Statement of problem: Inadequate polymerization diminishes the physical properties of composite and compromises material strength. Newly developed light-polymerizing units (plasma arc and high intensity halogen) operate at relatively high intensity and are purported to provide optimum properties to composites in a shorter time.
Purpose: The aim of this in vitro study was to examine the polymerization efficacy of 3 different polymerization units through a simulated ceramic restoration by determining degree of monomer conversion of a composite luting Material.
Material and methods: A conventional halogen light, a plasma arc light, and a high-intensity halogen light were used to polymerize a composite (Variolink II) with or without catalyst. A pressable ceramic block (5 mm in diameter, 2 mm in height) was used as an interface between the polymerizing light tip and composite. The composite/veneer combination was exposed to 2 different polymerization time protocols for each polymerization unit: (1) 20 or 40 seconds for conventional halogen light, (2) 3 or 6 seconds for plasma arc light, and (3) 10 or 20 seconds (under ramp mode) for the high-intensity halogen light. Using different light units, 120 composite specimens were evaluated (n = 5). Fourier transformation infrared spectroscopy was used to determine the level of light polymerization of the resin through the ceramic material with each of the light sources immediately after polymerization or after 24 hours. Degree of conversion was calculated as a percentage of experimentally polymerized versus maximally polymerized composite. The data were analyzed by 4-factor analysis of variance (alpha = .05). Independent t tests (alpha = .05) were used to detect differences between groups defined by the specific interacting variables.
Results: Degree of conversion values varied with the light source (P < .05), polymerization type (P < .05), testing time (P < .01), and exposure to each light source (P < .01). Significant interactions were present between light source and polymerization type (P = .00), light source and testing time (P = .007), and polymerization type and testing time (P = .004). The degree of conversion values were significantly higher in dual-polymerized specimens after 24 hours (P < .01), but there were no significant differences in light-polymerized specimens after 24 hours. The degree of conversion values achieved were significantly lower (P < .05) when specimens were dual polymerized by conventional halogen light and measured immediately (31.59 +/- 7.76). The degree of conversion values achieved were significantly (P < .05) higher with dual polymerization by high-intensity halogen measured after 24 hours (65.06 +/- 8.14). There were no other significant differences among groups.
Conclusion: The highest degree of conversion values of composite were achieved with polymerization by high-intensity halogen. The plasma arc light achieved similar polymerization of composite through ceramic material in a markedly shorter period than conventional halogen light.