Influence of proximal box depth associated with deep margin elevation and restorative materials on fracture resistance and repairability of endodontically treated teeth: an in vitro study

BACKGROUND: To evaluate the effects of different restorative materials under proximal box depth conditions associated with proximal deep margin (PDM) on fracture resistance and fracture mode on fracture resistance and fracture mode of endodontically treated mandibular molars. METHODS: Seventy extracted human mandibular molars were randomly assigned to seven groups (n = 10; control n = 10). Except for the control group, all specimens received standardized root canal treatment and mesial-occlusal-distal (MOD) cavity preparation. Restorations were performed with or without PDM using a short fiber-reinforced composite (SFRC), a bulk-fill composite (BFC), or a conventional resin composite (CRC). Specimens underwent 1,000 thermal cycles between 5 °C and 55 °C. Fracture resistance was measured under static loading using a universal testing machine. Fracture modes were classified as repairable or unrepairable. RESULTS: The control group demonstrated significantly higher fracture resistance than all restored groups (p < 0.001). Among restored teeth, specimens restored with SFRC showed significantly higher fracture resistance than those restored with BFC (p < 0.001). Proximal cavity depth conditions associated with PDM did not significantly affect fracture resistance (p > 0.05). Fracture repairability analysis showed no significant association between restorative approach and repairability (p = 0.309). CONCLUSIONS: Proximal cavity depth conditions associated with PDM did not adversely affect fracture resistance in endodontically treated mandibular molars. Short fiber-reinforced composite demonstrated superior fracture resistance compared with bulk-fill composite. These findings should be interpreted in relation to differences in cavity depth rather than the independent effect of PDM. CLINICAL SIGNIFICANCE: Restorative material selection, rather than proximal cavity depth conditions associated with PDM, is a key determinant of fracture resistance in endodontically treated molars, with fiber-reinforced composites providing a potential biomechanical advantage.