The purpose of this study was to evaluate the effect of ferrule design on the fracture resistance of endodontically treated mandibular first premolars after simulated crown lengthening restored with a fiber post-and-core system.

Twenty-four extracted and endodontically treated mandibular first premolars were decoronated at 2.0 mm above cemento-enamel junction, and then a title surface was made from lingual to buccal side. The roots were divided randomly into three equal groups, according to a table of random numbers. The control group was incompletely prepared in the cervical root, with 0 mm buccal and 2.0 mm lingual ferrule lengths (Group A) . Simulated surgical crown lengthening was used to obtain a ferrule preparation of 1.0 mm (Group B) and 2.0 mm (Group C) on the buccal side, and 3.0 mm and 4.0 mm on the lingual side, respectively. After restoration with a glass fiber post-and-core system and a cast Co-Cr alloy crown, each specimen was embedded at 2.0 mm below the crown margin in an acrylic resin block and loaded to fracture at a 135° angle to its long axis in a universal testing machine. Data were analyzed statistically using One-Way ANOVA with Tukey HSD tests, with α= 0.05.

Mean fracture loads (kN) for group A, B and C were 1.01 ± 0.26, 0.91 ± 0.29 and 0.73 ± 0.19, respectively. No significant difference was found for the effect of ferrule lengths (F= 2.588, P= 0.099) . The teeth with no buccal ferrule preparation in Group A had the highest fracture resistance. In contrast, with the increase of the height of buccal ferrule, the fracture strength of the roots decreased.

For lingual-to-buccal oblique fractures, increased apical ferrule preparation after simulated surgical crown lengthening may result in decreased fracture resistance of endodontically treated mandibular first premolars.