Thick hybrid layer in dental enamel without acid etching: birefringence helps in the characterization

   Buonocore (Journal of Dental Research, 34:849-853, 1955), in his breakthrough report of the use of acid etching to form a hybrid layer between enamel hydroxyapatite crystallites and resin, was more focused on the possibility of adhesion than on the transformation of normal enamel in a new tissue. In fact, the hybrid layer is a modified, new type of enamel, whose main pathways for acid transportation are blocked by the cured infiltrated resin. The shallow depth of the enamel hybrid layer rendered it prone to be removed relatively easily by oral mechanical forces, eventually exposing the inner enamel layers without the hybrid layer. 
   A recent report showed, for the first time, the formation of a thick hybrid layer (> 1 mm) in normal enamel without acid etching pretreatment (Gan et al., Enhanced Transport of Materials into Enamel Nanopores via Electrokinetic Flow, Journal of Dental Research, 2015; Doi:10.1177/0022034515572189;
 http://jdr.sagepub.com/content/early/2015/02/13/0022034515572189.abstract). The use of electrokinetic flow (EKF), a flow induced in nanochannels by electric field, has been able to form the deepest hybrid layer in enamel ever reported, in some cases with the same thickness of the enamel layer itself. Even if exposed to oral mechanical forces, the new hybrid enamel is expected to exist as long as the enamel layer exists. Optical birefringence was used to characterize the new hybrid enamel, which showed a higher negative birefringence than ordinary enamel due to the decrease in positive form caused by the fluid resin. The fluid resin, with a higher refractive index than water, reduces the the difference between the refractive indexes of the mineral and non-mineral phases, thus reducing form birefringence. The resulting extraordinary reduction in enamel permeability is expected to open new avenues for the application of hybridization in normal (sound) enamel.