Variations in the mechanical properties of Alouatta palliata molar enamel

Abstract

Teeth have provided insights into many topics including primate diet, paleobiology, and evolution, due to the fact that they are largely composed of inorganic materials and may remain intact long after an animal is deceased. Previous studies have reported that the mechanical properties, chemistry, and microstructure of human enamel vary with location. This study uses nanoindentation to map out the mechanical properties of Alouatta palliata molar enamel on an axial cross-section of an unworn permanent third molar, a worn permanent first molar, and a worn deciduous first molar. Variations were then correlated with changes in microstructure and chemistry using scanning electron microscopy and electron microprobe techniques. The hardness and Young's modulus varied with location throughout the cross-sections from the occlusal surface to the dentin-enamel junction (DEJ), from the buccal to lingual sides, and also from one tooth to another. These changes in mechanical properties correlated with changes in the organic content of the tooth, which was shown to increase from approximately 6% near the occlusal surface to approximately 20% just before the DEJ. Compared to human enamel, the Alouatta enamel showed similar microstructures, chemical constituents, and magnitudes of mechanical properties, but showed less variation in hardness and Young's modulus, despite the very different diet of this species.

Properties of tooth enamel in great apes

“…The horizontal spatial coordinate in this figure is normalized to the distance from the EDJ to the outer surface, to enable comparisons between species. There is a consistent upward rise in the data with distance from the EDJ, broadly consistent with the trends reported elsewhere [23,24]. By averaging the E and H datasets between 0.4 and 0.6 normalized distance for each of the individual species in Fig.…”

“…3 for all tooth enamel specimens tested show lower values of E and H near the EDJ relative to the outer surface. These gradients are smaller than the previously reported factor of 2 from digitized E and H maps across entire cross-sectional areas of dental enamel [23,24], although our data represent just one specimen per species. A correlation between such gradients and mineral composition over the sections has previously been demonstrated [23], but decussation and the presence Fig.…”

“…Modulus and hardness measurements were made across the sections for each primate. Instrumented nanoindentation tests with 50 nN noise resolution and 1 nm displacement noise floor (Nanoindenter XP, MTS Nano Instruments, Oak Ridge, TN) were carried out on the tooth sections with a Berkovich probe [23,24,[29][30][31][32][33]. A fixed penetration of 400 nm was used, sufficiently shallow as to avoid cracking and other serious damage to valuable specimens.…”

“…Material properties can also show substantial gradients across a tooth section. Such gradients have been most graphically demonstrated by instrumented indentation measurements of modulus and hardness on cross-sections of human and howler monkey teeth, in which point-to-point variations are presented as digitized maps [23,24]. Variations of as much as a factor of 2 in modulus and hardness are revealed between the outer surface and the EDJ [23].…”

The Wear and Tear of Teeth

“…Both the elastic modulus and hardness are linearly gradated, with the lowest values near the enamel-dentine junction and the highest at the tooth surface [40,41,42,43]. The properties of the enamel of other mammals vary slightly, but a gradient of hardness and elastic modulus is again usually present [44,45,46,47]. There are unlikely to be big surprises in enamels yet to be examined because both modulus and hardness are subject to a ‘rule of mixtures' wherein the composite tissue is constrained by the stiffest, hardest component of which it is made [48,49].…”