Investigation of Acid-Etched CO2 Laser-Ablated Enamel Surfaces Using Polarization-Sensitive OCT

Objectives: A CO₂-laser operating at the highly absorbed wavelength of 9.3µm with a pulse duration of 10-15µs is ideally suited for caries removal and caries prevention. The enamel thermally modified by the laser has enhanced resistance to acid-dissolution. This is an advantage for caries prevention; however it is often necessary to etch the enamel surface to increase adhesion to composite restorative materials and such surfaces may be more resistant to etching. The purpose of the study was to non-destructively measure the susceptibility of laser-ablated enamel surfaces to acid dissolution before and after acid-etching (37.5% H₃PO₄).

Methods: Thirty-two bovine enamel blocks were prepared and placed on delrin blocks. One set of blocks (n=15) was divided into five windows and two of those windows were ablated by the CO₂-laser. One of those two windows was etched (37.5% H₃PO₄). One of the remaining windows was covered with acid resistance varnish, another window was acid-etched and the last window was untreated. The samples were subsequently exposed to an acid-dissolution model. The remaining blocks (n=17) were divided into eight windows that were laser-irradiated at a fluence of 20J/cm² and acid-etched for various time periods before exposure to the acid-dissolution model. Automated methods of analysis were employed to calculate the depth and severity of the subsurface lesions after scanning with Polarization-Sensitive OCT. Sample groups were statistically analyzed (ANOVA, Tukey’s, p<0.05).

Results: PS-OCT imaging indicated that acid-etching significantly increased the dissolution rate of laser-irradiated and non-irradiated enamel surfaces compared to non-etched group (p<0.05). The outer layer of enamel modified by the laser was rapidly etched away after 5-10 seconds. 

Conclusion: This study suggests that the thermally modified enamel layer is rapidly removed by phosphoric acid etching after 5-10s and such etching also significantly accelerates the formation of subsurface lesions on both laser-ablated and non-laser irradiated surfaces.