Physical and chemical characterization of the surface layers formed on dentin following treatment with a fluoridated...

To characterize in vitro the formation and robustness of a layer formed on dentin following treatment with a fluoridated toothpaste containing calcium sodium phosphosilicate (NovaMin) using modem imaging and analysis techniques.

Calcium sodium phosphosilicate (CSPS)-containing toothpaste was brushed on to etched dentin specimens twice daily for up to five days. In between applications the samples were stored in artificial saliva. Additionally, certain samples underwent a chemical challenge in the form of a dietary acid, whereby samples were exposed to a cola or grapefruit juice beverage for five minutes on day 4 of the five-day study. The ability of the CSPS-containing formulation to occlude tubules was assessed visually by scanning electron microscope (SEM) imaging and compared to a water control. In a second experiment, the mechanical resistance of the layer was assessed using profilometry after controlled brushing for 200 brush strokes with a wet medium-bristled toothbrush. To visualize the layer and characterize the tubule occlusion, longitudinal cross-sections were prepared using a focused ion beam scanning electron microscope (FIB SEM), and analysis performed by energy dispersive x-ray spectroscopy (EDS) and electron diffraction. Owing to the complexity of the mixed material deposited after application of the toothpaste, material from inside a dentin tubule was selectively removed after five days of treatment, and the morphologically different materials imaged and analyzed by electron diffraction in the transmission electron microscope (TEM).

SEM inspection showed significant coverage of the dentin samples after application of CSPS toothpaste for all five days, in contrast to the water control where the majority of tubules remained open after all five days. Exposure of the NovaMin-treated samples to common dietary acids did not lead to re-exposure of the tubules. Profilometry measurements demonstrated an intact layer covering the dentin surface after one and five days. EDS analysis and electron diffraction indicated the layer and the material plugging the tubule to be a calcium phosphate material with a crystallographic structure similar to hydroxyapatite.

CSPS contained in toothpaste formulations adhered to exposed dentin surfaces. The layer formed was resistant to acid and mechanical challenges. Characterization of this layer indicated it was hydroxyapatite-like in nature.