Postoperative pain is a common clinical predicament that can occur from a few hours to several days after RCT of teeth with irreversible pulpitis. Poorly controlled pain can increase patient anxiety and distress. Dentists aim for less postoperative pain and faster symptom relief after RCT. To our knowledge, this prospective clinical study is the first to investigate the effect of the apical terminus of root canal preparation on postoperative pain in patients with teeth with irreversible pulpitis. The results showed that the distance between the apical terminus and AF affected pain; 0.5 mm short of the AF was most effective in terms of reducing the severity and duration of postoperative pain in symptomatic irreversible pulpitis, and 1 mm short of the AF was most effective in asymptomatic irreversible pulpitis.

It is a common belief that the apical extrusion of debris is inevitable during root canal preparation, and no method exists that completely eliminates this issue. However, there is considerable variability in the amount of debris extruded based on the techniques and instruments used, and these differences may be critical to the development of postoperative pain; techniques and instruments that extrude less debris are thought to reduce the risk of exacerbation [19, 28, 29]. To minimize postoperative pain and control all other clinical variables, the following mechanical preparation techniques and instruments were selected as standardized endodontic procedures in the present study.

Topçuoğlu et al. and Borges et al. showed that performing coronal preflaring before canal shaping significantly reduced the amount of apically extruded debris [30, 31]. The coronal canal space created by this procedure acted as a reservoir for the irrigant to coronally flush the debris resulting from root canal preparation, which minimized their extrusion beyond the AF [32]. Consistent with the in vitro findings, a recent randomized clinical trial also found that coronal preflaring resulted in less postoperative pain in mandibular premolars diagnosed with symptomatic irreversible pulpitis [33]. In this study, coronal preflaring during the crown-down preparation technique was performed with a large tapered nickel-titanium (NiTi) file specifically designed to enlarge the canal orifice [34]. Before coronal preflaring, the bleeding pulp tissue of the pulp chamber was irrigated with 2.5% NaOCl, which is the most widely chosen intracanal irrigant due to its ability to dissolve pulp tissue and its antimicrobial effects [35]. This procedure also helps to reduce the number of microorganisms that could be pushed into the periradicular tissues during apical instrumentation, as the majority of the microorganisms in teeth with symptomatic irreversible pulpitis are located in the pulp chamber [36].

Glide path preparation has been reported to guide the safer use of subsequent enlarging NiTi files and to prevent complications of root canal preparation such as transportation and ledge formation [32]. Compared to manual glide path preparation, creating the glide path using NiTi rotary files has been reported to result in less debris extrusion in laboratory studies [37,38,39] and a lower level and incidence of postoperative pain in two recent randomized clinical trials [40, 41]. Overall, these studies support the use of a rotary glide path instrument in this study, manufactured from memory NiTi wire with a 0.19 mm tip diameter and a constant 2% taper.

Although it remains controversial as to which NiTi instrument results in less debris extrusion during canal shaping, Bürklein et al. and Schäfer et al. compared debris extrusion in vitro produced by continuous rotary and reciprocating systems and concluded that multiple rotary-file systems were associated with less extruded debris [42, 43]. In vivo experiments indicated that root canal instrumentation using full-sequence rotary systems resulted in significantly less postoperative pain or showed no difference in postoperative pain compared with the use of reciprocating systems in teeth with vital pulp [44, 45]. Therefore, a full-sequence rotary system was used for root canal preparation in this study.

Previous studies have reported varying incidence rates of flare-ups following endodontic treatment [46, 47]; however, the absence of any flare-up in this study aligns with recent findings [48]. At 4 h postoperatively, the mean postoperative pain scores was below 2, with pain categories ranging from ‘no pain’ to ‘mild pain’ in 88% of patients, regardless of preoperative symptoms or the chosen apical terminus. This result seems logical considering the use of the aforementioned mechanical preparation techniques and instruments and also explains why no patients required analgesics.

All patients diagnosed with irreversible pulpitis during the recruitment period, regardless of preoperative symptoms, were consecutively enrolled in the study. The recruitment process provided a high degree of external validity to ensure that the results would be transferable to daily clinical practice.

Overall, 22.6% of cases diagnosed preoperatively with irreversible pulpitis were asymptomatic before receiving RCT. The incidence of asymptomatic pulpitis in this study is lower than that reported in a retrospective study by Michaelson and Holland [49] (38.8%) and in a prospective clinical study by Yu et al. [23] (38.1%). The percentage of patients with symptomatic irreversible pulpitis was higher than that of patients with asymptomatic irreversible pulpitis; however, none of the patients with symptomatic irreversible pulpitis reported severe preoperative pain. This may be due to the fact that patients tend to seek help only when experiencing pain, but if the pain is severe and acute, they are more likely to be treated at the Department of Dental Emergency at Beijing Stomatological Hospital, Capital Medical University, than at the Department of Endodontics.

In patients with irreversible pulpitis without preoperative pain, postoperative pain scores were highest at 4 h, with a gradual reduction observed at each subsequent follow-up. These findings are similar to those of a previous study that involved the same pulp diagnosis [44]. Additionally, the incidence of pain within the first 24 h was 48%, which decreased to 10% after 7 days, corroborating the recent findings reported by Pak and White [4] and Law et al. [3].

There is still no consensus on the optimal apical terminus for clinical use. Laboratory studies have reported contradictory results on the amount of debris extruded and the distance from the apical terminus to the AF. Two laboratory studies showed equal effectiveness in preventing apical extrusion of debris and bacteria when performing reciprocating instrumentation, whether used up to the AF or up to 1 mm short of the AF [50, 51]. Myers and Montgomery reported that a WL of 1 mm from the AF significantly reduced debris extrusion compared to setting this at the AF; however, they used hand files [52]. It appears that the amount of debris expelled is more closely linked to the instruments used for root canal preparation than to the apical terminus that was selected. To date, the effect of a continuous rotary system, as used in the present study, on the amount of extrusion is unclear when different apical terminus locations are used. However, our findings showed a significant difference in postoperative pain at 72 h in patients with asymptomatic irreversible pulpitis. The mean level of pain and the number of patients reporting pain were significantly lower when the procedure was performed 1 mm short of the AF, compared to those when the AF was chosen. At 72 h, all patients in the AF-1 mm group fell into the no pain category, whereas 25% of patients in the AF-0.5 mm group and 63% of patients in the AF group continued to report mild pain. Additionally, although no statistically significant difference in postoperative pain was observed among the three groups at 4 h, 24 h, and 1 week, the mean pain level in the AF-1 mm group remained lower than that in the other groups. Overall, the results indicated a significant positive effect on both severity and duration of pain when root canal preparation was performed up to 1 mm short of the AF in patients with asymptomatic irreversible pulpitis.

To date, no study has investigated the effect of using different apical terminus for root canal preparation on postoperative pain in teeth with irreversible pulpitis. The present findings could be more aptly compared with those reported by Yaylali et al., who also applied a continuous rotary system to mutirooted teeth without preoperative pain [53]. However, Yaylali et al. identified less postoperative pain in the first 2 days when preparation was performed 1 mm short of the AF; this duration of pain, with moderate pain still reported at 72 h, is longer than that reported in our study. In addition, Yaylali et al. reported that all patients in the AF-1 mm group had moderate pain on the first day. However, in the present study, 78% of the patients in the AF-1 mm group did not report any pain at either the 4- or 24-h assessment time points. This discrepancy could be attributed to several reasons. First, the present study involved teeth with vital pulps, whereas the study by Yaylali et al. involved necrotic pulps and chronic apical periodontitis. Differences in microbial infection patterns and the physical backpressure of periodontal tissues may lead to differences in apical extrusion of debris and microorganisms [36]. Additionally, other factors such as the virulence and species of microorganisms and host-dependent inflammatory response may affect postoperative pain [8, 29, 54].

In patients with symptomatic irreversible pulpitis with moderate preoperative pain, the percentage of patients reporting decreased pain at 4 h was higher in the AF-0.5 mm group than in the other groups. All patients in the AF-0.5 mm group fell into the no pain category by 1 week, whereas 27% of patients in the AF-1 mm group and 7% of patients in the AF group continued to report mild pain. The release of inflammatory mediators in cases of irreversible pulpitis triggers an increase in peripheral and central sensitization, thereby enhancing pain perception [55,56,57]. If the apical terminus of the preparation is advanced deeper, the local tissue pressure, concentration of inflammatory mediators, and number of nociceptive sensory free nerve endings may be substantially reduced or even eradicated [55], as opposed to instrumentation that stops 1 mm short of the AF. Additionally, patients with asymptomatic irreversible pulpitis in the AF-0.5 mm group experienced less pain compared with that experienced by patients in the AF group in this study, suggesting that there may be less debris extrusion apically at this length compared to when the file reaches the AF. The aforementioned reasons may account for the greater percentage reduction and quicker symptom resolution observed in the AF-0.5 mm group. In patients with mild preoperative pain, the percentage of patients who reported increased pain at 24 h was lower in the AF-0.5 mm group than in the other groups. Overall, the results indicated a significant positive effect on both severity and duration of pain when root canal preparation was performed up to 0.5 mm short of the AF in patients with symptomatic irreversible pulpitis.

The present study used a standardized procedure to determine the WL, which ensured reliable internal validity. Initially, coronal preflaring was performed before WL determination. The majority of teeth involved in this study were molars and premolars, which typically have narrow coronal orifices and/or curved canals. The coronal third of the root canals was preflared to remove cervical dentinal interference, thereby reducing the initial coronal curvature and providing straight access to the apical region of the canal. Several studies have indicated that this procedure 1) reduces the discrepancy between the initial apical file and the actual diameter of the minor AF and 2) increases the accuracy of WL determination [32]. Furthermore, accuracy was ensured using a standard ruler with a minimum scale of 0.5 mm for WL measurements, and the buccal cusp/incisal edge of each tooth was flattened as a stable reference point.

The most commonly used measures of pain intensity in endodontics studies are the visual analogue scale (VAS) and NRS-11 [9]. Previous evidence supports the validity and reliability of both VAS and NRS for detecting changes in pain intensity [45, 58]. However, based on other considerations, NRS-11 was preferred for this study. First, NRS-11 provides enhanced administration convenience over VAS, offering both verbal and written formats, along with a more straightforward scoring system [59]. Second, recent research has indicated that NRS-11 has greater sensitivity and stability [60] and exhibits lower failure rates than does VAS [61]. Lastly, patients were given a telephone reminder to ensure that they promptly and accurately recorded their pain scores, which may explain why there was no loss to follow-up in the study. Our results can be considered credible due to the 100% recall rate.

One limitation of this study is that the sample size was relatively small. Further studies with larger sample sizes are needed to confirm the present findings. Additionally, caution should be exercised in generalizing the present results as they apply only to the treatments described in the current protocol. Different mechanical preparation techniques and instruments might not yield the same postoperative pain intensity and/or duration.