Comparative evaluation of removal of smear layer using newer rotary endodontic files: A scanning electron microscope study.

Aim: The aim of this study is to evaluate the smear layer removal efficacy of newer rotary files using scanning electron microscopic analysis at the coronal, middle, and apical third of the root canal. Materials and
Methods: In this study, sixty permanent human single-rooted teeth with complete, mature root apices without any anatomic variation having straight patent root canal extracted for periodontal cause were included. The samples were cleaned to remove debris, calculus and rinsed with sodium hypochlorite to remove organic tissue and then stored in distilled water. All the samples were decoronated with the help of carborundum disc at root length of 12 mm to obtain segments similar in length. After this, all the samples were randomly divided into four groups -Protaper universal, Protaper Gold, EDM, and Revo-S. After cleaning and shaping, samples in each group were finally flushed with normal saline. Samples were then dried. Grooves were made into the root in the buccal and lingual portion of the root using a diamond bur and the samples were split longitudinally into two halves using stainless steel chisel and mallet. Preparation for scanning electron microscope was done and phototmicrographs at ×3000 were taken. Result: Chi square test and Kruskal-Wallis test was used for statistical analysis. Result of this study showed that Revo -S was found to be best amongst the groups and EDM was the poorest. Conclusions: Based on the present study, Revo-S files showed the best smear layer removal in the coronal, middle, and apical 1/3rd of the root canal when compared to other groups. Within the parameters of the study, Revo -S was found to be best amongst the groups and EDM was the poorest. The quality of various files according to ranks of smear layer removal was found to be: Group IV > Group II > Group I > Group III. Copyright:

INTRODUCTION

A fundamental aim of endodontic treatment is to prevent or cure apical periodontitis. Removal of nonvital and necrotic remnants of pulp tissues, microorganisms, and bacterial products from the root canal system is essential for endodontic success. Root canals are irregular and complex systems, with numerous dentinal tubules opening onto the root canal surface. When the root canal system becomes infected, microorganisms can invade the dentinal tubules and re-infect the root canal system.

Biomechanical preparation, disinfection, and obturation all together constitute equally important phases of endodontic treatment. The result of root canal preparation is dependent upon various factors, out of which, canal anatomy, is of prime importance. Keeping this in mind, over the years a variety of instruments and techniques have also been introduced to obtain a clean and debris-free canal fit for obturation.[1] Mechanical instrumentation alone cannot reduce the microbial population in the root canal system as it forms debris and smear layer that comprises inorganic and organic substances such as fragments of odontoblastic processes (Tomes fibers), necrotic tissues, microorganisms, and their metabolic by-products. Such tissue remnants and debris prevent the penetration of irrigants and medicaments into the dentinal tubules and avoid the close adaptation of root canal filling into canal walls.[2]

According to Mader et al. smear layer is found only on the instrumented portion of canal walls, being absent in dentin walls that have not been instrumented.[3] Afterward, this layer prevents penetration of root canal filling materials into dentinal tubules and might affect the microleakage. The main objective of cleaning and shaping of the root canal system is the removal of pulpal remnants, bacteria, and their by-products, and finally, sealing the canals.[4] Insufficient removal of debris and smear layer material can induce stresses on the cutting segment of endodontic instruments.

Considering the growing use of nickel-titanium (NiTi) rotary instruments in endodontic treatments and the introduction of new files, assessment of the ability of these instruments in the production of smear layer and debris (amount, thickness, and type) seems necessary.

ProTaper Universal (Dentsply Maillefer, Switzerland) includes three shaping files (Sx, S1, andS2) and three finishing files (F1, F2, and F3), with two additional new files for larger canals (F4 and F5). Their progressive taper design and a convex triangular cross-section are the main features which improve the flexibility and cutting efficiency while reducing torsional loading and file fatigue.[4]

ProTaper Gold (PG) (PG; Dentsply, Tulsa Dental Specialties, Tulsa, OK, USA) NiTi rotary system was developed with proprietary advanced metallurgy. PG rotary files feature the same exact geometries as ProTaper Universal (PU) (PU; Dentsply Maillefer, Ballaigues, Switzerland), but it may appear slightly curved when removed from the package due to their metallurgy. PG system has been metallurgically enhanced through heat treatment technology. PG files exhibit a convex triangular cross-section and progressive taper design that claimed to improve cutting efficiency and safety. PG files are available in eight sizes: SX (tip size 19 with a taper of 0.04), S1 (tip size 18 with a taper of 0.02), S2 (tip size 20 with a taper of 0.04), F1 (tip size 20 with a taper of 0.07), F2 (tip size 25 with a taper of 0.08), F3 (tip size 30 with a taper of 0.09), F4 (tip size 40 with a taper of 0.06), and F5 (tip size 50 with a taper of 0.05).

A new generation of nickel-titanium rotary instruments, Revo-S had been developed with asymmetrical cutting profile which facilitates root canal penetration by a snake-like movement, reduces instrument stress, and offers a root canal shaping which is adapted to the biological imperatives.

Hyflex EDM instruments are made from a specific NiTi alloy that has a lower per cent in weight of nickel (52.1% wt) than conventional NiTi alloys.[5] These instruments are characterized by a symmetrical cross-sectional design showing three cutting edges except the instruments with Size 40, 0.04 taper, which have a square cross-section with four flutes. A conventional grinding process is used to manufacture these instruments. A specific sequence of heat treatment is used during their manufacturing process leading to very flexible instruments. Fatigue resistance of Hyflex EDM was found to be markedly higher and working torque during canal instrumentation considerably lower compared with other NiTi instruments. The manufacturer states that permanently deformed instruments will regain their original shape when sterilized at approximately 134°C.[6] It is claimed that due to their increased flexibility, Hyflex EDM instruments are best suited to prepare curved root canals and possess a superior centering ability compared with conventional NiTi instruments.

Evaluation of fine debris and smear layer require higher magnification levels that are achieved only through the use of scanning electron microscope (SEM) which was proved to be an efficient method to investigate and compare the influence of different endodontic instruments on the morphology and cleanliness of dentin surfaces.[7]

Hence, the present study was undertaken to compare the removal of smear layer by newer rotary endodontic files using scanning electron microscopic analysis at the coronal, middle, and apical third of root canal.

MATERIALS AND METHODS

The study was conducted in the department of conservative dentistry and Endodontics. In this study, sixty permanent human single-rooted tooth with complete, mature root apices without any anatomic variation having straight patent root canal extracted for periodontal cause, were included. The samples were cleaned to remove debris, calculus and rinsed with sodium hypochlorite to remove organic tissue and then stored in distilled water. All the samples were decoronated with the help of carborundum disc at root length of 12 mm to obtain segments similar in length. After this, all the samples were randomly divided in four groups.

GROUP I- (PROTAPER UNIVERSAL) – In this group, Glide path was established using #15k-file (015/02). The coronal third was prepared using Protaper Universal SX in brushing manner. The final apical preparation was completed using F2 corresponding 025 size (Torque. 2 Ncm, speed.300 rpm). Samples were irrigated during instrumentation between each file change with 1 ml of 5% NaOCl followed by 1 ml of 17% Ethylenediamine tetraacetic acid (EDTA).

GROUP II- (PROTAPER GOLD) – In this group, Glide path was established using #15k-file (015/02). Using X-Smart endo motor, shaping files (SX, S1, and S2) were used with a brushing action on withdrawal stroke. Finishing files F1 and F2 were used with no brushing action.

GROUP III (EDM) – In this group, initially by 25/0.12 Orifice Opener file, followed by #10 K-file (Dentsply Maillefer, Ballaigues, Switzerland), and then consecutively 10/0.05 Glidepath File and 25/~ HyFlex one shaping file. All files were used at 500 rpm and at a torque of up to 2.5 Ncm except the Glidepath file which was used with 300 rpm and at a torque of up to 1.8 Ncm. samples were irrigated during instrumentation between each file change with 1 ml of 5% NaOCl followed by 1 ml of 17% EDTA.

GROUP IV (REVO-S) – Glide path was established using #15k-file (015/02). Initially, the SC1 file was used in the crown-down technique followed by SC2 and SU. All the files were used at the speed of 350 rpm and torque 3 N.cm. Samples were irrigated during instrumentation between each file change with 1 ml of 5% NaOCl followed by 1 ml of 17% EDTA.

After cleaning and shaping, samples in each group were finally flushed with normal saline. Samples were then dried. Grooves were made into the root in the buccal and lingual portion of the root using a diamond bur and the samples were split longitudinally into two halves using stainless steel chisel and mallet. Preparation for SEM was done and photomicrographs at ×3000 were taken. Scoring was done by two different observers using grids of (1 cm × 1 cm) [Figures 1-3].

Figure 1

Scanning electron microscopic image of smear layer removal in coronal third of different group at ×3000

Figure 3

Scanning electron microscopic image of smear layer removal in Apical third of different group at ×3000

Scanning electron microscopic image of smear layer removal in middle third of different group at 3000

The quality of rotary files was judged by the Gutmann rating system of smear layer removing scores given as follows:

Little or no smear layer; covering <25% of the specimen; most tubules were visible and patent or almost complete laser melting

Little to moderate or patchy mounts of smear layer; covering 25%–50% of the specimen, many tubules visible and patent, laser melting

Moderate amount of scattered or aggregated smear layer; covering 50%–75% of the specimen; minimal to no tubule visibility or patency; or no visible laser melting

Heavy smear layer covering >75% of the specimen; no tubule orifices were visible or patent; or no visible laser melting.

Statistical analysis

The results were analyzed using descriptive statistics and making comparisons between groups. Discrete (categorical) data were summarized as in proportions and percentages (%). Proportions were compared using the Chi-square test.

To calculate the Chi-square test, the generally used formula is:

where Oi are the observed frequencies and Ei are the expected frequencies. P value is then generated using the theoretical distribution of Chi-square at a given degrees of freedom.

OBSERVATION AND RESULTS

In coronal area

The quality of various rotary endodontic files according to mean smear layer removing score for the coronal area was found to be

Group IV > Group II > Group I > Group III.

According to Kruskal–Wallis test, the difference in ranks of smear layer remaining scores among various groups for the coronal area was found to be statistically significant (P = 0.011) [Table 1].

Table 1

Average smear layer scores among various groups for the coronal area

Groups	n	Mean±SD	Minimum-maximum	IQR	Mean rank
Group I	15	2.67±0.617	1-3	1.00	35.40
Group II	15	2.47±0.516	2-3	1.00	29.10
Group III	15	2.80±0.561	2-4	1.00	37.40
Group IV	15	2.00±0.756	1-3	2.00	20.10
Kruskal-Wallis Chi-square statistic	11.192
P	0.011

IQR: Inter quartile range, SD: Standard deviation

In the middle area

The quality of various rotary endodontic files according to mean removing smear layer score for the middle area was found to be

Group IV > Group I = Group II > Group III

According to the Kruskal–Wallis test, the difference in ranks of smear layer remaining scores among various groups for the middle area was not found to be statistically significant (P = 0.242) [Table 2].

Table 2

Average smear layer scores among various groups for the middle area

Groups	n	Mean±SD	Minimum-maximum	IQR	Mean rank
Group I	15	2.87±0.743	1-4	0.00	33.10
Group II	15	2.87±0.640	2-4	1.00	32.00
Group III	15	2.93±0.704	2-4	1.00	33.40
Group IV	15	2.47±0.640	1-3	1.00	23.50
Kruskal-Wallis Chi-square statistic	4.182
P	0.242

IQR: Inter quartile range, SD: Standard deviation

In the apical area

The quality of various rotary endodontic files according to the mean smear layer removing score for the apical area was found to be

Group IV > Group II > Group I > Group III [Table 3].

Table 3

Average smear layer scores among various groups for the apical area

Groups	n	Mean±SD	Minimum-maximum	IQR	Mean rank
Group I	15	3.07±0.704	2-4	1.00	32.87
Group II	15	3.00±0.535	2-4	0.00	31.37
Group III	15	3.20±0.561	2-4	1.00	36.13
Group IV	15	2.60±0.632	2-4	1.00	21.63
Kruskal-Wallis Chi-square statistic	7.475
P	0.058

IQR: Interquartile range, SD: Standard deviation

According to Kruskal–Wallis test, the difference in ranks of smear layer remaining scores among various groups for the apical area was not found to be statistically significant (P = 0.058) [Table 3].

The quality of various rotary endodontic files according to mean smear layer removing score for the total area including coronal, middle, and apical is found to be [Table 4].

Table 4

Average smear layer scores among various groups for total area

Groups	n	Mean±SD	Minimum-maximum	IQR	Mean rank
Group I	15	8.60±1.639	4-11	2.00	35.80
Group II	15	8.33±1.447	6-11	1.00	30.57
Group III	15	8.93±1.534	6-12	2.00	37.43
Group IV	15	7.07±1.387	5-9	2.00	18.20
Kruskal-Wallis Chi-square statistic	11.718
P	0.008

IQR: Inter quartile range, SD: Standard deviation

Group IV > Group II > Group I > Group III.

DISCUSSION

The goals of instrumentation include a continuously tapered preparation that maintains the canal anatomy, facilitates optimal irrigation, allows complete debridement and placement of local medicaments, and permanent root filling. Thus, mechanical instrumentation plays a major role in the cascade of treatment procedures that aim at the eradication of microbes from the root canal.

The canal cleaning ability of endodontic files can be evaluated from its ability to remove debris and smear layer which is an essential prerequisite for the successful outcome of endodontic treatment. Smear and debris layer leads to the following difficulties during endodontic treatment: an unpredictable thickness and volume due to greater water portion limit its removal and optimum penetration of disinfectants, contains bacteria, their by-products, and necrotic tissue which is a reservoir of microbial irritants allowing their deeper penetration in the dentinal tubules, compromises the seal of obturated material and its loosely adherent nature is a potential avenue for leakage. Debris and smear layer removal depends not only on the irrigation method but also on the design of the endodontic instrument (size, taper, cross-section, etc.), the way instrument is used (rotational or vibrational), and the method of preparation (step back or crown down).[8] Rotary NiTi systems differ in their cleaning ability, possibly owing to their flute design. Instruments having U-shaped blades with radial land have inferior cleaning efficiency, possibly due to their scraping action on the canal walls rather than a cutting action. Instruments with positive cutting angles may have a superior cleaning efficiency than those with neutral or negative cutting angles since they cut.

In the present study, the smear layer removal efficiency Of ProTaper Universal, ProTaper Gold, EDM, and Revo-S rotary instruments were examined based on a numerical evaluation scheme for presence/absence of smear layer, in the coronal, the middle, and the apical portions of the canals. The smear layer removal efficiency of four instruments was evaluated using a standard irrigation combination of NaOCl and EDTA-containing chelating agents routinely used in clinical situations. The result of this study showed that the mean smear layer remaining score in the coronal 1/3rd area was found to be minimum (2.00 ± 0.756) for Group IV and the results were also statistically significant (P = 0.011). Revo –S files showed better smear layer removal followed by ProTaper Gold, ProTaper Universal and then EDM.

In the middle third, the mean remaining smear layer score was found to be minimum (2.47 ± 0.640) for Group IV followed by Group I (2.87 ± 0.743) and Group II (2.87 ± 0.640) and it was statistically significant (P = 0.242). In this region, ProTaper Gold and ProTaper Universal showed equal results in the removal of the smear layer which does not correlate with the earlier study conducted by Arslan et al.[9]

In the apical third area, the mean remaining smear layer score was found to be minimum (2.60 ± 0.632) for Group IV. The smear layer efficiency of Revo-S files is better followed by ProTaper Gold, ProTaper Universal, and EDM and the result was not found to be statistically significant (P = 0.058).

In paired comparisons between the groups using Mann–Whitney U-test it was found that the differences in rank scores of Group I versus Group IV, Group II versus Group IV, and Group III versus Group IV were statistically significant (P ≤ 0.05).

On the basis of the total quality score, the quality of various rotary endodontic files can be categorized into four groups. 3.00 – 5.25 = very Good, 5.25 – 7.50 = Good, 7.50 – 9.75 = Fair, 9.75 – 12.00 = Poor. There was near to significance (P = 0.058) difference seen between the qualities of the four types of rotary endodontic files. The best one was found to be Group IV (Revo-S) file (13.3% very good and 46.7% good) followed by Group I (ProTaper Gold) file (6.7% very good, 6.7% good).

Revo-S files have asymmetrical cross-section larger taper (4% and 6%) which allows effective shaping and cleaning due to good cutting contact and due to continuous smooth funnel shape, if may allow penetration of needle and irrigating solution and sufficient irrigation flow that might have further allowed more removal of smear layer and debris during biomechanical preparations procedures. Positive rake angles of instruments cut more efficiently than negative or neutral rake angles, which may be one of the reasons for the better cleaning ability of Revo- S files.[1011]

CONCLUSIONS

Based on the present study, Revo S files showed the best smear layer removal in the coronal, middle, and apical 1/3rd of the root canal when compared to other groups. The quality of various files according to ranks of smear layer removal was found to be: Group IV > Group II > Group I > Group III.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.