Objectives: It was suggested in the dental literature that the shrinkage stress generated by a subsequent layer of higher modulus RBC can be absorbed by a flowable RBC intermediary layer, manifested clinically as a reduction in cuspal deflection. Furthermore, it was proposed that the higher strain capacity of flowable RBCs may be advantageous to protect the tooth-restoration complex from thermal stress. Consequently, the cuspal deformation patterns in extracted upper premolars were investigated during light-curing process of an incrementally applied higher modulus resin-based composite (RBC) with and without an intermediary flowable RBC layer. Methods: 40 upper premolars had standardized MOD cavities prepared. Restoration of the teeth involved the placement of Filtek™ Supreme RBC (3M ESPE) in eight increments with and without an intermediary flowable RBC layer (Filtek™ Flow, 3M ESPE). Cuspal deformation was recorded using a Twin Channel Deflection Measuring Gauge, in real time for 220 seconds from the start of light-exposure, for each increment of composite which was polymerized for 40 seconds with a halogen light-curing unit operating in standard mode at an intensity output of 740±29mWcm^-2. Data were statistically analyzed with Kruskal-Wallis and Student One-Tail/Two-Tail tests at a 95% significance level. Results: The cuspal extension during light-exposure of each composite restoration was significantly reduced compared to the cuspal deflection recorded post-irradiation (P<0.05). A significant reduction in cuspal deflection was evident when the higher modulus RBC material was placed with the intermediary flowable RBC (P<0.001). The premolars restored with and without the flowable displayed similar cuspal extension results. Conclusions: The light-curing process with a halogen curing unit caused in premolars cuspal extension followed by cuspal deflection. The results of the current study suggest that there was a benefit to the operator only in terms of a reduction in cuspal deflection not also in cuspal extension when a flowable was used.

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