Tooth enamel along with dentin, cementum, and dental pulp is one of the four major tissues which make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in human body. (1)
Ninety-six percent of enamel consists of mineral, with water and organic material composing the rest. In humans, enamel varies in thickness over the surface of the tooth, often thickest at the cusp, up to 2.5 mm, and thinnest at its border with the cementum at the cementoenamel junction (CEJ). (1) Enamel's primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. The large amount of minerals in enamel accounts not only for its strength but also for its brittleness. Enamel does not contain collagen, as found in other hard tissues such as dentin and bone, but it does contain two unique classes of proteins - amelogenins and enamelins. These proteins aid in the development of enamel by serving as a framework for minerals to form on, among other functions. (1)
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Understanding enamel orientation is very important in restorative dentistry, because enamel
unsupported by underlying dentin is prone to fracture. (2) Invented in 1955, acid-etching employs dental etchants and is used frequently when bonding dental restoration to teeth. (3,4) By dissolving minerals in enamel, etchants remove the outer 10 micrometers on the enamel surface and make a porous layer 5-50 micrometers deep. (3) This roughens the enamel microscopically and results in a greater surface area on which to bond.
The effects of acid-etching on enamel can vary. Important variables are the amount of time the etchant is applied, the type of etchant used, and the current condition of the enamel. (3)
The morphological changes produced in enamel surface using sweep electron microscope (SEM) was first reported by Gwinnet (1971) and Silverstone (1975), who identified the enamel micromorphology and classified enamel etching into 3 patterns. (5,6) Type 1 is a pattern where predominantly the enamel rods are dissolved; type 2 is a pattern where predominantly the area around the enamel rods are dissolved; and type 3 is a pattern where there is no evidence left of any enamel rods. Clinically, however one can only see a white, opaque surface, exhibiting the quantity but not the quality of the affected surfaces. (5,6) Silverstone, later showed that the most retentive etching patterns were types 1 and 2, because the porous surface offered retentive areas of greater size and depth. The type 3 pattern lacked the microretention offered by the other two.
Improving Tooth bonding
Different studies have been conducted in an effort to improve the retentive properties of the enamel for the best possible adhesions. Adhesion to enamel depends on the etching agent, acid concentration, etching time and composition of the enamel surface. (7) Mechanical elements such as air abrasion and laser have also been analyzed with no good results. (8,9) In one such effort, polishing the enamel surface with the intention to eliminate the organic components that hinder effective enamel etching was done by Roberto espinosa et al. (10)They found that the enamel deproteinization with sodium hypochlorite prior to phosphoric acid etching doubled the enamels retentive surface when compared with conventional phosphoric acid etching.
Sinohara MS (2006) carried out a study to analyze the etching pattern of SES self etching bonding system in comparison with 35% phosphoric acid etchant. (11) Self etching systems are aqueous mixtures of polymerizable acidic monomers and methacyrlate components while acidic monomers have been developed containing esters from phosphoric acid, carboxylic acid derratives. (12,13,14,15) It was found the self etching system was less aggressive than phosphoric acid etchants and the etching produced by them was less effective than that produced by phosphoric acid.
Fava M et al. (2003) evaluated the morphological aspects of the enamel of teeth after etching with 36% phosphoric acid or a non-rinse conditioner. Electron micrographic analysis showed that both etching agents were effective for etching the enamel of teeth causing formation of micro porosities on the enamel surface, though the etching pattern was more effective with the use of 36% phosphoric acid. (16) The common aim of all these investigations has been to improve the retention properties of the enamel for the best possible adhesion.
It has been established that firm adhesion is achieved by good acid etching, with a generalized retentive morphological area over the entire enamel surface. However recent studies have shown that enamel etching with H3PO4 is not achieved over the entire adhesion surface and that 69% of the treated surface had no etching whereas 7% presented tenuous etching and only 2% was ideally etched. These results are generally seen in clinical practices where sealants , adhesive restorations and orthodontic brackets are failing. (17) Deproteinization of enamel involves the removal of organic content i.e proteins from the enamel. A noninvasive technique successfully employed in endodontics, utilizes sodium hypochlorite (NaOCl) as an irrigating solution to disinfect, remove debris, as well as organic materials from the canals. (18,19)
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Till date few studies have been carried out utilizing NaOCl as a deproteinizing agent for the improvement of tooth bonding.One such study was carried out by Espinosa et al. (2008) who used 5.2% sodium hypochlorite (NaOCl) as a deproteinizing agent prior to phosphoric acid etching.It showed that prior deproteinization by NaOCl doubles significantly enamel's retentive surface to 94.47% and increased the type I and II etched enamel. (30) This technique seems promising in removing organic elements of both the enamel structure and acquired pellicle thus conserving the tooth structure and improving its adhesive properties. But the use of sodium hypochlorite prior to acid etching was put in doubt by Bhoomika et al. (2010) who carried out a study in accordance with the study done by Espinosa et al (2008) and observed that enamel deproteinization did not grossly alter the surface topographic features of enamel before acid etching. (20)
Acid etching is an important aspect in clinical dentistry but the surface area etched ideally is only 2% . This is generally seen in clinical practices where sealants , adhesive restorations and orthodontic brackets are failing, which results in repeated dental visits thus prolonging the dental treatment and increasing its cost. In order to improve the bonding of orthodontic brackets and restorative materials to enamel other techniques are now being developed. One such method is to remove the organic content i.e proteins (deproteinization) from enamel which has shown promising results in in-vitro studies. This method increases the enamel surface area and thus makes bonding more efficient. However, there is a need to validate these findings.
Sodium Hypochlorite solutions have been used as wound irrigants since 1915 and as an endodontic irrigant as early as 1920 due to its bactericidal and proteolytic properties. Irrigation of the root canals with sodium hypochlorite solutions (in the concentrations ranging from 1% to 5.25%) is now widely accepted Since the use of 5.2% NaOCl as a deproteinizing agent before acid etching is not conventionally used therefore further studies should be carried out to evaluate the effectiveness of NaOCl in improving the adhesive properties of enamel by increasing the etched enamel surface area.