Fracture resistance of endodontically treated maxillary premolars restored with different bi-layered core and occlusal overlay materials: an in vitro study.

OBJECTIVES: This study evaluated the reinforcing effect of various core and overlay materials used in bi-layered restorations on the fracture resistance of compromised endodontically treated maxillary premolars with mesio-occluso-distal (MOD) cavities. METHODS: Three core materials were assessed: a packable short fiber-reinforced composite (everX Posterior (EXP)), its flowable counterpart (everX Flow (EXF)), and a dual-cure core material (GRADIA CORE (GC)). Each core material was combined with one of three overlay restorations: an indirect overlay (ID) using CERASMART 270, a direct overlay (D), or a semi-direct overlay (SD) using Gænial A'CHORD. A total of 110 sound maxillary premolars were allocated to 11 groups (n = 10), including a positive (sound teeth) and a negative (endodontically treated teeth restored with direct composite, without cusp reduction) control groups. Standardized endodontic treatment and MOD cavity preparation were performed with 2 mm buccal and palatal cusp reduction. Fracture resistance was assessed using a universal testing machine at a crosshead speed of 1 mm/min. Failure modes were analyzed using Firth's logistic regression model, while fracture resistance was evaluated using two-way ANOVA followed by post hoc tests. The control groups were compared using an independent t-test. Statistical significance was set at p < 0.05. RESULTS: The overlay restoration, Core material, and their interactions significantly influenced fracture resistance (p = 0.043, p < 0.001, and p < 0.001, respectively). Within the direct overlay group, EXP exhibited the highest fracture resistance (1124.61 ± 213.32 N), significantly higher than EXF but not significantly different from GC. EXP also demonstrated relatively high fracture resistance across the tested overlay restorations. EXF exhibited the lowest fracture resistance in the direct overlay group (596.14 ± 171.40); however, in the indirect overlay group, both EXF and GC showed significantly higher values than EXP. No significant differences among core materials were observed in the semi-direct overlay group. Unfavorable failure mode predominated, with no significant differences observed among the experimental groups. CONCLUSIONS: The fracture resistance of bi-layered restorations was significantly influenced by both the core and overlay materials and their interaction. EXP maintained high fracture resistance across various bi-layered restorations, whereas EXF demonstrated technique-dependent performance. The indirect overlay group achieved high fracture resistance, but was not superior to all direct overlay restorations. CLINICAL RELEVANCE: Packable short fiber-reinforced composite, particularly everX Posterior, demonstrated a promising synergistic effect with direct overlay restorations, resulting in fracture resistance comparable to that achieved with high-performance indirect overlay restorations.