The ultrastructure of human dental enamel heat-treated in the temperature range 200 degrees C to 600 degrees C

Heating enamel in the temperature range 200 degrees C to 600 degrees C resulted in poor crystal packing due to void formation, permanent change in the sign of its birefringence (from negative to positive) in some areas, and an altered crystal morphology. Transmission electron microscopy of enamel heated in the temperature range 200 degrees C to 400 degrees C revealed that the distinction between the positively birefringent regions and the negatively birefringent regions (which were present up to 350 degrees C and occasionally up to 400 degrees C) at the tooth's surface was due to the greater volume of intra- and inter-crystalline voids within the positively birefringent regions. There was a significant increase in void volume at 400 degrees C, and above this all of the enamel was positively birefringent and opaque. Large remineralized crystals of beta-tricalcium phosphate (beta-TCP) phase (whitlockite) were initially formed at 400 degrees C, and their size and number increased at 500 degrees C and above. Both the greater solubility of beta-TCP crystals and the increased surface area due to the presence of voids would increase the rate of demineralization of heat-treated enamel.