Galvano crowns and bridges are restorations made of ceramics whose inner surfaces are made of a thin layer of fine gold produced by electroplating. The technique combines the esthetic advantages of a ceramic crown with the advantage of a cast gold crown, which is that it can be used with conventional luting cements such as phosphate or carboxylate cements. In this way, it is possible to bypass the dentin-adhesive (micro-mechanically adhering to the dentin) composite-based (resin-based) luting materials used in ceramic restorations, to which a few patients show intolerance. In addition, an electroformed crown has excellent accuracy of fit, which can exceed that of a cast gold crown. For the sake of completeness, the importance of double crowns made by electroforming for fixing combined dentures should be mentioned: Here, the fabrication of the so-called secondary crowns (synonyms: secondary telescopes, abutments) on the primary crowns can be done by electroforming. The abutments are incorporated into a removable denture and provide excellent denture retention due to their high precision. With the advent of zirconia – tooth-colored ceramic as a crown and bridge material that can also be incorporated with conventional cements – it is expected that ceramic-veneered electroformed crowns will be used less frequently in the future. Advantages of electroformed crowns
- High biocompatibility (biological compatibility) due to the use of pure fine gold.
- Gold savings compared to a full cast crown and also to a VMK crown (a veneered ceramic crown with a cast metal framework).
- Highest accuracy of fit
- The wall thickness of the fine gold framework is only about 0.2 mm, so the tooth can be prepared (ground) more gently
- Simple luting procedure through the use of conventional cements.
- Aesthetics
Indications (areas of application)
The indication for the restoration of a tooth with a crown results from its degree of destruction of the tooth to be restored. Bridge planning requiring the restoration of a tooth with a crown as a bridge anchor results from the gap situation in a jaw, as does the indication for combined dentures with telescoping double crowns. A ceramic-veneered electroformed crown can meet high esthetic demands, since only a narrow circumferential margin of its fine gold content is visible at the preparation margin in the tooth neck area. The ceramic itself promises the highest esthetics, although one must accept the limitation that, unlike an all-ceramic restoration, a chameleon effect – a color-matching interaction between the tooth stump and the ceramic – cannot be expected due to the fine gold framework. The following application options are available:
- For crown restoration in the anterior and visible posterior region.
- For the fabrication of abutments for removable double crown bridges.
- In case of incompatibility of composites (plastics) and thus also dentin-adhesive luting cements for ceramic crowns.
- In case of incompatibility against one of the components of an alloy for a gold casting or non-precious metal restoration.
- Adhesive cementation of ceramic crowns excluded because preparation margins are subgingival (below the gingival margin) and absolute drainage is not possible
Contraindications
- Very rare: Gold intolerance
- Suitable preparation form (way in which a tooth must be ground to accommodate the crown) is not feasible
The procedure
Two to three treatment sessions in the dental office and one to two laboratory runs are required before an electroformed crown or a fixed electroformed bridge can be placed. First treatment session
- Excavation (removal of caries) and, if necessary, placement of a subfilling, e.g., of phosphate or carboxylate cement for substance compensation of undercropping tooth areas.
- Preparation (grinding) – chamfer or shoulder preparation with a rounded inner edge and an approximately 6° converging angle of the smooth surfaces; the removal in the area of the smooth surfaces must be 1.2 mm, in the area of the occlusal surface or incisal edge about 2 mm. The smooth surfaces of abutment teeth for a bridge must have a common insertion direction and be ground accordingly.
- Impression – is used by the dental laboratory to make a working model from plaster in dimensions true to the original. It is taken, for example, with A-silicone (addition-curing silicone) in double paste technique: A higher viscosity (viscous) paste exerts stamp pressure on a low viscosity paste, which is thereby pressed into the gingival pocket and forms the preparation margin true to detail.
- Bite taking – to bring the upper and lower teeth in positional relationship to each other.
- If necessary, creation of a face bow – if the individual hinge axis (running through the temporomandibular joints) to be transferred to the dental laboratory.
- Temporary restoration – plastic transitional crowns made directly in the patient’s mouth are placed with easily removed temporary cement, thus protecting the dentin wound and preventing tooth migration until the definitive restoration is cemented.
First phase in the laboratory
- Pouring the impression with special plaster
- Making the master model (plaster model on which the restoration is made) – The model is socketed, the future working dies are fitted with pins so that they can be individually removed from the base and reset after sawing the model.
- Model assembly in the articulator (device for imitating the position of the jaw and the movements of the temporomandibular joint) based on the bite registration and the values determined by the facial arch.
- Fabrication of a duplicate die (working die on which the electroformed coping is fabricated). The die material, which is not conductive in itself, must be coated with an ultra-thin layer of conductive silver varnish for the electroforming technique. This creates a conductive metal layer.
- Electroplating – The duplicate die is placed as a cathode in the electrolytic bath of the electroplating equipment. In the bath, in addition to conductive salts and brighteners, there is the fine gold, which is deposited on the duplicate die by the flow of electric current. The result is a crown substructure of definable thickness made of fine gold, which reproduces the surface structure of the preparation extremely accurately. It is a maximum of 0.2 mm thin, but stable enough for a possibly required framework try-in on the patient.
- Fit of the fine gold framework on the die of the master model.
- Fabrication of a pontic – A pontic electroformed from a wafer-thin layer of fine gold could not withstand chewing pressure. Therefore, pontics for electroformed bridges must first be modeled in wax, then cast from a metal alloy (e.g., high-gold bio-firing alloy), and then bonded to the fine gold copings (by soldering, laser, special adhesive, and others). The cast metal alloy will be surrounded by ceramic all around when the bridge is completed.
Second treatment session (optional).
- Removal of the temporary restoration and cleaning of the tooth stump.
- Framework try-in – if a check of the internal fit of the fine gold copings or the bridge framework for tension-free fit is required. Corrections are possible at best with a very sharp milling cutter and only to a minimal extent due to the low layer thickness. The caps are carefully removed from the tooth stump again, for example, by grasping with a fine gauze.
- Renewed temporary restoration
Second phase in the laboratory
- The fine gold coping or bridge framework first receives a bonder firing, then a tooth-colored opaquer that is opaque and prevents the gold from shining through the ceramic.
- Firing of the ceramic veneer in several layers according to the coloration of a natural tooth.
- Glaze firing
Third treatment session
- Removal of the temporary restoration and cleaning of the cavity.
- Try-in with occlusion control (final bite and chewing movements).
- Cementing – Final cementation is performed with conventional cements such as phosphate or carboxylate cements, with care taken to select a particle size of less than 25 µm. The crown framework may only be spread thinly with the cement mixed to the appropriate consistency, but not filled, in order to minimize stresses during pressing onto the tooth stump.
- Removal of cement residues after setting.
After the procedure
- Follow-up appointment for reocclusion check.
Possible complications
- Fracture (breakage) of the crown during cementing.
- Deformation of the fine gold cap due to inadequate handling in the course of manufacture – e.g. during try-in, in the course of the soldering process and many more.
- Fracture after cementation
- Formation of marginal caries along the crown margin in case of insufficient oral hygiene.
