Role of Alcohol in the Fracture Resistance of Teeth
R.K. Nalla1,
J.H. Kinney2,
A.P. Tomsia1, and
R.O. Ritchie1,3,*
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA;
2 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA; and
3 Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA
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Figure 1. Typical load-displacement data (N = 5 each) for chemically dehydrated dentin (in 200-proof ethanol and 86-proof Scotch whisky) and hydrated dentin (in Hanks’ Balanced Salt Solution, HBSS), based on three-point bending tests. The initial (elastic) portion of the load-displacement curve is a measure of the stiffness and reflects the Young’s modulus; the maximum point on each curve is a measure of the ultimate bending strength. It is apparent that soaking the dentin samples in whisky or alcohol to dehydrate them led to a significant increase in the stiffness and strength of the dentin.
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Figure 2. Fracture toughness of dentin. (a) Fracture resistance data for dentin tested with continuously irrigation in whisky (N = 5), expressed in terms of crack-resistance curves (R-curves), measured on compact-tension, C(T) specimens. Each data point type represents a separate sample. The shaded regions on the R-curves indicate similar data obtained for hydrated dentin and dentin dehydrated in pure ethanol. The bar graphs (mean ± SD) show a significant increase in (b) crack initiation, (c) crack growth and (d) steady-state ("plateau") fracture toughness for dentin dehydrated in whisky and ethanol, as compared to hydrated dentin. Differences in the growth and plateau toughness were statistically significant (p < 0.05).
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Figure 3. Representative results for the "dehydrated/rehydrated/dehydrated" tests (N = 3) for whisky expressed in terms of the fracture toughness R-curve behavior. These experiments give a clear indication of how the toughness dropped when dehydrated dentin was exposed to water (HBSS), and that the fracture toughness of dentin was significantly higher in whisky. However, they also provide clear evidence that the effect was completely reversible.
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Figure 4. Toughening by crack bridging in dentin. (a) An optical micrograph showing the development of crack bridging in dentin dehydrated using whisky. Microcracks ("daughter cracks"), initiated primarily at tubules, form ahead of the main crack ("mother crack"); their inability to link perfectly with the main crack leads to regions of uncracked material which spans the crack. Such "uncracked ligaments" carry load that would otherwise be used to drive the crack, and thereby act to toughen the dentin. (b) A three-dimensional x-ray tomography reconstruction of a section of the crack in (a), showing the three-dimensional nature of the uncracked-ligament bridging. The nominal direction of crack growth is indicated in both cases.
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