Surface porosity of enamel white spot lesions and penetration of fluorapatite nanocrystals into their subsurface: proof-of-concept study

Abstract

Objectives: In this study, we used atomic force microscopy (AFM) to quantify the size of surface pore apertures of enamel white spot lesions and then demonstrated the penetration of fluorapatite nanocrystals (nFA) into the subsurface of these lesions.

Methods: For the porosity study, enamel lesions were created on three sound human teeth using a demineralizing gel for 8 days. The interface between sound enamel and the artificial lesion was analyzed by AFM. To visualize the penetration of nFA tagged with a calcium-binding fluorophore (Fluo-4) into the subsurface of white spot lesions, we used two-photon microscopy. Sixteen extracted human teeth with either active, natural, or in vitro-created carious lesions in enamel were randomly divided into three groups. The teeth were treated for 2 min with either a suspension of tagged nFA crystals, Fluo-4 alone, or deionized water, and left for 30 min before being washed with distilled water and examined microscopically.

Results: A greater concentration of surface pores with larger areas was observed on the in vitro demineralized enamel (29 % of pores greater than 1.0 µm²) when compared with the adjacent sound enamel (8 % of pores greater than 1.0 µm²) (p=0.012, Fisher exact test). In vitro and natural lesions treated with tagged nFA showed fluorescence at depths ranging from 50 to 170 µm, demonstrating penetration of the nFA into the lesion subsurface. The lesions treated with Fluo-4 alone with no crystals showed mostly surface fluorescence (restricted to the outer 25 µm), while those treated with deionized water showed minimal (restricted to the outer 20 µm) to no fluorescence.

Conclusion: We have demonstrated the use of AFM to quantify the surface pore apertures and two-photon microscopy to visualize nFA crystals in the subsurface of non-cavitated enamel lesions.

Clinical significance: The restoration of the subsurface of non-cavitated caries lesions is a clinical challenge. This study demonstrated that a 2 min application of nFA could penetrate through the surface apertures of non-cavitated enamel lesions into their subsurface.

Keywords: AFM; Non-cavitated lesion; Two-photon microscope; nFA.