|Year : 2008 | Volume
| Issue : 4 | Page : 309-314
|Crystal growth vs. conventional acid etching: A comparative evaluation of etch patterns, penetration depths, and bond strengths
Raghu Devanna, KM Keluskar
Department of Orthodontics and Dentofacial Orthopedics, Institute of Dental Sciences, KLE University, Belgaum, India
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|Date of Submission||18-Nov-2007|
|Date of Decision||24-Jun-2008|
|Date of Acceptance||07-Jul-2008|
| Abstract|| |
The present study was undertaken to investigate the effect on enamel surface, penetration depth, and bond strength produced by 37% phosphoric acid and 20% sulfated polyacrylic acid as etching agents for direct bonding. Eighty teeth were used to study the efficacy of the etching agents on the enamel surface, penetration depth, and tensile bond strength.
It was determined from the present study that a 30 sec application of 20% sulfated polyacrylic acid produced comparable etching topography with that of 37% phosphoric acid applied for 30 sec. The 37% phosphoric acid dissolves enamel to a greater extent than does the 20% sulfated polyacrylic acid. Instron Universal testing machine was used to evaluate the bond strengths of the two etching agents. Twenty percent sulfated polyacrylic acid provided adequate tensile bond strength. It was ascertained that crystal growth can be an alternative to conventional phosphoric acid etching as it dissolves lesser enamel and provides adequate tensile bond strength.
Keywords: Acid etching, crystal growth, penetration depth, tensile bond strength
|How to cite this article:|
Devanna R, Keluskar K M. Crystal growth vs. conventional acid etching: A comparative evaluation of etch patterns, penetration depths, and bond strengths. Indian J Dent Res 2008;19:309-14
Since Buonocore first introduced the use of phosphoric acid for enamel etching, various other alternative acids and chelating agents have been studied. None have proved to be having significant advantage, and most have shown poorer etch pattern production, lower bond strength, or even excessive enamel dissolution.  Iatrogenic effects of conventional acid etch bonding are: loss of enamel while debonding, increased plaque retention, loss of fluoride rich enamel with aging, etc. The efficacy of a number of organic and inorganic acids along with chelating agents of different concentrations and the duration of their application have been evaluated in an effort to obtain optimal etching of the enamel surface. ,,,,, In this perspective, the technique of crystal growth as demonstrated by Smith and Cartz receives attention.  Crystal bonding solutions are based on a mixture of polyacrylic acid and residual sulfate ions. The action of ionic salts like sodium sulfate in crystal bonding solutions has been investigated. , The bond strength for the group treated with polyacrylic acid was lower than those reported by Maijer and Smith.  Since several different ionic solutions produce crystal growth, it seems important to conduct further clinical testing of other crystal bonding agents to check if the results are comparable. 
|How to cite this URL:|
Devanna R, Keluskar K M. Crystal growth vs. conventional acid etching: A comparative evaluation of etch patterns, penetration depths, and bond strengths. Indian J Dent Res [serial online] 2008 [cited 2019 Sep 17];19:309-14. Available from: http://www.ijdr.in/text.asp?2008/19/4/309/44533
The present study was undertaken to compare the effect of 20% sulfated polyacrylic acid (in which the ion solution used was potassium sulfate) and 37% phosphoric acid on enamel surface, penetration depth, and tensile bond strength.
| Materials and Methods|| |
The sample comprised of 80 freshly extracted permanent maxillary first premolar teeth. They were used to study the efficacy of the two etching agents on enamel surface, for direct bonding of orthodontic attachments. The teeth that were fully erupted and free from caries and restorations were included in the study.
The etching solution of 37% phosphoric acid (solution I) used in the study was obtained from Libral trader's Pvt. Ltd. The 20% sulfated polyacrylic acid (solution II) was prepared at Deptartment of Pharmacochemistry, Jawaharlal Nehru medical college, KLE University, Belgaum.
Orthodontic stainless steel preadjusted edgewise brackets having standard bracket slot. 022" X .028" (GAC International Inc.) was used for evaluating the tensile bond strength.
Bonding agent used was OrthoSolo universal bond enhancer and Enlight light cure adhesive (Ormco Corp).
The study was split into three parts:
- Effect on enamel surface - a scanning electron microscope (SEM) study.
- Assessment of penetration depth - a 3-D profilometer study.
- Assessment of tensile bond strength - a universal instron testing machine study.
Effect on enamel surface analyzed by SEM
Twenty premolar teeth (maxillary) were taken as samples. Buccal surfaces of samples were polished with pumice paste prior to etching. Samples were etched with solution I and II for a given time. Etching time for every solution varied in order to compare the etching time and the quality of etch. Etchant was applied with applicator brush, after which the samples were washed off with 5 ml of distilled water, and then dried with compressed air. They were kept in a desssicator overnight and then observed under JSM-840A SEM, JEOL.
All the 20 samples were observed separately under SEM. The desired area was focused and scanned at a magnification of 750X and 1820X. Representative photomicrographs were then obtained.
Assessment of penetration depth by 3-D profilometer
The depth of etching on the polished enamel surface produced by solutions I and II was determined by surface profile recordings. The buccal enamel surfaces of the premolars were etched with solutions I and II for 30 and 60 sec, respectively. The surface profiles of these specimens were recorded with the help of WYKO NT100 optical profiling system. A computer provided the mean maximum peak to height velocity. The mean values obtained were in micron meters (µm). The statistical analysis was carried out with unpaired student's 't' test.
Assessment of tensile bond strength by universal instron testing machine
The exposed teeth surfaces were etched for a period of 30 seconds for both thesolutions (solution I and II). Meshed stainless steel preadjusted edgewise brackets were bonded onto the etched enamel surface with Enlight light cure adhesive (Ormco Corp).
Instron Model 4467 with a current load of 30 KN was allowed to warm up for 30 min prior to testing. The specimen was then checked for alignment and a load was applied with a cross head speed of 0.5 cm/minute. Care was taken to eliminate all forces except tensile load.
| Results|| |
Effect on enamel surface analyzed by SEM
Normal enamel surface showed featureless appearance when viewed in the SEM at 750X magnification [Figure 1]. Etching with 37% phosphoric acid for 30 sec showed a characteristic typical 'honey comb pattern' with a preferential dissolution of enamel prism cores and peripheries [Figure 2]. Etching for more than 30 sec revealed loss of superficial enamel and formation of clefts, pits, and ill-defined poor etch pattern [Figure 3].
Etching with 20% sulfated polyacrylic acid for 30 sec showed formation of crystals on the enamel surface and gave a comparable etch pattern compared to 37% phosphoric acid at 30 seconds [Figure 4]. When observed for the samples etched for 60 sec, it showed formation of clefts, pits, and ill-defined etch pattern [Figure 5].
Assessment of penetration depth
Surface profile measurements of teeth etched with solutions I and II for 30 and 60 sec were obtained [Figure 6],[Figure 7],[Figure 8],[Figure 9],[Figure 10]. Deepest penetration depth of 25.10 µm was obtained with 37% phosphoric acid etched for 30 sec [Figure 7], and the least was with 20% sulfated polyacrylic acid with measured depth of 6.39 µm etched for 60 sec [Figure 10]. [Table 1] shows the depth of etch from the solutions I and II at 30 and 60 sec.
The statistical analysis carried out with unpaired student's 't' test showed highly significant P values when solution I was compared with solution II at both 30 and 60 seconds [Table 2].
Assessment of tensile bond strength by universal instron testing machine
The 40 samples were divided into two groups, group I treated with phosphoric acid and group II with sulfated polyacrylic acid, respectively. Thereafter, meshed stainless steel preadjusted edgewise orthodontic brackets having a surface area of 11.25 mm 2 were bonded on to etched enamel. Samples were subjected to tensile stresses and load required to break the bond was recorded in kilogram [Figure 11]. Thereafter, tensile strength was calculated.
[Table 3] shows tensile strength in megapascals (MPa). The highest tensile bond strength values were 5.48 MPa (±0.62) obtained from group I, while for group II the recorded values were 4.13 MPa (±0.63). The tensile bond strength of group II was lesser than group I.
The 't' test values were found to be significant indicating that etching with solution II provided slightly lesser tensile bond strength compared to solution I. [Table 4] shows statistical comparison of tensile bond strengths of solution I and II.
| Discussion|| |
As a result of initial tooth preparation, bonding, debonding, and cleanup procedures, a loss of up to 55 µm of enamel can occur (3% of the total enamel thickness). ,, As enamel lacks regenerative power, it calls for a greater care in its maintenance. Smith  in his investigation of polycarboxylate cements found that they would adhere to dental enamel. Smith reported several advantages of this polyacrylic acid crystal growth technique compared to conventional acid etching with phosphoric acid: (1) the enamel surface is not significantly damaged, (2) debonding and cleanup are easier, (3) there is minimal loss of fluoride-rich outer enamel, and (4) few, if any, resin tags are left in enamel after debonding.  Direct bonding of attachments with this cement, however, was attempted with only partial success.  Further studies  revealed that polyacrylic acid containing residual sulfate ions reacted with the enamel surface to produce a deposit of white, spherulitic crystalline calcium sulfate.
Maijer and Smith  demonstrated that the crystalline structure produced a bondable surface to which orthodontic attachments were attached with sufficient strength to withstand tensile forces up to 11 kg. In 1980-1981, in abstracts of their ongoing research, Smith and Maijer reported crystals with a longer shape when the sulfate ion concentration was changed. The longer crystals provided a better retentive surface into which an unfilled resin could flow. They reported tensile bond strength with the new formulation similar to that obtained from conventional phosphoric acid etch. ,
In the present study, 20% sulfated polyacrylic acid, that is, 20% polyacrylic acid incorporated with potassium sulfate ionic solution was selected for comparison with 37% phosphoric acid for direct bonding. The comparison was to evaluate the etch pattern, penetration depth, and bond strength of the two acids.
The wettability of the enamel surface is increased by acid etching. , Etching with 37% phosphoric acid for 30 and 60 sec produced different etching patterns. Etching for 30 seconds produced Type I or good etch pattern, that is, preferential dissolution of enamel prism cores and boundaries [Figure 2].  Results are similar to that obtained by Retief et al. , Etching time of more than 30 sec resulted in formation of clefts, grooves, and ill-defined etching patterns [Figure 3].
Etching with 20% sulfated polyacrylic acid produced a comparable etch pattern at both 30 and 60 sec. Crystal growth was seen at 30 seconds [Figure 4]. The SEM photographs demonstrated crystal formations similar to those described by Smith. These crystals appear to form an adequate area for resin to flow into, forming a locking retentive mesh. Samples etched with 20% sulfated polyacrylic acid for 60 sec showed formation of clefts and pits [Figure 5].
The previously done studies, used a 2-D surface roughness testing machine (Retief 1974, Retief et al., 1976) wherein the diameter of the stylus was not so fine to record the finest details, so the conclusions drawn were of comparable type. But with 3-D profilometer used in this study, the measurements drawn were of finest details and significance.
Thirty seven percent phosphoric acid penetrated to a greater depth of about 25.10 µm at 30 sec and 16.88 µm at 60 sec. Twenty percent sulfated polyacrylic acid penetrated to a lesser extent at both 30 and 60 sec, as demonstrated by the profilometer study. Solution II penetrated to a depth of about 12.10 µm at 30 sec and 06.39 µm at 60 sec, respectively.
Bond strengths of the two acids were evaluated in order to find out if sulfated polyacrylic acid can be used with an efficacy comparable to that of phosphoric acid. A subcommittee on standard test methods for Dental Materials Group of the IADR decided in the year 1967 that a tensile test should be used in preference to other tests. The samples were oriented in the testing machine so that load applied to the occlusal bracket wings was at right angles to the enamel surface. The tensile force was measured in kilograms and the bond strength values were calculated by dividing the force values by the bracket base area.
Twenty percent sulfated polyacrylic acid removes significantly lesser amount of healthy enamel and at the same time provides lesser tensile bond strength compared to 37% phosphoric acid. An attempt was made to investigate 20% sulfated polyacrylic acid (in which the ion solution was potassium sulfate) for its use in clinical practice in the present study. Further studies with the change in polyacrylic acid concentrations and other ion solutions that can provide crystal growth and at the same time adequate bond strengths comparable to phosphoric acid can be investigated.
| Conclusion|| |
The conclusions drawn from the present study were:
- Effects on enamel surface produced by the two acids revealed through SEM.
- Application of 20% sulfated polyacrylic acid also produced a good etch pattern with the formation of crystals at 30 sec. A poor etch pattern was produced by 20% sulfated polyacrylic acid when etched at 60 sec.
- A comparable etch pattern was produced by 20% sulfated polyacrylic acid at both 30 and 60 sec.
- Assessment of penetration depth by 3-D profilometer revealed that 37% phosphoric acid dissolved more enamel than 20% sulfated polyacrylic acid at both 30 and 60 sec.
- Assessment of tensile bond strength reveals that, 37% phosphoric acid when applied on enamel for 30 sec provided adequate bond strength at adhesive-enamel interface. Sulfated polyacrylic acid applied for 30 sec provided lesser bond strength values.
- Twenty percent sulfated polyacrylic acid could be an alternative to phosphoric acid etching as it dissolves lesser amount of enamel but with a slightly lesser tensile bond strength.
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Department of Orthodontics and Dentofacial Orthopedics, Institute of Dental Sciences, KLE University, Belgaum
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]
[Table 1], [Table 2], [Table 3], [Table 4]
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