to our philosophy, have a positive impact during the disinfection phase of nonsurgical retreatment will be discussed.

The disinfection protocol in retreatment cases is similar to that used in conventional endodontic treatment. However, time plays an important role during these cases and should be carefully managed in order to achieve complete disinfection. Multiple visits and intracanal medicaments should be considered in difficult cases [81].

Instrumentation is still considered an important step in the removal of all previous filling materials and necrotic organic tissues from the inside of the canals. This process creates a suitable space for the irrigation solution to make contact with the canal walls. However, the use of irrigation and irrigants has become increasingly relevant as studies are demonstrating that a high percentage of the canal walls are still left untouched after instrumentation [49, 50, 52].

Before beginning retreatment and attempting to regain apical patency, diagnostic radiographs should be carefully examined for evidence of a previous perforation. Extrusion of caustic irrigants such as sodium hypochlorite beyond the apical foramen into the periodontal ligament, alveolar bone, and anatomical structures can cause undue harm and serious accidents. If a perforation is encountered, it is advisable to use a biocompatible solution for irrigation, such as physiologic saline or local anesthetics, until the perforation is sealed.

Numerous factors including delivery of the irrigant closer to the apex, larger irrigation volume, and narrower gauge irrigation needles have shown to improve the efficacy of root canal irrigation [23].

The ideal irrigant should preferably have disinfectant and organic debris dissolving properties. It should also act to remove the smear layer without causing irritation to the periradicular tissues. The irrigation solution should be delivered in copious amounts as close as possible to working length without extruding beyond the foramen. This procedure can be safely and effectively accomplished using a syringe that allows the solution to escape freely into the pulp chamber. It is also important that the irrigant is not delivered with excessive force. The solution may also be agitated while inside the canal using an ultrasonic or sonic system [22]. To date, an ideal irrigating solution that

possesses all of these desired properties does not exist; therefore the best protocol to achieve optimal disinfection is a combination of solutions.

Sodium Hypochlorite (NaOCl)

Sodium hypochlorite remains the solution of choice for disinfection throughout the cleaning and shaping procedure, especially in cases where persistent infection is present. NaOCl prepared at a concentration ranging from 2.5 to 6 % is indicated due to the antimicrobial properties, the ability to dissolve necrotic organic tissue, and the price and availability of this solution [80].

Du et al. [19] evaluated the antimicrobial activity of different endodontic disinfecting solutions on Enterococcus faecalis biofilms in dentin canals. Sodium hypochlorite at a concentration of 6 % was the most effective antibacterial solution. It has also been proposed that laser activation and passive ultrasonic irrigation of the sodium Hypochlorite solution may enhance the removal of biofilm in infected dentin [45].

Chelants

Ethylenediaminetetraacetic Acid (EDTA)

The advantages of using EDTA to remove the smear layer created by the debridement of the canal are well documented in the literature [2, 3, 78]. A study conducted by Keles et al. [32] indicated that during nonsurgical retreatment, the use of EDTA for removing smear layer assisted in the retreatment process by dissolving calcium hydroxide, polyketone, zinc oxide-eugenol, silicone, and two epoxy resin-based root canal sealers.

It is also noteworthy to state that chelating solutions such as citric acid and EDTA interfere with biofilm cohesion [8]. EDTA should be used after irrigating with NaOCl and before using chlorhexidine in order to avoid the formation of para-chloroaniline (PCA) [6].

Peracetic acid (PAA) in concentrations ranging from 0.5 to 1 % has been proposed as an alternative to the classic decalcifying agents (EDTA and citric acid) due to its capacity to