Evaluating the Microshear Bond Strength of an MDP-Containing Universal Adhesive Compared with Its Predecessor to Three Different Generations of CAD/CAM Fabricated Zirconia Ceramics

The long-term success of zirconia restorations depends on the durability of the resin–zirconia bond. This study evaluated the microshear bond strength (μSBS) of an MDP-containing universal adhesive, Scotchbond Universal Plus Adhesive, compared with its predecessor, Scotchbond Universal Adhesive, when bonded to three generations of CAD/CAM-fabricated zirconia ceramics. Sixty zirconia specimens were fabricated from 3Y-TZP (IPS e.max ZirCAD), 4Y-PSZ (IPS e.max ZirCAD MT), and gradient zirconia (IPS e.max ZirCAD Prime; 3Y-TZP/5Y-PSZ). Following polishing, sintering, and airborne-particle abrasion with 50 μm Al2O3 particles, specimens received either Scotchbond Universal Plus Adhesive or Scotchbond Universal Adhesive. Resin cement cylinders were bonded with a dual-cure resin cement and subjected to thermal cycling (20,000 cycles; 5–55 °C) prior to μSBS testing. Data were analyzed using one-way ANOVA, independent-samples t-tests, and Fisher’s exact test (α = 0.05). Significant differences in μSBS were observed among zirconia generations for both adhesive systems (p < 0.05). The highest bond strengths were recorded for 4Y-PSZ bonded with Scotchbond Universal Adhesive (15.36 ± 3.00 MPa) and Scotchbond Universal Plus Adhesive (14.67 ± 4.52 MPa), whereas the lowest values were obtained for 3Y-TZP (8.91 ± 3.53 MPa and 9.20 ± 3.52 MPa, respectively). No significant differences were detected between the two adhesive systems within any zirconia generation (p > 0.05). Failure mode analysis revealed predominantly adhesive and mixed failures, with no significant differences in the distribution of failure modes among groups (p > 0.05). In conclusion, zirconia generation significantly influenced bond strength, whereas Scotchbond Universal Plus Adhesive demonstrated bonding performance comparable to that of Scotchbond Universal Adhesive after thermocycling. Among the tested substrates, 4Y-PSZ exhibited the most favorable bonding performance.