INTRODUCTION:
Enamel
is the hardest biological tissue known which forms a protective covering over
the anatomic crown of the tooth. It is a highly mineralised structure which is
tightly packed with a mass of apatite crystals resulting in a highly organised
pattern of crystal orientation. The mineral content of enamel is about 96% and the
rest is organic content and water. The mineral present includes hydroxyapatite
crystals bound by a thin film of firmly bound water. The light microscopic view
of enamel reveals three structures namely, Rod or Prism, inter-prismatic region
and Prism sheath.(1) In case of restoration, the bond strength of
enamel depends on the etchant and the surface structure of enamel being
exposed. Etching can be done in two ways namely, total etch (etch and rinse)
and self-etch. Total etch is a process in which separate acid etching is done
and rinsed prior to application of primer and bonding agent whereas in
self-etch acidic resins contains etchant and primer which is applied prior to
bonding agent.(2,3) There may be various factors which affects the
etching capacity of the etchant thereby altering the adhesiveness of the
restoration to the tooth surface.
The
basic principle of adhesion is the replacement of organic tissue by synthetic
resin. There are two phases: first one includes the removal of calcium
phosphate to form micro-pores and the next is the hybridisation process leading
to polymerisation of the resin.(4) Adhesion plays a major role in
composite restorations by bonding with the tooth surface. Acid etching is an
integral part of composite restoration although various efforts have been taken
to modify this method. They can be crucial in some conditions and may require
special attention. Since acid etching is the most effective method, this review
aims at analysing some important aspects of enamel bonding using total etch
technique.(5)
Total
Etch Technique of Enamel:
The
main aim of etching enamel is to clean the surface pellicles in uncut enamel,
to remove smear layer in cut enamel and to partially dissolve crystalline
minerals to form retentive patterns. Optimal bonds between enamel and resin can
be obtained if the surface is free from saliva contamination.(3) Saliva
and blood contamination may affect bond strength. Hence use of gingival
retraction cords have been proven effective in increasing the bond strength of
total-etch adhesives when compared to self-etch adhesives.(6)
In
this technique, the enamel surface is first etched and rinsed, and then primer
is applied followed by the application of bonding agent. Phosphoric acid with a
concentration of 85% was considered appropriate. Ideal etching time for
this concentration of acid was found to be around 30 seconds initially which
were then increased to 60 seconds.(7) Now days most authors
recommend 15 seconds for etching with 32% to 40% phosphoric acid. (1) However
use of phosphoric acid for etching has limitations like etching only 50% of
enamel surface.(8,9)
Etching time for aprismatic enamel
seen in primary teeth was 120s initially which was reduced to 60s which is now
further reduced to 20-30s.(9)
Factors
Affecting Bonding ofTotal Etch Adhesives to Enamel:
Etching quality:
Etching
quality depends on the etching agent, acid concentration of the etchant, time
for which the surface is etched and composition of enamel surface being etched.
Failure of etching occurs during reduced quality of etching pattern as well as
the quantity of the etched surface. Polishing the enamel surface will help to
eliminate organic components that hinder effective etching of enamel.(8,
10)
Smear layer:
Smear
layer is a significant factor which affects bonding in bur-cut tooth surfaces
by masking the underlying prismatic enamel. It cannot be rinsed off with water
and resins applied to tooth covered by smear layer bonded to these weak smear
layers rather than the underlying hard tissues. Ultra-structural difference
exists between smear layers of enamel and dentin, however very little is known
about enamel smear layer. It has been seen that enamel smear layer consists of
tightly bound fractured apatite crystals which are larger than the original
crystals from which they are derived and they form a layer over the cut enamel
surface.(3,11)
Resin-enamel bonding:
Acid
etching increases resin-enamel bonding strength by creating micro-porosities in
the etched surface which help in resin infiltration and formation of resin
tags. (3)There are two types of tags namely: resin macro-tags formed
which penetrates the inter-prismatic structure and micro-tags which penetrated
the crystal-enamel prism space. Marginal adaptation has to be effective in
total etch than the other adhesives.However, enamel near margins of adhesive
restorations can be severely damaged by cariogenic acids and fluoride releasing
materials can prevent this in adhesive restorations. (7,12,13)Quantitative differences in tensile strength was seen in resin-enamel
bonds.(3)
Grinding
and abrading of the enamel surface increases retention.Deproteinisation with
5.25% NaOCl prior to etching showed better bonding and doubles retention.(6)
Etchant used for enamel contain unfilled Bis-GMA containing hydrophobic
resins forming resin tags into the enamel prism in the range of 5-10µ.
Hybridisation of dental hard tissues showed that the retention of resin
composites to enamel was independent of the intermediate coupling agent.
Adhesives with low viscosity produced both inter-prismatic and intra-prismatic
penetration into enamel after polymerisation whereas higher viscosity produces
only inter-prismatic penetration in enamel.(3,14)
Effects
of Bond Strength on Total Etch of Enamel:
The
strength of the bond formed on enamel by total-etch process accounts up to 40
MPa which is stronger than the other systems.(6)
Cut-enamel:
Etching with moist enamel showed with higher bond
strength. The hybrid layer on etched surface of cut-enamel renders it more
acid-resistant and provides better protection against demineralisation when
compared to un-bonded enamel. Due to the presence of water channels within the
resin-bonded enamel, there is lack of protection against demineralised. It was seen that bonding to cut enamel is adequate
even with a low phosphoric acid content of 3% or with etching time of
5s.However, etching for 15s creates sufficient micro-mechanical retention
without micro-leakage on the bonded enamel surface.(3,15,16)
Uncut-enamel:
There is lack of difference between bond strength of
cut and un-cut enamel in phosphoric acid etching however the ultrastructure of
un-cut enamel remains variable and difficult to interpret. The ability of the
total etch to bond to less retentive aprismatic enamel helps in achieving
better esthetics. The aprismatic surface is resistant to etching because of
high packed density of the crystallites and absence of inter-prismatic organic
substance. The formation of resin tags are reduced in acid etching by
phosphoric acid and retention is predominantly by micro-mechanical retention of
the admixed zone of aprismatic and prismatic enamel.(3)
Primary
tooth enamel:
In one of the experiment it was found that un-cut
primary enamel had increased thickness of hybridised aprismatic enamel which
was devoid of resin tags. Grinding of primary enamel revealed resin tags and
underlying hybrid layer in prismatic enamel in the resin-enamel interface
unlike permanent enamel. Primary enamel showed aggressive etching when compared
to permanent enamel but there was no difference between bond strength and micro-leakage
when etched with total etch and self-etch in primary enamel.(3,17,18)
Intact
and Ground permanent tooth enamel:
Intact enamel has varied surface morphologies when
etched with different acids. Non-rinse conditioner produces less destruction of
the unground enamel even with longerapplication time. Non-uniform loss of prism
was seen with shorter application of phosphoric acid. Etching the grounded or unprepared enamel surface with
phosphoric acid formed porous surface with enamel crystallites and dissolution
of both the inter-prismatic and intra-prismatic areas. Conditioning of these
surfaces is not required due to the aggressive etchant effect of phosphoric
acid. This deep etching pattern leads to formation of similar bond strength on
both the surfaces. (5,19, 20)
CONCLUSION:
Bonding
of the composite to the tooth plays an important role in adhesive
dentistry.Bonding to enamel has shown satisfactory results when compared to
that of dentin due to its complex structure.(20)There are several
factors which affect the bond strength, etching being one of them.Proper
etching of the tooth is essential for good bond strength and may be done
through various techniques.(22,23,24)Total etch is considered the gold standard
of the bond strength of adhesives to enamel surface.(25)Hence this article reviews the etching pattern of
total-etch and its advantages over self-etch for better quality of treatment.
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ISSN
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