| Post-and-core technique is the main method to restore theendodontically treated teeth. Beacause the strength of differerntpost-and-core restorations can determine their success rate, manyresearches have been done on the fracture resistance of differerntpost-and-core restorations. However, most of these researches onlyreflect the condition immediately after restored. Post-and-corerestoration as a permanent prosthesis, is subjected to cyclic forces inoral cavity. At present few researches were about the long-term effect ofdifferent post and core systems.The purpose of this in-vitro study is to compare the fracture resistance of teeth restored with different post-and-core systems bycyclic loading test and to prospect their long-term effect. This studyinclude two aspects: (1)to compare the fracture resistance of differentpost-and-core restorations with or without cyclic loading; (2)to explorethe effect of cyclic loading to the fracture resistance of differentpost-and-core restorations.In this study, a total of 40 recently extracted mandibular premolarswith their crowns removed were endodontically treated.Teeth wererandomly devided into two groups. Each group was divided into foursubgroups of 5 teeth each,Group A:restored with fiber post and resincore; Group B:with all-ceramic post and resin core; GroupC:with castmetal post and core; GroupD:with resin filling. All teeth were restoredwith full cast metal crowns and embeded in acrylic resin. Specimens ofone group were exposed to 300,000 cyclic loadings(95N～105N at afrequency of 2.33Hz) in a cyclic loading machine and 1000 thermalcycles between 5℃～55℃. Samples of the other group were notsubjected to cyclic loading and thermal cycling test. To explore theirfracture resistance, All samples were loaded in the AG-IS mechanicaltesting machine with a compressive load (N) applied at 135 degree to the long axis of the specimen at a crosshead speed of 1 mm/min untilfracture. The fracture load and fracture mode of each teeth wererecorded. Randomized block analysis of variance and SNK test wereused to compare the fracture resistance among each groups. Paired t testwas used to compare the fracture resistance with or without cyclicloading. A Fisher's exact test was conducted for evaluation of thefailure mode.The results of this study are as follows:1. Without cyclic loading, the fracture resistance of ceramic post andcast metal post and core restoration were higher than that of fiber postand resin filling restoration.2. With cyclic loading, the fracture resistance of each specimen wassignificantly lower, and teeth restored with fiber post and resin fillingexhibited significantly higher resistance to fracture than those withceramic post and cast metal post and core.3. The fracture mode of each kind of post restoration didn't changethrough cyclic loading(P＞0.05). Fractures that allow repair wereobserved in group A,B and D,whereas unrestorable,catastropic fractureswere observed in group C. From this study we can come to the following conclusions:1. With cyclic loading and thermal cycling test, the fracture resistanceand fracture mode of teeth restored with fiber post or with resin fillingwere superior to that with cast metal post and core,so when restoringendodontically treated teeth in clinic, we can choose the first tworestorations as better choices.2. With cyclic loading, the fracture resistance of ceramic postrestoration was lower, which implies that the long-term effect ofalumina-zirconia nano-composite infiltrated ceramic post is notprospecting.3. The fracture pattems of teeth restored with fiber post,ceramic postand resin filling allow repair.4. When restoring the endodontically treated teeth in clinic,we shouldpreserve the remaining tooth structure as much as possible.5. The results of this study was close to that of clinic,so that themethod of cyclic loading and thermal cycling can simulate the conditionin oral cavity to some degree.
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