Effect of different ferrule lengths on the fracture resistance of endodontically treated mandibular premolars restored with prefabricated fiber post-and-core system after simulated crown lengthening

Objective 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. Methods 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. Results 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. Conclusions 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. Key words: Crown lengthening; Post and core technique; Tooth fractures; Tooth root, residual; Fracture resistance; Ferrule