Ultrasonic versus sonic activation of the final irrigant in root canals instrumented with rotary/reciprocating files: An in-vitro scanning electron microscopy analysis

Abstract

Aim:

To compare the smear layer and debris removal in root canals instrumented with two different kinematic motions after ultrasonic and sonic irrigation activation.

Materials and Methods:

Eighty freshly extracted teeth were selected for the study and randomly divided the samples into two groups (n = 40) for instrumentation with either rotary ProTaper NEXT (PTN) or reciprocating WaveOne (WO) file systems. These (n = 40) were further divided into two groups (n = 20) where the final irrigant was activated using either Ultrasonics (Passive Ultrasonic Irrigation; PUI) or Sonics (EndoActivator; EA). Group 1: PTN + EA; Group 2: PTN + PUI; Group 3: WO + EA; and Group 4: WO + PUI. During instrumentation, a total of 4 ml of 5.25% NaOCl was used for irrigation. The final irrigation protocol included NaOCl and Smear Clear Solution. The samples were processed by scanning electron microscopic examination for debris and smear layer scoring, and statistical analysis was done.

Results:

The mean debris and smear layer score was less in the group instrumented by PTN with sonic activation of the irrigant.

Conclusion:

A combination of PTN instrumentation with sonic irrigation activation by EA is more effective in debris and smear layer removal in the groups tested.

INTRODUCTION

The ultimate goal of endodontic therapy is complete debridement of the entire root canal system with efficient disinfection.[1] Shaping the canal walls creates debris and a resultant smear layer comprising organic and inorganic substances, including microorganisms and their by-products.[2] This debris and smear layer not only prevent the penetration of root canal irrigants into the dentinal tubules but also intracanal medicaments [3] and sealers [4] that fail to effect an entire sealing of the root canal system. Therefore, it is essential to use a chemomechanical system that results in leaving the canal walls with the least amount of debris and smear layer.[5]

Recently, new generation nickel-titanium (Ni-Ti) instruments have been designed for better root canal instrumentation augering debris coronally.[6] The Wave One (Dentsply, Maillefer) NiTi single-file system is used in reciprocating motion. The counter-clockwise engaging angle is five times the clockwise disengaging angle. ProTaper NEXT (PTN; Dentsply Tulsa Dental, Tulsa, OK) files are uniquely designed, such that the center of mass and center of rotation are offset.[7] These instruments are manufactured with M-Wire technology, which increases flexibility and resistance to cyclic fatigue.[8]

These files tend to create a circular bore preparation irrespective of the canal shape and fail to reach buccal/lingual recesses and irregularities such as isthmi and canal fins.[9] Such areas might harbor bacteria caused by the packed debris and result in persistent periradicular inflammation.

It has been reported in literature that various irrigation activation systems might result in producing cleaner radicular dentinal walls, increasing the bond strength of canal-filling materials.

Hence, this study was formulated to evaluate the debris and smear layer removal after instrumenting them with rotary/reciprocating files and activating the final irrigant with ultrasonics (PUI)/sonics (EndoActivator; EA).

MATERIALS AND METHODS

Human mandibular premolars freshly extracted (for orthodontic treatment) with a single straight canal and fully formed apices were selected, and confirmation of single canal was carried out by buccolingual and mesiodistal angulated radiographs. The samples were decoronated with a diamond disk under water to a length of 16 mm. Hand instrumentation was done until #25k-file. The samples were randomly divided into four groups (n = 20):

• Group 1: PTN + EA

• Group 2: PTN + PUI

• Group 3: WO + EA

• Group 4: WO + PUI.

Group 1 and 2: Instrumentation with Protaper NEXT

Specimens were instrumented by PTN files until full sequence X4 (40/.06). These files were used in outward brushing mode at 300 rpm/2.6 nm. About 3% of sodium hypochlorite and ethylenediaminetetraacetic acid (EDTA) were used as an irrigant and lubricant alternatively between each file and recapitulation.

Group 3 and 4: Instrumentation with WaveOne

Specimens were instrumented by WO large files (40/.08) (Dentsply Maillefer, Ballaigues, Switzerland) in pecking motion, using X Smart Plus endomotor (Dentsply-Maillefer). These files were used in outward brushing mode at 300 rpm/2.6 nm. Again 3% of sodium hypochlorite and EDTA were used as an irrigant and lubricant alternatively between each file and recapitulation.

Final irrigation protocol

After instrumentation, 2 ml of 5.25% NaOCl was used as a final irrigant for 1 min, followed by 1 ml Smear Clear solution for 1 min, followed by 2 ml of 5.25% NaOCl. Each irrigant solution was activated either sonically or ultrasonically.

Group 1 and 3: Final irrigant activated by sonics (EndoActivator)

The EA (Dentsply, Mallifier) sonic handpiece with a size #25/0.04 taper activator tip was passively inserted to within 2 mm of the working length and used in a pumping action to move the tip for 1 min in short, 2–3 mm vertical strokes.

Group 2 and 4: Final irrigant activated by ultrasonics (PUI)

An ultrasonic tip 20/.04 (Satellec, Acteon) was passively inserted into the canal 1 mm short of working length and driven by an ultrasonic device (Satellec, Acteon) with power set at 5 for 1 min. A final flush with 5 ml distilled water was done in all the groups and the canals were dried with paper points.

Microscopic evaluation

To facilitate splitting of the roots, two longitudinal grooves were made with a diamond disk on the buccal and lingual surfaces. The roots were then split into two halves with a chisel, and the half containing the most visible part of the apex was conserved and coded. The specimens were dried, mounted on metallic stubs, and examined under scanning electron microscopy. Photomicrographs from the approximate center of the coronal (10 mm to the apex), middle (6 mm to the apex), and apical (2 mm to the apex) thirds of each specimen were taken at ×500 for debris and ×1500 for smear layer evaluation.

The photographs were blindly evaluated by two observer's using Schäfer and Lohmann criteria.[10]

The cleanness of each canal was evaluated by means of a numeric evaluation scale as follows.

Debris score

Debris score (dentinal chips, pulp remnants, and particles loosely attached to the canal wall):

1. Score 1: Clean canal wall, few debris particles

2. Score 2: A few small agglomerations

3. Score 3: Many agglomerations; <50% of the canal wall covered

4. Score 4: More than 50% of the canal wall covered by debris

5. Score 5: Canal wall completely covered by debris.

The SEM images depicting the debris scores are presented in Figure 1.

Figure 1.

Debris scores

Smear layer score

Smear layer score (dentin particles, remnants of vital or necrotic pulp tissue, bacterial components, and retained irrigant):

1. Score 1: No smear layer, orifices of the dentinal tubules patent

2. Score 2: Small amount of smear layer, some open dentinal tubules

3. Score 3: Homogeneous smear layer along almost the entire canal wall, with very few open dentinal tubules

4. Score 4: Entire root canal wall covered by a homogeneous smear layer, with no open dentinal tubules

5. Score 5: A thick homogenous smear layer covering the entire canal wall. The SEM images depicting the smear layer scores are presented in Figure 2.

Figure 2.

Smear layer scores

Statistical analysis

Kruskal–Wallis and Mann–Whitney U-tests were used for comparison of debris and smear layer scores. Friedman and Wilcoxon signed-rank tests were used to make comparisons between the studied parameters in different canal regions. The significance level was set at P - 0.05.

RESULTS

Statistical analysis of debris and smear layer scores for the tested groups instrumented canal walls exhibited varying amounts of remaining debris and smear layer along their entire length. The mean and standard deviation values for debris and smear layer scores are presented in Table 1.

Table 1.

Statistical analysis of debris and smear layer scores for the tested groups

Debris scores

For the middle and apical regions and the total debris score, there were significant differences among groups. At the middle and apical regions, Group 4 showed the highest mean. There were no significant differences among Groups 1, 2, and 3; all showed lower mean scores. A comparison between root regions: Group 1 showed a lower mean score in the middle region than the apical region. In Group 2, there was no significant difference between the middle and apical regions; both showed lower mean scores. In Groups 3 and 4, the middle region had a higher mean, but the apical region showed the highest mean debris score.

Smear layer scores

For the middle and apical regions and the total smear layer score, there were significant differences among groups. At the middle and apical regions, Group 4 showed the highest mean. There were no significant differences among Groups 1, 2, and 3; all showed lower mean scores. A comparison between root regions; Group 1 showed a lower mean score in the middle region than the apical region. In Group 2, the apical region showed the higher mean score. In Group 3, the middle region had a lower mean score, but the apical region showed a higher mean score. In Group 4, the middle region had a higher mean score, but the apical region showed the highest smear layer mean score.

DISCUSSION

The aim of endodontic treatment is the elimination of bacteria and the prevention of reinfection. This is accomplished by effective mechanical instrumentation and the use of irrigants and intracanal disinfectants.[11] Greater success in canal debridement can be achieved with increased apical size preparations and higher taper instruments; hence, all the samples were prepared until apical size 40/.06.[12,13]

In this study, the debris and smear layer were seen to increase from coronal to apical direction. This can be explained by the larger diameter of dentinal tubules in coronal regions exposed to increased volume of irrigants, and thus making debris and smear layer removal easier.[11,14]

Regardless of the final irrigation regimen used, the canal walls were cleaner in specimens prepared with PTN than those prepared with the WO system. This was attributed to the continuous forward motion of the rotary file which enabled constant exit of debris up the flute of the file. However, each backward motion of the reciprocating file might provide the opportunity for debris to accumulate in isthmus areas. Moreover, the reciprocating motion of the file might not allow the blade to cut as cleanly into the dentine, but push debris into recesses and isthmuses, resulting in a sort of burnishing effect.[15,16] This result is comparable to the results of other studies, showing that the reciprocating motion induces greater debris accumulation.[5,15]

It is also worth noting that the rotary group was irrigated more frequently, although the same volume of irrigant was used in both groups. With the single-file system, greater amounts of debris were packed laterally in isthmuses and protrusions.[15,17,18]

A file with an offset cross-section design, that is, the PTN affords more cross-sectional space for enhanced cutting, loading, and augering debris out of a canal. It decreases the probability of laterally compacting debris and blocking root canal system anatomy.[7,19] To improve cleanliness, irrigants must be in contact with the root canal. The conventional needle irrigation technique delivers solutions no more than 0–1.1 mm beyond the needle tip.[2,20] This is insufficient for complete cleaning of the intricate anatomy of the root canal system. A vapor lock, resulting in trapped air in the apical third of root canals, hinders the exchange of irrigants, and affects their debridement efficacy.[21]

Agitation with sonic and ultrasonic systems showed greater effectiveness in debris and smear layer removal.[22] PUI creates acoustic microstreaming which produces sufficient shear stresses to dislodge debris from instrumented canals. Along the length of an activated ultrasonic file, it has multiple nodes and antinodes.[23] Hence, PUI has been showed effective in smear layer removal.[21,24] However, it also creates the undesirable dampening effect of amplitude of its characteristic nodes and antinodes pattern, especially when the instrument touched the lateral walls of a shaped canal. This may be the reason in our study; PUI was less effective in smear layer removal than EA in the apical third. Sonic activation operated with one single positive and negative node. The movement of the vibratory sonic instrument was not influenced by lateral wall contact.[25] Similar results were also seen in other studies, showing that sonic irrigation is superior to ultrasonic irrigation activation [1,2,22] while in another study,[26] no significant difference was found among the two.

CONCLUSION

Complete cleanliness was not achieved in any of the groups. Based on the results, using the EA in canals prepared with PTN produced the cleanest canal walls. Results of the PTN system were superior to those of the WO system.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.