BackgroundThis in-vitro study aimed to evaluate the internal adaptation, marginal fit, and applicability of digital intraoral impression techniques for onlays fabricated using computer-aided design (CAD)-computer-aided manufacturing (CAM) and three-dimensional (3D) printing techniques using a stereomicroscope and micro-CT scan.MethodologyA total of 20 extracted mandibular first molars were selected for this study. The teeth were then divided into two groups. Onlay cavities were prepared involving the mesiobuccal cusp of the mandibular first molar in both groups. After preparation, both blocks were sent to the laboratory for fabrication of onlays using digital impressions (Shinning 3D scanner). Once the onlays were fabricated using CAD-CAM and 3D printing, a replica technique with monophase medium body impression material was used to assess the marginal fit and internal adaptation. The accuracy of internal adaptation was evaluated and compared using a stereomicroscope at 20× magnification. Measurements were taken at proximal margins, the inner axial wall, and the occlusal cavosurface area according to the Molin and Karlsson criteria. The same samples of both groups were studied for marginal fit using a micro-CT scan and values were recorded. The data collected were statistically analyzed using an independent Student’s t-test.ResultsIndependent Student’s t-test results demonstrated that the mean thickness values of the material in the CAD-CAM group at occlusal cavosurface area, proximal area, and axial area were significantly higher when compared to the 3D printing group at p <0.001 and 0.005, respectively.ConclusionsInternal adaptation and marginal fit of 3D-printed onlays were significantly lower than CAD-CAM onlays whereas the accuracy of 3D-printed onlays was significantly better than CAD-CAM onlays.