Effect of blue and violet light on polymerization shrinkage vectors of a CQ/TPO-containing composite

Objective To evaluate the effect of light-curing wavelengths on composite filler particle displacement, and thus to visualize localized polymerization shrinkage in a resin-based composite (RBC) containing camphorquinone (CQ) and Lucirin TPO (TPO). Methods Three light-curing units (LCUs) were used to light-cure a RBC containing CQ and TPO: a violet-only, a blue-only, and a dual-wavelength, conventional (Polywave^®, emitting violet and blue wavelengths simultaneously). Zirconia fillers were added to the RBC to act as filler particle displacement tracers. LCUs were characterized for total emitted power (mW) and spectral irradiant output (mW/cm²/nm). 2-mm high, 7-mm diameter silanized glass cylindrical specimens were filled in a single increment with the RBC, and micro-computed tomography (μ-CT) scans were obtained before and after light-curing, according to each LCU (n = 6). Filler particle movement identified polymerization shrinkage vectors, traced using software, at five depths (from 0 up to 2 mm): top, top-middle, middle, middle-bottom and bottom. Results Considering different RBC depths within the same LCU, use of violet-only and conventional LCUs showed filler particle movement decreased with increased depth. Blue-only LCU showed homogeneous filler particle movement along the depths. Considering the effect of different LCUs within the same depth, filler particle movement within LCUs was not statistically different until the middle of the samples (P > .05). However, at the middle-bottom and bottom depths (1.5 and 2 mm, respectively), blue-only LCU compared to violet-only LCU showed higher magnitude of displacement vector values (P < .05). Use of the conventional LCU showed filler displacement magnitudes that were not significantly different than blue-only and violet-only LCUs at any depth (P > .05). With respect to the direction of particle movement vectors, use of violet-only LCU showed a greater displacement when close to the incident violet LED; blue-only LCU showed equally distributed particle displacement values within entire depth among the samples; and the conventional LCU showed greater filler displacement closer to the blue LED locations. Significance Filler particle displacement in a RBC as a result of light-curing is related to localized application of light wavelength and total emitted power of the light emitted on the top surface of the RBC. When the violet LED is present (violet-only and conventional LCUs), filler particle displacement magnitude decreased with increased depth, while results using the blue-only LED show a more consistent pattern of displacement. Clinically, these results correlate to production of different characteristics of curing within a RBC restoration mass, depending on localized wavelengths applied to the irradiated surface.