Comparison of fracture resistance of mandibular molar teeth with different endodontic access cavities after thermocycling

The fracture resistance of endodontically treated teeth is lower than vital teeth. In traditional endodontic cavity design, the fracture resistance of teeth during function is reduced due to significant dentin loss. The aim of this study is to compare the effect of different endodontic cavity designs on fracture resistance in mandibular molar teeth which were restored with composite resin and thermocycled. A total of 50 freshly extracted, caries-free mandibular molar teeth were collected. All of the sampleswere visualized by cone-beam computed tomography, and the teeth were divided randomly into control (n=8) and 3 test groups (n=14) based on root canal morphology. No treatment was performed to Group1 (control). In Group 2; traditional endodontic cavity (GEK) was prepared, in Group 3; truss (cage) endodontic cavity (TREK) was prepared, and in group 4 ninja (ultraconservative) endodontic cavity (NEK) was prepared. After the access cavity preparation were completed as mentionedin the groups, root canals were prepared with WaveOne Gold files (Dentsply Maillefer, Ballaigues, Switzerland) and irrigated with 1.25% sodium hypochlorite. Root canals were filled with the matched cone technique using Waveone Conform Fit gutta-percha (Dentsply Maillefer, Switzerland) and AH-Plus canal sealer (Marka, şehir, ülke). Access cavities were sealed with composite resin. All teeth were aged for 6 months by subjecting to 1200 thermal cycles for 60 seconds between 5°C and 55°C in a chewing simulator. Fracture test was conducted immediately after aging with a Testometric test device (Testometric, M500-25kN, UK) at a cross-head speed of 1 mm/sec. with a 2.5 mm diameter round-tipped piston and the fracture values were recorded as Newtons. Data were analyzed by one-way ANOVA analysis of variance and Tukey post hoc analysis for multiple comparisons (P <.05).Fracture patterns were categorized as 'repairable' and 'irreparable'. The highest fracture resistance values were obtained in the control group. The control group showed significantly higher values than the GEK, TREK and NEK groups (p<0.0001, p=0.003 and p=0.017, respectively).While there was no difference between the TREK and NEK groups (p=0.900), the NEK group showed significantly higher values than the TREK group (p=0.035).There was no significant difference between the groups in terms of root canal shaping periods(p=0.112).When the total periods were considered, no difference was observed between the TREK and NEK groups (p=0.710). Root canal treatment procedure was completed in a shorter time period in the GEK group than in the TREK and NEK groups (p=0.003 and p<0.0001, respectively). The most repairable fractures were found in the control group (75%), however, similar repairable fractures rates were found among the tested groups (28.57%). Ninja "ultra-conservative" endodontic cavity access did not increase the fracture strength of teeth compared to those prepared with truss "cage" endodontic cavity access. Reducing the access cavity size increases the fracture resistance of the endodontically treated tooth. Different access cavities did not affect root canal shaping time periods. However, root canal treatment was completed in a shorter time in the GEK group.