title: [In vitro study evaluating the effect of different subgingival root exposure methods and ferrule designs on fracture resistance of residual root].
abstract: 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.
  
  
  METHODS
  Fifty-six extracted 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 2.0 mm apical to the margins of a cast Ni-Cr alloy crown, then loaded at 150 degrees from the long axis in a universal testing machine at a crosshead speed of 1.0 mm 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, P<0.0001). When the ferrule lengths were the same, fracture loads in the simulated forced eruption groups were significantly higher than those of the simulated crown lengthening groups (P<0.001). No statistical differences in fracture mode were observed among all groups (P=1.00).
  
  
  CONCLUSIONS
  More than 1.0 mm ferrule placement on the apical crown margin by using forced eruption significantly increases fracture resistance.
authors.forename: Qingfei
authors.initials: Q
authors.lastname: Meng
authors.forename: Lijuan
authors.initials: L
authors.lastname: Chen
authors.forename: Jian
authors.initials: J
authors.lastname: Meng
mesh_terms: D001641:Bicuspid; D003442:Crowns; D003799:Dental Stress Analysis; D006801:Humans; D066298:In Vitro Techniques; D011176:Post and Core Technique; D014082:Tooth Fractures; D014092:Tooth Root