الفهرس 
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Abstract
The objective of this study was to compare and evaluating the bond strength of three different ceramic materials; lithium disilicate, ultra-translucent zirconia after and hybrid ceramic Er,Cr:YSGG laser debonding.
Forty-two extracted human molars, fixed in acrylic resin block, its buccal surface were ground to expose dentin to provide flat surface for bonding then stored in distilled water at room temperature. Three different ceramic materials used in this study, lithium disilicate glass ceramics (IPS e.max CAD by Ivoclar Vivadent), hybrid-ceramic (Vita Enamic by VITA Zahnfabrik), ultra-Translucent Zirconia (BruxZir®).
A total of forty-two square shaped Samples 4x4mm & 1mm thickness resembling crowns prepared from CAD/CAM (computer aided design/computer aided manufacture) ceramic blocks of these ceramic materials, 14 sample each material in two subgroups; subgroup A (n=7), and subgroup B (n=7) samples each. Ceramic samples of both subgroups bonded to dentin using dual cure self-adhesive resin cement (TOTALCEM ITENA, France), and standard and recommended surface treatment by manufacturer for each material.
Laser debonding of ceramic samples of subgroups (B) of all three materials using Er,Cr:YSGG laser (Waterlase by Biolase), wave length 2780nm, using the following settings; 6W power, 20Hz frequency, 80% water and 60% air.
All samples of subgroups (B) of each material which debonded by laser, cleaned from residual cement, surface retreated and rebonded with the same steps done before laser debonding.
Shear bond strength test using universal testing machine were done for the three materials both subgroups. The failure mode was analyzed as adhesive, mixed and cohesive.
Two-way analysis of variance (2-Way ANOVA) was used to compare the mean bond and rebond strength of all three materials. Statistical analysis was performed with IBM® SPSS® Statistics Version 26 for Windows. The significance level was set at P ≤ 0.05.
Samples of hybrid ceramics VITA ENAMIC were all damaged by laser application while conducting pilot study so excluded from the study and discussed as separate finding.
Two-Way ANOVA showed that ceramic type had a significant effect on the rebond strength to dentin after laser debonding.
The results were the mean SBS in lithium disilicate bond group was 6.54±1.59 MPa, and 6.43±2.28 MPa for the ultra-translucent zirconia, and there was no statistical significance between them. There was significant difference in rebond SBS for both ceramic materials lithium dislicate and ultra-translucent zirconia, rebond strength mean values were 1.12±0.05MPa for e.max and 4.08±1.88 MPa for the BruxZir group, and there was statistical difference between the two re-bond groups.
Conclusion:
Within the limitations of this study, the following conclusions could be drawn:
1- The use of Er,Cr:YSGG laser for debonding lithium disilicate and ultra-translucent zirconia decreased the rebond strength.
2- The most frequent type of failure in this study was adhesive between the ceramic sample and tooth surface at dentin resin interface.
3- Using Er,Cr:YSGG laser for debonding of hybrid ceramics Vita Enamic is not recommended as it causes severe damage to the ceramic material.
Clinical implications:
Using Er,Cr:YSGG laser is safe for removal of lithium disilicate and ultra-translucent zirconia restorations, but improve bonding is needed, and not safe to use for removal of resin infiltrated hybrid-ceramics.
Recommendations:
Further studies are recommended using other types of laser with different parameters. As well as aging of samples and using full contour crowns can be done. Furthermore, using different types of dual-cured self-adhesive resin cement or using different types of bonding systems are recommended. Beside that studying the effect of laser on dentin after debonding of ceramics is recommended.