Shear Bond Strength of CAD/CAM Highly Translucent Zirconia Bonded to Occlusal Dentin after Different Extrinsic Modifications: An In vitro Study

The bond between highly translucent zirconia (HTZ) onlay restorations and the resin cement is challenging due to its non-glassy structure. This study compares the effect of different extrinsic modifications on the shear bond strength of HTZ to occlusal dentin after thermocycling.

Forty human molars were prepared and stored in 0.5% chloramine. Ceramic discs (n = 40) were distributed into four groups. HTZ discs underwent airborne-particle abrasion (Group ZA), 30 μm diamond abrasion (Group ZB), and no treatment (Group ZN). All HTZ discs were treated with 10-MDP primer. Lithium disilicate discs (Group LD-control) were treated with about 4.5% hydrofluoric acid and silane. All discs were cemented to dentin using resin cement. SBS was measured using a universal testing machine, and fracture types were assessed.

Group ZA showed the greatest SBS among HTZ groups (Tukey’s test; p-value < 0.05), though none exceeded the LD-control group (p-value < 0.0001). Most fractures occurred at the zirconia-resin cement interface, relatively less in group ZA (40%).

The current study highlights that combined mechanical and chemical treatment has the potential to strengthen the bond of HTZ to occlusal dentin. Moreover, it includes both the resin-zirconia and resin-dentin interfaces in the experiment that closely simulates the intracoronal restoration. One of the limitations is that the results may vary in the oral environment during the clinical trials because of external factors, such as saliva, food, and masticatory forces. Use of thermo-mechanical ageing could have overcome this limitation.

Combined mechanical and chemical treatment improved HTZ bonding to dentin. Further clinical studies are needed to establish optimal surface modification protocols.