Enamel birefringence in the study of tooth bleaching: enamel changes in relation to bleaching

          Commonly changes in dental enamel in relation to patologies and dental procedures are related to changes in mineral volume only. But enamel is composed of mineral, organic, and water contents. A deeper insight can be obtained when all three components are investigated. Interpretation of enamel birefringence combined with quantification of the mineral volume (from microradiography) is able to provide data on the organic and water volumes. One relevant research question is whether enamel incorporates organic matter when sujected to enlargement of its pores (by demineralization or frature formation). There is evidence indicating that fracture lines in enamel are infiltrated with extrinsic organic matter (Chai et al., PNAS, 106(18): 7289-93, 2009), and that artificial carious enamel incorporates salivary proteins when these later are in contact with enamel during the cariogenic challenge (Iizuka et al., Dent Mater J, 33:663-668, 2014). Recent study evaluated enamel changes after a short bleaching procedure (Ferreira et al., Arch Oral Biol, 65:52-58, 2016; http://dx.doi.org/10.1016/j.archoralbio.2016.01.016)
      After a single tooth bleaching session, tooth color changed and the changes in enamel componenet volumes followed a gradient both within component volume (mineral los > organic gain > water gain > decrease in permeability) and decreased from the enamel surface inward. Most of the mineral loss was restricted to the outer ~60 microns, indicating that most of the enamel layer was not affected by tooth bleaching. The mild enamel changes do not support previous report that 70% of changes in tooth bleaching were related to changes in enamel properties (Ma et al., J Dent 39:739-745, 2011; doi: http://dx.doi.org/10.1016/j.jdent.2011.08.005). The more intense enamel changes were related to a bleaching procedure performed with various sessions, which might have affected a thicker part of the enamel layer.
    The graded changes in enamel component volumes predict that organic matter is incorporated when enamel is demineralized by H+ ions released from hydrogen peroxyde. The resulted reduced permeability might protect enamel from further demineralization. An interesting question that worths investigation.