Purpose. The analysis of the complex model of fiber post and ferrule is given and studied in this paper. A novel approach and a solution to the evaluation of stress of post and core system within the ferrule effect are proposed. Methods. Sixty freshly extracted premolars were selected for the study. The following experimental groups were therefore defined (n = 10): (1) 5 mm, (2) 7 mm, (3) 9 mm, (4) ferrule-5 mm, (5) ferrule-7 mm, and (6) ferrule-9 mm. Preshaping drills (C) were used to prepare the root canals at 5, 7, and 9 mm in depth. In specimens of groups 3–6 a circumferential collar of tooth structure of 2 mm in height. Fluorocore 2 core build-up material (I) was used for fiber post luting. With the same material, a buildup of 2 mm in height was created. A controlled compressive load (crosshead speed: 0.75 mm/min) was applied by means of a stainless steel stylus (Ø 1 mm) at the coronal end of the post extruding out of the root. Results. In all the tests the level of significance was set at P < 0.05
. Significantly higher fracture strengths were measured in the presence of a ferrule effect. In groups 1, 2, and 3 (ferrule group), the mean fracture values were, respectively, 163,8 N, 270,9 N, and 254,7 N. These data are higher and statistically significantly different when compared with the three groups 4, 5, and 6 (no-ferrule group), in which the values obtained were, respectively, 40,5 N, 41,7 N, and 44,9 N. Conclusion. The ferrule effect in the endodontically treated teeth positively affects the fracture strength of the fiber post. Conversely, post depth insertion did not affect the resistance to fracture.
