Chemical and Visual Changes in Tooth Enamel Caused by Lactic Acid and Hydrofluoric Acid

¹ U. S. Naval Dental School, National Naval Medical Center, Bethesda, Maryland

Tooth slices cut from unerupted human teeth and cemented between sections of plastic were subjected to the intermittent effects of 0.007-0.009N lactic and hydrofluoric acids buffered by their sodium salts to a pH of 4.0-4.2. Reaction of the tooth surfaces to the acids was evaluated by measuring dissolved phosphorus and observing changes in appearance of the exposed enamel.

The lactic-acid buffer removed phosphorus from a normal tooth surface at a constant rate of dissolution for periods ranging up to 21 months, with the enamel becoming whitened from the point of exposure radially into the tooth slice.

The hydrofluoric-acid buffer removed phosphorus from a normal tooth surface at a relatively constant rate of dissolution for an observed period of approximately 2 months, during which time no visible change was detected in the appearance of the enamel.

Hydrofluoric acid, when applied to tooth slices whose surfaces had been whitened by a prior exposure to lactic acid, dissolved the whitened enamel completely.

Hydrofluoric acid provided a temporary protection against subsequent exposure to lactic acid when hydrofluoric-acid and lactic-acid buffers were applied alternately to the same tooth slices. This protective effect was evidenced by a decreased rate of phosphorus dissolution by the lactic acid, and by a delay in whitening of the enamel.

No direct association could be established between visual changes in enamel and the amount of phosphorus removed. Possible explanations, based on different mechanisms of action by lactic acid and hydrofluoric acid on tooth enamel, are discussed.