# The Influence of Different Cleaning Protocols on the Surface Roughness of Orthodontic Retainers
## 4 Discussion
This was expected, since they increase surface porosities [49] and, according to the literature, contain titanium oxide and hydrated silica, which are known to have abrasive properties [50; 51]. The value that explains a toothpaste's abrasivity is most commonly defined as relative dentin abrasivity (RDA). It is a value determined in vitro by comparing a toothpaste slurry's ability to remove radioactive denting during a brushing protocol with a standard abrasive or toothpaste formulation. According to the International Standards Organization (ISO), the abrasivity of a test formulation for denting should not be more than 2.5 times that of the reference abrasive, implying that the RDA should be less than 250 [52; 53]. The RDA values of whitening toothpastes on the market typically range between 98 and 120,with activated charcoal toothpaste having the lowest (RDA 50) and Colgate Max Fresh having the highest (RDA 175) [54; 55; 56; 51]. In the present study, Aquafresh toothpaste with an RDA of 113 was used. The final roughness of all investigated specimens on the top side was below 0.2 \(\upmu\)m, which is in concordance with the [PERSON] et al. study [43]. This is clinically significant because of potential bacterial adhesion [57; 58]. Moreover, the smooth surface of the vestibular part of the retainer is not only important in aspects of microbiology, but also for the patient's comfort. According to the literature, roughness values greater than 0.5 \(\upmu\)m can be felt by the patient's tongue [59; 60]. Furthermore, increased surface roughness can accelerate potential staining due to pigment/ dye accumulation in the porosities of the material [61]. Corega tablets and Corega in combination with brushing induced statistically significant increased surface roughness on the bottom side of Material B. Roughness was also increased on the bottom side of Material A; however, this difference was not statistically significant. The bottom sides of specimens initially showed greater roughness values than the top sides. This can be explained by the contact of the bottom side of the samples with the rough surface of dental casts during thermoforming. Thus, the irregular and bumpy surface of the plaster caused porosities on the bottom side of our prepared samples. These porosities, i.e., increased surface roughness on the bottom side of specimens, can increase the probability of the attachment of oxygen bubbles released from the Corega tablets. This is a result of the higher probability of the presence of micro and nanobubbles at the sample's surface [62]. This could be an explanation for the increased effect of Corega tablets on the bottom sides. This aforementioned phenomenon could have an influence on the retention and stability of orthodontic retainers because the investigated bottom sides of the specimens represent the inside of a retainer in clinical conditions. Since there was no statistical difference between the final values of the measured parameters, we can conclude that the two different brands acted similarly when exposed to various cleaning procedures. There was only a statistically significant difference in the upper side (Top) in the Corega + toothbrush group between Materials A and B, which is possibly due to different manufacturing processes between the two brands. Moreover, we need to take into consideration that the thermoforming process itself can affect the material surface. One study showed that the thermoplastic surface becomes statistically significantly rougher and more irregular after this process [63]. Limitations of this study include the shape of the polyethylene specimens, which were flat and did not completely reflect the shape of a real retainer. Additionally, in vivo environmental differences in abrasive wear may occur when a retainer is opposed by natural dentition versus the simultaneous wearing of two retainers [64]. A retainer's resistance to wear is also influenced by thermoforming and oral exposure [4]. The present report evaluated roughness.