Beyond a single scan: physicochemical modification of enamel subjected to multiple Nd:YAG laser irradiations in a preventive context

To investigate the surface modification (changes in roughness, morphology, and chemical composition) of human enamel subjected to multiple Nd:YAG laser irradiations over a photoabsorber, with and without the subsequent application of acidulated phosphate fluoride (APF), in a caries control protocol. 150 enamel blocks (4 × 4 mm) from human third molars were randomly distributed into 10 groups based on surface microhardness: control groups (CN, CF) and irradiated groups subjected to 1, 2, 3, or 4 successive scans. A pulsed Nd:YAG laser (1064 nm, 65 mJ/pulse, 10 Hz, 51.7 J/cm²) irradiated the surfaces using a motorized stage with calculated speed for partial pulse overlap. An aqueous coal powder suspension (equal parts of ultrapure water and 99% ethanol) was applied before each scan. APF 1.23% was applied post-irradiation for the fluoridated groups. The surfaces were analyzed by optical profilometry (Sa), Scanning Electron Microscopy (SEM), and Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR). Qualitative analysis of profilometry and SEM images revealed that one or two laser scans resulted in unaltered areas and spaces of non-melted enamel between pulses. Groups irradiated three or four times achieved more uniform areas of surface melting, also exhibiting significantly higher surface roughness. ATR-FTIR analysis, guided by multivariate models and semi-quantitative analysis, showed indications that laser irradiation promoted thermal decarbonation of the enamel and caused the narrowing of phosphate bands. One or two Nd:YAG laser scans were insufficient to modify the entire enamel surface. Successive irradiations with reapplication of the photoabsorber must be performed to ensure that the whole targeted area is modified, despite the localized increase in surface roughness.