Effect of dentin exposure on the shear bond strength of cast porcelain veneers

Objective To explore the best indication for veneers and to improve the repair success rate by investi⁃ gating the effects of different types of dentin exposure on the shear bond strength of cast porcelain veneers with two new veneer bonding systems. Methods Bonding interfaces with 0%, 25%, 50%, 75% and 100% dentin exposure were de⁃ signed and fabricated. The bonding interfaces were divided into groups A, B, C, D and E. Sixty 4⁃mm x 4⁃mm x 2⁃mm (length x width x thickness) ceramic specimens were bonded by using two bonding systems, The VN⁃A, VN⁃B, VN⁃C, VN⁃D and VN⁃E groups were bonded with Variolink bonding system, and the PF⁃A, PF⁃B, PF⁃C, PF⁃D and PF⁃E were bonded with Panavia F bonding system (six specimens per group). The bonded specimens were stored in a distilled water bath at (37 + 1)℃ for 24 hours. The fracture load was tested by a universal testing machine, and the fracture type was observed by scanning electron microscopy. Results The VN⁃A (25.14 ± 3.40 MPa), VN⁃B (22.54 ± 4.48 MPa), VN⁃C (19.59 ± 2.21 MPa), PF⁃A (20.61 ± 2.42 MPa), PF⁃B (18.08 ± 4.11 MPa), PF⁃C (17.06 ± 2.29 MPa) groups’shear bond strengths were above 17 MPa. The VN⁃A group had the highest shear bond strength value. There was no statistically significant difference in bond strength between the VN⁃A group and the VN⁃B and VN⁃C groups(P > 0.05) or the PF⁃A and PF⁃B groups (P > 0.05); however, the differences between VN⁃A and the VN⁃D and VN⁃E groups (P < 0.05) and between PF⁃A and the PF⁃C and PF⁃D and PF⁃E groups (P < 0.05) were statistically significant. The differences between the VN⁃A group and PF⁃A group (P < 0.05) were statistically significant. The fracture modes of the VN⁃A, PF⁃A, VN⁃B, PF⁃B, and VN⁃C groups mainly included resin cement cohesive failure and mixed failure; the VN⁃D, VN⁃E, PF⁃C, PF⁃D and PF⁃E groups were dominated by interface failure and mixed failure. Conclusion When the dentin exposure is greater than 50%, the shear bond strength value of the veneer was significantly affected. To ob⁃ tain a better clinical effect, the dentin exposure rate should be less than 25%.