Main aspects of preparation design are preservation of dentine, optimization according to the technology or material chosen and the expected stress distribution in the future restoration.Objectives: Aim of this study was the development of a 3D-method for the quantification of enamel and dentine loss due to preparation. Methods: Since teeth that need to be restored with crowns usually bear sizable amalgam fillings, upper and lower bicuspids and molars of a study model were prepared conventionally for that kind of restoration. Seven full mouth impressions were taken and 14 master casts were made. One cast each of maxilla and mandible remained unchanged and were taken as references. The other casts were prepared according to 6 designs, either conventional (full crowns) or simplified (partial crowns). In the simplified designs, undercuts were removed and cusps were reduced in height at least 1 mm unless antagonistic teeth required more reduction. The cusps were prepared either with a bevel or flat. References and prepared casts were optically digitized. CAD-models, which are identical to their respective casts were created. The volumetric differences before and after preparation were calculated with different algorithms. Results: For the tooth reduction, there was no significant difference to be found in conventional chamfer or shoulder preparation. As for one design, the simplified preparation designs resulted in about 50 % less reduction compared to the conventional preparation designs. In the case of 1.5 mm flat reduction of the bicuspids the volumetric difference found was about 30 % only. Conclusions: The method for quantifying volume loss due to preparation based on optical digitizing has shown reproducible results that did not depend on the algorithms used. It may be applied for optimizing preparation designs under equal consideration of tooth preservation and CAD/CAM machinability.

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