Immediately after setting (i.e., at 3 min after the start of light activation) or after storage in distilled water at 37°C for one day, excess composite restorative material on each restoration surface was removed by wet grinding on #600 carborundum paper. This was followed by polishing with linen and an aqueous alumina slurry (Alpha Micropolish, Buehler, Chicago, IL, USA). Then, distilled water was used for rinsing to avoid desiccation and erosion.
Each tooth was sectioned in a buccolingual direction through the center of the restoration with a low-speed diamond saw (IsoMet, Buehler, Lake Bluff, IL, USA). The presence or absence of interfacial gaps [14 (link)] was inspected and measured at 14 points, labelled as Point #1 to Point #14, each 0.5 mm apart (Fig 1 ), along the interface between the restoration and cavity walls (n = 10; total points measured per resin composite = 140). Inspection and measurements were carried out using a measurement microscope at ×1,000 magnification (Measurescope MM-11, Nikon, Tokyo, Japan), with the cavity walls and cavity floor of each half of the sectioned tooth facing up. At 3 min after the start of light activation or after one-day storage, the number of gaps at each measurement point (Point #1 to Point #14) in each tooth was summed up for each resin composite restorative material [14 (link), 15 (link)].
Each tooth was sectioned in a buccolingual direction through the center of the restoration with a low-speed diamond saw (IsoMet, Buehler, Lake Bluff, IL, USA). The presence or absence of interfacial gaps [14 (link)] was inspected and measured at 14 points, labelled as Point #1 to Point #14, each 0.5 mm apart (