Comparison of Commercial and Experimental Fibre-Reinforced Composites in Restoring Endodontically Treated Teeth with Minimal Coronal Dentine: An In Vitro Study

Aim: To compare the fracture resistance of teeth with varying degrees of residual coronal dentine after restoration using two fibre-reinforced composite core materials. Materials and Methods: Seventy extracted human lower premolars were divided into four groups: sound (control), one missing proximal wall (Cl-II), two missing proximal walls (MOD), and endocrown (EC). Subgroups were restored with either a short fibre-reinforced flowable composite (EverX Flow) or an experimental fibre-reinforced composite. Except for the control, teeth underwent endodontic treatment and were restored accordingly. Fracture resistance was tested using a universal testing machine. Statistical analysis compared fracture resistance across groups. Results: Teeth in EC exhibited the highest fracture resistance (1153.43 ± 332.52 N), comparable to sound teeth (1114.03 ± 185.58 N) and not significantly different from the experimental composite group (1006.89 ± 200.51 N) (p = 0.304). Cl-II restorations with EverX had significantly lower strength (652.48 ± 314.04 N) compared to MOD (773.02 ± 261.18 N) and EC (p < 0.05). The experimental composite showed a similar trend, with MOD having the lowest strength (408.6 ± 168.85 N). Significant differences were noted between materials in the MOD group (p = 0.009). Scanning electron microscopy revealed distinct fracture patterns. Conclusions: Endocrowns using direct fibre-reinforced composites provided protection for endodontically treated teeth with higher fracture resistance compared to teeth with MOD and Cl-II cavities. This gives direct composite endocrowns a potential for high-stress applications, though design and material selection remain critical.