 Results (Resin Tags) |
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This in vitro study was carried out to evaluate and compare the resin tags of self-etch adhesives formed on intact enamel with and without pre etching with 37% phosphoric acid.
The samples after completing the bonding procedures were sectioned using hard-tissue microtome, gold sputtered, and examined under the scanning electron microscope.
The resin tag analysis was done by measuring the depth of the penetration of the resin tags into the enamel in micrometers.
[Table 1] shows the values of the different samples of the group A and B regarding their range, mean, median values of the resin tag penetration depths in the micrometer.
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Table 1: Range, mean and median of resin tag penetration depths in micrometer of group A and B
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Group A: The range of depth of the penetration of the resin tags into the enamel in micrometers in group A was 0-8.14 mm with a median depth of penetration of 4.07 mm with a mean of 3.67 mm.
Group B: the range of depth of the penetration of the resin tags into the enamel in micrometers in group B was 9.3-27.9 mm with a median depth of penetration of 18.13 mm with a mean of 16.9 mm.
[Table 2] shows the comparison of the two groups regarding their Mean ± SD of the resin tag penetration depths in micrometer.
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Table 2: Mean and standard deviation of resin tag penetration depths in micrometer of group A and B
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The Mean ± SD of group A is 3.67 ± 2.85 mm
The Mean ± SD of group B is 16.9 ± 5.48 mm
Inter group comparisons
At the end of treatment the improvement in resin tag penetration depth of self-etch adhesive on intact enamel on pretreatment with 37% phosphoric acid was 95.4%.
The results showed that Group A demonstrated lower resin tag penetration depths than Group B exhibiting statistically significant difference with P value of <0.001 [Table 2].
| Discussion |
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The enamel bonding potency of self-etch adhesives over intact enamel is highly questioned mainly due to its decreased efficacy in creating a definitive etch pattern.
A bonding agent for giving better bond strength on a dental substrate has to form a uniform well-penetrating resin tags within the dental substrate. Bonding to the intact surface of enamel is important to prevent marginal leakage, and provide maximum bond strength. In most studies investigators flattened enamel surfaces using silicon carbide prior to application of adhesives. So an in vitro study was conducted to evaluate the resin tag formation of self-etch adhesives over intact enamel with and without pre-etching with 37% phosphoric acid.
"In this study, the investigations were carried out on intact enamel that is very important for certain procedures. The results of this study suggested that adequate bonding to intact enamel with self-etch adhesives alone cannot be achieved and will require additional phosphoric acid etching for 15 s at a concentration of 37% is used."
The intact enamel surface is hyperminerlized and contains more fluoride than ground enamel. It is reported that after teeth eruption, changes occur in the outermost enamel. Saturated calcium phosphate might hyper mineralize the enamel, and the fluoride ions might convert the hydroxyapatite into fluorapatite. Thick prismless enamel layer may prevent the permeation of self-etching primers and bonding agents and thus leaving some areas partially unetched, which may result in formation of shorter and poorly defined resin tags. Phosphoric acid etching removes the outermost enamel and there by resulting in formation of longer resin tags and there by increasing the bond strength. [2]
The aggressive etching effect of phosphoric acid on the enamel surface overcomes the difficulty of conditioning of intact or unprepared surfaces. The formation of a deep etching pattern leads to similar bond strength, when the adhesives are applied in both enamel surfaces. [3]
Self-etching systems contain acidic monomers based on esters from phosphoric acid, carboxylic acid, or their derivatives. Their etching efficacy and bonding formation depends on the type of acidic monomer, pH of adhesive solution, etching time, and application method. Acidic monomers are responsible for etching the dental substrates, whereas methacrylate components, such as HEMA, are available for monomer infiltration and polymerization of the bonding agent. [4]
Marina Di Francescantonio reported that the pH values of all self-etching systems were higher than phosphoric acid. In general, the demineralization effects of these systems are proportional to the acidity of the acidic primers or self-etching adhesive solutions. The self-etching primers are less aggressive than phosphoric acid etchants, and do not form a proper and defined acid etching pattern and the conditioning effects are also reduced in intact enamel surfaces, except for the Tyrian SPE primer, which is considered a strong self-etching adhesive with a very low pH (0.5). The etching pattern formed on both ground and intact enamel surfaces is similar to that promoted by phosphoric acid. Moderate self-etch systems include Optibond Solo Plus SE and Xeno III adhesives with pH 1.4 and 1.5, respectively, while Unifil Bond (pH 2.2) and One-Up Bond F (pH 2.6) systems are considered mild. The bonding mechanism of these self-etching systems to enamel is based on nanoretentive interlocking between crystallites and adhesive resin. These morphological features of the resin-enamel bonds are different from that formed with the etch and rinse adhesive systems. This thin hybridized complex of resin formed in enamel, produced by self-etchings without the usual micrometer-size resin tags can be responsible for lower bond strengths presented by some self-etching systems and the questionable effectiveness of this type of dental adhesives on enamel surfaces. Optibond Solo Plus SE and One-Up Bond F systems performed better on prepared enamel than on unprepared enamel. Although Optibond Solo Plus SE primer presents low pH (1.5), it did not bond to unprepared enamel, leading to pretesting debonding in all specimens. [4]
For One-Up Bond F, SEM analysis of treated enamel surfaces with the adhesive solution revealed an extremely mild etching pattern, regardless of the surface preparation. This probably occurred due to the very mild pH (2.6) of the system, which would not provide adequate demineralization for infiltration of monomers. This pattern helps us to explain the bond strength results, which were lower when compared to the ground surface. The higher bond strengths to the SiC-prepared surface can be attributed to the roughness of the surface that facilitates the activity of the self-etching adhesive to form a defined etching pattern for infiltration of adhesive resin. [4]
The removal of superficial, aprismatic layer, by wet-grinding with 600-grit SiC paper improves the etching effects. As the morphological structure and composition of the intact peripheral surface of the enamel is different from that of the middle enamel layer, these differences can be favorable for etching effects in the subenamel surface.
For Optibond Solo Plus SE and One-Up Bond F systems, the enamel surface preparation resulted in higher bond strength. Enamel preparation using 600-grit SiC paper is unlikely to affect resin enamel bond strengths. [4]
Pashley and Tay reported that the efficacy of self-etching primers in intact enamel does not depend solely upon their etching aggressiveness, but on monomeric composition of each material. It is also possible that the low enamel bond strengths might be caused by the high amount of unpolymerized acidic monomers remaining after curing. [3]
Self-etching primers contain acidic monomer and water, which decalcifies the inorganic parts of a tooth but generally its acidity is weak. The acidic portion of the primer is neutralized by the calcium and phosphate ions released during decalcification of enamel, and thereby the decalcification process of enamel is ceased. SEM analysis has revealed that phosphoric acid etching prior to application of the self etch adhesives, created a deeper etching pattern and thicker tag-like resin penetration than priming alone. [5]
SEPs have demonstrated a shallower etch pattern. This might be simply because of a poorer penetration of the SEP into the enamel porosities or the result of interference from mineral precipitates on the enamel surface that mask the etch pattern. Because the acidic primer is not rinsed off, phosphorous and calcium ions released from the hydroxyapatite crystal dissolution remain suspended in the primer solution. With the SEPs, the ionic precipitate remains embedded in the resin after polymerization. [6]
Kanemura et al. concluded stating that the phosphoric acid etching produced good resin adhesion to ground and intact enamel, whereas the self-etch adhesives clearfil liner bond II, Tokuso mac bond II produced good bonds on ground enamel, but had lower bond strength on intact enamel. The causes were attributed to the hypermineralized intact enamel with higher fluoride content and a thick layer of the prismless layer on intact enamel. It was also seen that the resin tags were shorter, poorly defined, and structurally incomplete. Phosphoric acid etching removes the outermost enamel layer and creates microirregularities and thereby results in good penetration of the resin etched intact enamel. [2]
Miguez PA study concluded saying that acid etching prior to application of self-etching primer clearfil SE bond, produced higher bond strength to enamel than self-etching priming only. The self-etch primer caused less demineralization of enamel compared to the total etch systems. The shallower pattern of demineralization is due to difficulty in penetration of the primer into enamel and due to the precipitation of minerals on enamel that might modify the depth of demineralization. [7]
Perdigao reported that, Single Bond, the total-etch adhesive, resulted in statistically higher microtensile bond strength than any of the other adhesives regardless of the enamel preparation. All self-etching adhesives resulted in higher microtensile bond strength when enamel was roughened than when enamel was left unprepared. The field-emission scanning electron microscope revealed a deep interprismatic etching pattern for the total-etch adhesive, whereas the self-etching systems resulted in an etching pattern ranging from absent to moderate. [8]
Marina Rotta concluded that all self-etching adhesives showed lower uTBS values than those obtained in phosphoric acid groups. Lower bond strength was mainly attributed to the low acidity of self-etch adhesives and less distinct etch pattern exhibited when the enamel was conditioned with self-etching adhesive. CSE exhibited the mildest etching pattern. All self-etching adhesives applied after phosphoric acid exhibited a more retentive etching pattern. It was concluded that the bond strength values of low-pH self-etching adhesives can be improved by the adjunctive use of phosphoric acid or replacement of the bonding resin. [9]
Nasr stated that the demineralizing capacity of a product depends on such factors as its pH (for strong acids) or pKa (for weaker acids), its chelating ability, or the solubility of its calcium salts and of the duration of application. The strongest demineralizations are yet less than the reference value observed with phosphoric acid. [10]
Pisol Senawongse compared three commercially available adhesive materials single bond (total etch), Clearfil SE Bond (two step self etch), One-Up Bond (one step self-etch) for their microshear bond strength on intact and ground enamel. The results showed that the two self-etching bonding agents showed lower bond strength than the total etching system on intact enamel, where as there was no significant differences in the bond strength on ground enamel. The resin tags in the single bond group revealed abundant longer resin tags on both intact and ground enamel. In the case of self-etch adhesives, short resin tags were seen in case of ground enamel, whereas on intact enamel, the short resin tags were rarely seen. [11]
Further studies testing these materials in vivo are warranted to determine whether the amount of bonding is the same as the in vitro study, is clinically relevant.
| Conclusion |
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Within the limitations of this study conducted and the results obtained, it can be concluded that
"Out of both treatment groups, pre etching the intact enamel with 37% phosphoric acid resulted in longer resin tags (higher depth of penetration of resin tags) and this can be the reason of low bond strength of self etch adhesives on intact enamel."
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