Method: Eighty teeth were sectioned in 16mm and prepared in 12mm with a custom drill. Fifty roots were weakened with a tapered diamond drill and 30 kept without weakening. The specimens were embedment with acrylic-resin in PVC cylinders until 3mm coronal (bone limit) and the periodontal ligament was simulated with polyether material. The experimental design was: G1 and G2: cast and post core (Golden League) with or without weakened roots, respectively; G3 and G4: cast and post core (Ni-Cr alloy) with or without weakened roots, respectively; G5 and G6: fiber-posts with or without weakened roots; G7: fiber-posts especially designed for weakened roots; G8: fiber-posts relined with composite-resin for weakened roots. All of posts were adhesively cemented. All of the restored specimens were submitted to the mechanical cycling (37ºC, 45º, 130N, 2.2Hz and 1.5 million pulses) and the specimens that survived the mechanical cycling were subjected to the fracture-resistance test under load application on palatal zone at 45º-inclination (cross-head speed= 1mm/min) until failure. Failure mode was classified in favorable (above the simulated bone limit) and catastrophic (below the simulated bone limit). Fracture resistance data were analyzed by the one-way ANOVA and Tukey test (p=0.05).
Result: Statistical differences (p<0.0001) were depicted among the groups weakening: G1- 541.4±227.4A, G3- 642.6±219.5A, G5- 282.2±64.7B, G7- 274.1±51.3B, G8- 216.6±63.6B. No differences was observed for groups without weakening: G2- 459.3±111.1A, G4- 422.0±151.9A and G6- 347.9±91.8A.
Conclusion: For weakened roots, fiber-posts (G5), fiber-posts designed for it (G7) and relined fiber posts (G8) were adequate strategies to restore weakened roots, revealing most favorable failures, on the contrary for cast and post cores. Pattern of failure for not weakened roots was favorable failures.