###### Abstract  
Irrigation represents a crucial step in endodontics for bacteria disinfection and smear layer removal. Several irrigation strategies have been proposed, although their effects are controversial. This study aims to assess the comparison of four different irrigation systems on the smear layer removal utilizing scanning electron microscopy (SEM). Forty sound monoradicular teeth were collected and casually allocated into four groups (\(n\) = 10): Group A, conventional irrigation; Group B, IriFlex irrigation; Group C, ultrasonic irrigation system; Group D, apical negative pressure irrigation. After chemo-mechanical preparation and longitudinal root separation, the three root thirds were analyzed by SEM. Micrographs were carried out at \(\times\)2000 to analyze the smear layer residual in each third of the root canal. Statistically significant differences were found between Group A and Group D (\(p\) < 0.05). Groups B and C showed satisfactory results in the coronal and middle thirds, while Group D was the only system that achieved satisfactory results in the apical third. However, the complete smear layer removal in the root canal was never achieved with any of the four systems tested. In conclusion, among the four tested irrigation systems, the greatest efficacy on smear layer removal was reached by Group D.  
endodontics; irrigation; smear layer; disinfection; dental material +
Footnote †: journal:  
long-term prognosis [9]. In this light, in order to improve the efficacy of root canal disinfection procedures, it is necessary to combine mechanical instrumentation with the chemical action of the irrigants [10; 11]. Indeed, this so-called chemo-mechanical preparation allows the irrigant solutions to act directly on the walls of the root canal and penetrate inside the dentinal tubules, considerably reducing the bacterial load [12]. Sodium hypochlorite (NaOCI) has been demonstrated to be the irrigating agent of choice for its antibacterial properties and its ability to dissolve necrotic tissues [13; 14; 15], despite its high toxicity [16] and inability to remove the inorganic component of the smear layer [12]. Therefore, NaOCI is usually combined with a chelating solution such as ethylenediaminetetraacetic acid (EDTA), which is effective in dissolving the smear layer from the inner surfaces of the root canal [17; 18]. However, the typical irrigants are administered into the root canal space using a conventional needle which has limitations related to the needle tip insertion [19] and their incapability to cleanse areas not prepared by mechanical instrumentation [20; 21]. Therefore, different systems and techniques have been provided to increase and facilitate the infiltration of the irrigant solution along the root canal system, with promising results. One of the most used techniques for intra-canal irrigation is the ultrasonic activation of the irrigant solution through a file or a vibrating tip inside the canal, whereby the solution can be injected intermittently or continuously [22; 23]. Another technique suggests preheating NaOCI solution as it more efficiently dissolves pulp tissue and cleans the canal [24]. Moreover, EndoVac (Discus Dental, Culver City, CA, USA), an apical negative pressure (ANP) irrigation system, is a safe dispensing system designed to allow irrigants to penetrate all inaccessible areas of the root canal system [25], preventing the extrusion of NaOCI out of the apex and hence reducing the probability of accidents [16; 26]. Several studies highlight the high performance of these two irrigation systems in regard to the delivery and penetration of the irrigating solution up to the most difficult areas of the root canal [27; 28; 29]. Despite these results, it is still debated in the literature which technique is the most effective in removing the smear layer [30; 31].