Minimal preparation designs for single posterior indirect prostheses with the use of the Cerec system

The advantages of minimally invasive dentistry are well established, especially for direct restorations. However, when it comes to indirect restorations traditional tooth preparation designs are still advised by most manufacturers. The purpose of this in vitro study was to investigate the ability of a CAD/CAM machine (Cerec) to produce minimal preparation designs and identify limiting parameters. Crown preparations based on the proposed minimal design were made using phantom teeth. Gauged burs (Intensiv SA; Meissinger, Germany) and a paralleling device were used to standardize preparations. Cerec Scan/Cerec 3D was used for scanning and designing. The materials tested were a resin composite (Paradigm MZ100, 3M ESPE) and two ceramic materials (ProCAD, Ivoclar Vivadent, and VITA Mark II, Vita). The morphology, marginal integrity, and materials' integrity were examined. The design was subjected to an interactive process as material property limitations and constraints imposed by the system became apparent. SEM, optical microscopy, and transillumination were used for the qualitative control of the crowns. The results of this study showed that only the composite material produced acceptable crowns with intact margins for the minimal design initially proposed. The ceramic materials required a wider preparation design in order to produce acceptable crowns. Within the limitations of this study, the null hypothesis was partially rejected as it was found that only the composite material could produce acceptable crowns based on the proposed minimal design. It was also found that the materials' properties, the milling mode, and cutting instruments are determining factors in establishing the extent of the minimal preparation.