Bridges

A bridge is a dental prosthesis supported by teeth or implants. It is used to fill one or more gaps between teeth. Natural teeth must be prepared (ground) beforehand to attach a bridge. Since the teeth intended to receive a bridge are often pre-damaged by caries (tooth decay), they are usually prepared circularly (ground all around) so that the laboratory-manufactured full crowns – comparable to a thimble – can be fitted. In the case of less pronounced tooth defects, bridge abutments can be prepared to receive a partial crown. Implants also serve as bridge abutments. A bridge is supported exclusively by teeth or implants. In this respect, it differs from a partial denture or combined denture, which are designed in such a way that the chewing load is transferred both to the teeth or implants and to the oral mucosa. A full denture for the restoration of an edentulous jaw, on the other hand, must completely dispense with tooth-supported support: the chewing load is transferred here exclusively to the oral mucosa and the underlying jawbone. In principle, a 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 designed according to hygiene criteria, whereby compromises are made in the visible area in favor of esthetics. A removable bridge also includes connecting elements that enable the bridge to be removed and held in place. While single-span bridges only serve to provide a gap caused by the loss of one or more immediately adjacent teeth, multi-span bridges bridge two or more gaps between several teeth.

Construction options and principles

I. Fixed bridge

In order to cement a fixed bridge to replace one or more teeth, the teeth intended to serve as bridge abutments must be largely aligned in the orientation of their long axes. 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 match 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, they are called composite bridges. II. split bridge

If the abutment teeth do not sufficiently match in the alignment of their longitudinal axis, so much tooth substance would have to be sacrificed during preparation for a common insertion direction of the teeth that damage to the pulp (tooth pulp) could not be ruled out and/or the retention (hold) of the crowns on the tooth stumps would no longer be sufficient. Too large axial differences are therefore compensated for by precision attachments incorporated into split bridges. A split bridge can be designed to be both fixed and removable. III. removable bridge

The design of a removable bridge (synonym: removable bridge) includes not only bridge abutments and pontics, but also connecting elements that make removal and retention possible. Double crowns are usually used as connecting elements. These are also used in combined dentures and consist of a so-called primary part, which is firmly cemented to the abutment tooth, and an abutment part, which together with the pontic(s) forms the actual bridge. The design of the outer surfaces of the primary parts compensates for axial differences. Friction is created by precisely matched telescoping surfaces between the primary and secondary crowns, thus enabling the bridge to be held in place. IV. Conditionally removable bridge

Conditionally removable bridges can only be removed from the patient’s mouth by the dentist. As a rule, these are superstructures on implants, which are fixed by screwing. The bridge is removed at regular intervals to check the implants and for cleaning. Repairs may also be possible in this way.V. Adhesive bridge (Adhesive bridge)

Adhesive bridges (synonyms: adhesive bridges, Maryland bridges) are micromechanically fixed to one or two only slightly prepared abutment teeth by means of an adhesive luting composite (resin-based cement) and are useful under certain conditions, especially for gaps in the adolescent dentition – for example, to bridge the waiting period until the completion of growth and implant placement (surgical placement of an implant). The minimally invasive preparation (which is gentle on the tooth substance) inevitably requires adhesive cementation with chemically curing composites (acrylics), as the preparation technique itself cannot achieve sufficient mechanical retention. In this case, retention is achieved by a micromechanical bond between the tooth surface and the bridge material. VI. Extension bridge

So-called extension bridges (synonyms: free-end bridges, trailer bridges) are to be distinguished from the conventional construction of a bridge, in which a pontic is suspended between two abutments. Extension bridges are made both for the restoration of tooth-limited gaps and for free-end situations, in which a pontic is attached to two interlocked (connected) crowns. Due to the less favorable statics because of the leverage force acting on it, the pendant may only bridge a short distance in the dental arch, a premolar width (width of a small anterior molar).

Materials

Not only are the design options for a bridge varied, but so are the materials available:

All-cast bridge made of precious or non-precious metal alloy (NEM) – e.g. in the posterior region for the restoration of a molar gap (gap caused by the absence of a posterior molar).
Plastic veneer bridge – For this, the aesthetically important buccal or labial side (cheek or lip side) of a metal framework is veneered (coated) with tooth-colored plastic. Since a plastic veneer is the limiting factor for the lifetime of the construction, this veneering option is usually abandoned.
Ceramic veneer bridge – metal framework with ceramic veneer.
All-ceramic bridge – designed as a monobloc (from one piece) or by ceramic veneering of a ceramic base framework, e.g. from zirconia, from aluminum oxide or lithium disilicate.

Luting options

Conventional cementation – using conventional cements such as zinc phosphate, glass ionomer or carboxylate cement: Fixed full-cast or veneer bridges are usually conventionally cemented to natural teeth. Oxide ceramics can also be cemented conventionally in principle.
Adhesive cementation – In addition to adhesive bridges, for which the micromechanical bond provided by a luting composite (resin) is mandatory, ceramic restorations are also adhesively cemented. Both the surfaces of the teeth and the bridge material must be chemically pretreated for this type of luting in order to generate the microscopically fine interlocking.
Screwing – for fixation of conditionally removable superstructures on implants.
Friction – fit by static friction between parallel walls achieved by the telescoping surfaces of precision attachments, so also double crowns – design element of split and removable bridges.

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
The root surfaces of the abutment teeth anchored in the alveolar bone must be at least 50% of the root surfaces of the teeth to be replaced

In addition, one of the following conditions is present when deciding on a split bridge:

Disparallel abutments – to compensate for different directions of insertion of natural teeth.
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

Absolute contraindications

Unsuitable abutment teeth – e.g. in case of severe damage to the periodontium (the tooth-supporting apparatus) and thus loosening or in case of apical osteolysis (inflammation-induced bone dissolution “down the tooth root“).
Large, arched bridge spans – e.g., when all upper anterior teeth are missing.
Insufficient number or unfavorable distribution of abutment teeth – Through abutment augmentation using implants, a fixed bridge can still be planned instead of a removable restoration, if necessary.
Loss of more than three consecutive teeth and not narrowed by tooth migration gap – an exception is the loss of four incisors, provided that the course of the dental arch is not arcuate.

Depending on the distribution and number of abutment teeth, it must be decided whether a fixed bridge is possible or a removable restoration should be planned. If necessary, an abutment augmentation by implant placement (surgical placement of one or more implants) can be performed, thereby enabling a fixed-fixed planning. Relative contraindications

Caries-free teeth limiting the gap – In this case, the provision of the gap with an implant or, especially in adolescents, with an adhesive bridge (adhesive bridge) should be considered as an alternative.
Condition after apicoectomy – 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. Adhesive cementation 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.
Root surfaces of the abutment teeth are less than 50% compared to the root surfaces of the teeth to be replaced – here the supply of a tight-fitting bridge still possible, but it is to be expected with a shorter retention time of the bridge.
Intolerance to components of a metal alloy – switch to compatible alternatives (eg, ceramics).

Before the procedures

Before the procedures, 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.