Dental caries inhibition by laser beam irradiation and chemical treatment

The effect of CO2 laser irradiation on the susceptibility of enamel to dissolution via acid challenge was investigated. A model based on a change in solubility was proposed to explain the observed dissolution rate reduction behavior after human enamel was laser irradiated. This model is consistent with the two-site model which has been proposed to explain the dissolution behavior of precipitated/digested synthetic hydroxyapatite. Laser irradiation reduces the effective driving force for dissolution of HAP from an ion activity product, based on (Ca)10]PO4)6(OH)2, of 10[-120] to an ion activity product of 10[-130]. Model calculations show that this decrease in driving force will lower the threshold pH for HAP dissolution by about 0.7 pH units. Temperature profiles during laser irradiation were studied to determine the optimal lasing parameters. Standard heat conduction equations have been modified to account for absorption of energy according to Beer's law and solved using Laplace transforms. The calculated solutions correlate well with surface temperatures measured by infrared pyrometer. The model allows arbitrary combinations of wavelength, pulse pattern, and energy density to be explored. The temperature profile was also correlated well with the laser induced enamel acid resistance assessed by X-ray microradiography. The additive effect of laser irradiation and chemical inhibitors on the dissolution rate reduction of dental enamel has been demonstrated. The effects of ethane-1-hydroxy-1,1-diphosphonic acid (EHDP), fluoride (F), and dodecylamine HCl (DAC) at various levels upon both the initial dissolution rate (IDR) and mineral density profiles were determined. After laser treatment at higher energies (130 J/cmsp2), the effects of DAC and/or EHDP are even more dramatic. No dissolution is observed via quantitative microradiography even after 24 hr of exposure to a sink buffer solution. Such a solution removes about 150 microns of enamel from unlased control samples in the absence of these agents.