Stress Distribution in Metal-Ceramic Crowns with a Facial Porcelain Margin

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Abstract
The use of metal-ceramic restorations with porcelain butt-joint facial margins has increased in the past several years. Although these crowns exhibit improved esthetics compared with metal-ceramic crowns which display a metal gingival collar or metal knife-edge margin, the effectiveness of this design in resisting intra-oral forces is not known. The objective of this study was to analyze the stress distribution induced by simulated intra-oral loads on crowns with variable coping configurations. The copings, with a thickness of either 0. 1 or 0.3 mm, were modeled with a facial termination of metal at three locations: at the gingival floor, 0.9 mm above the gingival floor, and 4.2 mm above the gingival floor. The coping and crown dimensions were based on a prepared maxillary central incisor with a facial shoulder and a lingual chamfer. Both Ni-Cr and Au-Pd alloy copings were employed in the crown models. Finite element stress analyses were performed on crowns which were subjected to several loading conditions. A cement film thickness of 0.030 mm was assumed. For all cases, the stresses which developed in porcelain and cement near the facial and lingual margins due to a vertical load of 200 N were predominately compressive in nature. For the crowns with Ni-Cr copings, the tensile stress in porcelain ranged from 11.0 MPa (for crowns with a facial metal thickness of 0.3 mm) to 12.5 MPa (for a metal thickness of 0.1 mm). The corresponding stresses for crowns with Au-Pd alloy copings were 8.3 MPa and 8.6 MPa, respectively. Maximum compressive stresses in cement near the lingual margin were 22.3 MPa for the Ni-Cr crowns and 30.0 MPa for the Au-Pd crowns. The maximum tensile stress in porcelain was 29.0 MPa and occurred near the area of loading for the Au-Pd case. The maximum tensile stress in the facial porcelain region (8.3 MPa) developed within the incisal third of the facial surface. Under the conditions assumed in this study, the alloy type, coping thickness, and coping termination point have a negligibly small effect on the stresses induced in porcelain or cement near the facial margin.

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