Effect of Smear Layer against Disinfection Protocols on Enterococcus faecalis-infected Dentin

Introduction: This study examined the effect of the smear layer on the antibacterial effect of different disinfecting solutions in infected dentinal tubules. Methods: Cells of Enterococcus faecalis were forced into dentinal tubules according to a previously established protocol. After a 3-week incubation period of infected dentin blocks, a uniform smear layer was produced. Forty infected dentin specimens were prepared and subjected to 3 and 10 minutes of exposure to disinfecting solutions including sterile water, 2% and 6% sodium hypochlorite (NaOC1), 2% chlorhexidine (CHX), 17% EDTA, and QMiX (Dentsply Tulsa Dental, Tulsa, OK). The following combinations were also included: 2% NaOC1 + 2% CHX, 2% NaOC1 + QMiX, 6% NaOC1 + QMiX, and 6% NaOC1 + 17% EDTA + 2% CHX. Four other dentin specimens similarly infected but with no smear layer were subjected to 3 minutes of exposure to 2% CHX and 6% NaOC1 for comparison. Confocal laser scanning microscopy and viability staining were used to analyze the proportions of dead and live bacteria inside the dentin. Results: In the presence of a smear layer, 10 minutes of exposure to QMiX, 2% NaOC1 + QMiX, 6% NaOC1 + QMiX, and 6% NaOC1 + 17% EDTA + 2% CHX resulted in significantly more dead bacteria than 3 minutes of exposure to these same disinfecting solutions (P < .05). No statistically significant difference between 3 and 10 minutes was found in other groups (P > .05); 6% NaOC1 + QMiX and 6% NaOC1 + 17% EDTA + 2% CHX showed the strongest antibacterial effect. In the absence of a smear layer, 2% CHX and 6% NaOC1 killed significantly more bacteria than they did in the presence of a smear layer (P < .05). Conclusions: The smear layer reduces the effectiveness of disinfecting agents against E. faecalis in infected dentin. Solutions containing 6% NaOC1 and/or QMiX showed the highest antibacterial activity.