Objective To evaluate the effect of different subgingival root exposure methods, namely, crown lengthening or forced eruption, and different ferrule lengths on fracture resistance of a residual root restored with a carbon fiber post-and-core system.MethodsFifty-sixextracted endodontically-treated mandibular first premolars were sectioned 1.0 mm coronal to the buccal cementoenamel junction. All the models were divided randomly into seven groups that each consist of eight roots. Group A was given non-ferrule as control. Simulated crown lengthening was performed for the dentin ferrule design in the cervical tooth structure for Groups B, C, and D with a ferrule length of 1.0, 2.0, and 3.0 mm, respectively. Simulated forced eruption was performed with a ferrule length of 1.0, 2.0, and 3.0 mm in the cervical tooth structure for Groups E, F, and G, respectively. After restoration with prefabricated carbon fiber post-and-core system, each specimen was embedded in a self-cured acrylic resin block from 2.0mm apical to the margins of a cast Ni-Cr alloy crown, then loaded at 150° from the long axis in a universal testing machine at a crosshead speed of 1.0mm•min-1 until fracture. Data of failure loads and fracture mode were recorded and analyzed.Results Mean failure loads for Groups A to G were (1.13±0.15), (1.27±0.18),(1.02±0.11), (1.05±0.12), (1.63±0.14), (1.92±0.19), and (1.93±0.15) kN, respectively. The effects of root exposure method and ferrule design differed significantly,and two factors exhibited significant interaction(F=33.396，Pwere the same, fracture loads in the simulated forced eruption groups were significantly higher than those of the simulated crown lengthening groups (PConclusion More than 1.0 mm ferrule placement on the apical crown margin by using forced eruption significantly increases fracture resistance.
