## 1 Introduction  
White-colored alterations may appear as a result of pre- or post-eruptive disorder [1]. The pre-eruptive lesions of importance in this regard result from fluorosis, traumatic hypomineralization and molar-incisor hypomineralization (MIH). Post-eruptive alterations take place when the demineralization process is stronger than the remineralization process and they are usually addressed as white spot lesions (WSLs) [1]. The initial development of WSLs is present as a mineral loss in the bulk of the enamel, while the lesion surface remains mostly intact. Later, the surface layer of the lesion becomes partially porous, which can result in a loss of transparency [2]. The fixed orthodontic appliances may enhance plaque accumulation and lead to the progress of both demineralization and caries lesions, especially in patients with inadequate oral hygiene [3; 4; 5]. Further, WSLs can be seen after just four weeks of a fixed orthodontic appliance being fitted [6]. Additionally, the development of WSLs is not only related to orthodontic treatment, since such changes can also be seen before treatment has started and after it has finished.  
The overall prevalence of WSLs ranges from 2-96% depending on the assessment procedure of assessment of the lesions, whether or not any such defects were present before treatment and whether or not fluoride supplements were used during treatment [7]. Moreover, a recent study showed that WSLs were 2.5 times more common in the maxillary arch than in the mandibular arch [4].  
The altered enamel structure of teeth with initial caries lesions may reduce the shear bond strength (SBS) between orthodontic brackets and the enamel surface at the beginning of orthodontic treatment, as well as during treatment if the rebonding of the brackets is required [8; 9]. In fact, the brackets rebonding is a regular procedure during orthodontic treatment [10] with the most common reasons for this intervention being bond failures on the part of the brackets and the need to reposition the brackets [11]. If bracket bonding is to be performed on demineralized enamel, it is advisable to stabilize and protect the demineralized enamel lesion prior to the reapplication of the brackets [12]. In general, there are two main types of interventions available for the treatment of WSLs. The first type involves non-invasive methods, like the application of topical high-concentration fluorides [13; 14] or casein phosphopeptide-amorphous calcium phosphate (CPP-ACP). The second type involves more invasive techniques, such as the hydrochloric acid-pumice microabrasion technique [8] or caries infiltration with resin [15]. Previous reports, demonstrated that low-viscosity resin is significantly able to penetrate initial caries lesions, which leads to their stabilization [16; 17]. The optical appearance of WSLs was also found to be improved by resin infiltration, with the achieved concealment being shown to remain stable in one-year interval in vivo [18].  
Observing limits of this technique it can be described as sensitive [19] and this can be a problem in vivo. In addition, some cases of WSL treated with the resin infiltrant experience no camouflage effect [20]. The explanation for this outcome can be the depth and activity of the lesion, the infiltration is not complete when the lesion is deeper or inactive and has a thicker surface [20].  
The aim of the present study was to assess the SBS of orthodontic brackets bonded with different adhesives to demineralized human enamel that had been treated with a low-viscosity resin infiltrant. It also aimed to compare the achieved bond strengths to those achieved in relation to sound enamel.  
The null hypotheses were: 1) treatment with caries infiltrant on demineralized enamel does not impair the SBSs of different adhesives in comparison to the bond strengths seen in relation to sound enamel and 2) the shear bond strengths recorded in demineralized specimens treated with caries infiltrant are not significantly enhanced by substrate pretreatment with a universal adhesive.