Fracture resistance of structurally compromised maxillary premolars restored with different bulk-fill base materials...

This in vitro study investigated the influence of base material type and CAD/CAM composite overlay fabrication technique on the fracture resistance and failure patterns of structurally compromised vital maxillary premolars. Sixty sound premolars were randomly divided into five groups (n = 12). One group served as an unprepared control, while the remaining teeth received standardized mesio-occluso-distal preparations with a 2-mm cuspal reduction. The prepared specimens were restored using either a short fiber-reinforced composite base or a bulk-fill flowable composite base, capped with either milled or three-dimensional (3D) printed hybrid composite overlays. All restorations were fabricated according to manufacturers' instructions, cemented with a self-adhesive dual-cure resin cement, and subjected to artificial aging through 10,000 thermocycles between 5 °C and 55 °C. Fracture resistance was evaluated using a universal testing machine by applying a compressive load at a crosshead speed of 1 mm/min with a 4-mm steel ball until failure. Failure modes were classified as favorable (restorable) when fractures occurred above the cemento-enamel junction (CEJ) and unfavorable (catastrophic) when fractures extended below the CEJ. Statistical analysis using two-way ANOVA and one-way ANOVA followed by Tukey's post hoc test showed that the overlay fabrication technique significantly affected fracture resistance (p < 0.001), whereas the type of base material had no significant influence (p > 0.05). Milled composite overlays demonstrated significantly higher fracture resistance than 3D-printed overlays. Although the difference was not statistically significant, 3D-printed restorations tended to exhibit more favorable failure patterns, while milled overlays were associated with more unfavorable fracture modes.