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

Area of Science:

• Materials Science
• Polymer Chemistry
• Spectroscopy

Background:

• Accurate monitoring of dental composite polymerization is crucial for restoration longevity.
• Light-curing protocols influence the degree of conversion and material properties.
• Infrared (IR) spectroscopy offers a non-invasive method for real-time reaction monitoring.

Purpose of the Study:

• To evaluate a novel approach using IR spectroscopy to monitor polymerization during light-curing.
• To compare continuous versus stepped light-curing intensity modes on conversion rates and reaction kinetics.

Main Methods:

• An IR spectrometer with an attenuated total reflectance (ATR) element was used as an IR-active substrate.
• Dental composite (Herculite XRV) was placed on the ATR crystal, and spectra were acquired during light exposure.
• Degree of conversion and maximum reaction rate were measured at various depths (0-3 mm) under different curing conditions (continuous vs. stepped intensity, 40s/60s exposures).

Main Results:

• Stepped intensity curing significantly reduced peak conversion rates at the surface and 1 mm depth compared to continuous exposure for both 40s and 60s durations (p < .05).
• Equivalent conversion values beneath the surface were achieved with longer exposure times (60s) in stepped mode.
• Despite reduced surface conversion, polymerization shrinkage in stepped mode caused debonding, indicating non-uniform stress development.

Conclusions:

• Stepped intensity curing, as utilized by the ESPE Highlight unit, results in lower conversion rates at superficial depths.
• Achieving comparable conversion to continuous curing requires extended exposure times with stepped intensity.
• The findings highlight potential challenges in achieving uniform polymerization and managing stress development with stepped curing protocols.