## 4 Discussion  
The techniques for eliminating white spots by means of HCl-based products described above may represent an alternative or coadjuvant treatment to the more commonly used techniques such as remineralization or restoration with resin composite. Hydrochloric acid at different concentrations has been used in esthetic dentistry for eliminating enamel marks since 1916, when [PERSON] used 36% HCl and heat to remove fluorosis stains. Due to the high concentration, the technique produced satisfactory results, but given the risk of destroying  
Figure 16: A digital volumetric analysis in a sagittal plane. (**a**) The volumetric change of the dental vestibular area after the application of 6.6% HCl. (**b**) The volume reduction in the vestibular area with the largest contour of the treated tooth. Figure 15: A digital volumetric analysis in a sagittal plane. (**a**) The volumetric change in the dental vestibular area after applying 15% HCl. (**b**) The volume reduction in the vestibular area with the greater convexity of the treated tooth. the enamel, reports of the treatment were not published. In 1941, [PERSON] was the first to describe the technique of fluorosis stain removal, who did so using HCl at a lower concentration than used by [PERSON] to ensure safety; 18% HCl was applied with a cotton wool swab, was left on the enamel for 10 min, and was followed by an application of sodium bicarbonate to neutralize the effect of the acid [37]. In 1989, a micro-abrasion technique was described by [PERSON] that involved using a solution that combined chemical erosion with 18% HCl and mechanical abrasion using a pumice stone, thus eliminating the surface layer of the enamel and the white spots within it [38]. This practice is still considered a safe, effective, and atraumatic technique for eliminating enamel defects derived from hypomineralization or minimal hypoplasia. An alternative to microabrasion is enamel infiltration with a low-viscosity photopolymerizable resin [30]. This involves pretreating the spot and completely eroding the enamel's impermeable surface layer to expose the lesion before resin infiltration. The surface layer can be removed mechanically with a bur or disc [39], but this generates a smear layer that obstructs the prisms making subsequent infiltration difficult. A better option is to remove the surface layer by means of acid etching, which allows controlling the reduction of the surface layer and leave the tubules open for later infiltration. The available literature on this topic includes several articles that aimed to compare the erosive capacity of various acids including orthophosphoric acid at 37% and HCl at various concentrations for eliminating the enamel surface layer. Different studies by [PERSON] et al. stated that enamel discoloration can only be eliminated by products based on 15% HCl due to its capacity to etch the tooth surface more effectively than orthophosphoric acid, although the latter is still used in regular practices [31; 40]. [PERSON] reported that the enamel surface etching capacity of HCl was 2.7 times greater than orthophosphoric acid and noted that the etching patterns also differed [41]. On the basis of the greater capacity of HCl to erode the tooth enamel surface [31; 40; 41], the present study's intentions were to conduct an in vitro comparison of two HCl-based products with concentrations of 6.6% and 15%. Our aim was to investigate the influence of the concentration and the application time of these products upon the enamel surface. Previous studies have yet to provide a clear report of the quantity of enamel eliminated by the HCl products with each application [42; 43; 44; 45; 46]. An in vitro study by [PERSON] et al. evaluated the erosive capacity of a 15% HCl solution by means of optical profilometry, reporting that treatment with 15% HCl will erode dental enamel to a greater or lesser degree depending on the application time. The present study obtained similar results, though with lower erosion values than [PERSON] et al.