Zirconia and titanium alloys are prevalent in prosthetic dentistry, requiring careful surface preparation (grit-blasting) for optimal micromechanical retention. Functional phosphorylated monomers in resin cements enhance chemical adhesion, while silane application is ineffective due to the absence of silica in these substrates. Three main bonding strategies exist: (1) universal primer, (2) universal adhesive, and (3) self-adhesive resin cements. Resin cements can be self-cured, light-cured, or dual-cured. The study aimed to compare the SBS of one extraoral and several intraoral resin cements on zirconia and titanium alloy under self-curing conditions and to measure their DC under both self-cure and dual-cure modes. The null hypotheses were that there would be no difference in SBS between protocols and no difference in DC between cements under different cure modes.

Existing literature highlights the importance of surface treatments and the role of functional monomers (like 10-MDP) in enhancing the bond strength of resin cements to zirconia and titanium alloys. Studies have explored different bonding strategies, including the use of universal primers and adhesives, but the relative efficacy of intraoral versus extraoral resin cements under various curing conditions remains less clear. A limited number of studies have directly compared bond strengths, particularly focusing on self-curing modes that can be clinically relevant when light curing is challenging.

Nine bonding protocols (Table 2) were tested on zirconia and titanium alloy specimens (n=180 total). Specimens were grit-blasted with Al2O3 (50 µm) for 10 s at 2 bars. A 3 mm-high cylinder of resin cement (7 mm²) was created using a Teflon mold. Self-curing was performed in the dark under 50 g pressure for 60 min, followed by 24 h storage in water at 37 °C. Shear bond strength was measured using a universal testing machine (LRX, Lloyd Instruments). Failure modes were classified (cohesive, adhesive, mixed). For DC measurements, cylindrical specimens (n=6 per cement) were made; 3 with light activation (60 s at 1200 mW/cm²) and 3 self-cured. DC was determined using FTIR spectroscopy by measuring changes in the height ratio of aliphatic and aromatic C=C peaks (formula provided). Statistical analysis included Shapiro-Wilk test, Levene test, one-way ANOVAs (SBS), and two-way ANOVA (DC), followed by post-hoc tests (Tukey’s). Significance level was p<0.05.

The extraoral resin cement, Monobond Plus + Multilink Hybrid Abutment (MP + MHA), showed significantly higher SBS values on both titanium alloy (35.1 MPa) and zirconia (32.9 MPa) compared to other groups (Table 3). Use of universal primers (GMP, MP) generally improved bond strength, particularly on titanium alloy. However, universal adhesives did not significantly improve bond strength on zirconia. Adhesive failures were observed across all groups, with no significant differences in failure occurrence. Table 3 presents the SBS data. Concerning DC, dual-cure mode resulted in significantly higher DC values compared to self-cure mode for all resins (Table 4). Nexus Universal (N-U) showed the highest overall DC (78.4%), whereas Totalcem (TTC) had the lowest (60.3%). There was no significant correlation between DC and SBS.

The superior performance of MP + MHA is likely due to the use of a specific primer (Monobond Plus), containing functional monomers (10-MDP) that have high affinity for zirconia and titanium. Universal primers generally showed better performance than universal adhesives used as primers, potentially because of the specific formulation and synergistic effects of monomers. Variations in the performance of intraoral self-adhesive cements are attributed to differences in their composition, specifically regarding the presence or absence of 10-MDP or GPDM. The higher DC observed in dual-cure mode is consistent with prior research, attributed to improved cross-linking due to both chemical and photopolymerization. This highlights the significance of dual-curing to maximize mechanical properties and long-term stability.

This in vitro study demonstrates that extraoral resin cement with a universal primer provided the highest bond strength to zirconia and titanium alloy. While some intraoral dual-cure cements showed comparable performance with universal primers, the results highlight variations among different brands and formulations. No correlation between DC and SBS was found. Dual curing is advised to improve DC, but further clinical research is needed to validate these findings.

The in vitro nature of the study limits the direct extrapolation of results to clinical situations. Long-term aging studies are needed to assess the long-term stability of the tested adhesive systems. Additional studies are warranted to evaluate the behavior of these self-adhesive cements on dental tissues.