In endodontically treated teeth, cuspal coverage plays a fundamental role in reducing the risk of fracture. However, the adhesive techniques with or without fiber post increased the possibilities in restoring root-filled teeth. The aim of this study was to determine the effect of the fiber post and/or post length and/or cuspal coverage on the fracture resistance of endodontically treated maxillary premolars. Seventy intact single-rooted maxillary premolars were selected and divided in seven groups of ten each: "intact teeth" (control), "inlay without fiber post" (G1), "inlay with long fiber post" (G2), "inlay with short fiber post" (G3), "onlay without fiber post" (G4), "onlay with long fiber post" (G5), and "onlay with short fiber post" (G6). Except for intact teeth, all specimens were prepared with a mesio-occluso-distal (MOD) cavity, endodontically treated and restored with or without long or short post, with or without cusp coverage. All specimens were thermal-cycled, exposed to a cyclic loading, and then submitted to the static fracture resistance test. Fracture loads and mode of failure were evaluated. A statistically significant difference in fracture resistance was found between group 1 and the other groups (p < 0.001). χ2 test showed statistically significant differences in the patterns of fractures between the groups (p < 0.001). The highest number of favorable fractures was observed in groups 3 and 4. Similar fracture resistance was detected in maxillary premolars endodontically treated with MOD cavity preparations, restored with either direct resin composite with fiber post or cusp capping. The "short post" direct restoration may be a valid alternative in the restoration of root-filled premolars.

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Liquid phosphoric acid applied with an endodontic needle yielded better canal wall smear layer removal and higher bond strength values when an etch-and-rinse system was used.

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