Thesis (M.Sc.)--Chulalongkorn University, 2013

Purpose: This in vitro study was to evaluate the effect of remaining tooth height of root-canal-treated incisors restored with fiber posts and direct composite resin build-up on fracture strength and mode of failure. Methods: Forty-eight extracted human maxillary central incisors were randomly assigned to 1 of 4 groups: group 1 (0mm+post), group 2 (2mm+post), group 3 (2mm+no post), and group 4 (control). All specimens were subjected to a fatigue-loading device at 40 N with a 135° angle. When 250,000 loading cycles were reached, the surviving specimens were subjected to a static load. The presence of differences was analyzed by 1-way ANOVA, Turkey HSD test, and Chi-square analysis (α = .05). Results: All specimens reached 250,000 cycles. ANOVA showed a significant difference in fracture strength (p-value < .0001). The highest mean fracture strength was recorded for group 4 at 1326.13 ± 145.25 N, followed by group 2 at 696.29 ± 191.75 N, group 1 at 592.80 ± 128.10 N, and group 3 at 234.65 ± 80.10 N. There was no significant differences in fracture strength between group 1 and group 2 (p-value > .05). Most failures in group 4 occurred due to root fracture. While in group 3, most fracture lines occurred in tooth structure above the CEJ. The coronal failures of composite resin build-up occurred only in group 1. The fractures in group 2 mainly involved tooth structure below the CEJ. When the mode of failure was evaluated, statistically significant differences were noted between groups 1 and group 2, also group 2 and group 3 (p-value < .05). Conclusions: The remaining coronal tooth structure did not increase the fracture strength of a direct composite resin build-up on root-canal-treated incisors. The presence of a fiber post improved the fracture strength of incisors restored with direct composite resin build-up, regardless of coronal height.