The principal effects of conditioning of dentin may be classified as
a) Physical changes
b) Chemical changes
Physical changes
Increases or decreases in the thickness and morphology of smear layer changes in the shape of dentinal tubules.
Chemical changes
a) Modification of the fraction of organic matter
b) Decalcification of the inorganic portion
Conditioning of dentin may be done by several means
1) Chemical
a) Acids
b) Calcium chelators
2) Thermal
a) Lasers
3) Mechanical
a) Abrasion
When dentin is cut for cavity preparation, the wrenched cutting debris of the dentin forms a thin smear layer on the surface. It is also driven into the dentinal tubule apertures displacing the odontoblast process and forming a smear plug at a depth of less than 10-micron meter. Etching dissolves the smear layer and part of the peritubular dentin, leaving tapered cylindrical holes of that depth.
In an experiment on monkey, dentin wall demineralized with a phosphoric acid jelly etchant for 60 sec was completely re-mineralized after 4 months. This results indicates that etching did not result in deleterious effect upon either the collagen fibers or the odontoblast processes, because the presence of collagen fibers maintaining their proper cross bonded structure as a base for apatite crystals to attach to and of the vital odontoblast processes to supply the calcium phosphate from the pulp is essential for remineralization of dentin.
A.J.Gwinnett and M.D.Jendresen (1978) have concluded from their experiments and observations that the surface of acid conditioned eroded dentin is significantly different from that of acid conditioned normal dentin. They further observed the depth of penetration of resin is also less in acid treated eroded dentin where many tubules remain partially occluded by intratubular insoluble deposits.
Ruse and Smith (1991) found when common conditioning agents were used, it has been found by X-ray photo electron microscopy that the outermost surface contains only 10% or less of the calcium and phosphorus initially present. They concluded that the treatment of dentin with acidic conditioners leaves the surface so depleted of calcium and enriched by organic residues that subsequently placed bonding systems should be based upon agents able to interact with organic components of dentin. Bonding agents that rely on chelation to calcium are unlikely to be successful when applied to acid etched dentin unless they penetrate into the demineralized matrix to reach normal, mineralized dentin.
Acid etching of dentin is not harmless but represents one more source of acute irritation to the pulpodentin complex in addition to the vibratory, thermal, mechanical and evaporative stimuli that accompany cavity a preparation. However, it is not as irritating as has been previously thought.
Nakabayashi (1982) introduced the concept of hybridization. The technique consists of applying an acid, ranging in concentration from 10% to 30% to the surface of dentin. Within 15 minutes the acid selectively dissolves away the inorganic component of the dentin to a depth of 5 to 10 microns. It then flows in the dentinal tubule for up to 100 microns at which point it diffuses laterally into the peri-tubular dentin for up to 10 microns.
As in the previous case the calcium component is selectively eliminated. Then these spaces are replaced by an insoluble resin component that completely encapsulates all exposed collagenous fiber.
He also reported that dentin conditioning by citric acid containing ferric chloride followed by a dentin bonding agent containing 4 META (methacryloxyethyl trimellitate anhydride) was effective method of dentinal bonding.
Concerning the bonding mechanism, he proposed that diffusion and impregnation of monomers into the subsurface of pretreated dentinal substrate and their polymerization, creating a hybrid layer of resin reinforced dentin. This newly formed hybrid layer may be thought of as an admixture of polymer and dentinal components, creating a resin dentin composite. This technique not only enhances the shear bond strength of the resin to the dentin but also increase the potential against micro leakage and postoperative sensitivity.
Nakabayashi (1985) suggested that the acidic treatment partially demineralized a zone of the dentin near the surface, facilitating an infiltration process of compatible monomers. The polymerized resin forms a reinforced zone of dentin on which a resin based restorative material can be bonded. The bond strength is not dependent upon interlocking at the dentinal tubules.
Kurosaki et al (1987) found that etching of dentin of the clinical cavity floor allows the chemically adhesive composite resin to produce resin tags of tapered, cylindrical or tubular form as well as impregnated dentinal layers. These changes will considerably improve the bond strength as well as the tubule aperture seal.
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Any discussion of the effects of acid conditioning of dentin must begin with the acid etching of enamel. This was first proposed by Buonocore (1955) as an attempt to clean enamel, increase the microscopic surface area for bonding, and infiltrate unfilled resins into enamel porosities. Many investigators were alarmed at what was then regarded as an unconventional and even reckless ap¬proach to the problem.
Buonocore, Wilernan and Brudevold (1956) not only intro¬duced the acid etching of enamel to dentistry, they also were among the first to attempt to bond resins to acid-etched (7% HCL, one minute) dentin.
Their success with acid etching of enamel led them to try to acid etch dentin. Unlike enamel, when dentin is etched, its surface becomes mineral -poor protein rich; and it tends to become wetter (Brannstrom and Nordenvall 1977). Unfortunately, Buonocore and his colleagues' suc¬cess with dentin was never realized, because the relatively crude resin materials that were available at that time would not wet dentin very well. Buonocore, however, was very much aware or the requirements for good bonding.
For clinical success, the conditioned dentin must be sealed to prevent sensitivity and to prevent the pathology (Brannstrom, 1981) associated with the increased permeability of the dentinal tubules. Conditioning of dentin will be defined as any alteration of dentin done after the creation of dentin cutting debris, termed the smear layer.
The objective of dentin conditioning is to create a surface capable of micro-mechanical and possible chemical bonding to a dentin-bonding agent.
Goals of acid conditioning of dentin
• Remove the intrinsic weakness of the smear layer to permit bonding to underlying dentin.
• Demineralize the superficial dentin matrix to permit resin infiltration into surface.
• Uncover both intertubular and peritubular dentin.
• Clean the dentin surface free of any biofilms.
It is important to define the purpose of the acid etching of dentin so that once identified, these goals can be tested in a systematic scientific manner.
As the smear layer is intrinsically weak, the first goal is to loosen it or remove it so that subsequently placed adhesive resins can interact with solid dentin adhesive resins can interact with solid dentin matrix. Most smear layers are 1-2 µm thick; they are composed of the cutting debris of the materialized tissue on which they lie. (Ruse and Smith 1991)
The reason for acid etching is to demineralize the solid dentin matrix (both intertubular and peritubular dentin) to increase the porosity of the dentin. While this is analogous to why enamel is etched, the porosities that are produced are of the order of 0.05 – 1-3 µm in peritubular dentin rather than the 5-7 µm diameter of enamel prisms. Further, acid-etched enamel can be thoroughly dried, while that foal is much more difficult in vital, normal dentin. Enamel contains little protein that is at risk of being denatured by acid treatment. Dissolving away hydroxyapatite mineral crystallites from the collagen component of dentin matrix creates dentin porosities. The crystals tend to stabilize collagen and prevent its denaturation.
There is a risk that the acid used to demineralize the dentin may denature or weaken the collagen. As denatured proteins generally change their dimensions, the pores may become smaller if the collagen is denatured. This may interfere with subsequent resin infiltration and prevent the formation of a hybrid layer (Nakabayashi, Nakamura and Yasuda 1991). Another danger in the etching step is that the demineralized zone may extend, for instance, 5 µm into the dentin, while the resin infiltration may only extend 4 µm, leaving a 1 µm demineralized zone at the base of the hybrid layer that is unpro-tected by mineral or resin and that may be structurally weak. If the pulpodentin complex can re-mineralize this unprotected basal 1 µm of demineralized dentin (Tatsumi, 1989; Tatsumi and others 1992), then the layer may become as strong as normal dentin, rather than be a zone of debonding that has been seen in vitro (Nakabayashi and others 1991).
Another purpose of acid etching dentin is to clean the dentin surface. Often dentin is inadvertently contaminated with blood during the cavity preparation. Acid etchant, by dissolving most of the smear layer, tend to float these biofilms on the dentin when it is rinsed. The low pH of the etchant may also denature the plasma proteins and hemoglobin. The purpose of acid etching may vary depending upon the material. If the intention is simply to remove the smear layer but leave the smear plugs in place, as when one uses glass-ionomer cements, then short etching times with dilute acids would seem to be indicated (Bowen 1978; Pashley and Others 1981; Hamlin and Others 1990a).
However, if one wants to create a resin hybrid layer (Nakabayashi and Others 1991) in the dentin rather than on the dentin, then one must demineralized more deeply and in the process, removes smear plugs. This can still be accomplished using dilute acids, but the etching time may have to be extended.
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Currently phosphoric acid is the acid of choice, but it is possible that other acidic etching agents such as pyruvic acid may be used in the future. A controversial issue, however, is the optimal concentration of phosphoric acid. The most widely used concentrations of phosphoric acid used in clinical practice exceed 30% phosphoric acid. This is partly based on the findings the phase diagram of the phosphoric acid +/- calcium hydroxide +/- water ternary system. They demonstrated that the application of phosphoric acid solutions greater than 27% phosphoric acid resulted in the formation of monocalcium phosphate monohydrate.
While dicalcium phosphate dihydrate was formed with phosphoric acid concentrations less than 27% phosphoric acid. The former product is readily soluble and would be completely washed away in the clinical situation, while the latter product is less soluble. The reaction products, if not completely removed after the etching procedure, may interfere with the bonding of composite resins to etched enamel surfaces.
The effect of phosphoric acid concentration on the tensile bond strength of a conventional composite resin to enamel surfaces etched with 10, 20, 30, 40, 50, 60, and 70% phosphoric acid was determined. The tensile bond strength to enamel surfaces etched with 70% phosphoric acid was significantly lower than the bond strengths recorded to enamel surfaces etched with other phosphoric acid concentrations.
The application of a phosphoric acid etching solution to freshly cut dentin may elicit a pulpal response. To prevent the flow of phosphoric acid applied to the enamel walls of preparations to the freshly exposed dentin at the floors of the preparations phosphoric acid gels were recently introduced. The objective was to confine the acid-etching agent to the intended site of application. It is recommended that the etching agent should be applied to the enamel surface using a dabbing action as opposed to rubbing. Another issue that has not been resolved is the optimal duration of etching with phosphoric acid.
It is surprising that some authors recommend that the etchant should remain on the tooth surface for at least 60seconds to develop an appropriate etched pattern. The etch duration is of particular importance in acid etching enamel prior to the direct bonding of orthodontic attachments, as it is practically impossible to confine the bonding site. Fluoride is not evenly distributed in enamel but allows a negative exponential distribution with fluoride concentration being in the surface enamel.
The loss of fluoride rich enamel surface during prolonged etching may make the adjacent enamel more susceptible to enamel decalcification during orthodontic treatment. The reaction products that are formed on the enamel surface after phosphoric acid etching should be removed completely, as incomplete removal may interfere with bond strength. Etched surface should be washed for at least 15 secs to remove the reaction products.
The tooth to be restored should be isolated with a rubber dam to prevent saliva contamination prior to acid etching and the placement of composite resin. It is generally recommended that saliva contaminated etched enamel should be washed and retched. O’Brien and others showed, however that it was not necessary to re-etch an enamel surface contaminated briefly with saliva, as a thorough washing of such a surface did not have a detrimental effect on bond strength.
Buonocore M (1955) introduced a simple conservative technique for bonding restorative resins to enamel. He placed a drop of self-curing acrylic resin on the labial enamel surface of upper central incisor of ten subjects. One surface was treated prior to resin placement with 85% phosphoric acid for 30 seconds. He noted that the acid conditioning of the enamel resulted in on uncontioned control surfaces lasted less than 12 hours. After three decades of laboratory and clinical research, Buonocore’s method is widely adopted and has added a new and exciting technical dimension to the practice of dentistry.
Gwinnett, Matsui and Buonocore (1969) suggested that formation of resin tags was the primary attachment mechanism of resin to phosphoric acid. Acid etching removes about 10 microns of the enamel surface and creates a porous layer ranging from 5-50 microns deep.
When a low viscosity resin is applied, it flows into the micro porosities and channels to this layer and polymerizes to form a micro mechanical bond with the enamel.
Fusayama et al (1979) introduced an etching technique for both the enamel and the dentin cavity wall using 37% phosphoric acid followed by a dentin-bonding agent containing methacryloxyethyl hydrogen phenyl phosphate (phenyl-P). This improved bond strength greater extent and dentinal etching has become fairly common practice in Japan. However, the concept of total etching only recently has gained acceptance in the United States.
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TIME
Increased Time Application
High fluoride content and primary teeth require longer etching time. The increased etching time is needed to enhance the etching pattern on enamel that is more a prismatic than that of permanent enamel. Currently 15 sec, a sufficient time to produce a bond equivalent to that produce by a 60 sec etching time is used routinely.
Shorter Etching Time
C.J. Guba et al (1994) highlighted that etching times and etchant consistency were not critical to enamel bond strengths. It yields acceptable bond strength. It conserves enamel and saves time.
They also found that on microscopic examination of a 10 secs etch versus a 60 sec etch showed that etching the enamel for 10 sec produced a very superficial etch compared to a very deep etch with a 60 sec etch. However this did not have significant impact on the tensile strength. Though some researchers suggested that the etching effect is reduced when the etching viscosity of the acid is high, this study showed no significant difference as related to their viscosities.
ACID CONCENTRATION
An interesting and important phenomenon is the existence of an inverse relationship between the etching effect of phosphoric acid and it’s concentration. The phenomenon was first observed and reported in 1965 and subsequently confirmed by others. The same etch time lower concentrations of acid tend to be more destructive of the enamel than higher concentrations
Concentrations of phosphoric acid over 65% tend to show minimal changes. The concentrations of acid, producing consistent, more or less evenly distributed relatively deep etch pattern, appear to be in the range of 30 to 50%. Bond strengths are greater with 30 to 50% acid concentration, the difference between their values and those obtained on surfaces etched with 10 to 70% acid were not as great. The higher concentration of acid may not produce a sufficient in depth etch to provide adequate resin penetration (tag formation) and / or sufficient bonding area to resist repeated long-term masticatory and other dislodging stresses encountered in the oral environment.
In an in vitro study carried out on bovine substrate by M.J.Shingi et al (2000) it was concluded that milder concentrations of phosphoric acid or less aggressive acids could be used to pretreated enamel for orthodontics adhesive systems and sealants if the diffusion potential of applied monomers is high enough.
According to Unos (1996) depths of demineralization increased by both acid concentration and conditioning times following a logarithmic relationship.
Chow and Brown (1973) demonstrated that the application of phosphoric acid solutions greater than 27% Phosphoric acid resulted in the formation of monocalcium phosphate monohydrate while dicalcium phosphate dehydrate was formed with phosphoric acid concentrations less than 27%. The former product is readily soluble and would be completely washed away in the clinical situations.
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During the last decades clinicians have been confronted with a continuous and fairly rapid turnover in adhesive materials. It started in the mid- ‘60s with the advent of first commercialized restorative resin composites, followed in the early ‘70s with the introduction of the acid etch technique in clinical practice. [Operative dentistry supplement 2001: 6 ; 119-144]
The bonding of restorative materials to teeth typically involves the use of acids to demineralize their surfaces. Changes in the surface due to acid treatment include the gross removal of smear layer, an increase in both permeability, micro porosity and chemical modifications of the surface composition. [Dental material 2002; 18; 26-35]
Bonding to enamel is now considered a durable and predictable clinical procedure. The acid etch technique relies on the micro mechanical retention obtained on the enamel surface by an acidic etchant and subsequent penetration of a blend of polymerizable monomers into the interprismatic spaces to form enamel resin tags (Gwinnett and matsui 1967)
In contrast, dentin bonding has become one of the most challenging topics of restorative dentistry. Recent developments in dentin bonding technology involve the simultaneous etching of enamel and dentin with acids. Dentin etching has become most common in Japan since the late ‘70s [Operative dentistry; 2000;25; 144-186].
A significant advance in dentin bonding was made by kanca who reasoned that if appropriate, chemically compatible, resin formulation was added following a total etch of the mineralized dental tissues [Esthetics].The evolution of adhesive systems has resulted in bond strengths to dentin that are very close to that of the celebrated union of enamel.
Possibly, in the not too distant future, dental adhesion will involve only one application of a self-etching system capable of satisfactorily bonding both to enamel and to dentin [Quintessence Int 2002; 33; 213 – 214]
HISTORY:
Adhesive dentistry originated from a simple clinical experiment and the vision and dedication of a unique dental scientist. In 1955, Buonocore reported the self-curing, methyl-methacrylate resin could be durably bonded to the enamel of human incisor teeth. Phosphoric acid was the agent of choice and was applied to the enamel surface for 30 seconds at a concentration of 85%.
The first clinical trails of the method were concerned with the sealing of pits and fissures of molar teeth to prevent their decay. (Dental adhesives – John Gwinneth)
FUSAYAMA et al [1979] introduced the concept of ‘Total etching’ advocating the treatment of both enamel and dentin with phosphoric acid prior to bonding. This technique has become relatively popular in Japan, but initially met with resistance in the USA. (Esthetics dentistry and ceramic restoration, Bernaud Touati]
Between 1982 and 1985, Nakabayashi described micro mechanical bonding mechanism of dentin bonding agents. He introduced the concepts of hybridization (Operating Dentistry 2003 28; (3); 287- 295). A significant step in improving the bond to dentin through smear layer removal came from the work of Bowen. He showed an improvement in bond strength of dentin with a combination of ferric oxalate conditioning and resin priming of the dentin surface.
The primers consisted of a comonomer of N- Tolyl glycineglycidil metharylate (NTG-GMA) and a coupling agent derived from an addut of pyromellitic acid dianhydriate and 2 hydroxyethyl methacrylate (PMDM). The oxalate was changed to the Aluminium salt to avoid staining invivo associated with the ferricion. The bonding procedure was clinically demanding and has since been simplified.
Further more, it has been documented that the presence of nitric acid in the oxalate conditioner was responsible for the improvements in bond strengths. Other products entered the market place were designed to remove dentin smear layer. The conditioning agents ranged from the chelation of EDTA (Gluma, Bayer) to the incorporation of maleic acid in a resin primer. (Scotchbond 2, 3m (Dental adhesives)
Perdiago and others (1998), found the application of Aqua – Prep to dried dentin surfaces to restore the bond strengths of acetone-based water free dentin adhesives to the same level as bonding to moist substrates. They have shown that rewetting the etched dentin with an aqueous HEMA solution re-established the level of bond strength obtained to moist dentin and severed as an effective means to reopen the interfibrillar spaces for penetration of resin.
Pilo and others (2001) concluded that pretreatment of etched dentin with either a disinfectant (2% chlorhexidise gluconate, tubulicid [2% EDTA and 1% benzylkonium chloride] or rewetting (Aquaprep 35% Hema) may have a positive effect on the shear bond strength of resin to dentin. Duke and Rhodes (2001) evaluated the effect of desensitizers (vivadent and Gluma) used as rewetting agents on dentin shear bond strength.
Vivadent –> 5% Glutaraldehyde and 35% polyethylene glycol dimethacrylate
Gluma –> Solution of glutaraldehyde and HEMA [Operative dentistry; 2003;28-3; 287-296].
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ACID ETCHING ON ENAMEL:
DEFINITION
The enamel in preparation for a micro mechanical attachment with resin. ( Graham J.Mount, W.R.Hume)
The enamel surface covered by an organic pellicle, which makes bonding difficult because of its low reactivity. Etching enamel with phosphoric acid raises the critical surface tension, increases the bonding area and roughness allowing the hydrophobic resins to penetrate the porosities of the dry etched enamel. (Aschhiem dale; Esthetic Dentistry; 2nd edition)
OBJECTIVES
1. To remove the contaminants.
2. To raise the energy and reactivity of the enamel surfaces.
STEPS IN ACID ETCH TECHNIQUE
ENAMEL PROPHYLAXIS:
Thorough dental prophylaxis for removing material A!ba and plaque is an important component of the conditioning regime. It has been observed that prophylaxis alone can double the bond strength,
The prophylaxis pastes, devoid of oils, flavouring agents and fluorides are recommended for this purpose. After cleaning, the enamel is thoroughly washed with water, the treatment dried and carefully isolated from oral fluids.
APPLICATION OF ETCHANT
Ever since Buonocore used phosphoric acid in the form of liquid or gel alternative acids and varying etching times. The acid is applied by one of several means including a cotton pellet brush or mini sponge.
Beginning with Buonocore's use , 85% phosphoric acid have been used to etch enamel, (Sturdevant). Application of 50% phosphoric acid for 60 secs result in the formation of a mono calcium phosphate dihydrate precipitate that can be rinsed off. However, 20% phosphoric acid may create a dicalcium phosphate dihydrate precipitate that cannot be easily removed and may interfere with adhesion. Sliverstone found that the applications of 30% to 40% phosphoric acid resulted in very retentive enamel surfaces.
Recent studies has shown that shorter etching time produce the same adhesive strength than the originally suggested 60 seconds ( Journal of operative dentistry 1986; 11:111). Morphologic studies has shown significant differences in etching results based on the viscosity of etchant.
Liquid or thin gel produced similar etch pattern than thick gel but thin gel seemed to have the best defined etched pattern ( Journal of prosthetic dentistry 1989;65:522).Etching for too long produces insoluble reaction products and a weak bond. An Etching time of 60 seconds was originally recommended for permanent enamel using 30% to 40% phosphoric acid.
WASHING : (Vimal sikiri)
Following acid application for a stipulated period, the area is thoroughly washed for 10-15 seconds to remove the reaction products of acid and mineral hydroxy apattite. Studies have concluded a 2 to 5 secs rinse of the tooth surface should sufficiently cleanse gel- etched enamel resulting in adequate shear bond strength.( Journal of prosthetic dentistry 1989 ; 62 : 522). Rinsing for 1 second from a smooth enamel surface resulted no micro leakage
DRYING :
The teeth are thoroughly dried with a oil free compressed air. An effectively etched surface on drying gives a matt white or frosted appearance. Even a minor exposure to saliva, blood or oil can restrict the potential for resin tag formation and bonding. All measures should therefore be taken to prevent any contamination. If accidental contamination occurs, the procedure should be repeated.
EFFECTS OF ETCHING ON ENAMEL:
Removes residual pellicle exposure to the inorganic crystallite component of enamel.
Creates a porous layer with the depth of the pores ranging from 5-10 µm
Increases as the wettublity and surface area of the enamel substrate.
Raises the surface energy of enamel with creation of reactive polar sites.
PATTERN OF ETCHING:
Silverstone et al (1975) studied the morphological changes in SEM produced on the acid etched enamel surface. Exposure of human enamel to conditioning solutions produces 3 basic etching patterns.
Type I: (Preferential prism center etching)
Etching pattern involves the preferential removal of enamel prism cores with prisms, peripheries remaining relatively intact, resulting in a honey comb appearance. The average diameter of the hollowed prism core is measured as about 3µm. This is the most commonest type of etching pattern.
Type II: (Preferential prism pheriphery etching)
The peripheral regions of the prism are dissolved preferentially, leaving the prism cores relatively intact, resulting in a cobblestone appearance.
Type III: (Mixed)
Etching pattern is less distinct and includes areas resembling type I and type II patterns as well as regions in which the etching pattern appears unrelated to prism morphology. This type of etching in general is associated with the presence of prism less enamel and appears as a generalized surface roughening.
Alternative acids have been tried as enamel etchants; mainly pyruvic acid and sulphuric acid. 2% sulphuric acid used for 30 seconds has shown to be as effective as phosphoric acid, where as higher sulphuric acid concentrations produce heavy crystal deposits which interfere with the bonding and cannot be washed away easily.
With the present concept of total etch techniques, acids such as 10% maleic acid, 10% citric acid, 10% phosphoric acid, 2.5% oxalic acids and 2.5% Nitric acid are used to etch enamel and dentin simultaneously (Quintessence !nt 2002; 33; 213-224).
FACTORS AFFECTING ETCHING ON ENAMEL:
Time:
Increase time application
High fluoride content and primary teeth require longer etching time. Etching for too long produces insoluble reaction products and a week bond (Aschhein dale; Esthetics dentistry).
Shorter etching time:
C.J. Guba et al (1994) highlighted that etching times and etchant consistency were not critical to enamel bond strengths, it yields acceptable bone strength, conserves enamel and saves time.
Acid concentration:
An interesting and important phenomenon is the existence of an inverse relationship between the etching effect of phosphoric acid and it's concentration. The concentrations of acid, producing consistent, more evenly distributed relatively deep etch pattern, appear to be in the range of 30 to 50%.
MICROSCOPIC APPEARANCE OF ETCHED ENAMEL
Clinically a uniform dull appearance is an indication that the tooth surface has been adequately etched.
Silverstone in 1974 showed that etched enamel surface under polarized light resulted in 3 zones.
1. Etched Zone
2. Qualitative Zone
3. Quantitative Zone
1. Etched Zone
This is the narrow zone of enamel at about 10µm in depth, that is removed by etching. The fully reacted mineral crystals are removed resulting in the exposure of more reactive surface. This increased surface area and a reduced surface tension allows resin to wet in the etched surface more readily.
2. Qualitative Zone:
This zone is about 20µm in depth and it is rendered porous during acid etching of the enamel when identified qualitatively using polarized light.
3. Quantitative Zone:
This third zone is almost up to 20µm depth. It is qualitatively indistinguishable from adjacent enamel and can be detected with quantitative polarized light. In human enamel, the pores may be spherical, elongated or sometimes as large chamber that are connected to smaller channels called ink bottle systems.
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