Laser applications in smear layer removal from posterior root canals: A comparative study.

Aim: The aim of the study is to evaluate erbium laser efficiency in removing smear layer from mesial roots of mandibular first molars at different laser settings. Subjects and
Methods: One hundred (100) mandibular first molars were selected. Ninety six (96) mesial roots were assigned to two experimental groups according to the laser wavelength applied: I. Er, Cr: YSGG and II. erbium-doped yttrium aluminum garnet laser system. Four samples were used as control group. Samples of each group were distributed into two subgroups (A and B), following specific irrigation protocol. Each subgroup was divided into four subcategories according to the applied power laser. Each subcategory consisted of six samples. Teeth were observed under scanning electron microscope. Statistical Analysis: Results were statistically analyzed with Kruskal-Wallis and Mann-Whitney test (SPSS statistics software).
Results: No statistically significant difference was observed between experimental groups I and ΙΙ. However, statistically significant difference was found among subgroups IA and ΙB and ΙIA and ΙΙΒ. In every root third, groups IIA and ΙΙB had better outcomes but it was not statistically significant. There was no statistically significant difference among subcategories of Group I (ΙΑ1-4 and ΙΒ1-4) and Group ΙΙ (ΙΙΑ1-4 and ΙΙΒ1-4).
Conclusion: Erbium laser with tested parameters did not completely remove the smear layer from the root canals. Chelating factors can contribute in the laser mechanism of smear layer removal from apical part of narrow and curved root canals. Copyright:

INTRODUCTION

Strong emphasis has been given for the improvement of endodontic therapy in cleaning and enlarging the root canals. Root canal morphology has primary importance in order to achieve a successful endodontic treatment in a long term. Root canal system is particularly variable, especially in molar teeth.

Mandibular first molar is the first posterior tooth that erupts and is Exposed to decay or caries for long time, thus in many cases it requires endodontic treatment. Anatomically, it typically displays a distal and a mesial root.[1] Canal configuration of mesial roots is particularly complicate including two or three root canals, isthmus, apical deltas, and inter-canal communications. Type IV and type II are the most common canal configuration (31.5% and 28%, respectively).[2]

Lasers have been proposed as a method of conventional root canal therapy in order to enhance the effect of irrigation solution and, thus, to improve the smear layer removal from the root canal system.[3] Erbium lasers are the most common lasers of mid-infrared spectrum used in dentistry[4] and exhibit high absorption in water and hydroxyapatite.[56] Erbium lasers remove worn dentine and smear layer through the process of thermo-mechanical ablation.[7] Considering that water content of the tissue plays a major role in the ablation process,[8] dentine

(water volume 20%) is easily removed.

However, there is no or little evidence on smear layer removal from narrow and curved roots using laser irradiation. Particularly, there is no relevant study on Er, Cr: YSGG laser, whereas there is only one study related to erbium-doped yttrium aluminum garnet (Er: YAG) laser efficiency in smear layer removal from mesial root canal. According to Lloyd et al.,[9] Er: YAG laser achieved in reducing the volume of smear layer 2.6 times more in comparison with conventional irrigation in mesial roots.

The present study was aimed to evaluate the efficacy of hard tissue lasers (Er, Cr: YSGG laser and Er: YAG laser) in smear layer removal from mandibular first molars with or without the aid of any chemical irrigant. A different approach toward smear layer removal of root canal system was introduced by means of laser, applying different wavelengths, laser tips, laser parameters, and irrigation protocols.

SUBJECTS AND METHODS

One hundred mandibular first molars (mesial roots) were selected. Cone-beam computed tomography (CBCT; Planmenca Pro-Max3Dmax operated at 96 kV, 10 mA, 12 s) was used to determine the sample selection. After scanned, mesial roots were mainly classified according to the configuration root anatomy of mesial root (types II and IV according to Vertucci classification).[10] Crowns were discarded and mesial roots were separated from distal roots and instrumented by means of Protaper Gold up size F2 (25/0.08) (Dentsply-Maillefer, Ballaigues, Switzerland).

Samples (n = 96) were assigned to two experimental groups according to laser wavelength applied: I. Er, Cr: YSGG laser system (2780 nm) (Waterlase MD, Biolase Technology, Irvine, CA, USA) and II. Er: YAG laser system (2940 nm) (Morita AdvErl Evo, Khioto, Japan). The other four samples were used as control group.

Each group was subdivided into subgroups A and B, following a specific irrigation protocol. In subgroup A, teeth were irrigated by 1-mL distilled water, between the instruments, and finally rinsed by 10-mL distilled water. In subgroup B, teeth were irrigated by 1-mL 5% NaOCl, between the instruments, and finally rinsed by 5-mL 17% ethylenediaminetetraacetic acid (EDTA) for 60 s, 5-mL 5% NaOCl, and 5-mL distilled water.

Each subgroup was assigned into four subcategories according to the applied power laser. Each subcategory consisted of six samples. Three samples were of configuration type II and the other three with configuration type IV).

The subcategories of every subgroup (Group I) included:

Er, Cr: YSGG laser (1.25 W, 25 mJ, 50 Hz, 140 μs H-mode, 80% water–30% air)

Er, Cr: YSGG laser (1.5 W, 30 mJ, 50 Hz, 140 μs H-mode, 80% water–30% air)

Er, Cr: YSGG laser (2 W, 40 mJ, 50 Hz, 140 μs H-mode, 80% water–30% air)

Er, Cr: YSGG laser (2.5 W, 50 mJ, 50 Hz, 140 μs H-mode, 80% water–30% air).

The subcategories of every subgroup (Group II) included:

Er: YAG laser (0.75 W, 30 mJ, 25 Hz, 300 μs H-mode, 7:7 water–air ratio)

Er: YAG laser (1.25 W, 50 mJ, 25 Hz, 300 μs H-mode, 7:7 water–air ratio)

Er: YAG laser (1.75 W, 70 mJ, 25 Hz, 300 μs H-mode, 7:7 water–air ratio)

Er: YAG laser (2 W, 80 mJ, 25 Hz, 300 μs H-mode, 7:7 water–air ratio).

A radial tip of 200 μm diameter for Er, Cr: YSGG laser and a radial tip of 300 μm diameter for E: YAG laser were used. Both laser tips were inserted into the root canal 1 mm short of the apex and were moving circularly from the apical to the coronal part of the root. The speed of the movement was 2 mm − 1 Each root canal was irradiated for four times.[11]

The control group was instrumented as experimental groups but not irradiated. The first two samples were irrigated by 1-mL distilled water, between the instruments, and were finally rinsed by 10-mL distilled water (subgroup A). The other two samples were irrigated by 1-mL 5% NaOCl, between the instruments, and were finally rinsed by 5-mL 17% EDTA for 60 s, 5-mL 5% NaOCl, and 5-mL distilled water (subgroup B). The sample classification is shown in diagram.

Roots were split into two parts. The mesial part was kept and distal part was rejected. The mesial parts were carbon coated so as to be observed under the scanning electronic microscope (SEM). All samples were analyzed at ×2000 magnification within an area (10 μm) at every third. Digital images were taken at the center of coronal (10 mm from apex), middle (6 mm from apex), and apical third (2 mm from apex) of both two root canals. Therefore, each sample had six scores.

Statistical analysis

The percentage of smear layer residues was estimated using Takeda et al.[12] rating system:

0: 0% residues

1: <20% residues

2: 20%–50% residues

3: <50% residues.

Evaluation was performed by two calibrated examiners independently and in a blind manner. If the examiners did not reach an agreement, a third examiner would assist in the scoring of the sample. The Cohen's kappa coefficient was used to analyze inter-rater agreement. Results were statistically analyzed with Kruskal–Wallis and Mann–Whitney test. Statistical analysis was performed with SPSS (2019, IBM, New York)statistics software and significance level was set at 1% (P < 0.001).

RESULTS

Statistical analysis revealed different results in experimental groups when comparing different laser parameters and irrigation protocol applied. Firstly, there was no statistically significant difference between experimental groups I (Er, Cr: YSGG laser) and II (Er: YAG laser) (Mann–Whitney test, coronal third U1 = 954.000 and U2 = 973.500, middle third U1 = 1012.000 and U2 = 988.500, and apical third U1 = 997.500 and U2 = 957.500, P < 0.001). Er: YAG laser showed higher efficiency, but this difference was not statistically significant.

However, there was statistically significant difference between subgroups IA and IB and IIA and IIB (Kruskal–Wallis test, coronal third U1 = 954.000 and U2 = 973.500, middle third U1 = 1012.000 and U2 = 988.500, and apical third U1 = 997.500 and U2 = 957.500, P < 0.001). There was no statistically significant difference among subcategories of Group I (ΙΑ1-4 and ΙΒ1-4) and Group ΙΙ (ΙΙΑ1-4 and ΙΙΒ1-4) [Table 1]. Statistical difference was present only in the middle third of one root canal in subcategory IA1-4.

Table 1

Kruskal-Wallis Test (Mean Ranks) for Subcategories IA(1-4), IB(1-4), IIA(1-4), IIB(1-4), P<0.001

Subcategories	Coronal 3rd		Middle 3rd		Apical 3rd
	1st root canal	2nd root canal	1st root canal	2nd root canal	1st root canal	2nd root canal
IA1	17.00	17.67	18.00	18.33	15.00	17.50
IA2	13.17	12.58	12.67	15.67	15.00	11.50
IA3	8.00	7.17	10.00	8.67	11.00	11.50
IA4	11.83	12.58	9.33	7.33	9.00	9.50
Kruskal Wallis H	5.471	6.311	6.537	7.290	11.294	5.914
IB1	8.50	9.00	7.42	7.83	8.33	9.50
IB2	14.50	11.00	13.75	15.75	12.83	12.42
IB3	12.50	15.00	12.17	9.42	11.83	11.58
IB4	14.50	15.00	16.67	17.00	17.00	16.50
Kruskal Wallis H	3.833	6.268	5.256	4.343	8.591	3.490
IIA1	15.17	15.17	13.50	14.25	13.25	12.83
IIA2	14.75	14.75	17.25	15.50	13.25	14.75
IIA3	4.50	4.50	4.50	4.75	13.25	14.75
IIA4	15.58	15.58	14.75	15.50	10.25	7.67
Kruskal Wallis H	11.358	12.156	1.038	11.358	11.024	5.600
IIB1	11.00	11.50	9.92	9.92	7.50	7.67
IIB2	11.00	11.50	13.75	11.83	13.17	14.83
IIB3	15.00	15.50	14.50	14.50	11.83	11.00
IIB4	13.00	11.50	11.83	13.75	17.50	16.50
Kruskal Wallis H	2.126	2.113	6.967	2.556	2.113	6.660

SEM pictures showed the presence of heavy smear layer (apical third) in samples irrigated only by distilled water, regardless of pulse energy applied. On the other hand, in samples irrigated following a specific irrigation protocol (5-mL 17% EDTA for 60 s, 5-mL 5% NaOCl, and 5-mL distilled water), there were areas in which smear layer was totally removed, even from apical third.

More specifically:

Group IA: In coronal and middle third, half of the samples were totally covered by smear layer (score 3) that obstructed the dentinal tubules. In apical third, all the areas were covered by smear layer.

Group IB: The samples presented cleaner surfaces under SEM. In coronal and middle third, samples had no or little remnants. In apical third, few samples were evaluated with score 2 or 1 [Figure 1].

Figure 1

Smear layer removal by Er, Cr: YSGG laser (2.5 W)

Group IIA: Most samples in coronal and middle had big parts of smear layer, whereas in apical third, smear layer covering was evident and dentinal tubules were closed (score 2 and 3).

Group IIB: Samples presented the best results. Nearly all the surfaces of the samples were relatively cleaner but not completely [Figure 2].

Figure 2

Smear layer removal by erbium-doped yttrium aluminum garnet laser (2 W)

Comparing the smear layer removal among all the thirds of root canal, results showed cleaner surfaces in coronal and middle than in apical third [Table 2].

Table 2

Percentages of smear layer in all root thirds

Smear layer	Coronal 3rd		Middle 3rd		Apical 3rd
	1st root canal	2nd root canal	1st root canal	2nd root canal	1st root canal	2nd root canal
0%	35.7	37.8	15.3	18.4	28.6	27.6
<20%	33.7	32.7	22.4	20.4	31.6	32.7
<50%	18.4	18.4	28.6	35.7	25.5	25.5
>50%	12.2	11.2	33.7	25.5	14.3	14.3
Total	100.0	100.0	100.0	100.0	100.0	100.0

DISCUSSION

Cleanliness of dentinal walls in all root thirds of mesial root canals after erbium laser irradiation was assessed. We used different laser parameters regarding the smear layer removal. It was supposed that there is no difference between groups irrigated with chelators and groups irrigated with distilled water before laser irradiation.

Erbium laser are well-investigated wavelengths and offer the greatest potential for removal of hard tissue and smear layer. Dentine removal can be achieved by two mechanisms: explosive ablation and explosive vaporization.[57] Explosive ablation (thermo-mechanical ablation) is the process that occurs with wavelengths between 2.7 and 3 nm.[7] In this process, light energy is absorbed by water molecules and is transformed into thermal energy. Water is heated and vaporized, creating shock waves which remove the damaged dentine.[5]

To date, sequence with EDTA and then NaOCl, as final irrigation, seems to remove the smear layer effectively[13] but not totally. In subgroups A, distilled water was the only irrigation liquid used between the instruments. The role of distilled water was to reduce the amount of variables and to allow evaluation of the contribution of erbium laser itself.

The results showed statistically significant difference between subgroups A and B. Irrigation protocol and chelating factor may play an important role in smear layer removal. This finding comes to agreement with George and Walsh,[14] de Moor et al.,[15] and Blanken et al.[16] On the contrary, there are studies that supported that there is no difference when laser is combined with irrigants (NaOCl, EDTA, NaOCl, and EDTA)[1718] or that Er: YAG laser is more effective in comparison with chelating factors (17% EDTA, 6% phosphoric acid, and 6% citric acid).[19]

As it was expected, in control group A, all samples were having a homogeneous smear layer. In control group B, there were areas where the smear layer was removed apart from apical third. Exclusive use of irrigants was less effective than combinative action of laser and irrigants (subgroups B).

In this study, there was no statistically significant difference between Er, Cr: YSGG laser and Er: YAG laser. Er: YAG laser showed higher efficiency in smear layer removal but this difference was not statistically significant. George and Walsh[14] also reported that both erbium lasers are equally effective.

In our study, increasing power and pulse energy applied does not affect positively smear layer removal. On the contrary, high power can act as source of smear layer because of structural destruction of dentine.[20]

In the study, a tip of 200-μm diameter (RFT2 was used for Er, Cr: YSGG laser and a tip of 300-μm diameter (R300T) was used for Er: YAG laser. There are no studies that use RFT2 (Er, Cr: YSGG laser) and R300T (Er: YAG laser) for smear layer removal from molars. The low efficiency of smear layer removal can be attributed to the increased incidence of narrow mesial root canal where laser beam cannot reach. Adequate canal preparation has been proposed such that the laser tip reaches the canal end. However, this can cause destruction of root canal anatomy.

More specifically, in subgroup A, the instrumentation of such narrow and curved root canals without EDTA was very difficult. Many samples were rejected due to broken file. Comparing the two laser tips, RFT can be applied in most narrow and curved root canals. On the other hand, R300T is often used in canals with diameter larger than ISO size 30.

Similarly, experimental groups showed better results in coronal and middle third than in apical.[212223] This is mainly attributed to small taper and diameter of apical third.[24]

Finally, there is great degree of differentiation with respect to the results in relevant studies. This is attributed to different methods applied – laser parameters, wavelengths, tips, irrigants, etc., In many studies, some laser parameters are omitted, so it is difficult to adopt their conclusions.

CONCLUSION

The present study showed that smear layer removal with an erbium laser with tested parameters was not performed for mesial root canals of molars. The presence of EDTA may be crucial in the mechanism of smear layer removal using laser from apical part. Increasing the average power of laser did not improve smear layer removal. Further research needs to be done in order to find a proper irradiation protocol for the laser systems, which can remove the smear layer effectively.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.