Effect of Different Light-Curing Units on Microhardness and Curing Depth of Bulk-Fill Composite Materials

Abstract

Background & Objective: Resin composite has undergone continuous developments like changes in the fillers and initiators. One such improvement is the new bulk-fill composites which are materials intended for bulk placement up to 4mm. However, an optimum polymerization to the full depth of the restoration is of utmost importance in order to obtain proper mechanical and physical properties of resin composites. The aim of this study was to measure the surface hardness of the top and bottom surfaces of the composites and to determine the depth of cure of bulk-fill composites according to Vickers Microhardness profiles (VHN) using two different types of light curing units.

Materials and Methods: A total of 90 specimens (n=15) were used in this study: three high viscosity bulk-fill composite materials were used, Filtek Bulk Fill (3M ESPE), X-tra fil Bulk-Fill (Voco) and SonicFill (Kerr). Two different curing light were used namely, LED ((Hemao Midical Instrument Co. Ltd, China)) at 1000 mW/cm2 and a Quartz Tungsten Halogen (QTH) curing unit (LA-500 Blue Light Apoza, CHINA) at 500 mW/cm2. cylindrical specimens (5 mm × 6 mm) of each material were prepared in black teflon molds.  To evaluate micro-hardness, Vickers hardness at top and bottom of each sample was measured after 24 hours post curing using Microhardness tester (Qualitest, Germany) load 200g/15 seconds. The mean hardness values obtained from the top and the bottom surface of each material were used to compare the micro-hardness of the various materials. The mean values obtained from the bottom surface were compared to the virespective values of the top surface of each material (bottom/top ratio) and used to calculate the depth of cure. Data were analyzed via T student, One-Way ANOVA, and Bonferroni tests (P>0.05).

Results: The micro-hardness and depth of cure tests showed a significant difference between the three materials after 24 hours post curing. When the curing lights were compared the LED Curing Unit  obtained significantly better depth of cure compared to QTH Units. Depth of cure ratios were found to be more than 0.80 for all composite types in LED group. The LED curing light showed greater micro-hardness values than the QTH unit.

Conclusions: In general LED curing light produced better hardness and depth of cure values than QTH curing light. Bulk-fill resin composites can be cured to an acceptable post-cure depth, according to the manufacturers' claims. HVN values that recorded after 24 h of cure for the specimens of the three materials were acceptable and suitable for clinical application.