Objective: To evaluate the influence of a grape seed extract (Proanthocyanidin)-based preconditioners on shear bond strength (SBS) of restorative resin-composites bonded to the dentin of human premolar teeth. Materials and Methods: unrestored extracted human premolars (n = 130) with sound buccal surfaces were divided into five groups: one control (n = 10) and four experimental groups (n = 30 each). Crowns of all teeth were cut and embedded horizontally in the middle of a plastic cylinder of a

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... lf-curing polymethyl methacrylate measuring 4 cm in diameter and 3 cm in depth. Exposure of the buccal dentin surface was done using a high-speed diamond bur with abundant cooling to avoid dentin burning and smoothed with wet silicon carbide (SiC) sandpapers, grits 180-320, and running tap water. Dentin surfaces of all specimens were etched with 37% phosphoric acid for 15 s, rinsed and gently dried with oil-free air. For the control group (I), a fully saturated brush tip of All-Bond Universal (BISCO, USA) was utilized to apply two coats to the dentin surface then light-cured. For the intervention groups, four concentrations of Proanthocyanidin (PA)-based preconditioners were applied to the etched dentinal surfaces: specimens of group II were treated with 5% PA-based gel, group III with 10% gel, group IV with 15% gel and group V with 20% gel. Specimens of each of the intervention groups were further subdivided into three subgroups (n=10) according to the application time of PA-based gel; 20 s, 40 s and 60 s. All treated surfaces were then washed and gently dried to receive the All-Bond Universal as the control group. A resin-composite (Grandio, VOCO GmbH-Cuxhaven, Germany) disc specimen (4 mm in diameter × 2 mm in thickness) was built to each dentin surface using Teflon molds and light-activated for 40 s and stored in distilled water at 37°C for 24 h before debonding that was carried out using a universal testing machine (model 3365, Instron, High Wycombe, UK) at a crosshead speed of 1 mm/min. A two-way ANOVA followed by one-way ANOVA and Bonferroni post-hoc test were applied for data analysis.