Fracture resistance and behavior of endodontically treated maxillary premolars with varying ferrule heights and post numbers: A laboratory study.

Purpose: To investigate how varying ferrule heights and the number of glass fiber posts affect fracture resistance and behavior of endodontically treated maxillary first premolars with substantial loss of tooth structure.

Materials and methods: Twenty-four extracted endodontically treated human maxillary first premolars were divided into three groups (n = 8) based on ferrule height and post number. The groups were as follows: premolars of 2 mm ferrule height that were restored with single posts (control group), premolars of 0.5 mm ferrule height that were restored with single posts in palatal canals (single-post group), and premolars of 0.5 mm ferrule height that were restored with double posts in palatal and buccal canals (double-post group). All groups were then restored with composite core and metal-ceramic crowns cemented with zinc phosphate cement. All specimens were subjected to 10,000 thermal cycles at temperatures of 5°C and 55°C, as well as 1.2 million cyclic loadings of 49 N load at 1.2 Hz using a chewing simulator. The specimens were loaded to fracture in a universal testing machine and fracture behavior was examined under stereomicroscope and divided into restorable and unrestorable fracture modes. Fracture loads and modes were analyzed by one-way ANOVA, Tukey's post hoc test, and Fisher's exact probability test (p ≤ 0.05).

Results: The control group showed the highest mean fracture load (1127 ± 172) of all groups (p < 0.001). The specimens in the single-post group showed numerically higher mean fracture load (522 ± 171) but with no significant difference compared to the ones in the double-post group (518 ± 157; p > 0.967). In the control group, 75% of specimens showed unrestorable fractures compared to 50% and 12.5% in single-post and double-post groups. The specimens in the control group had similar unrestorable fractures, starting from the coronal part and extending obliquely to the cervical third of the root below resin level. Two specimens in the control group showed unrestorable fractures in the middle third of the root. No middle-third root fractures were observed in single-post and double-post groups, and unrestorable fractures in both groups were similar to those of the control group. The single-post group had two specimens with core-post crown complex restorable fractures, while the control and double-post groups had one specimen each. The remaining specimens showed cervical-third root restorable fractures above resin level.

Conclusion: The ferrule height significantly influences the fracture resistance of endodontically treated premolars compared to the number of placed posts. Nonetheless, more restorable fractures are seen in endodontically treated premolars restored with double posts than single posts regardless of ferrule height.