Odontology DOI 10.1007/s10266-015-0203-8
Microtensile bond strength of a resin-based fissure sealant to Er,Cr:YSGG laser-etched primary enamel Elif Sungurtekin-Ekci1 • Nurhan Oztas2
Received: 19 September 2014 / Accepted: 15 March 2015 Ó The Society of The Nippon Dental University 2015
Abstract The aim of this study was to evaluate the effect of Er,Cr:YSGG laser pre-treatment alone, or associated with acid-etching, on the microtensile bond strength of a resin-based fissure sealant to primary enamel. Twenty-five human primary molars were randomly divided into five groups including (1) 35 % acid etching, (2) 2.5-W laser etching, (3) 3.5-W laser etching, (4) 2.5-W laser etching ? acid etching, and (5) 3.5-W laser etching ? acid etching. Er,Cr:YSGG laser was used at a wavelength of 2.780 nm and pulse duration of 140–200 ls with a repetition rate of 20 Hz. Following surface pre-treatment, the fissure sealant (ClinProTM, 3M Dental Products) was applied. Each tooth was sectioned and subjected to microtensile testing. Kruskal–Wallis test was used for statistical analysis. The level of significance was set at p \ 0.05. The microtensile bond strength values of group 1 were significantly higher than those of group 2, while no statistically significant difference was detected between groups 1, 3, 4, and 5. It was concluded that 3.5-W laser etching produced results comparable to conventional acid etching technique, whereas 2.5-W laser etching was not able to yield adequate bonding performance.
& Elif Sungurtekin-Ekci [email protected]
Nurhan Oztas [email protected]
Faculty of Dentistry, Department of Paediatric Dentistry, Yeditepe University, Bagdat Caddesi No. 238, Goztepe, Kadikoy, 34728 Istanbul, Turkey
Faculty of Dentistry, Department of Paediatric Dentistry, Gazi University, 8. Cd. 1.Sok., Emek, 06510 Ankara, Turkey
Keywords Acid etching Er,Cr:YSGG laser Fissure sealant Microtensile bond strength Primary teeth
Introduction Introduced to prevent caries almost 50 years ago, pit and fissure sealant application has been a proven approach for the prevention of caries on occlusal surfaces, thereby having a major role in preventive dentistry . The retention of pit and fissure sealants is undoubtedly essential for the effectiveness of the procedure, and the surface pretreatment regimen prior to placement is one of the most important factors affecting retention. Conditioning enamel surface with various concentrations of phosphoric acid is a conventional method for creating microporosities, providing micro-mechanical interlocking of the enamel–sealant interface . However, remaining debris and pellicle that cannot be removed from the base of fissures by the conventional prophylaxis and etching procedures as well as fissure morphology and aprismatic enamel structure can reduce etching performance and thus compromise adhesion [3, 4]. Another disadvantage attributed to acid etching is the demineralization of enamel, making it more vulnerable to acid attacks, especially if the demineralized enamel left at the resin restoration margin or around orthodontic attachments is not completely filled by resin monomers . The applicability of alternative methods for increasing the surface energy of enamel has been the research focus in recent years [6, 7]. Early observations of enamel surfaces prepared by erbium lasers demonstrated a similar etching pattern to those of acid etching . As these lasers emit energy in the mid-infrared region, they are highly absorbed by dental hard tissues that mainly contain water and hydroxyapatite . Laser irradiation of dental hard tissues has
been reported to create microirregularities and no smear layer. Furthermore, laser etching is simple and has the advantage of eliminating the need for isolation as well as leading to the formation of more stable and less acid-soluble compounds . The proposed mechanism for acid resistance is to change the calcium/phosphorus ratio with laser application [6, 10]. All these favorable properties seem to be potentially beneficial in pre-treatment of enamel prior to fissure sealant application, particularly in the case of unground primary enamel which involves an acid-resistant prismless superficial layer [6, 7, 11]. There has been a growing research interest in the conditioning effects of erbium, chromium:yttrium–scandium– gallium–garnet (Er,Cr:YSGG) laser on tooth surfaces. The etching effects of Er,Cr:YSGG laser on dental hard tissues have been investigated mostly via microleakage evaluation and mechanical tests made on composite restorations applied to primary and permanent tooth cavities [7, 12–15]. However, there are few studies regarding conditioning prior to fissure sealant application [11, 16] and, to date, results concerning the bond strength achieved with Er,Cr:YSGG laser etching in fissure sealing of primary molars are still not available. Therefore, the objective of this in vitro study was to evaluate the effect of Er,Cr:YSGG laser pre-treatment alone, or associated with acid-etching, on the microtensile bond strength of a resin-based fissure sealant to primary enamel.
Materials and methods Twenty-five human mandibular primary molars, free of caries and other macroscopic defects and extracted for orthodontic reasons, were collected and stored in sterile saline solution at 4 °C for up to 1 month. After debridement of buccal enamel surfaces with a scaler from calculus and tissue remnants and cleaning with a slow-speed handpiece and a brush with a non-fluoridated pumice, the teeth were randomly divided into five groups including (1) 35 % acid etching (30 s), (2) 2.5-W laser etching, (3) 3.5W laser etching, (4) 2.5-W laser etching ? acid etching, and (5) 3.5-W laser etching ? acid etching. Laser etching was done with an Er,Cr:YSGG laser system (Waterlase MD, Biolase Technology Inc.; SanClemente, CA, USA) operating at a wavelength of 2780 nm and having a pulse duration of 140–200 ls with a repetition rate of 20 Hz. The power output was set at either 2.5 W or 3.5 W, according to the study group. Air and water spray from the handpiece was adjusted to a level of 90 % air and 80 % water for 2.5 W, and 90 % air 85 % water for 3.5 W, to prevent the enamel surfaces from overheating. Laser energy was delivered through a fiberoptic cable to a sapphire tip terminal 600 lm in
diameter and 6-mm long and the laser beam spot size was 0.442 mm2. The laser beam was aligned perpendicular to the buccal surfaces in noncontact mode at 1-mm distance, in accordance with the manufacturer’s instructions for etching. The duration of exposure depended on the time needed to guide the laser beam evenly across the buccal enamel surfaces to be irradiated (10–15 s). Then, the etched surfaces were rinsed and air dried. For the acid etching procedure, the buccal surfaces of the teeth were first air dried and then etched for 30 s with 35 % orthophosphoric acid gel (ScotchbondTM Etchant Delivery System, 3M ESPE, St Paul, USA), rinsed for 15 s, and air dried for 10 s. Following these procedures, a layer of a resin-based fissure sealant (ClinProTM, 3M Dental Products) was applied to a thickness of approximately 1 mm by using a plastic mold with 2-mm diameter and 1-mm thickness. The sealant was handled according to the manufacturer’s instructions and lightcured for 20 s (Curing light XL 3000TM, 3M Dental Products). The intensity of the curing light was monitored daily and exceeded 500 mW cm-1. For the purpose of providing a gripping surface for the microtensile test, a universal hybrid composite resin (Filtek Z250, Shade A3.5, 3M ESPE) was applied and polymerized in four increments on the surface of the cured sealant to obtain an 8-mm-high composite buildup. Each increment was lightcured for 20 s. After a 24-h storage in a 37 °C sterile saline solution, teeth were mounted on a phenolic ring and sectioned parallel to the adhesive interface into a series of 1-mm-thick slabs with a water-cooled, slow-speed, diamond saw (Mecatome T201; Presi, France). Sticks of 1 mm 9 1 mm in cross-section were obtained by rotating the specimen 90° and sectioning it again lengthwise perpendicularly to the adhesive interface. The specimens with a bonded interface close to the enamel margin not be exactly perpendicular to the long axis of the stick and were discarded . Twenty slabs were produced for each group. Each tested specimen was assessed individually by attaching to the jig of the universal microtensile testing machine (Micro Tensile Tester T-61010K Bisco, USA) using cyanoacrylate glue (Zapit, DVA, Corona, CA, USA). The sticks were then subjected to a tension load at 1 mm/ min cross-head speed until failure occurred. The load at failure in newtons was recorded and the cross-sectional area at the fracture site was measured to the nearest 0.01 mm with a digital caliper to calculate the microtensile bond strength in MPa. The failure mode was also determined as adhesive, cohesive in enamel, cohesive in sealing material, or mixed by examining the fractured stick under a stereomicroscope (Olympus SZ-PT, Japan) at 209 magnification by one calibrated investigator unaware of the identity of the groups.
After drying for 24 h, some of the fractured specimens were mounted on aluminum stubs, sputter-coated with gold–palladium and observed under a scanning electron microscope (SEM) (Jeol 6060, Japan) at 15 kV of accelerating voltage and working distance of 20 mm to take representative photomicrographs (Figs. 1, 2, 3). The SPSS 11.5 program (Statistical Package for Social Sciences, Chicago, IL, USA) was used for all analyses. The normality of data was analyzed with Shapiro–Wilk test. Since whole data indicated non-normal distribution,
Kruskal–Wallis test was used for performing multiple comparisons. The level of significance was set at p \ 0.05.
Results The mean microtensile bond strength values and standard deviations are given in Table 1. The result of the power analysis indicated 99.9 % with statistically different results. The highest bond strength values were obtained in group 5 (3.5-W laser etching ? acid etching), although it was found to be statistically insignificant (p [ 0.05). Microtensile bond strength values of group 1 (acid etching) were significantly higher than those of group 2 (2.5-W laser etching) (p \ 0.05), while no statistically significant difference was detected between groups 1, 3 (3.5-W laser etching), 4 (2.5-W laser etching ? acid etching), and 5 (p [ 0.05). Therefore, the weakest bonds were obtained in group 2 (Fig. 1). With respect to failure mode, mixed type of failure was observed in all samples of all study groups (Figs. 2, 3, 4).
Fig. 1 The plot chart of MPa values. The box represents the distribution of the data between the lower and the upper quartile. The central horizontal line represents the median. The vertical lines extend to the minimum and maximum values
Fig. 2 a SEM image of a fractured specimen from the 2.5-W Er,Cr:YSGG laser etching group (980) revealing mixed type of fracture (M enamel, FS fissure sealant); b SEM image of a fractured
The efficiency and success of pit and fissure sealants have been mainly evaluated by microleakage tests [11, 16, 18], assessment of retention and caries-preventive effects , and bond strength tests [20, 21]. The bond strength of sealant materials to enamel was defined mostly via shear  and conventional tensile tests . Microtensile bond strength is also a preferred method for measuring the bond strength on the interface area between dental materials and tooth structure [21, 23]. The use of small-sized sample sections provides uniform distribution of stress leading a
specimen from the 2.5-W Er,Cr:YSGG laser etching group (9300) revealing mixed type of fracture (M: enamel, FS fissure sealant)
Fig. 3 a SEM image of a fractured specimen from the 3.5-W Er,Cr:YSGG laser etching group (980) revealing mixed type of fracture (M enamel, FS fissure sealant); b SEM image of a fractured
Table 1 The distribution of mean microtensile bond strengths (MPa) and standard deviations according to surface pre-treatment
specimen from the 3.5-W Er,Cr:YSGG laser etching group (9300) revealing mixed type of fracture (M enamel, FS fissure sealant)
Number of sticks (N)
Mean (MPa) ± SD
1 (35 % phosphoric acid etching)
12.18 ± 3.95b
2 (2.5-W laser irradiation)
8.30 ± 1.84a
3 (3.5-W laser irradiation)
11.57 ± 3.27b
4 (2.5-W laser irradiation ? 35 % acid etching)
12.67 ± 4.51b
5 (3.5-W laser irradiation ? 35 % acid etching)
13.04 ± 3.62b
Difference from group 1 is statistically significant (p \ 0.05)
Difference from group 2 is statistically significant (p \ 0.05)
Fig. 4 a SEM image of a fractured specimen from 35 % phosphoric acid etching group (980) revealing mixed type of fracture (M enamel, FS fissure sealant); b SEM image of a fractured specimen from 35 %
phosphoric acid etching group (9300) revealing mixed type of fracture (M enamel, FS fissure sealant)
more accurate evaluation of bond strength at the resin– tooth structure interface. Another advantage of this technique is minimizing the number of teeth required for testing . Enamel surfaces were prepared by grinding flat with silicon carbide papers prior to bond strength testing due to technical difficulties in measuring bond strength of intact
enamel surfaces [22, 24]. Hadad et al.  compared the bond strength of an adhesive system to surface and subsurface enamel after acid etching by microtensile testing and concluded that bond strength of surface enamel was significantly lower than subsurface enamel. This result was attributed to the finding of Poole and Johnson  that the middle enamel layer was prism rich and less mineralized
and had less inorganic material and fluoride than surface enamel. When the enamel surface is ground, a more homogenous enamel prism structure occurs, thereby increasing the efficiency of the etching procedure. On the other hand, taking into account that fissure sealants are applied to surface aprismatic enamel rather than subsurface enamel, whether or not preparing flat enamel surfaces prior to sealant placement will reflect the clinical situation is controversial. Therefore, flat enamel surfaces were not prepared for microtensile testing in an attempt to reflect the clinical situation. Enamel bond strengths achieved in the present study were generally lower than the bond strength values reported in previous studies for adhesive systems and composite resins bonded to enamel surfaces . The possible reasons thought to be responsible for this difference are as follows; prepared enamel surfaces were not ground flat for the present study. Hence, the bonding surfaces contain aprismatic enamel and this may prevent the penetration of the resin sealant. Furthermore, enamel bond strengths achieved with microtensile testing have been reported to be often lower than those of shear or tensile methods . Finally, the material used in the present study was an unfilled fissure sealant and an adhesive system was not used prior to enamel bonding. The results of the previous studies investigating the effect of laser etching on the bond strength of restorative materials remain controversial. Some authors reported that bond strengths obtained with acid etching were significantly higher than those obtained after etching with lasers [27, 28], while others suggested that laser etching produced similar  and even higher bond strengths  than acid etching. This diversity of the previous results may be attributed to the difference in the laser types (CO2, Nd:YAG and Er:YAG lasers) and the power settings preferred. Chimello-Sousa et al.  assessed the etching efficiency of Er:YAG laser in different power settings on permanent enamel. As a result, the bond strength obtained with the combination of Er:YAG laser and acid etching was found to be significantly less than that obtained with conventional acid etching. The authors stated that the chemical alterations of enamel induced by laser etching increased the resistance to demineralization with acids and this might explain the insufficiency of acid etching to effectively remove the laser-altered layer and produce an etching pattern similar to that created by phosphoric acid alone. In the present study, microtensile bond strength values achieved with laser and acid etching combination groups (regardless of different power settings) were not statistically different from those of the conventional acid etching group. The difference in these results may be explained by the difference in the laser types. Moreover, Er,Cr:YSGG laser beam has a pulsed character and this may create unaffected
areas between pulses. Acid etching after laser irradiation enables roughening of the unaffected areas, leading to a more uniform etching pattern and this might explain the statistical similarity of laser (in both power settings) and acid etching combination groups with conventional acid etching group. Usumez et al.  evaluated the effect of Er,Cr:YSGG laser etching on the tensile bond strength of orthodontic brackets in permanent teeth and concluded that laser etching produced statistically similar, but insignificantly less and inconsistent bond strength values compared with conventional acid etching. Bond strength values obtained with 1-W power setting were found to be significantly less than with acid etching, while those of 2 W were found to be statistically similar. These findings are consistent with the results of the present study for 2.5 and 3.5 W. Primary enamel involves an acid-resistant prismless superficial layer making it more resistant to the etching procedure [11, 26]. Therefore, higher power settings were preferred for effective etching of primary enamel similar to a study by Cehreli et al. . Basaran et al.  compared the shear bond strength of orthodontic brackets to Er,Cr:YSGG laser-etched permanent teeth at 0.5-, 1-, and 2-W power outputs with conventional acid etching. As a result, the mean shear bond strength values of all laser groups were found similar to acid etching except for the 0.5-W group and therefore it was concluded that Er,Cr:YSGG laser irradiation might be a good alternative to conventional acid etching. This finding is consistent with the results of the present study. In the present study, only the 2.5-W group had bond strength values significantly less than that in the acid etching group, whereas 3.5-W and both laser ? acid etching combination groups showed bond strength values similar to the acid etching group. As in the present study, laser irradiation and acid etching were not combined in the above-mentioned study. Er,Cr:YSGG laser ablation of dental hard tissues causes micro-explosions leading to macroscopic and microscopic irregularities. The laser energy in this wavelength is absorbed by water molecules and targets the hydroxyl groups in the enamel and dentin. The initial effect on dental hard tissues is evaporation of water and the other hydrated organic components. Internal pressure occurring during evaporation causes removal of inorganic substances by exploding without reaching melting point . This effect of Er,Cr:YSGG laser on enamel forms an irregular etching pattern different from conventional acid etching. This mechanism of effect might explain the diversity between the results of the laser-etching studies. Ramires-Romito et al.  evaluated the microtensile bond strength of various adhesive systems and a conventional fissure sealant. The authors claimed that no
relationship existed between the mode of failure and microtensile bond strength values, and mixed type of failure was the most common failure type in all groups. Cohesive mode of failure in enamel or resin material was not observed in any sample. This finding is in accordance with the results of the present study, indicating that the bond strength of fissure sealants to enamel surface is relatively less than that of the composite resins utilized together with bonding agents. Under normal circumstances, higher bond strength has been known to lead to more cohesive failure rather than adhesive failure . This phenomenon may possibly explain the above-mentioned findings. The results of the present study showed that 3.5-W Er,Cr:YSGG laser etching produced results comparable to conventional acid etching technique, whereas 2.5-W Er,Cr:YSGG laser etching was not able to yield adequate bonding performance in primary enamel for the use of a pit and fissure sealant. On the other hand, laser etching–acid etching combinations exhibited similar microtensile bond strength values compared with the conventional technique, regardless of the power output. Further studies should focus on the short- and long-term clinical outcomes of Er,Cr:YSGG laser etching in primary teeth. Conflict of interest The authors declare that there is no conflict of interests regarding the publication of this paper.
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