Mechanical properties
The hardness (H) describes the resistance to local plastic deformation induced by abrasion or indentation. The Young’s modulus (E) is the measure of the stiffness of a solid material and describes the relationship between tensile stress and axial strain.
In every tooth, the cusp (E mean values range between 10.27 and 15.95 GPa; H mean values range between 0.58 and 0.85 GPa) was always the stiffest and hardest region, followed by the terminal stylus (E=8.51–14.18 GPa; H=0.47–0.77 GPa), the basis of the stylus (E=7.58–12.09 GPa; H=0.41–0.64 GPa), the basis (E=2.64–4.23 GPa; H=0.14–0.23 GPa) and finally the bulge as the softest and most flexible region (E=1.75–1.84 GPa; H=0.09–0.10 GPa). The parts of the inner laterals were harder and stiffer, followed by the outer laterals A, B, C, D and finally E with the softest and most flexible parts (see Figure 7 and Supplementary Table 7 for all values).
Pairwise comparison by Wilcoxon method revealed, that the regions within each tooth showed highly significant differences (p<0.0001*; see Supplementary Tables 8 and 9 for all p-values). Most regions were also highly significantly different, when they were compared between the teeth (mostly p<0.0001*; see Supplementary Tables 8 and 9 for all p-values).
Despite the differences of E and H values along the various regions of each tooth and between teeth, the two values always exhibited a very high positive correlation (r = 0.99, p<0.0001*) within a specific locality.