NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 262

measurement of indentation imprint on test material due to highly elastic
recovery upon unloading. As a composite material, dentin mechanical
properties correspond to the relative proportions of mineral, collagen and
water content. Following demineralization, the dentin will contain more
water than the sound dentin and the remnant fluid-filled structure of
collagen with its high poroelastic deformability will become dominant,
particularly in the severely demineralized dentin. 86 The hardness and
elastic modulus of carious dentin were found to decrease significantly, of
which the outer carious surface normally has the lowest value. The
hardness and elastic modulus values for primary carious dentin ranged
from as low as 0.001 GPa and 0.015 GPa, respectively to just below the
sound dentin values. 86 Similar findings were reported for permanent
carious dentin. 73 The transparent, sclerotic layer which is assumed to be
hypercalcified, as a result of dental pulp attempts to block potential toxin
movement towards the pulp, has mechanical properties slightly lower
than the sound dentin. 73 In contrast to the changes in mineral levels
through a carious lesion, which appear to indicate a linear function with
lesion depth, 72,87 the deterioration in the mechanical properties of carious
dentin was found to exhibit a logarithmic function. 72
3.8. Nanoindentation on hypomineralised dental hard tissues
Developmental defects of dental hard tissues mainly include
amelogenesis imperfecta (defects of enamel) and dentinogenesis
imperfecta (defects of dentin). The etiology of these developmental
defects is complex, including genetic, environmental and systemic
factors. Amelogenesis imperfecta has two subtypes: hypoplastic type
(defective matrix production) and hypomineralized type (defective
maturation or mineralization). 81 In general, most of these defective
enamel and dentin have less mineral and higher organic contents.
Theoretically, from Spears' finite element model, 18 the change of the
mineral to organic ratio of the composite will decrease the mechanical
properties of the natural biocomposite significantly. In recent
nanoindentation investigations, the hardness and elastic modulus of the
hypomineralised enamel were reported as 0.53±0.31 and 14.49±7.56
GPa, respectively, which are much lower than the sound counterpart. 88