The influence of zirconia and fiber posts with different lengths on the fracture strength of maxillary central incisors restored with zirconia crowns: an in vitro study.

Aim: Endodontically treated teeth are more susceptible to fractures than vital teeth because of significant coronal and radicular dentin loss during endodontic therapeutic procedures. The objective of the present in vitro study was to estimate and compare the influence of the post length and type on the fracture strength of endodontically treated maxillary central incisors.

Materials and methods: Sixty extracted human maxillary central incisors were decoronated 2 mm above the cementoenamel junction. The teeth were selected and subjected to standard endodontic treatment, resulting in three groups with different preparation lengths: 5, 7.5, and 10 mm. Each group was randomly divided into two subgroups according to the post type: zirconia or fiber. After appropriate surface treatment, they were cemented with adhesive resin cement and restored with zirconia crowns. Thermocycling (5°C to 55°C, 60 s, 1500 cycles) was performed after cementing the zirconia crowns onto each tooth. Prepared samples were subjected to a compressive static load of 0.5 mm/min, at an angle of 130 degrees to the long axis of the roots, using a universal testing machine (Matest) at a crosshead speed of 0.5 mm/min, until fracture. The significance of the results was assessed using two-way analysis of variance (ANOVA) and the Tukey-Kramer test (α = 0.05).

Results: The ANOVA analysis indicated significant differences (P < 0.05) between the groups. The Tukey-Kramer test revealed no significant differences among the zirconia posts with lengths of 5 mm (26.5 N ± 13.4), 7.5 mm (25.2 N ± 13.9), and 10 mm (17.1 N ± 5.2). Also, in the fiber post group, there was no significant difference when the posts with lengths of 7.5 mm (13.4 N ± 11.0) were compared with those of 5 mm (6.9 N ± 4.6) and 10 mm (31.7 N ± 13.1). The 10-mm-long post displayed superior fracture strength, and the 5-mm-long post showed significantly lower mean values (P < 0.001).

Conclusions: The fracture strength of zirconia posts (with lengths of 5 and 7.5 mm) was found to be significantly higher than that of fiber posts (with lengths of 5 and 7.5 mm). The 10-mm-long fiber post group demonstrated significantly higher fracture strength values, and the 5-mm-long fiber post group showed the lowest values for the force resulting in root fracture; these groups were significantly different from each other (P < 0.001). The fracture strength analysis with a universal testing machine is the only method that enables the estimation of the differences between zirconia and fiber posts with different lengths in endodontically treated teeth.