1
SMEAR LAYER CONTENTS •
Introduction
•
History
•
Definition
•
Composition
•
Smearing phenomenon
•
Morphology of smear layer
•
Smear layer in restorative dentistry
•
Smear layer in endodontics
•
Removal V/S retention
•
Smear layer removal
•
Smear layer hybridization hybridization
•
Conclusion
2 INTRODUCTION Unknown and unrecognized for years, the smear layer has become a force to be reckoned with during the last decade. The full significance of the smear layer has been slow to be perceived. Its increasing importance has paralleled the interest in adhesive bonding to tooth structure. Its effect as a so called “cavity liner” is just beginning to be appreciated. As suggested by David Pashley, the Smear layer as a cavity liner may unquestionable have both beneficial and detrimental effect. Thus there is a need to alter the traditional procedures of restorative T/T to take advantages of its beneficial effect and to avoid its negative effects.
Definition: According to Schwartz: It is defined as 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: It
is
defined
as
an
amorphous
relatively
smooth
layer
of
microcrystalline debris whose featureless surface cannot be seen with the naked eyes. According to American Association of Endodontics (AAE) It is defined as a surface film of debris retained on dentin or other tooth surface like enamel, cementum after instrumentation with either rotary instruments or endodontic files. According to DCNA (1990): When a tooth structure is cut instead of being uniformly sheared the mineralized matrix shatters. Most of which is scattered even the enamel and dentins surfaces to form a layer; termed as SMEAR LAYER.
3 HISTORY
-
It is difficult to say by whom; the concept of the smear layer was first introduced.
-
Lemmie and Draycot in 1952 were the first to describe a “Grinding Debris” on teeth structure instrumented by burs and abrasives; using a light microscope.
-
Boyde, Switzer and Stewart in 1963 were the first to use SEM and coin the term “SMEAR LAYER”.
-
In 1972, Jones, Lozdan and Boyde showed that smear layers were common on enamel and dentin following instrumentation.
-
In 1970 Erich et al attempted to quantify and identify among the tooth surface instrumented with burs, the smear layer.
(i) Surfaces instrumented with diamond burs were rougher then with carbides. (ii) Dry cutting produced rougher surfaces in comparison to wet cutting. (iii) Smear layer was generally less than 0.5 µm in organic layer on which mineralized debris of upto 0.5-15 µm was coated.
-
Goldman in 1982 suggested that if a smear layer was allowed to remain within a pulp canal system; it might provide a reservoir of potential irritants.
COMPOSITION Consists of both organic and inorganic components. Inorganic component consists of non-specific inorganic components and hydroxyapatite crystals. The organic component consists of heated coagulated proteins, necrotic/viable dentin and/or pulp tissue, odontoblastic processes, collagen, saliva, microorganisms etc.
SMEARING PHENOMENON Ehlrich in 1976 stated that smearing occurs when “hydroxyapatite within the tissue is swept along and reset in the matrix”.
4 This smearing tends to lower the surface energy of the substrate, which makes interaction with bonding materials difficult if not impossible. Temperature upto 600 °c is reached when dentin is cut without a coolant. This temperature is lesser than the melting point of hydroxyapatite (1120 °C). Thus the postulation that melting and solidification of HA is the reason behind smear
layer
formation
stands
unacceptable.
Rather
the
smearing
phenomenon is suggestive of a physico chemical phenomenon rather than a thermal transformation. A typical smear superficial layer may range in thickness from 1 year to 5µm. While the smear plug may enter the dentinal tubule from a few years to upon 40µm. However the thickness of smear layer is influenced by 1.
Cutting instrument
2.
Force applied
3.
Dry/ wet cutting
4.
Chemical or irrigation if any.
DIAMOND POINTS V/S STEEL & TUNGSTEN CARBIDE BURS: •
Relative deep and
•
Lesser evidence of grooves
uniform grooves •
Rougher surface
•
Less rough
•
Grooves run parallel
•
Runs perpendicular
To direction of motion of cut •
Debris pushed deeper
•
Less
deep
(ejected
away from Into plugs (ahead of
cutting surface
Abrading surface) •
N/A
• Thicker smear layer and
•
Proportional to rake angle
•
less thick SL and shallower
Smear plugs.
WET VERSUS DRY CUTTING •
Reduced thickness
•
Thicker smear layer
5 •
More organized
•
Slightly
loose
superficial
Layers •
Not uniform but rather patchy
•
Structurally and constitutionally same for both.
•
Uniform thickness and distribution:
THE FORCE FACTOR Greater the force application the thicker and deeper the smear layer and smear plugs so formed.
CHEMICAL TREATMENT Varies from material being used as a chemical irrigant.
IMPACT OF SMEAR LAYER IN RESTORATIVE DENTISTRY: Clinically
untreated
smear
layer
has
been
showed
to
reduce
postoperative sensitivity by upto 86%. Smear layer and smear plugs physically form a superficial protective layer, which covers the cut dentin surface and seals the dentinal tubule orifices. Thereby reducing a)
Fluid hydrodynamic flow in the tubules.
b)
The penetration of irritation chemicals inside dentin.
c)
Preventing
further
entry
of
bacteria
into
tubules
and
resultant colonization and pulpal irritation. d)
Serves as an iatrogenically insulating layer. However the smear layer itself may become a cause of future insult
and damage to the pulp. It acts as a barrier to both the entry of harmful noxious agents into dentin and also as a barrier against proper bonding between the tooth structure and the restorative material. ‘Nanoleakage’ a term which refers to percolation and movement of microorganisms between the smear layer and restorative agent is always a possibility. As evident in SEM studies the smear layer is neither continuous in distribution nor in its structure. Patchy distribution of SL provides possible
6 entry points for penetration of bacteria and their toxins to cause pulpal harassment. The presence of nanochannels inside the smear layer also functions in the same manner. Because of its interference in the bonding mechanism and bond strengths so obtained; it may cause early failures in bonds between restorations and tooth structure. The weak nature of bonding of smear layer to underlying dentin and enamel also serves as an area of potential bound failure. Thus in summary bond failures can occur at 1) Smear layer-tooth surface 2) Within smear layer itself 3) Between smear layer and restorative material 4) Nanoleakage without bond failure. Any such alterations. from normal would lead to microleakage and result in failure.
IMPACT OF SMEAR LAYER IN ENDODONTICS In endodontics the perceived benefit of postoperative sensitivity is absent. Moreover the presence of bacteria within dentinal fins and tubules greatly increase the impact of the smear layer on successful endodontic outcome. If these bacteria and there by products should reach the periapical area some how; endodontic failure would result. Microleakage along the tooth wall smear layer interface and Nanoleakage within the smear layer and finally between the smear layer- sealer interface; all serve as areas for reinfection. The presence of smear layer moreover interferes with the penetration of sealer/ gutta percha into lateral and accessory canals and a close adaptation with the dentin walls. Lastly the presence of any viable bacteria within the smear layer may themselves led to endodontic failure.
THE PREDICAMENT? The benefits and drawbacks of smear layer became clear amongst the researchers and clinicians in mid 70’s.
7 Those people who were in favour of retaining the layer set their argument on its effect on postoperative sensitivity and its inability to affect the outcome of endodontic success and failure. While those who vociferously favoured
the
removal of this layer
argued
about the
ill-effects of
nanoleakage, poor bond strengths, and a reservoir of microorganisms. As more and more research and study date pecolated in; slowly and steadily the people who favored the retention of smear layer fell in minority and found themselves out numbered.
SMEAR LAYER REMOVAL Smear layer can be removed completely or partially by using a wide variety of elements and after liquid agents. Historically Water Hydrogen peroxide, Benzylkonium chloride, EDTA, Phosphoric acid, Critic acid, Maleic acid Sodium hypochlorite Urea peroxide Lactic acid Utrasonics etc. ....have been used. Brannstrom and colleagues published several articles describing the use of water, H 2O2, EDTA, Benzylkonium chloride, Citric Acid etc for removing the smear layer. He also formulated several commercially available products like Tubulicid Blue and Tubulicid Green for the removal of smear layer.
Inorganic Acids:
-
Phosphoric acid (H 3PO4)
8 Which the discovery of total-etch concept by Buonocore in 1955 complete removal of smear layer was noted using 85% H 3PO4 for 2 minutes. Over a period of time 37% H 3PO4 applied for 15 seconds was accepted a clinically efficacious for removal of smear layer and acid conditioning of tooth surface for receiving a resin bonding agent. However the smear layer removal was different for different etching time and also the depth of dentin demineralization. 5sec
1.1
±
0.14µm
15sec
1.9
±
0.1µm
30sec
2.7
±
0.2 µm
60sec
4.2
±
0.2 µm
120sec
8.1
±
0.6 µm
Organic acids:
-
25% Citric Acid removed smear layer better than the other acids such
as PAA, Lactic Acid and H 3PO4. When applied for 20 sec it produced acceptable results. Owever deposition of precipitated mineral crystals make it disadvantageous.
- 50% lactic acid produced cleaner walls however it was not effective in dissolving the smear plugs.
- 20% tannic acid and 20% polyacrylic acid were less effective than EDTA and other irrigating solutions in removing smear layer. But nevertheless were tried.
SODIUM HYPOCHLOKITE NaOCl alone in concerntrations varying between 0.5% to 5.25% is not effective in complete removal of smear layer. However, its combined usage with EDTA solutions completely removed the smear layer and plugs from the canal system.
HYDROGEN PEROXIDE (2% H2O2) Due to its effervescence it removes loose debris very quickly but it is not efective in removing smear layer and is only partially effective when used in conjunction with NaOCl.
9
CHELATING AGENTS EDTA solution (17% EDTA) EDTAC (EDTA +Cetrimide) REDTA (Original Goldmanns formula) Rc-prep (EDTA + urea peroxide + carbowax) either when used alone or in conjuction with 3% or 5.25% NaOCl with or without ultrasonics are the best known method for removal of the smear in endodonis. Ingle recommends soaking a tooth’s root a final flush of 5.25% NaOCl to achieve complete removal of the smear layer and disinjection of the canals; prior to observation. However use of RC-prep is associated with greater microleakage (upto 3.6 times) due to deposition of carbowax residue; and hence should not be used for removing the smear layer. Salvizol another chelating also proove to be equally effective in removing smear layer when used in combination with 5.25% NaOCl.
ULTRASONICS: Ulrasonic agitation using brands like Rispi Sonic, Cavi Endo, Enac, Piezon, Triosonic, Shaper sonic etc in conjuction with an irrigant helps obtain smear free canals faster and more reliaby. The action being facilitated by acoustic streaming and cavitation.
GLYOXIDE: (10% urea peroxide + glycerol) An irrigating solution was first first proposed by Stevart in 1961. It produces better action in comparison to
H 2O2. But is inferior to EDTA +
5.25% NaOCl. As time elapses and restorative dentistery and particularly adhesive dentistery grows by leaps and bounds. The controversy of smear layer has risen from the “Ashes like a Phoenix”. Buonocores total etch concept removed the smear layer completely; thereby creating a space for the bonding agent to fill up the plugs to form resin tags and make a hybrid layer with the exposed collagen networks.
10 This micro machanical interlocking of resin with collagen is termed as hybridisation and the layer so formed “HYBRID LAYER”. The hybrid layer was first discribed by Nakabayashi in 1992. However in 1994 Kurraray in Japan introduced a Self Etching Prinmer named
“Clearfil”
wich
itself
incorporated
the
acidic
primer
thereby
elemenating the need for an etching and rinsing step in the total etch concept. Later on several companies intensified research into this new and exciting concept of dentin bonding and came up with a wide variety of self etching primers namely Prompt L Pop
(ESPE)
Syntax 3
(Vivadent)
Scotechbond
(3M)
Singlebond
(3M)
Clearfil liner BondSE
(Kurraray)
Clearfil SE
(Kurraray)
Xenoll
(Dentsply)
Adhese
(Ivoclar)
The total eteh concept consists of An Etchant An dentin primer
A three step procedure
An unfilled resin A two step total etch concept, combines the primer and the adhesive resin Acid etchant
- 1st step
Primer + Adhesive
- 2nd step
The self-etching primers are available in either as a. Two step procedure
or
b. Single step procedure In the two step self-etching concept the etchant and primer are combined and an unfilled resin is applied separately. (NRC and Prime and Bond NT) (Dentsply) In the one-step self-etching concept the etchant, primer and the unfilled resin all are combined into one container (PROMPT- L POP) ESPE.
11 These
self-etching
primers
contain
varying
percentage
of
methacrylated phosphoric esters. Action of H3PO4 seems to diminish as demineralization process progresses and ceases after about 10 seconds. These SEP depending on their depth of penetration into subsurface (below the smear layer) dentin may be classified as Mild
- 0.4-0.5 µm (MegaBond)
Moderate
- 1.2-2.2 µm (NRC + Prime ‘n’ Bond NT)
Aggressive
- 2.5-5.0 µm (Prompt L pop)
The incorporation of the Smear Layer into the bonding process and hybrid layer formation directly translates into reduced postoperative dentin sensitivity. Thus the SEP’s incorporate the smear layer into the hybrid layer; and at the same time maintain an acceptable dentin bond strength of 10-15 MPa comparable with that of the total etch concept. However the bond strengths to enamel are still inferior (~ 8-10 MPa) when compared to the total etch concept. (~17-20 MPa); but the future looks promising.
CONCLUSION As far as endodontics is concerned smear layer removal is mandatory and is best achieved with a combination of 17% EDTA and 5.28% NaOCl wash. Smear layer
hybridization
rather than removal
is fast gaining
acceptance in restorative dentistry especially with the introduction of SEP’s; though the smear bond strengths to enamel are yet to be comparable with those of total etch concept.
12 On the conclusion it is still not finalized whether the smear layer should be removed/ retained or modified to best serve a given preference and thus the controversy continues…
13 REFERENCES •
Operative Dentistry
- Sturdevant
•
Operative Dentistry
- Vikram Sikri
•
Endodontics
- Ingle
•
Operative Dentistry
- Schwartz
•
Management Alternatives
- Operative Dent. Suppl 6, 2001
for carious lesions
•
Dental Materials 17(2001)
- Pg 296 August
•
Dental Materials 17(2001)
- Pg 298 August
