| Abstract|| |
Introduction: The aim of the study is in vitro evaluation of the shear bond strength of composite resin bonded to enamel which is pretreated using acid etchant and Er,Cr:Ysgg.
Materials and Methods: 40 extracted human teeth were divided in two groups of 20 each (Groups A and B). In Group A, prepared surface of enamel was etched using 37% phosphoric acid (Scotchbond, 3M). In Group B, enamel was surface treated by a an Er, Cr: YSGG laser system (Waterlase MD, Biolase Technology Inc., San Clemente, CA, USA) operating at a wavelength of 2,780 nm and having a pulse duration of 140-200 microsecond with a repetition rate of 20 Hz and 40 Hz. Bonding agent ((Scotchbond Multipurpose, 3M) was applied over the test areas on 20 samples of Groups A and B each, and light cured. Composite resin (Ceram X duo Nanoceramic restorative, Densply) was applied onto the test areas as a 3 × 3 mm diameter bid, and light cured. The samples were tested for shear bond strength.
Results: Mean shear bond strength for acid-etched enamel (26.41 ± 0.66MPa, range 25.155 to 27.150 MPa) was significantly higher (P < 0.01) than for laser-etched enamel (16.23 ± 0.71MPa, range 15.233 to 17.334 MPa).
Conclusions: For enamel surface, mean shear bond strength of bonded composite obtained after laser etching were significantly lower than those obtained after acid etching.
Keywords: Enamel etching; laser etching; shear bong strength; phosphoric acid etching; erbium; chromium:yttrium, scandium, gallium, garnet laser
|How to cite this article:|
Hoshing UA, Patil S, Medha A, Bandekar SD. Comparison of shear bond strength of composite resin to enamel surface with laser etching versus acid etching: An in vitro evaluation. J Conserv Dent 2014;17:320-4
|How to cite this URL:|
Hoshing UA, Patil S, Medha A, Bandekar SD. Comparison of shear bond strength of composite resin to enamel surface with laser etching versus acid etching: An in vitro evaluation. J Conserv Dent [serial online] 2014 [cited 2019 Sep 21];17:320-4. Available from: http://www.jcd.org.in/text.asp?2014/17/4/320/136438
| Introduction|| |
Current understanding of the adhesion of dental restorative materials is based on two fundamental theories. One theory is based on chemical adhesion, describing intermolecular forces at the interface and the other theory is based on micromechanical retention; attributing adhesion to the interpenetration of components of the two surfaces.  Cavity preparation with rotary instruments or manual scalers leaves a smear layer on the dental surface. The low surface energy of this layer hinders the impregnation of the tissue with the adhesive agent and thus prevents adequate adhesion. Since the report of Buonocuore, the standard approach to this problem has been acid etching. 
Acid etching of the enamel appears to improve the retention by selectively eroding certain hydroxyapatite formations and facilitating the penetration with the development of resin tags of about 6-12 mm in length.  Various procedures for acid etching have been proposed, though the most widely used at present for enamel is 37% phosphoric acid for 15 s. 
Development of laser technology has enabled its use in multiple dental procedures, such as soft tissue operations, composite restorations, tooth bleaching, caries removal, and tooth preparations with minimal pain and discomfort.  The use of lasers like high power diode laser and neodymium-doped yttrium aluminum garnet (Nd:YAG) in endodontics is an innovative approach for disinfection, providing access to formerly unreachable parts of the tubular network, due to their ability to penetrate dental tissues better than irrigant solutions.  Laser etching has become available as an alternative to acid etching of enamel and dentin. Laser etching is painless and do not involve either vibration or heat, making this treatment highly attractive for routine use. Furthermore, laser etching of enamel or dentin has been reported to yield an anfractuous surface and open dentin tubules, both apparently ideal for adhesion.  Various lasers are used in restorative dentistry such as the initially developed carbon dioxide laser (CO 2 laser) and the Nd:YAG laser however Nd:YAG laser is not well-absorbed by dental hard tissue and CO 2 laser although well-absorbed by dental tissue is not suitable because it can cause an increase in the temperature of the dental pulp.  Thus to avoid these drawbacks, erbium (Er) lasers with two different wavelengths were introduced.
Like Er:YAG laser the Er, chromium:yttrium, scandium, gallium, garnet (Er, Cr:YSGG) laser of wavelength 2780 nm is effective in cutting the tissue and preparing the cavity.  There is no or little (2°C) temperature rise in the dental pulp when the laser is used along with a water-air spray.  Enamel preparation using the Er, Cr:YSGG laser produces chalky surface. Scanning electron microscope (SEM) evaluation shows that the laser irradiation produces the surface with high retention for restorative material, making it suitable for composite filling.  Er:YAG laser irradiation vaporizes the water content of hard immediately, which results in irregular surface. These irregularities work as a mechanical retention increasing adhesion of restoration to tooth hard tissue, which can be a substitution for acid etching technique, not only in microscopic dimensions, but also in macroscopic and clinical appearance. 
The purpose of this study is to compare the shear bond strength (SBS) of composite resin to enamel surface using conventional acid etching and laser etching.
| MaterialS and Methods|| |
Forty intact, noncarious freshly extracted human permanent central incisors were collected. Teeth that exhibited any evidence of cracking or fissuring were rejected. All teeth were stored in a phosphate-buffered 5% solution of formalin at 4°C to minimize bacterial growth. Prior to use teeth were cleaned with gauge piece and allowed to dry in air.
Before the experiments, teeth were embedded in a resin block leaving the coronal portion of the tooth exposed with the help of metal block of 2″ × 2″. Forty teeth were used for assessment of bonding to enamel after acid etching (Group A) and after laser etching (Group B). Allocation of the teeth to the two groups (n = 20 per group) was random.
Bonding to enamel (Group A), the facial surfaces of the 20 teeth in Group A were cleaned with a pumice water mixture with the aid of a rubber prophylaxis cup in a low-speed handpiece for 30 s, then washed with abundant distilled water for 30 s and dried with oil-free and dry air for 20 s. Teeth assigned to Group A were coated with acid resistant nail varnish leaving 3 mm × 3 mm of labial surface free for acid etching procedure. Later, all the specimens were etched with a 37% phosphoric acid gel (3M Scotchbond etchant, 3M Dental Products, St. Paul, MN, USA) for 15 s followed by 15 s of washing with distilled water and air dried for 10 s.
Bonding to Enamel (Group B), laser etching was completed with an Er, Cr:YSGG laser system (Waterlase MD, Biolase Technology Inc., San Clemente, CA, USA) operating at a wavelength of 2780 nm and having pulse duration of 140-200 μs with a repetition rate of 20 Hz. Laser energy was delivered through a fiberoptic system to a sapphire tip terminal that was 600 μm in diameter and 6 mm in length. The power output was set at 2 W according to the study group. Air and water spray from the handpiece was adjusted to a level of 85% air and 85% water for 2 W to prevent the enamel surfaces from overheating. Moreover, the average exposure time was set at 10 s. Laser etched enamel surface of 3 mm × 3 mm was obtained.
After acid or laser etching, the teeth were dried with oil-free air for 20 s; the light-cured bonding resin (Scotchbond Multipurpose, 3M, Lot No. 19971001) was then applied immediately with a brush, and the coating was cured with a quartz-tungsten-halogen light source (XL 155, 3M) for 20 s. Composite resin (Ceram × duo Nanoceramic restorative, Dentsply) was bonded as a bead of 3 mm × 3 mm size on prepared surface.
Shear bond strength analysis, all the specimens were transferred to the Instron universal machine individually and subjected to SBS analysis at constant crosshead speed of 1.0 mm/min. For each preparation the peak shear load (kilogram) was recorded. The order of testing of teeth was randomized with respect to the groups as Group A followed by Group B.
For SBS analysis, data were expressed as means ± standard deviation. All data was processed by SPSS software version 19.0 (SPSS Inc., Chicago, IL, USA). To investigate whether SBS was significantly affected by treatment option, a Student's t-test with the Bonferroni correction was used. To obtain a better homogeneity of variances, data were normalized by means of a square-root transformation. Differences were considered significant for a value of P < 0.01.
| Result|| |
Shear bond strength values obtained for acid-etched enamel were in the range of 25.155-27.150 MPa and for laser-etched enamel were in the range of 15.233-17.334 MPa. Mean SBS for acid-etched enamel was 26.41 ± 0.66 MPa, which was significantly higher (P < 0.01) than for laser-etched enamel 16.23 ± 0.71 MPa. Comparison of mean values of SBS of acid etch group and laser etch group (n = 10) was shown in [Table 1].
|Table 1: Comparison of mean values of shear bond strength of acid etch group and laser etch group (n = 10)|
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Shear bond strength analysis
By applying Student's unpaired t-test, there is a highly significant statistical difference between mean values of SBS in acid etch enamel and laser etch enamel group (i.e., P < 0.01).
| Discussion|| |
Phosphoric acid etching is one of the best methods to bond resins to enamel. The smear layer is removed by applying acid on the enamel surface. Microscopic roughness and enamel surface energy are enhanced by removing prismatic and interprismatic mineral crystals. In general, 10-37% orthophosphoric acid is applied to both enamel and dentin.  In this study, we used a 37% orthophosphoric acid gel and found that acid etching was the application that led to the highest SBS.
Currently, laser etching of enamel surfaces is popular because of the potential disadvantages of acid etching. Acid etching results in chemical changes that can modify the organic matter and decalcify the organic component. As a result of this demineralization, enamel becomes more susceptible to caries attack, which is induced by plaque accumulation around the bonded composite resin. 
Recently, Er:YAG and Er; Cr:YSGG lasers were introduced in dentistry. For physical and medical reasons, they are used for the treatment of hard tissue. The advantage of an Er wave is that it is well-absorbed by water and dental hard tissue. The strong absorption of water reduces the level of heat during tooth preparation. As water absorbs laser radiation better than dental hard tissue, it reduces the increasing temperature of the tissue during the preparation. Water reaches the boiling point and causes micro-explosion of the tooth. This action breaks up the surrounding tissue into small pieces and dissipates them at the same time. As this explosion occurs in water, it is so-called a preparation induced by water. Although most radiation is absorbed in water, a certain amount of heat transmission is unavoidable. Therefore, a water spray is used for cooling. Lack of adequate water spray causes pulpal damage. This laser can also be used for etching of enamel surfaces for the purpose of bonding the composite resin to enamel surface. , Thus, aim of our study was to compare the SBS of composite resin to enamel surface with conventional acid etching versus laser etching.
When the effects of Er, Cr:YSGG lasers on dental tissues were examined, cavities were found to be unaffected by thermal damage. This pattern may enhance the bonding of adhesive restorations.  The literature contains conflicting findings regarding surface treatments and cavity preparations with lasers. One laser used for this purpose is Er, Cr:YSGG, which has a high absorption coefficient in water and enamel. This led researchers to explore its use in enamel conditioning. For cutting enamel, the clinician can use high-irradiation outputs varying from 2.5 to 6 W. In this study, we used a lower output 2W to etch the enamel. Recently, Berk  observed by means of a SEM analysis that 1 W, 1.5 W, and 2W Er, Cr:YSGG laser irradiation produced etching patterns similar to those of acid etching. Moezizadeh et al. in their study they found that surface treatment of Signum Plus indirect composite with Er, Cr:YSGG laser showed greater bond strength with 1W irradiation than 2W irradiation output. 
The time for laser etching used in this study was 10 s which was shorter than that required for acid etching that is, 15 s.  Baygin et al.  found that 10 s of 2W laser etching might be an alternative to enamel acid etching. Obeidi et al.  found that increasing the laser etching time to 40 s may increase the bond strength to the level of conventional acid etching.
Several of the findings concerning the use of lasers for enamel etching are contradictory. Some researchers stated that laser irradiation was not capable of etching enamel. Martνnez-Insua found weaker adhesion forces in an Er:YAG laser-etched enamel surface than an acid etched enamel surface.  This was related to sub-surface cracks observed in (SEM) images. Tarηin et al. found that the microtensile bond strength was significantly lower in the acid-etched group than the Er, Cr:YSGG and Nd:YAG laser-etched enamel group for both bonding agents used.  Borsatto et al. verified that Er:YAG laser irradiation did not eliminate the need to etch the enamel surface with acid before applying the sealant. 
The results obtained lead us to affirm that the research hypothesis of an expected similar or better adhesive force after the laser treatment is not correct. The laser treatment had some deleterious effects in the tissue surface that severely weakened the adhesion. The SBS obtained in our study was 16.23 ± 0.71 MPa for laser etching procedure, which is less than acid etching procedure, which is found to be 26.41 ± 0.66 MPa. Such variability may be due to undetected weaknesses at the tissue surface, differences in surface curvature or trapping of air bubbles within the resin.
On the other hand, some researchers stated that laser applications give similar results to acid-etching techniques. Ozer et al. investigated the SBS of brackets that they applied on enamel prepared with 0.75 W Er, Cr:YSGG, 1.5 W Er, Cr:YSGG, 37% orthophosphoric acid or self-etching primer. They found that the 0.75 W laser-applied groups was significantly less in regard to SBS than all other groups, although there was no statistically significant difference among the other groups.  Basaran et al. compared the SBS of brackets using an Er, Cr:YSGG laser in 0.5 W, 0.75 W, 1 W, 1.5 W, 1.75 W, and 2 W. They stated that 1.5 W, 1.75 W, and 2 W etching yielded similar success rates as 38% phosphoric acid etching. 
Frequency in laser units connotes for how many seconds the pulse creates the wave. The unit of frequency is the hertz. A laser with a wide spectrum of frequencies was used in our study because of its clinical feasibility.  Another advantage of the Er, Cr:YSGG laser is that the frequency settings are adjustable. Studies examining the frequency settings are generally aimed at evaluating thermal increments. Baygin et al.  found no statistical difference between SBS values between 2 W 20 Hz laser and 40 Hz laser.
Data from our study demonstrated that bonding to Er; Cr:YSGG laser-etched surfaces provided markedly weaker values than bonding to acid-etched surfaces. In the laser-etched enamel preparations, the high prevalence of cohesive tooth fractures suggests that disruption as a result of "micro-explosions"  weakened the enamel and gave rise to a more heterogeneous surface than that obtained by acid etching. Acid etching typically produced a repeating surface pattern, with cracks and fissures no deeper than 12 mm that are readily filled with resin.  In contrast to acid etch treatment, laser etching produced extensive surface fissuring and less regular and less homogeneous surface patterns arising from the union of different craters. Fissuring may be related to the orientation of enamel rods, because enamel is an anisotropic material. One of the potential disadvantages of enamel acid etching is that the acid causes demineralization of the most superficial layer. As a result, this surface becomes more susceptible to long-term acid attack and caries, especially when resin impregnation is defective because of air bubbles or saliva contamination. Such effects are particularly important given that plaque tends to accumulate at interfacial surfaces. 
The physicochemical changes caused by laser etching can be expected to decrease long-term susceptibility to acid attack and caries. This reduction may be related to changes in Ca:P ratio, reduced carbonates, and pyrophosphate formation,  together with reduced water and organic component contents.  It has also been suggested that laser etching may create remineralization microspaces that trap free ions.  Nevertheless, our results suggest that these putative advantages of laser etching may be outweighed by the extensive fissuring caused by the treatment and by consequently poor bonding strength. In addition, larger samples by means of SEM should be examined in future bond strength studies.
| Conclusion|| |
Within the limits of this study, the following conclusions were drawn:
For enamel surface, mean SBS of bonded composite obtained after laser etching were significantly lower than those obtained after acid etching.
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Upendra A Hoshing
Department of Conservative Dentistry and Endodontics, Vasantdada Patil Dental College and Hospital, Kavalapur, Sangli - 416 306, Maharashtra
Source of Support: None, Conflict of Interest: None