Shade Dependent Change in Dental Composite Light Transmittance During Curing

Objective: During curing, the resin in dental composites changes refractive index, typically allowing more light transmittance as cure proceeds.  This study evaluated the effect of shade on the change in transmittance of light through various thicknesses of one dental composite during curing.

Materials and Methods: Twelve shades of Venus Diamond (Heraeus) were light cured in 1, 2, and 3 mm thick Delrin disks (6 mm diameter; n=3) with the bottom surface placed atop a MARC Resin Calibrator (Blue Light Analytics).  Irradiance at the bottom during light curing (1500 mW/cm²; Valo, Ultradent) was measured continuously for 10 seconds; then specimens were re-irradiated 3X-10s while continuously recording irradiance.  Groups were compared with 2-way ANOVA/Tukey's (α=0.05).

Results: Total light energy reaching the bottom vs. top was 13, 9 and 5% at 1, 2 and 3 mm, resp.  For combined shades, increases in bottom irradiance during the 10s initial cure varied significantly with depth: 49mW/cm²@1mm > 32mW/cm²@2mm > 16mW/cm²@3mm (Figure 1), providing evidence for improved refractive index match of the resin and filler during curing.  Shades visually perceived as lighter (Figure 2) generally showed greater increase in bottom irradiance than darker shades at all depths (Figure 1). Additional changes in irradiance during subsequent exposures were reduced (<0, ~5%, ~10-20% for 1, 2, 3 mm, resp.).  This was likely due to the small additional changes in degree of conversion, which were accentuated somewhat at greater depths due a lower initial bottom cure for these specimens. 

Conclusions: All shades and all thicknesses (up to 3 mm) of one specific composite experienced some increase in light transmittance during curing, but the amount was shade dependent, with darker shades and greater depths of material showing less improvement.  This study confirms that for one specific resin composite formulation, some increase in translucency with cure occurs independent of shade.