Power distribution across the face of different light guides and its effect on composite surface microhardness

The aim of this study was to examine the influence of light guide type on the distribution of irradiant emission from a light-emitting diode (LED) curing light and to measure the effect of light dispersion on surface microhardness across the top and bottom surfaces of two types of composite resins.

A laser beam analyzer (LBA-700, Spiricon) was used to evaluate light distribution (Top Hat factor [THF]) across the distal surface of a standard and turbo light guide from an LED curing light (Bluephase, Ivoclar Vivadent). Composites (Z100 [hybrid], A110 [microfill]; 3M ESPE) were placed in blackened rings (2 x 11 mm) and exposed at 0 mm for 5 seconds (Z100) or 15 seconds (A110) using the light guides at similar irradiance, energy density, and exit diameters (N = 5). Similar irradiance values were produced by using the turbo light guide on the "low power" setting of the curing light and the "high power" setting when using the standard light guide. THF values were analyzed with an unpaired t-test. Knoop hardness (KHN) was determined on the top and bottom surfaces (Leco) in 1-mm lateral increments from the specimen center and proceeding 4 mm in both east-west and north-south directions. The effects of the major factors (light guide type and lateral distance) on the hardness of each composite were analyzed using multiple analysis of variance (ANOVA), and a two-tailed, unpaired Dunnett's t-test determined when lateral hardness values significantly differed from that at the specimen center. The percentage difference between maximum and minimum (max-min) hardness values for each specimen, with respect to distance from specimen center, and the percentage decrease for the standard and turbo light guides, with respect to both composite resin types, were compared using ANOVA and the Tukey's post-hoc test.

The standard guide had a significantly higher (i.e., more uniform light distribution) THF than did the turbo tip (p < 0.001). For the microfill, significant differences in hardness were found based on the distance from the specimen center (p < 0.0001), and with respect to the top or bottom surfaces (p < 0.0001). However, no difference was found between the two types of light guides (p = 0.939). For the hybrid, significant differences in hardness were found based on lateral distance (p < 0.001), surface (p < 0.001), and light guide type (p = 0.045).However, for both composites, significant interactions were present. Significant differences were found between hardness at the specimen center and at various lateral distances, depending on composite type, surface, and light guide type. The percent age max-min hardness decrease across the surface was significantly less for the standard light guide (p < 0.0001) and at the top surface (p < 0.02) with both composite resin types. Also, the least percentage max-min hardness decrease occurred on the top surface of the microfill material (p < 0.001).

The standard light guide produced a more homogeneous distribution of light across the tip end compared with the turbo light guide, based on the THF. Composite surface hardness patterns correlated with the applied irradiance distribution profiles, yielding greater hardness at higher irradiance locations for both top and bottom surfaces.

Light guide selection may influence the uniformity of surface and subsurface hardness when light-curing composite resin restorations.