and heterogeneous material properties. Therefore, tests should be
performed on multiple regions and in several orientations to illustrate a complete description of a
tissue’s biomechanical properties.
Most biomechanical experiments performed on cartilaginous tissues can be categorized as
either compressive, tensile, shear, or friction tests. In a compressive test, the tissue is exposed to either
indentation, where the force is applied through a small indenter or confined/unconfined compression,
where a platen is used to compress the entire specimen.It is categorized as either confined or unconfined
depending on whether the tissue is supported or allowed to expand freely on its lateral sides. A test
where the sample is pulled is called a tension test. This type of test is inherently more complicated
and prone to error than a compression test due to the need to grip the sample in some fashion.These
tests are classified as either uniaxial or biaxial depending on whether the sample is pulled along one
axis or two axes. A shear test is one where a stress is applied parallel to a face of a material.To perform
this type of test, a small compressive tare load is applied to a sample situated between two parallel
platens. One platen is then translated or rotated parallel to the surface of the sample while recording
the load and displacement in that direction. A friction test can be performed using a device similar
to a shear apparatus, but instead allowing the surfaces to slide against the each other. The coefficient
of friction is obtained as the ratio of the forces applied parallel and perpendicular to the surface.
A stress-strain curve from a uniaxial tension test is shown in Figure 1.6. In this example, a
tissue has been pulled at a constant rate until failure, while measuring force with a load cell. Stress is
defined as load (force) divided by the samples cross section, and has units of pressure (force/length
2
).
Strain is a non-dimensional quantity defined as the change in length divided by initial length. An
appropriate constitutive model can be fit to a stress-strain curve to obtain material properties. In
material mechanics, a constitutive equation is one that relates stress to strain. The most basic model
is that of a linearly elastic solid, which assumes a linear stress-strain relationship. The elastic or Young’s
modulus is defined as the ratio of stress to strain in the range of elastic deformation (the linear region
of the curve before the yield point in Figure 1.6), and is a measure of material stiffness.Deformations

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