The Smear Layer

The Smear Layer in Endodontics and Conservative Dentistry

1. The Smear Layer Aaron Sarwal MDS 2 nd Prof

2. CONTENTS Introduction Definition History Components Of Smear Layer Bonding & Smear Layer Functional Implications Methods Of Removal Conclusion

3. INTRODUCTION Smear layer - a force to be reckoned with . Increasing importance - paralleled interest in adhesive bonding.

4. INTRODUCTION Natural “Cavity liner ” . David Pashley - the smear layer as a cavity liner is both beneficial and detrimental.

5. DEFINITION According to Operative Dentistry Journal (1984) the term smear layer 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 ”.

6. DEFINITION Cohen defined smear layer as: “ an amorphous, relatively smooth layer of microcrystalline debris whose featureless surface cannot be seen with the naked eye ”.

7. DEFINITION 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 ”.

8. HOW IS SMEAR LAYER FORMED? 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” .

9. HISTORY The earliest studies on the effects of various instruments on dental tissues - reported by Lammie and Draycott in 1952 and Stret (1953) . Limited principally to light microscope.

10. HISTORY Charbeneou , Peyton and Anthony were among the first to quantify and rank the differences between burs and abrasives by using a profilometer to record the surface topography of cut and abraded dental tissues.

11. HISTORY In 1961 Scott and O’Neel used transmission electron microscopy to study the nature of the cut tooth surface.

12. HISTORY Advent of SEM - grinding debris was first referred to as the smear layer by Boyde , Switsur and Stewart in 1963.

13. HISTORY Eick and others in 1970 attempted to quantify and identify cutting debris on tooth surfaces. They confirmed that : Surfaces abraded with diamonds were rougher than those cut with tungsten carbide burs . Surfaces cut dry were rougher and more smeared than those in which water was used as coolant . The smear layer is composed of an organic film less than 0.5 microns thick . Included with in it were particles of opacity ranging from 0.5 – 15 microns . Such layers were present on all surfaces though they were not necessarily continuous .

14. HISTORY In 1972, Jones, Lozdan and Boyde showed that smear layers were common on enamel and dentin following the use of instruments . SMEAR LAYER ON ENAMEL

15. HISTORY Erich and co-workers in 1976 discussed the role of friction and abrasion in the drilling of teeth . They accounted for the formation of smear layers, especially in dentin by a brittle and ductile transition and alternating rupture and transfer of apatite and collagen matrix into the surface .

16. HISTORY In 1982 , Goldman and others studied smear layers after the use of endodontic instrumentation.

17. SMEAR DILEMMA

18. IN SUPPORT OF RETAINING SMEAR LAYER Blocks the dentinal tubules, preventing the exchange of bacteria and other irritants by altering permeability ( Michelich et al. 1980, Pashley et al. 1981, Safavi et al. 1990, Drake et al. 1994, Galvan et al. 1994 ). Barrier to prevent bacterial migration into the dentinal tubules ( Drake et al. 1994, Galvan et al. 1994, Love et al. 1996, Perez et al . 1996). Vojinovic et.al. showed that bacteria could not penetrate into dentin in the presence of smear layer.

19. IN SUPPORT OF REMOVING SMEAR LAYER Smear layer - unpredictable thickness and volume, because a great portion of it consists of water ( Cergneux et al. 1987 ). It contains bacteria, their by products and necrotic tissue ( McComb & Smith 1975, Goldberg & Abramo - vich 1977, Wayman et al. 1979, Cunningham & Martin 1982, Yamada et al. 1983).

20. Bacteria may survive and multiply ( Brannstrom & Nyborg 1973) and can proliferate into the dentinal tubules ( Olgart et al. 1974, Akpata & Blechman 1982, Williams & Goldman 1985, Meryon et al. 1986, Meryon & Brook 1990 ), which may serve as a reservoir of microbial irritants ( Pashley 1984).
 It may act as a substrate for bacteria, allowing their deeper penetration in the dentinal tubules (George et al. 2005). IN SUPPORT OF REMOVING SMEAR LAYER

21. It may limit the optimum penetration of disinfecting agents ( McComb & Smith 1975, Outhwaite et al. 1976, Goldberg & Abramovich 1977, Wayman et al. 1979, Yamada et al. 1983 ). It can act as a barrier between filling materials and the canal wall and therefore compromise the formation of a satisfactory seal ( Lester & Boyde 1977 , White et al. 1984, Cergneux et al. 1987, Czonstkowsky et al. 1990 , Foster et al. 1993, Yang & Bae 2002 ). IN SUPPORT OF REMOVING SMEAR LAYER

22. COMPONENTS OF SMEAR LAYER The exact proportionate composition of the smear layer has not been determined but SEM examinations have disclosed that its composition is both organic and inorganic .

23. COMPONENTS OF SMEAR LAYER The inorganic material in the smear layer is made up of tooth structures and some non-specific inorganic contaminants . The organic components may consist of heated coagulated proteins , necrotic or viable pulp tissues and odontoblastic processes plus saliva, blood cells and microorganisms .

24. SMEAR PHENOMENON Eirich (1976) stated that smearing occurs when “ hydroxy apatite within the tissue is either plucked out or broken or swept along and resets in the smeared out matrix ”. Temperature rises upto 600 o C in dentin when it is cut without a coolant. This value is significantly lower than the melting point of apatite (1500-1800 o C )

25. SMEAR PHENOMENON Smearing is a physico -chemical phenomenon rather than a thermal transformation of apatite involving mechanical shearing and thermal dehydration of the protein . Plastic flow of hydroxy apatite is believed to occur at lower temperature than its melting point and may also be a contributing factor to smearing.

26. MORPHOLOGY OF SMEAR LAYER Smear layer - two separate layers – a superficial layer and a layer loosely attached to the underlying dentin . Dentin debris enters the orifices of the dentinal tubules and acts as plugs to occlude the ends of the tubules . The smear layer is not always firmly attached and neither is it continuous over the substrate. Smear layers found on deep dentin contain more organic material than those found on the superficial dentin .

27. Disc of human dentin cut with a fine-grit diamond blade on a metallurgical saw. Note the uniformity and amorphous nature of the smear layer. Water was the coolant. SCANNING ELECTRON MICROGRAPH

28. Scanning electron micrograph of dentine surface with typical amorphous smear layer with granular appearance and moderate debris present . Dry Cutting. SCANNING ELECTRON MICROGRAPH

29. Scanning electron micrograph of smeared surface of dentine. The crack shapes are processing artefacts overlying dentinal tubules. Un treated surface. SCANNING ELECTRON MICROGRAPH

30. Scanning electron micrograph of dentine surface showing smear plugs occluding tubules. The surface has been treated for 60 s with Tubulicid Blue Label SCANNING ELECTRON MICROGRAPH

31. Clinically produced smear layers have an average depth of from 1-5 microns (Goldman et.al. 1981, Mader et.al. 1984 ). The depth entering the dentinal tubule may vary from a very few microns to 40 microns . Cengiz et.al. (1990) proposed that the penetration of smear layer into dentinal tubules could be caused by Capillary action as a result of adhesive forces between the dentinal tubules and the smear material. MORPHOLOGY OF SMEAR LAYER

32. Several factors may cause the depth of the smear layer to vary from tooth to tooth: Dry or wet cutting of the dentin. The type of instrument used and The amount and chemical make of the irrigation solution. MORPHOLOGY OF SMEAR LAYER

33. Filing a canal without irrigation or cutting without a water spray will produce a thicker layer of dentin debris The use of coarse diamond burs produces a thicker smear layer than the use of carbide burs . MORPHOLOGY OF SMEAR LAYER

34. TOPOGRAPHICAL DETAIL OF CUT DENTIN Steel and tungsten carbide burs produce an undulating pattern , the trough of which run perpendicular with the direction of movement of the hand piece.

35. Fine grooves can be seen running perpendicular to the undulations and parallel with the direction of rotation of the bur. Such a phenomenon is referred to as “ galling ”. TOPOGRAPHICAL DETAIL OF CUT DENTIN

36. Scanning electron micrograph showing the galling pattern on a dentin surface cut with a water-cooled, tungsten carbide bur SCANNING ELECTRON MICROGRAPH

37. Scanning micrograph of the cutting anomalies on dentin following the use of a cross-cut steel bur. Note the debris and evidence of smearing (arrow). SCANNING ELECTRON MICROGRAPH

38. The galling phenomenon appears more masked with tungsten carbide burs run at high speed. An examination of both steel and tungsten carbide burs shows a rapid deterioration of the cutting edges through what appears to be a brittle fracture . Brittleness significantly diminishes the cutting efficiencies of the bur - increases frictional heat - causes smearing. TOPOGRAPHICAL DETAIL OF CUT DENTIN

39. Scanning electron micrographs of the flutes of, tungsten carbide bur. At higher magnification evidence of brittle fracture (arrow) of the cutting edge is seen together with the formation of facets. TOPOGRAPHICAL DETAIL OF CUT DENTIN

40. Steel and tungsten carbide burs - obliterate normal structural detail of the tissue . Debris , irregular in shape and non-uniform in size and distribution , remains on the surface even after thorough lavage with H 2 O 2 TOPOGRAPHICAL DETAIL OF CUT DENTIN

41. Scanning electron micrograph of dentin cleaned to show that deformation after abrading and cutting is confined to a few superficial micrometers of the tissue. TOPOGRAPHICAL DETAIL OF CUT DENTIN

42. The mechanism of dental tissue removal for burs and diamond is different significantly . As burs rotate, the flute undermines the tissue, the amount being determined by such factors as the angle of attack of the flute . TOPOGRAPHICAL DETAIL OF CUT DENTIN

43. On the other hand, abrasive particles , passing across the tissue , ploughs through in which substrate is ejected ahead of the abrading particle and elevated into ridges parallel with the direction of travel of particle. TOPOGRAPHICAL DETAIL OF CUT DENTIN

44. Scanning electron micrograph showing grooves traversing a dentin surface abraded with diamond. TOPOGRAPHICAL DETAIL OF CUT DENTIN

45. TOPOGRAPHICAL DETAIL OF CUT DENTIN Scanning electron micrographs of the grooves left by a diamond stone on dentin . Fine grooves run within the deeper grooves and pitting is also evident .

46. Scanning electron micrograph of dentin abraded with 600-grit silicon carbide abrasive paper. Note the occluded tubules and the prominent peritubular dentin mounds . Surface cleaned with 3% hydrogen peroxide TOPOGRAPHICAL DETAIL OF CUT DENTIN

47. Several factors govern the size of the grooves , including particle size, pressure and hardness of the abrasive related to the substrate . TOPOGRAPHICAL DETAIL OF CUT DENTIN

48. Scanning electron micrograph of a diamond stone in situ. Note the abrasive particles and the grooves left by them in the tissue. TOPOGRAPHICAL DETAIL OF CUT DENTIN

49. A significant difference exists between diamond burs used with and without coolant or water spray . In the absence of coolant smeared debris does not form a continuous layer but exists rather as localised islands with discontinuities exposing the underlying dentin . Coolant of the water spray does not prevent smearing but appear to significantly reduce the amount and distribution of it. TOPOGRAPHICAL DETAIL OF CUT DENTIN

50. Scanning electron micrograph showing considerable smearing of dentin after the use of a clogged diamond using water as a coolant. TOPOGRAPHICAL DETAIL OF CUT DENTIN

51. An SEM of surface of dentin ground with a diamond cylinder, high speed and then cleaned with a detergent ( Tubulicid , Bli Label). The smear layer is removed, the per tubular dentin intact , and amorphous material remains in the apertures of the tubules . TOPOGRAPHICAL DETAIL OF CUT DENTIN

52. BONDING THE SMEAR LAYER Diamonds , through the introduction of grooved anomalies - greater surface area than burs . Increased surface area - offers large number of retentive sites . These sites in enamel are primarily m icromechanical and the retention mechanism for the tissue lies in the multitude of superficial micropores enhanced following acid conditioning of the tissue.

53. Acids - remove the smear layer. For enamel - H 3 PO 4 phoshoric acid in gel solution ranging from 30-50 % . Branstrom , Nordel Wall and Gwinnett – conditioning facilitates the penetration of resin into the dentinal tubules - increases bond strength . BONDING THE SMEAR LAYER

54. Etched dentin. BONDING THE SMEAR LAYER

55. 35% O-Phosphoric Acid in non silica gel and in silica gel. BONDING THE SMEAR LAYER

56. TOTAL ETCH VS SELF ETCH Chihiro C, Finger WJ. Effect of smear layer thickness on bond strength mediated by three all-in-one self-etching priming adhesives . Oper Dent 2002;4:283-289

57. RESTORATIVE DENTISTRY FUNCTIONAL IMPLICATIONS

58. Smear Layer of Dentin

59. INFLUENCE OF CONDITIONING OF SMEAR LAYER ON SENSITIVITY OF DENTIN Etching the dentin of roots, whether done therapeutically or by the action of microorganisms of plaque can remove the thin layer of covering cementum or smear layer or both. Conditioning with acids will remove the smear layer plugs exposing patent dentinal tubules to the oral cavity. This can lead to sensitivity of the dentin to the point whereit interferes with the oral hygiene

60. Several studies indicate that most of the resistance to the flow of fluid across dentin is due to the presence of smear layer. Etching dentin greatly increases the ease with which fluid can move across dentin . This is accompanied clinically by increased sensitivity of dentin to osmotic, thermal and tactile stimuli. INFLUENCE OF CONDITIONING OF SMEAR LAYER ON SENSITIVITY OF DENTIN

61. INFLUENCE ON PERMEABILITY OF DENTIN Substances diffuse across dentin at a rate that is proportional to their concentration gradient and the surface available for diffusion . The removal of smear layer increases the dentin permeability by 5-6 times in vitro by diffusion but increases it by 25-36 times by filtration .

62. INFLUENCE ON PERMEABILITY OF DENTIN

63. BACTERIA UNDER SMEAR LAYER UNDER RESTORATIONS Water cleaned cavities with the smear layer remaining underneath the composite restoration showed the presence of numerous bacteria, whereas in the antiseptically cleaned cavities , bacteria were absent. Micro organisms get sufficient nourishment from the smear layer and dentinal fluid .

64. BACTERIA UNDER SMEAR LAYER UNDER RESTORATIONS These considerations favour the opinion that most of the smear layer should be removed and any smear layer remaining for instance at the tubule should be antiseptically treated before the application of lining or a luting cement. There is no evidence that common permanent restorative materials are sufficiently antibacterial to kill bacteria entrapped within the smear layer, especially when a fluid filled contraction gap , 5-20 microns wide separates the restoration from the smear layer .

65. Bases of ZnOE and Ca(OH) 2 may have good antiseptic effects but unfortunately under permanent restorations , these bases cannot be placed on all cavity walls . Pure Ca(OH)2 is an excellent antibacterial temporary dressing and should be applied under temperature fillings . Ca(OH)2 may reinforce the remaining smear plugs in the outer apertures of dentinal tubules. BACTERIA UNDER SMEAR LAYER UNDER RESTORATIONS

66. THE PROTECTIVE EFFECT OF SMEAR PLUGS IN APERTURES Etching the cavity prior to the placement of the composite resin resulted in a massive invasion of bacteria in dentinal tubules . This was seen in all teeth after 3-4 weeks. The corresponding cavities, cleaned by water and with smear layer had a bacterial layer on the cavity walls but practically no invasion into the dentinal tubules . Obviously smear plugs in the apertures of the tubules had prevented bacterial invasion. Pashley - Natural Cavity Liner.

67. THE CONSEQUENCE OF REMOVING THE PLUGS From opened tubules, bacteria may easily reach the pulp and multiply therefore removal of smear plugs should be avoided . Another important consequence of etching and removal of smear plugs and peritubular dentin at the surface is that the area of wet tubules may increase from about 10-25% of the total . Subsequently it is difficult to get the dentin dry because fluid continues to be supplied from below through the tubules. This moisture would not seem to favour adhesive or mechanical bonding to dentin .

68. THE CONSEQUENCE OF REMOVING THE PLUGS Inflammation was present under all infected cavities, being somewhat more pronounced on the etched cavities, but the difference was not great . Thus smear plugs did not prevent bacterial toxins from diffusing into the pulp.

69. ENDODONTICS FUNCTIONAL IMPLICATIONS

70. The presence of smear layer on the surface of an instrumented root canal. SMEAR LAYER ON ROOT CANAL

71. The presence of several bacteria in a dentinal tubule of a tooth with necrotic pulp . BACTERIA IN DENTINAL TUBULES

72. ROLE OF SMEAR LAYER IN APICAL LEAKAGE The smear layer’s presence plays a significant part in an apical leakage. Its absence makes the dentin more conducive to a better and closer adaptation of the gutta percha to the canal wall. With the smear layer intact, apical leakage will be significantly increased , without the smear layer, the leakage will still occur but at a decreased rate . Plasticized GP can enter the dentinal tubules when the smear layer is absent. This can establish a mechanical block b/w the GP and the canal wall.

73. EFFECT OF SMEAR LAYER ON SEALERS The type of sealer used has different implications once the smear layer is removed . A powder liquid combination, the most of which is grossmans sealer contains small particles in the powder that could enter the orifices of the dentinal tubules and help create a reaction interface between sealer and canal wall . Ca(OH) 2 based sealers have the advantage of promoting the apposition of cements at the canal apex and sealing it off against microleakage.

74. POST CEMENTATION GICs are effective in post cementation after smear layer removal because the glass ionomer has a better union with tooth structure.

75. METHODS OF REMOVAL FUNCTIONAL IMPLICATIONS

76. METHODS OF REMOVAL Braunsternis - described the use of H 2 O, H 2 O 2 benzalkonium chloride, EDTA and other agents to remove the smear layer . Commercially available products like tubulicid blue label , tubulicid red label - remove most of smear layer without removing smear debris that has fallen into the orifices of the tubules to form plugs on the cut surface of the dentin.

77. METHODS OF REMOVAL Smear layer removed by etching with acid - does not injure the pulp , especially if dilute acids are used for short periods of time. Etching dentin with 6% citric acid for 60 seconds removes all of the smear layer as does 15 seconds of etching with 37 % phosphoric acid .

78. ADVANTAGES OF REMOVAL BY ETCHING The smear layer is entirely removed . The tubules are open and available for retention. The surface collagen is exposed for possible covalent linkage with new experimental primers for cavities . Further with the smear layer gone, one does not have to worry about it slowly dissolving under a leaking restoration or being removed by acid produced by bacteria, leaving a void b/w the cavity wall and the restoration might permit bacterial colonization.

79. DISADVANTAGES OF REMOVING THE SMEAR LAYER In its absence, there is no physical barrier to bacterial penetration of dentinal tubules

80. SODIUM HYPOCHLORITE The capacity of NaOCl to remove the smear layer from the instrumented root canal wall has been found to be insufficient. Even the combination of NaOCl and H 2 O 2 proved to be ineffective. NaOCl cannot destroy bacteria within the tubules closed by a smear layer covering.

81. Scanning electron micrograph of "strings" of resin which had penetrated deep into the dentinal tubules after conditioning with phosphoric acid and sodium hypochlorite . Resin was disclosed by tissue dissolution. RESIN IN DENTINAL TUBULES

82. SODIUM HYPOCHLORITE Scanning electron micrographs show dentin etched for 10 seconds with 50 % phosphoric acid. A significant morphological difference exists following additional treatment for 60 seconds with 5.25% sodium hypochlorite.

83. SODIUM HYPOCHLORITE Scanning electron micrographs showing tubules exposed in longitudinal section. After 60 seconds of 50% phosphoric acid and 60 seconds of 5.25% sodium hypochlorite treatment, the surface appears smooth. Increasing the time of application of sodium hypochlorite brings about a roughening of the surface and the exposure of numerous lateral canals.

84. CHELATING AGENTS The most common chelating solutions are based on ethylenediamine tetra acetic acid (EDTA 17% 10 minutes) which reacts with Ca ++ in dentin and forms soluble calcium chelates . While Fehr and Nygard Ostby (1963) found that EDTA decalcified dentin to a depth of 20-30 microns in 5 minutes . Fraser (1974) stated that chelating effect was almost negligible in the apical third of the root canals.

85. CHELATING AGENTS Different preparations of EDTA have been used as a root canal irrigants . Root canal preparations i.e. a mixture of EDTA and urea peroxide left a residue of this mixture after instrumentation . This may have disadvantages in the hermetic sealing of root canals . The combination of EDTA + cetrimide (a quarternary ammonium bromide ) left no smear layer except in the apical part of the canal.

86. Scanning electron micrograph of dentine following 60 s exposure to 18% ethylene diamine tetra acetic acid solution ( Ultradent Products Inc., South Jordan, UT, USA). 18% EDTA

87. Erosion of the dentinal tubule after placement of EDTA in the root canal for 5 minutes . EFFECT OF EDTA

88. GLYOXIDE Glyoxide - 10% urea peroxide ( carbamide peroxide ) in a vehicle of anhydrous glycerol . In 1961 Steward proposed glyoxide to be an effective adjunct to instrumentation for cleaning of the root canal

89. GLYOXIDE Glyoxide - greater solvent action than 3% H 2 O 2 . It enhances root canal lubrication without softening dentin . Other root canal chelating agent - solvizol which is based on aminogerinaldium diacetate and EDTAC i.e. EDTA + Ceterlon .

90. ORGANIC ACIDS Citric acid removed smear layer better than many acids such as polyacrylic acid, lactic acid and phosphoric acid and phosphoric acid except EDTA . Yamada et.al. in 1983 observed that the 25% citric acid – NaOcl combination was not effective as 17% EDTA- NaOcl combination . Citric acid left precipitated crystals in the root canal - disadvantageous in the root canal obturation.

91. After etching with 6% citric acid and for two minutes. The orifices of the patent dentinal tubules are flared due to removal of peritubular dentin . Scanning electron micrograph. ETCHING WITH 6% CITRIC ACID

92. With 50% lactic acid , the canal walls were generally clean but the openings of the dentinal tubules did not appear completely patent . 25 % tannic acid introduced by Bitter in 1989 was better than NaOcl – H 2 O 2 combination . 20 % polyacrylic acid was less effective than REDTA according to a study conducted by Mc Cough and Smith . ORGANIC ACIDS

93. SODIUM HYPOCHLORITE &EDTA Goldman et.al. in 1982 - most effective working solution - 5.25 % NaOCl and the most effective final flush - 10 ml of 17 % EDTA followed by 10 ml of 5.25% NaOCl . NaOCl removes organic material including the collagenous matrix of dentin and EDTA removes the mineralized dentin , thereby exposing more collagen .

94. ULTRASONICS Used in conjunction with a solution of NaOCl can eliminate the smear layer . The apical region of the canal showed less debris and smear layer than the coronal aspects depending on the acoustic streaming which was more intense in magnitude and velocity at the apical region of the file.

95. LASERS Takeda et al - lasers - vaporize tissues in the main canal, remove the smear layer , and eliminate the residual tissue in the apical portion of the root canals . Effectiveness of lasers depends on - power level, the duration of exposure , the absorption of light in the tissue, the geometry of the root canal, and the tip-to-target distance.

96. Smear Layer of Dentin - Operative Dentistry, Supplement 3,1984. Pathways of the Pulp - Cohen, 6th edition. Ingle’s Endodontics - 6th edition. Gunnar’s Textbook of Endodontics - 2nd edition. Grossman’s Endodontic Practice - 12th edition. REFERENCES 96