Objectives: 1) To compare the polymerization effectiveness of 6 different posterior composites; 2) to compare the polymerization effectiveness of 2 and 4 mm increments in two composites. Methods: Silicone molds were used to form 30 composite slabs, 2 mm thick, divided in 6 groups (n=5). Composites used were Pyramid Dentin (Bisco), Solitaire (Heraeus/Kulzer), Filtek P60 (3M), Synergy Compact (Coltène/Whaledent), Prodigy Condensable (Kerr) and Surefil (Dentsply/Caulk). For each of the two composites whose manufacturers claim effective polymerization in increments greater than 2mm (Prodigy Condensable and Surefil), 5 additional 4 mm thick slabs were made. The specimens were polymerized with a 3M Curing Light 3000XL (600 mW/cm²) for the time recommended by the manufacturers. To indirectly assess the effectiveness of composite polymerization, microhardness (VHN) was determined immediately after polymerization on the bottom and top surfaces of the composite slabs. A Hardness Ratio (HR) was calculated using the following formula: HR=VHN of bottom surface/VHN of top surface. Ideally, HR should be 1, when the VHN of the bottom is similar to the top. It has been suggested that an effective polymerization is achieved when HR³0.8. Results: Mean HR values are showed in the following table:

Statistical analysis (ANOVA) made for the 2mm specimens, showed significant (p<0.0001) differences between groups. HR values sharing superscript letters are not significantly different. The comparison (t-test) between 2mm and 4mm showed significant (p<0.001) differences for Prodigy Condensable and Surefil. Conclusions: 1) Two composites (Pyramid Dentin and Solitaire) had HR inferior to 0.8, suggesting inadequate polymerization at 2mm. The other composites showed effective polymerization (HR³0.8). 2) Increasing cavity depth to 4 mm resulted in significant drop of the HR, suggesting inadequate polymerization effectiveness.