Introduction

Accurate working length determination or Odontometry is one of the most important parameters that influences the outcome of non-surgical root canal treatment. Working length is defined as the distance from a preselected coronal reference point to an apical landmark at which root canal preparation as well as obturation should be terminated (AAE 2012). The classic concept of apical root canal anatomy is based on five landmarks: anatomic apex, radiographic apex, cementodentinal junction and minor apical constriction as well as the apical foramen.  Every effort is done by clinicians to locate the minor apical constriction to fulfill the resistance form and maximize the treatment outcome. Unfortunately, minor apical constriction is an anatomical landmark that is rarely localized by expert clinicians ((Dummer et al 1984). Overestimation of the working length may lead to over-instrumentation which will, in turn, lead to destruction of the natural integrity of the minor apical constriction with subsequent postoperative pain and lower treatment outcome. Underestimation of the working length may cause persistent discomfort due to incomplete disinfection and subsequent underfilling of the root canal system (Ricucci and Langeland 1998).

Two widely used methods for working length determination are Ingle’s radiographic method and electronic apex locators. Recently, cone beam computed tomography has been promoted for working length determination (Jeger et al 2012), but this is not commonly used to avoid excessive patient exposure to radiation. The Radiographic method is the most used method for working length measurement in non-surgical root canal treatment and has many advantages such as direct observation of the anatomy of root canal system, number, and curvature of roots and, in addition, acts as an initial guide for working length estimation (Olson et al 1991). There are, however, several disadvantages like increased exposure to radiation for both the patient and clinician, image distortion, being a two-dimensional image of three-dimensional object and variation in radiographic interpretation which may result in erroneous judgment as well as its limitations such as use during pregnancy and patients with severe gagging reflex (Bramante and Berbert 1974).

The electronic apex locator is an indispensable aid for contemporary endodontic therapy. The principle behind most electronic root canal measuring devices that human tissues have certain characteristics that can be detected by combination of electrical components. Then, by measuring the electrical properties of the tissues (resistance, impedance) it should be possible to detect the canal terminus (Nekoofar et al 2006). An electronic method for working length determination was first introduced by Custer in 1918. In 1948, Suzuki introduced a new electronic apex locator based on a constant electrical resistance value of 6.5 K Ohms between periodontal ligament and oral mucosa (Mancini et al 2011). The earlier generations of apex locators did not function properly in the presence of fluids or pulp tissue in the root canal. Due to these disadvantage, new apex locators have been developed to provide accurate readings regardless to the root canal content (Dunlap et al 1998).  

The electronic method of working length determination is superior to the radiographic method in that it allows for continuous monitoring and localization of anatomic apex (Nekoofar et al in 2006), capable of implementing and maintaining apical patency with minimal periodontal trauma, and finally it is considered an objective method rather than the subjectivity of radiographs (Fouad et al 1990). Several studies reported the accuracy of electronic methods in localizing apical foramen to have been achieved in 96.2% of cases (Felippe and Soares 1994, Shabahang et al 1996).

Unfortunately, postoperative pain is a common sequela after endodontic therapy with 3-58% reported incidence (Sathorn et al 2008). Several factors may influence the degree of postoperative pain following root canal treatment such as psychological condition of patient, gender, preoperative pulpal and periapical status, intensity of preoperative pain, change in the ecosystem of root canals and surrounding periodontium, root canal preparation technique, irrigation protocol, method of working length determination and finally the obturation technique (Torabenjad et al 1988, Topçuoğlu et al 2018). Flare-up is defined as an acute exacerbation of a pulpal or periapical pathosis represented clinically as severe pain and/or swelling following endodontic therapy.

Several articles investigated the influence of preoperative pulpal and periapical status (Negm Mostafa 2021), irrigant agitation (Topcuoglu et al 2018), and number of sessions (Sathorn et al 2008) as well as instrumentation technique (Vera et al 2012) upon postoperative pain. However, to the best of our knowledge, only a few studies investigated the influence of the working length determination method upon postoperative pain (Tuncer and Gerek 2014, Abu Naeem et al 2017). The author hypothesized that working length determination has no influence on the intensity of post-operative pain following root canal treatment.

Subjects and Methods

This clinical study was conducted with approval of the Ethics committee of the Faculty of Oral and Dental Medicine at Ahram Canadian University-Egypt under approval number IRB00012891#2. An informed consent form was formulated to include the aim of study, sequence of steps, benefits, and risks. Upon consent to participate in the study, all data of the participants were stored securely in locked files in areas with limited access to ensure patients' confidentiality.

Eligibility criteria

The inclusion criteria were as follows:

1. Healthy person between the ages of 20 and 60 years old.
2. Both males and females were included.
3. Asymptomatic patients with molars indicated for root canal treatment either due to asymptomatic pulpal disease or intentional root canal treatment for restorative purposes.
4. One molar for every patient.

The exclusion criteria were the following:

1. The offending tooth has previous attempts of pulp therapy or root canal treatment.
2. The offending tooth showing clinical or radiographic evidence of periapical pathosis.
3. Patients received systemic antibiotic in the last month.
4. Patient received analgesic 12 hours before treatment.
5. Offending molar with mobility score ≥ 2.
6. Offending molar with pocket depth ≥4mm.
7. Immature molars.
8. Nonodontogenic pain.
9. Patients with more than one tooth requiring endodontic intervention.

Sample size and grouping:

Seventy-eight patients were included in this study, the power analysis used for pain (Verbal Rating Scale) as the primary outcome. The effect sizes w1 (effect size of GP I) = (0.56) and w2 (effect size of GP II) = (0.44) were calculated based on the results of Tuncer and Gerek 2014. Using alpha (α) level of (5%) and Beta (β) level of (20%) i.e., power = 80%; the minimum estimated sample size was a total of 67 subjects. The number increased to a total of 78 subjects (39 subjects per group) to compensate for a dropout rate of 15%. Sample size calculation was performed using G*Power Version 3.1.9.2.

Demographic data including age, sex, address, phone number as well as medical and dental history were recorded in the diagnostic charts. The chief complaint was obtained in the patient's own words.  Detailed dental history was obtained to reach a tentative diagnosis.

The offending tooth was inspected visually for any caries, cracks, or color change in the clinical crown. Periradicular tests were also performed followed by thermometric evaluation of pulpal neural element using both cold and hot sensibility tests. The offending molar and its attachment apparatus were assessed by a periapical digital radiograph (ATECO sensor, Kaso group, England). 

Clinical procedures

All patients were treated by the same clinician in a single session approach irrespective of the group. All molars were anesthetized either through infiltration in case of maxillary molars or inferior alveolar nerve block in case of mandibular molars using Lidocaine 2% with epinephrine 1:
100,000(Lidocaine HCl, Novocol Pharmaceutical, Ontario, Canada.). A rubber dam was applied, and complete caries removal was followed by opening access using the Endo access bur. The mechanical glide path was established using rotary Ni-Ti Proglider file (DENTSPLY MAILLEFER, Baillagues, Switzerland) in a reciprocation maneuver and then early coronal flaring was performed using Gates Glidden drill #3 in a brushing motion away from the dangerous zone. Supplemental intrapulpal or pressure anesthetic technique was applied for any patient suffering from pain during scouting of the root canals.

Working length determination

In the radiographic group (group I), the estimated working length was calculated from a preoperative periapical radiograph taken using the parallel technique following the Ingle’s radiographic method. One millimeter (mm) was subtracted from the estimated length (for safety factor) and then transferred to K-file #15 which was introduced inside root canal in a watch winding maneuver. Finally, a periapical radiograph was taken using digital periapical radiography (ATECO sensor, Kaso group, England) and receptor holding device (DURR DENTAL, Bietigheim- Bissingen, Germany) for the parallel technique. The difference between the radiographic apex and file tip was measured on the radiograph and the working length was readjusted to be 0.5 mm short from radiographic apex followed by confirmatory periapical radiograph.  A cone fit radiograph was made in all cases of the radiographic group before starting the obturation phase.

In the electronic group (group II), root canals were irrigated using 2.5% sodium hypochlorite NaOCl (Clorox; Egyptian Company for household bleach, Egypt) delivered using 30 Gauge safety Steri Irrigation Tip (DiaDent Group International, Burnaby, BC, Canada) 3 mm below the cementoenamel junction to improve efficiency of the electronic apex locator Root ZX II (J. Morita Mfg. Corp, Kyoto, Japan) during the determination of the working length using manual patency stainless-steel K-file #10 or #15 according to the apical width of root canal. The patency file used was advanced till “apex” point or the “beginning of red zone” on EAL to ensure patency and then drawn back to the “end of green zone” and finally working length was adjusted at 0.5 mm short of the anatomic apex or end of green zone. Working length was continuously monitored until the beginning of the obturation phase.

Root canals were shaped using the ProTaper Next rotary Ni-Ti files (DENTSPLY MAILLEFER, Baillagues, Switzerland) in a continuous rotation maneuver. After apical gauging, root canal preparation completed using X3, X4, or X5 according to initial clinical apical width of root canal. Canals were chemically disinfected using 2ml 2.5% NaOCl between each successive rotary file irrespective to the group. Irrigant was delivered at 3mm coronal to anatomic/radiographic apex, taking into consideration that the needle would not be bound to the canal wall. After complete shaping of the root canals, 17% EDTA was left for 1 min inside root canal for smear layer removal.  Final irrigation with 2ml/canal 2.5% NaOCl was hydro-dynamically agitated using the EndoActivator device (Dentsply Tulsa Dental, Tulsa, OK) with red tips #25/04 inserted and finally root canals were dried with paper points. Root canals were obturated using the cold lateral compaction technique and epoxy resin-based sealer (AH plus). The remaining amount of tooth structure was evaluated to decide on permanent restoration for each molar. Some molars were restored with an intraradicular metallic post (Unimetric, DENTSPLY MAILLEFER, Baillagues, Switzerland) and coronal dual cure composite restoration (PARKELL, Directa Dental Group, USA) prepared for receiving zirconia crowns. Other molars were restored through zirconia endo crown fabrication. Occlusion was then checked and adjusted in all types of restorations performed. All patients received a prescription of 400 mg ibuprofen every 6 hours in case of severe postoperative pain. All participants were instructed to call for emergency treatment at any time in case of severe postoperative pain not relieved by analgesics intake.

Patient questionnaire:

The postoperative pain was recorded using a verbal rating scale (VRS) with well-defined categories at the 5-time intervals after root canal treatment by: 4, 6, 12, 24 and 48 hours (Abu Naeem et al 2017). The postoperative pain assessment will be defined as no pain, mild pain, moderate pain, and severe pain or flare-up, suggesting the acute exacerbation of an asymptomatic pulpal and/or periradicular pathological condition occurring after the root canal treatment. Regarding the level of discomfort, each patient will be asked to categorize his/her pain according to the following criteria:

1. No pain: the treated tooth felt normal.

2. Mild pain: the tooth involved was slightly painful for a time, regardless of the duration, but there was no need to take analgesics.

3. Moderate pain: the tooth involved caused discomfort and/or pain, which was either tolerable or was rendered tolerable by analgesics.

4. Severe pain: the pain caused by the treated tooth disturbed normal activity or sleep necessitating an emergency appointment as analgesics had little or no effect.

Statistical analysis:

Numerical data were explored for normality by checking the distribution of data and using tests of normality (Kolmogorov-Smirnov and Shapiro-Wilk tests). Age data showed normal (parametric) distribution while pain scores data showed non-normal (non-parametric) distribution. Data were presented as mean, standard deviation (SD), median and range values. For parametric data, Student’s t-test was used to compare between age values in the two groups. For non-parametric data, Mann-Whitney U test was used to compare between the two groups. Friedman’s test was used to study the changes within each group. Dunn’s test was used for pair-wise comparisons when Friedman’s test is significant. Qualitative data were presented as frequencies and percentages. Chi-square test was used for comparisons between the groups regarding qualitative data. The significance level was set at P ≤ 0.05. Statistical analysis was performed with IBM SPSS Statistics for Windows, Version 23.0. Armonk, NY: IBM Corp.

Results:

Seventy-eight patients were enrolled in this study. Two patients from group I (5.1%) and one patient from group II (2.5%) were lost to follow-up and didn’t show up for the second visit. Please refer to Figure 1. No significant difference was found between groups regarding the age (P-value = 0.736), gender (P-value = 0.907) and offending arch (P-value=0.734) as well as clinical diagnosis (P-value =0.210). Please refer to Table 1.

Table 1: Mean, standard deviation (SD), frequencies (n), percentages and results of Student’s t-test and Chi-square test for comparisons between base line characteristics in the two groups.

Figure 1: CONSORT 2010 flow diagram of the study.

Pain scores and changes within each group:

In the radiographic group, 94.9% participants were available for follow-up while 97.5% participants in the electronic group were available for follow-up. After four hours, Group I showed statistically significant higher pain score than Group II (P-value <0.001, Effect size = 1.608). After six hours, Group I showed statistically significant lower pain score than Group II (P-value = 0.012, Effect size = 0.274). After 12 hours; Group I showed statistically significant higher pain score than Group II (P-value <0.001, Effect size = 1.126). After 24 as well as 48 hours; there was no statistically significant difference between the two groups. Please refer to Tables 2 and 3.

Table 2: Descriptive statistics and results of Mann-Whitney U test for comparison between pain (VRS) scores in the two groups.

Table 3: Descriptive statistics and results of Friedman’s test for comparison between pain scores at different times within each group

Analgesics intake:

Group I showed statistically significant higher percentage intake of analgesic tablets than Group II (P-value <0.001, Effect size = 3.305). Group I was 3.305 folds prone to intake of analgesics than Group II. Please refer to figure 2.

Figure 2: Bar chart representing percentage intake of analgesic tablets in the two groups.

Discussion:

A special emphasis was given in the current study to the influence of the radiographic and electronic methods of working length determination upon postoperative pain incidence and the intensity as well as the frequency of analgesic intake. Some studies investigated the accuracy of the radiographic and electronic methods of working length determination (Vieyra et al 2010, Martins et al 2014). The findings of other studies reported excellent length determination through hybridization between electronic and radiographic method (Brunton et al 2002, Hoer and Attin 2004) whereas; several other studies reported that working length can be accurately detected using electronic method alone (Saad and al-Nazhan 2000, Kim et al 2008).

Postoperative pain was calibrated using the numerical 4-points verbal rating scale with scores of no pain, mild pain, moderate pain relieved by analgesic and severe pain and/or flare-up non-responsive to analgesics/seeking an emergency appointment (ElMubarak et al 2010, Erdem et al 2018, Negm M 2021). Several studies evaluating pain intensity used a visual analogue scale (Oshima et al 2009), but the author used verbal rating scale in the current study due to its reliability and validity. We evaluated pain intensity at 5-time intervals within 48 hours in agreement with Tuncer and Gerek in 2014. Molars only were included in the current study as previous studies showed increased pain intensity in molars following root canal treatment (Nagendrababu, Gutmann 2017).

All treatments were performed by the same clinician who has more than 15 years of experience in endodontics to exclude the operator as a variable influencing postoperative pain and to be able to overcome molars' complexities and complete the treatment in a single session approach (Nagendrababu, Gutmann 2017). Several studies reported that presence of preoperative pain is a significant predictor of postoperative pain and that’s why we recruited only asymptomatic patients in the current study (O'Keefe 1976, Negm M 2021). A mechanical glide path was established rather than manual glide path to minimize postoperative pain (Pasqualini et al 2012). Because apically extruded debris is one of the major etiologies of postoperative pain, early coronal flaring was performed to minimize apically extruded debris through the creation of wider space for irrigant delivery at an early stage of treatment. This is in addition to straight line access either to primary curvature or physiologic terminus of the root canal (Borges et al 2016). Working length determination was postponed till after coronal flaring as several studies reported a decrease in working length by a fraction of the mm following coronal flaring (Schroeder et al 2002). Any patient who received analgesics 12 hours prior to treatment has been excluded from the current study to avoid influence upon postoperative pain (Jalalzadeh et al 2010).

 In the radiographic method group we used radiographic apex as our landmark and then subtracted 0.5 mm to adjust the working length as many studies reported the presence of minor apical constriction 0.5 mm short from the apex (Green D 1955, Martos et al 2009). Dr. Schilder mentioned in 2006 that he did not encounter a case of endodontic failure due to root canal system overfilling but many cases of failure had been encountered due to underfilled root canals (Schilder 2006). Root ZX was used in our study due to it being considered the benchmark of apex locators with reported accuracy between 75-97.3% (Tselnik et al 2005, Plotino et al 2006). In the electronic group, we reached the apex point and then have withdrawn the file back to the end of the green zone as per the manufacturer recommendations to obtain an accurate reading (J.Morita corp. 2004). Reliable and accurate readings cannot be obtained electronically without the implementation of the apical patency concept (Jorge et al 2012). In the electronic group, we also subtracted 0.5 mm as ElAyouti et al found that the minor apical constriction exists 0.2 mm short from the major apical diameter regardless of the case being of a maxillary or mandibular molars (ElAyouti et 2014).

Interestingly, the radiographic group has shown a statistically significant higher pain score and more analgesic consumption than the electronic group at the 4 hours interval following root canal treatment. The null hypothesis was thus rejected. The higher pain score in the radiographic group can be attributed to the variation between the radiographic and the anatomic apex to which the apex locators are concerned (Gordon and Chandler 2004). Nekoofar et al in 2006 mentioned that electronic apex locators can localize the point where periodontal ligament begin outside the root and that the use of generation (X) to describe and classify the devices is neither beneficial nor scientific and used for marketing issues.  Also, this statistically significant difference in pain score can be attributed to the lack of apical patency concept implementation in the radiographic group (Jorge et al 2012).  After 6 hours postoperative; the radiographic group showed a statistically significant lower pain score than the electronic group and this is logically accepted due to the analgesic effect which usually lasts for 4-6 hours (Dionne et al 2002). After 12 hours postoperative, the radiographic group showed a statistically significant higher pain score than the electronic group and this can be attributed to the termination of the analgesic effect that only lasts that long.  After 24 as well as 48 hours; there was no statistically significant difference between the two groups. None of the participants called for an emergency appointment at these time intervals.

The results of the present study contrasted with past studies that showed no significant difference between radiographic and electronic group (Tuncer and Gerek 2014, Abu Naeem et al 2017). The difference in results can be attributed to the difference in the apical termination point of the preparatory and obturation phases. Tuncer and Gerek in 2014 terminated their preparation and obturation 1mm short from the radiographic apex, but in the current study, it was terminated 0.5 mm short from radiographic apex. Our study shows the resolution of pain 24 hours after treatment in agreement with Pak and White in 2011. Maximum pain score occurred at 4 hours after treatment, and this can be attributed to the natural disappearance of the anesthetic effect and irritation of periodontium from the mechanical procedure.

Conclusions:

Under the limitations of the current study, the following conclusions could be withdrawn:

1. Both postoperative pain and analgesic intake were influenced by the method of working length determination.
2. Electronic method of working length determination is a non-therapeutic method for reducing postoperative pain.

Recommendations:

1. An observational follow-up cohort studies are needed to determine the long-term effect of working length determination method upon the clinical and radiographic outcome.
2. More clinical studies are required to examine the influence of apical patency implementation upon the clinical and radiographic outcome.

Conflict of Interest

The authors declare that there is no conflict of interest

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.