Influence of post fit and post length on fracture resistance: an in vitro study

As root filled teeth often have insufficient coronal tooth structure, placement of a post is occasionally necessary to provide adequate retention for the core and final restoration. The aim of the present study was to investigate (i) the impact of post fit (form-congruence) and (ii) the influence of post length on the fracture resistance of severely damaged root filled extracted teeth. Forty single-rooted human teeth were root filled and divided into four groups (n = 10 per group). Post spaces were prepared with a depth of 6 mm (groups 1, 3) and 3 mm (groups 2, 4). Form-congruence with a maximal fit of the post within the root canal space was obtained in groups 1 and 2, whereas there was no form-congruence in groups 3 and 4. In all groups, glass fiber reinforced composite (FRC) posts were adhesively cemented and direct composite crown buildups were fabricated without a ferrule. Specimens were subjected to thermocycling and cyclic loading followed by application of static load until failure. Loads-to-failure [in N] were compared among the groups. Post fit did not have a significant influence on fracture resistance, irrespective of the post length. Both groups with post insertion depths of 6 mm resulted in significantly higher mean failure loads (group 1: 274.27 N; group 3: 277.16 N) than the groups with post space preparation of 3 mm (group 2: 250.40 N; group 4: 255.48 N). Within the limitations of this study, the fracture resistance of teeth restored with FRC posts and direct resin composite crowns without ferrules was not influenced by post fit within the root canal. These results imply that excessive post space preparation aimed at producing an optimal circumferential post fit is not required to improve fracture resistance of roots.