Effect of citric acid concentration and etching duration on the surface roughness of prepared human tooth enamel: an in vitro study

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Abstract
BACKGROUND: Citric acid, commonly encountered through the consumption of fruits and juices, is a frequent factor affecting teeth in daily life. It contributes to enamel dissolution, exposing the underlying dentin in the crown region and potentially leading to significant tooth surface loss. On the other hand, acid etching is widely used in restorative dentistry to enhance the adhesion of restorative materials to tooth surfaces. This process partially dissolves and demineralizes the enamel’s inorganic matrix, creating micropores and microgrooves that increase surface roughness. Pre-etching of enamel improves the bond strength of resin-based adhesives compared to non-etched enamel. Orthodontists also employ acid etching to create retention surfaces, which enhances bracket adhesion but simultaneously weakens adjacent enamel prisms, increases permeability, promotes biofilm adhesion, and reduces enamel’s protective properties. AIM: To evaluate the effect of citric acid exposure time and etching duration on the surface properties of dental hard tissues, including enamel and dentin. MATERIALS AND METHODS: Three enamel samples were obtained from a maxillary molar extracted from a 21-year-old male patient. The tooth was embedded in epoxy resin for easier sectioning. Sectioning parameters on the IsoMet 4000 (Buehler, Switzerland) were set to a disk rotation speed of 2500 rpm and a feed rate of 10 mm/min. Three distinct areas were selected for analysis. Each sample was embedded in epoxy resin again for subsequent grinding and polishing, which was performed simultaneously for all three samples using the MetaServ 250 (Buehler, Switzerland). The prepared enamel surfaces were examined using an optical microscope (Stemi 305; Carl Zeiss, Germany) equipped with a color video camera (Axiocam 105; Carl Zeiss, Germany) under reflected light. The samples were etched as follows: Sample 1 with 0.5 wt.% citric acid (pH = 2.3), Sample 2 with 3 wt.% citric acid (pH = 1.86), and Sample 3 with 5 wt.% citric acid (pH = 1.71). The pH was measured using Cobra4 Mobile-Link (PHYWE, Germany). Atomic force microscopy of enamel surface topography before and after etching was performed using the NANOEDUCATOR microscope (NT-MDT, Russia) with a tungsten probe operating in non-contact mode. Surface roughness (Ra) was measured at each etching stage for each sample using the Gwyddion software (Czech Metrology Institute, Czech Republic). RESULTS: Optical microscopy provided images of enamel, dentin, and the dentinoenamel junction, confirming the healthy condition of the examined samples. The findings suggest that 5 wt.% citric acid is more suitable for controlled etching than 37% phosphoric acid, as prolonged exposure to phosphoric acid can cause severe enamel damage. AFM analysis demonstrated the surface characteristics of each sample before and after etching at different time intervals. A linear increase in surface roughness was observed in all samples after 45 s of etching. CONCLUSION: For achieving significant surface roughness and visualizing enamel prisms within a reasonable timeframe, etching with 5 wt.% citric acid is recommended. However, further studies on the adhesive properties of enamel post-etching with citric acid are necessary for clinical implementation.

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