Fabio Hiroyuki Ogata Mitsui

Effect of remaining dentine structure and thermal‐mechanical aging on the fracture resistance of bovine roots with different post and core systems

AIM To evaluate the influence of remaining dentine thickness around post and core systems and the thermo-mechanical stresses on fracture resistance of bovine roots. METHODOLOGY This study involved 288 bovine incisor roots with standardized dimensions. Roots were randomly distributed into 24 groups (n = 12) according to root conditions [intact, semi-weakened, or weakened] and post and core systems [custom cast core, composite resin core, prefabricated metallic post, or prefabricated carbon fibre post], submitted or not to thermomechanical aging [5000 thermal cycles and 100 000 mechanical cycles at a 135-degree angle to the long axis of the root]. Specimens were submitted to a tangential compressive load (135 degrees angle) at a crosshead speed of 0.5 mm min(-1) until failure. Fracture resistance data were analyzed using 3-way anova and Tukey test: alpha = 5%. RESULTS Roots restored with composite resin cores demonstrated no resistance to mechanical aging. No statistically significant difference was observed between aged and nonaged specimens involving all post-systems. Roots restored with custom cast cores had the highest fracture strength, followed by prefabricated metallic posts and carbon fibre posts, regardless of root conditions and thermomechanical aging. The remaining dentine thickness affected significantly roots restored with custom cast cores; weakened roots had a lower fracture resistance. CONCLUSIONS Although custom cast cores had a higher fracture resistance when compared to the other techniques, the results were highly dependent on remaining dentine thickness. Prefabricated posts performed in a similar manner in intact, semi-weakened and weakened roots reinforced with composite resin.

Evaluation of dentin hardness and bond strength at different walls of class II preparations.

PURPOSE The aim of this study was to evaluate the Knoop hardness number (KHN) of dentin and the microtensile bond strength (microTBS) at different walls of MOD preparations, and the correlation between microTBS and the KHN of dentin. MATERIALS AND METHODS Human molars with MOD preparations were allocated into 3 groups according to the preparation wall (axial, gingival, pulpal). Each tooth was sectioned into halves, resulting in two similar preparation walls: one was used for measuring the KHN of dentin and the other for microTBS testing. Flat surfaces of the respective walls were obtained by cutting away adjacent walls. For the KHN measurement, twelve indentations were performed along the preparation wall (50 g/15 s). For microTBS testing, specimens were distributed into 3 groups according to the adhesive system (Adper Single Bond Plus/SB, Clearfil SE Bond/SE, Adper Prompt/AP). Composite blocks were built over surfaces; specimens were sectioned (3 slabs/wall) and trimmed to an hourglass shape (1 mm(2)). RESULTS The KHN of dentin walls was significantly different (ANOVA/Tukey); pulpal wall > axial wall > gingival wall. The effect of preparation walls on microTBS was dependent on the bonding system (2-way ANOVA/Tukeys test). SB had higher bond strength than the other systems at the gingival wall; and higher than SE at the axial wall. At the pulpal wall, SE presented a higher mean compared to AP. A significant negative correlation between microTBS and KHN was observed at the gingival wall. CONCLUSION It could be concluded that dentin structure can affect bond strengths, but the impact of the substrate varies according to the type of adhesive system.

Effect of cyclic loading on the bond strength of class II restorations with different composite materials.

This study evaluated the effect of cyclic loading on the bond strength of Class II restorations using different composite materials. Class II preparations with gingival margins located in dentin were performed on the mesial surface of 80 bovine incisors. The teeth were randomly allocated to eight groups (n=10) according to resin composite (Filtek Z250, Filtek Supreme, Tetric Ceram HB and Esthet-X) and use of cyclic loading. The restorations were bonded with the Single Bond adhesive system. Simulated aging groups were cyclic loaded for 200,000 cycles with 80N load (2Hz). The specimens were vertically sectioned (two slabs per restoration) and further trimmed into an hour-glass shape at the adhesive interface to obtain a final bonded area 1 mm2. Samples were placed in an apparatus and tested under tension using a universal testing machine. The data were analyzed using two-way ANOVA and Tukey test with a 95% confidence level. Aged groups presented significantly lower means when compared to the groups that were not aged (p=0.03). However, significant differences among composite materials were not observed (p=0.17). Regardless of the restorative composite material used, it could be concluded that the bond strength of Class II restorations at the gingival wall was affected by simulated cyclic loading.