Successful root canal therapy depends mainly on controlling pulp space infection, which can be achieved by good cleaning and shaping followed by three-dimensional filling of the entire root canal system [1,2,3, 6]. However, these steps are frequently challenged by the characteristics of root canal anatomy such as isthmuses. It is reported that the apical 5 mm region of the mandibular first molar mesial root canals tends to have the highest prevalence of isthmuses, which appear long oval in cross-sectional shape with the largest diameter in the buccolingual direction at 1 to 5 mm, which is not evident on periapical radiographs [4, 9,10,11]. Several studies have pointed out that increased risk of failure of endodontic treatment is associated with the presence of isthmus-containing canals, owing to the fact that this irregular area may harbor a significant number of microorganisms that might lead to treatment failure [22,23,24]. In the present study, the null hypothesis was rejected since significant differences were found among the obturation techniques.
Consequently, the majority of previous similar studies have used extracted natural molar teeth to reproduce the clinical situation of isthmus filling [9, 15, 18]. However, the wide variability of isthmuses makes standardization between groups difficult [10, 18]. Instead, custom-made 3D-printed artificial molar teeth, which had a fused mesial root with canals and a band-shaped isthmus in the apical 5 mm, were used in the present study.To the best of our knowledge, this is the first report to use these artificial teeth in a band-shaped isthmus study.The artificial teeth selected have the same type, volume, length and dimensions, guaranteeing the homogeneity of experimental groups, which makes it possible to obtain more reliable results in terms of the void percentages in the isthmus regions after obturation.
Although uniform samples were used in the experiment, the prepared canal shapes may have an influence on the obturation results. Therefore, in order to assess whether the shape of the isthmuses in the mesial root after preparation were consistent, three prepared artificial teeth were randomly selected to obtain micro-CT images as previously reported [21, 25]. The acquired images of the three samples were superimposed, and the results showed no obvious displacement in the location of the isthmus’ roof and floor of specimens. Thus the tooth-to-tooth variation can be considered negligible. This may probably because the original canal and isthmus space were quite wide, thus, the conventional root canal preparation had a limited impact on the root canal space of 3D-printed artificial teeth. Consequently, we could maintain a consistent isthmus space volume in all specimens.
Voids percentage has been used as a method to evaluate the quality of root canal filling [14,15,16,17,18,19,20], because bacteria and their byproducts may remain at a site in non-obturated areas and affect endodontic treatment outcome. Therefore, adequate sealing of the root canal system is required to prevent the spread of microorganisms and toxins [26].
Previous studies have reported slightly different criteria for dividing the root area to calculate the void percentage. Iglecias et al. [15] divided the root into three areas: apical, middle, and cervical, to assess the void percentage. Kim et al. [25] divided the apical region into 1–5 mm and 5–9 mm areas for measurement, because they considered this measurement to be more clinically relevant in root canal treatment success than measurements of the full canal length. Somma et al. [26]analyzed the entire root canal system as a whole without division into thirds. In the present study, a similar approach [26]was preferred and a statistically significant difference was detected between different obturation techniques. It is worth noting that according to the findings of the present study, all three obturation techniques were subject to failure, i.e., none were able to completely fill the isthmus region, which is similar to the findings of most previous studies [7, 14,15,16,17,18,19,20].
In this study, there were significantly more voids in the samples treated by the SC technique than with the CWC and LC methods in filling of isthmuses, with a percentage of 22.98 %. This result was consistent with that of several earlier studies [7, 17]. This may be because, unlike the other two techniques, the SC technique lacks vertical and lateral pressure during the obturation procedure. It just barely allows the use of a GP cone tapered according to the final shape of the canal, working as a key-and-lock system. Without pressure, it is difficult for the filling materials to enter the isthmus regions, therefore, there was a possibility of more and larger voids forming in the isthmus in the SC technique group. This may be the main reason why more obvious voids can be observed under the operating microscope compared to the CWC and LC groups. In this study, the middle of the isthmus, which was 3 mm away from the apex,were horizontally crossed to observe voids. Through the microscope observation,it is obviously to find that among three obturation techniques, the middle part of the isthmus was mostly filled with root canal sealer even in CWC group, which indicated that the filling technique based on sealer was worthy of attention.
However, other authors reached different conclusions in their studies compared with this research. Iglecias et al. [15]measured the volume of voids in the mesial root of the human mandibular molar in the apical one-third showing Vertucci Type II configuration canals, and found no significant difference between the SC and CWC groups in the apical area. In addition, a similar study conducted by Keles et al. [18] used micro-CT to measure the volume of voids in the band-shaped isthmuses in the mesial root of human mandibular first molars filled with AH plus sealer, and found similarities between the SC and CWC groups. Somma et al. [26] assessed straight canals, and they found no differences in root canal obturation or void distribution when comparing these two techniques. Voids and gaps within root canal fillings can be influenced by several factors [18], such as the experience of the clinician, the root canal mechanical preparation technique, differences in irrigation methods, the selected filling technique, the physical properties of the selected sealer, the focused anatomy configuration of the root canal system, the use of different scanning devices, diverse calculation softwares and so on. We speculated that these factors may be reasons for the differences between other studies and the outcomes of this research.
In addition, it has been reported that the SC technique provides inadequate obturation in oval root canals [7, 27, 28]. A micro-CT analysis of band-shaped isthmuses revealed that both the roof and floor of this anatomy exhibited a long, oval cross-sectional shape [10], thus lacking a full adaptation to the oval canal walls would complicate the obturation of isthmuses using the SC technique. The CWC technique has been reported to provide a more homogenous filling with significantly fewer voids when compared to either the SC or the LC technique [7, 18, 19, 21, 26, 28]. Homogeneous thermoplasticized GP with a certain liquidity, suitable vertical pressure, and tightly condensed backfilling make it more effective in filling complex root canal system [26,27,28,29]. In the present study, significant differences in the voids and more complete filling quality were found in the band-shaped isthmus region when comparing CWC with the cold-filling techniques. The better sealing,the better outcome for root canal therapy. Therefore, CWC technique may lead to a better prognosis for the tooth with band-shaped isthmus, which needed additional clinical studies to investigate.
However, Li et al. [30]demonstrated that the obturation quality, long-term outcome, and postoperative pain prevalence were similar between warm GP and cold LC obturation through a systematic review and meta-analysis. In addition, a retrospective analysis [31] reported that the overall success rate of nonsurgical endodontic treatment using EndoSequence Bioceramic Sealer and a single cone obturation technique was 90.9 %, which was higher than the reported success rate of initial root canal treatment (89.1 %) and retreatment (85.6 %) in a study with a large sample size [32]. Moreover, in a recent non-randomized clinical trial [33],the author used CBCT images and PA radiographs as loose criteria to compare the success rate of root canal treatments undertaken using a single cone technique and a calcium silicate sealer (BioRoot™ RCS) with a warm vertical condensation technique and AH plus, and the results showed no difference between those two groups. These studies showed that although the CWC technique achieved a lower void percentage in isthmus areas in the present study, cold filling techniques still resulted in a good clinical outcome in some aspects, especially the SC technique. Thus, further research is needed.
There were several limitations to this study. The 3D-printed artificial teeth lacked the microscopic structures characteristic of the dentin of natural teeth such as dentinal tubules, therefore, the adhesion between the endodontic filling material and the root canal wall could not be completely reproduced. Another consideration was that heating could alter the properties of the sealer, thus impacting on results of obturation. In addition, the outcome of this study mainly based on one band-shaped isthmus type, but the different isthmus types such as V-shaped or other irregular configurations still remained important factors for the void distribution produced by root canal filling techniques.Therefore,further investigations on different isthmus types should be conducted to get more information on obturation efficacy with different techniques for treatment of these anatomically challenging variations.