Year : 2019 | Volume
: 9 | Issue : 2 | Page : 71--81
The smear layer in endodontic: To keep or remove – an updated overview
Ruaa A Alamoudi
Department of Conservative Dentistry, Division of Endodontics, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia
Dr. Ruaa A Alamoudi
Department of Conservative Dentistry, Division of Endodontics, Faculty of Dentistry, King Abdulaziz University, P.O. Box: 1119, Jeddah 21431
During mechanical preparation, the use of hand or rotary instruments results in the production of considerable amount of smear layer. The smear layer consists of two parts: a superficial layer that covers the dentinal wall and a smear plug which occludes that dentinal tubules. Researchers had reached to different conclusions on the importance of removing or maintaining this layer. Removing the smear layer allows for more cleaning and disinfecting root canal walls and better adaptation of root canal filling materials. However, the presence of smear layer can act as a seal to the dentinal tubules and minimize the ability of bacteria and its toxins from penetrating the dentinal tubules. The ability to remove smear layer depends primarily on chemomechanical preparation. There are three main methods to remove smear layer: chemically, mechanically (ultrasonically), laser, or combinations. No one single irrigant has the ability to kill microorganisms, dissolve organic tissues, and demineralize smear layer. Thus, alternating between organic and inorganic solvents and the use of different methods and techniques have been recommended. Indeed, there is little relevance attributed to the influence of smear layer on the clinical treatment outcomes. Moreover, there is critical lack of clinical studies to determine the role of smear layer since all previous studies were carried out on laboratory based. Further experimental model with a longitudinal observational characteristic should be applied.
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Alamoudi RA. The smear layer in endodontic: To keep or remove – an updated overview.Saudi Endod J 2019;9:71-81
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Alamoudi RA. The smear layer in endodontic: To keep or remove – an updated overview. Saudi Endod J [serial online] 2019 [cited 2019 Apr 21 ];9:71-81
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During mechanical preparation, the use of hand or rotary files for instrumentation will result in the production of considerable amount of mineralized debris what is called smear layer. Eick et al. were the first who identified the smear layer using scanning electron microscope (SEM) and found that smear layer is made from different size of particles ranging from <0.5 to 15 μm. The presence of smear layer on instrumented root canals was first reported by McComb and Smith in 1975. They showed that this layer is made of remnants of dentin, odontoblastic processes, necrotic or viable pulp tissues, and bacteria. Lester and Boyde reported that smear layer is a mineralized collagen matrix made up of entrapment of organic matter within inorganic dentin. Other studies showed that the smear layer has an amorphous granular and irregular particle under SEM.,,
The smear layer consists of two parts: a superficial layer that covers the dentin surface and a smear plug that occludes the dentinal tubules. Mader et al. showed that this superficial layer consists of a thin layer of mineralized tissue and is about 1–2 μm in thickness. Goldman et al. agreed with a previous study and reported that the smear layer is about 1 μm. Brännström and Johnson reported that the thickness of smear layer could range between 2 and 5 μm in thickness. Other studies reported that the smear plug is about 40 μm.,,,, and can reach up to 110 μm. Variation in the thickness of smear layer depends on whether the dentin was instrumented in wet or dry field and the type and sharpness of the cutting instruments., The increase in the centrifugal forces resulted from the proximity of the endodontic instrument to the dentin wall forms a thicker and more resistant smear layer, thus Gates-Glidden and postdrills produce more and greater volume of smear layer than that produced by hand-filing. Several studies explain the phenomenon of tubular packing. Brännström and Johnson and Mader et al. reported that smear plug occurred due to the rotational movement of the burs and rotary instruments leading to scattering of the smear debris and subsequently plugged inside the tubules. Meanwhile, Cengiz et al. advocated that it could be due to the sucking force of the capillary resulting in an adhesion between the smear layer and the dentinal tubules. Other factors such as whether the dentin is wet or dry during instrumentation,, and the cutting efficiency of the instruments determine the thickness of smear layer.
The Significance of Smear Layer
Researchers have reached to different conclusions on the importance of removing or leaving this layer. Some authors advocated the significance of removing the smear layer since it contains necrotic tissue, bacteria, and its by-products.,,,, This content can act as a reservoir for further microbial irritants and may serve as a substrate for microorganisms to survive, multiply, and then proliferate deeply inside the dentinal tubules.,,,, Although several studies reported the presence of microorganisms inside the dentinal tubules after chemomechanical preparation,,, Brännström advocated that these microorganisms inside the dentinal tubules can easily be destructed once the smear layer is removed. Meryon and Brook also reported the negligible effect of smear layer on the penetration ability of three microorganisms.
In addition, the smear layer can minimize the ability of disinfecting agents to penetrate the dentinal tubules.,,,, Other studies showed that it can also minimize the ability of intracanal medicaments to penetrate deeply.,, These findings contradicted with that of Haapasalo and Ørstavik, who reported that removing the smear layer may allow liquid-camphorated monochlorophenol solution to completely disinfect the dentinal tubules. Yet, it failed to eradicate Enterococcus faecalis with calcium hydroxide. Therefore, smear layer can delay but did not completely eliminate the effect of disinfectant agent or intracanal medicament.
Furthermore, Okşan et al. stated that smear layer decreases the ability of sealer to penetrate the dentinal tubules and adhere properly. Gençoǧlu et al. stated that smear layer minimizes the ability of gutta-percha to adapt well to canal wall regardless of the condensation techniques used; cold laterally or thermoplastic vertically. Their studies were in accordance with that of Gutmann who also reported that thermoplastic gutta-percha adapted well to canal wall after smear layer removal regardless of the presence of sealer. Smear layer acts as a sealing barrier between the canal wall and root filling materials and may compromise the ability to form a satisfactory seal.,,,,, Several authors advocated that smear layer is a loosely nonhomogeneous adherent structure that can easily dislodged from the underlying dentin and potentially lead to bacterial contaminant and leakage between the filling material and the dentinal walls.,, A study by Pashley et al. showed the presence of a microchannel between the root filling material and dentinal wall in the presence of smear layer which is between 1 and 10 μm in thickness. These channels may negatively affect the apical and coronal seal and they may disintegrate leaving voids in poorly filled root canal, consequently altering the root canal treatment outcome.,,,,,,,,,,,,,,,,,,,,,,,,,
Conversely, other investigators advocated the importance of maintaining the smear layer after canal preparation, and some studies provide strong evidence to prove that smear layer acts as a seal to the dentinal tubules and minimizes bacterial and its toxin from invasion by altering dentinal permeability.,,,, Pashley reported that the presence of a smear layer may limit bacteria present in the infected canal to enter the dentinal tubules in case of inadequate canal disinfection or recontamination of the canal between treatment sessions. However, a study by Williams and Goldman reported that this layer cannot act as a complete barrier and its presence could only delay bacterial invasion. Another study by Madison and Krell using a chelating agent, ethylenediaminetetraacetic acid (EDTA) solution, found no difference in the leakage properties regardless of the presence of smear layer. This study was in accordance with that of Chailertvanitkul et al. who found the same result. However, a major disadvantage of these studies is that the experiments did not mimic the clinical condition and were undertaken using cross-sectional root models or dentin discs. This limitation was overcome by a study of Drake et al. who evolved a more clinically relevant model using extracted human teeth to examine the effect of smear debris on bacterial retention on canal wall. They suggested that smear layer formed during mechanical instrumentation can prevent bacterial colonization of root canals as it limits bacterial penetration into dentinal tubules.
A systematic review and meta-analysis by Shahravan et al. tested whether removal of the smear layer can prevent the leakage of root-filled teeth, 54% of the comparisons stated no significant effect with or without smear layer removal, 41% advocated toward the removal of smear layer in order to prevent leakage, and 5% reported keeping it. They concluded that smear layer removal can promote an excellent fluid-tight seal, while other factors such as type of the sealer or the filling technique cannot produce significant effects.
The Effect of Instrumentation Technique on the Amount of Smear Layer
Though little research highlight the effect of different instrumentation techniques and materials on the amount of smear layer remaining in the dentinal wall, it is well known that mechanical preparation produces considerable amount of smear layer. Endodontic hand files such as K-reamers and K-files created similar surfaces compared to rotary files. Meanwhile, postburs and Gates-Glidden showed a higher volume in producing smear debris compared to hand instruments.
A study by Poggio et al. demonstrated the effect of two different nickel-titanium (NiTi) rotary systems on the amount of smear layer debris present in the middle and apical third of the root canal: Reciproc (VDW GmbH, Munich, Germany) and Mtwo rotary files (VDW, Munich, Germany). Mtwo group presented significantly less smear layer and promoted more clean canal walls compared to Reciproc group. A recent study by Kar et al. compared the amount of smear debris remained after using two multifile rotary systems (MTwo and Silk MANI, INC, Tochigi, Japan) and two single-file rotary systems (F6 Skytaper, KometBrasseler GmbH & Co., Lemgo, Germany and NeoNiTi, Neolix Creative Dental Instruments, Châtres-la-Forêt, France). The result of this study showed that F6 Skytaper single-file rotary instrument had the maximum cleaning efficacy followed by Mtwo multifile rotary instrument in the apical area of root canals. This result may be attributed to the fact that both instruments share similar file design, i.e. S-shaped cross section with two sharp cutting edges, small core diameter, and greater chip space. Another study reported no significant difference in the amount of smear layer between canals with different tapers: 30/0.02 files and 30/0.4 files.
The Effect of Smear Layer on the Bonding Efficacy of Different Endodontic Obturation Materials
Several studies demonstrate the importance of smear layer removal and its effect on material-to-dentin bond strength, which promotes fluid-tight seal and minimized leakage. White et al., reported effective penetration of different endodontic sealers and root filling materials into dentinal tubules after removal of the smear layer. They also reported that Roth 801, AH26, pHEMA, and silicone sealers extended consistently inside the dentinal tubules when the smear layer was removed. This study was in accordance with that of Sonu et al. which showed that removal of smear layer allows AH plus sealer to penetrate deeply into the dentinal tubules at cervical and middle thirds of the root. Sisodia et al. advocated that removal of smear layer helps in better resistance to bacterial penetration and less leakage along Apexit Plus (Vivadent, Schann, Lichtenstein) root canal sealer. However, few other investigations contradicted the effect of smear layer on bond strength. Goldberg et al. reported that smear layer did not produce any difference on the sealing ability of Ketac-Endo (ESPE. GmbH, Seefeld, Germany)and Tubliseal endodontic sealers (Kerr Italia S.p.A., Salerno, Italy). Another study by Saleh et al. showed that removing the smear layer did not necessitate the improvement of bacterial resistance to penetrate along different types of sealers.
Methods to Remove the Smear Layer
The ability to clean effectively an endodontic space and remove smear layer depends primarily on chemomechanical preparation. Several aspects related to the smear layer removal have been discussed in the last decade, such as the use of different chelating agents, the volume and concentration of the solution used, the interaction between chelators and other irrigants, the ideal time effective to eliminate smear layer without causing an extensive destruction to the dentin matrix, and the influence of ultrasonic agitation. The following three main methods are used to remove smear layer: chemically, mechanically, laser, or combination.
This solution is well known to dissolve organic tissues,, and kill microorganisms. Meanwhile, it lacks the ability to remove smear layer.,,,
This solution has been considered a substantive antibacterial agent through its excellent long-lasting adherence to dentin wall, but it did not show any dissolving capability to organic material or removing effect to smear layer.
These agents interact with calcium ions which are present in the dentin wall and form soluble calcium chelates. EDTA acids are considered one of the most common chelating irrigants in endodontics. McComb et al. reported that application of EDTA resulted in effective opening to the dentinal tubules with very little superficial smear debris. Another study showed that 17% EDTA can decalcify dentin up to a depth of 20–30 μm in 5 min. However, Fraser reported that the chelating effect of EDTA was almost neglected in the apical part of root canals. Furthermore, the use of 24% EDTA gel was believed to prevent the extrusion of the material to periapical tissues compared to liquid form, increase the permeability of dentin, and enhance the cleaning ability.
Different materials were added to EDTA to enhance its effect. RC-Prep (Premier Dental Products, Plymouth Meeting, PA, USA) is an EDTA product with urea peroxide to enhance the floating effect of smear debris out of the root canal. However, this product contains a wax that remains on canal walls and decreases the hermetic seal between canal wall and filling material. Many other studies have advocated that paste-type EDTA did not eliminate the smear layer as effective as liquid type. Another recent investigation examined the addition of surfactants to liquid EDTA to minimize the surface tension and enhance the cleaning efficacy; however, no additional result was noted. Quaternary ammonium bromide (cetrimide or Cetavlon) is another solution that has been added to EDTA solutions. Fehr and Nygaard recommended the addition of 0.84 g of a quaternary ammonium bromide (Cetavlon or cetrimide) to transform EDTA to EDTAC. Cetrimide reduces the surface tension and increases the penetrating capacity of the solution. Goldberg and Abramovich reported the presence of smooth canal surface and regular dentinal tubules with the use of EDTAC. However, Frithjof et al. reported no difference in the behavior of EDTA and EDTAC. The ideal working time of EDTAC is recommended to be <15 min and no further chelating effect could be expected after this period. Smear Clear® (SybronEndo, Orange, CA, USA) is a recently introduced chelating agent that contains 17% EDTA solution, cetrimide, and two additional surfactants (polyoxyethylene and isooctylcyclohexyl). A study by Dua and Uppin showed that final irrigation with either 17% EDTA solution or Smear Clear followed by 1% sodium hypochlorite (NaOCl) was effective in removing the smear layer in coronal and middle thirds. However, Smear Clear was more effective when compared with 17% EDTA solution in the apical third.
Another chelating agent called Bis-dequalinium-acetate (BDA) consists of a dequalinium compound and an oxine derivative. This agent has been known for its ability to remove smear debris throughout the entire length of root canal., It has been reported that this agent has a low surface tension, less toxic, and well tolerated by periodontal tissues. Commercial forms of BDA, Solvidont (De Trey, A.G., Zurich, Switzerland) and Salvizol (Ravens Gmbh, Konstanz, Germany), were introduced in the 1980s and have both inorganic and organic debridement actions.,,,, Kaufman et al. showed that Salvizol had better cleaning effect compared to EDTA. Meanwhile, Berg et al. demonstrated less patent dentinal tubules using Salvizol compared to EDTA.
Another chelator is ethylene glycol-bis (ß-aminoethyl ether)-N, N, N¢, N¢-tetraacetic acid (EGTA). It showed to bind more specifically to calcium ions. Calt and Serper compared the effects of EGTA to EDTA on the removal of smear layer and found that this layer was completely eliminated using EDTA solution although it caused more destruction and erosion to the peritubular and intertubular dentin, while EGTA was not as efficient as EDTA in the apical area of root canals.
Tetracycline (including tetracycline hydrochloride, minocycline, and doxycycline) is a broad-spectrum antibiotic. Tetracycline can act as a calcium chelator because of its low pH. A study by Barkhordar et al. showed that 100 mg/ml of doxycycline hydrochloride was sufficient in eliminating smear layer. Haznedaroǧlu and Ersev showed no difference between 1% tetracycline hydrochloride and 50% citric acid in removing smear layer though tetracycline showed less demineralization to peritubular dentin than citric acid. Recently, Torabinejad et al. introduced a solution containing a mixture of a tetracycline isomer, an acid, and a detergent called MTAD™. This solution is an excellent irrigant for the removal of smear layer and killing microorganisms.,,,
Citric acid is an organic acid that demonstrates its effectiveness in removing smear layer.,, It has been reported that citric acid eliminates smear layer much better than other acids such as polyacrylic acid, lactic acid, and phosphoric acid. Wayman et al. reported that 10% citric acid produces the best effect in removing smear layer compared to higher concentration of citric acid (25% and 50%). A study by Machado et al. compared the effectiveness of smear layer removal when either 17% EDTA or 10% citric acid was used. They found that sealer penetrates the dentinal tubules equally with both chelating solutions. pH and time of exposure are the main factors to determine the amount of removal.
Polyacrylic acid is another type of organic acid that can be used as a chelating solution. Studies by McComb and Smith, compared the effect of commercial liquid EDTA preparation (REDTA) to 5%, 10%, and 20% polyacrylic acid and reported that there is no difference between all solutions in eliminating or even preventing the formation of smear layer. Although 40% polyacrylic acid (Durelon™ liquid and Fuji II liquid) is an effective potent solution, it should not be applied for more than 30 s to avoid extensive damage of the dentin surface.
Moreover, Bitter introduced 25% tannic acid irrigant as a root canal-chelating solution and reported that it produces clean smooth canal wall. However, Sabbak and Hassanin contradicted previous findings and reported that tannic acid increases the organic cohesion. Their explanation was because of the presence of collagen cross-linking between smear layer and dentin.
Chitosan is a biopolymer derived by the partial deacetylation of chitin obtained from crustacean shells. A study by Geethapriya et al. advocated that combination of chitosan-EDTA (1:1) exhibits excellent smear layer removal with less erosion to the coronal and middle thirds of the root compared to 17% EDTA alone. This study was in accordance with previous studies, which had reported the ability of chitosan/chitosan nanoparticles to eliminate smear layer and inhibit bacterial recolonization when used as a final irrigant during root canal treated on dentin., Silva et al. recommended using 0.2% chitosan for 3 min to remove the smear layer without causing dentinal erosion.
Combination between Different Solutions
No one single irrigant has been yet found to act as an antimicrobial agent, a tissue dissolvent, and a smear layer demineralizer. Thus, alternating between organic and inorganic solvents has been advised.,,
Different studies recommended the sequential application of NaOCl and EDTA during chemomechanical preparation,,, to help eliminating microorganism and remove soft-tissue debris and smear layer. Goldman et al. tested the result of using single irrigant or various combinations to thoroughly clean the entire root canal. The result showed that the most effective final rinse was when 10 mL of 17% EDTA was applied into the canal followed by 10 mL of 5.25% NaOCl as a final rinse. Brännström et al. confirmed that this mixture has the ability to remove most of the smear debris without extensive opening to the dentinal tubules or erosion to peritubular dentin.
Sequential irrigation with either NaOCl and MTAD or with NaOCl and EDTA produces similar effect in the ability of bacteria to penetrate filled canals. However, NaOCl and MTAD mix can demineralize dentin faster than NaOCl and EDTA.
QMix is an endodontic irrigant recently introduced to remove smear layer and kill microorganisms. It contains EDTA, chlorhexidine (CHX), and a detergent. Stojicic et al. reported that alternating between NaOCl and QMix was superior to CHX and MTAD in killing microorganisms and removing smear layer.
Another study by Yamada et al. showed that combination of 25% citric acid and NaOCl solution can result in crystal precipitation.
Ultrasonic smear removal
Several previous studies related to smear layer removal were conducted in straight wide root canal. Meanwhile, most of the human teeth present some degree of curvature. Achieving smear layer removal in a curved apical third is difficult and challenging. A continuous flow of irrigant solution activated by an ultrasonic delivery system was advised in order to produce a highly clean canal surface.,,,, Martin and Cunningham advocated the use of a biotechnological approach using ultrasound in order to produce canal with effective debridement and disinfection. This was achieved by a handpiece and an instrument file that were both energized by ultrasound. Their technique explained the highly intense magnitude and velocity applied on endodontic file. Later, Ahmad et al., reported that the free movement of ultrasonic tip inside the root canal produced an intense acoustic streaming effect that enhances the cleaning efficacy and the direct physical contact of the instrument to the canal walls may reduce the effect. Lumley et al. also showed the importance of using a small-sized instrument to maximize microstreaming effect, leading to cleaner wall. Prati et al. advocated that the best smear layer removal was achieved with the use of ultrasonic activation. A recent study by Kowsky and Naganath concluded that the application of EndoVac system could enhance smear layer removal at the apical portion of curved canals.
Several studies showed the significant effect of ultrasonic delivery system with different irrigation solutions in cleaning canal wall. Walker and del Rio, reported no significant difference between the different types of irrigation solutions. Tap water gives the same result compared to NaOCl when used with ultrasonic irrigation. Cameron showed that 2%–4% NaOCl irrigant with ultrasonic energy can eliminate smear debris, compared to other irrigations. Ahmad et al. claimed that modified ultrasonic instrumentation with low concentration of 1% NaOCl removed smear layer debris more efficiently and produced clean apical region. Yeung et al. showed that a combination of 5 mL of 17% EDTA with the endo activator eliminated smear layer from a curved apical third of root canals more efficiently.
Furthermore, Cameron reported that the best time period for removing smear layer using ultrasonic is 3–5-min activation compared to 1 min to ensure smear-free canal walls.
A recent systematic review and meta-analysis done by Virdee et al. concluded that irrigant activation techniques – passive ultrasonic irrigation, sonic irrigation, apical negative pressure, and manual dynamic activation – improve intracanal cleanliness and smear layer removal compared to conventional needle irrigation. Therefore, it is recommended to be used throughout root canal preparation. However, no individual technique showed superiority than another.
These findings were contradicted by other studies which showed ultrasonic system's inefficiency to eliminate smear layer.,, Researchers who found the cleaning effects of ultrasonic system to be effective used this technique at the end of chemomechanical preparation and after completion of hand instrumentation.,, Moreover, Baumgartner and Cuenin reported that irrigation with NaOCl and ultrasonic did not enhance smear layer removal from root canal walls. Guerisoli et al. also reported that the use of ultrasonic irrigation did not add any additional effect to smear layer removal and the use of 15% EDTAC with either distilled water or 1% NaOCl was found to be more effective to achieve the desired result. In addition, a study by Ahmetoglu et al. evaluated the effectiveness of different irrigation devices, namely passive ultrasonic irrigation, apical negative pressure irrigation (EndoVac), and conventional needle irrigation systems on smear layer removal. The results found that removal of smear layer depends primarily on the solution used and not on the irrigation system.
Lasers have many applications in dentistry. In endodontics, it was used to eliminate the smear layer and vaporize tissues in the main straight root canal till the apex.,, The effectiveness of laser energy was based on several aspects such as the anatomy of the root canal, power level of the laser machine, the duration of exposure to laser light, the absorption ability of dental tissues, and the distance between the tip of the laser and the targeted tissue.,,, Although application of laser during endodontic treatment is safe, yet it has some limitations since it cannot access small curved canal spaces with the large straight probes that are provided.
Dederich et al. and Tewfik et al. used neodymium–yttrium-aluminum-garnet (Ne:YAG) laser and showed that different changes occur on the surface of dentin ranging from no effect to actual melting and recrystallization of the underlying dentin. Takeda et al.,, advocated the use of erbium: YAG (Er:YAG) laser to remove the smear layer. It resulted in effective smear layer removal without causing any side effect to the dentin; melting, charring, or vaporization that can be associated with other types of laser. Kimura et al. confirmed the positive effect of using Er:YAG laser on the smear layer. Other studies showed the use of different types of lasers such as argon laser,, argon fluoride laser, and carbon dioxide laser. They all give the same pattern of dentin disruption. Moreover, Saraswathi et al. reported that 940 nm diode laser irradiation of root dentin along with NaOCl and EDTA irrigation resulted in better removal of smear layer without significant additional loss of mineral content. Yet, another study aimed to demonstrate the effectiveness of different techniques and lasers on smear layer removal using- NaOCl, 17% EDTA, MTAD, Nd:YAG, or Er:YAG. They reported that smear layer removal by EDTA solution demonstrated the best irrigation technique in all regions, and the effect of EDTA was statistically significant in the coronal and middle thirds only compared to MTAD. Thus, although alternative materials and techniques were used to improve smear layer debridement, still the combination of EDTA and NaOCl remains the best technique.
The Effect of Smear Layer on the Outcome of Root Canal Therapy
Violich and Chandler concluded that removing smear layer allowed for a more thorough cleaning and disinfecting of root canal wall and better adaptation of filling materials. Yet, there are no clinical trials to support this. Until today, no study has shown the long-term outcome of the effect of removing smear layer. Only one study by Nischith et al. showed that smear layer removal increases the long-term apical seal and further success of the root canal therapy, leading to improve the outcome. Other clinical trials performed on primary teeth have been found. The complex morphology of the root canal system of posterior primary teeth and the close relationship of the apex to the developing permanent tooth buds make mechanical instrumentation alone difficult to eliminate the infected tissue and microorganisms, especially in the apical area to avoid damaging of the permanent tooth bud. For that, further investigation with randomized controlled clinical trials should be performed to demonstrate the effect of smear layer into the root canal treatment outcome.
Indeed, there is little relevance attributed to the influence of smear layer on clinical treatment outcomes and a major lack of clinical studies to determine the role of smear layer since all previous studies were carried out based on laboratory. Further experimental model with a longitudinal observational characteristic should be applied.
Upon mechanical root canal preparation, organic and inorganic debris are formed and named smear layer. This layer contains necrotic tissue and microorganisms that overlay the canal walls. This layer limits the penetration ability of disinfecting agents and intracanal medicaments into the dentinal tubules and compromises the adherence of filling materials to canal wall.
Different methods have been used to remove smear layer including chemical, mechanical, and laser which were applied aiming to enhance the cleaning effect to canal wall, yet no single method has been obtained universally to clean throughout the entire length of root canals. However, if the smear layer is required to be eliminated, the ultimate method to be used is sequential application of EDTA and NaOCl solutions.
Conflicting studies exist with regard to the benefit of maintaining or eliminating the smear layer before filling root canals. The data presented demonstrate the importance of removing the smear layer to enhance the adaptation of filling materials and proper disinfection of the canal system. Since new sealer and core materials have recently been introduced, further researches are necessarily to point out the role of the smear layer in the outcome of treatment through conducting a clinical trial.
Moreover, it has been reported that using SEM to study the smear layer is not trustworthy and reproducible. The magnification used in different studies differed broadly, making the result inconsistent.
Further investigations should be conducted to determine the effect of chelating solution on an uninstrumented canal wall since there are 35% of dentin surface untouched after mechanical instrumentation as reported by Peters et al. It is also important to understand the change that occurs to the root dentin after the application of chelating agent. Moreover, research about the thickness of demineralization layer occurred by chelating agent is lacking and further research is indicated in this field.
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|1||Eick JD, Wilko RA, Anderson CH, Sorensen SE. Scanning electron microscopy of cut tooth surfaces and identification of debris by use of the electron microprobe. J Dent Res 1970;49:1359-68.|
|2||McComb D, Smith DC. A preliminary scanning electron microscopic study of root canals after endodontic procedures. J Endod 1975;1:238-42.|
|3||Lester KS, Boyde A. Scanning electron microscopy of instrumented, irrigated and filled root canals. Br Dent J 1977;143:359-67.|
|4||Brännström M, Nordenvall KJ, Glantz PO. The effect of EDTA-containing surface-active solutions on the morphology of prepared dentin: An in vivo study. J Dent Res 1980;59:1127-31.|
|5||Yamada RS, Armas A, Goldman M, Lin PS. A scanning electron microscopic comparison of a high volume final flush with several irrigating solutions: Part 3. J Endod 1983;9:137-42.|
|6||Pashley DH, Tao L, Boyd L, King GE, Horner JA. Scanning electron microscopy of the substructure of smear layers in human dentine. Arch Oral Biol 1988;33:265-70.|
|7||Mader CL, Baumgartner JC, Peters DD. Scanning electron microscopic investigation of the smeared layer on root canal walls. J Endod 1984;10:477-83.|
|8||Goldman LB, Goldman M, Kronman JH, Lin PS. The efficacy of several irrigating solutions for endodontics: A scanning electron microscopic study. Oral Surg Oral Med Oral Pathol 1981;52:197-204.|
|9||Brännström M, Johnson G. Effects of various conditioners and cleaning agents on prepared dentin surfaces: A scanning electron microscopic investigation. J Prosthet Dent 1974;31:422-30.|
|10||Cameron JA. The use of ultrasonics in the removal of the smear layer: A scanning electron microscope study. J Endod 1983;9:289-92.|
|11||Moodnik RM, Dorn SO, Feldman MJ, Levey M, Borden BG. Efficacy of biomechanical instrumentation: A scanning electron microscopic study. J Endod 1976;2:261-6.|
|12||Cengiz T, Aktener BO, Piskin B. Effect of dentinal tubule orientation on the removal of smear layer by root canal irrigants. A scanning electron microscopic study. Int Endod J 1990;23:163-71.|
|13||Aktener BO, Cengiz T, Pişkin B. The penetration of smear material into dentinal tubules during instrumentation with surface-active reagents: A scanning electron microscopic study. J Endod 1989;15:588-90.|
|14||Barnes JE. The production of inlay cavity bevels. Br Dent J 1974;137:379-90.|
|15||Gilboe DB, Svare CW, Thayer KE, Drennon DG. Dentinal smearing: An investigation of the phenomenon. J Prosthet Dent 1980;44:310-6.|
|16||Jodaikin A, Austin JC. Smear layer removal with chelating agents after cavity preparation. J Prosthet Dent 1981;46:171-4.|
|17||Czonstkowsky M, Wilson EG, Holstein FA. The smear layer in endodontics. Dent Clin North Am 1990;34:13-25.|
|18||Cameron JA. The use of ultrasound for the removal of the smear layer. The effect of sodium hypochlorite concentration; SEM study. Aust Dent J 1988;33:193-200.|
|19||Goldberg F, Abramovich A. Analysis of the effect of EDTAC on the dentinal walls of the root canal. J Endod 1977;3:101-5.|
|20||Wayman BE, Kopp WM, Pinero GJ, Lazzari EP. Citric and lactic acids as root canal irrigants in vitro. J Endod 1979;5:258-65.|
|21||Cunningham WT, Martin H. A scanning electron microscope evaluation of root canal débridement with the endosonic ultrasonic synergistic system. Oral Surg Oral Med Oral Pathol 1982;53:527-31.|
|22||Pashley DH. SmearLayer: physiological consideration. Oper Dent 1984;3:13-29.|
|23||George S, Kishen A, Song KP. The role of environmental changes on monospecies biofilm formation on root canal wall by Enterococcus faecalis. J Endod 2005;31:867-72.|
|24||Brännström M, Nyborg H. Cavity treatment with a microbicidal fluoride solution: Growth of bacteria and effect on the pulp. J Prosthet Dent 1973;30:303-10.|
|25||Olgart L, Brännström M, Johnson G. Invasion of bacteria into dentinal tubules. Experiments in vivo and in vitro. Acta Odontol Scand 1974;32:61-70.|
|26||Akpata ES, Blechman H. Bacterial invasion of pulpal dentin wall in vitro. J Dent Res 1982;61:435-8.|
|27||Williams S, Goldman M. Penetrability of the smeared layer by a strain of Proteus vulgaris. J Endod 1985;11:385-8.|
|28||Meryon SD, Jakeman KJ, Browne RM. Penetration in vitro of human and ferret dentine by three bacterial species in relation to their potential role in pulpal inflammation. Int Endod J 1986;19:213-20.|
|29||Meryon SD, Brook AM. Penetration of dentine by three oral bacteria in vitro and their associated cytotoxicity. Int Endod J 1990;23:196-202.|
|30||Byström A, Sundqvist G. Bacteriologic evaluation of the efficacy of mechanical root canal instrumentation in endodontic therapy. Scand J Dent Res 1981;89:321-8.|
|31||Byström A, Sundqvist G. Bacteriologic evaluation of the effect of 0.5 percent sodium hypochlorite in endodontic therapy. Oral Surg Oral Med Oral Pathol 1983;55:307-12.|
|32||Bystrom A, Sundqvist G. The antibacterial action of sodium hypochlorite and EDTA in 60 cases of endodontic therapy. Int Endod J 1985;18:35-40.|
|33||Brännström M. Communication between the oral cavity and the dental pulp associated with restorative treatment. Oper Dent 1984;9:57-68.|
|34||Outhwaite WC, Livingston MJ, Pashley DH. Effects of changes in surface area, thickness, temperature and post-extraction time on human dentine permeability. Arch Oral Biol 1976;21:599-603.|
|35||Cameron JA. The synergistic relationship between ultrasound and sodium hypochlorite: A scanning electron microscope evaluation. J Endod 1987;13:541-5.|
|36||Haapasalo M, Orstavik D.In vitro infection and disinfection of dentinal tubules. J Dent Res 1987;66:1375-9.|
|37||Orstavik D, Haapasalo M. Disinfection by endodontic irrigants and dressings of experimentally infected dentinal tubules. Endod Dent Traumatol 1990;6:142-9.|
|38||Okşan T, Aktener BO, Sen BH, Tezel H. The penetration of root canal sealers into dentinal tubules. A scanning electron microscopic study. Int Endod J 1993;26:301-5.|
|39||Gençoǧlu N, Samani S, Günday M. Evaluation of sealing properties of Thermafil and Ultrafil® techniques in the absence or presence of smear layer. J Endod 1993;19:599-603.|
|40||Gutmann JL. Adaptation of injected thermoplasticized gutta-percha in the absence of the dentinal smear layer. Int Endod J 1993;26:87-92.|
|41||White RR, Goldman M, Lin PS. The influence of the smeared layer upon dentinal tubule penetration by plastic filling materials. J Endod 1984;10:558-62.|
|42||Cergneux M, Ciucchi B, Dietschi JM, Holz J. The influence of the smear layer on the sealing ability of canal obturation. Int Endod J 1987;20:228-32.|
|43||Foster KH, Kulild JC, Weller RN. Effect of smear layer removal on the diffusion of calcium hydroxide through radicular dentin. J Endod 1993;19:136-40.|
|44||Yang SE, Bae KS. Scanning electron microscopy study of the adhesion of Prevotella nigrescens to the dentin of prepared root canals. J Endod 2002;28:433-7.|
|45||Cameron JA. The use of 4 per cent sodium hypochlorite, with or without ultrasound, in cleansing of uninstrumented immature root canals; SEM study. Aust Dent J 1987;32:204-13.|
|46||Pashley DH, Depew DD, Galloway SE. Microleakage channels: Scanning electron microscopic observation. Oper Dent 1989;14:68-72.|
|47||Madison S, Krell KV. Comparison of ethylenediamine tetraacetic acid and sodium hypochlorite on the apical seal of endodontically treated teeth. J Endod 1984;10:499-503.|
|48||Goldberg F, Bernat MI, Spielberg C, Massone EJ, Piovano SA. Analysis of the effect of ethylenediaminetetraacetic acid on the apical seal of root canal fillings. J Endod 1985;11:544-7.|
|49||Goldberg F, Artaza LP, De Silvio A. Apical sealing ability of a new glass ionomer root canal sealer. J Endod 1995;21:498-500.|
|50||Evans JT, Simon JH. Evaluation of the apical seal produced by injected thermoplasticized gutta-percha in the absence of smear layer and root canal sealer. J Endod 1986;12:100-7.|
|51||Kennedy WA, Walker WA 3rd, Gough RW. Smear layer removal effects on apical leakage. J Endod 1986;12:21-7.|
|52||Saunders WP, Saunders EM. The effect of smear layer upon the coronal leakage of gutta-percha fillings and a glass ionomer sealer. Int Endod J 1992;25:245-9.|
|53||Saunders WP, Saunders EM. Influence of smear layer on the coronal leakage of Thermafil® and laterally condensed gutta-percha root fillings with a glass ionomer sealer. J Endod 1994;20:155-8.|
|54||Gençoǧlu N, Samani S, Günday M. Dentinal wall adaptation of thermoplasticized gutta-percha in the absence or presence of smear layer: A scanning electron microscopic study. J Endod 1993;19:558-62.|
|55||Karagöz-Küçükay I, Bayirli G. An apical leakage study in the presence and absence of the smear layer. Int Endod J 1994;27:87-93.|
|56||Tidswell HE, Saunders EM, Saunders WP. Assessment of coronal leakage in teeth root filled with gutta-percha and a glass of ionomer root canal sealer. Int Endod J 1994;27:208-12.|
|57||Lloyd A, Thompson J, Gutmann JL, Dummer PM. Sealability of the trifecta technique in the presence or absence of a smear layer. Int Endod J 1995;28:35-40.|
|58||Behrend GD, Cutler CW, Gutmann JL. An in vitro study of smear layer removal and microbial leakage along root-canal fillings. Int Endod J 1996;29:99-107.|
|59||Chailertvanitkul P, Saunders WP, MacKenzie D. The effect of smear layer on microbial coronal leakage of gutta-percha root fillings. Int Endod J 1996;29:242-8.|
|60||Vassiliadis L, Liolios E, Kouvas V, Economides N. Effect of smear layer on coronal microleakage. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1996;82:315-20.|
|61||Taylor JK, Jeansonne BG, Lemon RR. Coronal leakage: Effects of smear layer, obturation technique, and sealer. J Endod 1997;23:508-12.|
|62||Timpawat S, Sripanaratanakul S. Apical sealing ability of glass ionomer sealer with and without smear layer. J Endod 1998;24:343-5.|
|63||Economides N, Kokorikos I, Kolokouris I, Panagiotis B, Gogos C. Comparative study of apical sealing ability of a new resin-based root canal sealer. J Endod 2004;30:403-5.|
|64||Economides N, Liolios E, Kolokuris I, Beltes P. Long-term evaluation of the influence of smear layer removal on the sealing ability of different sealers. J Endod 1999;25:123-5.|
|65||von Fraunhofer JA, Fagundes DK, McDonald NJ, Dumsha TC. The effect of root canal preparation on microleakage within endodontically treated teeth: An in vitro study. Int Endod J 2000;33:355-60.|
|66||Fróes JA, Horta HG, da Silveira AB. Smear layer influence on the apical seal of four different obturation techniques. J Endod 2000;26:351-4.|
|67||Goya C, Yamazaki R, Tomita Y, Kimura Y, Matsumoto K. Effects of pulsed Nd:YAG laser irradiation on smear layer at the apical stop and apical leakage after obturation. Int Endod J 2000;33:266-71.|
|68||Timpawat S, Vongsavan N, Messer HH. Effect of removal of the smear layer on apical microleakage. J Endod 2001;27:351-3.|
|69||Clark-Holke D, Drake D, Walton R, Rivera E, Guthmiller JM. Bacterial penetration through canals of endodontically treated teeth in the presence or absence of the smear layer. J Dent 2003;31:275-81.|
|70||Cobankara FK, Adanr N, Belli S. Evaluation of the influence of smear layer on the apical and coronal sealing ability of two sealers. J Endod 2004;30:406-9.|
|71||Park DS, Torabinejad M, Shabahang S. The effect of MTAD on the coronal leakage of obturated root canals. J Endod 2004;30:890-2.|
|72||Michelich VJ, Schuster GS, Pashley DH. Bacterial penetration of human dentin in vitro. J Dent Res 1980;59:1398-403.|
|73||Pashley DH, Michelich V, Kehl T. Dentin permeability: Effects of smear layer removal. J Prosthet Dent 1981;46:531-7.|
|74||Safavi KE, Spangberg LS, Langeland K. Root canal dentinal tubule disinfection. J Endod 1990;16:207-10.|
|75||Drake DR, Wiemann AH, Rivera EM, Walton RE. Bacterial retention in canal walls in vitro: Effect of smear layer. J Endod 1994;20:78-82.|
|76||Galvan DA, Ciarlone AE, Pashley DH, Kulild JC, Primack PD, Simpson MD, et al. Effect of smear layer removal on the diffusion permeability of human roots. J Endod 1994;20:83-6.|
|77||Pashley DH. Dentin-predentin complex and its permeability: Physiologic overview. J Dent Res 1985;64:613-20.|
|78||Shahravan A, Haghdoost AA, Adl A, Rahimi H, Shadifar F. Effect of smear layer on sealing ability of canal obturation: A systematic review and meta-analysis. J Endod 2007;33:96-105.|
|79||Subramaniam P, Girish Babu KL, Tabrez TA. Effectiveness of rotary endodontic instruments on smear layer removal in root canals of primary teeth: A scanning electron microscopy study. J Clin Pediatr Dent 2016;40:141-6.|
|80||Poggio C, Dagna A, Chiesa M, Scribante A, Beltrami R, Colombo M, et al. Effects of NiTi rotary and reciprocating instruments on debris and smear layer scores: An SEM evaluation. J Appl Biomater Funct Mater 2014;12:256-62.|
|81||Kar PP, Khasnis SA, Kidiyoor KH. Comparative evaluation of cleaning efficacy using four novel nickel-titanium rotary instruments: An in vitro scanning electron microscope study. J Contemp Dent Pract 2017;18:1135-43.|
|82||Zarei M, Javidi M, Afkhami F, Tanbakuchi B, Zadeh MM, Mohammadi MM, et al. Influence of root canal tapering on smear layer removal. N Y State Dent J 2016;82:35-8.|
|83||White RR, Goldman M, Lin PS. The influence of the smeared layer upon dentinal tubule penetration by endodontic filling materials. Part II. J. Endod 1987;13:369-74.|
|84||Sonu KR, Ponnappa KC, Kishan KV, Thameem PK. Comparative evaluation of dentinal penetration of three different endodontic sealers with and without smear layer removal- Scanning electron microscopic study. Saudi Endod J 2016;6:16-20.|
|85||Sisodia R, Ravi KS, Shashikiran ND, Singla S, Kulkarni V. Bacterial penetration along different root canal fillings in the presence or absence of smear layer in primary teeth. J Clin Pediatr Dent 2014;38:229-34.|
|86||Saleh IM, Ruyter IE, Haapasalo M, Ørstavik D. Bacterial penetration along different root canal filling materials in the presence or absence of smear layer. Int Endod J 2008;41:32-40.|
|87||Rubin LM, Skobe Z, Krakow AA, Gron P. The effect of instrumentation and flushing of freshly extracted teeth in endodontic therapy: A scanning electron microscope study. J Endod 1979;5:328-35.|
|88||Goldman M, Goldman LB, Cavaleri R, Bogis J, Lin PS. The efficacy of several endodontic irrigating solutions: A scanning electron microscopic study: Part 2. J Endod 1982;8:487-92.|
|89||Baker NA, Eleazer PD, Averbach RE, Seltzer S. Scanning electron microscopic study of the efficacy of various irrigating solutions. J Endod 1975;1:127-35.|
|90||Berg MS, Jacobsen EL, BeGole EA, Remeikis NA. A comparison of five irrigating solutions: A scanning electron microscopic study. J Endod 1986;12:192-7.|
|91||Baumgartner JC, Mader CL. A scanning electron microscopic evaluation of four root canal irrigation regimens. J Endod 1987;13:147-57.|
|92||Mohammadi Z, Abbott PV. The properties and applications of chlorhexidine in endodontics. Int Endod J 2009;42:288-302.|
|93||McComb D, Smith D, Beagrie G. The results of in vivo endodontic chemomechanical instrumentation – A scanning electron microscopic study. Int Endod J 1976;9:11-8.|
|94||Frithjof R, Östby BN. Effect of EDTAC and sulfuric acid on root canal dentine. Oral Surg Oral Med Oral Pathol 1963;16:199-205.|
|95||Fraser JG. Chelating agents: Their softening effect on root canal dentin. Oral Surg Oral Med Oral Pathol 1974;37:803-11.|
|96||Bighetti Trevisan RL, Scatolin RS, Castro Raucci LM, Raucci Neto W, Froner IC. Effects of EDTA gel and chlorhexidine gel on root dentin permeability. Microsc Res Tech 2018;81:191-7.|
|97||Stewart GG, Kapsimalas P, Rappaport H. EDTA and urea peroxide for root canal preparation. J Am Dent Assoc 1969;78:335-8.|
|98||Biesterfeld RC, Taintor JF. A comparison of periapical seals of root canals with RC-prep or Salvizol. Oral Surg Oral Med Oral Pathol 1980;49:532-7.|
|99||Lui JN, Kuah HG, Chen NN. Effect of EDTA with and without surfactants or ultrasonics on removal of smear layer. J Endod 2007;33:472-5.|
|100||Fehr F, Nygaard OB. Effect of EDTAC and sulfuric acid on root canal dentin. Oral Surg Oral Med Oral Pathol 1963;16:199-205.|
|101||Goldberg F, Spielberg C. The effect of EDTAC and the variation of its working time analyzed with scanning electron microscopy. Oral Surg Oral Med Oral Pathol 1982;53:74-7.|
|102||Dua AD, Uppin VM. Evaluation of the effect of duration of application of smear clear in removing intracanal smear layer: SEM study. Saudi Endod J 2015;5:26-32.|
|103||Kaufman AY, Binderman I, Tal M, Gedalia I, Peretz G. New chemotherapeutic agent for root canal treatment. A preliminary electron microscopic study on an in vivo and in vitro endodontically treated tooth. Oral Surg Oral Med Oral Pathol 1978;46:283-95.|
|104||Kaufman AY. The use of dequalinium acetate as a disinfectant and chemotherapeutic agent in endodontics. Oral Surg Oral Med Oral Pathol 1981;51:434-41.|
|105||Kaufman AY. Solvidont – A new chemotherapeutic and bacteriocidal agent for endodontic use (I). Quintessence Int Dent Dig 1983;14:71-9.|
|106||Kaufman AY. Solvidont – A new chemotherapeutic and bacteriocidal agent for endodontic use (II). Quintessence Int Dent Dig 1983;14:235-44.|
|107||Chandler NP, Lilley JD. Clinical -trial of a Bis Dequalinium Acetate solution as an endodontic irrigant. J Dent Res 1987;66:842.|
|108||Lilley J, Russell C, Chandler N. Comparison of bisdequalinium-acetate and sodium hypochlorite solutions as endodontic irrigants. J Dent Res 1988;67:300.|
|109||Mohd Sulong MZ. The incidence of postoperative pain after canal preparation of open teeth using two irrigation regimes. Int Endod J 1989;22:248-51.|
|110||Schmid R, Reilley CN. New complexon for titration of calcium in presence of magnesium. Anal Chem 1957;29:264-8.|
|111||Calt S, Serper A. Smear layer removal by EGTA. J Endod 2000;26:459-61.|
|112||Bjorvatn K. Antibiotic compounds and enamel demineralization. An in vitro study. Acta Odontol Scand 1982;40:341-52.|
|113||Barkhordar RA, Watanabe LG, Marshall GW, Hussain MZ. Removal of intracanal smear by doxycycline in vitro. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997;84:420-3.|
|114||Haznedaroǧlu F, Ersev H. Tetracycline HCl solution as a root canal irrigant. J Endod 2001;27:738-40.|
|115||Torabinejad M, Khademi AA, Babagoli J, Cho Y, Johnson WB, Bozhilov K, et al. Anew solution for the removal of the smear layer. J Endod 2003;29:170-5.|
|116||Torabinejad M, Shabahang S, Aprecio RM, Kettering JD. The antimicrobial effect of MTAD: An in vitro investigation. J Endod 2003;29:400-3.|
|117||Shabahang S, Pouresmail M, Torabinejad M.In vitro antimicrobial efficacy of MTAD and sodium hypochlorite. J Endod 2003;29:450-2.|
|118||Mozayeni MA, Javaheri GH, Poorroosta P, Ashari MA, Javaheri HH. Effect of 17% EDTA and MTAD on intracanal smear layer removal: A scanning electron microscopic study. Aust Endod J 2009;35:13-7.|
|119||Loel DA. Use of acid cleanser in endodontic therapy. J Am Dent Assoc 1975;90:148-51.|
|120||Tidmarsh BG. Acid-cleansed and resin-sealed root canals. J Endod 1978;4:117-21.|
|121||Baumgartner JC, Brown CM, Mader CL, Peters DD, Shulman JD. A scanning electron microscopic evaluation of root canal debridement using saline, sodium hypochlorite, and citric acid. J Endod 1984;10:525-31.|
|122||Meryon SD, Tobias RS, Jakeman KJ. Smear removal agents: A quantitative study in vivo and in vitro. J Prosthet Dent 1987;57:174-9.|
|123||Machado R, Garcia LD, da Silva Neto UX, Cruz Filho AM, Silva RG, Vansan LP, et al. Evaluation of 17% EDTA and 10% citric acid in smear layer removal and tubular dentin sealer penetration. Microsc Res Tech 2018;81:275-82.|
|124||Morgan LA, Baumgartner JC. Demineralization of resected root-ends with methylene blue dye. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997;84:74-8.|
|125||Berry EA 3rd, von der Lehr WN, Herrin HK. Dentin surface treatments for the removal of the smear layer: An SEM study. J Am Dent Assoc 1987;115:65-7.|
|126||Bitter NC. A 25% tannic acid solution as a root canal irrigant cleanser: A scanning electron microscope study. Oral Surg Oral Med Oral Pathol 1989;67:333-7.|
|127||Sabbak SA, Hassanin MB. A scanning electron microscopic study of tooth surface changes induced by tannic acid. J Prosthet Dent 1998;79:169-74.|
|128||Geethapriya N, Subbiya A, Padmavathy K, Mahalakshmi K, Vivekanandan P, Sukumaran VG, et al. Effect of chitosan-ethylenediamine tetraacetic acid on enterococcus faecalis dentinal biofilm and smear layer removal. J Conserv Dent 2016;19:472-7.|
|129||Silva PV, Guedes DF, Nakadi FV, Pécora JD, Cruz-Filho AM. Chitosan: A new solution for removal of smear layer after root canal instrumentation. Int Endod J 2013;46:332-8.|
|130||Del Carpio-Perochena A, Bramante CM, Duarte MA, de Moura MR, Aouada FA, Kishen A, et al. Chelating and antibacterial properties of chitosan nanoparticles on dentin. Restor Dent Endod 2015;40:195-201.|
|131||Silva PV, Guedes DF, Pécora JD, da Cruz-Filho AM. Time-dependent effects of chitosan on dentin structures. Braz Dent J 2012;23:357-61.|
|132||Koskinen KP, Meurman JH, Stenvall H. Appearance of chemically treated root canal walls in the scanning electron microscope. Eur J Oral Sci 1980;88:505-12.|
|133||Ghoddusi J, Rohani A, Rashed T, Ghaziani P, Akbari M. An evaluation of microbial leakage after using MTAD as a final irrigation. J Endod 2007;33:173-6.|
|134||De-Deus G, Reis C, Fidel S, Fidel R, Paciornik S. Dentin demineralization when subjected to bioPure MTAD: A longitudinal and quantitative assessment. J Endod 2007;33:1364-8.|
|135||Stojicic S, Shen Y, Qian W, Johnson B, Haapasalo M. Antibacterial and smear layer removal ability of a novel irrigant, QMiX. Int Endod J 2012;45:363-71.|
|136||Griffiths BM, Stock CJ. The efficiency of irrigants in removing root canal debris when used with ultrasonic preparation technique. Int Endod J 1986;19:277-84.|
|137||Alaçam T. Scanning electron microscope study comparing the efficacy of endodontic irrigating systems. Int Endod J 1987;20:287-94.|
|138||Martin H, Cunningham W. Endosonic endodontics: The ultrasonic synergistic system. Int Dent J 1984;34:198-203.|
|139||Ahmad M, Pitt Ford TJ, Crum LA. Ultrasonic debridement of root canals: Acoustic streaming and its possible role. J Endod 1987;13:490-9.|
|140||Ahmad M, Pitt Ford TR, Crum LA. Ultrasonic debridement of root canals: An insight into the mechanisms involved. J Endod 1987;13:93-101.|
|141||Lumley PJ, Walmsley AD, Walton RE, Rippin JW. Effect of precurving endosonic files on the amount of debris and smear layer remaining in curved root canals. J Endod 1992;18:616-9.|
|142||Prati C, Selighini M, Ferrieri P, Mongiorgi R. Scanning electron microscopic evaluation of different endodontic procedures on dentin morphology of human teeth. J Endod 1994;20:174-9.|
|143||Kowsky DK, Naganath M. Effectiveness of different irrigation devices on the smear layer removal in the apical portion of curved root canals: A scanning electron microscopy study. Saudi Endod J 2018;8:111-6.|
|144||Walker TL, del Rio CE. Histological evaluation of ultrasonic and sonic instrumentation of curved root canals. J Endod 1989;15:49-59.|
|145||Walker TL, del Rio CE. Histological evaluation of ultrasonic debridement comparing sodium hypochlorite and water. J Endod 1991;17:66-71.|
|146||Yeung W, Raldi DP, Cunha RS, Mello I. Assessment of smear layer removal protocols in curved root canals. Aust Endod J 2014;40:66-71.|
|147||Virdee SS, Seymour DW, Farnell D, Bhamra G, Bhakta S. Efficacy of irrigant activation techniques in removing intracanal smear layer and debris from mature permanent teeth: A systematic review and meta-analysis. Int Endod J 2018;51:605-21.|
|148||Cymerman JJ, Jerome LA, Moodnik RM. A scanning electron microscope study comparing the efficacy of hand instrumentation with ultrasonic instrumentation of the root canal. J Endod 1983;9:327-31.|
|149||Baker MC, Ashrafi SH, Van Cura JE, Remeikis NA. Ultrasonic compared with hand instrumentation: A scanning electron microscope study. J Endod 1988;14:435-40.|
|150||Goldberg F, Soares I, Massone EJ, Soares IM. Comparative debridement study between hand and sonic instrumentation of the root canal. Endod Dent Traumatol 1988;4:229-34.|
|151||Baumgartner JC, Cuenin PR. Efficacy of several concentrations of sodium hypochlorite for root canal irrigation. J Endod 1992;18:605-12.|
|152||Guerisoli DM, 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;35:418-21.|
|153||Ahmetoglu F, Keles A, Yalcin M, Simsek N. Effectiveness of different irrigation systems on smear layer removal: A scanning electron microscopic study. Eur J Dent 2014;8:53-7.|
|154||Takeda FH, Harashima T, Kimura Y, Matsumoto K. A comparative study of the removal of smear layer by three endodontic irrigants and two types of laser. Int Endod J 1999;32:32-9.|
|155||Takeda FH, Harashima T, Kimura Y, Matsumoto K. Comparative study about the removal of smear layer by three types of laser devices. J Clin Laser Med Surg 1998;16:117-22.|
|156||Takeda FH, Harashima T, Kimura Y, Matsumoto K. Efficacy of er:YAG laser irradiation in removing debris and smear layer on root canal walls. J Endod 1998;24:548-51.|
|157||Dederich DN, Zakariasen KL, Tulip J. Scanning electron microscopic analysis of canal wall dentin following neodymium-yttrium-aluminum-garnet laser irradiation. J Endod 1984;10:428-31.|
|158||Onal B, Ertl T, Siebert G, Müller G. Preliminary report on the application of pulsed CO2 laser radiation on root canals with AgCl fibers: A scanning and transmission electron microscopic study. J Endod 1993;19:272-6.|
|159||Tewfik HM, Pashley DH, Horner JA, Sharawy MM. Structural and functional changes in root dentin following exposure to KTP/532 laser. J Endod 1993;19:492-7.|
|160||Moshonov J, Sion A, Kasirer J, Rotstein I, Stabholz A. Efficacy of argon laser irradiation in removing intracanal debris. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1995;79:221-5.|
|161||Al-Zand SA, Mahmood AS, Al-Karadaghy TS. Temperature elevation investigations on the external root surface during irradiation with 940 nm. Saudi Endod J 2018;8:14-8.|
|162||Kimura Y, Yonaga K, Yokoyama K, Kinoshita J, Ogata Y, Matsumoto K, et al. Root surface temperature increase during Er: YAG laser irradiation of root canals. J Endod 2002;28:76-8.|
|163||Harashima T, Takeda FH, Zhang C, Kimura Y, Matsumoto K. Effect of argon laser irradiation on instrumented root canal walls. Endod Dent Traumatol 1998;14:26-30.|
|164||Stabholz A, Neev J, Liaw LH, Stabholz A, Khayat A, Torabinejad M, et al. Effect of arF-193 nm excimer laser on human dentinal tubules. A scanning electron microscopic study. Oral Surg Oral Med Oral Pathol 1993;75:90-4.|
|165||Saraswathi MV, Padinjaral I, Bhat S. Ultra morphological changes of root canal dentin induced by 940 nm diode laser: An in vitro study. Saudi Endod J 2012;2:131-5.|
|166||Kalyoncuoǧlu E, Demiryürek EÖ. A comparative scanning electron microscopy evaluation of smear layer removal from teeth with different irrigation solutions and lasers. Microsc Microanal 2013;19:1465-9.|
|167||Violich DR, Chandler NP. The smear layer in endodontics – A review. Int Endod J 2010;43:2-15.|
|168||Nischith KG, Srikumar GP, Razvi S, Chandra RV. Effect of smear layer on the apical seal of endodontically treated teeth: An ex vivo study. J Contemp Dent Pract 2012;13:23-6.|
|169||Ahmed HM. Anatomical challenges, electronic working length determination and current developments in root canal preparation of primary molar teeth. Int Endod J 2013;46:1011-22.|
|170||Gulabivala KP, Evans G, Ng YL. Effects of mechanical and chemical procedures on root canal surfaces. Endod Top 2005;10:103-22.|
|171||Peters OA, Peters CI, Schönenberger K, Barbakow F. ProTaper rotary root canal preparation: Effects of canal anatomy on final shape analysed by micro CT. Int Endod J 2003;36:86-92.|