Microhardness of a new flowable composite liner for posterior restorations

Objectives: A recently introduced flowable composite liner for I and II class restorations is supposed to be efficiently polymerized up to 4mm thick increments according to manufacturer’s indications. This preliminary study investigated the microhardness of SDR liner in relation to thickness and storage. Materials and methods: Four moulds (thickness: 2, 3, 4, and 5mm, diameter: 4mm) were prepared. Ten SDR (Dentsply Caulk) discs for each thickness subgroup (total sample size = 160)were polymerized following 4 different protocols by using halogen (Elipar 2500, 3M ESPE) or LED (BluePhase G2, Ivoclar Vivadent) lamp, for 20 or 40 s. Six indentations per specimen (3 on top and 3 on bottom surface) were performed with a Vickers indenter (VHMT 30, Leica). Three measurement times were considered: t0, immediately after polymerization; t1, after 24 h in dry environment; t2, after further 24 h in artificial saliva. Statistical analysis was performed by nonparametric tests. Results: Differences among groups were found as follows: top vs. bottom surface (most comparisons p < 0.005); LED vs. halogen lamp (most comp. p < 0.001); 40 s vs. 20 s (most comp. p < 0.05); increased thickness led to decreased hardness only on bottom surfaces (most comp. p < 0.005); hardness increased at t1 (all comp. t0 vs. t1, p < 0.05), then decreased at t2 (all comp. t1 vs. t2, p < 0.05); however, the t2 values remained greater than those at t0 (most comp. t0 vs. t2, p < 0.05). Conclusions: SDR showed low hardness values, which decreased after 24 h storage in artificial saliva; further investigations are required before its clinical use.