Effectiveness of etching with phosphoric acid when associated with rubbing technique

Introduction
Some dental adhesive systems require an etching step using phosphoric acid in order to prepare dentin and enamel surfaces. This treatment modifies the surface characteristics and the chemical compositions of these hard tissues. Objectives: The aim of this in vitro study was to analyze chemical and microscopic effects of acid etching with rubbing technique on dental tissues.

Material and methods
Sixteen extracted teeth were obtained. Two etching techniques (with and without rubbing) were used to treat the enamel and dentin surfaces. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to evaluate morphological and chemical changes of the enamel and dentin surfaces. Collected data were statistically analyzed by c2 and Mann-Whitney rank sum tests.

Results
The enamel that was etched with rubbing technique for 30 s showed a great dissolution at inter-prismatic enamel level. The dentin, which was etched with rubbing technique for 15 s, revealed a larger tubules diameter and more open tubules than without rubbing technique. There were more silica particles on dentin etched by rubbing than without rubbing technique. Chemical analyses (EDX) showed that a dental surface etched using 37% phosphoric acid with rubbing technique revealed a surface rich in calcium, with a Ca/P ratio = 1.66 ± 0.05 near to Ca/P ratio = 1.67 of hydroxyapatite.

Conclusions
Although phosphoric acid cleans the dental surface, it causes salts precipitation on the etched dental surface; these salts can be removed more efficiently by rubbing the acid and rinsing it, providing an optimal interface for the bonding.