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KMID : 0988420030150030223
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Journal of Dental Science (C.N.U.)
2003 Volume.15 No. 3 p.223 ~ p.234
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Surface Hardness of the Dental Composite Cured by Light That Penetrate Tooth Structure According to Thickness of Tooth structure, Light Intensity and Curing Time
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Abstract
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For clinical success of light polymerized composite resin, it needs sufficient application of light intensity and curing time. Composite resin can be polymerized by a direct light projected on the surface of resin inside the real cavity, but it can also be polymerized by projecting light indirectly through enamel or dentin. This study measured the amount of light energy that was projected on a tooth material and analyzed the degree of light polymerized by measuring the surface hardness.
Optilux 501 (Demetron, USA), which uses a halogen light source, was used as a light polymerizing unit. For polymerizing light guide, two types were used for this study: a 12 mm diameter light guide with 840mW/㎠ light intensity and a turbo light guide with 1100mW/㎠.
Slices of tooth disc, which include enamel and dentin, were prepared as tooth materials. The size of the disc was 0.7 × 0.7㎝, and the thickness was 1 mm and 2 mm. Light polymerized composite resin with A1 color was filled in a mold of 5 mm in diameter and 2 mm in thickness, and it was polymerized by light according to the conditions specified for each group.
Specimens were divided into three groups; group 1 was investigated through a tooth material of 1mm thickness, group 2 through a tooth material of 2 mm thickness, and control group through a transparent glass slab of 1 mm thickness. Each group was further divided into four subgroups, and each subgroup was examined by a turbo light guide for 20 seconds and 40 seconds and by a light guide of 12 mm diameter for 20 seconds and 40 seconds. Fifteen specimens were fabricated for each group, and they were kept in a container for 24 hours, away from the light. Vickers´ hardness was measured by using a microhardness tester, and the following results were obtained.
1. The specimens from the groups 1 and control group, which were polymerized by a turbo light guide for 40 seconds, showed the highest hardness values. The specimens from group 2, which were polymerized for 20 seconds, demonstrated the lowest hardness regardless of the types of light guides(p<0.05).
2. From control group, the specimens polymerized for 40 seconds showed higher hardness than those for 20 seconds, regardless of the types of light guides (p<0.05). From group 1, the hardness values were decreased in the order of the specimens polymerized by a turbo light guide for 40 seconds, by a 12 mm light guide for 40 seconds, and by a 12 mm light guide for 20 seconds (p<0.05). From group 2, the specimens by a turbo light guide for 40 seconds showed the highest hardness(p<0.05), whereas the specimens polymerized for 20 seconds, regardless of the types of light guides, did not show any significant differences (p>0.05).
3. Group 1, which was polymerized by permeating tooth material 1mm thick, showed significant interaction between light intensity and curing time(p<0.05); but group 2, which was permeated by the tooth material 2mm thick, did not show any significant interaction between light intensity and curing time.
The results of this study suggest that, when polymerizing composite resin by permeating thin tooth material below 1mm thick, polymerizing degree increases as light intensity and curing time increase. When projecting tooth material over a specified thickness, the increase of polymerization will be limited even if light intensity or curing time is increased.
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