  Background: Direct dental restoration implies a drying of the dentin
substrate. This drying may induce significant strain in the dentin, affecting
the bonding efficiency of the restoration. Objective: We measure the dilatation
of dentine under changes of relative humidity as well as the impact of humidity
on dentin elastic properties. This investigates the role of relative humidity
variation during dental surgery on restoration lifetime. Methods: We have
coupled an environmental chamber to control both temperature and humidity on
the sample, with an optical microscope to measure precisely the strain on the
sample surface, after a quantification of the measurement noise. This set-up is
used on carefully prepared samples placed on a compression device to measure
the elastic parameters. Results: Dentin dilates when the relative humidity
increases, with a coefficient of hygroscopic expansion of typically 6.10-3
%.(%RH)-1. This dilatation occurs in about ten minutes. Young modulus and
Poisson's ratio are not modified by the variation of relative humidity.
Conclusions: Hygroscopic expansion is an order of magnitude larger than thermal
expansion during dental surgery: around 0.3% with respect to 0.03%. These
levels are low with respect to dental rupture, but may induce a significant
decrease of the life-expectancy of a restoration.
