Onlays are dental fillings that are usually fabricated indirectly (outside the mouth) and placed in the tooth that has been previously prepared (ground) using special luting materials matched to the onlay material. The spatial limits of the preparation are on the cusp tips of the tooth. In terms of preparation technique, the onlay thus occupies an intermediate position between an inlay and an overlay: the former does not completely cover the occlusal surface, while in the latter the cusp tips are included in the preparation in the sense of a chewing edge protection. The transitions between these three forms of preparation can definitely be regarded as fluid.
Indications (areas of application)
The indication for the preparation of an onlay results from the degree of destruction of a dental crown and, depending on the material used, from the position of the tooth in the mouth. Gold alloys have been used as the material for decades; however, due to the insufficient esthetics of cast gold fillings, tooth-colored materials such as glass-ceramic composites and, above all, ceramics have been used in the majority of cases in recent years. The areas of application must therefore be differentiated according to the material used:
Indications for gold casting onlay
- Proven amalgam intolerance;
- Proven intolerance of materials for adhesive cementation technique of tooth-colored onlays;
- Subgingival cavities extending into cervical dentin or root dentin (defects extending into the dentin of the cervix or root), for which adhesive cementation techniques of tooth-colored onlays are no longer feasible;
- Cavity walls (walls of the tooth defect) that are too thin and not sufficiently stable, requiring cusp protection;
- Bridge anchors;
- Restoration of maxillary premolars (anterior molars) as an esthetically acceptable compromise to the overdome overlay;
- Restoration of molars (posterior molars) and mandibular premolars;
- Defects with large buccolingual extension (large extension from the cheek to the tongue).
Indications for tooth-colored ceramic or glass-ceramic composite onlay.
- Proven amalgam intolerance;
- Very rare proven gold intolerance;
- Approximal cavities (tooth defects in the interdental space) that can still be restored with adhesive techniques, i.e. do not extend into the cervical or root region;
- Cavity walls that are too thin and not sufficiently stable for inlay restoration, which require cusp protection;
- Defects with large buccolingual extension;
- Esthetic aspects especially in the restoration of premolars (anterior molars).
Contraindications
- Circular decalcification (surrounding the tooth in a band-like manner); this is where the indication for a crown arises;
- Missing wall; here a partial crown or crown is indicated;
- Too short clinical crown; this limitation applies only to the conventionally cemented gold cast-onlays, whose retentive fit would be insufficient; for tooth-colored restorations, it is not decisive due to the adhesive cementation technique used.
The procedure
Unlike direct filling therapy, restorations with indirectly (outside the mouth) fabricated fillings are divided into two treatment sessions, unless they are one-time chairside (at the dental chair) ceramic restorations milled with the CAD-CAM method. 1st session:
- Caries removal;
- Preparation (grinding):
- In principle, any preparation technique must be as tooth tissue sparing as possible, ie: sufficient water cooling (at least 50 ml/min), rounded preparation shapes, no excessive roughness depths, lowest possible substance removal and protection of neighboring teeth.
- An essential difference between fillings placed directly in the tooth and the indirect (made outside the mouth) onlay is the preparation technique (technique of grinding); because the latter must be able to be removed from the tooth or placed in/on it without jamming or creating undercuts (unprovided cavities). This is made possible by a preparation angle slightly divergent in the direction of removal of the restoration.
- However, especially in the case of gold casting restoration, good retention (primary fit without cement layer) must be given despite divergence, because the cement does not serve here as an adhesive, but only increases the retention.
- Occlusal preparation (in the occlusal area): layer thickness min. 2 mm;
- A gold casting preparation receives a max. 1mm wide feather margin occlusally at an angle of 15° to the enamel surface, which serves to protect the enamel prisms in the marginal area of the preparation and minimizes the distance casting object-to-tooth. No spring margin for tooth-colored restorations!
- Proximal preparation (in the interdental area): slightly diverging box-shaped, free of undercuts in the marginal area; while a defined step is prepared in the cervical (tooth neck area) for the ceramic or glass-ceramic composite onlay, a tooth receives a step with a defined bevel in the sense of the spring margin technique here to accommodate a gold cast onlay;
- Approximally, the use of sonic preparation attachments instead of rotating instruments is advantageous.
- Proximal contact (contact with the adjacent tooth): is solved, i.e. it must be in the area of the onlay to be produced and not in the tooth substance area;
- Impression: it is used by the dental laboratory to produce a working model in dimensions faithful to the original;
- Temporary (transitional) restoration to protect the tooth and prevent tooth migration until the onlay is cemented; for placement, zinc oxide eugenol cement must not be used for an adhesively planned onlay, as it inhibits (prevents) the curing of the adhesive final cement.
2nd session:
- Removal of the temporary restoration;
- Rubber dam to protect against saliva ingress and prevent swallowing or aspiration of the onlay;
- Cleaning of the cavity ( of the ground defect);
- Try-in of the onlay, if necessary with the help of thin-flowing silicone or colored spray to find interfering areas in the internal fit;
- Control of the proximal contact.
In the further procedure, a distinction is made between gold cast-on and adhesively cemented restorations:
Gold castonlay:
- Checking the occlusion (contacts with the teeth of the opposing jaw in the final bite position) and articulation (contacts during the lateral movements and advancement of the mandible); if necessary, corrections by grinding;
- Final polishing of the onlay;
- Disinfection of the tooth, e.g. with chlorhexidine digluconate;
- Placement of the gold cast-onlay with, for example, zinc phosphate, glass ionomer, carboxylate or dual-curing composite cements (whose curing is light-induced and then chemically continued).
- Finishing: this is done after the removal of excess cement finally cured, driving the spring edges to the enamel with the finest Arkansas stones, polishing wheels and rubber polishers.
Adhesively luted onlay:
- Here, occlusion and articulation are better checked after final cementation and corrected if necessary; for this purpose, it is helpful to mark the occlusal contacts on the adjacent teeth with colored foil before placing the rubber dam, so that they can be used as a comparison after cementation;
- Disinfection of the tooth, e.g. with chlorhexidine digluconate – not with hydrogen peroxide, as this inhibits (prevents) the curing of the luting composite;
- Preparation of the tooth: conditioning of the enamel margins for 30-60 sec with 35% phosphoric acid gel; dentin etching for 15 sec and subsequent application of a dentin bonding agent to the previously only carefully dried dentin;
- Preparation of the onlay:
- Ceramics: etching the lower surface with 5% hydrofluoric acid 2 min ; spray off; dry; silanize;
- Composite: Clean lower surface; silanize;
- Insertion in adhesive technique with a luting composite, preferably with a dual-curing (light- and chemical-curing) cement, which sets faster due to light polymerization; excess cement removal before light curing!
- Correction of occlusion and articulation with ultra-fine diamond burs;
- Finishing and smoothing the edges with ultra-fine grit diamonds and polishers;
- Fluoridation.
Possible complications
These arise from the large number of intermediate steps of the procedure that must be carefully performed, such as:
- Loss of a gold cast onlay due to poor retentive preparation technique or incorrectly mixed luting cement;
- Fracture of a ceramic or composite onlay due to insufficient tooth substance removal in the occlusal area (masticatory area);
- Tooth sensitivity or pulpitides (tooth pulp inflammation) due to errors in adhesive cementation;
- Marginal caries due to insufficient application of luting cement in the marginal areas.