The effect of an Er,Cr:YSGG laser on the micro-shear bond strength of composite to the enamel and dentin of human permanent teeth

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1 Lasers Med Sci (2012) 27: DOI /s ORIGINAL ARTICLE The effect of an Er,Cr:YSGG laser on the micro-shear bond strength of composite to the enamel and dentin of human permanent teeth Zahra Jaberi Ansari & Reza Fekrazad & Saideh Feizi & Farnaz Younessian & Katayoun AM Kalhori & Norbert Gutknecht Received: 30 November 2010 / Accepted: 29 June 2011 / Published online: 2 August 2011 # Springer-Verlag London Ltd 2011 Abstract The bond strength of resin composite to Er,Cr: YSGG laser-irradiated enamel and dentin has been evaluated in only a few studies. Therefore, we measured and compared the micro-shear bond strength of composite restorations to enamel and dentin using two different cavity-preparation tools and conditioning methods. One hundred and seventy-five caries-free human third molars were sectioned longitudinally into two different thicknesses and randomly assigned to seven subgroups (n=25). Enamel groups included laser-cut without etching (LO), laser-cut Z. Jaberi Ansari Iran Center for Dental Research, Dental School of Shahid Beheshti University of Medical Sciences, R. Fekrazad (*) Dental Department, AJA University of Medical Sciences, dr_rfekrazad@yahoo.com S. Feizi Department of Restorative Dentistry, School of Dentistry, Shahid Beheshti University of Medical Sciences, F. Younessian Dental School, Shahid Beheshti University of Medical Sciences, R. Fekrazad : K. A. Kalhori Laser Research Center of Dentistry, Tehran University of Medical Sciences, N. Gutknecht University of Aachen Medical Faculty, Clinic of Conservative Dentistry, Aachen, Germany and laser-etched (LL), laser-cut and acid-etched (LA), burcut and laser-etched (BL1), and bur-cut and acid-etched (BA1-comparison group). Dentinal groups included burcut and laser-etched (BL2) and bur-cut and acid-etched (BA2-comparison group). The specimens were bonded by Single Bond and Tygon tubes and were restored with Z100 composite. Failure patterns were evaluated using a stereomicroscope, and a shear bond test was performed at 0.5 mm/min. The mean shear bond strength values (MPa) for the LO, LL, LA, BL1 and BA1 enamel groups were 23.14, 23.77, 23.51, 19.30, and 28.99, respectively, whereas for the BL and BA dentinal groups, these values were and 26.15, respectively. In enamel specimens, BA1 and LL groups presented the highest shear bond strength values, and the bur-cut and laser-etched (BL1) group showed the lowest values. In the laser-etched groups, bond strength values for bur-cut surfaces were significantly higher than those for laser-cut surfaces. Moreover, there was a significant difference between the BL2 and BA2 dentinal groups. The results of this study indicate that re-etching with acid phosphoric would be recommended if an Er,Cr:YSGG laser is used for tooth preparation or surface treatment. Keywords Er,Cr:YSGG. Shear bond strength. Composite Introduction Despite improvements to resin composite formulations over the years, polymerization shrinkage of the resin metric is still considered highly relevant to the failure of direct resin composite restoration [1]. Therefore, pretreatment of the tooth surface is essential for establishing a strong bond between the resin and both the enamel and dentin [2].

2 762 Lasers Med Sci (2012) 27: Acid etching has been the standard approach for enamel pre-treatment since the publication of Bounocuore s report. Effective adhesion to enamel has been achieved with relative ease and has repeatedly proven to be a durable and reliable clinical procedure for routine applications in modern adhesive restorative dentistry [3]. In cavity preparation, bonding of resin to enamel is achieved via micromechanical retention on the roughened surface, whereas the retention to dentin is based mainly on hybrid layer formation and, to a lesser degree, on the micromechanical retention offered by the resin tags embedded in dentin [4]. Therefore, the formation of a hybrid layer and resin tags is essential for the establishment of a strong bond at the dentin level [5, 6]. One way of achieving this strong bond is the complete dissolution of the smear layer and demineralization of intertubular and peritubular dentin by means of acid etching, resulting in an exposed collagen matrix that is infiltrated by resin that polymerizes in situ. In recent years, there has been a growing debate on the use of lasers for various applications in dentistry including cavity preparation due to their efficient removal of dentin and enamel [7 14]. The prerequisites for conditioning the hard tooth tissues are best met by pulsed erbium lasers due to their thermomechanical ablation process that produces, like the etching process, a rough structure in dental tissue. Consequently, the primary indication for these systems is the preparation of cavities for composite restorations [13, 15]. Previous articles have mainly dealt with the Er:YAG laser, and to date, there have been few investigations to determine the micro-shear bond strength of the cavities prepared with an Er,Cr:YSGG laser. Therefore, the purpose of this study was to measure and compare the micro-shear bond strength of composite restorations to human enamel and dentin with two different cavity preparation tools (bur versus Er,Cr:YSGG laser) and with two different conditioning methods (acid etching versus Er,Cr:YSGG laser). Materials and methods One hundred and seventy-five recently extracted human third molars that displayed neither caries nor fillings were collected and stored in 0.5% chloramines-t solution for a week. These molars were subsequently pumiced and washed. The enamel specimens were prepared in two different thicknesses from buccal and lingual surfaces of the samples using a thin-sectioning machine (Hamco machine, IW, Rochester, NY, patent pending). The enamel was also removed from the occlusal surface of specimens in groups that underwent cooled high-speed headpiece preparation (W&H Topair, 190BC, Austria) with a fissure diamond bur (Kavo, USA) and were subsequently abraded with 400-grit silicon carbide (SiC) paper (Matdor, Germany) until a uniform layer of peripheral dentine was observed. The Er, Cr:YSGG laser that was used was the Waterlase from Biolase technologies (San Clemente, CA, USA), which operates at a wavelength of 2.78 µm and has a pulse duration of 140 µs with a repetition rate of 20 Hz. Laser energy is transported through a fiber optic system to a G6-tip terminal that is 0.6 mm in diameter and 6 mm long. The average output power can be adjusted from 0 to 6 W depending on the tissue to be cut. Three different power settings were used in this study: 5.5 W (85% water and 95% air spray) and 1.5 W (55% water and 65% air spray) were used for enamel preparation and surface treatment, respectively, and 1 W (55% water and 65% air spray) was used for dentinal etching. The teeth were sectioned longitudinally to obtain two halves of different thicknesses and were randomly assigned to seven subgroups of equal size (n=25) according to their thickness and surface. Enamel groups included laser-cut without etching (LO), laser-cut, and laser-etched (LL). In this group, we also etched the surface with laser after cutting it, laser-cut and acid-etched (LA), bur-cut and laser-etched (BL1), and bur-cut and acid-etched (BA1; as a comparison). Dentinal groups included bur-cut and laser-etched (BL2) and bur-cut and acid-etched (BA2; control) (Table 1). Two uniform layers of adhesive (Single Bond, 3M ESPE Dental Products, USA) were applied to the surface of the specimens with a brush. Tygon tubes (R-3603, Norton Performance Plastic Co., Cleveland) that were 0.8 mm in diameter were placed on the treated tooth surface and lightcured for 20 s. Tubes were subsequently filled with Z100 composite (3 M, Dental Product, USA) to a 0.5-mm height and were light-cured for 40 s. The samples were stored in distilled water at 37 C for 24 h before and after the test. Failure patterns were observed using a stereomicroscope (Olympus Optical Co. Ltd, Model SZX111 b 2000 Japan) at 12.5 magnification. The shear bond strength was then measured by a micro-shear bond tester (Bisco Inc., USA) at a cross-head speed of 0.5 mm/min. Data were subjected to Tukey and t test at the 95% confidence level to examine the effects of preparation type on each of the two substrates (enamel and dentine). Data were subjected to Tukey and t test at the 95% confidence level to examine the effects of the preparation method and each method on the two substrates (enamel and dentine) separately. Furthermore, the Kolmogorov-Smirnov test was used to verify the normal distribution of data in each group. Results Mean shear bond strength data of composite bonded to conventionally prepared or Er,Cr:YSGG laser-prepared

3 Lasers Med Sci (2012) 27: Table 1 Experimental groups Group Type of specimens Thickness of cut Preparation type Surface treatment LO Enamel 3 mm Er,Cr:YSGG laser None LL Enamel 3 mm Er,Cr:YSGG laser Er,Cr:YSGG laser LA Enamel 3 mm Er,Cr:YSGG laser Acid BL1 Enamel 1.5 mm Bur Er,Cr:YSGG laser BA1 Enamel 1.5 mm Bur Acid BL2 Dentin 1.5 mm Bur Er,Cr:YSGG laser BA2 Dentin 1.5 mm Bur Acid dentin and enamel that was etched with either 37% phosphoric acid or laser-etched at 1 or 1.5 W are presented in Tables 2 and 3. Among the enamel groups, the bur-cut, acid-etched group (group BA1) yielded the highest mean debonding force (28.99 MPa), which was followed by the laser-cut and laser-etched group (LL; MPa) and the laser-cut and acid-etched group (LA, MPa). ATukey test of couples also revealed significant differences between BL1 and LL, LA and BL1, BL1 and BA1, BA1 and LO, and LL and BA1 as shown in Table 4. However, after performing a Kolmogorov-Smirnov test, which revealed the normal distribution (p>0.001) of data in each group, a parametric Student s t test was used to compare two subgroups of dentinal groups, and a one-way ANOVA according to Leven's test assumption of equality of variances was used to compare subgroups of the enamel groups. Based on the significant results of the ANOVA analysis, the Tukey test was used for pair-wise comparisons of the data from the enamel groups. Discussion Regarding the results of the t test, there were significant differences in the mean shear bond strength between the two dentinal groups (BA2 and BL2), indicating that the mean shear bond strength of group BA2 was significantly higher than that of BL2 (p<0.01). Enamel preparation by the Er,Cr:YSGG laser results in a characteristically chalky surface. Scanning electron microscopic images showed that laser irradiation produces a surface that increases the restorative material retention, which in turn makes the surface suitable for the application of composite and composer filling materials [16, 17]. Some studies have assessed the ability of different settings of erbium lasers to improve marginal seal and bond strength, reporting a wide range of results [18]. Line et al. found that the bond strength for nonetched enamel was much higher for the Er,Cr:YSGG laser-cut surfaces compared to bur-cut surfaces, although no difference was reported between the two dentinal groups [19]. Remos et al. reported that all lased subgroups showed a decrease in bond strength compared with control groups, especially when using single-bottle bonding agents [20]. Regarding the determination of laser wavelength, Usumez and colleagues recently published two studies on the bonding of orthodontic brackets and porcelain veneers to lased enamel surfaces [21, 22]. They found that the results of enamel conditioning with the Er,Cr:YSGG laser at a power output of 2 W (20 Hz, 100 mj) were similar to those of acid etching. Cutting the power by half (20 Hz, 50 mj) significantly decreased the bond strength of the irradiated surface compared to acid etching; however, individual results varied greatly in each case. Thus, they suggested that the Er,Cr:YSGG laser by itself cannot be considered a successful alternative to conventional methods of increasing the bond strength to enamel. In their most recent study, Usumez and Aykent pointed out that porcelain laminate veneers bonded to laser-etched tooth surfaces has a bond strength that is equivalent to orthophosphoric acid- or maleic acid-etched tooth surfaces [22]. In the LO group, in which the enamel surfaces were laser-cut and unetched, the shear bond strength was significantly less than that of surfaces in the bur-cut and Table 2 Descriptive statistics of the mean shear bond strength (MPa) of the enamel groups Group Mean SD SE Min. level of bond strength Max. level of bond strength LO LL LA BL BA

4 764 Lasers Med Sci (2012) 27: Table 3 Descriptive statistics of the mean shear bond strength (MPa) of the dentin groups Group n Mean SD SE Min. level of bond strength Max. level of bond strength BL BA acid-etched (control) group (p<0.005). Yu et al. claimed that the ablation of enamel with the Er,Cr:YSGG laser could produce a rough structure and subsequently increase the bond strength of composite restorations [23]. However, the laser irradiation of enamel surfaces produced surface fissures and a union or blending of a distinctive etch pattern normally seen in acid-etched enamel. This blending effect likely prevented the penetration of resin into enamel, resulting in lower enamel bond strength values [24]. The laser-cut and -etched group (LL) showed a significantly higher mean shear bond strength (23.77 MPa) compared to the non-etched, laser-cut surfaces; however, these values were still lower than those of acid-etched enamel, and there was still a statistically significant difference compared to the control group. These results are in agreement with the findings of Delme et al. who postulated that the acid-etching procedure is essential after laser ablation [25]. Interestingly, the subsequent addition of acid etchant to laser-cut enamel (the LA group) produced a delicate etch pattern that assumed the appearance of a more retentive surface than that created by laser etching alone [25 27]. The minimum level of shear bond strength was attributed to bur-cut and laser-etched enamel specimens (the BL1 group) that showed a significant difference compared to the control group. In this study, a power of 1.5 W (55% water and 65% air spray) was used for enamel etching. These results do not correspond with those of Usumez, in which the etching procedures were performed using laser ablation with a power level of 1 and 2 W [22]. Table 4 The result of couple comparisons of the shear bond strengths between the enamel groups Groups Groups Significance level LO LL LA BL BA LL LA BL BA LA BL BA BL1 BA This disparity may be due to a wide range of standard deviations in laser groups, making it difficult to draw conclusions by comparing these results. However, bur-cut and acid-etched enamel specimens (BA1) showed the highest level of mean shear bond strength among enamel subgroups. This result is in agreement with the result obtained in the investigations of Lin et al. [19]. Furthermore, when considering bonding to irradiated dentin surfaces, the odontoblastic tubules are opened up during erbium family laser procedures, and therefore, laserprepared dentin revealed surface scaling that may lead to flaking and peritubular cuffing. This unusual appearance of laser-irradiated dentin can be understood by gaining insight into the ablation process. Lin et al. explained that the Er,Cr: YSGG laser ablates hard tissues thermomechanically by making micro-explosions within inorganic structures in teeth [19]. Consequently, we did not consider the processes related to laser preparation in this study [27]. Although some previous investigations have reported that the bond strength of composite to laser-etched, bur-cut dentin (BL2) was higher than that of acid-etched, bur-cut dentin (BA2), data from our study as well as other studies demonstrate that the bond strength is significantly weaker when tooth surfaces are prepared with the Er,Cr:YSGG laser compared to a diamond bur [26]. Sennou et al. proposed that the ablation of dentin fuses collagen fibrils together, resulting in a lack of interfibrillar space that restricts resin diffusion into the subsurface of intertubular dentin [28]. A cross-sectional observation of resin bonded to laser-ablated dentin revealed a lack of penetration of the resin and even peeling off of the resin layer from the ablated dentin surface, which supports the theory of resin restriction. However, acid etching procedures improve micromechanical bonding by completely removing nonorganic material and exposing collagen fibers [29]. Based on these diverse results from previous bond strength studies, several issues are worth noting. The diversity of the results may be due to different factors including the physical parameters of the laser, the material used for filling the cavities, or the combination of the laseretched surface with or without acid etching. Conclusions The micro-shear bond strength of groups prepared by burcutting and acid-etching were higher than that of groups

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