How Curing-Light’s age, Irradiance, and Curing-Distance Affect Composite Polymerization

Objectives: To determine the effect of curing light’s age, irradiance, and material-to-probe distance on the relative hardness (RH) of two resin-based composites (RBC) used in dental practices
Methods: In participating practices (n=110), light-curing units (LCU) were tested using a spectroradiometer (CheckUp, BlueLight-Analytics) to obtain 10s irradiance, which was recorded alongside their service age. Disc-specimens: TPH Spectra (Dentsply) and Tetric EvoCeram (Ivoclar-Vivadent) were prepared. Specimens received top-surface light-polymerization at 1mm and 4mm material-to-probe distance. Knoop-hardness numbers (KHNs) for each sample were obtained from 8 (4-top/4-bottom) measurements. Calculated RH ratios (average bottom/top KHN) were obtained. Results were analyzed using Pearson correlations, Chi-square tests, and independent-samples t-tests, with a pre-set alpha of 0.05
Results: Moderate negative (r=-0.328), but significant correlation was observed between LCU’s age and material-to-probe distance. Moderate positive (r=0.442), but significant correlation was observed between LCU’s irradiance and material-to-probe distance. An average 0.80 RH polymerized at 1mm material-to-probe distance was obtained by 69.0% of Tetric EvoCeram specimens and 82.3% of TPH Spectra specimens, respectively. An average 0.80 level of RH at a 4 mm material-to-probe distance was obtained by 28.3% of Tetric EvoCeram and 37.2% of TPH Spectra specimens. A significantly higher percentage (p<0.001) of LCUs produced 0.80 RH level with TPH Spectra compared to Tetric EvoCeram at both material-to-probe distances.
Conclusions: Irradiance and LCUs’ age significantly affected the Knoop hardness profile and mechanical properties of RBCs. Participating LCUs failed to achieve the 0.80 RH desired ratio at 4mm material-to-probe distance, a clinically relevant distance: 71.7% for Tetric EvoCeram and 62.8% for TPH Spectra, respectively. The magnitude of these relationships suggests that other factors are contributing to RBC microhardness, such as LCU’s spectral irradiance output and beam profile differences. These findings can raise the clinically pertinent question of the efficacy of polymerization in the proximal boxes of Class II RBC restorations.