This study compared the bond strengths of fiber-reinforced composite (FRC) posts luted into oversized dowel spaces with FRC posts luted into precisely fitting dowel spaces using five different resin cements or build-up composites. The hypotheses examined were that bond strength does not present vast variations according to the width of the gap between the root canal and post and that bond strength increases for FRC posts luted with build-up composites compared with FRC posts luted with resin cements. Dowel space preparations (ER post-restoring system, 12 mm in length) were performed on 100 human anterior teeth up to ISO 90 and ISO 110 of 50 roots each. FRC posts, all ISO size 90, were inserted into the precisely fitting as well as into the oversized dowel spaces (n = 10) using five composite materials (Calibra, Multicore Flow, Rely X Unicem; resin cements and Build-It, Rebilda DC; build-up composites). The manufacturers' instructions of the composite materials were strictly followed except for Rebilda DC, which was used with Adhese for dentin bonding. Following water storage (37 degrees C, 24 hours) and thermocycling (5000 cycles, 5 degrees C-55 degrees C, 30 seconds) tensile strength testing was performed and fracture modes were assessed using SEM. Data were analyzed statistically (one-way and two-way ANOVA, Bonferroni/Dunn correction, alpha <0.05). The retentive bond strengths of FRC posts in oversized dowel spaces decreased significantly for all luting composites except for Rebilda DC, compared with the respective groups with precise fitting (p <0.011). Yet, the bond strengths of RelyX Unicem (454 N +/- 64 N), Build-It (422 N +/- 43 N) and Calibra (408 N +/- 50 N) showed significantly higher values for the precisely fitting posts than Rebilda DC (267 N +/- 54 N). RelyX Unicem revealed significantly higher values than Multicore Flow (296 N +/- 73 N). The bond strengths of FRC posts in oversized dowel spaces varied from 301 N +/- 78 N for Build-It to 152 N +/- 37 N for Calibra. The values of Build-It were significantly higher compared with Calibra, Multicore Flow (180 N +/- 47 N) and Rebilda DC (186 N +/- 52 N), as well as those of RelyX Unicem (256 N +/- 115 N) were significantly higher than Calibra. Fracture modes were shown to be mainly adhesive at the post surface or cohesive for precisely fitting posts and to occur between post and composite except for RelyX Unicem with cohesive fractures for the oversized dowel spaces. The build-up composite Build-It showed superior bond strengths in oversized canals, which were still not as high as those of posts in precisely fitting dowel spaces using common resin cements.
