The effect of citric acid concentration and etching time on the roughness of the enamel surface of a prepared human tooth: in vitro study
Authors: Maksyukov Stanislav , Maksyukova Ekaterina Stanislavovna , Risovannaya Olga Nikolaevna 
Published: January 2025

BACKGROUND. Teeth are exposed to various acids during daily functioning. A very common source of exposure is citric acid, associated with the consumption of fruits or juices. Despite the exceptional strength properties of human enamel covering the underlying dentin in the area of the tooth crown, acidic foods and beverages contribute to its dissolution. This can lead to significant loss of the tooth surface. On the other hand, acid-localized etching is widely used in clinical practice to increase the adhesion of restoration materials to the tooth surface. Etching provides partial dissolution and demineralization of the inorganic matrix of the enamel surface, forming microporeities and micropases. This process leads to an increase in the roughness of the enamel surface. Pre-etching of enamel can improve the adhesion strength of resin-based bonding systems with enamel compared to using the same systems without pre-etching. Various acids can be used for these purposes. Orthodontists also use acid to form retaining surfaces, which, on the one hand, improves the fixation of the bracket, but, on the other hand, weakens the structure of adjacent enamel prisms, increases the permeability and adhesion of biofilm to enamel and reduces its protective properties.
AIM: to establish the informativeness of the use of acid etching, which allows to visualize the structural elements of tooth enamel for further study.
MATERIALS AND METHODS: Three samples of human enamel were obtained from the maxillary molar, which was removed from the patient (male, 21 years old). The tooth was filled with epoxy resin for ease of cutting. The cutting parameters on the Isomet 4000 machine (Buehler, Germany) were as follows: the rotation speed of the disc is 2500 rpm, the sample feed rate is 10 mm/ min. As a result, three areas were selected as samples for research. Each sample was again filled with epoxy resin to allow subsequent grinding and polishing, which was carried out simultaneously for all three samples on the MetaServ 250 machine (Buehler, PRC). After sample preparation, the tooth surface was examined using an optical microscope Stemi 305 Greenough (Zeiss, China) equipped with a color video camera Axiocam 105 (Zeiss, Germany) in reflected light. Sample 1 was etched with acid with a concentration of 0.5 wt. % (pH = 2.3), sample 2 with a concentration of 3 wt. % (pH = 1.86), sample 3 with a concentration of 5 wt. % (pH = 1.71). pH measurements were performed (Cobra4 Mobile-Link, PHYWE, Germany). Atomic force microscopy of the enamel surface topography was performed for each sample before etching and after etching on an AFM Nanoeducator (NT-MDT, Russia) equipped with a tungsten probe. The device was working in contactless mode. Surface roughness Ra was measured at each etching step for each sample using Gwyddion software (Czech Metrological Institute, Czech Republic).
RESULTS: The results of optical microscopy of enamel, dentin and dentinoemal boundary are presented, images that indicate the healthy state of the studied samples were clearly visible. AFM examination shows the surface of each of the three samples before and after etching at different time intervals. After the study, it was concluded that citric acid of the specified concentration is more suitable for controlled etching than 37% phosphoric acid. It is also known that with prolonged exposure to phosphoric acid, the enamel can be seriously damaged.
CONCLUSION: For the purpose of obtaining a significant amount of surface roughness and visualization of an enamel prism in a reasonable time, etching of 5 wt can be recommended.% citric acid. After an etching time of 45 s, a linear increase in surface roughness was observed on all samples. For further clinical implementation of the obtained results, additional studies of the adhesive properties of enamel after pickling with citric acid are necessary.
KEY WORDS: citric acid; etching; tooth enamel.