Mapping of tooth deformation caused by moisture change using moiré interferometry

Bonding of composite restorative materials to teeth usually involves surface etching and drying of both dentin and enamel. These two hard tissues have very different water contents.

The purpose of this study was to evaluate the effects of changes in humidity on the dimensional changes in dentin disks constrained by enamel and in unconstrained dentin.

Changes in humidity were used to induce changes in the moisture level of the hard tissues, which created a mechanical load across the dentin-enamel junction (DEJ). High-sensitivity moiré interferometry was used to measure the magnitudes of the strains. Diffraction gratings (with a sensitivity of 2400 lines/mm) were transferred to moist polished sections of human teeth, which were subsequently dehydrated and then hydrated again. The fringe patterns from moiré interferometry recorded the change in deformation between the moisture states.

The results indicated that there were wide variations in strain between the two specimen geometries. The strains were fairly constant through the bulk of the dentin in both specimen types. However, the strain gradients were very high across the dentin-enamel interface into the dentin in the unconstrained geometry, and nearly zero in the constrained geometry.

Moiré interferometry is a powerful tool to study the deformation of materials that are not isotropic and are not linearly elastic. It provides full-field, high-resolution deformation fields in tooth specimens. Deformations observed in this study indicate that the DEJ zone is a unique material interface that needs to be better understood in terms of normal tooth function, and that its material properties be taken into account when natural hard tissues are restored.