Effect of light exposure time on the depth of curing in various composite resin systems.

Hardness of both the top and bottom surfaces were determined for five different composite resins varying the thickness and exposure time to light. Specimens of the five different composite resins (Durafill®, Visio-Dispers ®, Prisma-Fil®, Estilux®, and Visio-Fil ®) were prepared in brass rings with thicknesses of 1.5, 2.0, and 2.5 ram. A Barcol Hardness Number (BHN) of the top or bottom surfaces of the specimens was obtained immediately after exposure to light (20, 40, 60 sec). The mean and standard deviations of the individual readings were calculated. For each thickness, the hardness of the top surface increased with time. Although this was also true for the bottom surface, the difference in the hardness of the top and bottom was increased with the increase of thickness. The maximum difference always occurred with the shortest time intervals for light exposure. Thus, as the thickness of composite restoration increases, longer exposure time is required. For all the materials, the hardness number of composite resin after 20and 40-sec exposure to light were always significantly lower than the 60-sec exposure. At BHN 50, it was difficult to mar the surface of composite resin. Thus, BHN 55 was selected as a conservative baseline for the minimal hardness of the surface. Visible light-curing composite resins are being used in dentistry for a variety of restorative and preventive procedures. These materials have the following advantages over the self-curing composite resins: (1) the single paste formulation requires no mixing -resulting in less porosity, (2) they have adequate working time, (3) they cure faster, and (4) they better color stability since there is no amine accelerator present. Visible light-curing composite resins also have certain advantages that the ultraviolet light-curing composite resins do not possess: deeper curing depth, and effective light penetration through tooth structure. There are only a few disadvantages found in the visible light-curing composite resins that chemical-curing composite resins do not have: (1) the technique is sensitive [polymerization can begin under an operating light], (2) additional equipment is necessary, and (3) there is limited cure depth. The advantages listed for these systems are dependent on inherent properties in the composite resins. In composite resins with chemically induced polymerization, the reaction takes place almost uniformly throughout the bulk of material. However, it has been found that the degree of polymerization of visible light-curing composite resin is generally dependent on the thickness of the restoration. 1,2 Incomplete polymerization in the inner part of the restoration may lead to retention failures and adversely affect the pulp tissue. 3"4 To determine the cure depth, the following variables were studied: (1) time of light exposure, (2) resin shade, (3) distance of the light from the resin surface, and (4) postexposure time. Ruyter and Oysaed 5 showed that there is an increase in maximal curing depth with increasing time. Leung et al. 6 investigated the polymerization of visible light-curing composite resins after a short exposure to light (10-60 sec). They measured hardness immediately following cessation of exposure to the light source, and at a later time. They found that the extent of polymerization at the top and bottom surfaces increased with postirradiation time. The general trend for the hardness of the surface after a specific exposure time followed the typical saturation curve; there was a slow increase in hardness values for the first 20 rain after exposure, followed by gentle leveling off up to 60 min, and then slightly higher hardness values at one and seven days. The size and amount of the inorganic particles are also important variables. Ruyter and Oysaeds also PEDIATRIC DENTISTRY: March 1985/Vol. 7 No 1. 19 concluded from their study that greater curing depths were attained with materials containing larger inorganic fillers than those containing smaller inorganic fillers. In clinical situations, it would be an advantage to know the hardness of composite restorations immediately after insertion since patients may begin mastication immediately following their dental appointments. The purpose of this study was to determine the hardness of top and bottom surfaces among five different composite resins of various thicknesses immediately after being cured by the same visible light device as a function of the light exposure time. Method and Materials