## 4 Discussion  
During tooth polishing, the impact of various rubber cup rotational speeds on the depth of damage to the sound tooth surface was evaluated. Specifically, we compared the extent of damage to sound teeth, teeth with initial caries, and cracked teeth resulting from high-speed rotation of the rubber cup. Our study used three different rubber cup rotational speeds that hold particular clinical relevance. A speed of 100 rpm represented the lowest speed that can minimize damage to the tooth surface while achieving effective polishing. The 3000 rpm speed, recommended in dental hygiene practice books, served as a standard, guideline-based speed for comparison. Lastly, the 10,000 rpm speed, commonly used in clinical settings, represented the higher end of the spectrum, allowing for the evaluation of more aggressive polishing approaches. According to our findings, polishing sound teeth at 10,000 rpm resulted in significantly deeper damage (3.56 times and 2.65 times) compared with polishing at 100 rpm and 3000 rpm, respectively (Figure 4). While most previous studies recommended maintaining the lowest possible speed to minimize frictional heat [28; 29], it is difficult to uniformly maintain the lowest speed in the dental clinical setting where the speed of the rotating rubber cup cannot be visually confirmed. Particularly, the speed considered the lowest by clinicians can vary significantly among individuals. Polishing at 10,000 rpm can wear away sound teeth up to a depth of about 100 \(\upmu\)m, severely removing the outermost enamel layer (Figures 4 and 7), which contains the highest fluoride content [30]. Therefore, polishing can pose a significant risk of exposure to dental caries or erosion [31]. In addition, polishing at 3000 rpm reduced sound tooth surfaces by approximately 37.45 \(\upmu\)m, with the depth of damage increasing by 1.45-fold and 1.75-fold in teeth with early caries or cracks, respectively, compared with that in sound teeth (Figure 5). Moreover, polishing at 10,000 rpm reduced sound tooth surfaces by 99.36 \(\upmu\)m, with the depth of damage in cracked teeth increasing by 1.60-fold compared with that in sound teeth (Figure 6), with an average damage depth of \(>\)100 \(\upmu\)m on surfaces with early caries and cracks. Scanning electron microscope observations revealed that the outer surfaces of the early carious teeth were worn such that the enamel rod pattern was visible (Figure 7). Early carious lesions are characterized by demineralization occurring beneath the surface of the tooth, leading to increased porosity and optical changes [32; 33]. This indicates that the enamel at the surface remains intact, and demineralization occurs below the surface layer. Initial stages of carious lesions can undergo remineralization and halt lesion progression, restoring tooth structure, if they have not progressed to irreversible caries [33; 34; 35]. However, using excessively high speeds when polishing tooth surfaces with early carious lesions can lead to the loss of the outermost layer, resulting in the loss of enamel structure that could have been recovered through remineralization [36]. The loss of the outermost enamel layer that can be remineralized can accelerate the progression to a state of dental caries requiring restorative treatment [37]. Additionally, cracked teeth exhibited deep destruction patterns along the crack lines (Figure 7). Microcracks, which are incomplete fractures that extend into the dentin or pulp, can cause sharp pain during mastication and sensitivity to cold and sometimes hot foods [38; 39; 40; 41]. Performing rubber cup polishing on cracked teeth can cause severe damage and accelerate the progression to a complete fracture. Thus, polishing at excessively high speeds on surfaces with hard-to-detect enamel damage, such as early carious lesions and cracks, can cause more significant damage than that on sound tooth surfaces.