NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 246

a prominent elastic-plastic transition point. This is mainly caused by the
unique composite rod structure of enamel and its organic rich interrod
which makes the whole material more deformable than pure inorganic
hydroxyapatite. The stress-strain curves, especially the inelastic part for
the top surface, lie well above those for the cross section surface. For
indentations on the top surface, the force direction is parallel to the prism
with the yield behavior primarily determined by shear deformation of the
very thin protein “glue” between the individual apatite crystallites. In the
case of the cross-section surface, the loaded rods deflects under the force
because the contact load is supported not only by the rod structure but
also by the more deformable and thicker interrod structure surrounding
the rods parallel to the surface.
Anisotropic properties of enamel, which arise from the rod like
prism-sheath structure, are well reflected in the stress-strain curves. The
top surface is stiffer and has higher stress-strain response than an
adjacent cross section surface because of the greater influence of the
prism sheaths in the latter behaviour.
From a comparison of stress-strain curves for a range of dental
materials ( Fig. 8-5 ) , enamel was found to behave more like a metallic
material rather than a ceramic. A comparison with pure hydroxyapatite
disk (HAP), which is the main component of the enamel, indicated that
enamel had a much different stress-strain response. This suggests that the
minor organic components in enamel influence the mechanical behaviour
of this highly mineralized biocomposite significantly. From this point of
view and the above results, traditional criteria of dental material selection
based on elastic modulus and hardness may not be appropriate for good
restorative materials which can protect opposing teeth from excessive
attrition. The nanoindentation stress-strain evaluation should also be
involved in future dental restorative material selection and development
studies.
3.3. Energy absorption of enamel
Another approach that further reinforces the significant role of the minor
organic component on the mechanical properties of enamel is the issue of