Light Beam Profile Influence on Composite Bond Strength

Objectives: To evaluate the beam profiles of two light-curing units (LCUs) and their influence on the micro-shear bond strength (µSBS) of resin composite.
Methods: Two LCUs were evaluated (VALO Grand and Bluephase Style). Their spectral irradiance was measured with a spectrophotometer and their beam profile images were recorded using a beam profiler camera. Sixteen fused-silica-cylinders were polished and their top surfaces were treated with hydrofluoric acid and silane. A template was used to delimitate a circle area 10mm in diameter. Nine holes were made on the template to consistently orient the position of the µSBS rods in relation to the LCU light tip. Both the cylinder and the LCU maintained their position with a silicone jig. The rod positioning was assigned as one rod at the center and eight rods on the periphery of the 10mm circle area diametrically opposed to each other. A universal adhesive (Adhese Universal) was applied to the surface of each hole and light-cured for 10s. Tygon tubes (inner diameter of 0.8mm) were placed in each hole. The tubes were filled with flowable composite (Natural Elegance) and cured for 20s simultaneously. After dark storage for 12h, µSBS testing was performed with a portable tensile-machine. Data were analyzed using a split-plot ANOVA and Tukey’s test (α=0.05; b=0.2).
Results: Both LCUs emitted a non-uniform light beam profile. However, the VALO Grand had a more homogeneous light emission than the Bluephase Style as reflected in the µSBS values. For the VALO Grand, no differences were found in the µSBS of the rods at different positions (p=0.87). For the Bluephase Style, rods near the areas where the beam profile showed lower irradiance had significantly lower µSBS values (p=0.008).
Conclusions: The light beam profile of LCUs can significantly influence the µSBS of resin composites.