Abstract
For ceramic crowns, the recommended depth of tooth reduction from the incisal edge of anterior teeth is 1.5 mm to 2. 0 mm. Although some prosthodontists have suggested that incisal heights of ceramic which exceed 2 mm are associated with dangerously high intra-oral stresses, this theory has not been verified. The objective of this study was to test the hypothesis that the stress distribution in ceramic crowns designed for a prepared maxillary central incisor which are subjected to applied loading is relatively insensitive to the incisal length of ceramic. Finite element stress analyses were performed on three crown designs loaded with a horizontal or vertical force of 200 N along the lingual surface near the incisal edge. Ceramic crowns for maxillary central incisors were modeled with incisal lengths of 1.0 mm (Case I), 1.9 mm (Case II), and 4.0 mm (Case III). Zinc phosphate cement with a film thickness of 30 μm was included for each case. Plane-stress finite element analyses indicated that tensile and compressive stresses which were induced in cement and ceramic due to a vertically applied load of 200 N were comparable in magnitude for all three cases within the gingival third of the crowns. For Cases I, II, and III, tensile stresses at the facial region were 6.7, 5.4, and 6.5 MPa, respectively, in cement and 46.2, 48.6, and 49.2 MPa, respectively, in the ceramic. The results of this study tend to support the hypothesis that the amount of tooth reduction (1 to 4 mm) in an incisogingival direction does not significantly influence the stress distribution in the crown or cement layer. In fact, the orientation of the applied load is a more significant factor than the geometry of the crown or prepared tooth.
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