Effects of Photodynamic, Nanoparticle-Based, and Chemical Final Post-Space Conditioning on Smear Layer Removal and Push-Out Bond Strength of Glass Fiber-Reinforced Composite Posts

To evaluate and compare the effect of different final post space treatments, i.e., methylene blue-mediated antimicrobial photodynamic therapy (aPDT-MB), superparamagnetic iron oxide nanoparticles (SPIONs), and apple cider vinegar (ACV), against the established benchmark of 17% EDTA on smear layer (SL) removal and push-out bond strength (PBS) of glass fiber posts (GFPs) bonded to post space dentin. Endodontic treatment was conducted on eighty human mandibular premolars with single roots, which were obturated. Post spaces were uniformly prepared, and specimens were randomly divided into four groups based on the final post space treatment protocol (n = 20 per group): Group 1 (NaOCl + 17% EDTA), Group 2 (NaOCl + aPDT-MB), Group 3 (NaOCl+ SPIONs), and Group 4 (NaOCl + ACV). All teeth received conventional root canal treatment, including a final 17% EDTA rinse, before post-space preparation. Within each group, ten teeth were allocated to SL evaluation by scanning electron microscopy (SEM) and ten to PBS testing, in which GFPs were luted with a dual-cure self-adhesive resin cement. The two outcomes were therefore measured in different teeth, and antimicrobial efficacy was not assessed. The teeth underwent thermocycling (10,000 cycles, 5–60 °C) and were sectioned into cervical, middle, and apical segments. These segments were subjected to PBS testing at a speed of 0.5 mm/min using a universal testing machine (n = 10), and failure modes were determined through stereomicroscopic analysis. PBS data were analyzed using a two-way mixed ANOVA followed by simple-effects analyses with Tukey- and Bonferroni-adjusted comparisons; ordinal SL scores were analyzed using Kruskal–Wallis (between protocols) and Friedman (between root thirds) tests with Bonferroni-adjusted pairwise comparisons (α = 0.05). Group 3 (NaOCl + SPIONs) showed the lowest SL scores (medians 1.20–1.32) and the highest PBS values (10.70 ± 0.76 to 11.20 ± 0.84 MPa; maximum at the cervical third). Group 2 (NaOCl + aPDT-MB) showed the highest SL scores (medians 2.60–3.54) and the lowest bond strength (3.90 ± 0.52 to 6.10 ± 0.58 MPa). PBS ranged from 7.20 to 10.90 MPa in Group 1 and from 6.10 to 7.90 MPa in Group 4. No statistically significant difference was detected between NaOCl + EDTA and NaOCl + SPIONs at the cervical and middle thirds, whereas at the apical third NaOCl + SPIONs showed lower SL scores and higher PBS (mean difference 3.50 MPa; 95% CI 2.75–4.25). Combined NaOCl and SPIONs irrigation did not differ significantly from NaOCl + EDTA in the cervical and middle thirds and outperformed it in the apical third; this promising approach requires further validation, including assessment of residual nanoparticles, biological safety, and long-term bond durability.