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Quintessence International



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Quintessence Int 30 (1999), No. 10     1. Oct. 1999
Quintessence Int 30 (1999), No. 10  (01.10.1999)

Page 661-672

Design optimization and evolution of bonded ceramics for the anterior dentition: A finite-element analysis
Magne, Pascal / Douglas, William H.
Objective: Finite-element method was used to explore the stress distribution of incisors restored with porcelain veneers. The design of the incisal palatal finish line was analyzed as a function of incisal overlap and initial tooth substance loss (coronal fractures). Method and materials: The treatment of intact and fractured incisors was investigated using 8 different designs of porcelain veneer. The palatal finish line varied from butt margins to extended chamfers. The stress distribution was assessed in a 2-dimensional finite-element model, reproducing a buccolingual cross section of an incisor. A palatal 50-N horizontal force was applied to the incisal edge to simulate an extreme functional load. The palatal surface tangential stresses were calculated. Results: Considerable differences were detected in the stresses at the level of the incisal-palatal restoration margin. The margins of restorations with limited incisal overlap (butt margin or minichamfer) showed low tensile stresses or even compressive stresses. Restorations with a long chamfer extending into the palatal concavity were subjected to the highest tensile stresses. In the presence of moderate crown fractures (incisal one third) or severe wear, butt margins limited the palatal extension of ceramic, thus reducing the amount of stress at the restoration interface. In the presence of severe crown fractures (incisal two thirds), the margins (either butt or chamfered) were subjected to low tensile forces when located in the smooth convex area of the cingulum. Conclusion: Because of the geometry and natural elastic modulus of mineralized tooth structures, a concentration of tensile stresses is formed at the palatal concavity of teeth restored with porcelain veneers. Long chamfers extending into the palatal concavity are unfavorable because thin extensions of ceramic are generated in an area of maximum tensile stresses. Minichamfers or butt margins are generally recommended, especially in the presence of moderate crown fractures or severe wear.