The use of 50 per cent phosphoric acid as an etching agent in orthodontics: a rational approach.

The chemical treatment of enamel surfaces is an approach used in obtaining increased adhesion of dental materials to tooth surfaces. One of the conditioning solutions extensively used prior to the direct bonding of orthodontic attachments is a 50 per cent phosphoric acid solution. The object of this investigation was to study the effect of various concentrations of phosphoric acid on the bond strength of an epoxy adhesive formulation developed for the direct bonding of orthodontic attachments. Furthermore, an attempt was made to quantitate the etching action of the phosphoric acid solutions by determining the depth of etch on polished enamel surfaces and recording the surface profiles of etched enamel surfaces by means of a surface-roughness measuring instrument. The use of 50 per cent phosphoric acid solution as a conditioning agent prior to the use of the epoxy adhesive formulation is advocated. The rationale supporting this recommendation includes (1) the bond strength of the epoxy adhesive system to enamel surfaces etched with 50 per cent H3PO4 is not significantly improved by the use of phosphoric acid at any other concentration; (2) the depth of etch obtained with 50 per cent H3PO4 is less than that observed with H3PO4 solution at concentrations which produce comparable bond strengths; (3) the surface roughness of enamel surfaces etched with 50 per cent H3PO4 is greater than that obtained with more dilute acid solutions. The optimal phosphoric acid concentration should be determined for each adhesive system.