Effect of Distance on Power Density from Curing Lights
Howard E. Strassler
Felix CA and Price RB. J Dent Res. 2006; 85 (Special Issue B):Abstract #2468.
OBJECTIVES: Turbo light guides and focusing lenses are used to boost the power density at the tip of the light guide but, as distance from the curing light increases, these power-boosting devices may have a detrimental effect on power density. This study investigated the effect on power density as the distance increased from 0 mm to 10 mm. The hypothesis was that all curing lights would behave similarly as distance from the tip of the light guide increased. METHODS: The spectra and power densities from 12 curing lights, (Optilux501, Sapphire, Allegro, Bluephase, Bluephase 16i, LEDemetron I, LEDemetron II, UltraLume 5, SmartLiteIQ, FlashLite 1401, RadiiPlus, and Freelight 2) were measured from 0 mm to 10 mm from the end of the light guide using a calibrated laboratory grade spectroradiometer attached to a cosine corrector. Three examples of each light (total 36 lights) were tested in a random sequence so that valid conclusions could be made concerning each brand of light. RESULTS: The hypothesis was rejected. The power density decreased for all lights as the distance increased (P < .01), but the rate and extent of this decrease was not similar for all lights (P < .01). At 0 mm the Sapphire delivered the greatest irradiance followed by the Bluephase 16i light. However, the power density from the Bluephase 16i declined rapidly as the distance increased. At a distance of 6 mm from the light guide, only the Allegro and Sapphire lights delivered more than 1,000 mW/cm2, and some lights delivered less than 200 mW/cm2. The Sapphire and Allegro curing lights were the best at maintaining a high power density (P < .01). CONCLUSIONS: The design of the curing light affects the rate at which power density decreases as the distance from the light guide increases. High power densities measured at 0 mm do not necessarily mean that the light will deliver a sufficient power density over clinically relevant distances to adequately cure the resin.
In the past 5 years there have been a number of research studies reported on the depth of cure of composite resin with a variety of curing lights and light probes. In these studies, the tip of the light is placed on top of the composite resin that has been placed in a hollow tube (effectively no distance away from the composite being cured). The depth of cure is measured and reported. Unfortunately this is not how we cure composite resin intraorally. When we are placing and light-curing a posterior composite resin because of the height of the cusp tips, the light guide tip is usually at least 1 mm to 2 mm away from the occlusal cavosurface margin. For a Class 2 preparation, the light source can be as far as 6 mm to 7 mm from the gingival margin. Based on the principles of light power density (amount of light energy measured from the source) and distance (depth in a proximal box), there is a significant drop off of light-curing energy based on the fact that the further the light tip is from the surface needing light-curing, the less energy is available to cure the adhesive and composite resin. This power density is also complicated by the fact that when light is directed into a proximal box, there is light scatter that also diminishes the amount of light available to cure that most important gingival margin and gingival area of the proximal box.
This study addresses an extremely important issue: is your curing light really curing composite resin at the gingival margin? Based on this study and others evaluating the potential to cure posterior Class 2 composite resin restorations, Sapphire and Allegro (both manufactured by Den-Mat® Corporation, Santa Maria, CA) meet the challenges of light-curing Class 2 composites. A complete cure at the gingival margin should contribute to resistance to microleakage and recurrent caries at the gingival margin of posterior composite resin restorations.
Howard E. Strassler, DMD