## 4 Discussion  
Sealers with gutta-percha determine a tight seal of the whole space of the root canal system with the apical delta [2; 5]. The robustness of such sealing aims at hermetic closing from remains and bacteria as well as prevention of re-infection [2; 5; 6; 7]. The failure of the endodontic obturation may result in periapical inflammation leading to surgical treatment or even tooth loss, the latter requiring replacement with dental implants [25; 26]. One of the parameters that defines the sealer interaction with the dentin is the sealer penetration range into the dentinal tubules [5; 6; 7]. It depends on factors such as the presence of the smear layer, the chosen filling technique, the physio-chemical properties of the sealer (e.g., its thickness, stickiness to the root canal tissues and hydrophilic properties) as well as its possible interaction with dentin [5; 6; 7; 27; 28; 29; 30]. An epoxy sealer was used in this investigation as it is the most frequently used type of sealer in endodontic therapy and to this day is considered as the golden standard in endodontics. According to [PERSON] et al. a type of sealer used, may also differ in its penetration into dentinal tubules, when the smear layer is present or absent [31]. The smear layer, created by a mechanical preparation, establishes a mechanical barrier against irrigating solutions and sealers. Its removal is obligatory for effective decontamination of the dentin and for the sealers penetration into the dentinal tubules [4; 5; 6; 7; 22; 23]. In the above studies, the smear layer was removed in experimental groups (2 and 3) using two different irrigation protocols with citric acid and sodium hypochlorite. The other studies of the authors revealed that this method of canal irrigation was the most effective way to remove the smear layer [22; 23]. During the alternating irrigation with sodium hypochlorite (NaOCl) and citric acid (CA), probably irrigation with NaOCl after the first cycle of CA increases the effectiveness of the subsequent CA irrigation cycle. The partial removal of the smear layer (inorganic matter) during the first irrigation cycle with the chelating agents resulted in the exposure of the organic compounds in the deeper parts of the smear layer and the dentin (collagen). Subsequent irrigation with NaOCl most likely dissolves exposed organic substances (organic matter in the deeper parts of the smear layer), enhancing the effect of subsequent application of CA. This phenomenon was tested by the authors twice with the use of different methodologies [22; 23]. There are some recently invented solutions for the final rinse that can provide higher sealer penetration and more opened dentinal tubules than sequent application of EDTA and chlornetidine [32]. Although there is no consensus on the sequence of the irrigation protocols, the most common protocol is using sodium hypochlorite (NaOCl) during the whole mechanical preparation. When used alone, NaOCl can dissolve pulpal remnants and dentin debris. However, many studies have revealed its ineffectiveness in removing the entire smear layer when used as the only irrigant. Other studies proved that alternating the use of EDTA and NaOCl is an effective method for smear layer removal [33; 34; 35]. [PERSON] et al. researched the effect of NaOCl concentration, its active duration and temperature on its penetration into the dentinal tubules while in direct contact with dentine. The results proved that the potential of sodium hypochlorite to penetrate into the dentinal tubules was dependent on time, its percentage and temperature, but the comparative effect of the these three factors was less effective than supposed [36]. In a systematic review of the literature, [PERSON] et al. evaluated the irrigating solutions and activation methods used in clinical endodontics. They concluded that existing protocols have not yet been defined for the real clinical state and context among those actually available.