## 1 Introduction  
Conventionally, the cleaning of the root canal system (RCS) is done with a chemical-mechanical preparation using endodontic instruments and irrigating the area profusely with disinfecting chemical solutions [1]. The basis for a successful endodontic treatment is the complete removal from the RCS [2] of both necrotic and vital pulp substrates, together with microorganisms and their toxins. In this context, the complexity of the morphology of the RCS makes their complete mechanical debridement a major challenge for clinicians. It has been shown that over 35% of the RCS surface remains untouched by endodontic instruments after instrumentation [3; 4], highlighting the importance of efficient instrumentation and irrigation to ensure the proper chemical debridement and disinfection of the RCS.  
One of the most important factors for the clinical success of an endodontic treatment is the proper preparation of the apical third of the root canal. Inappropriate preparation and disinfection of this portion can lead to the persistence of organic remnants (i.e., microorganisms and necrotic tissue). These, in turn, may cause periapical inflammation, disfavouring tissue repair and resulting in the consequent failure of the treatment, especially in non-vital and infected teeth [5; 6].  
When analysing the amount of enlargement needed in the preparation of the apical portion of infected root canals, several aspects must be considered, such as the action of the irrigant solutions [5; 7], the bacterial penetration into dental tubules [5; 8; 9], and the irregular and complex morphology of the RCS [5; 7; 10; 11].  
It has been demonstrated that the higher the apical diameter the better disinfection of root canals, mainly because of the mechanical elimination of contaminated dentin and the penetration of the irrigant solution in the apical region [5; 12]. Despite great advances in mechanised instrumentation systems and their alloys, in complex anatomies it is difficult to reach high apical diameters during instrumentation. Although these last-generation instruments may allow for higher diameter coverage, they may also lead to the risk of procedural mistakes, such as transportations and foramen deformation, among others. In these situations, the effect of the irrigant solution becomes fundamental in order to remove debris, dissolve remnant tissue and act as a lubricant [4; 10; 11].  
On the other hand, as far as the irrigation of the root canal is concerned, it has been shown that PUI is more effective than CI for removing organic tissue, because of its ability to disintegrate more bacteria and dentin debris [13; 14; 15; 16; 17]. Recently, [PERSON] et al. (2019) have demonstrated that a higher amount of organic remnant is eliminated using PUI during the ex vivo preparation of vital root canals when using instruments with low apical diameters [18]. However, they state that their conclusions are not completely applicable to infected root canals. Thus, our hypothesis states that PUI enhances the removal of dentin debris and organic remnant during the preparation of infected root canals, especially when instruments with low apical diameters are used.  
The objective of this study was to evaluate the effect of PUI on the removal of the organic remnant tissue from narrow, infected and curved root canals during their instrumentation with apical enlargements of #25 and #35.