piezoelectric oscillation. Grischke et al. [29] compared the efficiency of sonic, ultrasonic, and hydrodynamic devices in the removal of a root canal sealer from the surface and simulated irregularities of root canals. Again, the passive ultrasonic irrigation was seen to be effective in removing sealer from the root canal.

During endodontic therapy, dental instruments may separate within the root canal and impede the renegotiation of the canal path. As such, during radiographic examination in preparation for retreatment, the clinician may unexpectedly encounter one or more retained endodontic instrument fragments. In clinical studies, the incidence of this accident has been reported to range from 0.39 % to 5 % [18, 51].

In a systematic review, Panitvisai et al. [48] assessed the prognosis of teeth after instrument fracture during endodontic therapy and found no statistically significant difference in healing rates between teeth with and without retained instrument fragments. However, the odds of treatment failure are higher when fragments prevent a thorough cleaning and shaping of the entire canal system and when periradicular lesions are present preoperatively [13, 33, 66]. When infection is present, removing or bypassing the fractured instrument is essential to ensure that the irrigation solution reaches the working length in order to obtain disinfection and the associated increased predictability of the outcome. Dental operating microscopes (DOM) and ultrasonic tips have allowed clinicians to obtain access to separated instruments and can assure higher success rates in the removal of instrument fragments, as reported by [43].

Carrier-Based Filling Materials

Carrier-based filling materials provide a straightforward approach to the obturation procedure; however, removal of these materials can be particularly challenging especially when retreating small and curved canals as the plastic core is not soluble in common solvents [4] (Fig. 17.8).

More recently, a 3rd generation of carrierbased obturators named GuttaCoreTM (Dentsply Tulsa Dental) was developed and employs crosslinked gutta-percha instead of a plastic carrier.

According to a recent research study, this system is easier to remove than those containing plastic carriers [7, 44] (Fig. 17.9).

Solvents: How Effective/Safe

Are They?

The use of mechanical techniques and solvents to remove filling materials from previously root canal-treated teeth has been tested throughout the years [9, 11, 68]. The assertion that usage of an organic solvent is necessary for the removal of filling material can be considered inappropriate as several published articles demonstrate both rootfilling remnants on the root canal surface and the formation of an artificial smear layer after using these agents [55, 64] (Fig. 17.10). A further disadvantage is the cytotoxic property of organic solvents, which is especially of concern when they are extruded into the periradicular area [5, 60].

Solvents were studied more frequently in the 1980s and 1990s than they are in the present period [30, 70, 76]. Barbosa et al. [5] examined the effects of halothane, turpentine oil, and chloroform solvents on the fibroblastic cells of rats. These authors concluded that the use of solvents should be avoided because all of the agents analyzed were found to be toxic.

In certain cases, the hardening of the sealer is accentuated to such a degree that it is very difficult to remove the gutta-percha in its entirety or even establish a glide path through the guttapercha, especially in curved canals [30, 70].

A consensus has not yet been reached regarding whether solvents are helpful during the process of gutta-percha removal. Despite this uncertainty, the issue of the cytotoxicity of the solvents used in endodontic retreatment needs to be analyzed more accurately. Although chloroform is generally considered highly effective, the claim that it is cytotoxic has led to the testing of some “alternative” solvents, such as halothane, eucalyptol, orange oil, and xylene [24, 59]. Wilcox [75] and Bueno et al. [9] have reported that chloroform is highly efficient. Recently, a similar study was performed by Sağlam et al. [56] in extracted molars with curved roots. The ProTaper Universal and Self-Adjusting File were used in conjunction with chloroform,

Fig. 17.8 (a) A 48-year-old patient had a nonsurgical root canal treatment where the obturation was performed using a carrier-based technique in all four canals. A 7-year follow-up radiograph showed a periradicular lesion in both mesial and distal roots; (b) during the attempt to retrieve Thermafil from inside the canals, a ledge and subsequent perforation occurred in the mesial-buccal canal. The carrier inside the distal-lingual canal separated at the

apical portion; (c) even though a portion of the previous obturation material was kept inside the canals and the perforation inside the mesial-buccal canal perforation, the treatment was concluded and the final restoration was placed; (d) a 10-month follow-up showed signs of healing and the patient was asymptomatic. Despite the technical difficulties in this case, the disinfection protocol was essential for a successful outcome

Fig. 17.10 Artificial smear layer adhered to the canal’s walls after the usage of chloroform during an endodontic retreatment in a mandibular lower premolar

Endosolv, or no solvent and the residual root-fill- ing material was evaluated using micro-computed tomography (μCT), a noninvasive technology. No significant differences were found between the groups in terms of the percentage volume of residual root canal filling.

Regarding solvent cytotoxicity, McDonald and Vire [39] found that chloroform had no effect on the clinical staff or on the patient who had undergone the treatment involving chloroform. In addition, it was reported that if used under normal conditions, chloroform does not cause any irreversible cytotoxic effects. Chutich et al. [12] analyzed the toxicity of chloroform, halothane, and xylene by quantifying the apically extruded solvent and found that the amount of solvent that was carried through the apical foramen is much lower than the permitted dose. Nevertheless, the US Food and Drug Administration (FDA) specify that chloroform may not be used as an ingredient in drug products or in pharmaceutical compounding.

Fruchi et al. [26] used μCT imaging to evaluate the amount of filling material remaining after instrumentation with reciprocating files and again after passive ultrasonic irrigation (PUI) with xylene. The study concluded that both instruments efficiently, but not completely, removed the filling material from the inside of the curved mesial-buccal canals of extracted maxillary molars. Although the use of xylene with PUI slightly increased the removal of the filling material, this finding was not statically significant. Cavenago et al. [11] evaluated the percentage of remaining filling material in the mesial root canals of mandibular molars after retreatment

in which 3 procedures were performed sequentially. The first step involved removal of the filling material, enlargement of the root canals to size 40, and instrumentation with a 0.04 tapered instrument. In the second step, the root canals were irrigated with xylene and an attempt was made to clean the root canals with paper points. In the third step, the PUI technique was performed using 2.5 % sodium hypochlorite. The authors concluded that the filling materials were not completely removed by any of the retreatment procedures alone; however, the use of xylene and PUI after mechanical instrumentation enhanced removal of the materials during endodontic retreatment of anatomically complex teeth.

The antimicrobial effectiveness of chloroform on Enterococcus faecalis has also been evaluated. Edgar et al. [21] collected bacterial samples for analysis after gutta-percha had been removed using either chloroform or saline. Negative cultures were obtained in 11 of 17 chloroform samples and 0 of 17 saline samples, demonstrating that the use of chloroform during endodontic retreatment significantly reduced intracanal levels of cultivatable Enterococcus faecalis. Martos et al. [38] evaluated the antimicrobial activity of three different solvents solutions (chloroform, eucalyptus oil, and orange oil) alone and in association with various concentration of cetrimide (CTR) against Enterococcus faecalis biofilms in dentine. The results obtained in this study suggested that the antimicrobial ability of the solvent agents combined with CTR contributes to the disinfection in endodontic retreatment.

In conclusion, the usage of organic solvents should be avoided in retreatment cases in which the filling material can be easily removed. However, if in the attempt to reach the apical foramen and achieve patency no progress is made, solvents should be used cautiously.

Disinfection Protocol in Nonsurgical

Retreatment

In order to obtain an in-depth understanding of the irrigation solutions available for disinfection in endodontic treatment, referral to Chap. 5 is recommended. Briefly, the solutions that, according