Influence of obturating techniques on root dentin crack propagation: A micro-computed tomography assessment.

AIM: The aim is to assess and compare the microcrack formation in radicular dentin after obturating the root canals with cold lateral condensation (CLC), warm vertical condensation (WVC), and injectable gutta-percha (IGP) techniques using micro-computed tomography (CT).
MATERIALS AND METHODS: Human extracted mandibular premolar teeth (n = 60) were haphazardly assigned based on the obturation technique into three experimental groups (n = 20 each). Root canals are cleaned and shaped with M Two rotary files and 3% sodium hypochlorite irrigant. Cross-sectional images were taken with Micro-CT to record the baseline defects present on root samples. After root canal obturation either with CLC or WVC or injectable obturation techniques, micro-CT images were captured again to analyze the increase in the number and type of dentinal defects. Statistical analysis of data was performed using the Mann-Whitney U test and the Mcnemar test at 5% significance level.
RESULTS: An increase in the number of radicular micro-cracks was identified in samples obturated with lateral condensation technique (1.66%). No change in the percentage of micro-cracks was recorded after obturation with warm vertical or injectable guttapercha (IGP) techniques (P > 0.05). The three obturation techniques were not statistically different in the occurrence of micro-cracks after obturation.
CONCLUSION: The three obturating techniques tested showed no significant increase in radicular dentin defects' occurrence or propagation. Copyright:

INTRODUCTION

An important goal of contemporary primary endodontic treatment is to achieve and maintain sterile root canals by chemical and mechanical cleaning and by complete sealing of the canals both coronally and apically.[1]

Procedures in endodontics such as biomechanical preparation, obturation, retreatment, and preparation of post space are likely to develop microcracks in the root dentin.[2] Rotary kinematics could be attributed to most of the microcracks observed after the instrumentation of the canals.[23] These micro-cracks can propagate upon any additional stress concentration during obturation or post endodontic restorative procedures and result in vertical root fractures (VRFs). VRFs can appear in any tooth, but upper premolars are the vulnerable teeth because of their high convex-shaped roots.[4]

The compaction forces applied to achieve a good seal during obturation can impact root canals by generating tensile stresses. When these applied compaction forces are above the dentin's structural limit, the formation of craze lines leading to VRFs may result.[4]

Compaction techniques such as cold lateral condensation (CLC) and warm vertical condensation (WVC) are the most proposed procedures to enhance the overall obturation quality. Even though CLC was proven clinically effective and used for many decades, forces generated by spreader while creating space for accessory cones may exert tensile stress on root dentin, leading to microcracks development in the root dentin.[5] In warm vertical compaction procedure, an improved adaptation of obturation materials to root canal walls can be seen, but the forces generated during vertical compaction using pluggers can create wedging forces in all directions leading to the propagation of cracks in the root dentin.[6] Currently, many clinicians perform root canal obturations utilizing injectable thermoplasticized gutta-percha (GP) filling procedures due to their advantage of faster obturation and better adaptation of the GP to canal walls than the lateral compaction technique.[7]

Few researchers studied the effect of obturation techniques on microcracks in root dentin using sectioning and microscopic examination.[26] This destructive methodology did not permit the assessment of preoperative cracks in root dentin and cracks throughout the length of the root. The root sectioning technique for observation of cracks yields fewer slices per tooth to evaluate, which might result in information loss.[2]

Advances in imaging technology and the invention of 3-dimensional (3-D) micro-computed tomography (CT) lead to the confirmatory knowledge of microcrack development and propagation.[5] Micro-CT is an accurate and reproducible method that enables the analysis of more slices per sample, explaining why micro-CT evaluation shows a high frequency of dentinal micro-cracks compared to root sectioning models.[4] Very few studies are available on micro-crack propagation and its relation to obturation techniques using the non-destructive evaluation method. Hence, the present study assessed and compared the radicular dentinal defects formation while obturating the canals either with CLC, WVC, or injectable GP (IGP) obturating techniques using micro CT evaluation.

MATERIALS AND METHODS

The study protocol was accepted by the biomedical research and ethics committee of the university with reference number D178601023. Sixty single-rooted mandibular premolars recently extracted for orthodontic or periodontal reasons were selected. Collected teeth were preserved in 0.1% thymol solution at 35°C. After microscopic and radiological examination, teeth free of caries, cracks, or defects, and roots that closely matched in mesiodistal and buccolingual measurements were included in the study. Teeth with previous root canal obturation, resorption defects, and severe curvatures were excluded from the study. All the samples were mounted by embedding roots in acrylic resin, and polyvinyl siloxane was used as a spacer to simulate periodontal ligament.

Sample size estimation

The sample size was estimated at 5% significance and the power of the study at 95%. It was decided to research with 20 samples per group.

Canal preparation

For all the samples, uniform oval-shaped accesses were prepared using #245 bur (Mani Inc, Tochigi, Japan). The canal patency was checked using endodontic explorer, DG-16 (Dentsply Maillefer, Switzerland), and #10K file (Mani Inc, Kiyohara, Tochigi, Japan). The working length was confirmed radiographically. Cleaning and shaping of the root canals were performed with MTwo Ni-Ti rotary files up to size 35/.04 using X-smart (Dentsply Maillefer, Japan) torque control endodontic motor. After using each instrument, canals were irrigated using 2 ml of 3% sodium hypochlorite (Prime Dental Products Pvt Ltd., India). Root canals were then irrigated with 17% EDTA for 60 s to clear the smear layer. The final rinsing was carried out with 5 ml of sterile physiologic saline.

Micro-computed tomography evaluation

Specimens were scanned after the biomechanical preparation, using a micro-CT scanner (GE Phoenix V |tome| x S, Germany). The scan was performed with 20.0 lm isotropic voxel size, 200 lA tube current, 90 kVp energy, and 300 ms integration time with a 0.5 mm aluminum filter. The 3-D data were then analyzed using VG Studio Max 2.2.6 version software (VGL Kernel technology, Germany) to detect micro-cracks at the apical, middle, and coronal third of the root dentin.

Obturation protocol for the study groups

Group A-cold lateral condensation group (CLC)

After coating the canal walls with AH Plus sealer, standardized GP (master apical cone) of size 35/.04 coated with sealer was placed till working length. By utilizing a #20 finger spreader and accessory GP cones, lateral compaction was performed until the spreader no longer penetrates beyond the canal's coronal third. The heated pluggers were used to remove overabundance material, and the GP was vertically compacted up to 1 mm underneath CEJ. The access cavity was sealed with IRM.

Group B-warm vertical condensation technique

An appropriate plugger tip with a .04 taper that penetrated 5–7 mm short of the working length was selected. AH-Plus sealer was coated inside the canal, and GP cone of size 35/.04 (master cone) was introduced till working length. The heat source of the obturation unit (Elements system; System B™, SybronEndo, CA, USA) was adjusted to 200°C. The heated plugger was introduced through the GP till 4 mm short of working length. Warm vertical compaction was continued till completion upto 1 mm below the CEJ, and the access cavity was sealed with IRM.

Group C-thermoplasticized injectable gutta-percha technique

The i-Fill GP obturation system (Denjoy dental co., Ltd, Changsha, China) having a .04 taper needle was activated to the temperature of 200°C. The needle was placed in the canal to reach <3-5 mm of working length, and GP was introduced passively till the canals were completely filled. Compaction was done utilizing the I-Fill pen. The compacting force was maintained until the GP was cooled and solidified totally. The access cavity was sealed with IRM.

Micro crack evaluation

All the three micro-CT images in the three groups were assessed at 0.25 mm intervals, 4030 slices in total were evaluated. If there were no microcracks detected, they were noted zero (no crack). If there were any microcracks noted, the samples were included, and the total number of cracks was calculated.

All root samples were imaged again after obturation using a micro-CT scanner with the same settings used earlier. Two blinded observers compared 3-D reconstructed pictures of the samples twice (after biomechanical preparation and after obturation) to identify the increase in number and progression of micro-cracks. The number of microcracks found was calculated as a percentage for each group.

Statistical analysis

All the collected data of the percentage of cracks in the root specimens were submitted to statistical analysis using the SPSS software (Version 20; IBM; Chicago; IL, USA). The Mcnemar test was used to identify any significant differences in the occurrence and propagation of root dentinal defects before and after obturation. The Pairwise comparison was done using the Wilcoxon Mann–Whitney test to assess the association of microcracks between the three groups at apical, middle, and coronal sections.

RESULTS

After biomechanical preparation, 20 samples (33.33%) exhibited dentinal micro-cracks, of which 15% were in the cold lateral compaction group, 8.33% in warm vertical, and 10% in injectable technique groups. After obturation, only the CLC group showed an increase in micro-cracks by one number (1.66%), which was not statistically significant from other groups [Table 1]. This new crack formation after obturation with cold lateral compaction was seen in the apical section of root dentin [Figure 1]. No change was observed in the percentage of micro-cracks formation after obturation with warm vertical or IGP techniques [Figures 2 and 3]. The pair-wise comparison of microcracks formation after obturation in the apical root section showed no significant difference between the three groups [Table 2].

Table 1

Percentage of microcracks observed before and after obturation within three groups

Type of obturation	Cold lateral (%)	Warm vertical (%)	Injectable (%)
After bio mechanical preparation	15%	8.33%	10%
After obturation	16.66%	8.33%	10%
The difference from pre to post obturation	1.66%	0%	0%
P	1.000	1.000	1.000

P>0.05 indicates statistically no significance

Figure 1

Micro-computed tomography images of cold lateral condensation group indicating new crack formation after obturation in apical section.(Blue arrow shows microcrack after obturation)

Figure 2

Micro-computed tomography images of warm vertical condensation group indicating pre-obturation cracks not propagated after obturation (Red arrow shows microcrack before obturation and blue arrow shows microcrack after obturation)

Figure 3

Micro-computed tomography images of injectable condensation group indicating pre-obturation cracks not propagated after obturation (Red arrow shows microcrack before obturation and blue arrow shows microcrack after obturation)

Table 2

Pair-wise comparisons of three groups with New Micro-cracks after obturation in the Apical section of the tooth

Groups	Mean	SD	Median	IQR	Mean rank	U	Z	P
Group CL	0.3	0.7	0.0	0.0	21.1
Group WV	0.2	0.4	0.0	0.0	19.9	188.5	−0.3111	0.7557
Group CL	0.3	0.7	0.0	0.0	20.6
Group injectable	0.2	0.4	0.0	0.0	20.4	198.0	−0.0541	0.9569
Group WV	0.2	0.4	0.0	0.0	20.0
Group injectable	0.2	0.4	0.0	0.0	21.0	190.0	−0.2705	0.7868

P>0.05 indicates statistically no significance. SD: Standard deviation, IQR: Interquartile range

DISCUSSION

Following endodontic therapy, VRF is one complication that leads to poor prognosis of the obturated teeth.[28] Yan et al. reported that VRF is most likely to occur by the transmission of an existing dentin defects such as micro-cracks or craze lines when they are continuously subjected to stresses.[9] Although many iatrogenic and non-iatrogenic factors are suggested to plays a vital role in the occurrence of dentin microcracks in root canal treated teeth, mechanical and chemical disinfection, obturation, post-placement, and post endodontic restorative procedures are considered as potential causes.[101112]

From previous literature, it is evident that biomechanical preparation with hand or rotary files causes stress and strain on root dentin walls and might result in microcrack occurrence. The degree of microcrack formation may depend on the number of files used, taper, pitch, cross-section design, flexibility, and other metallurgical features of the instruments.[131415] Obturation techniques used in endodontics force the root filling materials into the root canal. These forces may result in microcracks or craze lines, which may develop into VRFs.[16] From previous studies using sectioning and microscopic examination, it was observed that CLC and WVC techniques were associated with dentin crack formation.[1718] These studies evaluated the occurrence of dentinal micro-cracks after the CLC and WVC (both require active force application), while no study investigated the association between passive injectable technique and crack formation. As the force application during obturation is considered a critical factor in microcrack formation, CLC and WVC techniques were compared with the passive injectable obturation technique in the current study. Sectioning and microscopic examination were used in earlier studies associated with false-positive results, hence in the current study, nondestructive micro-CT evaluation was chosen.

The outcome of the current study shows that only the lateral compaction technique was associated with one new crack formation after obturation. Warm vertical compaction and injectable methods were not related to the initiation of new defects in dentin. Statistically, no significant difference was found between all three groups. Micro-cracks noticed after the biomechanical preparations were consistent with the postobturation micro-CT images for all the groups without propagating root defects. A slight increase in new cracks in lateral compaction group could be due to the high compaction force used when compared to the other two groups. Telli et al. reported that when WVC performed skilfully, the forces did not cause root fractures.[19] As the injectable technique is a passive compaction technique with minimal forces; it is not associated with an increase in microcracks after obturation. The present study results were similar to De-Deus et al. study, where they also reported that the obturation technique did not induce new crack formation in root dentin.[20]

The present study results are contradictory to a few previous studies, in which they quoted a direct correlation between the obturation technique and initiation of microcracks in dentin.[17182122] Shemesh et al. reported that the lateral compaction technique resulted in more dentinal cracks than roots filled passively.[17] In another study Shemesh et al. showed that lateral compaction resulted in more cracks, followed by a continuous-wave technique.[18] They reported that the CLC technique has a strong impact on dentin crack formation, which is not correlative with the results of the present study. Kumaran et al. showed that that CLC and WVC resulted in more dentinal cracks than passive obturation techniques.[23] Similarly, Capar et al. observed that obturation techniques have a direct relationship with crack formation and found that lateral and continuous wave techniques lead to more crack generation than single cone technique.[22]

With respect to the location of cracks occurrence, Jain et al. and Aydinbelge et al. reported micro-cracks at 6 mm and 9 mm sections, whereas in this study, one new crack after lateral condensation was identified at the apical 3 mm section.[1516] They reported that crack originated from the contact of pluggers at 20°C with root canal walls by sectioning evaluation technique.[16] In the present study, this contradictory result can be attributed to the technical differences where coronal flaring of the root canal was done in this study before initiating the shaping of canals, which was not done in other studies.

The difference in the results of this study with previous studies may be due to differences in the methodological design. The potential damage caused by sectioning of the samples, chemo-mechanical preparation, obturation protocol, aging of samples were not considered in previous studies; this may have contributed to varied outcomes from the current study. Methodological flaws with conventional evaluation techniques were minimized with the usage of micro-CT as the same sample can be used as a test and control. Preexisting cracks and their location were examined with micro-CT in the current study to avoid false-positive results. Since very limited literature is available on the effect of obturation on root dentin crack formation using micro-CT, further studies need to be conducted to obtain more evidence.

CONCLUSION

Within the limitations of the study, obturation techniques, i.e., CLC, WVC, and injectable obturation techniques, do not form or propagate the dentinal defects.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.