Extension Bridge

An extension bridge (synonyms: free-end bridge, trailer bridge) is used to restore a shortened or interrupted row of teeth by attaching a pontic to two interlocked crowns. The extension of the bridge is strictly limited by the special features of the bridge statics.

Bridge statics

Due to the structural requirements of an extension bridge explained below, such designs have lower survival rates than so-called end-pier bridges. Nevertheless, under certain conditions, an extension bridge may be the treatment option of choice to still allow for fixed prostheses and to avoid removable restorations or surgical procedures such as implantation of artificial tooth roots. First of all, a comparison should be made with an end abutment bridge. In this case, the so-called pontic hangs between the teeth serving as bridge abutments, unlike in the case of the extension bridge. The gap between the teeth is therefore enclosed by two teeth. If the pontic of an end abutment bridge is loaded by masticatory pressure, the transmission of the compressive forces takes place statically favorably in the axial direction to the abutment teeth. The static requirements on the abutment teeth of an extension bridge, on the other hand, are much greater. Here, the pontic is attached to the last abutment tooth, whereby strong tensile forces act on the abutment tooth far from the load when the bridge pendant is masticatory loaded, while the abutment near the load is intruded (pressed in) into the alveolus (the bony tooth socket). The tensile forces can result in loosening of the bridge anchor. To withstand such a load, the tooth axes of the abutment teeth must be aligned largely parallel and the teeth must be sufficiently dimensioned. This results in a strict indication for endodontically (root) treated abutment teeth whose tooth structure is severely reduced due to previous history. The higher requirements for retention (the mechanical hold of the bridge on the abutment teeth) are met on the one hand by an almost parallel-walled preparation (grinding). On the other hand, an inclination of the abutment teeth opposite to the position of the bridge abutment has a positive effect on the bridge statics. Such retentive preparation forms can usually only be created by restoring full crowns, not partial crowns. In addition, the teeth must be anchored in the bone via a healthy periodontium (periodontal apparatus) in order to withstand the unavoidable tensile forces. Due to the tensile forces applied, the width of the bridge pendant in the dental arch is limited to a maximum of one premolar width. Materials

All-cast bridge made of precious metal alloys or non-precious metal alloy (EMF, NEM) or titanium – in the posterior region for the restoration of a shortened row of teeth (caused by the absence of a posterior molar).
Plastic veneer bridge – A metal framework receives a tooth-colored plastic coating in the visible area. Since the plastic veneer is the limiting factor for the lifetime of the construction, this veneer option is used only in exceptional cases.
Ceramic veneer bridge – metal framework with ceramic veneer.
All-ceramic bridge – e.g. made of zirconia, made of alumina or lithium disilicate.

Fastening options

Conventional luting – The permanent bond between bridge material and bridge abutments is created by means of a conventional one cement (e.g. zinc phosphate, glass ionomer or carboxylate cement). The cement as such only serves to fill the cement joint, which must be kept as thin as possible. The actual hold of the bridge is provided by so-called friction (fit by static friction between parallel walls). – In addition to metal bridge frameworks, oxide ceramics can also basically be fixed conventionally.
Adhesive cementation – After conditioning (chemical pretreatment) of the surfaces to be bonded, i.e. the prepared teeth and the inner surfaces of the crowns, a micromechanical bond is generated by means of chemically curing composites (plastics), thereby increasing the retention (mechanical hold) of the crowns on the abutment teeth. – Ceramic materials are often cemented using the more complex adhesive technique.

Indications (areas of application)

To build up a premolar occlusion in the case of a shortened dentition.
On two directly adjacent abutment teeth only if the abutment is sufficiently long and far away from the load, and if the axis alignment is largely the same and the preparation shape is thus almost parallel-walled
On vital (living, not root-treated) and stably dimensioned abutment teeth.
For lengthening a shortened row of teeth with a pontic attached distally (behind the last abutment tooth) in the width of no more than one premolar (an anterior small molar)
At the end of an interrupted row of teeth with mesial (in front of the last abutment tooth) attached pontic in the width of a maximum of one premolar – e.g. to avoid the preparation of a canine tooth.
To prevent tooth migration – e.g. the elongation of an antagonist (outgrowth of a tooth in the opposing jaw from its bone compartment).

Contraindications

Absolute contraindications

Free-end bridge with only one abutment – The special form of the single-abutment adhesive bridge (synonyms: adhesive bridge, Maryland bridge) is an exception.
Endodontically treated abutment teeth with excessive loss of substance.
Periodontopathy – abutment teeth with previous disease of the periodontium, which can not permanently withstand the special static load caused by an extension bridge.
Apical osteolysis (inflammatory bone dissolution around the root apex).
Short clinical crowns – The resulting lack of retention (mechanical hold) of the crown restoration on the prepared abutment teeth leads to loosening of the bridge.

Relative contraindications

Caries-free abutment teeth – Here, the restoration of the gap with an implant or, especially in adolescents, with an adhesive bridge should be considered as an alternative.
Inefficient oral hygiene – Since the abutment teeth and thus their periodontium (periodontal apparatus) are exposed to special loads by an extension bridge, must be counteracted with adequate hygiene techniques the establishment or progression of periodontal disease.
Lack of compliance – The lack of willingness to regular dental check-up appointments, supplemented by PZR (professional dental cleaning) or even UPT (Supportive Periodontal Therapy) puts the success of the bridge restoration in question.
Condition after root tip resection – Surgically induced shortening of the root can lead to an unfavorable crown-root relationship.
Intolerance to components of a metal alloy – Evasion to compatible alternatives (e.g., high-gold alloy or ceramic).
Incompatibility against PMMA-based plastics (polymethyl methacrylate) – Evasion to bridge material, which can be fixed with conventional cements.

Before the procedure

Sensitivity test
X-ray diagnostics
If necessary, surgical, conservative and periodontal rehabilitation of abutment teeth and estimate their prognosis.
Evaluation of the abutment teeth in terms of the feasibility of a retentive preparation form, which can counteract the load of the bridge sufficient mechanical support.

The procedure

The procedure for fabricating an extension bridge is explained using an all-cast bridge as an example. Any additional procedural steps for fixed ceramic veneer bridges, resin veneer bridges, adhesive bridges and constructions fabricated using the CAD/CAM method are merely referred to here. I. First treatment session

Impression of the opposing jaw and the jaw with the future abutment teeth for the subsequent temporary fabrication.
Excavation – Carious tooth structure is removed, the tooth is provided 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 (dentine)) 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 slightly subgingivally (below gingival level) in the form of a chamfer or step with a rounded inner edge. Build-up fillings must be sufficiently grasped by the preparation (barrel ripening effect).
Insertion direction – An important procedural step that makes a fixed bridge design possible in the first place is the alignment of the preparation angles of the abutment teeth. To ensure a common insertion direction of the subsequent crowns, it may be necessary to deviate slightly from the ideal of 6° preparation.
Placement of retraction threads – Before taking an impression of the abutment teeth, the surrounding gingiva (gums) is temporarily displaced with a retraction thread (from Latin retrahere: to pull back) placed in the sulcus (gingival pocket), thereby representing the preparation margin on the impression. The thread is removed immediately before taking the impression.
Preparation impression – e.g. two-phase impression with A-silicone (addition-curing silicone) in double paste technique: a higher viscosity (viscous) paste exerts a stamp pressure on a low viscosity mass, which is thereby pressed into the gingival pocket and forms the preparation margin true to detail.
Facial arch unit – for transferring the individual hinge axis position (axis through the temporomandibular joints) to the articulator (dental device for imitating temporomandibular joint movements).
Bite registration – e.g., made of plastic or silicone; brings the upper and lower jaws into positional relationship with each other
Temporary restoration – The impression taken at the beginning is filled with chemically curing acrylic in the area of the preparation and placed back in the mouth. The resin 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) which is easy to remove. If adhesive cementation is planned, a eugenol-free (clove oil-free) temporary cement must be used, as eugenol inhibits (inhibits) the setting reaction of the luting composites. – The design of a temporary pontic is possible and useful to prevent tooth migration until the definitive restoration is cemented.

II. dental laboratory

II.1. pouring the preparation impression with special plaster.

II.2. 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 put back after sawing the model. II.3. model assembly in the articulator – on the basis of the facial arch and bite registration

IÍ.4. wax-up – first the crowns, then the bridge pendant are shaped by applying liquid wax in layers according to anatomical and functional aspects. Casting channels made of wax are attached to the finished wax model. II.5. metal casting – The wax model is embedded in a casting muffle. In the hot furnace, the wax is burned out without residue, creating cavities inside the investment. Liquefied metal (gold or non-precious metal alloy) is introduced into the cavities via the casting channels using centrifugal and vacuum processes. After cooling, the casting is devestigated and then finished to a mirror polish. III. second treatment session

Removal of the temporary restoration and cleaning of the abutment teeth e.g. with chlorhexidine.
Trying in the bridge while checking the static and dynamic occlusion (final bite and chewing movements) with the help of different colored occlusal foils to mark it
Control of proximal contacts – contact points to adjacent teeth must be as tight as between natural teeth, but must not create a feeling of tension
Definitive cementation – Before cementation (e.g., with conventional zinc phosphate or carboxylate cement), the abutment teeth are dried, but not overdried. The crowns are spread thinly with cement and placed on the teeth under slowly increasing contact pressure to make the cement joint as thin as possible.
Waiting for the setting phase, keeping the bridge in situ (in the correct position) in a controlled manner.
Removing all excess cement after setting.
Occlusion control

After the procedure

Recall (follow-up appointment) promptly for recheck.
Thereafter, regular recalls with oral hygiene skills refresher to prevent bridge loss due to caries or periodontal disease (tooth decay or periodontal disease).

Possible complications

Bridge loosening due to lack of mechanical retention (the mechanical hold of the bridge on the abutment teeth).
Fracture (tooth fracture) of one or more abutment teeth, especially endodontically treated teeth (with root filling).
Technical failures – fracture of the bridge framework.
Loosening of the cement joint on an abutment tooth – especially on the abutment far from the load.
Inadequate oral hygiene – resulting in the establishment of periodontal disease or the development of marginal caries along the crown margin.
Preparation-related pulpitis (pulp inflammation).
Tooth sensitivities (hypersensitivities) due to adhesive luting technique or material.