Deep-layer integrity in bulk restorations: Effects of placement technique on fiber- and particulate-reinforced...

This study investigated the influence of bulk-fill versus incremental placement techniques on the mechanical properties of the bottom layer of fiber- and particulate-reinforced bulk-fill resin composites following long-term water storage. Five materials were evaluated: a conventional composite, a high-viscosity bulk-fill, a fiber-reinforced flowable bulk-fill, and two particulate-reinforced flowable bulk-fills. Specimens were prepared using a custom mold to simulate clinically relevant placement techniques, either incremental or bulk-fill techniques. Specimens stored in distilled water for either 24 h or 30 d. The incremental technique produced similar flexural strength between top and bottom layers, whereas the bulk-fill technique showed higher top-layer strength, except for the fiber-reinforced composite, which maintained comparable performance across layers and conditions. Long-term storage reduced differences among bulk-fill materials. Hardness was generally higher in packable composites, with the fiber-reinforced material outperforming other flowable types. Extended water storage only reduced the hardness of the conventional composite. The fiber-reinforced bulk-fill composite exhibited stable and superior mechanical performance under all conditions, whereas particulate packable composites remained the strongest overall. Incremental placement improved deep-layer properties in particulate bulk-fill composites.