Influence of Adhesive Strategies on the Bond Strength and Fracture Behaviour of Biodentine™ XP to Flowable Bulk-Fill Composite

The bonding performance of restorative composites to Biodentine™ XP remains insufficiently characterised, particularly regarding the influence of adhesive strategies on mechanical behaviour and fracture characteristics. This in vitro study evaluated the shear bond strength between Biodentine™ XP and a flowable bulk-fill composite using four restorative protocols: acid etching with Prime&Bond® NT (G1), acid etching with Scotchbond™ Universal (G2), Scotchbond™ Universal without prior acid etching (G3), and no adhesive treatment (G4). Cylindrical specimens measuring 4.7 mm in diameter and 7 mm in total height were prepared, comprising 3 mm of Biodentine™ XP and 4 mm of composite. Uncured composite was placed directly onto the Biodentine™ XP surface and light-cured in situ following one of four bonding protocols. Specimens were stored for 72 h at 37 °C and 100% relative humidity before shear bond strength testing. Fracture patterns were assessed by optical microscopy, and representative surfaces were characterised using scanning electron microscopy and energy-dispersive X-ray spectroscopy. Significant differences in bond strength were detected among groups (p = 0.002), with G4 showing significantly lower values than G1 and G2. No significant differences were observed among the adhesive-treated groups. Fracture-pattern distribution also differed significantly (p < 0.001); acid-etched groups showed no adhesive failures, whereas adhesive failures occurred in G3 and were most frequent in G4. SEM-EDS revealed morphological and elemental differences among the selected fractured regions. Within the limitations of this short-term in vitro study, adhesive treatment improved bonding between Biodentine™ XP and the bulk-fill composite. Phosphoric acid etching did not significantly increase bond strength among adhesive-treated groups but was associated with a different failure-mode distribution. The mechanisms underlying these findings require direct chemical and long-term interfacial investigation.