Ceramic Partial Crown

A partial ceramic crown is a tooth-colored restoration fabricated indirectly (outside the mouth) for which the tooth to be restored is prepared (ground) using a specific technique and adhesively cemented (by mechanical anchorage in microscopic pores) with special materials matched to the ceramic material and tooth hard tissue. Over many decades, cast restorations have established and proven themselves as the so-called “gold standard” for the restoration of dental defects. Due to the desires for:

Better esthetics,
After tooth substance sparing procedures and
Biocompatible (biologically compatible) materials.

ceramic restorations have made their way into dentistry. This was made possible not only by the ceramic materials themselves, but also by the improvements in the micromechanical bond between ceramic and tooth structure through adhesive technology. Partial crowns made of ceramics are now considered scientifically accepted. A partial crown is required when the loss of hard substance of a tooth is so extensive that the occlusal surface has to be reshaped and one or more tooth cusps have to be stabilized by overcoupling with the partial crown. Accordingly, the preparation limits (extent of the milled tooth areas) are extended beyond the occlusal and proximal surfaces (the masticatory and interdental surfaces). As a rule, several tooth cusps are overcoupled (encompassed on the outside). Unlike a full crown, however, not all cusps are included, so the preparation margin is not lowered circularly (circumferentially) to gingival level (the level of the gum line). Today, glass-ceramics, feldspar ceramics, glass-infiltrated aluminum oxide ceramics or zirconium oxide ceramics are generally used for partial 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 partial 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 overlay. The decision to use ceramics as the material and adhesive technique for cementation is made:

For reasons of esthetics, which cannot be achieved with metallic partial crowns.
For reasons arising from the pre-treatment of a tooth – the thin dentin walls (dentin walls) of endodontically treated teeth (with root filling) require the setting and stabilization by the adhesive technique.
Due to a proven incompatibility (intolerance) against alloys based on precious or non-precious metals.

Contraindications

Small tooth substance defects
Circular decalcification (surrounding the tooth in a band) – indication for a full crown.
Cavities (cavity in the tooth) that extend deep to subgingival (deep into the gingival pocket), so that drainage for adhesive luting technique is not guaranteed. In this case, a partial gingivectomy (surgical removal of gums to reduce a gingival pocket) may be helpful to enable the adhesive cementation method of the ceramic restoration after all. Alternatively, it is necessary to switch to conventional cementing, for example, zirconium oxide.
Pronounced bruxism (grinding and pressing).
Incompatibilities (incompatibilities) to the luting components.

The process

Unlike the direct filling technique, restorations made indirectly (outside the mouth), such as inlays, onlays and partial/crowns, are divided into two treatment sessions, provided that the restoration is made in the dental laboratory.As an alternative, ceramic restorations are increasingly being used which are milled chairside (at the dental chair) using the CAD-CAM process in a single treatment session immediately after tooth preparation. 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 gently as possible, with sufficient water cooling and with as little substance removal as possible.
Preparation angle: must diverge slightly in the direction of extraction (lat. divergere “strive apart”), so that the future partial crown can be removed from or placed on the tooth, without jamming or leaving undercut areas unprovided for
Occlusal substance removal (in the occlusal surface area).
Approximal preparation (in the interdental area): slightly diverging box-shaped.
Proximal contact (contact with the adjacent tooth): must be in the area of the partial crown, not in the tooth substance area.
Bite registration: serves the spatial allocation of both jaws and the design of the occlusal relief of the partial crown.
In case of two-stage (two different treatment appointments) procedure Fabrication of a temporary crown made of resin: this temporary restoration must be used with eugenol-free cement, since eugenol (clove oil) inhibits (prevents) the curing of the adhesive definitive luting composite (resin for final luting).

2nd manufacturing phase of the partial ceramic crown:

2.I. One-stage procedure: Instead of an impression, the dentition is prepared for optical scanning: A “digital impression” is created. Factory-made ceramic blanks (feldspar ceramic, leucite-reinforced glass-ceramic or zirconium oxide) are used for the CAD-CAM milling technique (copy milling). After optical scanning of the tooth to be restored, the partial crown is designed on the computer and then milled out of the blank in a three-dimensional milling process. The advantage of this procedure lies in the one-time nature and homogeneous material properties of the factory ceramic. 2.II. Two-stage procedure: The preparation is followed by the taking of impressions of both jaws with precision impression materials layered in impression trays, which serve the dental laboratory for the fabrication of a working model in true-to-the-original dimensions and an opposing jaw model for the occlusal surface design. After that, the following alternatives arise in terms of laboratory technology:

Laboratory-fabricated partial ceramic crowns are sintered in several layers – and thus also color layers – on a refractory duplicate of the tooth to be restored. In the sintering process, the ceramic mass is usually heated under pressure almost to melting temperature. This significantly reduces porosity and volume, so that the dental technician must 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 press ceramic method has become established: a heated, plasticized glass-ceramic blank is pressed into a hollow mold, into which a wax model of the partial crown to be produced has previously been embedded and burned out. Following firing, the rather milky-light pressed ceramic partial crown is given a sintered layer of ceramic stain to improve its esthetics. With regard to the lack of translucency (light transmission), significant technical progress has been made recently. The accuracy of fit of a pressed ceramic partial 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 ceramic in its stability.
The data of a digital impression are transmitted to the dental laboratory, which mills the partial crown using CAD-CAM technology (see 2.I.).

3. second treatment session or second treatment phase in the chairside procedure:

Control of the completed partial 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 partial crown.
Cleaning the prepared tooth
Try-in of the partial crown, if necessary with the help of thin flowing silicone to find places that hinder the internal fit.
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 (etching of the dentin) for a maximum of 15 sec, then application of a dentin bonding agent to the dentin, which has only been carefully dried or slightly moistened again.
Preparation of the partial crown – etching of the lower surface with hydrofluoric acid, thorough spraying and silanization (application of a silane compound as a chemical bonding agent).
Insertion of the partial 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 basic building blocks of the material chemically combine to form a polymer), during which the partial 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 can arise from the multitude of intermediate steps in the manufacturing process, such as:

Fracture (breakage) of the partial ceramic crown during the fitting phase.
Fracture after adhesive cementation – e.g. due to insufficient removal of tooth structure, rounded preparation not suitable for ceramics or disregard of functional conditions.
Tooth sensitivities (hypersensitivities) or pulpitides (tooth pulp inflammations) due to errors in adhesive cementation.
Lack of biological compatibility of the luting material; the decisive role here is 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