Novel approach to measure composite conversion kinetics during exposure with stepped or continuous light-curing

Purpose: The objective of this research was to evaluate a novel approach to monitor the polymerization reaction during a light-curing exposure by using infrared (IR) spectroscopy. Materials and Methods: An IR spectrometer was equipped to use an attenuated total reflectance (ATR) element as an IR-active substrate. The uncured composite (Herculite XRV, Shade A2, Kerr, Orange, California) was placed against the crystal, and the IR spectrum was continuously obtained during various exposure scenarios. The degree of conversion and the maximum rate of reaction were monitored at 0 mm (top surface), 1 mm, 2 mm, and 3 mm beneath the surface. The exposure conditions included continuous 40-second or 60-second exposures at 100% intensity (800 mW/cm²) or a stepped output of 10 seconds at 17% maximal output (133 mW/cm²) followed by full output for the remainder of the 40-second or 60-second exposure (Elipar Highlight, ESPE, Norristown Pennsylvania). The results were analyzed using MANOVA with appropriate post hoc tests (p <.05). Results: For 40-second exposures, the peak conversion rates were significantly reduced (p <.05) when using the stepped exposure mode compared to the continuous exposure: 40-second top surface: stepped = 5.5%/s ± 0.4, continuous = 10.5%/s ± 1.0; 1 mm step = 3.6%/s ± 0.4, continuous mode = 4.8%/s ± 0.2. The same trend was noted when using the 60-second exposure. Equivalent conversion values (p >.05) beneath the surface between stepped and continuous exposure modes at similar depths 60 seconds after light initiation were only attained at 3 mm 4for the 40-second exposure. However, using the 60-second exposure, equivalent conversion values between step and continuous exposure modes at similar depths were obtained. Even with a reduced conversion rate at the surface using the stepped cure mode, polymerization shrinkage forces were sufficient to debond the specimens from the test crystal after only 20 seconds into the exposure. This result indicated that stress development in the curing composite was non-uniform, and stress values developed at the surface of the restoration were the greatest.