SEM observations of Retzius lines and prism cross-striations in human dental enamel after different acid etching regimes

Abstract

Objective and design: Sectioned and ground enamel surfaces need to be etched with acid in order for the enamel structure to be exposed for study in the SEM. In the present study Retzius lines and prism cross-striations were observed in variably ground specimens after etching with nitric acid of different concentrations (0.1, 1, 2.5 and 5%) and for different time periods (15, 30, 45, 90 and 180 s). The aims were to improve our interpretation of these structures and to find the best etching regime. Results and conclusions: Retzius lines in the superficial enamel were more readily exposed than Retzius lines in deeper parts of the enamel. The former showed discontinuity defects that facilitated their identification, while the latter lacked such defects. Few deep Retzius lines were exposed and could first be identified with certainty after etching for 30 s in 1% acid. The structural and compositional basis for the visibility of deep Retzius lines may be subtle and difficult to identify. Scratches from the sectioning/grinding procedures may disturb their identification, but in general the grinding procedure was of minor importance for interpretation of enamel structure. Prism cross-striations were more readily exposed with the three stronger acid concentrations than with the weakest. They appeared as alternate light and dark bands, with a lower crystal concentration in the dark bands. No clear association with the prism varicosities/undulations was found. Etching with 1% nitric acid for 30 s seems to be a good compromise for the study of Retzius lines and prism cross-striations.