Enamel alterations from bracket bonding and debonding: a study with the scanning electron microscope.

Abstract

The central problems of the direct-bonding technique were investigated with a comprehensive scanning electron microscope study. Labial surfaces of extracted teeth were etched for a study of enamel conditioning. Starting from the appearance of a 2-minute etching with 50 percent phosphoric acid, the influence of different factors on the etching patterns was investigated: variation of etching time, of acid concentration, and of acid dispersion; etching of fluoride-treated enamel; and variation of the rinsing time with the air/water syringe. Furthermore, the effect of contamination with saliva, mucosa, oil, and finger contact was demonstrated. The penetration depth of resin tags was measured by means of adhesive matrices and of cross sections through bracket/adhesive/enamel which were partially demineralized. The etching patterns generally were variable. On the same enamel surfaces, sparsely etched areas were found near zones that showed etching types with preferential dissolution of prism cores or prism peripheries and sometimes with poorly structured morphology. The findings of varying the etching time suggested that the peripheral etching pattern is an advanced state of acid attack, after fracturing or breaking down of the marginal ridges. A 2-minute etching with gel-like 50 percent phosphoric acid (H 3 PO 4 ) was found optimum to provide an equal distribution of retentive microroughnesses on the entire conditioned enamel surface. The water-spraying time that follows the etching process should not be less than 5 seconds per tooth in order that all superficial acid residues and crystal precipitates may be surely wiped off. The resin tags generally reached a depth of 80 μm, sometimes extending to about 100 to 170 μm in length. The repair procedures on etched enamel surfaces not covered by adhesive were investigated in vitro and in vivo. The repair is effected by defect-filling organic/inorganic precipitates from saliva and by grading. (brushing habits, abrasive foodstuffs, etc.). Even after a 4-month exposure of etched enamel surfaces in the mouth environment, no restitutio ad integrum was found. The etching lesion is still visible, sometimes showing a coblike surface structure. Examination of clinically and experimentally removed brackets revealed a heterogenous site of fracture, partially running along the bracket/adhesive interface, within the adhesive, partially along the adhesive/enamel interface, and within the enamel. The location of the fracture site depends on the strength of micromechanical retention produced by the acid pretreatment. Localized terraced or ribbed enamel tear-outs reached to a depth of 100 μm. The clinical consequences of the morphologic findings are discussed.