Effects of surface treatments on bond strength of laminate veneers to tooth surfaces prepared in different depths

Effects of Surface Treatments on Bond Strength of Laminate Veneers to Tooth Surfaces Prepared in Different Depths The aim of this in vitro study is to evaluate the shear bond strength of a light-cure resin cement block cemented on teeth with three different laminate veneer preparation depths (0 mm, 0.5 mm, 0.7 mm) and four different surface treatments (37.5 % phosphoric acid, femtosecond laser, acid+laser, laser+acid). A total of 204 exracted human maxillary central teeth without restoration, fracture or caries were used in this study. One hundred and eighty teeth were subjected to shear bond strength test and 24 teeth were examined with SEM. One hundred and eighty teeth were mounted individually in an acrylic resin using metal molds then randomly divided into 3 groups (n=60). Porcelain laminate veneer preparations in different depths (0 mm (non-prep.), 0.5 mm, 0.7 mm) were done. Then each group were randomly divided into 5 subgroups (n=12) according to surface treatments (control, acid, femtosecond laser, acid+femtosecond laser, femtosecond laser+acid). After finishing surface treatments, light-cure resin cement blocks (2.3 mm in diameter and 3 mm in height) were cemented on teeth surface with a total-etch adhesive system. After bonding procedures, all specimens were stored in distilled water at 370C for 24 hours and shear bond strength measurement was performed after thermocycling (50-550C, 1000 cycle). To evaluate the shear bond strength, the specimens were tested using an universal testing device. The composite resin cement blocks were loaded to failure using a notched loading head at a cross head speed of 0.5 mm/min. Failure mode of each specimen was examined under an optic microscope at x10 magnification. After the examination of failure modes sagittal sections were taken from the specimens under water cooling using a diamond saw and the bonding areas of resin cement blocks (enamel, dentin, enamel-dentin complex) were examined under optic microscope at x20 magnification. After the laminate veneer preparation was done and surface treatments were applied to 12 of the specimens which reserved for SEM analysis, the surface topographies of these samples were examined. The other reserved 12 samples were prepared in the same way and resin cement blocks were cemented on tooth surface and then the sagittal sections were taken from the samples and the adhesive-tooth interfaces were examined with SEM. The data obtained were analyzed by 2-way ANOVA and Tukey HSD tests. According to surface treatments, the highest bond strength values were recorded in acid etched groups (17.88 MPa), while the lowest values were recorded in laser+acid etched groups (10.38 MPa) after the control group (7.58 MPa). The difference between acid etched groups and all groups except acid+laser group (p>0.05) was found statistically significant (p<0.05). The difference between negative control group, laser and laser+acid etched groups was not found statistically significant (p>0.05). According to the preparation depths, the highest bond strength values were observed in non-prep. groups (15.79 MPa), while the lowest bond strength values were observed in 0.7 mm preparation depth groups (9.09 MPa) (p<0.05). While the highest bond strength values were recorded in the non-prep. and acid etched group (24.25 MPa), the lowest bonding values were recorded in the control group prepared at 0.7 mm depth (6.18 MPa) followed by the group prepared at 0.7 mm depth and etched with laser+acid (6.35 MPa). According to the fracture mode analysis, cohesive failures was most common in non-prep. and acid etched group (9/12, % 75), while the adhesive failure was most common in the negative control group prepared at a depth of 0.7 mm (10/12, % 83.33). SEM images also showed a regular hybrid layer and a continuous, homogeneous bonding interface in the acid etched groups, supporting the shear bond strength test results. With the laser parameters determined in this study, femtosecond laser application was not found as effective as 37.5 % phosphoric acid etching in improving the bond strength of the resin cement cemented on the tooth surface prepared for PLV. Keywords: Porcelain laminate veneer, acid-etching, femtosecond laser, bond strength.