Effect of distance on the power density from two light guides.

PURPOSE: This study determined the effect of distance on the power density from standard and Turbo light guides (Demetron/Kerr, Danbury, Connecticut).

MATERIALS AND METHODS: Power density was measured from 0 to 10 mm away from the tip of standard 8-mm curved light guides and 13/8-mm Turbo curved light guides. To determine the effect of distance on power density, a polynomial regression line was fitted. The Kolmogorov-Smirnov (K-S) statistic and the Wilcoxon rank sum (WR) tests were used to determine if there was a difference in the rate at which the power density decreased for the standard and Turbo light guides as the distance from the tip increased. Photographs of the light dispersion from each tip were also taken.

RESULTS: At 0 mm, the mean (+/- SD) power density from the two standard light guides was 743 +/- 6.1 mW/cm2 and from the four Turbo light guides was 1128 +/- 22.1 mW/cm2. As the distance from the tip of the light-guide tip increased, the power density decreased, but the rate of decrease was greater from the Turbo light guides than from the standard light guides. At 6 mm the power density from the standard light guides fell to 372 mW/cm2 (50% of the original value) and the power density from the Turbo light guides fell to 263 mW/cm2 (23% of the original value). Both the K-S statistic and the WR sum test indicated that the distribution of light intensities was significantly different from the two light guides (WR p-value = .0246, K-S p-value < .0001). The two estimated polynomials intersected at 3.66 mm, and the 95% prediction intervals intersected at about 2.8 and 4.8 mm. Therefore, beyond 5 mm away from the tip of the light guide, the standard light guides gave higher power density readings than the Turbo light guides. Photographs showed that the light dispersed at a wider angle from the Turbo light guides than from the standard light guide.

CLINICAL SIGNIFICANCE: The design of the light guide of a light curing unit affects light dispersion, power density, and ultimately the dentist's ability to properly cure composite. For these reasons, manufacturers should report the power density at the tip of the light guide and 6 mm from the tip of the light guide, since significant differences exist between light guide designs.