All-ceramic crowns can create some of the most aesthetically-pleasing restorations available today. Using a no metal substructure allows light to be transmitted through the crown, closely replicating the translucency of a natural tooth. An all-ceramic crown also promotes good tissue response, and only mild reduction of the facial surfaces is required. However, significant tooth reduction is necessary on the lingual and proximal surfaces.
Wear may also develop on the functional surfaces of natural teeth opposing the all-ceramic crown. All-ceramic crowns may not be suitable for discoloured teeth or teeth with enamel defects, teeth with bilateral or unilateral decay. During fitting, the appearance of the restoration can be modified by the colour of the luting agent. To be successful, an all-ceramic crown should have a relatively-even thickness circumferentially. There are only minor differences in preparation between the various all-ceramic crown materials. Proper design is critical for ensuring the mechanical success of the restoration.
Ensuring the preparation has a 90° cavosurface angle helps to prevent unfavourable distribution of stresses and minimises the risk of the crown fracturing. The preparation must be designed to provide the correct support for the porcelain along its entire incisal edge, unless an all-ceramic crown with a strong core (i.e. zirconia) is chosen. The tooth should have a relatively intact coronal structure that will provide sufficient support for the restoration, particularly in the incisal area. Ideally, this area shouldn’t exceed a thickness of 2 mm. Otherwise more brittle all-ceramic restorations may fail. When preparing posterior restorations, the occlusal load should be evenly distributed, so that contact is in an area where the porcelain is supported by the tooth structure.
For an IPS Empress or e.max crown, and for zirconia anterior crowns, a tooth must be reduced by between 1 mm and 1.5 mm to create an aesthetically-pleasing restoration. Facial reduction should be between 1 mm and 1.5 mm, while incisal edges should be reduced between 1.5 mm and 2 mm to ensure sufficient incisal translucency can be created.
To reduce the facial surface, depth orientation grooves should be placed at 0.8mm deep; after they are finished, this depth will become 1mm. Once placed, the area between the grooves should be reduced and facial reduction should extend around to the facial-proximal angles. Margins must be precisely prepared with a 1-mm-wide circumferential shoulder or chamfer with rounded inner edges. To reduce the incisal edges, three depth grooves of 1.3mm should be created and the tooth structure between them should be carefully reduced. Care should be taken to avoid creating undercuts at the junction of the shoulder finish line and the axial walls. Feather edges and sharp transitions must be avoided and the shoulder should be as smooth as possible. A football-shaped bur can be used to reduce and shape the lingual surfaces.
For posterior crowns, occlusal surfaces should be reduced between 1.5mm and 2mm, with a 1.5mm axial reduction. Internal line angles should be rounded and a tapered, flat-ended diamond should be used to create a good shoulder margin.
When preparing teeth for all-ceramic crowns, a uniform reduction will help result in optimal ceramic strength. Ensuring sufficient tooth structure is removed will lead to better aesthetics. The smoother the edges, the lower the stresses placed on the porcelain crown which in turn decreases the potential for fracturing. Additionally, scanners can read smooth preparations more accurately.
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