In order to place a bridge to replace one or more teeth, the teeth intended as bridge abutments must largely match in the alignment of their long axes. If the difference is too great, there is a risk that the pulp (tooth pulp) will be damaged by the preparation (grinding). This can be avoided by split bridges, as these compensate for the difference in axis by means of incorporated attachments. In principle, a bridge consists of at least two abutment teeth (bridge anchors) and one or more pontics (pontics). To accommodate a bridge, the abutment teeth must be circularly prepared (ground) in such a way that laboratory-produced crowns – comparable to a thimble – can be placed on them. However, this requires that the abutment teeth largely match in their axial direction. If the tooth axes diverge too much, so much tooth substance of the abutment teeth would have to be sacrificed for a common insertion direction, which is indispensable for an undivided bridge, that the vital preservation of the pulp (tooth pulp) would be endangered and/or the retention (hold) of the crown on the tooth stumps would no longer be sufficient. A split bridge solves the problem of diverging axial directions with the help of an incorporated precision attachment. An attachment consists of an enclosing portion, called the matrix, and the patrix enclosed by it. While the pontic is firmly connected to one of the two abutment crowns, the attachment establishes the connection to the second abutment crown, whereby a rigid connection is established by an additionally inserted screw connection. The attachment has the same insertion direction as the crown to which the pontic is rigidly connected.
Indications (areas of application)
The indication for fabricating a bridge, whether split or unsplit, arises for the following reasons:
- To close the gap
- To prevent tooth migration – tipping into the gap, elongation of the antagonist (outgrowth of a tooth of the opposing jaw from its bone compartment) into the gap.
- To restore phonetics (phonation) and aesthetics.
- To restore the chewing function
- To preserve the support zones (the posterior teeth support the upper and lower jaw against each other, thus preserving the bite height).
In addition, when deciding to use a split bridge, one of the following conditions is present:
- Disparallel abutments – to compensate for different directions of insertion of natural teeth, for example, after tilting into a longer existing gap.
- Disparallel abutments – to compensate for different insertion directions of composite bridges (bridges between natural teeth and implants).
- Disparallel implant abutments
- Abutment teeth with reduced retention (with poorer hold of cemented crown due to short crown or preparation angle).
- To connect several small units of different insertion direction in multi-span bridges.
- To compensate for physiological mandibular mobility or different abutment mobility – stress breaker attachment.
Contraindications
The insertion of a bridge should generally be avoided in the case of:
- Strongly loosened abutment teeth
- Large, arched bridge spans – e.g., when all upper anterior teeth are missing
Contraindications to movable connections.
Stress-breaker attachments in mandibular bridges are intended to compensate for stresses that act on the bridge structure during the natural opening movement of the mandible due to its elastic deformation. A major disadvantage of these constructions is the lack of calculability of the load acting on the abutment teeth. In addition, since the actual necessity for such a construction has not been sufficiently proven on the basis of corresponding studies, the indication should be very narrowly defined. Movable connections (interlocks, resilience attachments) are also not recommended for composite bridges. Although a natural, periodontally healthy and thus not loosened abutment tooth nevertheless has a physiological inherent mobility in contrast to an artificial implant abutment that is firmly fused to the bone, the elastic behavior of the bone, the opposing dentition and the bridge construction itself contribute to the mobility adaptation between the abutments.Rigidly connecting, bolted attachments are therefore also preferable here.
The process
First treatment session
- Impression of the opposing jaw and the jaw with the future abutment teeth for later temporary fabrication.
- Shade selection
- Excavation – carious tooth structure is removed, the tooth is treated with build-up fillings if necessary, to medicate areas close to the pulp (near the pulp) ( for example, with calcium hydroxide preparations, which stimulate the formation of new dentin) and to block out areas that go under themselves.
- Preparation (grinding) – reduction of the crown height by about 2 mm and circular grinding of the smooth surfaces at an angle of about 6° converging towards the coronal. The circular removal must be about 1.2 mm and ends at the gingival margin or subgingivally (below gingival level) in the form of a chamfer or shoulder with a rounded inner edge. Unlike an unsplit bridge, where it may be necessary to deviate from this angle in favor of a common direction of insertion of the teeth, the abutments of a bridge planned as split can each be ground separately without coordination with the axis of the other abutment teeth.
- Preparation impression – e.g. with A-silicone (addition-curing silicone) in double paste technique: a higher viscosity (viscous) paste exerts plunger pressure on a low viscosity paste, which is thereby pressed into the gingival pocket and impresses the preparation margin in detail.
- If necessary, facial bow creation – to transfer the individual hinge axis position (axis through the temporomandibular joints) into the articulator (dental device for imitating the temporomandibular joint movements).
- Bite registration – e.g., made of plastic or silicone; brings the upper and lower jaws into positional relationship with each other
- Fabrication of temporaries – The impression taken at the beginning is filled with self-curing acrylic in the area of the preparation and placed back in the mouth. The acrylic hardens in the cavity created by the preparation. The temporary crowns are finely contoured and placed with temporary cement (e.g. zinc oxide-eugenol cement) that is easy to remove. The design of a pontic is also possible and useful to prevent tooth migration until the definitive restoration is cemented.
First phase in the laboratory
- Pouring the preparation impression with special plaster.
- Making the working model (plaster model on which the bridge will be made) – The model is socketed, the future working dies are pinned so that they can be individually removed from the base and reset after sawing the model.
- Model assembly in the articulator – based on the facial arch and bite registration.
- Wax modeling first of all of the crown framework with the (factory-made) matrix part of the attachment, taking into account the direction of insertion.
- Metal casting – conversion to metal: casting channels made of wax are attached to the wax model. Subsequently, the modeling is embedded in a casting muffle. The wax is burned out in a hot furnace. In the resulting cavities, liquid metal (gold or non-precious metal alloy) is introduced in the centrifugal process, the matrix is connected to the crown by sprueing.
- After devestment, the finishing of the first bridge part, then insertion of the patrix part in the attachment and connecting with the modeling of the second abutment crown.
- Sprue of the patrix to the second abutment crown.
- Finishing the metal framework
- If necessary: manufacture of registration templates for the jaw relation determination.
Second treatment session
- Removal of the temporary restoration and cleaning of the abutment teeth.
- If necessary: jaw relation determination – to maintain or redefine the distance of the maxillary and mandibular bases from each other.
- Framework try-in – check for insertion direction, tension-free fit and marginal fit.
- Renewed provisional cementing of the temporary restoration
Second phase in the laboratory
- If necessary, model assembly based on the jaw relation determination.
- Ceramic veneering – Ceramic masses are applied in layers to the metal frameworks using the finest brush technique and finally fired on in several phases.
Third treatment session
- Removal of the temporary restoration and cleaning of the abutment teeth.
- Try-in of the bridge under control of occlusion (final bite and chewing movements).
- Definitive cementation – cementing (e.g. with conventional zinc phosphate or carboxylate cement) first of the bridge portion supporting the matrix. The crown framework is spread thinly and placed on the tooth under strong pressure. After removing any interfering excess cement, the second bridge part is placed immediately.
- Removal of all excess cement after setting.
- Establishing the rigid connection by screwing.
- Occlusion control
After the procedure
- Follow-up appointment for recheck
Possible complications
- Intrusion (pushing into the jaw) of the abutment far from the slide when using movable connections – A split bridge must be loaded as it is slid through the antagonists (teeth of the opposing jaw). If, on the other hand, only the abutment far from the attachment is loaded during the chewing connection, it may be intruded (gradually pressed into the jawbone) to such an extent that the matrix and patrix of the attachment become detached from each other. A screw-retained attachment can prevent this.
- Intrusion of the natural abutment in composite bridges when using movable connections – To prevent intrusion of the natural tooth compared to the implant-supported abutment, screw-retained attachments should be preferred here as well.
- Chipping of ceramics
- Loosening of the cement joint on an abutment tooth.
- Formation of marginal caries along the crown margin in case of insufficient oral hygiene.