Saudi Endodontic Journal

REVIEW ARTICLE
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

Correspondence Address:
Dr. Ruaa A Alamoudi
Department of Conservative Dentistry, Division of Endodontics, Faculty of Dentistry, King Abdulaziz University, P.O. Box: 1119, Jeddah 21431
Saudi Arabia

Abstract

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.



How to cite this article:
Alamoudi RA. The smear layer in endodontic: To keep or remove – an updated overview.Saudi Endod J 2019;9:71-81


How to cite this URL:
Alamoudi RA. The smear layer in endodontic: To keep or remove – an updated overview. Saudi Endod J [serial online] 2019 [cited 2019 Aug 23 ];9:71-81
Available from: http://www.saudiendodj.com/text.asp?2019/9/2/71/255375


Full Text

 Introduction



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.[1] 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.[2] They showed that this layer is made of remnants of dentin, odontoblastic processes, necrotic or viable pulp tissues, and bacteria. Lester and Boyde[3] 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.[4],[5],[6]

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.[7] showed that this superficial layer consists of a thin layer of mineralized tissue and is about 1–2 μm in thickness. Goldman et al.[8] agreed with a previous study and reported that the smear layer is about 1 μm. Brännström and Johnson[9] 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.[4],[7],[10],[11],[12] and can reach up to 110 μm.[13] 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.[14],[15] 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,[16] thus Gates-Glidden and postdrills produce more and greater volume of smear layer than that produced by hand-filing.[17] Several studies explain the phenomenon of tubular packing. Brännström and Johnson[9] and Mader et al.[7] 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.[12] 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[14],[15],[18] 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.[2],[5],[19],[20],[21] This content can act as a reservoir for further microbial irritants[22] and may serve as a substrate for microorganisms[23] to survive, multiply,[24] and then proliferate deeply inside the dentinal tubules.[25],[26],[27],[28],[29] Although several studies reported the presence of microorganisms inside the dentinal tubules after chemomechanical preparation,[30],[31],[32] Brännström[33] advocated that these microorganisms inside the dentinal tubules can easily be destructed once the smear layer is removed. Meryon and Brook[29] 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.[2],[5],[19],[20],[34] Other studies showed that it can also minimize the ability of intracanal medicaments to penetrate deeply.[7],[29],[35] These findings contradicted with that of Haapasalo and Ørstavik[36],[37] 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.[38] stated that smear layer decreases the ability of sealer to penetrate the dentinal tubules and adhere properly. Gençoǧlu et al.[39] 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[40] 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.[3],[17],[41],[42],[43],[44] 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.[7],[29],[45] A study by Pashley et al.[46] 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.[42],[47],[48],[49],[50],[51],[52],[53],[54],[55],[56],[57],[58],[59],[60],[61],[62],[63],[64],[65],[66],[67],[68],[69],[70],[71]

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.[72],[73],[74],[75],[76] Pashley[77] 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[27] reported that this layer cannot act as a complete barrier and its presence could only delay bacterial invasion. Another study by Madison and Krell[47] 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.[59] 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.[75] 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.[78] 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.[79] Meanwhile, postburs and Gates-Glidden showed a higher volume in producing smear debris compared to hand instruments.[17]

A study by Poggio et al.[80] 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.[81] 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.[82]

 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.[41],[83] 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.[84] 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.[85] 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.[49] 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.[86] 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.

Sodium hypochlorite

This solution is well known to dissolve organic tissues[20],[87],[88] and kill microorganisms. Meanwhile, it lacks the ability to remove smear layer.[8],[89],[90],[91]

Chlorhexidine

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.[92]

Chelating agents

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.[93] Another study showed that 17% EDTA can decalcify dentin up to a depth of 20–30 μm in 5 min.[94] However, Fraser[95] 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.[96]

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.[97] However, this product contains a wax that remains on canal walls and decreases the hermetic seal between canal wall and filling material.[98] 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.[99] Quaternary ammonium bromide (cetrimide or Cetavlon) is another solution that has been added to EDTA solutions. Fehr and Nygaard[100] 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[19] reported the presence of smooth canal surface and regular dentinal tubules with the use of EDTAC. However, Frithjof et al.[94] 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.[101] 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[102] 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.[103],[104] 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.[105],[106],[107],[108],[109] Kaufman et al.[103] showed that Salvizol had better cleaning effect compared to EDTA. Meanwhile, Berg et al.[90] 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.[110] Calt and Serper[111] 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.[112] A study by Barkhordar et al.[113] showed that 100 mg/ml of doxycycline hydrochloride was sufficient in eliminating smear layer. Haznedaroǧlu and Ersev[114] 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.[115] 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.[115],[116],[117],[118]

Organic acids

Citric acid is an organic acid that demonstrates its effectiveness in removing smear layer.[119],[120],[121] It has been reported that citric acid eliminates smear layer much better than other acids such as polyacrylic acid, lactic acid, and phosphoric acid.[122] Wayman et al.[20] 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.[123] 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[124] 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[2],[93] 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.[125]

Moreover, Bitter[126] introduced 25% tannic acid irrigant as a root canal-chelating solution and reported that it produces clean smooth canal wall. However, Sabbak and Hassanin[127] 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

Chitosan is a biopolymer derived by the partial deacetylation of chitin obtained from crustacean shells. A study by Geethapriya et al.[128] 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.[129],[130] Silva et al.[131] 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.[5],[121],[132]

Different studies recommended the sequential application of NaOCl and EDTA during chemomechanical preparation[5],[12],[41],[91] to help eliminating microorganism and remove soft-tissue debris and smear layer. Goldman et al.[88] 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.[4] 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.[133] However, NaOCl and MTAD mix can demineralize dentin faster than NaOCl and EDTA.[134]

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.[135] 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.[5] 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.[10],[35],[45],[136],[137] Martin and Cunningham[138] 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.[139],[140] 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.[141] also showed the importance of using a small-sized instrument to maximize microstreaming effect, leading to cleaner wall. Prati et al.[142] advocated that the best smear layer removal was achieved with the use of ultrasonic activation. A recent study by Kowsky and Naganath[143] 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[144],[145] 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[18] showed that 2%–4% NaOCl irrigant with ultrasonic energy can eliminate smear debris, compared to other irrigations. Ahmad et al.[139] 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.[146] 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[10] 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.[147] 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.[148],[149],[150] 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.[18],[137],[139] Moreover, Baumgartner and Cuenin[151] reported that irrigation with NaOCl and ultrasonic did not enhance smear layer removal from root canal walls. Guerisoli et al.[152] 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.[153] 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.

Laser removal

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.[154],[155],[156] 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.[157],[158],[159],[160] Although application of laser during endodontic treatment is safe,[161] yet it has some limitations since it cannot access small curved canal spaces with the large straight probes that are provided.

Dederich et al.[157] and Tewfik et al.[159] 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.[154],[155],[156] 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.[162] 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,[160],[163] argon fluoride laser,[164] and carbon dioxide laser.[158] They all give the same pattern of dentin disruption. Moreover, Saraswathi et al.[165] 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[166] 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[167] 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.[168] 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.[169] 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.

 Conclusion



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.[170]

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.[171] 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.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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