In vitro shear bond strength over zirconia and titanium alloy and degree of conversion of extraoral compared to intraoral self-adhesive resin cements

Zirconia and titanium alloys are widely used in prosthetic dentistry and require surface pretreatments (e.g., grit-blasting) and functional phosphate-based monomers (e.g., 10-MDP, GPDM) to enhance chemical adhesion. Silane is ineffective on these oxide materials due to lack of silica. Current bonding approaches include: (1) applying a universal primer containing functional phosphorylated monomers before resin cement; (2) using a universal adhesive in place of a prosthetic primer before resin cement; or (3) directly using a resin cement formulated with functional monomers. Zirconia bonding is performed intraorally to tooth structure or extraorally to Ti-base abutments, with some cements dedicated for intraoral use and others for extraoral use. Polymerization may be self-, light-, or dual-cure; many clinical/laboratory situations limit effective light curing, necessitating reliable self-cure performance. The study aimed to evaluate SBS of one extraoral and multiple intraoral resin cements to zirconia and titanium alloy across nine bonding protocols in self-curing mode (least favorable polymerization), and to measure their DC in self-cure and dual-cure modes. The null hypotheses were that: (1) no SBS differences exist between protocols for each substrate; and (2) no DC differences exist among cements between self- and dual-cure modes.

Design and materials: Nine bonding protocols were tested on two substrates: zirconia 4Y-TZP (Katana Zirconia STML) and titanium alloy Ti-6Al-4V, yielding 18 groups (n=10 per group; total n=180). Seven resin cements were evaluated: one extraoral (Multilink Hybrid Abutment, MHA) and six intraoral (G-CEM One, GC-O; SpeedCem Plus, SCP; RelyX Universal, R-U; Panavia SA Universal, PSA-U; Nexus Universal, N-U; Totalcem, TTC). Universal primers/adhesives included Monobond Plus (MP), G-Multi Primer (GMP), Scotchbond Universal Plus (SBUP), Clearfil Universal Bond (CUB), and Optibond Universal (OBU). Sample preparation: Zirconia and titanium blocks were embedded in self-curing acrylic resin and ground with 800-grit SiC under water to a flat surface (>7 mm²). All samples were grit-blasted with 50 µm Al2O3 for 10 s at 2 bar, rinsed, cleaned with 99% ethanol, and dried. Bonding procedures: A Teflon mold defined a resin cement cylinder (diameter 3 mm; height 3 mm; base area 7 mm²). Protocols followed manufacturers’ instructions: MP+MHA; GC-O; GMP+GC-O; MP+SCP; SBUP+R-U; PSA-U; CUB+PSA-U; OBU+N-U; TTC. All cements were allowed to self-cure in the dark under 50 g pressure for 60 min, mold removed, excess trimmed, and specimens stored in demineralized water at 37 °C for 24 h. Shear bond strength testing: SBS measured using a universal testing machine (LRX, Lloyd Instruments) with a chisel blade applied parallel to the interface at 0.5 mm/min. Failure mode assessment: After debonding, specimens were examined under a stereomicroscope (30×) and classified as: cohesive in substrate, adhesive at interface, mixed, or cohesive in cement. Degree of conversion (DC): For each resin cement (n=7), six cylindrical specimens (6 mm diameter × 3 mm height) were prepared: three dual-cure (immediate 60 s light activation at 1200 mW/cm² with Valo Grande) and three self-cure (mixed in the dark). All were stored in the dark in distilled water at 37 °C for one week. FTIR-ATR (Nicolet IS10) was used to record spectra (500–4000 cm−1; 4 cm−1 resolution; 32 scans). Unpolymerized and polymerized spectra were collected (three reads/specimen). DC (%) was calculated from the change in the aliphatic C=C (1638 cm−1) to aromatic C=C (1608 cm−1) peak-height ratio: DC = [1 − (Abs1638/1608)P / (Abs1638/1608)NP] × 100. Statistics: Normality (Shapiro–Wilk) and homoscedasticity (Levene) were verified. For SBS, two one-way ANOVAs (one per substrate) with Tukey post hoc tested group differences; failure modes analyzed by Fisher’s exact and pairwise comparisons. For DC, two-way ANOVA (factors: resin, polymerization mode) with Tukey post hoc. Significance p<0.05 (R v3.6.1).

- SBS: The highest SBS for both substrates was with Monobond Plus + Multilink Hybrid Abutment (MP+MHA): Ti-6Al-4V 35.1±1.6 MPa; zirconia 32.9±1.9 MPa. The lowest was Totalcem (TTC): Ti-6Al-4V 15.9±1.4 MPa; zirconia 12.7±1.4 MPa. On zirconia, adding universal primer/adhesive did not significantly improve SBS for GC-O (29.1±2.8 MPa vs GMP+GC-O 29.4±3.2 MPa) or PSA-U (24.7±1.6 MPa vs CUB+PSA-U 24.3±2.3 MPa). On Ti-6Al-4V, GC-O showed increased SBS with its universal primer (GMP+GC-O 32.0±2.7 MPa vs GC-O 25.9 MPa, as reported). Adhesive failures occurred in every group with no significant differences in failure mode distributions between groups. - Degree of conversion: Dual-cure yielded significantly higher DC than self-cure for every resin. Highest overall DC was Nexus Universal (N-U) at 78.4% (dual-cure). In dual-cure, DC (%) means included: N-U 78.4, GC-O 75.2, MHA 74.2, PSA-U 73.6, R-U 73.3, SCP 73.1, TTC 70.2. In self-cure, DC (%) means included: N-U 69.1, R-U 68.4, SCP 68.4, MHA 65.8, GC-O 64.3, PSA-U 62.3, TTC 60.3. - No direct correlation was observed between DC and SBS across the tested materials and protocols.

The study addressed whether intraoral self-adhesive resin cements can match an established extraoral protocol and how polymerization mode affects DC. The extraoral protocol MP+MHA produced the highest SBS on both zirconia and Ti-6Al-4V, rejecting the first null hypothesis. Using universal primers (containing 10-MDP and related functional monomers) generally yielded higher SBS than using universal adhesives as primers, particularly on titanium, likely due to stronger chemical interactions of phosphoric acid ester monomers (e.g., 10-MDP) with metal oxides. Differences among intraoral cements reflect monomer chemistry: materials with 10-MDP tended to outperform those relying on GPDM for zirconia; TTC lacked 10-MDP/GPDM and showed the lowest SBS and DC. Dual-curing significantly improved DC for all materials, rejecting the second null hypothesis. Enhanced DC with light is attributed to activation of photoinitiators and thermal effects improving network crosslinking. Notably, MHA, though designated self-cure, contains camphorquinone and benefited from additional light, suggesting potential laboratory optimization. Despite higher DC, SBS did not directly correlate with DC, indicating that interface chemistry and primer-cement synergy are critical determinants of adhesion beyond bulk polymer conversion.

The extraoral self-curing resin cement Multilink Hybrid Abutment, when used with Monobond Plus universal primer, achieved the highest bond strength on zirconia and Ti-6Al-4V. Some intraoral dual-cure resin cements demonstrated comparable performance when paired with appropriate universal primers. Simplified bonding approaches using self-adhesive cements, with or without universal adhesives, produced variable outcomes dependent on brand and formulation but may be clinically sufficient as they approached conventional protocol bond strengths. No direct correlation was found between degree of conversion and shear bond strength on the tested substrates. Additional light curing is recommended whenever possible to enhance degree of conversion. Further clinical studies are needed to validate and generalize these findings.

- In vitro study design limits direct clinical translatability; long-term clinical studies are needed. - No artificial aging was performed; the durability of simplified intraoral adhesives over time (e.g., under thermomechanical cycling or water storage) was not assessed. - Universal adhesives used as primers were not light-cured to isolate self-cure effects, which may have reduced their performance relative to clinical protocols. - Substrate-specific behaviors and monomer chemistries may not generalize across all brands and formulations; only selected products were tested.