## 4 Discussion
However, the abundance of free radicals increases the likelihood of their mutual termination, thereby reducing the activation of additional monomers and ultimately lowering overall polymerization efficiency [20; 21]. The conventional and rapid curing protocols resulted in lower short-term (1.3-5.9%, Figure 2) and 24 h DC values (1.7-4.7%, Figure 4) in the MBG compared to the CG. Other studies also confirmed that lower DC values obtained by curing in the presence of the bracket [23; 31; 42]. Indirect curing underneath a metal bracket increases the complexity of the polymerization conditions, as the presence of the bracket attenuates the curing light and lowers the total energy delivered for polymerization [9; 23; 39], resulting in lower DC values regardless of curing mode. Under clinical conditions, it is common to cure the orthodontic adhesive when bonding the metal bracket from two directions, occlusally/gingivally or mesially/ distally, under a 45\({}^{\circ}\) angle [23; 39]. In this study, for rapid curing, samples in the MBG were cured for 6 s, i.e., 3 s mesial and 3 s distal to the bracket. Additionally, some studies [5; 9; 10] reported that efficient bracket bonding could not be guaranteed unless the exposure time was more than 4 s in total, irrespective of the power density. In continuation, the factor \"Bracket\" had a more pronounced impact on short-term DC and the least impact on 24 h DC. This could also be explained by a lower energy received at the beginning, which resulted in fewer free radicals and activated monomers, leading to a slower rise of viscosity of the reaction medium and leaving more reactants available for post-cure DC development [31], which in turn compensated for the presence of the bracket at the beginning of the reaction. The 24 h DC values were higher by 18.4-21.4% (Figure 3) than the corresponding short-term DC values, suggesting post-cure DC development. Post-cure DC development is typical for methacrylate resin-based dental composites [14; 43]. Free radicals generated during curing remain active, allowing continued addition polymerization of monomers. Immediately after curing, the polymer network is macroscopically rigid, yet the mobility of unreacted monomers and free radicals allows for continuation of the reaction, leading to greater post-cure DC values [37]. Post-cure development could also explain the observed lesser difference in DC values due to rapid curing for the 24 h DC measurements compared to the short-term DC measurements in both groups (0.5-4.7%, Figure 5). Additionally, the samples for the 24 h DC measurements were stored in a laboratory incubator at 37 \({}^{\circ}\)C to mimic intraoral conditions. The exposure to simulated body temperature promotes the mobility of free radicals and monomers, sustaining the continuation of the polymerization reaction \"in the dark\" [44]. The viscosity of the material can also play a role in the final DC value attained by a specific material formulation [19]. Following prior literature, exposure reciprocity seems to be more valid for highly filled materials [22] like Enlight and Transbond XT. The lower sen sitivity of these more highly filled materials to negative effects of bimolecular termination could be less significant, allowing them to reach DC values that are closer to those attained with conventional light curing [45]. While a high filler volume fraction may impede light transmission [16; 45], small, well-distributed filler particles can enhance light scattering, improving light penetration and supporting ongoing polymerization [45; 46; 47]. Since light scattering increases exponentially with layer thickness [48; 49], its effect is limited in the thin layers typically used for orthodontic adhesives systems. That may explain why Enlight and Transbond XT, compared to Heliosit, showed much higher 24 h DC values than at the end of the short-term measurements in both groups (Figures 2 and 4). These findings are consistent with a previous study where the same observation was noted [31].