Ceramic Inlay for Teeth

A ceramic inlay (synonyms: ceramic inlay; ceramic inlay;) is a tooth-colored dental filling produced indirectly (outside the mouth), for which the tooth to be restored is prepared (ground) using a specific technique and adhesively bonded (by clamping in microscopic pores) with special materials matched to the ceramic material and the tooth hard substances. In rare cases, the spatial extent of an inlay is limited only to the occlusal area (occlusal surface area) with its fissures (dimples in the occlusal relief of the posterior teeth); as a rule, the inlay also includes one or both approximal space surfaces (interdental space surfaces facing the adjacent teeth). Ceramics consist of quartz crystals bonded in a matrix (base material) of feldspar. The ceramic inlay can be seen in comparison to the plastic inlay. With a few exceptions, the ceramic material is used more frequently, partly because it is biologically inert (inert to reaction) and thus has a clear advantage over the plastic material, to which hypersensitivity reactions can develop. However, ceramic inlays are also generally adhesively bonded to the tooth with resin-based luting material, so it is only possible to significantly reduce the use of resin in quantitative terms.

Indications (areas of application)

The indications for ceramic inlay result from:

• The patient’s desire for tooth-colored, durable esthetics;
• The degree of destruction of the tooth to be restored. While for small to medium defects the placement of a direct filling, e.g. made of composite, makes sense in order not to sacrifice tooth substance unnecessarily to the preparation technique (grinding technique) of an inlay, for medium to large defects the restoration with an inlay is the means of choice, whereby the restoration with a ceramic inlay involves a greater expenditure of time and considerable additional financial costs for the patient and therefore compromises must sometimes be made in favor of a direct filling;
• A proven amalgam intolerance;
• A – very rare – proven gold intolerance, which prohibits the provision of a gold cast inlay;
• The need to restore a posterior tooth. Incisors and canines are not usually restored with inlays.

Contraindications

• Minor defects;
• Circular decalcification (surrounding the tooth in a band-like manner). Here arises the indication for the crown;
• Missing buccal (tooth surface, which is directed to the cheek, i.e. to the outside) or oral cavity wall (the hole to the cheek or to the inside of the mouth bounding wall). Here, too, a crown or partial crown, if necessary, is indicated;
• Cavities that extend proximally deep to subgingivally (in the interdental space deep into the gingival pocket), so that drainage for the 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 allow ceramic restoration after all;
• Severe bruxism (grinding and pressing); in this case, a gold cast or glass-ceramic composite restoration should be considered;
• Intolerance to the luting components.

The process

Unlike the direct filling technique, the restoration with fillings made indirectly (outside the mouth) is divided into two treatment sessions, provided that it is an inlay made in the dental laboratory. More and more frequently, ceramic restorations are used as an alternative, which are milled chairside (at the dental chair) in one treatment session using the CAD-CAM process. 1st session or 1st treatment phase in the chairside procedure:

• Excavation (caries removal) and, if necessary, placement of a build-up filling for substance compensation;
• Preparation (grinding of the tooth), as tooth tissue sparing as possible, with sufficient water cooling and with the least possible substance removal;
• Preparation angle: must diverge slightly in the direction of extraction, so that the future inlay can be removed from or placed on the tooth without jamming or leaving undercut areas unprovided;
• Occlusal substance removal (in the occlusal surface area): at least 2 mm;
• approximal preparation (in the interdental space): slightly diverging (deviating) box-shaped; sonic preparation approaches are also used here instead of rotating instruments;
• Proximal contact (contact with the adjacent tooth): must be in the area of the inlay, not in the tooth substance area.

Manufacturing phase of the inlay

Two-stage procedure: at this point follows the impression of both jaws, which serves 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:

1. Laboratory-produced ceramic inlays 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.
2. Alternatively, the press ceramic process has become widely accepted. Plasticized mass of leucite-reinforced glass ceramic is pressed into a hollow mold, in which a wax model of the inlay to be produced was previously embedded and burned out. After firing, the rather milky-light pressed ceramic inlay 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 press-ceramic inlay is very good because the volume shrinkage of the ceramic is compensated for by appropriately dimensioned investment materials. In addition, the press ceramic is superior to the layered in its stability.

One-stage procedure: instead of an impression, the dentition is prepared for optical scanning. For the CAD-CAM milling technique, factory-made material blanks of feldspar ceramic or leucite reinforced glass-ceramic and zirconium oxide are used. After optical scanning of the tooth to be restored, the inlay is designed on the computer and then machined out of the blank in a three-dimensional milling process. – The advantage of this procedure lies in the one-time nature and material properties of the factory ceramic. 2nd session or 2nd treatment phase in the chairside procedure:

• Control of the completed inlay;
• Rubber dam (tension rubber to shield the tooth to be treated from the rest of the oral cavity) to protect against saliva ingress and against swallowing or aspiration (inhalation) of the inlay;
• Cleaning the cavity (of the ground defect);
• Try-in of the inlay, if necessary with the help of thinly flowing silicone to find areas that hinder the internal fit;
• Control of the proximal contact;
• Preparation of the tooth for adhesive cementation: Conditioning of the enamel margins for 30-60 sec with 35% phophoric acid gel; dentin etching for 15 sec, then application of a dentin bonding agent to the dentin, which has only been carefully dried – not dried out!
• Preparation of the inlay: etching of the lower surface with hydrofluoric acid; after thorough spraying, silanization takes place;
• Insertion of the inlay using an adhesive technique, e.g. with a dual-curing (both light-initiated and chemically curing) and high-viscosity luting composite; excess cement is removed before light curing; a sufficient polymerization time (time during which the monomeric building blocks of the material chemically combine to form a polymer) of, for example, 60 sec. 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

These can arise from the large number of intermediate steps in the manufacturing process, such as:

• Fracture of the ceramic inlay during try-in due to accidental biting (leaving) in the not yet cemented phase;
• Breakage after cementation due to insufficient tooth substance removal in the occlusal area (occlusal surface area);
• Tooth sensitivity or pulpitides (pulpitis) due to adhesive luting errors;
• 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 into the pulp can lead to pulpitis (pulp inflammation);
• Marginal caries due to insufficient application of luting material in the joint between tooth and inlay – not to be confused with marginal caries caused by poor oral hygiene.