Fracture strength and failure patterns of different restoration approaches in endodontically treated maxillary premolars: an in vitro study with fractographic analysis

The restoration of endodontically treated premolars has been a controversial topic. Whether a post or a more conservative option should be used remains inconclusive. The present in vitro study aimed to evaluate the fracture strength and failure pattern of different partial coverage restoration approaches for endodontically treated upper premolars in comparison to the post, core and crown approach. Thirty-five maxillary premolars were endodontically treated and assigned to five groups (n = 7): endocrown (E), endocrown with buccal veneer (EB), overlay (O), overlay with buccal veneer (OB), and post, core, and crown (P). All restorations were fabricated from lithium disilicate ceramics and subjected to vertical static loading until failure. Fracture strength was recorded, and failure patterns were examined using a stereomicroscope and scanning electron microscopy. Data were analyzed with Welch one-way ANOVA and Games–Howell post hoc test (α = 0.05). Significant differences were found among groups (p < 0.001) with a large effect size (partial eta squared = 0.83 (95% CI; 0.59 to 0.91). The highest mean fracture strength was observed in group P (1676.29 ± 191.25 N), followed by OB (1475.00 ± 198.72 N), E (1374.71 ± 371.91 N), EB (1047.41 ± 163.90 N), and O (987.99 ± 125.01 N). Groups P and OB demonstrated significantly higher strength than EB and O. Group P exhibited only repairable restoration fractures, whereas the other groups predominantly showed catastrophic failures. Within the limitations of this in vitro study, post, core, and crown restorations provided superior fracture strength and the most favorable failure patterns under static axial loading. While overlays with buccal veneers demonstrated promising mechanical resistance thresholds, their clinical application should be considered with caution. These designs may serve as a potential conservative alternative, although further research involving dynamic fatigue and oblique loading is required to confirm their long-term clinical durability in the complex oral environment. The findings of this in-vitro study provide insight into the fracture strength and failure tendencies of different restorative approaches for endodontically treated maxillary premolars. While post, core, and crown restorations demonstrated high load-bearing capacity and predominantly favourable failure patterns, overlay designs with a buccal veneer showed comparable strength but a greater tendency toward catastrophic fractures. These observations highlight a potential balance between structural preservation and failure reparability. Given that static axial loading was used, direct clinical interpretation should be approached cautiously. Treatment planning should integrate these biomechanical considerations with patient-specific factors, functional demands, and clinical judgment when selecting restorative approaches.