Effect of delay in layering particulate filler resin composites

Objectives: Adhesion of new layer of particulate filler composites (PFC) to the already polymerized ones may be affected when the time between application of subsequent layers is prolonged. The aim of this study was to compare bond strength of two PFCs relayered after four time points. Methods: Disc shaped PFC (PFC1=Sinfony, PFC2=Targis) (N=96) were fabricated and processed according to each manufacturer's instructions. They were randomly assigned to 4 groups. PFC surfaces were conditioned with their corresponding activator (Sinfony, 3M ESPE) or wetting agent (Targis, Ivoclar-Vivadent). Specimens were stored in dark. After time points of 5 minutes, 24 hours, 1 week, a new layer of PFC of same kind with the substrate was adhered to the substrate in polyethylene molds. In control groups, PFCs were adhered to substrate immediately after application of the adhesive resin or wetting agent. Final polymerization was made in polymerization devices assigned for each PFC. Shear force was applied to adhesive interface in a universal testing machine. Data were analyzed with 1-way, 2-way ANOVA, Tukey's tests. Results: Both the PFC types and relayering time points significantly affected the bond strength results. While PFC1-PFC1 adhesion was not affected when relayering was performed at the tested time points (34.5±5.3 - 32.2±4.4 MPa) (P=.939), it affected the results for PFC2 significantly (38.9±6.9 - 25±5.7 MPa) (P=.0008). When compared to 5 minutes, 24 hours (P=.0028), 1 week (P=.0025) delay showed significant decrease in the results for PFC2. Conclusions: Delay in relayering did not affect the PFC1-PFC1 adhesion but PFC2 showed decreased bond strength especially after 1 week. Restorations made of Sinfony can be relayered at delayed time points up to 1 week but Targis should be processed preferably in one day in order not to experience decrease of bond strength between the layers which may induce clinical chipping problem.