On the mechanistic role of the dentin-enamel junction in preventing the fracture of human teeth Page: 5 of 18
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toughness of the DEJ; further, we identify the microstructural mechanisms by which the
DEJ functions to inhibit cracks from traversing the interface to cause catastrophic tooth
To first assess the variation in properties across the interface, we measured Vickers
hardness and indentation toughness profiles under hydrated conditions normal to the
DEJ (Fig. 1). Similar to previous results11,13,16,18,20, this indicated that the hardness of
the enamel falls quite rapidly within a millimeter of the optical DEJ to reach a minimum
in the mantle dentin in close proximity to the DEJ. Corresponding indentation
toughness measurements show that that the toughness of the enamel also has a
minimum close to the DEJ, but then rises steeply over the final -200 m into the DEJ.
These profiles clearly indicate that cracks in the enamel experience a region of
decreasing hardness yet increasing toughness as they impact the DEJ.
To quantitatively evaluate the toughness of the DEJ, we placed a series of Vickers
microhardness indents (-40-50 per tooth) in polished sections of 13 non-carious
extracted human molars (11 axial and two occlusal sections), each tooth being unique to
a single patient. Indents were made under hydrated conditions at -20-50 m from the
optical DEJ on the enamel side (Fig. 2) such that cracks emanating from the corners of
the indents would propagate toward the dentin and impinge onto the DEJ at differing
angles of incidence. We then observed whether the cracks penetrated the interface,
arrested or deflected along the DEJ. Knowing the modulus and toughness of the two
phases on either side of the interface, we deduced the interface toughness using an
"interface impingement" technique developed by Becher et al. for ceramics1. The
basis of this method is the linear-elastic solutions of He and Hutchinson22 which govern
whether a crack, which is incident on a bimaterial interface, will deflect along, or
penetrate through, the interface; this event depends specifically on (i) the angle of
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Imbeni, V.; Kruzic, J.J.; Marshall, G.W.; Marshall, S.J. & Ritchie, R.O. On the mechanistic role of the dentin-enamel junction in preventing the fracture of human teeth, article, September 1, 2004; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc785232/m1/5/: accessed May 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.