Meng Q, et al. - During this study, authors discerned the effect of ferrule design on the fracture resistance of endodontically treated mandibular first premolars after simulated crown lengthening and orthodontic forced eruption methods restored with a fiber post-and-core system. Forty extracted and endodontically treated mandibular first premolars were decoronated to create lingual-to-buccal oblique residual root models, with a 2.0 mm height of the lingual dentine wall coronal to the cemento-enamel junction, and the height of buccal surface at the cemento-enamel junction. Findings illustrated that increased apically complete ferrule preparation led to decreased fracture resistance of endodontically treated mandibular first premolars, irrespective of whether surgical crown lengthening or orthodontic forced eruption methods were used.