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The core build-up procedure is utilized to restore teeth with limited remaining coronal tooth structure. However, voids have been observed radiographically within composite resin- and glass ionomer-based core build-ups, potentially compromising the mechanical strength of a fully restored tooth and requiring build-up replacement before a final restoration can be delivered. The purpose of this in vitro study was to determine whether applying ultrasonic vibration during core build-up placement reduces the presence of radiographically detectable voids. A total of 120 acrylic resin mandibular premolar analogs were fabricated using a 3-dimensional printer and randomly allocated into 4 groups (n = 30). Dual-cured composite resin or glass ionomer core build-ups were placed with or without vibration. The final build-ups were assessed radiographically and rated by 3 independent calibrated clinicians based on a 4-category scale for the severity of voids. In an ordinal logistic regression model with the void severity rating as the outcome, a significant interaction was found for glass ionomer, composite resin, and the use of ultrasonic vibration (P = 0.03). Vibration was associated with worse void severity ratings in glass ionomer specimens (P < 0.01). No effect of vibration was found in the composite resin specimens. The Fleiss kappa score (κ = 0.36) indicated fair agreement in all severity ratings among the 3 raters. These results suggest that the application of ultrasonic vibration during core build-up placement may not be clinically advantageous for improving restorative outcomes., Competing Interests: No conflicts of interest reported.