## 1 Introduction  
Despite its well-recognized benefits, orthodontic treatment remains associated with adverse effects, including the associated enamel surface and structural detrimental changes such as decalcification, cracks, and abrasion [1]. Moreover, orthodontic treatment is considered as a risk factor for tooth discorrelation, ranging from imperceivable changes and loss of translucency, to white spot lesions, to brown color changes depicting overt carious lesions [2]. Studies have shown that different etiologic factors could directly or indirectly affect the color changes of enamel related to orthodontic treatment, including the type of adhesive system used in the bonding of the orthodontic brackets. Etch and rinse (E&R) and self-etch (SE) adhesive systems are used for the bonding procedures, each with different penetration lengths of the formed resin tags within the enamel. Since these changes occur in the enamel subsurface, they cannot be reversed after debonding of the brackets with the finishing procedures; therefore, adverse effects could be long-lasting [1, 3, 4, 5].  
The type of orthodontic bracket, metal, or ceramic is another variable that might affect enamel color changes related to orthodontic treatment [6]. The orthodontic bracket material has been reported to affect the capacity of microorganisms to adhere to and grow on the bonded tooth surface. In an in vitro study, [PERSON] et al. reported that enamel demineralization was higher in teeth bonded with ceramic brackets compared with those bonded with metal brackets [6]. The increased demineralization may be because of a greater bacterial accumulation around ceramic brackets [7]. However, adherence of lipopolysaccharideacharides (LPS) from Gram-negative bacteria has been shown to be higher to steel brackets than to porcelain, plastic, and gold brackets [8].  
The finishing and polishing method after bracket debonding could also affect the final tooth color change. Following the debonding of the brackets, composite resin remnants cannot be completely removed without causing structural defects to enamel. A systemic review has reported that tungsten carbide burs damage enamel surfaces less than Arkansas or green stones, diamond burs, steel burs, or lasers [9]. Post-finishing polishing has been recommended to minimize the damage and the subsequent color changes of enamel [10]. Factors, related to patient habits, such as the use of mouthwash or bleaching agents, or the consumption of chromogenic food during or after orthodontic treatment, may also affect tooth color changes related to orthodontic treatment [5, 11, 12, 13].  
Of the factors under the orthodonticist's control, controversy still exists regarding which type of orthodontic brackets and adhesive systems would cause the least amount of tooth surface discoloration. Therefore, the aim of this in vitro study was to assess the effect of the type of orthodontic bracket and adhesive system on tooth surface discoloration. The null hypotheses were that, regarding tooth surface discoloration, no difference would be found between the metal or the ceramic orthodontic brackets and that no difference would be found between the three-step and two-step adhesive systems.