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Smear layer removal

 

 

Abstract:

 

      Studies have shown that current methods of cleaning and shaping root canals produce a smear layer that covers the instrumented walls. This layer contains inorganic and organic substances that include fragments of odontoblastic processes, microorganisms, and necrotic materials.

     The advantages and disadvantages of the presence of the smear layer created by root canal instrumentation, and if it should be removed from instrumented root canals are still controversial issues. Nowadays, the consensus is toward smear layer removal. Various techniques and chemicals were used for smear layer removal including the use of chelating agents, acids, ultrasonic agitation and laser…

 

 

Introduction:

 

     Root canal treatment can be summarized as a series of procedures access, cleaning, shaping and three dimensional filling of the root canal system to prevent reinfection.

     One of the principal goals of root canal treatment is the cleaning of the entire root canal system through the removal of pulpal debris, smear layer, and smear plugs …

 

Structure of Smear Layer:

 

     It has been recognized that root canal instrumentation produces a smear layer that cover the surfaces of the prepared canal walls. It is composed of debris compacted into the surface of dentinal tubules by the action of instruments. It is burnished into the surfaces as the edges of instruments slide by.  

     This layer contains inorganic and organic substances that include fragments of odontoblastic processes, microorganisms and necrotic materials.

 

 

Advantages and Disadvantages of Smear Layer:

 

     Despite the controversy regarding the effect of the smear layer on the quality of instrumentation and obturation, several investigators have found that the smear layer itself may be infected and may protect the bacteria already present in the dentinal tubules. Presence of this smear layer prevents or delays penetration of intracanal medication, intracanal irrigants and antimicrobial agents into the irregularities of the root canal system and the dentinal tubules and also prevents complete adaptation of obturation materials to the prepared root canal surfaces. Because of these concerns, one may deem it prudent to remove the initially created smear layer in infected root canals and to allow the penetration of intra canal medication into dentinal tubules.

     Clinicians in favor of leaving the smear layer intact argue that it may be a clinical factor that actually enhances endodontic success. It appears to plug the dentinal tubules, microbes and tissue included preventing bacterial egress from the tubules after treatment.

     Currently the consensus is toward smear layer removal in order to reduce the microflora and associated endotoxins, enhance the sealing capability of obturating materials and decrease the potential of the bacteria to survive and reproduce.

 

 

Methods of Smear Layer Removal:

 

     Various acids, ultrasonic instruments, and lasers have been used to remove the smear layer.

 

·             Chemical Removal:

 

v           Combination of Sodium Hypochlorite and EDTA:

     The components of the smear layer are very small particles with a large surface-mass ratio, which make them very soluble in acids. Because of this characteristic, acids have been used to remove the smear layer. At present, chemical conditioning is the only way to obtain complete cleaning of dentinal walls. Smear layer removal requires a combination of sodium hypochlorite (an organic solvent) and substances active on inorganic compounds, including chelating agents (EDTA or REDTA) or acids (orthophosphoric, polyacrylic, tannic, maleic or citric acid) to remove both organic and inorganic components.

     Sodium hypochlorite (NaOCl) is the most commonly used irrigant in root canal treatment, and has proven to be an excellent irrigating solution, due to its tissue dissolving capability and microbicidal activity. However, its action does not affect inorganic material. EDTA complements the action of sodium hypochlorite, by chelating calcium ions in dentine and making instrumentation of the root canal easier, and is effective at neutral pH.

      O’Connell et al (1) evaluated three solutions of EDTA for their ability to remove the smear layer. All solutions were adjusted to pH 7.1 using either sodium hypochlorite or HCl. When the EDTA solutions were alternately used for root canal irrigation with 5.25% sodium hypochlorite, they completely removed the smear layer in the middle and coronal thirds of canal preparations, but were less effective in the apical third.

     None of the EDTA solutions by themselves were effective in completely removing the smear layer at any level. A final irrigation of the root canal system with sodium hypochlorite after EDTA irrigation seems to produce the cleanest walls. The alkaline tetra sodium salt, pH adjusted with HCl, is more effective and performed equally as well as the more commonly used disodium salt (1).    

     The main disadvantages of the use of EDTA include its destructive effects on coronal and middle thirds of root dentin and its limited antibacterial effects (2).

 

v          Tetracyclines:

     Tetracyclines are broad spectrum antibiotics that are effective against a wide range of microorganisms. Tetracyclines have many unique properties in addition to their antimicrobial effect .They have low pH in concentrated solution and thus can act as a calcium chelator, and they can cause enamel and root surface demineralization. The ability of the tetracycline family of antibiotics to remove the smear layers has also been studied. They have been used to demineralize dentin surfaces, uncover and widen the orifices of dentinal tubules, and expose the dentinal collagen matrix.

     Torabinejad et al (2) investigated the effect of a mixture of a tetracycline isomer, an acid, and a detergent (MTAD) as a final rinse on the surface of instrumented root canals. Forty-eight extracted maxillary and mandibular single-rooted human teeth were prepared by using a combination of passive step-back and rotary 0.04 ProFile taper nickel-titanium files. Sterile distilled water or 5.25% sodium hypochlorite was used as intracanal irrigant. The canals were then treated with 5 ml of one of the following solutions as a final rinse: sterile distilled water, 5.25% sodium hypochlorite, 17% EDTA, or a new solution, MTAD. The presence or absence of smear layer and the amount of erosion on the surface of the root canal walls at the coronal, middle, and apical portion of each canal were examined under a scanning electron microscope. The results showed that MTAD is an effective solution for the removal of the smear layer and does not significantly change the structure of the dentinal tubules when canals are irrigated with sodium hypochlorite and followed with a final rinse of MTAD(2) .

     In contrast to the destructive effects of 5-minute EDTA exposure, they observed no significant dentinal erosion in a pilot project when the surfaces of the root canals were in contact with MTAD for periods ranging from 1 to 20 minute (2).

 

v          Organic acids:

     Citric acid (a week organic acid) has been applied previously on the root surfaces altered by periodontal disease and instrumentation in order to increase cementogenesis and to accelerate healing and regeneration of a normal periodontal attachment after flap surgery. In operative dentistry citric acid has been proposed as a mild etchant for dental hard tissue. In endodontic research, substitution of EDTA with an aqueous citric acid solution as an endodontic irrigant has recently been proposed (3).

 

     Di Lenarda et al (3) evaluated in vitro the cleansing and smear layer removal capability of alternate canal irrigation with citric acid and Sodium Hypochlorite. Eighty one teeth were divided into three groups on the basis of the type of instrumentation then they were divided on the basis of irrigation protocol: 5% sodium hypochlorite alone, sodium hypochlorite alternated with 1 mol/L citric acid solution or a combination of 15% EDTA and Cetrimide solution. Results showed that 1 mol/L citric acid solution was as effective in removing smear layer as10% EDTA, but was superior in specimens treated with ProFile 0.04 taper instruments. 10% EDTA and 1mol/L citric acid, both alternated with sodium hypochlorite had an efficacy that varied with the duration of application. The simple preparation, low cost, good chemical stability if correctly used and their effectiveness even with short application times suggests citric acid irrigant suitable for clinical use (3).

 

Risk associated with citric acid use:

 

     One of the main problems associated with using citric acid is its very low pH, whilst an EDTA solution is almost neutral. Irrigation of the canal with both citric acid and sodium hypochlorite ensures neutralization of the previous irrigant, with a drastic modification of the pH inside the canal and the liberation of gaseous chloride (3).

 

 

 

·             Mechanical Removal (Ultrasonic): 

      

    Sodium hypochlorite and EDTA must be in direct contact with the surface for effective action. Due to the small diameter of root canals, it is often difficult for the irrigating solutions to reach the apex of the tooth. Ultrasonically activated files may be a means of reaching the entire length of the root canal with irrigating solutions. The cleaning ability of such files is assisted by acoustic microstreaming.

 

     Guerisoli et al (4) evaluated smear layer removal by different irrigating solutions under ultrasonic agitation. They used twenty recently extracted mandibular incisors with a single root canal and divided them into four equal groups. Each group was irrigated with either distilled water, 1% sodium hypochlorite alone or associated with 15% EDTAC between each file size. The final group was not instrumented but irrigated with 1.0% sodium hypochlorite and 15% EDTAC. A size 15 file energized by ultrasound was used with small amplitude filing movement against the canal walls in all groups.  Results showed that groups irrigated with 1% sodium hypochlorite alone and that irrigated with distilled water had canal walls covered with smear layer. Canals irrigated with 1.0% sodium hypochlorite and 15% EDTA had less smear layer throughout the canal, indicating that under ultrasonic agitation, sodium hypochlorite associated with EDTAC removed the smear layer from root canal walls, whereas irrigation with distilled water or 1.0% sodium hypochlorite alone did not remove the smear layer. No statistical differences between root thirds when groups were examined separately. The lack of differences between root thirds may be explained by the low surface tension of the solution provided by the cationic surfactant and the use of ultrasound.

 

 

 

 

·             Laser Removal:

 

     The use of the laser beam, especially Nd:YAG laser, in the dental field is well known and has been investigated by numerous researchers. In endodontic applications, Nd:YAG laser can stop bleeding after pulpectomy or apicectomy, promote disinfection and sterilization, prevent pain after treatment, seal dentinal tubules and remove debris and smear layer from instrumented root canals.

 

     Goya et al (5) evaluated the removal of smear layer at the apical stop by pulsed Nd:YAG laser irradiation with or without black ink, and the degree of apical leakage after obturation in vitro. Sixty extracted human single rooted teeth were used, instrumented up to a size 40 K-file and then divided into three groups: group one was unlased, group 2 was treated with laser and the third group was treated with laser and black ink. The laser was operated at 2 W and 20 pp for 2 seconds, which is suitable for infected root canals without any injury to periapical tissues. Results showed that all root canal surfaces became melted, fused, and recrystallized and in most cases the smear layer was also melted and fused to the dentinal walls. 60% leakage was found in group1, 20% in group2 and no leakage in group 3. Furthermore, the results demonstrated that the use of black ink enhanced the effects of the Nd:YAG laser. Therefore, after obturation, intimate contact between obturating materials and root canal wall dentine was achieved and a considerable reduction of apical dye penetration was observed.

 

 

 

Conclusion:

 

     It can be concluded that current methods of root canal instrumentation produce a layer of organic and inorganic material (smear layer) that may also contain bacteria and their byproducts. This layer covers the instrumented walls and may prevent the penetration of intracanal medications into the dentinal tubules and may affect close adaptation between root canal filling materials and the root canal walls. This layer is removed by lasers as Nd:YAG laser which its effect is enhanced by the use of black (Indian) ink, by ultrasonic agitation with 1% sodium hypochlorite combined with 15% EDTAC, by organic acids as citric acid combined with sodium hypochlorite, by chelating agents as the different salts of EDTA combined with sodium hypochlorite and by the newest of all  the MTAD which is effective when used as a final rinse and cause no harm to the tissues…

 

 

 

 

 

 

 

References:

 

1.          O’Connell MS, Morgan LA, Beeler WJ, Baumgartner JC.: A comparative study of smear layer removal using different salts of EDTA.

 J Endodon 2000; 26: 739-743.

 

2.          Torabinejad M, Khademi A, Babagoli J, Cho Y, Johnson W, Boshilov K, Kim J, Shabahang S.: A new solution to remove the smear layer.

J Endodon 2003; 29: 170-175

 

3.          R.Di Lenarda, M. Cadenaro, O. Sbaizero.: Effectiveness of 1 mol/L citric acid and 15% EDTA irrigation smear layer removal.

 Int Endod J 2000; 33: 46-52

 

4.          Guerisoli DMZ, Marchesan MA, Walmsley AD, Lumley PJ, Pecora JD.: Evaluation of smear layer removal by EDTAC and sodium hypochlorite with ultrasonic agitation. Int Endod J 2002; 35: 418-421.

 

 

5.          C. Goya, R. Yamazaki, Y. Tomita, Y. Kimura & K. Matsumoto.: Effects of pulsed Nd:YAG laser irradiation on smear layer at the apical stop and apical leakage after obturation. Int Endod J 2000; 33: 266-271.