Stress distribution in ceramic crown forms as a function of thickness, elastic modulus, and supporting substrate

The design of ceramic dental crowns is related to esthetic and fracture resistance requirements. Clinicians assume that approximately 1.5 to 2 mm of tooth structure must be removed to provide sufficient thickness of ceramic to meet these requirements. However, little scientific evidence is available on the stress distributions in crowns as a function of variable dimensions and substrate materials to optimize ceramic crown designs. The objective of this study was to analyze stress patterns in crown forms as a function of ceramic thickness and elastic moduli of ceramic and substrate. Version 5.3 of the ANSYS finite element program was used to model the crown forms based on plane 82 elements (8 node, axisymmetric quadrilateral and triangular solid elements) and 12,000 to 14,000 nodes. Three ceramic crown forms were analyzed: a full-length (FL) crown, a half-length (HL) crown, and an occlusal plate (OF), each with a 1.5 mm thickness. Pressure loading of 200 MPa was applied to an area of 2 mm/sup 2/ on the occlusal surface. Four different loading sites were selected. The elastic modulus of the ceramic was varied from 70 to 200 GPa and the modulus of the supporting substrate to which ceramic was bonded was varied between 4.5 and 18.6 GPa. The maximum principal stresses generally develop at the lower surface of the ceramic near the ceramic-cement interface. The maximum stresses within the FL crown and the HL crown differed by less than 2%, but the stress in the occlusal plate was 20% higher than that in either the FL or HL crowns. As the elastic modulus of the ceramic increases, the stress difference between the OP design and the HL and FL crowns also increased. As the thickness of the ceramic crown increases from 0.5 mm to 1.5 mm, the maximum stress decreases from approximately 343 to 101 MPa, but it remains relatively constant for thicknesses between 1.5 mm and 2 mm. The maximum principal stress in ceramic near the ceramic-cement interface decreased as the modulus of the substrate material increased.