Elastic Anisotropy of Human Dentin

Objectives: Firstly, to calculate the elastic anisotropy of both wet and dry human dentin determined by others using resonant ultrasound spectroscopy to have transverse isotropic symmetry, thus acquiring the five transverse isotropic elastic stiffness coefficients (elastic constants) for both. Secondly, to apply a model calculation, that replaces these five tensor elastic stiffness coefficients by two scalar constants, the percentage shear, As*(%), and compressive, Ac*(%), anisotropies, in order to understand how dentin's elastic properties relate to those of human cortical bone as well as apatite, the inorganic component of both dentin and bone. Methods: The calculation of the scalar compressive and shear anisotropy factors developed for single crystal refractory compounds has been adapted to the anisotropic elastic stiffness coefficients determined by a number of ultrasonic measurements of both wet and dry human dentin, human cortical bone and apatites based on transverse isotropic symmetry. Results: These calculated compressive Ac*(%), and shear, As*(%), anisotropy factors respectively are as follows: dentin: wet 1.305, dry 0.000; wet 0.377, dry 0.011; human femoral cortical bone: 1.036, 1.055; HAp: 0.995, 0.686; FAp: 0.867, 0.630. Conclusion: The value of Ac*(%)is zero and that of As*(%)is very close to zero for dry dentin. Elasticity theory for this case indicates that dry dentin thus is isotropic. This isotropic-like behavior of the dry dentin may have clinical significance in patients with decreased saliva output or “dry mouth” syndrome in which there is an associated drying of tooth structure. The authors gratefully acknowledge partial support for this project from the following National Institutes of Health grants: R01DE014392 and S10RR16710.