Initial Composite Increment Conversion Enhancement by Exposure Through Second Increment

OBJECTIVES: To determine the effect of exposure through a second composite increment on monomer conversion (MC) of the bottom composite layer of the first increment.

METHODS: Three types of light curing units (QTH, Optilux 501); PAC (ARC IIM, AirTechniques), and LED (VALO, Ultradent) were each used for 20s to photocure combinations of composite (Kerr Sybron) increments, VITA shade A3: Prodigy Condensible (PC)) and Prodigy (P). A brass ring (2mm high, 6mm dia) was placed over a horizontal ATR element in a Fourier transform infrared (IR) spectrometer and filled with composite. IR spectra were collected of the uncured paste, after which the top surface was exposed to a light source held 4mm above the ring top. Spectra were recorded, another ring placed over the first and filled (or not (X)) with either composite, and another spectrum obtained (dark). The curing unit was activated (tip 2 mm above second ring surface) and another spectrum taken. N=5 for each combination. MC was calculated using changes in absorption ratios between aliphatic C=C (1636 cm^-1) and aromatic C=C (1608 cm^-1) in the cured and uncured states. Data were analyzed using a 2-way ANOVA and Tukey's post-hoc test at 0.05 pre-set alpha.

RESULTS: Table displays presence (yes) or absence (no) of a significant conversion difference between dark and second cure values at bottom of first increment.  (second increment type/first increment type), X =no second increment.

For PC as first increment, second exposure through either overlaying composite did not increase first increment bottom surface conversion.  MC of first increment only increased when no second increment (X) was placed, or P/P and the QTH.

CONCLUSIONS: In general, conversion of the bottom surface of a first 2-mm thick composite increment is not significantly raised by exposure of light through a second, overlying increment when it is being photocured.