Evaluation of smear layer removal from ultrasonically prepared retrocavities by three agents.

OBJECTIVES: To evaluate the efficacy of 35% orthophosphoric acid, 24% ethylenediaminetetraacetic acid (EDTA) and 10% citric acid in the removal of smear layer from retrocavities prepared with ultrasonic retro-tips using scanning electron microscopy.
MATERIALS AND METHODS: Root canals of fifty single-rooted teeth were cleaned, shaped, and obturated with gutta-percha and AH plus sealer. The apical 3 mm of each root was sectioned with a diamond disc and retrograde cavities were prepared with Kis # 1 ultrasonic retro-tips to a depth of 3 mm. Retro cavities in Group I were treated with a gel of 35% orthophosphoric acid for 15 seconds, Group II were treated with a gel of 24% EDTA at neutral pH for 2 minutes, and Group III were treated with a gel of 10% citric acid for 2 minutes, followed by 1 minute rinsing with distilled water for all groups. The samples were prepared for scanning electron microscopic observation. Scoring was performed for the presence of the smear layer on the walls of the retrocavity.
RESULTS: In the orthophosphoric acid group, it was observed that all dentinal tubules were open in 70% of the samples. The majority of analyzed samples in EDTA and citric acid group showed dentinal tubules covered with the smear layer.
CONCLUSIONS: Application of 35% orthophosphoric acid gel for 15 seconds on retrocavities prepared with ultrasonic retro-tips is the most effective means for removal of smear layer.

INTRODUCTION

Endodontic treatment of teeth is a routine therapy to treat inflammation or necrosis of the pulp. It is established that the success rate of conventional endodontic therapy is 85% to 95%.[1] Inspite of this high success rate, root canal therapy occasionally fails for which retreatment is needed. However, some cases cannot be retreated conservatively because of various limitations and therefore an endodontic surgery is required to save the affected tooth.[2]

The management of the root during endodontic surgery includes root end resection, root-end preparation, and placement of a root end filling to seal the root canal.[3]

During root end cavity preparation with ultrasonic retro-tips, smear layer is produced on the walls.[4] Smear layer contains microorganisms and necrotic pulpal tissues. Bacteria can survive and proliferate inside this smear layer.[56]

The presence of smear layer has been postulated as an avenue for leakage and source for bacterial growth and ingress, particularly following root end preparation. Also, it seems to support growth of bacteria remaining in the dentinal tubules.[7] The debris created during root-canal instrumentation should be removed from the dentine surface of the canal wall and the dentine tubules.[8]

Therefore, it would be advantageous to remove the smear layer from the retrocavity. This enhances root end filling material adaptation and potentially eliminates or minimizes apical leakage.[7]

Various gels and solutions such as citric acid, tannic acid, polyacrylic acid, phosphoric acid, chelating agents such as ethylenediaminetetraacetic acid (EDTA) or REDTA, doxycycline, and tetracycline isomers have been used to remove the smear layer formed on the walls of the retrocavity following preparation.[9]

Hence the objective of this study was to evaluate the efficacy of 35% orthophosphoric acid gel, 24% EDTA gel and 10% citric acid gel in removal of smear layer from retrocavities prepared with ultrasonic retro-tips using scanning electron microscopy.

MATERIALS AND METHODS

Sample selection

Fifty human single-rooted teeth, extracted for orthodontic/periodontal reasons, were employed in the study.

Teeth preparation

To facilitate instrumentation, the crowns of all teeth were decoronated at the level of the cementoenamel junction using diamond disc (Horico, Germany). The working length was determined by inserting a #10 K-file (Mani, Japan) into the canal until it just exits the apical foramen and then subtracting 1 mm from the obtained length.

Canal preparation technique

The root canals were prepared with the ProTaper (Dentsply, Maillefer, Switzerland) rotary file system according to manufacturer's instructions in a crown down manner. The final apical size at the working length was F2 having an apical diameter 0.25 mm and taper of 0.08. During biomechanical preparation the canals were irrigated with 2 ml of 3% sodium hypochlorite (NaOCl) after each instrumental change. At the end of the instrumentation, the root canals were dried and irrigated with 17% EDTA. The root canals were dried again and irrigated with 2 ml of 3% NaOCl.

Apicoectomy and retrograde cavity preparation

The specimens were then divided into two groups. Preliminary control group (n = 10) and experimental group (n = 40). The preliminary control group did not receive any obturation, apicoectomy or retrograde cavity preparation. The experimental groups were obturated with gutta-percha (Dentsply-Maillefer, Ballaigues, Switzerland) AH Plus sealer (Dentsply/ De Trey, Konstanz, Germany) using lateral condensation technique. After obturation, the apical 3 mm of each root was resected at an angle of 900 with respect to the long axis of the root with a diamond disc. Retrograde cavities were prepared with Kis # 1 diamond-coated surgical ultrasonic retrotips (Dentsply, Maillefer, Switzerland) with ultrasonic handpiece (Mini Piezo; EMS, Nyon, Switzerland) set at full power to a depth of 3 mm into root canal.

Preliminary control group

In this group, specimens were split longitudinally in the buccolingual plane. To facilitate separation of the root into two halves, all roots were grooved longitudinally on the external surface with a diamond disc (Horico, Germany) and split into two halves with a chisel. The half containing the most visible portion of the apex was selected for SEM observation (Cambridge S60, Germany).

Experimental group

The specimens were divided into four groups of 10 teeth each. Retro cavities in Group I were treated with a gel of 35% orthophosphoric acid for 15 seconds, followed by 1 minute rinsing with distilled water. Group II retrocavities were treated with a gel of 24% EDTA at a neutral pH for 2 minutes, followed by 1 minute rinsing with distilled water. Group III retrocavities were treated with gel of 10% citric acid for 2 minutes, followed by 1 minute rinsing with distilled water. Group IV (Negative control group) did not receive any treatment of the reretrocavity.

Analysis with a scanning electron microscopy

The specimens were immersed in liquid nitrogen for one hour. All roots were grooved longitudinally on the external surface with a diamond disc (Horico, Germany), then split into two halves with a chisel. The half containing the most visible part of the retrocavity was mounted on a brass stub and gold sputtered to examine under scanning electron microscopy (Cambridge S60, Germany).

Sample observation and classification

An evaluation was performed to record the presence of smear layer on the surface of the retrocavity walls based on the following score described by Hulsmann et al.,[9](1997):

Score 1: Dentinal tubules completely open

Score 2: More than 50% of dentinal tubules open

Score 3: Less than 50% of dentinal tubules open

Score 4: Almost all dentinal tubules covered with smear layer.

Statistical analysis

The data were then analyzed statistically using a non-parametric test (Kruskall-Wallis test) and post-hoc analysis, using Statistical Package for the Social Sciences software (SPSS 17) data analyzing software.

RESULTS

The analyzed samples that showed a cleanliness of the walls with dentinal tubules completely open (score 1) were all from Group I. In this group, 70% of the observed samples had this score [Figure 1a]. The majority of analyzed samples in Group II (5 samples of 10 [50%]) showed dentinal tubules covered with the smear layer (score 4) [Figure 1b] and in the Group III, 40% of samples, all dentinal tubules were completely covered with smear layer [Figure 1c]. Eighty percent of samples in preliminary control group showed root canal walls covered with smear layer [Figure 1d]. All the analyzed samples of the negative control group showed dentinal tubules covered with smear layer [Figure 1e] [Table 1].

Figure 1

SEM images of (a) orthophosphoric acid group, (b) EDTA group, (c) citric acid group (d) preliminary control group and (e) negative control group

Table 1

Percentage distribution of scores for smear layer removal and inter-group comparison

Statistical inter-group comparison was done between orthophosphoric acid group and citric acid group. Orthophosphoric acid (Group I) was significantly superior in removing smear layer when compared with citric acid (P value-0.001). Also, phosphoric acid proved to be more effective than EDTA in removal of smear layer. This difference was statistically significant (P value-0.0001) [Table 1].

However, there was no significant difference in smear layer removal between EDTA and citric acid (P value-0.59) [Table 1].

Analysis of the results reveal that (Group I) orthophosphoric acid was more efficacious (statistically significant) than (Group II) citric acid and (Group III) EDTA in removing smear layer from retrocavities prepared with ultrasonic retro-tips.

DISCUSSION

Retrocavities have traditionally been prepared with burs and micro hand pieces. Currently, ultrasonic retro-tips are more popular for this purpose. Ultrasonic technique of root end preparation allows optimal root end preparation in terms of dimension and direction.[7]

An evaluation of the technique of root end preparation by Wuchenich et al., (1993) showed that ultrasonically created cavities had more parallel walls, deeper depths for retention, and preparation which followed the line of the root canal, and cleaner surfaces than those created with burs.[7]

The use of ultrasonic retro-tips during root end cavity preparation produces a smear layer on the walls of the retrocavity.[10] Smear layer is an amorphous, irregular entity containing organic (pulp tissue, bacteria) and inorganic (dentin) materials with microorganisms and necrotic pulpal debris.[411]

Pashley et al, 1997 believed that the smear layer contains bacteria and bacterial by-products and thus it must be completely removed from the root canal system.[12]

While the removal of smear layer subsequent to coronal cavity preparation and restoration has been advocated in some clinical situations (Brannstrom 1984), the removal of smear layer from apical retrocavity preparations has not been clinically addressed.[7] However, because smear layer may serve as either an avenue for leakage or as a bacterial substrate (Pitt Ford and Roberts 1990), removal of the smear layer prior to filling the retrocavity may be clinically appropriate.[7]

Effective removal of smear layer has been reported using a combination of 5.25% NaOCl and 17% EDTA solution (Baumgartner et al, 1987).[9] Although NaOCl remains gold standard as a result of its anti-microbial effect and tissue dissolution properties, it has no effect on inorganic portion of smear layer. Therefore, NaOCl has been used in association with EDTA, which acts on inorganic debris formed in instrumented root canals.[13] Other agents such as citric acid, tannic acid, polyacrylic acid, phosphoric acid and REDTA are also used for the removal of smear layer.[14]

Root end resection is determined by a number of factors. Since most ramifications are within the apical 3 mm of the root, this is the least amount of root end that should be removed. Von Arx and Colleagues, described the concept of “therapeutic length”. They recommended resection of the apical 3 mm, followed by the preparation of a root end cavity 3 mm deep, thus making the “therapeutic length” to 6 mm.[15]

During root end resection, the apical 3 mm of roots are resected at a 90° angle with respect to the long axis of the tooth. This minimizes exposure of dentinal tubules on the resected surface when compared to the 45° angle root end resection.[16]

There are no comparative studies on the different methodologies of studying smear layer removal from the walls of retrograde cavities during surgical endodontics, for this reason, we used the scoring criteria developed by Hulsmann et al (1997) to evaluate the endodontic smear layer after instrumentation of the canal.[17]

Seventy percent of the teeth in othophosphoric acid group showed cleanliness of the retrocavity walls with dentinal tubules completely open. Majority of analyzed samples of EDTA and citric acid groups showed dentinal tubules covered with smear layer. Eighty percent of samples in the preliminary control group showed root canal walls covered with smear layer. All the analyzed samples of negative control group showed dentinal tubules covered with smear layer.

The results of this study were consistent with those of Cristiano Fabiani et al (2011) who compared removal of smear layer from retrocavities using 35% orthophosphoric acid and 24% EDTA gel. They found that orthophosphoric acid was more effective in removing smear layer than EDTA.[16]

Also findings of this study were in accordance with Maira Prado et al (1995) who reported 37% phosphoric acid solution to be effective in removing smear layer when compared with that of 17% EDTA and 10% citric acid at 30 seconds, 1 and 3 minutes experimental time period.[18]

Overall, the results of our study suggested that orthophosphoric acid gel is more effective in removing smear layer when compared with citric acid and EDTA with lesser application time. Removal of smear layer enhances adhesion by intimate adaptation of the retrofilling material, making the root surface biocompatible and optimizing periodontal healing.

CONCLUSION

The following conclusions can be drawn from this study:

Application of 35% orthophosphoric acid gel for 15 seconds on retrocavities prepared with ultrasonic retro-tips is the most effective means for removal of smear layer.

35% orthophosphoric acid is more effective than 10% citric acid gel and 24% EDTA gel. Its action requires less time; hence, may be preferred for removal of smear layer.

There is no significant difference with the application of 24% EDTA and 10% citric acid in removal of smear layer on retrocavities prepared with ultrasonic retro-tips.