Changes in Human Lased Enamel after Immersion in Human Saliva

Abstract

Changes in lased human dental enamel after immersion in human resting whole saliva were investigated by the determination of Vickers hardness number (VHN) and amount of calcium released into acid solution, and also by polarized light microscopy and scanning electron microscopy (SEM). Lased enamel had a lower VHN than unlased enamel, but it rehardened significantly after immersion in saliva for 5 days. The VHN of lased enamel became larger as the period of immersion in saliva increased.
A high positive birefringence of lased enamel was reversed during the immersion in saliva. After immersion for 10 days, the lased area showed a low negative birefringence which was similar to that of the unlased enamel. The rehardening and the change in birefringence of lased enamel were attributed to deposition of salivary ions in microspaces created in dental enamel by laser irradiation.
The amount of calcium dissolved from enamel which was lased and subsequently immersed in saliva for 10 or 20 days was higher than that from enamel which was lased only. However, the lased enamel still retained significantly higher acid resistance than unlased enamel even after immersion in saliva for 20 days. No remarkable change of the surface of lased enamel was observed by SEM after immersion in saliva.
Those results suggested that lased enamel gets a structural stability by rehardening and it retains high resistance to an acid attack in the oral cavity.