Influence of Root Canal Retreatment Protocols on Initial Fracture Force and Colour Changes in Internally Bleached Mandibular Anterior Teeth

This in vitro study evaluated the effects of root canal retreatment protocols (with or without solvents) on the fracture resistance, failure patterns and colour behaviour of stained and internally bleached mandibular anterior teeth. Ninety specimens were allocated to six groups: control, access cavity, primary treatment, mechanical retreatment, chloroform-assisted retreatment and orange oil-assisted retreatment. Following coffee staining and sodium perborate bleaching, fracture force and colour differences were analysed. The results showed significant group × time interactions for colour (p < 0.001); retreatment groups exhibited greater staining than control groups. Mechanical retreatment specimens yielded the lowest fracture resistance, whereas access cavity specimens yielded the highest. Lower failure loads significantly correlated with restorable fracture patterns (p < 0.001). Retreatment reduced fracture force in bleached teeth, particularly via mechanical methods. Solvent-assisted protocols showed numerically higher fracture resistance than mechanical retreatment alone. Retreatment-related structural changes increase susceptibility to discolouration more than they impact bleaching efficacy.