Etching of teeth enamel in dentistry - in Detail
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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