A major goal in fixed prosthodontics is minimizing the discrepancy between the prepared tooth and it's restoration. Up to now, the evaluation of internal fit has been accomplished mainly by sections rendering two-dimensional information. Objectives: The aim of the study was to evaluate the reliability of both the existing replica-technique as well as the innovative computer-aided technique for three-dimensional analysis. Methods: Sixteen titanium copings for a prepared maxillary canine were CAD/CAM-made. Restorations were produced with accurate and purposely enlarged fit on the basis of high-density and low-density data respectively. For each restoration, five light-body silicone films representing the space between inner crown surface and prepared tooth (replicas) were analyzed by measuring the marginal, axial and incisal thickness using a light-microscope. Another five replicas each were optically digitized. The replicas' and copy dies' 3D-data-sets of the canine were aligned and discrepancies were calculated and visualized in a color-coded picture. Two independent examiners performed both techniques. Results: Light microscope measurements showed an intrapersonal reliability coefficient from 0.963 to 0.999 and an interpersonal reliability between 0.963 and 0.998. For optical 3D measurements, the interpersonal reliability coefficient ranged from 0.963 to 0.998. Probably due to the application of CEREC powder prior to optical measuring, the results achieved tended to show greater values than in the light microscope group. Restorations made from the low-density data set showed better fit than those made from the high-density data. In contrast to the group of well-fitting restorations, which showed less dispersion, within the group of enlarged restorations the seating resulted in tilting. The direction of tilt depended on the direction of enlargement. Conclusions: Both replica techniques have shown to be reliable with accurately fitting restorations. Compared with the conventional technique, detailed information about the 3D-deviation can be achieved with the developed 3D replica-technique.

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