Disinfection of the root canal system and its three-dimensional tubular network is necessary for successful root canal treatment [1]. This may be achieved by mechanical instrumentation, irrigation with disinfecting solutions and the application of inter-appointment intracanal medicaments. Studies have reported that mechanical instrumentation alone cannot completely irradiate the microbes present in accessory canals, anastomoses and deep dentinal tubules [2]. Likewise, approximately 35–53% of the root canal surface remains untouched during mechanical instrumentation [3], which results in the retention of biofilms in-situ. For these reasons, thorough debridement of the root canal system is paramount to remove bacterial biofilms and infected hard tissue debris.

Sodium hypochlorite (NaOCl) in the range of 0.5 to 5.25% is considered the gold standard in root canal irrigation because of its broad antimicrobial spectrum and its ability to dissolve organic tissues [4]. Mechanical instrumentation of root canal system invariably promotes the formation of a smear layer. The latter consists of inorganic and organic particles, coagulated proteins, pulp tissue, blood cells, bacteria and their by-products [5,6]. Combined application of ethylenediaminetetraacetic acid (EDTA) and NaOCl is frequently recommended for the effective removal of smear layer from the instrumented canal walls [7]. Chlorhexidine gluconate (CHX) has been shown to be an effective antimicrobial irrigant. Because of its substantivity, CHX prevents reinfection of the dentine for up to 72 h [8,9]. However, CHX lacks the ability to dissolve pulp tissues [10]. To date, there is no single root canal irrigant that can remove the smear layer and disinfect the root canal system simultaneously. Hence, manufacturers have come up with the concept of combining a chelating agent and a disinfectant into a single irrigating solution (2-in-1 irrigant). The use of a 2-in-1 irrigant purportedly reduces treatment time and increases the effectiveness of root canal therapy.

QMix® 2 in 1 (Dentsply Sirona, Charlotte, NC, USA) is one such irrigant. This irrigant has been reported to remove canal wall smear layers as effectively as 17% EDTA [11,12] and is an effective antimicrobial agent against Enterococcus faecalis (E. faecalis) biofilms [12]. SmearOFF (Vista Dental Products, Racine, WI, USA) is a recently introduced smear layer removal agent in endodontics. According to the manufacturer, mixing SmearOFF with NaOCl does not cause precipitation. This negates the need for a second rinse after NaOCl irrigation. The manufacturer also claims that SmearOFF outperforms EDTA in terms of calcium suspension and clears more debris from dentinal tubules. A recent study reported that SmearOFF had better smear layer removal efficacy than 17% EDTA, when used as a final rinse along with 2.5% NaOCl [13]. Irritrol (Essential Dental Systems, Hackensack, NJ, USA) is also a recently introduced single-step, disinfecting endodontic irrigation solution that consists of EDTA and CHX. The manufacturer of Irritrol claims that it removes smear layer efficiently and less aggressively than EDTA, causing less demineralization of the root canal dentine. The manufacturer also claims that the antimicrobial activity of Irritol against E. faecalis is better than 2% CHX. However, contrary to the manufacturer's claim, a recent study reported that 17% EDTA had better smear layer removal efficacy than Irritrol when used as a final irrigant [14]. The composition and concentration of the 2-in-1 irrigating solutions used in the present study are summarised in Table 1.

To date, no comparative studies are available that evaluate the physical and chemical properties of 2-in-1 irrigating solutions. Accordingly, the objectives of the present ex-vivo study were: a) to evaluate the efficacy of QMix, SmearOFF and Irritrol when used as a final irrigant for removal of canal wall smear layer; b) to evaluate whether precipitation occurs when QMix, SmearOFF or Irritrol is combined with NaOCl; c) to determine the antimicrobial efficacy of QMix, SmearOFF and Irritrol; and d) to evaluate the cytotoxicity of QMix, SmearOFF and Irritrol. The null hypotheses tested were: (1) there are no differences in the ability of QMix, SmearOFF and Irritrol to remove canal wall smear layer when these solutions are used as final root canal irrigants; (2) there is no precipitate formation when QMix, SmearOFF or Irritrol solutions is mixed with NaOCl; (3) there are no differences in the antimicrobial efficacy of QMix, SmearOFF and Irritrol; and (4) there are no differences in the cytotoxicity of QMix, SmearOFF and Irritrol.