Degree of conversion and adhesion of methacrylate-based resin cements with phosphonic or phosphoric acid acrylate to glass fiber posts at different regions of intraradicular dentin

Abstract

This study evaluated the degree of conversion (DC) and adhesion of methacrylate-based resin cements to glass fiber posts at different regions of intraradicular dentin. Single-rooted teeth (N = 24, n = 12 per group) were cut at the cement–enamel junction (CEJ), endodontically treated and post space (depth = 8 mm) was prepared. Teeth were randomly divided into two groups according to the resin cements: (a) Group ML: methacrylate-based cement with phosphonic acid acrylate (Multilink Automix, Ivoclar Vivadent); (b) Group RXU: methacrylate-based cement with phosphoric acid acrylate (RelyX Unicem 2 Automix, 3 M ESPE). Fiber-reinforced composite root posts (RelyX Fiber Post, 3 M ESPE) were cemented according to the manufacturers’ instructions of the resin cements. Root slices of 2-mm thickness (n = 3 per tooth) were cut below the CEJ 1, 3, and 5 mm apically. The DC of each section was analyzed with micro-Raman spectrometer and push-out test was performed in the Universal Testing Machine (0.5 mm/min). After debonding, all specimens were analyzed using optical microscope to categorize the failure modes. While data (MPa) were statistically evaluated using Kruskal Wallis, Mann–Whitney U tests for DC data 3-way ANOVA and Tukey’s tests were used (α = 0.05). Regardless of the resin cement type, the mean push-out bond strength results (MPa), were significantly higher for the coronal slices (ML: 9.1 ± 2.7; RXU: 7.3 ± 4.1) than those of the most apical ones (ML: 7 ± 4.9; RXU: 2.89 ± 1.5) (p = 0.002). Resin cement type and (p < 0.001) root level (p = 0.002) significantly affected the DC values, while the interaction terms were not significant (p = 0.606). Overall, DC was significantly higher for ML (67 ± 8.2%) than RXU (26 ± 8.8%) (p < 0.001). Adhesive failures at the cement–dentin interface were more commonly experienced in RXU than in ML, whereas ML presented more incidences of adhesive failures at the cement–post interface. Considering the push-out bond strength, DC and failure types, methacrylate-based cement with phosphonic acid acrylate should be preferred to those containing phosphoric acid to adhere glass fiber posts in the root canal.