Using Beam Profiles to Predict the Curing Time of MOD Restorations

Objectives: Determine the minimum exposure time to light cure a Class II MOD restoration based on lateral beam-profiles and composite absorption curves.
Methods: Lateral beam-profiles plotting areas of equal irradiation were obtained using a coherent optical tank (110x80x30mm) filled with 0.025% rhodamine B solution and placing 9 different light curing units (LCU) on top and registering the lateral beam-profile from the side with an ASI ZWO ASI 174MM CMOS camera. Best fit absorption profiles of two bulk fill composites (Filtek Supreme Bulk fil A2, Tetric Powerfil IVA) and from the rhodamine solution were obtained using three replications and the Marc resin calibrator with a 10 mm aperture. The radiant exposure required to reach the maximum degree of conversion was determined on thin composite films using an FTIR (Nicolet iS20) device with two scans per second and an experimental LCU (400mW/cm²). In PowerPoint, an ideal MOD cavity (2mm occlusal depth, 4mm depth in the 2mm wide boxes) was superimposed on the lateral beam-profile in the assumption that the restoration would be cured in a single irradiation from the center of the tooth.
Results: Both composites and the 0.025% rhodamine solution showed similar absorption curves. The FTIR showed that approximately 8J/cm² is needed to cure the composite fully. Applying this value to the absorption curve of the composite grossly underestimates the time required for complete polymerization. Using the lateral beam-profile, which usually shows a decrease of irradiation towards the periphery to estimate the curing time, shows that very long curing times are needed for curing the bottom of the box when trying to cure a composite in a molar MOD cavity in one exposure.
Conclusions: Even if an LCU has a lens larger than the diameter of the molar, both boxes should be irradiated separately, centering the LCU on the box.