An In Vitro Comparison of Different Irrigants Used for the Removal of Calcium Hydroxide from Root Canal Walls

Introduction

Chemomechanical debridement and chemical disinfection of the root canal system is one of the chief factors that determines the prognosis of endodontic therapy. Despite considerable advancements, none of the methods or materials invented till date could achieve complete disinfection of the root canal system. Hence, chemical disinfection with intracanal irrigants and medicaments has become inevitable to achieve a successful end result of endodontic therapy.^1,2

The sole purpose of using intracanal medicament is to prevent or inhibit the growth of micro-organisms between appointments and to render an environment that is conducive to healing. Calcium hydroxide is one of the most widely used intracanal medicament in root canal treatment due to its potent antimicrobial activity, high alkalinity, tissue dissolution property, ability to degrade lipopolysaccharides and inhibit tooth resorption. The use of calcium hydroxide has also been suggested for teeth with pulp necrosis and apical periodontitis.³ The positive outcomes obtained with the calcium hydroxide is primarily due to its highly alkaline pH and its potential to rapidly split into hydroxyl and calcium ions. Calcium hydroxide has been employed as an intracanal medicament for numerous clinical applications, where it is left within the root canal system for varying periods of time ranging from seven days, to six months and even up to two years as the case scenario demands.⁴

Before obturating the cleaned and shaped root canals, calcium hydroxide placed within the root canal systems must be completely removed. Besides hindering the sealer penetration into dentinal tubules, the residual calcium hydroxide on the root canal dentin has undesirable effects on dentinal bond strength, which significantly compromises the quality of the seal. These remnants could also chemically react with the sealer hampering its flow and altering other properties. Therefore, calcium hydroxide removal prior to the root filling becomes mandatory.^5,6

Currently there are no methods or materials that can completely or predictably remove calcium hydroxide medicament from the root canal walls. Rotary or hand instrumentation of the canals up to the working length along with copious irrigation with saline, sodium hypochlorite (NaOCl) or ethylenediaminetetraacetic acid (EDTA) is one of the most frequently sought methods for eliminating residues of intracanal calcium hydroxide. Several studies have demonstrated the potential of EDTA to eliminate calcium hydroxide residues, but very few researchers have studied the efficacy of another potent calcium chelator – citric acid, in removing the calcium hydroxide residues.^5,7 A recent study has stated that using alcoholic solutions as the final rinse of root canals showed an enhanced sealer penetration and improved wettability of radicular dentin.⁸

Therefore, the present study aimed to compare the efficacy of saline, 3% NaOCl, 70% ethanol and 10% citric acid in removing calcium hydroxide dressing from the root canal walls under passive ultrasonic irrigation using a stereomicroscope.

Materials and Methods

The present research was approved by the local ethics committee. Forty straight and single rooted premolars were used for this study. Teeth with caries, restorations, cracks, root deformities or resorption were excluded. Ultrasonics was used to clear the calculus and soft tissue debris from the sample teeth. The samples were then stored in normal saline that was replaced every day.

A diamond disk was used to decoronate the sample teeth under constant irrigation. Crown -down technique was followed to prepare the root canals, using the ProTaper Universal system (up to size F3) which facilitated optimum irrigation. During preparation, 3% NaOCl was used as the irrigant. Final irrigation was done using 5 mL of 17% EDTA and 3% NaOCl for a minute, followed by 5 mL of saline in the end.

Thereafter, paper points were used to dry the root canal walls and calcium hydroxide paste with saline vehicle was applied on the walls until the material was seen extruding through the apex. The access cavities were temporized with a cotton pellet and intermediate restorative material (IRM). After this period, two coats of colored nail varnish were applied on the specimens, including the apical foramen, to prevent leakage of the irrigant. The specimen teeth were then incubated at room temperature for a week.

The samples were randomly allotted into four experimental groups (n=10) according to the type of irrigant used with a side vented needle and passive ultrasonic activation for one minute.

• Group 1: 6 mL saline

• Group 2: 6 mL 3% NaOCl

• Group 3: 6 mL 70% ethanol

• Group 4: 6 mL 10% citric acid (freshly prepared just before use)

Ultrasonic activation was achieved with a non-cutting 20.01 stainless steel ultrasonic file inserted at 2 mm from the working length and 1–2 mm up-and-down motion was applied.

Under dry conditions, the buccal and lingual surfaces of the root were cut with a diamond disk. This ensured the preserving of inner layer of radicular dentin surrounding the canal. With the help of a chisel, the roots were split and root halves of each sample were obtained. The amount of residual calcium hydroxide left on the root canal walls was observed under a stereomicroscope at a magnification of 30X. The percentage of the overall root canal surface area covered with Ca(OH)₂ residue was recorded. Initially, surface area of residual calcium hydroxide was calculated in terms of percentile, after which these values were scored from zero to five. This scoring system has been a basis to determine the calcium hydroxide residues in previously conducted in vitro studies.⁹

The scoring system was as follows:

• Score 0: clean root canal walls, without any residue

• Score 1: 0–10% of the root canal walls were covered with residues of Ca(OH)₂

• Score 2: 11–20% of the root canal walls were covered with residues of Ca(OH)₂

• Score 3: 21–50% of the root canal walls were covered with residues of Ca(OH)₂

• Score 4: 51–99% of the root canal walls were covered with residues of Ca(OH)₂

• Score 5: root canal walls were completely covered with residues of Ca(OH)₂

Statistical analysis

The data obtained was subjected to statistical analysis that used the SPSS software package (Statistical Package for Social Sciences, version 11.5, SPSS Inc). The nonparametric tests, Kruskal–Wallis and Dunn post hoc tests were used to make comparisons between the groups at a significance level of p<0.05.

Results

None of the irrigants could completely eliminate the residues of calcium hydroxide. Stereomicroscopic images showed that no statistically significant differences were observed between Group 1 (Saline) and Group 2 (NaOCl), with a score of 4 showing large amounts of residues.

Group 4 (10% citric acid) produced significantly cleaner walls with a score of 1, followed by Group 3 (70% ethanol) which showed a score of 2.

Discussion

The use of hand or rotary files to eliminate the residues of calcium hydroxide dressing should be avoided to prevent over preparation that could weaken root dentin. Effective techniques to eliminate intracanal medicament should include improved irrigation methods, which increases the penetration of irrigants into dentinal tubules. Several studies have stated that NaOCl or saline does not effectively eliminate the calcium hydroxide dressing from the root canal.^10,11

The effect of ultrasonic agitation of the irrigants has been evaluated with controversial results in this regard. Passive ultrasonic irrigation works on the basis of transmission of energy from an ultrasonically oscillating instrument to the irrigant within the root canal system.¹² It has been stated that irrigant solution in tandem with ultrasonic vibration is straightaway associated with the eradication of organic and inorganic debris from the walls of the root canal system. Thus, the effectiveness of irrigation is dependent on both mechanical flushing out and the chemical potential to dissolve tissue material. A study conducted by Zorin et al., denoted that PUI could significantly improve the removal of calcium hydroxide dressing. The study also stated that an increased volume of irrigant provided a better clearance of calcium hydroxide residues, irrespective of the solution activation.¹³

In the current study, citric acid led to maximum elimination of calcium hydroxide, which is due to its superior chelating property. It chemically reacts with calcium hydroxide and forms complexes with calcium, which gets easily flushed away on subsequent irrigation with saline or NaOCl. It is advantageous to avoid the use of agents that are known to cause dentinal erosion and root weakening, such as EDTA and NaOCl, to remove calcium hydroxide dressing and residues from the root canal walls.^5,14

Irrigation with 70% ethanol yielded better results than with 3% NaOCl and saline in the removal of calcium hydroxide which was statistically significant. 70% ethanol increased surface-free energy of the radicular dentin, significantly improving the wettability of the root canal sealer without altering the inorganic content of dentin. This increase in the wettability justified the present results.

Irrigation with ethanol might prove to be a more conservative approach to get rid of calcium hydroxide dressing, with fewer disadvantages. Ethanol causes desiccation and coagulation of proteins, acting as a tissue fixation agent. But this occurs only when the tissue is subjected to a high volume of ethanol for a long duration of time. In this study, a low volume of ethanol (6 mL) was used, for a very short period (3 minutes), along with side vented needles which further limits the apical extrusion.⁸

Conclusion and Clinical Significance

The present study denotes that citric acid and ethanol enhanced the cleansing of the root canal walls, leaving least amount of residual calcium hydroxide when compared to the most widely used irrigant solutions - saline and sodium hypochlorite. Calcium hydroxide residues adversely affect the obturation quality; hence ethanol and citric acid when used judicially can render dentinal walls that improve the sealer penetration. However, further studies and clinical trials with ethanol should be performed to confirm its efficacy in eliminating calcium hydroxide residues and to validate its safety towards periapical tissues.

Conflict of interest

Nil