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Laser, Resin Cements, Root Canal Therapy, Er, YAG, Bond Strength, IntroductionTeeth with insufficient residual coronal structure, caused by caries lesion, fracture, or extensive cavity preparation can be treated with various endodontic posts. One of the main functions of endodontic posts is to provide retention and stability for favorable coronal restoration [ 1,]. Disadvantages associated with metal posts have led to the development of different esthetic posts among which prefabricated posts have gained popularity due to their adhesive capacity and ability to form a gap-free single unit. Prefabricated posts can be made of zirconia, glass, quartz or polyethylene fibers [ 2,]. To select an appropriate material for prefabricated posts, fracture resistance should be considered as an important physical property. Carbon and glass fiber posts are favorable in this manner [ 3,]. Satisfactory survival rates have been reported in teeth that have been restored with fiber reinforced composite (FRC), posts made of quartz or glass fibers embedded in a matrix of epoxy or methacrylate resin [ 4,].However, considerations for appropriate post selection should not be limited to fracture resistance and must include efficiency of post removal. In several incidences, post removal and endodontic retreatment is required due to unsatisfactory post length, diameter, or apical seal [ 5,]. Different techniques for post removal are selected based on post material and device availability, including burs to drill, ultrasonic vibration, and solvents used with endodontic files [ 6,]. These methods can be relatively challenging, increase the risk of fracture [ 7,], and cause pain and discomfort to the patient [ 8,].Since their introduction in the 1990s, lasers have revolutionized modern dental practice. Lasers can be applied for several purposes, such as diagnostic applications (caries detection), tooth whitening, resin curing, and cavity preparation [ 9,]. Erbium, yttrium aluminum garnet (Er, YAG) laser, is readily used for cutting enamel, dentin, and gingival depigmentation procedures [ 10,], due to its bactericidal effects and ability to cause minimal pain [ 11,]. Moreover, Er, YAG has been used in the removal of glass ceramic brackets and porcelain veneers [ 12,- 14,]. Namely, Oztoprak et al. [ 12,] reported the effectiveness of Er, YAG laser on thermal softening and degradation of adhesives, resulting in debonding of ceramic brackets. Moreover, Morford et al. [ 14,] conducted a study on IPS Empress and IPS e.max porcelain veneers, concluding that the application of Er, YAG laser can be effective in debonding and preserving tooth structure. Despite these developments, the debonding properties of Er, YAG laser on different post materials remains largely unexplored.Therefore, in this study, the effects of Er, YAG laser on debonding cemented glass fiber posts in endodontically treated teeth is investigated. The null hypothesis was that laser irradiation could not reduce the bond strength of glass fiber posts.Materials and MethodA minimum sample number of 36 (power of 80%) was calculated. However, for higher accuracy, 40 samples were recruited and consented for the study. Samples included single-rooted mandibular premolars with the same length, extracted due to periodontal, prosthetic or orthodontic reasons, which were divided into two groups (20 in each group).For disinfection, each tooth was stored in 5% chloramine T (Mina Tajhiz-co, Tehran, Iran) for 48 hours, before placing in distilled water, which was changed every day until examination. Soft tissue and dental plaques were removed from the root surface using periodontal scaler. Two radiographs were taken to assess the anatomic structure. Any tooth with internal/external resorption, two or more roots or canals, calcification, fracture lines and cracks detected by stereomicroscope was excluded from the study. Each tooth was sectioned from the crown with a water-cooled diamond disk (Buehler, Lake Bluff, Illinois, USA) to reach a standard root height of 15 mm.Root canal therapy was performed for each tooth using the step back method, which then obturated using a gutta-percha (DiaDent, DiaDent Group International, Chongju, Korea) and sealer (AH Plus Dentsply De Trey GmbH, Konstanz, Germany), applying the cold lateral condensation method [ 15,]. The access hole was filled with temporary restorative material (Cavisol, Gpl Chai co., Alborz, Iran) and teeth were placed in distilled water.After 24 hours, the gutta-percha was removed using Gates Glidden drills (Dentsply Maillefer, Ballaigues, Switzerland), leaving a 5mm of the gutta-percha at the end of the canal. Fiber posts (Whitepost DC 2 FGM Produtos Odontologicos, Joinville, Brazil) with a height of 20mm, end diameter of 1.8mm and tip diameter of 1.05mm were used in this study. For fiber post placement, the root canals were prepared using drills according to the manufacturer&,rsquo s instructions. The depth of 10 mm was prepared for the post space.The prepared post space then rinsed with 2% chlorhexidine (Drogal Farm&,aacute cia de Manipula&,ccedil &,atilde o Ltda., Piracicaba, Brazil), before drying with paper points (DiaDent, DiaDent Group International, Chongju, Korea). Excess length of the fiber posts were cut outside the canal (by drilling), and then the fibers surface was dipped in ProsilSilane (FGM, Joinville, SC, Brazil) and dried. The fiber posts were cemented using Panavia F2.0 cement (Kuraray Inc., NY, USA) according to the manufacturer&,rsquo s instructions. Er,YAG laser (wave length of 2,940 nm, 20 Hz) was used in the contact mode for 60 seconds. The selected parameters were based on Hoteit et al. [ 16,] to prevent extreme heat generation and decrease crack formation. A pilot study was conducted to determine the best parameters for laserirradiation. Six extra samples were randomly selected for the pilot study. Table 1, presents the tested parameters. According to the resultsof the pilot study, the best effects were obtained using 7 W, interrupted pulse duration and discontinued rinsing spray. For the main study,40 samples were divided into two groups by simple randomization method (using a hexagonal dice). The first group received no laser and thesecond group was irradiated with the laser. The laser tip was placed on the coronal part, exactly on the fiber post, at the cut site.CoolingPulse durationPotencyContinuous water spray(5mL/min)Continuous4 WattContinuous water spray(5mL/min)Continuous6 WattContinuous water spray(5mL/min)Continuous7 WattInterrupted water spray(5mL/min)Interrupted4 WattInterrupted water spray(5mL/min)Interrupted6 WattInterrupted water spray(5mL/min)Interrupted7 WattTable 1. Laser parameters used for the pilot studyPush-out test was used to evaluate tensile resistance. After laser irradiation, 40 teeth (20 premolars in each group) were sectioned under a microscope (Leica S8 APO Leica Microsystems Inc., IL, USA) with a disk (Buehler, Lake Bluff, Illinois, USA) to obtain 1.5 mm thick slices (Figure 1,) according to the penetration depth of Er,YAG laser [ 17,] from the coronal, middle and apical thirds of the root (120 slices in total). The diameter of each cut was measured with a digital caliper(Mitutoyo digital caliper Mitutoyo, IL, USA). For the push-out test, the cut samples were placed on the universal testing machine (Instron, MA, USA),with an empty space under the post. A load with a 1mm/min cross head speed [ 18,] was applied to the post and the load at which the fracture occurred was recorded. Tensile strength was calculated by dividing the load of fracture(N) by bonding area (A), and the value was reported in MPa. Figure 1. Samples cut from 1/3 coronal, 1/3 middle, and 1/3 apical sectionsThe bonding area was calculated according to the equation [ 18, ], [(R+r)*h2 + (R&,minus r)2]where R is the coronal diameter of the post, r is the apical diameter of the post and h is the slice thickness in mm.Five samples from each group were randomly selected, which were prepared for scanning electron microscopy (SEM) analysis. Samples were coated with gold and evaluated using SEM (Zeiss Evo 50 Carl Zeiss, Oberkochen, Germany) at a magnification of &,times 100. Finally, the fracture mode was reported as cohesive fracture in post, cements, dentin, adhesive failure between cement and post, adhesive failure between cement and dentin, and mixed.Statistical analysisThe effects of laser irradiation and location on bond strength were evaluated using two-way ANOVA. As the interaction effect was significant, subgroup analysis was applied. Independent sample t test was used to compare lasered and non-lasered groups in each location and one -way ANOVA and Tukey&,rsquo s HSD post hoc test were used to compare the strength between locations in each group.ResultsIn this study, 20 teeth in each group and 3 cuts on each tooth resulted in 120 samples. Tensile strength measurements are presented in Table 2,. Statistically significant difference in push-out bond strength was noted between the groups. Tensile strengths at different sections were higher in the control group than the laser-irradiated group (p&,lt 0.001). Within group comparison analysis, significant differences in the control group were noted the highest tensile strength was related to the coronal section (10.303 MPa) and the lowest was belonged to the apical section (6.842 MPa). There was a significant difference between coronal and middle sections (p=0.022) and between middle and apical sections (p&,lt 0.001 Table 2,). In the laser-irradiated group, the highest tensile strength was belonged to the coronal section (4.023MPa) and the lowest to the apical section (3.308MPa), which were not significantly different (p= 0.09). Also no significant differences were noted between the coronal and middle (p= 0.680) sections and the middle and apical sections (p= 0.399), as shown in Table 2,. SEM images of the fractures in each site are shown in Figure 2,.Study groupStudied sectionsMean&,plusmn SDMinimum- MaximumComparison withMean differencep ValueLower limitUpper limitLaser irradiated group 1/3 coronal4.023&,plusmn 0.9481.222-6.109Compared with middle0.2801500.680&,minus 0.522991.08329Compared with apical0.7152000.090&,minus 0.087941.518341/3 middle3.743&,plusmn 1.1191.127-5.218Compared with coronal&,minus 0.2801500.680&,minus 1.083290.52299Compared with apical0.4350500.3990.368091.238191/3 apical3.308&,plusmn 1.0901.056-5.157Compared with coronal&,minus 0.7152000.090&,minus 1.518340.08794Compared with middle&,minus 0.4350500.399&,minus 1.238190.36809Control group1/3 coronal10.303&,plusmn 1.1397.805-12.259Compared with middle0.895600*,0.0220.110451.68075Compared with apical3.461650*,&,lt 0.0012.676504.246801/3 middle9.408&,plusmn 0.9727.231-11.647Compared with coronal0.895600*,0.022&,minus 1.68075&,minus 0.11045Compared with apical2.566050*,&,lt 0.0011.780903.351201/3 apical6.842&,plusmn 0.9744.029-8.230Compared with middle&,minus 3.461650*,&,lt 0.001&,minus 4.24680&,minus 2.67650Compared with coronal&,minus 2.566050*,&,lt 0.001&,minus 3.35120&,minus 1.78090*Significant at &,lt 0.05, according to the results of t-test |