Posttreatment endodontic diseases caused by intraradicular infection in most cases can be treated by nonsurgical endodontic re-treatment. Despite technical advances in our disinfection armamentariums, anatomically complex areas in root canals such as isthmuses and fins that potentially harbor pathogenic microorganisms are not easily accessible due to the limitations of our instrumentation techniques during nonsurgical therapy [16–20]. Iatrogenic errors such as canal transportation and separated instruments that impair thorough root canal disinfection may be another significant factor contributing to failure of nonsurgical re-treatment [21, 22].

While endodontic surgery provides less opportunity to disinfect the entire root canal system than nonsurgical root canal treatment, in clinical situations where sufficient intracanal preparation and sealing cannot be obtained, endodontic surgery is a preferred alternative for teeth with apical periodontitis.

If the source of infection or the cause of apical periodontitis resides in extraradicular areas [23–29], endodontic surgery is essential for healing. The strategy to cure apical periodontitis in these situations is rather straightforward. The elimination of the origin or cause of the periapical lesion by surgical debridement can lead to the resolution of apical periodontitis.

With the introduction of molecular methods to detect the microorganisms, a growing body of evidence exists to support the view that microorganisms exist in the extraradicular space [30, 31]. The existence of biofilms on the extraradicular surfaces also has been clearly demonstrated in microscopic observational studies [27, 28]. Biofilms can have up to 1,000 times more resistance than their planktonic counterpart [32–34]. Foreign materials beyond the apical foramen may also cause persistent apical periodontitis [35]. Lentil beans [36, 37], cellulose from paper points and cotton wool [38, 39], and small gutta-percha particles [40] may induce chronic inflammatory reaction and foreign body reaction because these materials are not easily degraded by host immune cells [35, 41]. Cholesterols derived from dead host blood and immune cells and plasma lipids [42, 43] are endogenously induced but also cause similar inflammatory reaction and result in persistent apical periodontitis (Fig. 4.1) [44, 45].

The incidence of cysts in periapical pathosis varies greatly between 6 and 54 % [46–50], and it is generally agreed that cystic lesions are less likely to be resolved by nonsurgical endodontic treatment [50–52]. A pocket cyst (bay cyst) where the cyst lumen is continuous with the root canal may resolve with nonsurgical root canal therapy [50, 53, 54]. On the other hand, a true cyst, which is not associated with root canals and is self-sustaining, is less likely to heal by nonsurgical endodontic treatment [50, 53, 54].

Peterson and Gutman [98] reported in their systematic review that only 35.7 % of 350 patients healed successfully after resurgery with 26.3 % uncertain and 38 % failed. This finding is consistent with Gagliani et al. [99] who showed that teeth that underwent resurgery had a poorer outcome compared to that of initial surgery, although this prospective study reported 76 % success rate (59 % complete healing and 17 % incomplete healing). Gagliani et al. [99] used a modern surgical technique including ultrasonic tips, EBA as the root-end filling material, and 4.5× loupes to aid visibility. Another prospective clinical study by Song et al. [100] showed a 92.9 % success rate after endodontic resurgery based on 42 patients with a 77.8 % recall rate. Song et al. [100] used ultrasonic tips and MTA and EBA as root-end filling materials with a surgical operating microscope. The success rate in this study is equivalent to those of first-time EMS and is thought to be due to the ability of EMS to address the causes of posttreatment endodontic disease. Therefore, resurgery may not be considered to affect the outcome negatively if microsurgical techniques are used.

Higher magnification aids in identifying the etiology of persistent apical periodontitis and provide a more precise control of surgical procedures. Surgical operating microscopes and endoscopes are considered to be state-of-the-art magnification tools for endodontic surgery. Any less magnification and illumination is thought to be associated with the lower success rates in TES compared to EMS. However, it should be noted that in order to investigate the effect of magnification on the outcome, magnification should be the only variable to compare, while the other variables such as surgical techniques and materials are controlled. Del Fabbro and Taschieri [101] reported that there was no significant difference in success rates among loupes, endoscopes, and microscopes. Setzer et al. [102] performed a meta-analysis based on 14 outcome studies, which applied the same ultrasonic root-end preparation and root-end filling with biocompatible materials but different magnification tools, and showed that endodontic surgery with the endoscope or the microscope had a statistically greater success rate than those with insufficient magnification. Interestingly, this systematic review showed that no significant difference was observed for anteriors and premolars, but there was a significant difference for molar surgery [102]. This finding is perhaps due to the low statistical power during the subgroup analysis. However, it may be also due to the easier accessibility and less complex root-end intricacies in anteriors and premolars. A recent meta-analysis by Tsesis et al. [59] showed that significantly higher positive outcomes were found when endoscopes or microscopes were used as a magnification tool compared to when loupes were used. This result is consistent with von Arx et al. [103], who reported that teeth that underwent endodontic surgery with the use of an endoscope had a higher success rate than teeth without the use of an endoscope. Therefore, higher magnification using the endoscope and microscope may be considered a prognostic factor that affects the outcome of endodontic surgery positively.

The limited root canal disinfection can be achieved in endodontic surgery by root-end resection and root-end preparation. Therefore, remaining microorganisms in the root canal system may cause a recurrent apical pathosis if the apical seal is incomplete. In order to have an adequate apical seal, proper root-end filling materials should be used. MTA and EBA appear to have a better sealing ability than amalgam and other filling materials such as Cavit and zinc oxide eugenol [92]. In addition, the depth of root-end filling is an important factor for an adequate apical seal and should be at least 3–4 mm when MTA is used as a root-end filling material [104, 105]. MTA was reported to also have better biocompatibility compared to EBA and amalgam [96]. These results from preclinical studies give us considerable insights into selecting root-end filling materials for endodontic surgery, but we do need clinical outcome studies to validate whether superior sealing ability and biocompatibility of MTA can translate into higher success rates compared with other materials. In a systematic review by von Arx et al. [103], a significantly higher estimated healed rate was found in studies using MTA (91.4 %) as compared to amalgam (57.9 %) and glass ionomer cement (51.2 %). Notably, no significant differences in estimated healed rates were found among MTA, EBA (69.8 %), and IRM (71.6 %) [103]. A meta-analysis by Tsesis et al. [59], however, showed that use of MTA was associated with significantly higher success rates than the use of IRM or EBA, although MTA did not significantly differ from EBA in success rates when studies with low risk of bias were selected and analyzed. A recent retrospective study by Song et al. [106] demonstrated a significant difference between MTA and IRM, but no difference between MTA and EBA. In contrast, two randomized controlled study showed no significant difference between MTA and IRM [85, 107]. Clearly, there is no doubt as to the better clinical outcomes with MTA compared to other materials, although MTA, EBA, and IRM are considered clinically acceptable root-end filling materials. Therefore, the material selection for root-end filling is considered to affect the outcome significantly.

When the periapical lesion is limited to the apical area, endodontic surgery with microsurgical techniques offers an excellent outcome as evidenced by the previous systematic reviews [6, 58