The Smear layer in endodontics

Smear layer is a controversial topic in the field of operative dentistry and endodontics. This presentation includes composition, concepts, structure, advantages, disadvantages, and removal methods of smear layer.

1. THE SMEAR LAYER IN ENDODONTICS Presented By : Dr. Arpit Viradiya Guided By : Dr. Shubhprabhat Gupta Dr. Sandeep Metgud

2. Introduction The term ‘Smear layer’ is used most often to describe the grinding debris left on dentin by cavity preparation. However the term applies to any debris produced iatrogenically by the cutting, not only of dentin, but also of enamel, cementum and even the dentin of the root canal.

3. The complete cleaning of the root canal system has always been a problem in endodontics . Although successful endodontics is based on the fundamentals of diagnosis, instrumentation and obturation, it is generally recognized that debridement of the root canal is the most important aspect of successful endodontic treatment.

4. Smear layer differ from dusty pattern of the superficial debris in that it is the layer of muddy material composed of an amorphous layer of organic and inorganic debris and sometimes bacteria and it is assumed that debris and bacteria left inside the canal can affect the prognosis of root canal treatment.

5. CONCEPTS of SMEAR LAYER : It is difficult to say when or by whom, the concept of the smear layer was first introduced. The earliest studies on the effects of various instruments on dental tissues were those reported by Lammie , Draycott in 1952. Their attempts were limited principally to light microscope and it has consistently failed to identify the smear layer

6. Boyde et al (1963) were the first to describe and demonstrate the presence of a “smear layer ”. They concluded that an organic layer containing apatite particles was deposited or smeared on the enamel surface, through functional heat generated during cutting.

7. Provenza and Sardana (1996) evaluated means of removing debris from enamel and dentin after the use of steel burs, diamond stones and hand instruments. They reported variations in the degree to which debris was removed. Detergents were relatively ineffective, EDTA left behind a film , hydrochloric acid was considered too destructive in its action, hydrogen peroxide appeared to be most effective.

8. Researchers become aware of the endodontic smear layer by about 1975. It was first reported by Baker. Tidmarsh in 1978, treated instrumented teeth with 50% citric acid and found the dentin clear of smear layer and the dentinal tubules wide open.

9. Goldman in 1979 , tested various solutions individually and in combination and concluded that chelating agent EDTA and sodium hypochlorite was the best to remove the debris when used as a final flush . Evans, injected thermoplasticized gutta-percha into canals after smear layer removal and concluded that the presence or absence of the smear layer had no significant effect on the apical seal.

10. Definition According to Shwartz – “Any debris, calcific in nature, produced by reduction or instrumentation of dentin, enamel or cementum or as a contaminant that precludes interaction with the underlying pure tooth tissue”. According to Cohen – “An amorphous, relatively smooth layer of microcrystalline debris whose featureless surface cannot be seen with the naked eye.”

11. The American Association of Endodontists defined smear layer as a “surface film of debris retained on dentin or other tooth surfaces like enamel, cementum after instrumentation with either rotary instruments or endodontic files ”. According to DCNA (1990) “when tooth structure is cut, instead of being uniformly sheared, the mineralised matrix shatters. Existing on the strategic interface of restorative materials and the dentin matrix most of the debris is scattered over the enamel and dentin surface to form what is known as smear layer”.

12. In endodontics , the smear layer results directly from the instrumentation used to prepare the canal wall and is found only where the wall is instrumented and not in uninstrumented areas. Because it is a very thin layer and is soluble in acid it is not very apparent. It cannot be seen in demineralized teeth as it dissolves in the process of demineralization. It is only visible under SEM or TEM.

13. The smear layer has an amorphous, irregular and granular appearance when viewed under the scanning electron microscope. This appearance may be formed by translocating and burnishing the superficial components of the dentin walls during endodontic instrumentation . The smear layer consists of two separate layers: A superficial layer. Loosely attached layer to dentin.

14. The smear layer is made up of tooth particles ranging from less than 0.5  m to 15  m. The particles are composed of globular subunits approximately 0.05 – 0.1  m in diameter which originated from mineralized fibers. The thickness of this layer is 1-5  m. The depth entering the tubules may be from a few  m upto 40  m.

15. Several factors may cause the depth of the smear layer to vary from tooth to tooth – Dry or wet cutting of the dentin. The size and shape of the cavity or root canal. The type and sharpness of instrument used. The amount and chemical make up of the irrigating solution .

16. If there is a difference in the rate of flow of fluid across dentin before and after removal of the smear layer, the magnitude of rate change is an indication of the thickness or density of the smear layer.

17. Dentin is composed of 2 different layers. Superficial dentin (near the enamel). Deep dentin (near pulp ). Smear layer on deep dentin contains more organic material than superficial dentin. This is because of greater number of proteoglycans lining the tubules or the greater number of odontoblastic processes near the pulp.

18. The movement of fluid across dentin meets a resistance directly proportional to the quantity and quality of smear layer present. In vital teeth the smear layer restricts the dentinal fluid from flushing the dentin surface. It also hinders the chemical process that produces marginal seal. The presence of smear layer, however, does not appear to restrict the adaptation of freshly condensed amalgam to cavity surface.

19. According to some investigators, after a canal is instrumented the smear layer produced can harbour bacteria and bacterial products that can be a reservoir of potential irritants. The smear layer is a separate structure from the underlying dentin and may crack open and pull away from the underlying dentinal tubules.

20. A situation like this could be harmful to the foundation of gutta-percha obturated over the smear layer. Hence they thought it best to remove the smear layer, though controversy still remains.

21. Factors affecting formation of smear layer in endodontics Type of Instruments : Increased centrifugal forces resulting from the movement and the proximity of the instrument to the dentine wall forms a thicker and more resistant smear layer ( Jodakin and Austin, 1981) and thus the amount produced during automatic preparations, as with Gates-Glidden or post drills, will be greater in volume than that produced by hand filling.

22. Dentin cut wet / dry : Filing a canal without irrigation (or cutting without a water spray) will produce a thicker layer of dentin debris and smear layer. Proximity of the instrument of dentin walls : More closely the instrument to dentinal walls more is the centrifugal forces producing thicker smear layer.

23. Size and shape of root canals The authors explained that, on account of the reduced dimension of the root canal, air bubbles frequently remain trapped and prevent total filling with the irrigant . Mechanical stirring with a lentulospiral removes the air bubbles, favors improved contact of EDTA with the canal walls, and takes the solution to areas that are not reached by the irrigating needle.

24. COMPONENTS OF THE SMEAR LAYER Though the exact proportion of the composition is not certain. Is composed of Organic component. Inorganic component. Inorganic component is made up of tooth structure and some non-specific inorganic components.

25. Organic component consists of heated coagulated proteins (gelatin formed by the deterioration of collagen heated by cutting temperatures), necrotic or viable pulp tissue, odontoblastic processes, saliva , blood cells and micro-organisms.

26. Advantages of smear layer in endodontics Reduction of dentin permeability to toxins and oral fluids. Reduction of diffusion (usually inwards by convection and outwards by hydrostatic pressure) of fluids and prevents wetness of cut dentin surface. Bacterial penetration of dentinal tubules is prevented.

27. Disadvantages of smear layer It may harbour bacteria, either from original carious lesion or saliva, which may multiply taking nourishment from smear layer or dentinal fluid. Smear layer is permeable to bacterial toxins . Smear layer itself is infected. (Presence of bacteria) The smear layer may prevent the adhesion of composite resin system, bonding agents, GIC and polycarboxylate cements.

28. It blocks the antimicrobial effects of intracanal disinfectants and increases disinfecting tissue. Presence of smear layer would necessitate use of higher concentration and / or amount of anti bacterial agents. Acts as a intermediate physical barrier, interfere with adhesive penetration of sealers ( Obturating materials) with dentinal tubules.

29. PHYSICAL BARRIER FOR BACTERIA AND DISINFECTANTS When pathologic changes occur in the dental pulp, the root canal system can harbour several species of bacteria, their toxins and by products. These bacteria are predominantly gram-negative anaerobes.

30. The morphology of the root canals is very complex therefore the mechanically prepared canals contain areas not accessible by endodontic instruments and bacteria will be found more in number in these areas.

31. Available evidence shows that bacteria and its by products present in infected root canals may invade the dentinal tubules. Investigators have reported the presence of bacteria in the dentinal tubules of infected teeth at approximately half the distance between the root canal walls and the cemento -dentinal junction.

32. Bacterial penetration into the dentinal tubules is seen upto 150  m. in the apical 2/3 rd of the root. Thus even after chemomechanical instrumentation of the root canal, some bacteria still remain in the canal and dentinal tubules, for this reason, chemomechanical cleansing is often supported by the use of disinfectants.

33. Drake et al showed that removal of the smear layer opened the tubules, allowing bacteria to colonize in the tubules to a much higher degree (10 fold) compared with roots with an intact smear layer, removal of smear layer facilitates passive penetration of bacteria.

34. According to some authors the presence of smear layer may block the antimicrobial effects of intracanal disinfectants into the tubules, various medicaments have been proposed for disinfection of root canals, they are: Traditional phenolic or fixative agents like camphorated mono chlorophenol (CMCP), formacresol . Non – phenolic compounds like iodine potassium iodide & calcium hydroxide.

35. Researchers found that in absence of smear layer, liquid camphorated monochlorophenol disinfected the dentinal tubules rapidly and completely but calcium hydroxide failed to eliminate enterococcus faccalis even after 7 days of incubation and hence concluded that smear layer did delay but not abolish the action of the disinfectants. However following removal of smear layer, bacteria in dentinal tubules can be easily destroyed.

36. Smear layer and microleakage An important consideration in endodontics is the ultimate seal of root canals in order to prevent possible microleakage which may be the cause of future failure of the root filling. Prepared dentin surfaces should be very clean to increase the sealing efficiency of obturation.

37. Smear layer on root canal walls acts as an intermediate physical barrier and may interfere with adhesion and penetration of sealers into dentinal tubules.

38. Investigators observed that plastic filling materials and sealers penetrated into the dentinal tubules after removal of smear layer, and its presence obstructed their penetration. The penetration in smear free groups ranged from 40-60  m . They concluded that tubular penetration may increase the interface between the filling and the dentinal structures and thus may prevent leakage.

39. Apical leakage According to Evan et al., the use of injected thermoplasticized gutta-percha should be accompanied by the use of sealer regardless of whether or not the smear layer has been removed. But Kennedy stated that an absence of the smear layer causes less apical leakage than gutta-percha filled canal with the smear layer intact.

40. Sealers Endodontic sealers act as a glue to ensure good adaptation of gutta-percha to the canal walls. If the smear layer is not removed then the gutta-percha is not firmly attached to the dentin and the smear layer may laminate off the canal wall and create a false seal, voids in the fill and an environment for microleakage .

41. The type of sealer used has different implications once the smear layer has been removed. For example Grossman sealer which is a powder liquid combination, contains small particles in the powder that enter the dentinal tubules and create a secure interface between sealer and canal wall.

42. After the removal of smear layer, calcium hydroxide based sealers promote the apposition of the cementum at the canal apex and seal it off against microleakage by the formation of osteoid or dentoid type material.

43. Circulation of blood is needed for the calcium ion to promote new tissue thus the calcium hydroxide sealers are effective for sealing only at the root apex. If more cementum is going to form to create a better apical seal, dentin chips at the apex of a root canal acts as a nidus for formation of hard tissue. Bacterial contamination by the presence of a smear layer can prevent this.

44. Urethane dimethacrylate (UDMA) based root canal sealers have been introduced. Their aim is to provide a better bond to allow less microleakage and increase the fracture resistance of root filled teeth through the creation of monoblocks , when a core material such as Resilon replaces gutta-percha.

45. Some studies indicate that smear layer removal leads to higher tubule penetration, increased sealer to dentine bond strength and enhanced fluid-tight seal.

46. Post cementation Removal of smear layer increases the cementation bond and the tensile strength of the cementing medium for post cementation. Glass ionomer cements are effective in post cementation after smear layer removal because the glass ionomer has better union with tooth structure.

47. When the smear layer was removed by flushing with EDTA and sodium hypochlorite rinse, the unfilled BISGMA resin (cementing media) flowed into the exposed dentinal tubules and into serrations on the post, improving retention vastly, and with the removal of smear layer and an unfilled resin bonding agent, shorter posts can be used.

48. Functional implications Dental materials: The presence of smear layer masks the underlying dentin matrix and may interfere with the bonding of adhesive dental cements such as polycarboxylates and glass ionomer that reacts chemically with the dentin matrix.

49. The cements that react chemically to smear layer rather than the matrix of sound intertubular dentin produce a weaker bond as the smear layer can be torn away from the underlying matrix, and when these cements are tested for tensile strength, the failure can be either adhesive (between cement and smear layer) or cohesive (between constituents of smear layer).

50. To increase the tensile strength of a cement dentin interface there are several approaches: Remove the smear layer by etching with acids. This procedure does not injure the pulp if dilute acids are used for shorter periods of time ex: etching dentin with 6% citric acid for 60secs removes all the smear layer as does 15secs of etching with 37% phosphoric acid.

51. The advantages are that the smear layer is entirely removed, the tubules are open and available for increased retention and the surface collagen is exposed for covalent linkage with new experimental primers for cavities. The disadvantage is that there is a physical barrier for bacterial penetration and the permeability of dentin increases.

52. Methods to remove the smear layer Chemical removal: The quantity of smear layer removed by a material is related to its pH and the time of exposure (Morgan & Baumgartner 1997). A number of chemicals have been investigated as irrigants to remove the smear layer. According to Kaufman & Greenberg (1986), a working solution is the one which is used to clean and irrigate the canal.

53. Chlorhexidine , whilst popular as an irrigant and having a long lasting antibacterial effect through adherence to dentine, does not dissolve organic material or remove the smear layer.

54. Sodium hypochlorite: The ability of NaOCl to dissolve organic tissues is wellknown (Rubin et al. 1979) and increases with rising temperature ( Moorer & Wesselink 1982). However , its capacity to remove smear layer from the instrumented root canal walls has been found to be lacking. The conclusion reached by many authors is that the use of NaOCl during or after instrumentation produces superficially clean canal walls with the smear layer present (Baker et al. 1975).

55. Chelating agents Smear layer components include very small particles with a large surface, which makes them soluble in acids. The most common chelating solutions are based on EDTA which reacts with the calcium ions in dentine and forms soluble calcium chelates. It has been reported that EDTA decalcified dentine to a depth of 20–30 um in 5 min

56. Different formulations of EDTA have been used as root canal irrigants . In a combination, urea peroxide is added to encourage debris to float out of the root canal. Many studies have shown that paste-type chelating agents, whilst having a lubricating effect, do not remove the smear layer effectively when compared to liquid EDTA.

57. A quaternary ammonium bromide ( cetrimide ) has been added to EDTA solutions to reduce surface tension and increase penetrability of the solution. McComb & Smith (1975) reported that when this combination (EDTAC) was used during instrumentation, there was no smear layer remaining except in the apical part of the canal. This study also showed that EDTAC was the most efficient irrigating solution for removing smear layer.

58. Bis - dequalinium -acetate (BDA), a dequalinium compound and an oxine derivative has been shown to remove the smear layer throughout the canal, even in the apical third. Salvizol (Ravens Gmbh , Germany) is a commercial brand of 0.5% BDA and possesses the combined actions of chelation and organic debridement. Kaufman et al. (1978) reported that Salvizol had better cleaning properties than EDTA.

59. When comparing Salvizol with 5.25% NaOCl , both were found comparable in their ability to remove organic debris, but only Salvizol opened dentinal tubules.

60. Tetracylines are antibiotics effective against a wide range of microorganisms. Tetracyclines have unique properties in addition to their antimicrobial aspect. They have low pH in concentrated solution, and because of this can act as a calcium chelator and cause enamel and root surface demineralization.

61. The surface demineralization of dentine is comparable with that of citric acid. Barkhordar et al. (1997) reported that doxycycline hydrochloride (100 mg mL-1) was effective in removing the smear layer from the surface of instrumented canals and root-end cavity preparations . Hazne (2001) showed that 1% tetracycline hydrochloride or 50% citric acid can be used to remove the smear layer from surfaces of root canals.

62. In an effort to produce an irrigant capable of both removing the smear layer and disinfecting the root canal system, Torabinejad et al. (2003) developed a new irrigating solution containing a mixture of a tetracycline isomer, an acid, and a detergent (MTAD ).

63. Their work concluded MTAD to be an effective solution for the removal of the smear layer. It does not significantly change the structure of the dentinal tubules when the canals are irrigated with sodium hypochlorite and followed with a final rinse of MTAD.

64. Components of MTAD- Acids – EDTA, acetic acid, citric acid, tannic acid, polyacrylic acid, bi- dequalinium acetate. This helps to remove smear layer Tetracycline— Bacteriostatic and prevents production of endotoxin. Detergent- Decrease surface tension and increase penetrability.

65. Organic acids The effectiveness of citric acid as a root canal irrigant has been demonstrated ( Loel 1975, Tidmarsh 1978) and confirmed to be more effective than NaOCl alone in removing the smear layer. Citric acid removed smear layer better than polyacrylic acid, lactic acid and phosphoric acid but not EDTA.

66. Citric acid, in addition to removing the smear layer, is a powerful antimicrobial agent, but its antimicrobial action is not as great as that of 5.25 percent sodium hypochlorite, which acts as a broad spectrum antibiotic effectively against bacteria, bacteriophages, spores, yeasts, and viruses. Combining the two, sodium hypochlorite followed by 6 per cent citric acid would give an ideal endodontic irrigant .

67. Wayman et al. (1979) showed that canal walls treated with 10%, 25% and 50% citric acid solution were generally free of the smeared appearance, but they had the best results in removing smear layer with sequential use of 10% citric acid solution and 2.5% NaOCl solution, then again followed by a 10% solution of citric acid.

68. However, Yamada et al. (1983) observed that the 25% citric acid– NaOCl group was not as effective as a 17% EDTA– NaOCl combination . Berry et al reported Polyacrylic acid a t 40% to be very effective for removal of the smear layer. They have been used to demineralize dentin surfaces, uncover and widen the orifices of dentinal tubules and expose the dentin collagen matrix.

69. Sodium hypochlorite and EDTA When irrigating a root canal the purpose is twofold: T o remove the organic component, the debris originating from pulp tissue and microorganisms, A nd the mostly inorganic component, the smear layer. As there is no single solution which has the ability to dissolve organic tissues and to demineralize the smear layer, the sequential use of organic and inorganic solvents has been recommended.

70. Numerous authors have agreed that the removal of smear layer as well as soft tissue and debris can be achieved by the alternate use of EDTA and NaOCl . Goldman et al. (1982) examined the effect of various combinations of EDTA and NaOCl , and the most effective final rinse was 10 mL of 17% EDTA followed by 10 mL of 5.25% NaOCl , a finding confirmed by Yamada et al. (1983).

71. Ultrasonic smear removal Following the introduction of dental ultrasonic devices in the 1950s, ultrasound was investigated in endodontics .

72. NaOCl activated by an ultrasonic delivery system was used for the preparation and irrigation of canals and smear-free canal surfaces were observed using this method . Researchers who found the cleaning effects of ultrasonics beneficial used the technique only for the final irrigation of root canal after completion of hand instrumentation.

73. This has given the term passive ultrasonic irrigation. Ahmad et al. ( 1987) claimed that direct physical contact of the file with the canal walls throughout instrumentation reduced acoustic streaming. Acoustic streaming is maximized when the tips of the smaller instruments vibrate freely in a solution.

74. Baumgartner & Cuenin (1992) also observed that ultrasonically energized NaOCl , even at full strength, did not remove the smear layer from root canal walls. Guerisoli et al. (2002) evaluated the use of ultrasonics to remove the smear layer and found it necessary to use 15% EDTA with either distilled water or 1% sodium hypochlorite to achieve the desired result.

75. Laser removal Lasers can be used to vaporize tissues in the main canal, remove the smear layer and eliminate residual tissue in the apical portion of root canals.

76. The effectiveness of lasers depends on many factors, including the power level, the duration of exposure, the absorption of light in the tissues, the geometry of the root canal and the tip-to-target distance . Takeda et al. using the erbium – yttrium – aluminium – garnet ( Er : YAG) laser demonstrated optimal removal of smear layer without the melting, charring and recrystallization.

77. The main difficulty with laser removal of the smear layer is access to the small canal spaces with the relatively large probes that are available.

78. Conclusion The problem of smear layer is yet a controversy. To keep it or remove it is still a problem. However, if the smear layer is to be removed the method of choice seems to be the alternate use of EDTA and sodium hypochlorite solutions . It is upto the dentists judgement , knowledge and understanding to treat the smear layer or not . Clinical investigations are needed to determine the role of smear layer in the outcome of root canal therapy.

79. References Ingle’s Endodontics 6 th Edition. Violich DR, Chandler NP. The smear layer in endodontics –a review. International Endodontic Journal. 2010 Jan 1;43(1):2-15. Torabinejad M, Handysides R, Khademi AA, Bakland LK. Clinical implications of the smear layer in endodontics : a review. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology, and Endodontology . 2002 Dec 31;94(6):658-66 . Czonstkowsky M, Wilson EG, Holstein FA. The smear layer in endodontics . Dental Clinics of North America. 1990 Jan;34(1):13-25 . Şen BH, Wesselink PR, Türkün M. The smear layer: a phenomenon in root canal therapy. International Endodontic Journal. 1995 May 1;28(3):141-8.

80. Thank you