Fit of crowns, accuracy of an innovative measuring method

Methods: Two steel cones (precise tilt of α=5°) were machined and exactly fitting gold caps were constructed using galvanization. Each cone was embedded in a plaster duplicate of a real dentition in region of the second premolar. By placing distance plates (height h) on top of the cone between the cone and the cap, different constant spacings (s=h∙sinα) between the cone and the cap were simulated along the circumferential surface. In this in-vitro study, three different spacings (s=100μm, 200μm, and 300μm) were simulated twice for each cone, resulting in a total of n=12 trials. After applying silicone (Xantopren Comfort Light, Heraeus) between cone and cap ("fit-checking"), a regular monophasic impression (Impregum Penta, 3M ESPE) was taken. Cap and silicone remained in the impression when it was pulled off the plaster model. Afterwards, two casts of the impression were fabricated (Exakto-Form, Bredent): one cast with the silicone inside the cap and one without. The casts were digitized using a 3D-laser-scanner (Laserscan 3D PRO, Willytec GmbH) and then matched (DentalForm, INUS Technology/3D Alliance). In a final step, the difference between the two digitized casts, i.e. the measured spacing, was evaluated over the area of interest. Descriptive statistics (SPSS15.0) were used to describe the mismatch of the measured data.

Results: The mean error of the reconstructed spacings amounted to 15μm, the 95%-confidence-interval for data gained over the whole circumferential surface was given by ±29μm with respect to the mean value. Since a preliminary calibration test resulted in an accuracy of about 20μm for the scanning and matching procedure, this will probably be the main error source.

Conclusions: Fit of crowns can be adequately determined with the presented non-destructive method, which is applicable in-vivo, also.