INTRODUCTION

One of the main aims of endodontics is to provide obturations with three-dimensional sealing of the root canals as a basis for successful treatment ^1-2 . However, three-dimensional sealing is not always easy to achieve. Surgical shaping cannot always adapt to the wide range of existing tooth anatomies ^3-8 , and chemical preparation does not always eliminate 100% of the contents of the main, lateral, accessory and recurrent canals or apical deltas ^9-12 . Successful treatment requires extensive knowledge of materials and different techniques to achieve proper obturation of the entire volume of the root canal system.

For years, obturation techniques used cold-laterally compacted gutta percha cones as the main component, sealed with cements, usually of acid-base composition. Later, techniques were developed for gutta percha mechanical plasticization or direct injection into the system. These changes in canal-sealing techniques were mainly motivated by the need to ensure a 3D seal, i.e., an obturation mass without voids, with gutta percha and/or sealer filling any lateral and/or accessory canals ^13-15 .

When 3D sealing is not successful, voids form. Voids are either closed, when they are within the obturation mass, or open, when in contact with the dentin wall. Open voids may connect to the periodontal space through the dentinal tubules. The presence of voids reduces the quality of the final obturation, promoting an environment in which microorganisms can survive, and connection between the periodontal space and the interior of the root canal system. This is why it is so important to ensure a three-dimensional seal, without voids or spaces in which bacteria may develop ¹⁶ .

Currently, with the advent of bioceramic sealers, the concept of obturation has changed. Gutta percha has ceased to be the main component, having been replaced by the sealer itself, which has excellent mechanical and biological properties ^17-18 .

These new techniques usually use single, more tapered gutta percha cones that almost exactly fit the root canal that was prepared and shaped using rotary and/or oscillatory instruments ¹⁹ .

In 2018, Kim proposed an obturation technique based on the use of a bioceramic material with a single gutta percha cone, to which vibration was applied. The vibration transfers ultrasound mechanical energy through a cotton plier that is holding the gutta percha cone inside the root canal, thereby transferring the energy to the bioceramic sealer. This eliminates voids or bubbles, and enhances sealer fluidity, enabling it to penetrate canals that are difficult to access. This technique provides three-dimensional sealing thanks to the sealer rather than the gutta percha ²⁰ .

X-ray micro-CT scanning is an imaging technique based on X-ray absorption by different types of tissues or materials. It can be used to observe and analyze ex vivo samples on a very small scale without causing deterioration ²¹ . Specimens are positioned in the micro-CT scanner with a holder. Scanning settings (voltage, amperage, rotation, filter, pixel size, grey-scale density, image format, among others) are established, and multiple 2D images are taken while the sample rotates 180° or 360° on its own axis, as programmed. Software is used to recognize the images and convert them into a three-dimensional volume. The volumes thus obtained enable observation, measurement and calculation of many specimen properties, such as volume, surface area, density, presence or absence of voids, etc ^22-33 . Identifying the presence, size, type and location of voids in an endodontic obturation is of great clinical importance because it enables evaluation of the three-dimensionality of the sealing techniques, which can be related to the likelihood of healing periradicular lesions. Micro-CT scanning is the only technique that provides images in which the above parameters can be measured without destroying or cutting the specimens. It is important to avoid cutting specimens because doing so may smear the obturation material, preventing proper evaluation ²² . The possibility of observing voids as small as 7 pm enables different sealers and sealing techniques to be studied and compared with the aim of improving obturation three-dimensionality.

The aim of this study was to analyze by micro-CT the presence of voids in the root canal obturation of lower single-rooted premolars prepared using the single cone and ultrasound vibration technique.

MATERIALS AND METHOD

RESULTS

Micro-CT scans showed presence of closed and open voids in both groups ( Fig. 1A-B ).

Fig. 1 Micro-CT images, with original colors and contrast, show presence of voids in the endodontic obturation. A) closed voids and B) open voids. 

Volume of closed voids

Comparison of the global volume of closed voids between groups showed no significant difference; Group 1 without vibration: 0.003 (0.0069) / Group 2 with vibration: 0.004 (0.006). Values expressed as mean (SD) ( Fig. 2 ).

Fig. 2 Global volume of closed voids. Scatter diagram. Horizontal red line: median. p: Mann-Whitney’s U-test. 

In both groups, the volume of closed voids differed significantly between thirds (p < 0.05), with the cervical third having the highest volume of voids ( Fig. 3 A and B ).

Fig. 3 Volume of closed voids according to third. A) Group 1, Without Vibration, and B) Group 2, With Vibration. Scatter diagram. Horizontal red line: median. p: Kruskal-Wallis test. Different letters indicate significant differences in post-hoc comparisons (Conover). 

The volumes of closed voids per group, global and per third, are set forth in Table 1 .

Table 1 Volume of closed voids in treatments without and with vibration, in each third and global 

Volume of closed voids (mm3)
Third	Vibration														p*
	No							Yes
	N	min	Max	Me	Md	SD	Q1 - Q 3	N	min	Max	Me	Md	SD	Q1 - Q 3
Cervical	10	0.000	0.027	0.007	0.003	0.008	0.000 - 0.010	10	0.002	0.024	0.008	0.007	0.007	0.002 - 0.009	0.39
Middle	10	0.000	0.009	0.001	0.000	0.003	0.000 - 0.001	10	0.000	0.009	0.002	0.001	0.003	0.000 - 0.002	0.35
Apical	10	0.000	0.013	0.001	0.000	0.004	0.000 - 0.000	10	0.000	0.003	0.000	0.000	0.001	0.000 - 0.000	0.69
Global	30	0.000	0.027	0.003	0.000	0.006	0.000 - 0.004	30	0.000	0.024	0.004	0.001	0.006	0.000 - 0.004	0.32

The volumes of open voids per group, global and per third, are set forth in Table 2 .

Table 2 Volume of open voids in treatments without and with vibration, in each third and global 

Volume of closed voids (mm3)
Third	Vibration														p*
	No							Yes
	N	min	Max	Me	Md	SD	Q1 - Q 3	N	min	Max	Me	Md	SD	Q1 - Q 3
Cervical	10	4.339	13.062	8.378	7.502	3.139	5.459 - 10.717	10	6.069	10.916	8.565	8.652	1.822	6.493 - 10.465	0.87#
Middle	10	2.102	5.858	3.459	3.155	1.210	2.531 - 4.327	10	2.584	5.974	3.784	3.313	1.165	3.083 - 4.054	0.55#
Apical	10	1.204	3.274	1.706	1.521	0.613	1.340 - 1.837	10	1.004	2.142	1.555	1.496	0.356	1.440 - 1.758	≈1
Global	30	1.204	13.062	4.514	3.489	3.447	1.837 - 5.858	30	1.004	10.916	4.635	3.313	3.215	1.758 - 6.493	0.80*

Global volume of open voids was 4.514 (3.447) in Group 1, and 4.635 (3.215) in Group 2; values expressed as mean (SD); without significant differences. ( Fig. 4 ).

Fig. 4 Volume of open voids according to treatment. Scatter diagram. Horizontal red line: median. p Mann-Whitney’s U-test. 

The volume of open voids differed significantly between thirds (p < 0.05), both in the group without vibration ( Fig. 5 ) and in the group with vibration ( Fig. 5 ). In both cases, the cervical third had the highest volume of voids and the apical third had the lowest.

Fig. 5 Volume of open voids according to thirds. A) Group without vibration and B) Group with vibration. Scatter diagram. Horizontal red line: median. p Kruskal-Wallis test. Different letters indícate significant differences in post-hoc comparisons (Conover). 

Total volume of voids

In Group 1, global volume of voids ranged from 1.204 to 13.076 mm³, with a median (Q₁-Q₃) of 3.489 (1.850 - 5.867) and a mean (SD) of 4.517 (3.449). In Group 2, it ranged from 1.007 to 10.919 mm³, with a median (Q₁- Q₃) of 3.313 (1.758 - 6.499) and a mean (SD) of 4.638 (3.218) ( Fig. 6 ). There was no significant difference between the groups without and with vibration ( Table 3 ), either within the thirds studied: cervical (p = 0.87), middle (p = 0.55) and apical (p ~ 1), or global (p = 0.80).

Fig. 6 Total volume of voids according to treatment. Scatter diagram. Horizontal red line: median. p: Mann-Whitney’s U-test. 

The total volume of voids differed significantly ( Fig. 7B ). In both cases, the cervical third had the between thirds (p < 0.05), both in Group 1 Without highest volume of voids, and the apical third had the Vibration ( Fig. 7A ) and in Group 2 With Vibration lowest.

Fig. 7 Total volume of voids according to thirds. A) Group Without Vibration and B) Group With Vibration. Scatter diagram. Horizontal red line: median. p Kruskal-Wallis test. Different letters indícate significant differences in post-hoc comparisons (Conover). 

Table 3 Total volume of voids in treatments without and with vibration, in each third and global 

Volume of closed voids (mm3)
Third	Vibration														p*
	No							Yes
	N	min	Max	Me	Md	SD	Q1 - Q 3	N	min	Max	Me	Md	SD	Q1 - Q 3
Cervical	10	4.340	13.076	8.384	7.508	3.139	5.463 - 10.717	10	6.077	10.919	8.573	8.655	1.820	6.499 - 10.483	0.87#
Middle	10	2.102	5.867	3.461	3.155	1.212	2.531 - 4.332	10	2.584	5.974	3.786	3.313	1.166	3.083 - 4.056	0.55#
Apical	10	1.204	3.274	1.707	1.521	0.614	1.340 - 1.850	10	1.007	2.143	1.555	1.496	0.355	1.440 -1.758	≈1*
Global	30	1.204	13.076	4.517	3.489	3.449	1.850 - 5.867	30	1.007	10.919	4.638	3.313	3.218	1.758 - 6.499	0.80*

DISCUSSION

Many different obturation techniques are currently in use, all of which seek to achieve a three-dimensional seal that will increase the likelihood of successful endodontic treatment, free from recontamination. One of the most popular techniques in recent years

uses a bioceramic as the main material, along with a more tapered gutta-percha cone that matches the last instrument used. In 2018, Kim introduced a variation of this technique by adding the use of ultrasound to make the gutta percha cone vibrate, which transmits the waves to the sealer, thereby improving its flow towards the more irregular parts of the root canal ²⁰ . The present study used micro-CT scans to evaluate 20 teeth obturated with bioceramic sealer and single, more tapered cone. Ultrasound was used to apply vibration to half the specimens. Statistical data showed no significant difference in sealing capacity between techniques. There were, however, differences between the thirds evaluated. As expected, the cervical third had the most voids, regardless of the obturation technique, because it contains more sealer due to the anatomical change in the canal, and the gutta percha cone has poorer fit. The data recorded in the current study are consistent with the report published by Kim et al. in 2018 ²⁰ , which found no significant difference between groups with and without vibration, although many sealer-free spaces (voids) were observed, mainly in the cervical third, also in agreement with the current study. Iglecias et al. ²⁸ also reported similar results to the current study. They performed single-cone obturations and continuous wave obturations. Micro-CT analysis showed no difference between groups regarding the presence of voids, except in the cervical third.

Ho et al . ³² conducted a study observing endodontically treated samples obturated using different techniques (cold lateral condensation, hot vertical condensation and thermoplasticization by ultrasound). The amount of gutta percha present in each sample was evaluated. The results showed that vertical compaction and ultrasound thermoplasticization achieved comparable obturation qualities with gutta percha in the canal system, with significantly higher density than the samples obturated using cold lateral condensation. This demonstrates better results with the use of ultrasound applied on the gutta percha, reducing voids in the mass, but the article does not specify results on the evaluation of the presence of voids in the endodontic sealer.

The data reported by Somma et al. ³⁵ in 2011 on the evaluation of three different sealing methods also agree with the current study. They analyzed two thermoplasticization techniques (Thermafill and System B) and the single cone technique, by means of 19.1 pm pixel size micro-CT scan. All measurements found similar values for the quantity and distribution of voids and the percentage of occupation of the endodontic space, regardless of the technique.

In 2014, though without using ultrasound, Kele§ ³⁶ et al. measured the volumes of filling and voids in oval root canals by means of 12.5 pm micro-CT. They found that in the apical third, there was no difference between cold lateral compaction and hot vertical compaction techniques, but in the other two thirds, there was a difference in favor of hot compaction.

In 2021, Pérez-Alfayate et al. ³⁰ analyzed the obturation of 200 lower molars with curved roots, using six sealers (AH Plus Thermafil, GuttaCore, GuttaFusion, Cold Side Compaction, AH Plus and EndoSequence BC sealer). Half of the samples from each group were activated with an Endoactivator sonic system tip size 25/06. Three sections were cut from each molar, representing the cervical, medium and apical thirds. Images taken with confocal laser microscope and operative microscope showed no statistically significant differences between sealers, techniques or thirds. It was concluded that the use of sonic activation was unrelated to the three-dimensional seal of the obturation, at least according to the data recorded under fluorescent laser and operative optical microscopes. These results are consistent with the present study.

Ba§er Can et al. ²⁹ also used micro-CT to evaluate 30 single-rooted teeth obturated with EndoREZ, Activ GP or AHplus with gutta-percha. The images were acquired at 13.68 pm and showed significantly more voids in the apical third with Activ GP than with the other two techniques (EndoREZ or AHplus and gutta percha). As in the current study, micro-CT scans enabled determination of whether there were significant differences between different obturation methods.

Celikten et al. ¹⁹ evaluated and compared the presence of voids in endodontic obturation with different sealers (EndoSequence BC Sealer, Smartpaste bio, Activ GP and AH Plus) by means of 13.47 pm pixel size micro-CT scans. A large quantity of voids was found in the samples obturated with bioceramics, but there was no statistical difference between them, or compared to the other sealers. This paper suggests that micro-TC scanning is essential for evaluating this type of parameters.

Huang et al. ²⁷ evaluated two endodontic sealers (EndoSequence BC sealer and AH Plus) for obturation of lower premolars. Although the SEM study showed that both sealer were better adapted at crown level, micro-CT scans showed the presence of closed voids within the sealer mass, with no difference between them. Moreover, the number of voids in the cervical third was significantly higher than in the other two thirds, in direct relation to the oval anatomy of root canals in these thirds. These results support the current study.

All of the above demonstrates the importance of using technology that enables parameters to be assessed at very high resolutions.

CONCLUSIONS

The results suggest that the volume of closed, open and total voids does not differ between treatments with and without ultrasound vibration. In the cervical third, the highest volume of voids is related to the oval geometry in the teeth evaluated in this study.