Marginal integrity produced by quaternary ammonium methacrylate-based dental adhesive tested under physiologically...

Endogenous metalloproteinases (MMPs) lead to dentin collagen degradation when exposed to demineralizing events, such as during the cariogenic process. Quaternary ammonium compounds are known to be antibacterial, and there is also evidence for their action as a MMP inhibitor. The aim of this study was to evaluate a quaternary ammonium-based methacrylate (dimethylaminohexadecyl methacrylate - DMAHDM, QAM) as an experimental adhesive, tested for bond stability and gap formation under physiologically-relevant conditions using a bioreactor system.

The inhibition of MMP activity was assessed using a fluorescence-based assay, in the presence of serial concentrations of chlorhexidine (CHX) or QAM (n = 3). Shear storage modulus (G') was measured on demineralized dentin slices before and after incubation into the inhibitor's solutions, using a rheometer (n = 5). Collagen solubilization was quantified through hydroxyproline assay, using powdered dentin obtained from extracted human third molars (n = 6). Adhesives were formulated with BisGMA/HEMA, added of either 2 % CHX or 10 % QAM, and the degree of conversion was evaluated by near-IR (n = 6). Microtensile bond strength (μTBS) was assessed on restored dentin beams after storage in dH₂O or bacterial challenge (n = 6). Biofilm growth and viability were tested on adhesive disks exposed to Streptococcus mutans, with biofilm growth, viability and morphology assessed through optical density, luminescence, and crystal violet staining, respectively (n = 6). Restorative procedures of simulated dental preparations involved etching, adhesive application, and composite restoration in standardized cavities, followed by biofilm challenge tests in an incubator or in a modified bioreactor system (n = 5). Epoxy replicas of the samples were analyzed using scanning electron microscopy for gap measurements and confocal laser scanning microscopy (n = 5).

Dentin disks treated with either 2 % CHX or 10 % QAM exhibited a significant increase in shear storage modulus after 72 h (p = 0.005 and p = 0.007, respectively), indicating enhanced mechanical stability. The hydroxyproline assay demonstrated that both CHX and QAM effectively reduced collagen degradation, with QAM showing superior inhibition (p = 0.02). In terms of the degree of conversion, the QAM adhesive achieved significantly higher values compared to the control and CHX groups (p = <.001). Biological assays showed that QAM markedly inhibited planktonic bacterial growth (p = <.001), biofilm biomass (p = <.001), and biofilm viability (p = <.001), outperforming CHX. In the S. mutans biofilm challenge model, both 2 % CHX and 10 % QAM groups showed a significant increase in perimeter gap length, PGL (p = <.001 and p = 0.039, respectively) and occlusal gap width, OGW (p = <.001 for both) from initial to final measurements, although no differences were observed across groups at each time point. In contrast, when exposed to mechanical loading in the bioreactor (in water), none of the groups, including QAM, exhibited significant changes in either PGL or OGW (p = 0.989, and p = 0.118). These findings indicate that biofilm activity, rather than mechanical stress, is the primary driver of marginal breakdown in the early stages.

In summary, this study demonstrates that QAM-based adhesives outperform CHX by providing stronger protection against MMP activity, improving dentin's mechanical properties, and offering robust antimicrobial effects. The fact that these results were obtained in a clinically-relevant model increase the confidence that this approach can be employed as a tool to extend the lifespan of restorations.

Results obtained in a simulated cavity preparation subjected to mechanical and bacterial challenges demonstrated the potential for QAM-based materials to serve as both antimicrobial and collagen-preserving.