Effects Of Different Tooth Preparations On The Fracture Behavior Of Tooth With Severe Wedge-Shaped Defect Restored With Post And Core Crown

Purpose:To investigate the effects of different tooth preparations on the fracture strength and pattern of failure of tooth with severe wedeg-shaped defect restored with post and core crown.Methods:64 human mandibular premolar teeth were chosen which were extracted for orthodontic treatment in the stomatological hospital of Shandong University and department of stomatology of Hospital of Shandong Province from August 2010 to October 2010, All the teethwere randomly divided into 8 groups,each group 8 teeth. All the teeth were endodontically treated, artificial severe wedeg-shaped defect were maded.Group A1-1:teeth overhang above the wedeg-shaped defect was not removed, ferrule next the wedeg-shaped defect was not prepared, cast post-and-core(Co-Cr) was chosen.Group A1-2:teeth overhang above the wedeg-shaped defect was not removed, ferrule next the wedeg-shaped defect was not prepared, glassfiber post and resin core was chosen.Group A2-1:teeth overhang above the wedeg-shaped defect was not removed, 1.5mm ferrule next the wedeg-shaped defect was prepared, cast post-and-core(Co-Cr) was chosen.Group A2-2:teeth overhang above the wedeg-shaped defect was not removed, 1.5mm ferrule next the wedeg-shaped defect was prepared, glassfiber post and resin core was chosen.Group B1-1:teeth overhang above the wedeg-shaped defect was removed, ferrule next the wedeg-shaped defect was not prepared, cast post-and-core(Co-Cr) was chosen.Group B1-2:teeth overhang above the wedeg-shaped defect was removed, ferrule next the wedeg-shaped defect was not prepared, glassfiber post and resin core was chosen.Group B2-1:teeth overhang above the wedeg-shaped defect was removed,1.5mm ferrule next the wedeg-shaped defect was prepared, cast post-and-core(Co-Cr) was chosen.Group B2-2:teeth overhang above the wedeg-shaped defect was removed,1.5mm ferrule next the wedeg-shaped defect was prepared, glassfiber post and resin core was chosen.Each specimen was mounted on the electronic pressureniversal testing machine and subjected to a load at 30 degrees angle to the long axis of the teeth until failure. The cross head speed was 1 mm/min. Recored the maximum fracture strength and The pattern of failure.Results:1. Fracture strength:Group A1-1:1.55±0.49KN; Group A1-2:1.35±0.55 KN; Group A2-1:1.22±0.42 KN; Group A2-2:1.11±0.43 KN; Group B1-1:0.78±0.14 KN; Group B1-2:0.87±0.39 KN; Group B2-1:1.28±0.46 KN, Group B2-2: 0.75±0.29 KN. Variance analysis of the fracture strength of the 8 groups showed That F=3.863,P=0.002<0.05, the difference was significant. The comparisons of Every two groups according to LSD showed that the fracture strength GroupAl-1 >Group B1-1, Group A1-2>Group B1-2, Group B2-1>Group B1-1,Group B2-1 >Group B2-2, the difference was significant, there was no significant difference between any other two groups. 2. The pattern of failure:For the teeth with no ferrule next the wedeg-shaped defect, repairable fracture pattern were defined as root fracture above the 1.5mm deep below the shoulder, adhensive failure or post fracture with no root fracture. For the teeth with 1.5mm ferrule next the wedeg-shaped defect, repairable fracture pattern were defined as adhensive failure or post fracture with no root fracture.Other type of fracture pattern were defined as unrepairable fracture pattern. The patterns of failure in Group A1-2 and B1-2 were both 37.5% repairable,other groups were all 0%. Fisher exact probability analysis results: a=0.05, P=0.03<0.05, the difference was significant.Conclusion:1. The maintenance of the overhang above the severe wedeg-shaped defect can help to improve the fracture strength of tooth restored with post and core crown, but it has no influence to the fracture pattern.2. The preparation of 1.5mm ferrule next the wedeg-shaped defect for tooth with severe wedeg-shaped defect restored with post and core crown makes the fracture pattern unrepairable, but it is not necessarily capable of improving the fracture strength.3. The fracture strength of tooth with severe wedeg-shaped defect restored with cast post-and-core is higher than restored with glassfiber post and resin core under the same condition, but the ratio of repairable fracture pattern of tooth restored with glassfiber post and resin core is bigger than tooth restored with cast post and core.4. For the tooth with severe wedeg-shaped defect, the fracture strength is maximum When the tooth overhang above the wedeg-shaped defect is not removed, ferrule next the wedeg-shaped defect is not prepared, cast post-and-core is chosen, the fracture strength is only a little smaller when glassfiber post and resin core, but the pattern of failure is better than that of tooth resored with cast post-and-core.5.When teeth with severe wedeg-shaped defect need restoration, we should Consider comprehensively in order to choose a approprite method of tooth preparation.