Degree of water transport in enamel inversely related to the water volume to organic volume ratio: an evidence

    As normal enamel dehydrates in air at room temperature, a plateau in the water volume remaining into enamel pores (firmly bound water) is reached after a certain time. This water volume represents the equilibrium moisture content. Water is the main vehicle in enamel for the transport of materials, which occurs by diffusion. It was recenly highlighted that as enamel dehydrates, the ratio of water volume to organic volume decreases in the pores (Medeiros et al., J Micros, 2013; DOI: 10.1111/jmi.12037). This reduced ratio represents an increased viscosity, according to the Einstein equation for diffusion, slowing down diffusion. Medeiros et al. (2013) hipothesized that transport of materials is slower in dried than in wet enamel because of the expected higher viscosity in the former. As enamel dehydrates, the organic volume remains the same while the water volume decreases.
   There is preliminary published evidence supporting this hipothesys. Houwink B.(The limited usefulness of Thulet's solution in imbibition experiments in dental enamel. British Dental Journal, v.126: 447-450, 1969) reported (in vitro study with ground sections using polarizing microscopy; see Table Ib on page 448) that normal human enamel dried at room temperature for 1.5 h recovered the original water content after 2 h of imbibition in water; while enamel dried for 2.5 days took 2.5 days of water imbibition to recover the original water content. This suggests that the time required to reach maximum infiltration  of a given material in normal enamel is inversely proportional to the water volume to organic volume ratio remaining in the pores. This has important implications for pathogenesis, diagnosis, remineralization and infiltration of enamel caries. For caries pathogenesis, the faster inward diffusion rate of water in wet enamel is expected to play a role in the depth of artificial and in situ caries lesions. For diagnosis, opacity of enamel should be determined after equilibium moisture content is established only, for consistency between studies. For resin infiltration, it is expected to be slower in dried than in wet enamel. Sousa et al. (Caries Res, 47: 183-192, 2013; Doi: 10.1159/000345378) reported how permeability can be measured at histological points of enamel caries lesion and its relationship to the expected amount of infiltrant that can fill the pores.