Influence of curing light power and energy on shrinkage force and acoustic emission characteristics of a dental composite restoration

Abstract

Purpose: To evaluate the density effects of light power and energy on the volumetric polymerization shrinkage and acoustic emission (AE) characteristics of a dental resin composite in the cavities of human teeth. Methods: Two experiments were performed at different power levels (1,000 and 4,000 mW/cm(2)) using a light curing unit: (1) cylindrical cavities with diameters of 4 nun and depths of 2 ram were constructed using two symmetric steel molds. The cavities were filled with resin, and the shrinkage force during polymerization was measured using a load cell attached to the mold. Polymerization shrinkage forces were measured under four conditions (1,000 mW/cm(2) x 10 seconds, 1,000 mW/cm(2) x 20 seconds, 4,000 mW/cm(2) x 3 seconds, and 4,000 mW/cm(2) x 5 seconds); (2) tooth specimens with cavity diameters of 6 mm and depths of 2 mm were made from human molars. AE signals during polymerization shrinkage were monitored in real time for 10 minutes after irradiation and two AE factors (amplitude for defect size and hit number for defect number) were assessed in the examination of defects. Two levels of light energy (20 J/cm(2) = 1,000 mW/cm(2) x 20 seconds and 12 J/cm(2) = 4,000 mW/cm(2) x 3 seconds) were used. Results: Shrinkage occurred more quickly at 4,000 mW/cm(2) than at 1,000 mW/cm(2) during the initial phase. The shrinkage force became almost the same for equivalent light energy as time increased. Higher light energy (20 J/cm(2)) under low-power conditions (1,000 mW/cm(2)) caused larger cumulative numbers of AE hits than did lower light energy (12 J/cm(2)) under high-power conditions (4,000 mW/cm(2)). At 4,000 mW/cm(2) and 12 J/cm(2) (i.e., high power, low energy), the average amplitude of the AE signals was larger than at 1,000 mW/cm(2) and 20 J/cm(2) (low power, high energy).