[Composite curing with new LED equipment].
Standardized holes ("cavities") measuring 4 mm in diameter and 8 mm in depth were prepared using plexiglass cubes. The following LCUs were tested: Astralis 10 Halogen light curing units (LCU), Bluephase, Elipar Freelight 2 and SmartLite LED LCUs. Each LCU was examined according to the following test conditions. Light exposure time of 20 s and 40 s. Polymerization directly onto the composite, with a distance of 4 mm or through a 3 mm thick ceramic disk, respectively. The composite Tetric Ceram (A3) was inserted in one batch and after a seven-day incubation period the cubes were sectioned into two halves and polished. Knoop microhardness was determined 100, 200, 500, 1000, 1500, 2500, 3000, 4000, 5000 and 6000 microm from the occlusal surface. The total degree of polymerization of a composite specimen for any given curing time and curing light was determined by calculating the area under the hardness curve. Multiple linear regression revealed that the distance of the light tip from the composite, curing time and the LCU had a significant association with the degree of polymerization (p < 0.001). The same pattern of cure profile was shown by all LCUs during the course of the test. The hardest composite was usually encountered around 200 microm from the occlusal surface. It was concluded that without irradiation through ceramic and a light tip close to the composite a light exposure time of 20 s will sufficiently polymerize an increment of at least 2 mm. Polymerization through ceramic needs a curing time of 40 s from each side.