An all-ceramic crown is a full crown made of tooth-colored, ceramic material that completely encases the remaining tooth structure of the natural tooth crown so that the crown margin is level with or below the gum line. Over many decades, full-cast crowns or veneered ceramic crowns (metal frameworks veneered with ceramic materials) have been established and proven for the restoration of extensive dental defects. Due to the desire for better esthetics and biocompatible materials, full ceramic restorations have found their way into dentistry. This has been made possible not only by the further development of ceramic materials themselves, but also by the improvement of the micromechanical bond between ceramics and tooth structure through adhesive technology. A full crown is required when the loss of hard substance of a tooth is so extensive that its occlusal surface needs to be reshaped and the cusps of the tooth need to be stabilized by edging. Accordingly, the preparation margin (circumference of the milled tooth areas) of a full crown runs circularly at or below gingival level (at the level or below the gingival margin) depending on the depth of the defects (holes). Unlike a partial crown, all cusps are covered. Today, glass-ceramics, feldspar ceramics, glass-infiltrated alumina ceramics or zirconia ceramics are increasingly used for full crowns. One advantage of ceramic materials is that they are biologically inert (inert to reaction). However, in the case of adhesive cementation, hypersensitivity reactions to the methacrylate-based luting resin can negate this advantage. Glass-infiltrated and zirconia-based advanced ceramics can also be luted with conventional (conventional) cements such as zinc phosphate or glass ionomer cement, but they do not achieve the bond that is achieved by micromechanical anchorage using adhesive technology. Ceramic materials have a higher microhardness than enamel, so this can result in increased abrasion of the antagonists (abrasion of the teeth of the opposing jaw), especially during bruxism (teeth grinding).
Indications (areas of application)
The indication for a full crown in itself results primarily from the loss of tooth structure, which makes it impossible to restore the tooth with a filling, inlay, onlay or partial crown. The restoration of the jaw relationship (positional relationship of upper and lower jaw) and a support zone build-up required in this context may also necessitate extensive crown planning for occlusal reshaping. If a gap is to be restored by a bridge, the abutment teeth are prepared for full crowns in the majority of cases. The decision to use ceramic as the material and adhesive technique for cementation is made
- For reasons of esthetics
- On the basis of a proven incompatibility with alloys based on precious or non-precious metals.
Contraindications
- Small tooth substance defects
- Pronounced bruxism (grinding and pressing).
A relative contraindication is the intolerance / allergy to adhesive luting material based on PMMA (polymethyl methacrylate). In this case, it is necessary to switch to ceramic materials that allow the luting of the crown with conventional cements.
The procedure
Restoring a tooth with indirectly (outside the mouth) fabricated restorations is divided into two treatment sessions, provided it is a restoration fabricated in the dental laboratory. As an alternative, ceramic restorations are increasingly being used that are milled chairside (at the dental chair) in one treatment session using the CAD-CAM process. 1st treatment session or first treatment phase in the chairside procedure:
- Excavation (caries removal) and, if necessary, placement of a composite build-up filling (made of plastic) for substance compensation.
- Preparation (grinding of the tooth), as tooth tissue sparing as possible, with sufficient water cooling and with as little substance removal as possible.
- Preparation angles must diverge slightly in the direction of extraction, so that the future crown can be removed from or placed on the tooth, without jamming or leaving undercut areas unsupplied
- Occlusal substance removal (in the occlusal surface area).
- Preparation margin – rounded step or chamfer.
- Bite registration and opposing jaw impression – serve to spatially match both jaws and design the occlusal relief of the crown.
- In the case of a two-stage procedure, fabrication of a temporary acrylic crown – this temporary restoration must be placed with eugenol-free cement if adhesive cementation is planned, as eugenol (clove oil) inhibits (prevents) the curing of the adhesive definitive luting composite (acrylic for final cementation)
2nd manufacturing phase of the all-ceramic crown:
2.I. One-stage procedure: Instead of an impression, the dentition is prepared for optical scanning: A “digital impression” is made. Factory-made ceramic blanks (feldspar ceramic, leucite-reinforced glass-ceramic or zirconium oxide) are used for the CAD-CAM milling technique (copy grinding). After optical scanning of the tooth to be restored, the crown is designed on the computer and then machined out of the blank by a three-dimensional milling process. The advantage of this procedure is the one-time nature and the homogeneous material properties of the factory ceramic. 2.II. Two-stage procedure: The preparation is followed by the impression of both jaws, which is used by the dental laboratory to produce a working model in true-to-the-original dimensions and the opposing jaw model for the occlusal surface design. After that, the following options arise in terms of laboratory technology:
- Laboratory-fabricated ceramic crowns are sintered on a refractory duplicate of the tooth to be restored in several layers – and thus also color layers; in the sintering process, the ceramic mass is usually heated under pressure almost to enamel temperature. In this process, porosities and volumes are significantly reduced, so that the dental technician has to compensate for this volume shrinkage by applying multiple layers of ceramic and sintering. This complex technique inevitably provides the best aesthetic results due to the possibility of color layering.
- Alternatively, the pressing process has become widely accepted. A heated, plasticized glass-ceramic blank is pressed by means of a vacuum-pressure process into a hollow mold, in which a wax model of the crown to be produced was previously embedded and burned out. Following firing, the rather milky-light pressed ceramic crown is given a sintered layer of ceramic stain to improve its esthetics. With regard to the lack of translucency (partial light transmission), significant technical progress has been made recently. The accuracy of fit of a press ceramic crown is very good, since the volume shrinkage of the ceramic is compensated by appropriately dimensioned investment materials. In addition, the press ceramic is superior to the layered in its stability.
- The data of a digital impression are transmitted to the dental laboratory, which mills the crown using CAD-CAM technology (see 2.I.).
3. second treatment session or second treatment phase in the chairside procedure:
- Control of the completed crown
- Provided that the preparation margins allow this: Installation of rubber dam (tension rubber) to protect against saliva ingress and against swallowing or aspiration (inhalation) of the crown.
- Cleaning the prepared tooth
- Trying in the crown
- Control of the proximal contact
- Preparation of the tooth for adhesive cementation: Conditioning of the enamel margins for approx. 30 sec. with 35% phophoric acid gel; dentin etching for a maximum of 15 sec., then application of a dentin bonding agent to the only carefully dried or slightly moistened dentin again.
- Preparation of the crown – etching of the undersurface with hydrofluoric acid, thorough spraying and silanization.
- Insertion of the crown in adhesive technique – with a dual-curing (both light-initiated and chemically curing) and high-viscosity luting composite (resin); excess cement is removed before light curing; sufficient polymerization time (time during which the monomeric building blocks of the material chemically combine to form a polymer), during which the crown is exposed from all sides, must be observed
- Control and correction of occlusion and articulation (final bite and chewing movements).
- Finishing the margins with ultra-fine grit polishing diamonds and rubber polishers.
- Fluoridation to improve the surface structure of the enamel after conditioning with acid.
Possible complications
Possible complications may arise from the large number of intermediate steps in the manufacturing process, such as:
- Fracture (fracture) of the crown in the phase of fitting.
- Fracture after adhesive cementation or cementation – e.g. due to insufficient removal of tooth structure, rounded preparation not suitable for ceramics or disregard of functional conditions.
- Tooth sensitivity (hypersensitivity) or pulpitides (tooth pulp inflammation) due to errors in adhesive cementation.
- Lack of biological compatibility of the luting material; the decisive role here is played by the unavoidable low residual content of monomer (individual components from which the larger and thus hardened polymers are formed by chemical combination) in the finished polymerized material; diffusion of monomer into the pulp can lead to pulpitis (pulp inflammation)
- Marginal caries in the area of the joint between tooth and restoration due to washout of luting material.
- Marginal caries caused by poor oral hygiene – bacteria preferentially adhere to the luting material in the cement joint
