## 4 Discussion
Most of the studies included in this review did not find a significant difference between the two groups. Some studies have shown higher flareups and pain in single visit [22; 36] contrary to those which show increased incidence in multiple visits [19; 23; 25; 28; 30; 31; 35]. The two studies [23; 36] which found significant differences between groups did not have any characteristics in common. Both studies favoured multiple visit root canal therapy. It is interesting to note that both these studies were carried out at the beginning of our selected period (2004 and 2006). The studies that have been done since, till April 2020, found no significant difference between the two procedures. This is possibly due to evolution of the techniques of root canal preparation, endodontic irrigation and obturation that improved the quality of single visit treatment, i.e., apical extrusion is an issue which has been addressed. Currently, evidence on precautions to minimize postoperative flare ups is scarce [44]. Studies have examined various factors that contribute, such as operator skills and pulp vitality. In our findings, we found similar results in studies regardless of pulp vitality. Studies included in the present review and previous studies have not found any link between pulp vitality and postoperative pain and flare ups. Again, the studies that found a significant effect of vitality in our review were [23; 36] which are older. Periapical inflammation and microbial colonization is easier in necrotic tissue [3; 15]. This can cause inflammation in a single visit procedure due to apical extrusion of necrotic tissue during cleaning and shaping. Since present day techniques minimize this phenomenon, it is possible that tooth vitality does not play a role in postoperative pain and flare up. The included studies were unclear on the matching of the baseline characteristics of the included groups. These should be matched to the best possible extent to avoid confounding. Variation in tooth anatomy leads to difficulty in the adequate endodontic treatment of teeth. Molar teeth especially, have a complex anatomy and are difficult to access due to their location, which has been linked to postoperative pain [27; 45]. However, some studies show that postoperative pain does not have any correlation with the number of roots [18; 19; 46] and gender [7; 20; 22]. The studies included in the review had variation in several characteristics. Multiple visit groups used intracanal medicaments in some studies, i.e., calcium hydroxide, the most popular option [47]. A medicament is used to provide anti-inflammatory and antimicrobial effect. Evidence suggests that calcium hydroxide may not be able to maintain sterile canals [48] and aid healing [21] in all cases. According to a study [49], it may infect, allow growth of microbes. The studies in this review that used calcium hydroxide medicament as a separate group included [23] which suggested a possible role and [34] that established no relation between the use of calcium hydroxide and postoperative pain and flare ups. Cleaning and shaping procedures also varied among studies using both manual and rotary techniques. Different irrigants such as sterile saline, EDTA, sodium hypochlorite and chlorexidine were used. Using the correct technique of mechanical preparation may play a more important role as studies suggest no relation between the irrigant used and postoperative pain [50]. The variations mentioned above led to considerable clinical heterogeneity in studies. The meta-analysis was done on cleaned data that included only 14 studies with quantified results. Most of the studies showed very little variation in the outcome, with the forest plot showing few studies that favoured either outcome. The weight distribution was such that no single study was able to influence the overall risk ratio. The overall risk ratio was 0.96, which favoured the single visit treatment, but the confidence interval extended from 0.75 to 1.22. Thus, we cannot favour either modality.