This in vitro study evaluated the survival rate and fracture resistance of maxillary central incisors restored with different post and core systems. The post and core systems investigated were a prefabricated high precious metal post with cast core (group A), zirconia post with a prefabricated bonded ceramic core (group B), and a resin-ceramic interpenetrating phase composite post (experimental) with a prefabricated bonded ceramic core (group C). The all-ceramic copings were cemented using Panavia 21 TC. In the group without coronoradicular reinforcement, the access cavity was closed with a light-cured composite in combination with a dentine-bonding agent (group D). Each specimen was intermittently loaded and thermocycled before final stress tests in a Zwick machine. The survival rates after 1,200,000 cycles in the artificial mouth were 90% (group A), 80% (group B), 60% (group C), and 100% (group D). Statistically significant differences were found between all groups with the exception of A and B, when failure during cyclic loading was included (Kruskal-Wallis multiple comparisons test). Samples restored with a cast post and core demonstrated more vertical root fractures. It was concluded that the preservation of both internal and external tooth structure is of utmost importance when restoring endodontically treated teeth.

The survival rate and fracture resistance of 40 decapitated endodontically treated maxillary central incisors using four different post and core systems covered with all-ceramic copings was evaluated after exposure in the artificial mouth. Ten samples of the following post and core systems were investigated: high precious metal post (Permador) and core (Olympia) (A), zirconia post (Cerapost) with a pre-fabricated bonded ceramic core (Ceracap) (B), resin-ceramic post (experimental) with a pre-fabricated bonded ceramic core (Ceracap(R)) (C) and a zirconia post (Cerapost) with a custom made ceramic core (Cosmo Ingots) (D). The all-ceramic copings (Procera) were cemented using Panavia TC. The survival rates after 1 200 000 cycles in the artificial mouth are as follows: 90% (A), 80% (B), 60% (C) and 100% (D). The results of the means and standard deviations (s.d.) of the fracture resistance during static loading are: 1270 +/- 312.5 (A), 1494.5 +/- 333.5 (B), 1146.7 +/- 182.6 (C) and 463.3 +/- 46.2 (D). There are statistically significant differences between all groups with the exception of A and B, and A and C (Wilcoxon test). None of the zirconia posts with custom made ceramic cores covered with all-ceramic copings fractured during dynamic loading in the artificial mouth. The mean fracture strength during static loading was less favourable than that of groups A, B and C but above the clinical necessary level.

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*Double teeth occur infrequently and may be distinguished from fusion, gemination, concrescence and dental twinning. *Several malformations may be present in a single tooth. *Orthograde root canal treatment may be an adequate treatment option even in teeth with a complex internal anatomy.

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