Fixed Bridge

A bridge is used to restore a gap between teeth. In order to cement a fixed bridge to replace one or more teeth, the teeth intended as bridge abutments must be prepared (ground) to receive a crown or partial crown. The abutment teeth must largely match in the alignment of their longitudinal axis. In principle, a fixed bridge consists of at least two abutment teeth (bridge anchors) and one or more pontics (pontics) in the area of the teeth to be replaced. The pontics are usually designed according to hygienic criteria as so-called tangential pontics (with a convex or egg-shaped base surface), whereby compromises are made in the visible area in favor of esthetics and phonetics. A bridge is supported exclusively by teeth or implants. In this respect, it differs from a partial denture or combined denture, which are designed to be supported by both teeth and mucosa. A full denture, on the other hand, must completely forgo tooth-supported support: the chewing load is transferred here exclusively to the oral mucosa. Since the teeth are often pre-damaged by caries (tooth decay), they are usually prepared circularly (ground all around) to accommodate a bridge, so that the laboratory-manufactured full crowns – comparable to a thimble – can be fitted. In the case of less pronounced tooth defects, partial crowns can also serve as bridge abutments. Design principles

  • Since the masticatory force acting on the pontics of a bridge is transmitted to the abutment teeth, the root surface of the abutment teeth anchored in the bone should at least correspond to the surface with which the teeth to be replaced were formerly anchored.
  • Fixed bridge restorations are made not only on natural teeth, but also on implants. If the bridge abutments are combined from natural teeth and implants, we speak of composite bridges.
  • While single-span bridges serve only to supply a gap caused by the loss of one or more immediately adjacent teeth, multi-span bridges bridge two or more gaps between several teeth.
  • The conventional design of a fixed bridge, in which a pontic hangs between two abutments, is an end-abutment bridge. To be distinguished from this are so-called extension bridges (free-end bridges, trailer bridges). These are fabricated both for the restoration of tooth-limited gaps and for free-end situations, whereby a pontic is attached to two interlocked (connected) crowns. Due to the less favorable statics because of the strong acting leverage force, the pendant may only bridge a short distance in the dental arch, a premolar width (width of a small anterior molar).

Attachment 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.

Materials

  • Full-cast bridge made of precious metal alloys or non-precious metal alloy (EMF, NEM) – e.g. in the posterior region for the restoration of a molar gap (gap caused by the absence of a posterior molar).
  • Resin veneer bridge – A metal framework receives a tooth-colored resin 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.
  • Adhesive bridge – adhesively attached metal or ceramic framework with veneer.

Indications (areas of application)

The indication for the fabrication of a bridge arises for the following reasons:

  • To replace missing teeth – gap closure
  • To prevent tooth migration – tipping into the gap, elongation of the antagonist (outgrowth of a tooth in the opposing jaw from its bone compartment).
  • To restore phonetics (phonation).
  • To restore the 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) and restore occlusion (chewing closure and chewing movements).
  • As a superstructure on implants
  • On abutment teeth with largely matching axial alignment.

Contraindications

Absolute contraindications

  • Severe damage to the periodontium (the tooth-supporting apparatus) and thus loosening.
  • Apical osteolysis (inflammation-induced bone dissolution around the root apex).
  • Large, arch-shaped bridge spans – e.g., in the absence of all upper anterior teeth; if necessary, abutment augmentation with implants.
  • Insufficient number or distribution of abutment teeth – if necessary, abutment augmentation with implants.
  • Loss of more than three consecutive teeth and gap not narrowed by tooth migration – an exception is the loss of four incisors, provided that the course of the dental arch is not arcuate.
  • Defects of the edentulous alveolar bone – e.g. after injuries or operations such as covering a cleft lip and palate: If the bridge surfaces close to the mucosa cannot be made hygienic, so that chronic inflammation is to be expected, hygienicity should be made possible by a removable construction.

Relative contraindications

  • Caries-free teeth limiting the gap – In this case, the restoration of the gap with an implant or, especially in adolescents, with an adhesive bridge should be considered as an alternative.
  • Condition after root tip resection – Surgically induced shortening of the root can lead to an unfavorable crown-root relationship.
  • Short clinical crowns – For reasons of mechanical retention (crown hold) on the prepared tooth, this must be at least 3 mm high for preparation angles of 3° to 6°, and at least 5 mm is required for angles between 6° and 15°. If these minimum dimensions cannot be implemented, surgical tooth lengthening must be considered. An adhesive luting procedure is preferable to improve retention (hold of the crown on the tooth).
  • Inadequate oral hygiene – secondary caries in the crown margin area calls into question the long-term success of a bridge restoration.
  • Difficult access during preparation – A restricted mouth opening, for example, can make it difficult or impossible to apply the rotary instruments for grinding an abutment tooth at the correct angle.
  • Law according to ante – root surfaces of the abutment teeth are less than 50% compared to the root surfaces of the teeth to be replaced – here the restoration with a tight-fitting bridge still possible, but a shorter retention time of the bridge is to be expected.
  • Intolerance to components of a metal alloy – switch to compatible alternatives ( eg high gold alloy or ceramics).
  • Incompatibility against PMMA-based plastics (polymethyl methacrylate) – Evasion to bridge material that can be fixed with conventional cements.

Before the procedure

Before the procedure, it must be ensured that the abutment teeth to be crowned are clinically and radiographically healthy or, after restoration by conservative, endodontic, surgical or periodontal therapy measures (by caries removal and filling therapy, root canal treatment, root tip resection or treatment of periodontal diseases), their load-bearing capacity by the planned bridge is given.

The procedure

The procedure for fabricating a fixed bridge is explained using an all-cast bridge as an example. Any additional procedural steps for fixed ceramic veneer bridges, plastic 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 later 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) ( e.g. 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. Circular ablation must be about 1.2 mm and ends at the gingival margin or slightly subgingival (below gingival level) in the form of a chamfer or step with a rounded inner edge.
  • Insertion direction – An important procedural step that makes a fixed bridge design possible in the first place is the matching 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 plunger 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) 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
  • 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 pontic 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 cementing (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

  • Loosening of the cement joint on an abutment tooth – especially with extension bridges.
  • Inadequate oral hygiene – resulting in periodontal complications or marginal caries along the crown margin.
  • Preparation-related pulpitis (pulp inflammation).
  • Tooth sensitivities (hypersensitivities) due to adhesive luting technique or material.
  • Fracture (fracture)