## 2 Materials and Methods  
### Description of the Clinical Process to Remove White Spots Located in the Enamel  
To begin, a clinical protocol was followed to treat white spots using one of two commercially available HCl-based products that involved either a chemical erosion technique or an abrasive erosion technique.  
Secondly, an intraoral scanner was used to generate Standard Tessellation Language (STL) files using specialized software to quantify the enamel loss produced with each technique. Then, the following steps were followed in sequence:1. Isolation of the operative field using anchorage clamps in the posterior regions and ligatures in the anterior regions to ensure unvarying isolation during treatment.
2. Initial scan of affected teeth from which an STL file was generated. Scans were taken before and after different treatment applications to evaluate the quantity of tissue eliminated.
3. Application of the HCl-based product to white spots: 1. In cases where the 15% HCl product was applied (ICON etch; DMG), each affected tooth received three applications of chemical erosion treatment for two minutes each. The product was placed with the tip of a sponge applicator on the enamel surface for 2 min. When 2 min had passed, the enamel was washed with abundant water for 30 s (Figures 1, 3 and 5). 3.2. In this study, the abrasive erosion treatment (Opalustre; Ultradent) contained a 6.6% HCl slurry with silicon carbide microparticles; this combination offered a chemical stain removal along with gentle mechanical abrasion. In cases using 6.6% HCl, teeth with white spots were treated with various applications according to the manufacturer's instructions. The product was spread on the enamel surface in a 1 mm thick uniform layer. Afterwards, it was rubbed homogeneously for 1 min using Opal Cups (Utradent) provided by the manufacturer. The enamel was then washed in abundant water for 30 s (Figures 2, 4 and 6).
4. Affected teeth were then scanned with an intraoral scanner (TrueDefinition, 3M-ESPE) and the data were used to generate STL files.
5. Volumetric change was analyzed by means of superimposing the STL files obtained from the scans. The STL files were imported to use the reverse engineering software Geomagic Wrap (3D system), which aligns the data in each file using the best fit algorithm. To do this, the initial file reference mesh was selected, and subsequent meshes were aligned over it as independent floating meshes. For the alignment process, the vestibular area of the teeth presenting variations from one scan to another was omitted due to HCl application.
6. After the final alignment, each tooth was exported in a binary STL format to a program called GOM Inspect (Precise Industrial 3D Metrology). This program generates a gradient color map representing variations from 250 \(\upmu\)m to \(-\)250 \(\upmu\)m so that any point exhibiting a difference between the initial mesh and the succession of the HCl application is represented by a specific color.
7. After quantifying the volume of eliminated enamel, the data were processed to calculate the depth affected by the treatment within the enamel layer. In this way, the denti was able to know whether to continue applying HCl, to terminate treatment, or to carry out more invasive treatment.  
Figure 1: The initial situation of a white spot from a traumatic etiology.  
Figure 4: The treatment of the same white spot with 6.6% HCI (Opalustre).  
Figure 3: The treatment of the same white spot with 15% hydrochloric acid (HCl) (ICON etch).  
Figure 2: The initial situation of a molar incisor hypomineralisation (MIH) white spot.