Influence of the internal anatomy on the leakage of root canals filled with thermoplastic technique
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The aim of this paper is to evaluate the influence of the internal anatomy on the leakage of root canals filled with the thermoplastic technique.
Materials and methods
The upper central incisors (UCI) and mesial roots of the lower molars (MRLM) (n = 12 each) were tested regarding leakage using the gas-enhanced permeation test (GEPT) after root filling. The quality of the root fillings was assessed using micro-computed tomography (μCT) by superimposing scans before and after treatment to calculate unfilled volume. The calculated void volume was compared between the groups and correlated to the measured leakage values. Data were analyzed using t test and Pearson’s correlation tests (p < 0.05).
The mean void volume did not differ between UCI and MRLM (13.7 ± 6.2% vs. 14.2 ± 6.8%, respectively). However, significantly more leakage was evident in the MRLM (p < 0.001). While the leakage correlated highly to the void volume in the MRLM group (R 2 = 0.981, p < 0.001), no correlation was found in UCI (R 2 = 0.467, p = 0.126).
MRLM showed higher leakage values, which correlated to the void volume in the root canal fillings.
Care should always be taken while doing root canal treatments, but attention to teeth with known/expected complex root canal anatomy should be considered.
KeywordsEndodontics Leakage μCT Root canal anatomy
The controversy about the importance of leakage testing in root-filled teeth has been initiated two decades ago and has remained a continuous source of academic dispute ever since. So far, this debate did not openly discuss all problematic aspects and how to solve them but instead was blocked by some endodontic communities. Some scientific journals even abandoned any publishing regarding this potentially important topic . This discussion dates back to 1993, when the efficacy of endodontic leakage testing was questioned for the first time . In their original investigation, a fluid filtration method was used, and they found that the rate of leakage decreased over time. They concluded that substrate infiltration might be influenced by the entrapment of the substrate used throughout the path of leakage, resulting in a potential blockage of this path. In addition, it was reported that the temperature increase facilitate or even enhance the leakage values. Another important observation or—more exactly—claim was that gas bubble entrapment could retard the leakage testing process. This late observation led to the suggestion to apply vacuum on the counterpart to overcome such problems.
Another method to test leakage of filled root canals is the bacterial leakage setup using two chambers. This model was first adapted in the field of endodontics in 1980 , and ever since, many studies were carried out based on this original model [4, 5, 6, 7, 8, 9]. Leakage was indicated to happen within weeks . However, these findings were not corroborated by histological studies, which indicated that the bacterial presence in the apical canal portion was negligible or not present at all, even if the root obturation was exposed for a long time period, once the root canal filling was properly performed [10, 11]. A systematic testing of this method  indicated a possible bias resulted in false-positive leakage phenomena, which were detected and influenced by unwanted routes, such as the sample-embedding interface. This may have resulted in unwanted gaps and pathways, leading to measurement and interpretation bias.
Micro-computed tomography (μCT) is able to assess root canals in vitro, at different stages throughout the course of the treatment. This technique can assess three-dimensionally the volume, which is either removed or added to root canal spaces . It allows therefore for a quality assessment of root canal fillings by means of their capability of occupying the space within the root canal system . However, the method per se cannot express the leakage status of the sample, but it can indicate the radiographic quality of the root canal filling and serve as a valuable surrogate parameter as such for the internal seal and therefore indirectly for leakage, as one could hypothesize.
In summary, there is no doubt that current leakage testing methods as described above are still lacking validated setups and have never addressed possible leakage routes nor assessed the initial status of tested samples. In addition, leakage testing in vitro must correspond to in situ findings of properly filled root canals. For these reasons, recently published editorial in the International Endodontic Journal has invited and encouraged investigators to establish new experimental models for assessing root canal filling quality, in terms of techniques and materials, using more reliable and reproducible methods .
Based on different shortcomings on leakage evaluation techniques as described above, the current investigation aimed to compare the leakage after root canal filling of the teeth with simple root canal anatomy to corresponding values in counterparts with complex canal anatomy. In addition, the leakage values were correlated to the corresponding root canal filling quality as assessed by μCT evaluation. It was hypothesized that root canals with complex anatomy would have more voids when obturated, which results in increased leakage.
Material and methods
To study the influence of root canal anatomy on the root filling quality and sealability, teeth with two different root canal morphologies were selected based on μCT pre-scans. A group of upper central incisors (UCI) with a single root canal (n = 12) and a group of lower molars (MRLM) with the complex anatomy of mesial roots containing two canals and an isthmus in between (n = 12) were selected for this study. All the teeth were selected from the department’s collection of extracted teeth. Written informed consent was obtained from all donors according to the recommendation of the Swiss Academy of Medical Science . All these teeth were extracted for reasons not related to this study and stored in 0.2% thymol at 5 °C. Personnel handling the teeth applied all necessary precautions for infection control. Ethical guidelines were followed , and anonymization was performed in accordance with state and federal laws . The teeth were intact with no previous root canal treatment and visible signs of caries or cracks in their roots. All roots had a fully formed apex and were extracted for reasons that were not related to this study. All the teeth were pre-scanned with a μCT device (μCT 40: Scanco Medical, Brüttisellen, Switzerland) with an isotopic resolution of 72 μm at 70 kV and 114 μA, to confirm their suitability for the study purpose as mentioned above.
Root canal preparation
A dentist who was not experienced in root canal treatment and was not aware of the study aims did all root canal treatments. The dentist was taught the technique of rotary root canal preparation (ProTaper Universal system) and the continuous wave of condensation technique for root canal filling.
Root canal preparation was then accomplished under magnification using a stereomicroscope (Stemi 1000, Zeiss, Oberkochen, Germany). An access cavity was prepared through the crown with an 80-μm grit-size diamond bur (Bur 837 KR, 8614, Intensive SA, Grancia, Switzerland) attached to a high-speed handpiece (Sirius, Micro Mega, Besancon, France). The initial working length was determined by the first appearance of a Hedström file size 10 (Dentsply-Maillefer, Ballaigues, Switzerland) observed at the apical foramen. The working length was further confirmed by digital X-rays (Digora Optime, Soredex, Münchenstein, Switzerland). The canals were then prepared using a chemo-mechanical preparation approach according to the manufacturer’s recommendations with the ProTaper rotary system (ProTaper Universal, Dentsply-Maillefer, Ballaigues, Switzerland), ran on a rotary motor (Endo-Mate TC2, NSK, Tochigi, Japan). Irrigation was performed using a side-vented needle of 0.30-mm diameter (Max-i-Probe; Hawe-Neos, Dentsply, Gioggio, Switzerland) inserted to 1-mm shorter than the working length, with repetitive rinsing of 1 ml 1% hypochlorite solution after each file size preparation. Canals were prepared to file size F3. After a final irrigation with 5 ml 17% EDTA over 3 min, the canals were dried with paper points and the buildup was accomplished as described in the “Sample embedding” section.
A cylindrical coronal buildup of 11-mm diameter and 10-mm height was cast in a custom-made Teflon mold, which covered the coronal 7 mm of the tooth (Fig. 1c). Before proceeding with the buildup and embedding process, the pulp chamber was covered with a cotton pellet and a temporary filling (Cavit, 3M ESPE, Seefeld, Germany), to facilitate finding the canals. The coronal part was then conditioned with a bonding system (Clearfil SE Protect, Kuraray America Inc., New York, USA) and light cured for 20 s in a light cure chamber (Spectramat, Ivoclar-Vivadent, Schaan, Liechtenstein). The buildup was made using a dual-cured composite buildup material (Luxa Core Automix, DMG, Hamburg, Germany). Samples were then light cured for 5 min.
All tooth samples were then embedded in custom-made PVC rings, with an outer diameter of 15 mm and an inner counterpart of 10 mm and a thickness of 3 mm. These rings were conditioned on their inner surface by sandblasting with 50-μm aluminum oxide (Benzer-Dental AG, Zurich, Switzerland).
The teeth, with their buildups, were embedded with a light-cured nail buildup material kit (Sina, Shenzhen Cyber Technology Ltd., Mainland, China). The nail buildup gel material consisted of a primer, gel, and glaze. The teeth, as well as the rings, were primed on their inner surface and subsequently light cured for 4 min in a light-cure chamber (Spectramat, Ivoclar-Vivadent, Schaan, Liechtenstein). The parts were then held together in position in a custom-made rubber carrier, made of a putty material (Optosil, Heraeus Kulzer GmbH, Hanau, Germany). The gel was applied in one increment on the topside to fill the space between the ring and sample and was then light cured for 4 min. The sample was turned upside down, the gel was optimized and extended on the root surface, leaving the last 3 mm free. It was then light cured again for another 4 min. Care was taken not to allow excess material formation on the upper and lower ring surfaces. Finally, a glaze layer was applied to both the upper and lower gel surfaces, to strengthen and eliminate any imperfections in the embedding. The entire embedded sample was finally light cured for 4 min one final time.
μCT analysis of root canal treatments
Root canal filling
The root canal filling was performed after the buildup was fabricated and the samples were scanned for the first time with the μCT and the baseline measurement determined with gas-enhanced permeation test device (GEPT). An access through the buildup was then established, and the canals were recapitulated to the full working length. Patency of the canal was ensured by inserting a headström file size 10 (Dentsply-Maillefer, Ballaigues, Switzerland) to 1 mm beyond the apex followed by irrigation with 5 ml EDTA 17%. The root canal filling was made using the continuous wave of condensation technique. After confirming the master point gutta-percha fitting to full working length (F3 GP, Dentsply-Maillefer, Ballaigues, Switzerland) with an X-Ray, it was soaked into an epoxy resin root canal sealer (MM Seal, Micro Mega, Besancon Cedex, France) and was then presented to the full working length in the canal. After cutting the master point down to 3–4 mm coronal to the apex with the aid of a vertical thermal plugger (Xtra Fine, 0.04 taper, System B, Sybron Endo, Ca, USA) a gutta-percha back fill was applied (Obtura III, Sybron Endo, Ca, USA).
The access cavity was sealed with a temporary filling (Cavit, 3M ESPE, Seefeld, Germany) with a cotton pellet underneath and left to dry in a humid box at 35 °C for 24 h. In between tests, the samples were kept in a humid box and at a temperature of 35 °C (Heraeus UT6420, Thermo Fisher Scientific, Dreieich, Germany).
Gas-enhanced permeation test
After the sample embedding in the PVC rings and before access cavity reopening, to determine the baseline of the sample and to ensure the embedding tightness
After root canal filling with the access cavity opened, to measure the leakage through the root canal filling
After the root apex was sealed from its outer surface, to ensure no leakage happened through unwanted routes within the embedding components. This took place at the end of testing. The last free 2 mm of the apex was sealed with composite (Filtek Supreme, 3M ESPE, Seefeld, Germany) after a standardized conditioning and bonding (Syntac Classic, Ivoclar, Vivadent, Schaan, Liechtenstein). Subsequently, the GEPT was assessed again for a last time to ensure the baseline value could be re-established
Data extraction for GEPT
Pressure at 1200 s
Pressure at 2400 s
The baseline value was then subtracted from the value after treatment to calculate the absolute leakage of each sample.
The collected water volume in the Eppendorf tube was also weighed and converted into volume (ml).
To compare groups for the resulting filled canal volume, as well for their performance under GEPT, t test was applied (p ≤ 0.05). To test for correlation between the GEPT measurements and the root canal filling defective volume, as well the GEPT measurements and the permeated fluid volume in milliliters, the Pearson correlation coefficient was applied for both situations with the probability of type I error set at α = 0.05.
ᅟRoot canal filling defects as well as the GEPT performance for both groups
Root canal filling defect (%)
Defect range (%)
Effective GEPT (hPa/min)
GEPT range (hPa/min)
Effective fluid infiltration (ml)
Fluid infiltration range (ml)
13.74 ± 6.23 A
0.102 ± 0.072 A
0.049 ± 0.036 A
14.17 ± 6.83 A
0.321 ± 0.154 B
0.144 ± 0.073 B
The correlation between the measured pressure difference slope and the infiltrated water volume was high (R 2 = 989, p < 0.001). All the samples exhibited leakage values close to their baseline values once the apices were sealed again, with the differences barely detectable and ranging between 0.00 and 0.01 hPa/min.
The aim of a root canal treatment is to obtain a tight root canal obturation (seal) after properly debriding and disinfecting the root canal system . Leakage has always been a controversial issue in endodontics. Clinical studies suggested different opinions concerning the importance of the coronal seal and the root filling tightness, or comparing their role preventing leakage against each other. A retrospective cohort study highlighted the importance of the root canal treatment quality as the determinant factor for success . Another study  observed better success rates once a coronal seal was achieved, regardless of the quality of the root filling. Recently, retrospective clinical studies emphasized the importance of both seals to be established to ensure the best outcome of the root canal treatment [23, 24]. A systematic review  found evidence of better healing outcomes when both, root canal treatment and coronal restoration, were adequately provided. On this basis, a proper root canal filling can form a second defense line against root canal recontamination. All dentists agree that micro-leakage is one of the largest risk factors contributing to the development of periapical periodontitis . Some overlap between failure related to root canal disinfection and proper root canal filling can be observed clearly in clinical practice. Those root canals show signs and/or symptoms of periapical disease, although the radiographs show a properly executed root canal filling.
The present study aimed to shed light on the cause and effect between root canal filling quality and leakage. The detected root canal filling defect measured as void volume, in both UCI and MRLM, was relatively high accounting for 13.7 and 14.2%, respectively. This finding corroborated a previous study where the root canal filling defect was also assessed . Another study validating the percentage of canal-filled area in oval-shaped canals with different techniques found it to range between 37.1 and 98.5% . Thus, the percentage of canal-filled area is comparable to other studies and can be considered reliable. The high defect volume in the present study may be mainly to the limited experience of the operator who did the root canal filling. On the other hand, this fact has resulted in an equal and normal distribution of the detected voids in both groups redundant and therefore not necessary (equal and normal distribution says that there were no differences). Another observation was the significantly higher leakage in the MRLM group. The leakage in this group was highly correlated to the detected void volume, in contrast to the UCI group, where no correlation was found. This indicates that voids do not necessarily have to correlate with leakage, unless they are through and through. This finding corroborates the suggestion of Wu and companions that a minimum void diameter through the whole root filling length is necessary to allow for a bacterial leakage [2, 5]. In the teeth with complex anatomy, as can be found in the mesial roots of the lower molars for example, the voids are most likely located at areas that are hard-to-reach by the root canal filling (fins and isthmus). Hence, they maintain their continuity along the whole root length (Fig. 2). In his series of periradicular surgeries, von Arx demonstrated a high frequency of isthmuses in resected lower molar mesial roots at the resection level. None of the inspected isthmuses were filled, emphasizing the difficulty of orthograde instrumentation and root filling of the canal isthmus . In contrast, in a simple root canal anatomy like in the UCI group, a void may exist but a seal can still be established at any level within the root canal, i.e., the void can be entrapped within the root filling itself (Fig. 2).
There was a slight deviation from zero in the correlation between the measured slope values and the collected fluid volume. This may be explained by the loss of some infiltrated fluid within the sample or by some evaporation of the infiltrated fluid under pressurized conditions in the lower chamber. However, the correlation was still high (R 2 = 989, p < 0.001).
The applied test methods have been previously validated and proved to be non-destructive, accurate, and reproducible [19, 29]. Combining both the leakage permeation test and the micro-computed tomography helped in establishing a correlation between the root filling and its sealing ability. The 3D reconstruction gave an overall idea about the defect distribution and the possible defect continuity, which is the determinant factor in the leakage process.
The theory that applying pressure during the leakage test might create voids within the root canal filling can be ignored, at least within the current setup, for the following reasons: first, the applied pressure is minimal and close to the atmospheric pressure; second, the pressure was applied after 24 h, i.e., after full setting of the sealer used; and finally, suggested voids will happen in both groups equally and will result in no difference between the groups.
The tested samples acquired their initial leakage status (before the establishment of the access cavity through the buildup) once their apices were properly sealed again. This indicates the detected leakage can only be explained by one pathway, namely through the canal itself.
Though these were in vitro tests, they offer a good alternative to clinical studies where 3D radiography is not precise enough to establish the right correlation. A major limitation of the current study is the link to daily clinical practice. The exact leakage threshold with clinical relevance is not known. These studies help in providing an overall view of the problem and its possible causes and solutions. In clinical practice, each case must be dealt with individually, taking into consideration that each tooth is a possible candidate for failure after a root canal treatment.
In summary, within the limitations and under the conditions of the current investigation, the mesial roots of the lower molars showed higher leakage values, which highly correlated to the root canal filling quality (i.e., loads of voids). This corroborates with clinical findings where apical periodontitis was significantly more often detected in the molars with root canal filling not extending to the radiographic root apex . Di Filippo and companion found a high correlation between poor root canal filling quality and the presence of periapical periodontitis. This emphasizes again the importance of root canal filling quality . A high-quality root canal filling will ensure a higher success rate of the root canal treatment. Further, it is of outmost importance to develop adequate laboratory models to measure these conditions.
Under the current conditions of this investigation, the hypothesis was accepted with regard to the leakage performance in root canals with complex anatomies but not for the quality of root canal fillings. The mesial roots of the lower molars, presenting a complex anatomy, showed the highest leakage values and correlated to the root canal filling quality. The combination of the current leakage setup and the quantitative μCT analysis seems to present a promising way to improve the understanding of and to confirm the pathways through which leakage occurs under different anatomical variations and simulated clinical conditions. Therefore, it merits further investigation and the field is open for scrutiny.
The work was supported by the Clinic of Preventive Dentistry, Periodontology and Cariology, Center of Dental Medicine, University of Zurich, Zürich, Switzerland.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All procedures performed in studies involving human teeth were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The current research protocol was according to the guidelines of Good Clinical Practice (ICH, Geneva, Switzerland) and did not alter the treatment plan of any of the involved patients. The institutional ethics committee approved the procedures.
The patients gave an informed consent that their extracted teeth could be used for study purposes anonymously.
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