Rebond strength of bonded lingual wire retainers.

There is no consensus in the literature concerning the rebonding procedure for orthodontic retainers. The aim of this in vitro study was to evaluate the bond and rebond strength of retainers bonded to enamel surfaces with and without composite remnants. The retainers were bonded with Excite and Tetric Flow on three different surfaces: clean enamel, enamel where the composite had been removed by a tungsten carbide bur, and with cured composite remnants roughened by a tungsten carbide bur. The bond strength was determined by means of a cantilever-tensile bond strength test using a repeated crossover design. Each tooth was rebonded twice and tested three times (N = 114). The surface was examined by scanning electron microscopy (SEM), electron backscatter diffraction and micro X-ray fluorescence (EDAX), and scored using the adhesive remnant index (ARI). Two-way analysis of variance of the mean bond strengths did not show significant differences between the three different enamel surface treatments. However, the specimens with cured composite remnants showed a higher standard deviation. This was confirmed by Weibull analyses. The ARI score showed that 96.5 per cent of bond fractures occurred at the retainer-resin interface. In contrast to the ARI score obtained in this study, the clinical ARI scores also showed failures at the resin-enamel interface. Based on these results, it is recommended that for rebonding the bond site is controlled, and the enamel surfaces are free of old composites remnants.

A comparison of finite element analysis with in vitro bond strength tests of the bracket-cement-enamel system.

The aim of this study was to determine the in vitro shear bond strength (SBS) and tensile bond strength (TBS) of 45 metal brackets bonded with Transbond XT to bovine enamel. The SBS was determined by loading the short and the long sides of the bracket base. Testing took place after storage of the specimens for 72 hours in water at 37°C. Fractures were analysed with the adhesive remnant index (ARI) and scanning electron microscope (SEM). The stresses in the system were analysed with finite element (FE) analysis models of the experimental set-up to identify the initial fracture point and the stress distribution at fracture. Statistical analysis of bond strengths was performed using analysis of variance (ANOVA) and the Tukey's post hoc test (P < 0.05). The ARI scores were analysed using Kruskal-Wallis one-way ANOVA on ranks. ANOVA showed significant differences between the three experiments. Loading the short side of the bracket resulted in the highest average bond strength. Tensile loading gave the lowest results. FE models supported the bond strength findings and SEM. FE analysis revealed peak stresses in the cement during loading, confirming that shear testing is sensitive to loading angles. The stress distribution over the bracket-cement-enamel system is not homogeneous during loading. Fractures are initiated at peak stress locations. As a consequence, the size of the bonding area is not predictive of bond strength. The bracket design and the mode of loading may be of greater relevance.

The influence of accelerating the setting rate by ultrasound or heat on the bond strength of glass ionomers used as orthodontic bracket cements.

Conventional glass ionomer cements (GICs) may be a viable option for bracket bonding when the major disadvantages of these materials, such as the slow setting reaction and the weak initial bond strength, are solved. The aim of this in vitro study was to investigate the influence of ultrasound and heat application on the setting reaction of GICs, and to determine the tensile force to debond the brackets from the enamel. A conventional fast-setting GIC, Fuji IX Fast, and two resin-modified glass ionomer cements (RMGICs), Fuji Ortho LC and Fuji Plus, were investigated. Three modes of curing were performed (n = 10): (1) according to the manufacturer's prescription, (2) with 60 seconds application of heat, or (3) with 60 seconds application of ultrasound. The tensile force required to debond the brackets was determined as the tension 15 minutes after the start of the bonding procedure. The mode of failure was scored according to the Adhesive Remnant Index (ARI) to establish the relative amount of cement remnants on the enamel surface. Curing with heat and ultrasound shortened the setting reaction and significantly (P < 0.05) increased the bond strength to enamel. The ARI scores showed an increase for all materials after heat and ultrasound compared with the standard curing method, most notably after heat application.