Comparison of the retentive and photoelastic properties of two prefabricated endodontic post systems

This in. vitro study compared the retention and photoelastic stress patterns from two loading conditions, vertical (133.2 N, 30 pounds) and oblique at a 26 degrees angle (133.2 N, 30 pounds) for two prefabricated post systems. The post studied were: (1) Flexi-Post(R) (a split-shank threaded post) and (2) C-Post(R) (a carbon fibre bound in an epoxy resin, passive double taper tier post). Two groups with 10 specimens per group were subjected to retentive forces with a universal testing machine (MTS 810 Material Testing Machine). In addition, two photoelastic test blocks were prepared with simulated root canals for each post studied. After cementation the photoelastic blocks were photographed before vertical and oblique loading and after loading. One-way analysis of variance (ANOVA) for retention data revealed a highly significant difference (P < 0.0001) between groups. The Flexi-post(R) had a statistically higher mean retention force of 1180.6 N (265.9 pounds), while the C-Post(R) had a mean of 171.8 N (38.7 pounds). Photoelastic analysis indicated minimal stresses for both the Flexi-Post(R) and C-Post(R) in the unloaded state. The C-Post(R) showed asymmetrical apically stress patterns, while loaded in both states. The multi-tiered system of the Flexi-Post(R) clearly distributes stress symmetrically, while the C-Post(R) distributes stresses asymmetrically. The symmetric, even stresses and statistically higher retentive strength for Flexi-Post(R) are more favourable than the asymmetric, uneven stresses and relatively low retentive strength for the C-Post(R).