Figure 7. The Young’s modulus of dentin as a function of orientation of the tubules as calculated from the elastic constants of Lees and Rollins (1972). There was a pronounced hexagonal (transverse isotropic) anisotropy in the Young’s modulus, a result that was forced by a priori assumptions of the symmetry. The Young’s modulus was greatest in the direction of the tubules (36 GPa) and much less in the orthogonal direction (29 GPa). The Young’s modulus is compared with the values from the resonant ultrasound spectroscopy (RUS) measurements of Kinney et al. (2002). The RUS measurements, which were made without assumption as to material symmetry, also show hexagonal symmetry, but the stiffest orientation is now perpendicular to the axis of the tubules. This result is more consistent with what is known about the orientation of the collagen fibrils, which lie in a plane perpendicular to the tubule axis.