Marília Regalado Galvão

Effects of stress relaxation in beta-titanium orthodontic loops

INTRODUCTION This study evaluates the changes in the force system of the beta-titanium T-loop spring (TLS) caused by stress relaxation. METHODS Ninety TLSs with dimensions of 6 × 10 mm, made of 0.017 × 0.025-in beta-titanium alloy and preactivated by concentrated bends, were randomly distributed into 9 groups according to the time point of evaluation. Group 1 was tested immediately after spring preactivation and stress relief by trial activation. The other 8 groups were tested after 24, 48, and 72 hours, and 1, 2, 4, 8, and 12 weeks. By using a moment transducer coupled to a digital extensometer indicator adapted to a universal testing machine, the amounts of horizontal forces and moments and the moment-to-force ratios were recorded at every 0.5 mm of deactivation from 5 mm of the initial activation in an interbracket distance of 23 mm. RESULTS The horizontal forces and moments were higher (P <0.001) for group 1 compared with the other 8 groups, which were not different among themselves. All groups produced similar moment-to-force ratios (P = 0.600), with no influence of time. CONCLUSIONS The TLSs preactivated by concentrated bends had progressive load decreases over time, and this effect is critical in the first 24 hours.

Shear bond strength of repairs in porcelain conditioned with laser

The aim of this study was to evaluate the shear bond strength of repairs in porcelain conditioned with laser. Sixty porcelain discs were made and six groups were formed (n = 10): G1: conditioning with laser with potency 760 mW; G2: conditioning with laser with potency 760 mW and application of 37% phosphoric acid for 15 s; G3: conditioning with laser with potency 900 mW; G4: conditioning with laser with potency 900 mW and application of 37% phosphoric acid for 15 s; G5: application of 37% phosphoric acid for 15 s (group control) and G6: application of 10% hydrofluoric acid for 2 min. The composite resin was insert of incremental layers at the porcelain surface aided with a metal matrix, and photoactivation for 20 s each increment. The specimens were submitted to a thermal cycling by 1000 cycles of 30 s in each bath with temperature between 5 and 55°C. After the thermal cycling, specimens were submitted to the shear bond strength. The results were evaluated statistically through analysis of variance and Tukeys tests with 5% significance. The averages and standard deviation founded were: G1, 11.25 (±3.10); G2, 12.32 (±2.65); G3, 14.02 (±2.38); G4, 13.44 (±2,07); G5, 9.91 (±2,18); G6, 12.74 (±2.67). The results showed that the femtosecond laser produced a shear bond strength of repairs in porcelain equal to the hydrofluoric acid and significantly superior to the use of phosphoric acid. Microsc. Res. Tech., 2012.

Compressive strength of dental composites photo-activated with different light tips

The aim of this study was to evaluate the compressive strength of microhybrid (Filtek??Z250) and nanofilled (Filtek??Supreme XT) composite resins photo-activated with two different light guide tips, fiber optic and polymer, coupled with one LED. The power density was 653?mW?cm?2 when using the fiber optic light tip and 596?mW?cm?2 with the polymer. After storage in distilled water at 37???2??C for seven days, the samples were subjected to mechanical testing of compressive strength in an EMIC universal mechanical testing machine with a load cell of 5?kN and speed of 0.5?mm?min?1. The statistical analysis was performed using ANOVA with a confidence interval of 95% and Tamhane?s test. The results showed that the mean values of compressive strength were not influenced by the different light tips (p?>?0.05). However, a statistical difference was observed (p?<?0.001) between the microhybrid composite resin photo-activated with the fiber optic light tip and the nanofilled composite resin. Based on these results, it can be concluded that microhybrid composite resin photo-activated with the fiber optic light tip showed better results than nanofilled, regardless of the tip used, and the type of the light tip did not influence the compressive strength of either composite. Thus, the presented results suggest that both the fiber optic and polymer light guide tips provide adequate compressive strength to be used to make restorations. However, the fiber optic light tip associated with microhybrid composite resin may be an interesting option for restorations mainly in posterior teeth.