Opaque dentin in relation to caries (Gottlieb, Dental Caries, 1946)

       Bernhard Gottlieb (one of the fathers of Oral Histology; former professor of the College of Dentistry at Baylor University, Texas, USA, and College of Dentistry at University of Vienna, Austria) studied dentin reactions to caries in his book “Dental Caries. Lea & Febiger, 1946” (out of print). Ground sections of carious teeth were analyzed by both transmission light microscopy and microradiography (known as Grenz rays at that time), and some sections presented opaque dentin in relation to caries:

Figure. Light microscopic (left) and microradiogarphic (right) images of the same ground section of the tooth crown showing that some areas of opaque dentin (area B in the figure on the left) are sclerotic dentin (A, B, and C areas in the figure on the right) and another area is demineralized dentin (area E in the figure on the right).

       This rare evidence is important for the understanding of the nature of caries lesions.

Polychromatic polarization microscopy: new technique for studying weak birefringent structures

           A new technique for analyzing structures at nanoscale with polarizing microscopy has been developed: polychromatic polarization microscopy (Shribak, Scientific Reports, 5:17340, 2015; doi:10.1038/srep17340). One of the limitations of exisitng polarizing microscopic techniques was that samples with small retardances (like biological soft tissue samples) required digital processing to be rendered visible in the field of view. The new technique allows the visualization of birefringent samples without digital processing and the structures appear colored regardless their orientation in relation to the diagonal position (position at which strutuctures present maximun brightness in conventional polarization microscopy). The resulted image of the new polychromatic polarization microscopy shows birefringent objects with different brightness (relative to their retardances) and colors (relative to the orientaiton of their slow axis). The new technique is independent of sample orientation and the image can be seen through the eyepieces without digital processing.
  The new technique opens the possiblity of studying biological samples with small retardances, with applications in the diagnosis of various diseases and imaging of low birefringent crystals (like in carious, fluortic and developing enamel).

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.