Digitization of crowns with destructive 3D-volume scanning

Objectives: For the 3D-analysis of the internal and marginal fit of cemented crowns and FPD's as well as for the analysis of 3D-sintering shrinkage of all-ceramic restorations, digitalization with a high precision is needed. Optical digitalization methods cannot capture the inner surface; mechanical digitization is limited in areas with sharp edges. Aim of this study was to evaluate the precision of the 3D-volume scanning procedure, a destructive method, for the digitalization of crowns.

Methods: Three crowns, manufactured with a direct shaping process (ce.novation) were embedded with a resin with sufficient contrast for optical measurement commonly used in materialography. The samples were reduced in height with a CNC milling machine. For the 2D-contour measurement a CNC multi-sensor-coordinate-measuring-machine with image processing sensor and special pattern recognition software was used. In-between the measurement steps, surface layers of 100 µm in height were removed from the samples. The 2D-data slices were added to 3D-data sets and aligned to and compared with the underlying CAD-model of the crowns.

Results: Mean positive deviations of 21.7 µm (SD 7.3) were found for the 3D-data sets from the underlying CAD-model. The mean negative values were -20.6 µm (SD 7.9). Maximum positive deviations of 107.3 µm and maximum negative deviations of -111.1 µm were found. The qualitative analysis of color-coded graphs showed an elongation and slight reduction in width.

Conclusion: The results were influenced by the manufacturing error, the sintering shrinkage and the measurement error of the destructive 3D-volume scanning. It seems that the destructive 3D-volume scanning procedure is suitable for the 3D-analysis of the internal and marginal fit of crowns and FPD's and for the analysis of 3D-sintering shrinkage of all-ceramic restorations. In an optimization process, the thickness of the slices will be reduced and the accuracy of the 2D-contour measurement will be improved.