**1. Introduction**

Beyond just straightening teeth and achieving better esthetics, proper orthodontic intervention can result in greatly improved oral and psychological health [1,2]. A major undesirable outcome during orthodontic treatment is the development of white spot lesions (WSLs) around orthodontic brackets, especially near the gingival margin [3–6]. The boosted accumulation of biofilms around the brackets further lowers pH around these sites. Despite advancements in patient education, WSLs can still develop rapidly and could compromise the successful outcome of the treatment, even resulting in premature termination of treatment in severe cases [7–9]. It has been reported that between roughly 26% and 70% of patients using fixed orthodontic appliances exhibited various levels of WSLs during the orthodontic treatment [4,6,7]. In another study, the prevalence of WSLs was reported to be 38% in the 6-month group, whereas it was 46% in the 12-month group [9].

Due to the more retentive, complicated surfaces around brackets and higher occurrences for plaque or biofilm adherence, it has become quite challenging to maintain good oral hygiene, especially for less compliant patient groups [8,10–12]. It has been reported that fixed orthodontic appliances can affect the self-cleansing capabilities of the intraoral system, owing to the interactions of saliva and teeth surfaces. Even for patients with clear aligners, wearing aligners 20~22 h daily could limit the natural cleansing and neutralizing e ffects of saliva [13]. The fixed orthodontic appliances can even alter the oral microflora and increase the levels of acidogenic plaque bacteria, i.e., *Streptococcus mutans* (*S. mutans*), and *lactobacilli* in saliva and dental biofilm during active wearing of such appliances throughout lengthy orthodontic treatment procedures, with an average duration around two to three years [14–19] .

The interface between tooth and orthodontic bonding adhesive or cement, many of them based on (meth)acrylate chemistry, could also become the victim of bacterial attack and subsequent degradation [20,21]. Finer et al.'s findings revealed increased degradation of resin-based composite and bonding agen<sup>t</sup> by *S. mutans* UA159 vs. control. Esterase activities at levels that could degrade adhesives and resin composite have also been linked to *S. mutans* [20]. While bonding the irregularly shaped orthodontic appliances onto the tooth surface, the residual excess of orthodontic cements can also stimulate the extra accumulation of dental biofilm around brackets. The biofilm degradation of this important tooth–cement–bracket interface may contribute to the premature debonding of the brackets, especially given the compounding interactions with the microorganisms that could lead to biocorrosion on the surfaces of metal-based brackets and appliances [22].

Given the extensive prevalence of WSLs occurring during orthodontic treatments, various strategies have been utilized to prevent or mediate demineralization and occurrence of WSL formation. The use of fluoride via various forms, i.e., mouth rinse, gel, topical varnish, toothpaste, etc., has been the predominant remedy applied and studied [7,14,23,24]. Fluoride ions have been well documented to enhance remineralization of enamel and mitigate mineral loss during acid dissolution, by replacing the hydroxyl groups in hydroxyapatite and forming more acid-resistant fluorapatite [25]. The findings consolidated by Bergstrand and Twetman concluded that the use of topical fluoride varnish along with fluoride toothpaste has been the best evidence-based approach to prevent WSLs. The mean prevented portion based on six clinical trials was 42.5% with a range from 4% to 73% [23]. These studies provided convincing support for routine professional applications of fluoride varnish around the bracket base during orthodontic treatment [24–28].

The application of quaternary ammonium compound (QAC) has been actively investigated to introduce antibacterial activity into the orthodontic bonding system. A unique type of antimicrobial polymer is demonstrated by incorporating quaternary ammonium polyethyleneimine (QPEI) nanoparticles. QPEI nanoparticle-containing composites have been reported to exhibit antibacterial activity against salivary bacteria and in vivo antibiofilm activity [29]. Experimental orthodontic bonding cements with 1% incorporated insoluble QPEI and polycationic polyethyleneimine nanoparticles were reported to exhibit stable, long-lasting antibacterial properties against *S. mutans* [30,31]. A variety of QAC-containing functional monomers have been systematically studied, including the widely studied methacryloyloxydodecylpyridinium bromide (MDPB) [32,33] ; dimethylaminohexadecyl methacrylate (DMAHDM), a mono-methacrylate QAC with an alkyl chain length of 16 [34,35]; dimethylaminododedecyl methacrylate (DMADDM), a mono-methacrylate QAC with an alkyl chain length of 12 [36–38]; and 2-methacryloxylethyl hexadecyl methyl ammonium bromide (MAE-HB), a crosslinkable, di-methacrylate QAC with an alkyl chain length of 16 [39,40]. Significant antibacterial activities and biofilm reductions have been demonstrated.

Recently, a series of polymerizable imidazolium-based antibacterial resins (ABRs) were successfully designed in our laboratory [41,42]. To the best of our knowledge, no such novel crosslinkable imidazolium-based monomers have been investigated for their application towards an orthodontic bonding system with the potential to prevent and mitigate WSLs. The aims of this study were to investigate imidazolium-based ABR's antibacterial activities and impacts on the adhesive and mechanical properties of experimental orthodontic bonding cement formulations.
