Short fiber-reinforced composite versus fiber posts in endodontically treated premolars with MOD cavities: effect of intracanal extension depth

The restoration of endodontically treated premolars with extensive coronal destruction requires a balance between biomechanical reinforcement and preservation of remaining dentin. Short fiber-reinforced composites (SFRCs) have been proposed as minimally invasive alternatives to conventional fiber post systems, but the optimal intracanal extension depth of flowable SFRC remains unclear. This study evaluated the effect of two intraradicular extension depths of flowable SFRC on the fracture resistance and failure patterns of endodontically treated premolars with mesio-occluso-distal (MOD) cavities, compared with coronal SFRC and fiber post restorations. Fifty extracted human single-rooted premolars were randomly assigned to five groups ( n = 10): intact controls; coronal packable SFRC (everX Posterior); 2-mm intracanal flowable SFRC (everX Flow) with coronal packable SFRC; 4-mm intracanal flowable SFRC with coronal packable SFRC; and fiber post with coronal packable SFRC. Standardized MOD cavities and endodontic treatment were performed in all restored groups. After thermocycling (5,000 cycles, 5–55 °C), specimens underwent oblique compressive loading (30°, 1 mm/min) until fracture. Fracture patterns were classified as repairable or non-repairable. Maximum loads were compared using Welch’s ANOVA with Games–Howell post-hoc tests, and fracture modes using Fisher’s exact test, with p < 0.05 considered statistically significant. Intact teeth (445.30 ± 169.02 N) showed significantly higher fracture resistance than restored groups ( p = 0.014). No significant differences were observed among restored groups (coronal SFRC: 285.97 ± 60.04 N; 2-mm intracanal SFRC: 258.72 ± 71.36 N; 4-mm intracanal SFRC: 248.86 ± 82.66 N; fiber post: 329.33 ± 57.16 N). Increasing intracanal flowable SFRC extension from 2 to 4 mm did not improve fracture resistance. The proportion of repairable fractures was numerically higher in the SFRC groups (80–90%) than in the fiber post group (50%), although the difference was not statistically significant ( p = 0.327). Within the limitations of this in vitro study, deeper intracanal extension of flowable SFRC provided no additional biomechanical benefit beyond conservative coronal SFRC reinforcement. SFRC-based restorations achieved fracture resistance comparable to fiber post restorations with a trend toward more favorable failure patterns, supporting conservative, dentin-preserving post-endodontic restorative strategies.