NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 256

another study showed that upon the removal of dentin collagen using
sodium hypochlorite, the dentin had relatively unchanged hardness and
elastic modulus, but the opposite outcome was seen when the dentin was
etched beforehand. 68
In conclusion, considering the microstructural variations of the dentin
and their impact on the mechanical properties measured, any attempt
to investigate dentin mechanical properties has to take into account
sample selections, that is, whether permanent or primary teeth, age of the
tooth, young teeth are less mineralized and have bigger tubules, testing
sites, dentin near the pulp has decreased mechanical properties due to
increased tubule numbers and dimensions, 69 less mineralized intertubular
dentin 60 and the decreased dimension of highly mineralized peritubular
dentin, 70 direction of load whether it is perpendicular or parallel to
tubules 71 and whether the test is conducted on hydrated or dehydrated
dentin. 72,73 Furthermore, storage media have been shown to have a
detrimental effect on the mechanical properties of dentin, 38 but this may
be minimized by preparing the specimens just before testing.
The micromechanical properties of dentin are of relevance to the
issue of etch-based pretreatment of dentin prior to use of a composite
resin restorative. Bonding between adhesive restorative materials and
dentin is normally facilitated through partial ionic bonding of the
polymer of the restoration to the calcium of hydroxyapatite and the
collagen of the dentin, as well as micro-mechanical retention of the resin
into the etched dentin. Toledano et al . 74 showed that etched dentin
prepared for restorative bonding had a significantly reduced hardness,
which could not be fully restored following the impregnation of bonding
resin into it, thus the hardness of the resin-dentin interdiffusion zone was
considerably lower than that of unaltered dentin. A gradient of moduli of
elasticity was observed from the stiffer sound dentin to a less stiff resin-
dentin interdiffusion zone, and adhesive resin layer to the restorative
composite. 66 This is simply explicable as the restorative materials used
have an elastic modulus far inferior to sound dentin, typically ranging
from 5.8 to 9.2 GPa. Further investigation may lead to more appropriate
technique and material selections that can create a mechanically
sustainable dentin-resin bonding.