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All-ceramic crowns and preparation characteristics: a mathematic approach.

PURPOSE
The purpose of this investigation was to compare two mathematically calculated preparation parameters, ie, retentive capacity and finish line type, with a visual characterization based on a 3-D projection of the same prepared teeth on a PC screen.


MATERIALS AND METHODS
Data from 400 teeth prepared for all-ceramic crowns recorded and manufactured by the Procera system, ie, 100 teeth each of types 11, 13, 14, and 16, were selected. Teeth were visually characterized for retentive capacity as good, acceptable, or insufficient, and type of finish line was characterized as deep chamfer, chamfer, or knife edge. For the mathematic calculations, data points located along vertical lines every 10 degrees around the tooth from finish line to occlusal/incisal midpoint were selected to calculate retentive area, defined as the part of the axial walls with an angle of less than 10 degrees to the preparation midline, and the area at the finish line with an angle of more than 45 degrees to the midline.


RESULTS
Significant differences were found between the calculated size of surface areas for all types of prepared teeth visually characterized as having good retention and the two other categories (acceptable and insufficient). The difference between the latter two categories was not significant. Significant differences were also found between calculated surface areas for teeth visually characterized as having a deep chamfer and the two other finish line categories, ie, small chamfer and knife edge, but not between the latter two categories.


CONCLUSION
The mathematic program allows a characterization of preparation parameters and may be further developed for use in prospective or retrospective studies of CAD/CAM restorations.