LECTURE-6-TOOTH PREAPARATIONS FOR ALL CERAMIC.pdf

All-Ceramic Crowns The all-ceramic crown differs from other cemented veneer restorations because it is not cast in gold or some other metal. It is capable of producing the best esthetic effect of all dental restorations. However, because it is made entirely of ceramic, a brittle substance, it is...

All-Ceramic Crowns The all-ceramic crown differs from other cemented veneer restorations because it is not cast in gold or some other metal. It is capable of producing the best esthetic effect of all dental restorations. However, because it is made entirely of ceramic, a brittle substance, it is more susceptible to fracture. The development of dental porcelain reinforced with alumina in the 1960s created renewed interest in the restoration. 41 In recent years, this interest has mushroomed with the use of new reinforcement materials such as lithium disilicate and zirconia.42,43 Preparations for this type of crown should be left as long as possible to give maximum support to the porcelain. An overshortened preparation will create stress concentrations in the labiogingival area of the crown,3 which can produce a characteristic half-moon fracture in the labiogingival area of the restoration.44–46 A shoulder of uniform width (approximately 1 mm) is used as a gingival finish line to provide a flat seat to resist forces directed from the incisal.44,47 The incisal edge is flat and placed at a slight inclination toward the linguogingival to meet forces on the incisal edge and prevent shearing.48,49 Finally, all sharp angles of the preparation should be slightly rounded to reduce the danger of fracture caused by points of stress concentration.44,48,49 The position of the tooth in the arch, factors relating to occlusion, and morphologic features of the tooth all should be weighed when an all-ceramic crown is considered for a restoration. All-ceramic crowns are best suited for use on incisors. If they are used on other teeth, patients should know that there is an increased risk of fracture. Use of the all-ceramic crown should be avoided on teeth with an edge-to-edge occlusion that will produce stress in the incisal area of the restoration. It likewise should not be used when the opposing teeth occlude on the cervical fifth of the lingual surface. Tension will be produced, and a half-moon fracture is likely to occur. Teeth with short cervical crowns also are high risks for all-ceramic crowns because they do not have enough preparation length to support the lingual and incisal surfaces of the restoration. Armamentarium Handpiece Coarse-grit flat-end tapered diamond (6847-016) Coarse-grit football-shaped diamond (6379-023) Fine-grit flat-end tapered diamond (8847KR-016) CP-11/12 binangle chisel Technique Depth-orientation grooves are placed on the labial and incisal surfaces with the coarse-grit flat- end tapered diamond before any reduction is done (Fig 10-42). Without grooves it is impossible to accurately gauge the depth of reduction done on the labial surface. The grooves are 1.2 to 1.4 mm deep on the labial and 2.0 mm deep on the incisal. Three labial grooves are cut with the diamond held parallel to the gingival one-third of the labial surface. A second set of two grooves is made parallel to the incisal two-thirds of the uncut labial surface. The labial surface of an all-ceramic preparation is done in two planes to achieve adequate clearance for good esthetics without encroaching on the pulp.48 Incisal reduction is done with the coarse-grit flat-end tapered diamond so that it will be possible for instruments to reach the finish line area of the preparation in subsequent steps. Approximately 1.5 to 2.0 mm of tooth structure is removed (Fig 10-43). The tooth structure still remaining between the depthorientation grooves on the incisal portion of the labial surface is planed away (Fig 10-44). The gingival portion of the labial surface is reduced with the coarse-grit flat-end tapered diamond to a depth of 1.2 to 1.4 mm. This reduction extends around the labioproximal line angles and fades out on the lingual aspects of the proximal surfaces (Fig 10-45). The end of the coarse-grit flat-end tapered diamond bur will form the shoulder finish line, while the axial reduction is done with the sides of the diamond. The shoulder should be a minimum of 1.0 mm wide. Lingual reduction incisal to the cingulum is done with the coarse-grit football-shaped diamond, with care taken not to overreduce the junction between the cingulum and the lingual wall (apical to the cingulum) (Fig 10-46). Overshortening the lingual wall will reduce the retention of the preparation. Fig 10-45 Labial reduction (gingival half): coarse-grit flat-end tapered diamond. Fig 10-46 Lingual reduction: coarse-grit football-shaped diamond. Fig 10-47 Lingual axial reduction: coarse-grit flat-end tapered diamond. Fig 10-48 Axial wall and radial shoulder finishing: fine-grit flat-end tapered diamond. Fig 10-49 Features of an all-ceramic crown preparation and the function(s) served by each.  Reduction of the lingual axial surface is done with the coarse-grit flat-end tapered diamond (Fig 10-47). The wall should form a minimum taper with the gingival portion of the labial wall. The radial shoulder is at least 1.0 mm wide and should be a smooth continuation of the labial and proximal radial shoulders. All-ceramic crowns made over shoulder finish lines exhibit greater strength than those made over chamfers.11,50 All of the axial walls should be smoothed with a fine-grit flat-end tapered diamond, accentuating the shoulder at the same time (Fig 10-48). All sharp angles should be rounded over at this time. The CP-11/12 modified binangle chisel is used to smooth the shoulder, removing any loose enamel rods at the cavosurface angle. 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