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RESEARCH REPORT |
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
* corresponding author, roritchie{at}lbl.gov
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: dentin • fracture resistance • alcohol • toughening • R-curves
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Water is vital in developing and maintaining the structure of the molecules comprising the collagen fibrous network. It forms a highly ordered inner hydration layer that creates hydrogen bonds along the underlying peptide chains (Ramachandran and Chandrasekharan, 1968; Chapman and McLauchlan, 1969; Chapman et al., 1971; Lazarev et al., 1992). It also forms hydrogen-bonded "bridges", which further contribute to the structure of collagen by forming intra- and inter-chain links within molecules, along with intermolecular bridges between neighboring triple helices (Bella et al., 1994, 1995).
Certain polar solvents, such as acetone and methanol, are known to dehydrate dentin chemically by replacing the water bonded to the collagen. This behavior is of interest, because polar solvent-based adhesive monomers are often used in clinical dentistry to help achieve micromechanical retention of resin composites (Nakabayashi, 1998). Such dehydration causes shrinkage of the tissue, and has also been reported to increase the tensile moduli and strength of dentin (Maciel et al., 1996; Pashley et al., 2001, 2003). Indeed, our recent studies have showed that the fracture resistance, i.e., toughness, of fully mineralized dentin is also increased by the presence of such solvents, specifically acetone, methanol, and ethanol (Nalla et al., 2005). This suggests that dehydration by alcohol may actually strengthen teeth. Accordingly, in the present study, we examined whether 86-proof Scotch whisky had a similar effect on the mechanical properties of dentin.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Deformation Behavior Testing
To evaluate the stiffness and strength properties of dentin, we conducted bending strength tests. Beams of dentin, ~ 1.65 x 2.9 x 20 mm (N = 5), were sectioned such that their length was nominally parallel to the long axis of the tubules, and soaked in 86-proof Scotch whisky (Black & White, James Buchanan and Co., London, UK) for ~ 24 hrs at room temperature. The beams were loaded to failure (displacement rate = 0.015 mm/s) under three-point bending (center-to-end loading span = 7.62 mm) with the use of a servo-hydraulic testing machine (MTS 810, MTS Systems Corp., Eden Prairie, MN, USA), while the loads and load-line displacements were monitored. We analyzed these data to assess differences in the deformation behavior in terms of the initial stiffness (reflective of Young’s modulus) and ultimate (bending) strength.
Fracture Toughness Testing
To measure the fracture toughness of dentin, we machined compact-tension, C(T), specimens (N = 5), from the shards with specimen thicknesses of ~ 1.7–2.7 mm, widths of ~ 12.3–17.1 mm, and initial notch lengths of ~ 3.5–4.3 mm, oriented such that crack growth was perpendicular to the long axis of the tubules, and the crack plane was in the plane of the tubules; further details are given in our previous studies (Kruzic et al., 2003; Nalla et al., 2004). The specimens were dehydrated prior to actual testing by being soaked in the whisky for 24 hrs at room temperature. Crack resistance-curve (R-curves) were then measured while the specimens were continuously irrigated with whisky. This approach involved measurement of the crack resistance as a function of crack extension, KR(
a), and has been shown to be the most appropriate means of evaluating the fracture toughness of mineralized tissues such as bone and dentin (Vashishth et al., 1997; Kruzic et al., 2003; Malik et al., 2003; Pezzotti and Sakakura, 2003; Nalla et al., 2004). Specimens were loaded at a displacement rate of ~ 0.015 mm/s in an MTS 810 testing machine, until the onset of cracking from the notch. At this point, the sample was unloaded by 10–20% of the peak load so that we could record the sample compliance at the new crack length. This process was repeated at regular intervals until the end of the test, at which point the compliance and loading data were analyzed for the determination of fracture resistance, KR, as a function of a; crack lengths, a, were calculated from the load-line compliance data according to standard compliance calibrations (Saxena and Hudak, 1978), while we periodically corrected for any errors arising from crack-bridging (Kruzic et al., 2003; Nalla et al., 2004). The data were compared with those for ethanol (200-proof alcohol) and water (Hanks’ Balanced Salt Solution, HBSS) (Nalla et al., 2005), and analyzed statistically by the non-parametric Kruskal-Wallis test. After specimens were tested, crack paths were examined by optical microscopy (Olympus STM-UMS, Olympus America Inc., Melville, NY, USA) and three-dimensional synchrotron x-ray tomography at the Advanced Light Source (Berkeley, CA, USA). The latter technique was performed with monochromatic 16-keV x-rays, with the tomographic data converted into three-dimensional images by means of the Fourier-filtered back-projection algorithm; full details are described elsewhere (Kinney et al., 2001; Kruzic et al., 2003).
"Dehydration/Rehydration/Dehydration" Testing
To understand the change in toughness with hydration and dehydration, we performed "dehydration/rehydration/dehydration" testing on specimens (N = 3) previously used for R-curve testing. An R-curve test was started in whisky (first dehydration step) and interrupted after some crack extension, and the specimens were dried in ambient air for 24 hrs. The samples were then rehydrated in HBSS for 24 hrs and tested while being continuously irrigated with HBSS (rehydration step). After further crack extension, the samples were again dried in ambient air for 24 hrs, dehydrated for 24 hrs in whisky, and tested while being continuously irrigated with whisky (second dehydration step).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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) revealed that both the initial stiffness, which reflects Young’s modulus, and the bending strength were markedly enhanced due to dehydration in whisky: The stiffness increased some 75–100% and the strength ~ 40–50% compared with hydrated dentin. These values, however, were still 10–20% lower than those reported previously for pure (200-proof) ethanol (Nalla et al., 2005).
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) can be seen to display a steep rise in toughness over the initial 1–2 mm of crack growth, followed by a flat "plateau" region of nearly constant fracture resistance. Qualitatively, such behavior was similar in all 3 environments. Quantitatively, 3 measures of the fracture resistance—the crack-initiation toughness (the initial point on the R-curve), the crack-growth toughness (the slope of the R-curve), and steady-state ("plateau") toughness—were extracted from these data. As with the strength and stiffness (Figs. 2b-2d), there were significant differences. Although there was only a small change in the initiation toughness, all other measures of the toughness were significantly higher for dentin dehydrated with whisky, as compared with hydrated dentin; differences in the growth and plateau toughness were statistically significant (p < 0.05). However, values measured in whisky were again lower than those for pure ethanol.
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). Indeed, the effect of the whisky appeared to be completely reversible, since the elevated strength and toughness could be re-established upon re-exposure to whisky.
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). The bridges, which tomography verified as being three-dimensional and not just a surface phenomenon (Fig. 4b), were comprised of unbroken material, often up to several hundred micrometers in size, which spanned the crack behind the crack tip. Such "uncracked-ligament" bridges were primarily formed along the crack path by the imperfect linking of microcracks ("daughter" cracks) that initiated ahead of the tip of the main ("mother") crack, invariably at the tubules, as reported previously (Kruzic et al., 2003; Nalla et al., 2004).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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So what role does whisky play in this mechanism? First, it is likely that the increased stiffness and strength of dentin exposed to polar solvents have their genesis in additional hydrogen bonds between adjacent collagen peptide chains within the collagen fibers (Pashley et al., 2003). Water forms hydrogen-bond bridges across adjacent chains, and when the water is replaced with a weaker hydrogen-bond-forming solvent, like ethanol, fewer of the hydrogen-bonding sites are occupied by the solvent; additionally, the structure of the collagen molecule is likely to be disrupted from the loss of the hydration layer and change in bonding patterns. The resulting increase in direct collagen-collagen hydrogen-bonding between molecules due to dehydration then led to a stiffer and stronger material, as shown by the load-deformation behavior.
The higher stiffness and strength of the dentin in whisky associated with increased collagen-collagen hydrogen-bonding led, in turn, to stiffer and stronger crack bridges than in hydrated dentin. We believe that it is this enhanced ability of the crack bridges to sustain loads that is the source of the increased fracture toughness of dentin in whisky. This notion is consistent with our experiments showing that these changes in fracture properties are reversible, since the breaking and reformation of hydrogen bonds would be both a relatively easy and a reversible process.
It is interesting to note that, whereas the elastic modulus was the same whether dentin was vacuum-dried or solvent-dried, vacuum-drying decreased the toughness, while alcohol drying increased the toughness. In Kruzic et al.(2003), we observed that crack-blunting occurred in hydrated dentin, but was absent in air- and vacuum-dried dentin. Such blunting decreased the driving force of a dominant crack by reducing the stress intensity at the crack tip; it further facilitated bridge formation, which led to a rising R-curve in normal hydrated dentin. Rising R-curves were not seen in air-dried dentin, consistent with the absence of crack-tip blunting, but were seen in alcohol-dehydrated dentin, implying that blunting occurs in alcohol. The feature common to both hydrated and alcohol-saturated dentin is the presence of fluid. We conjecture that a fluid layer must facilitate the blunting of cracks in dentin, consistent with the significantly different toughness properties in alcohol-dried vs. vacuum-dried dentin.
In summary, we have used alcohol to probe fundamental questions in restorative dentistry and mineralized tissue research. For many years, there has been clinical debate over whether endodontically restored teeth are more "brittle" than untreated teeth. Many have argued that endodontically restored teeth are less moist, and therefore would be more prone to brittle fracture (Helfer et al., 1972). Indeed, several attempts have been made to measure the moisture content of teeth, often with contradictory findings. Here, we demonstrated that partial removal of water, and its replacement with whisky, actually increased the fracture resistance of dentin. However, since removal of water by testing in vacuo conversely lowers the toughness of dentin (Kruzic et al., 2003), we believe that it is not water per se that may be important; rather, it appears that the presence of a fluid is more critical for the proper mechanical function of the tooth.
Finally, our observations that changes in water content can have pronounced effects on both the elastic and fracture properties of a mineralized tissue indicate that processes that occur at the molecular level are important in regulating mechanical behavior at all length scales. Thompson et al.(2001) have recently showed that collagen properties at the molecular length scale in bone are affected by changes in the fluid chemistry; this work demonstrates that these changes can indeed influence fracture, but over much larger length scales.
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ACKNOWLEDGMENTS |
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Received August 29, 2005; Last revision June 29, 2006; Accepted August 7, 2006
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REFERENCES |
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