Crowns and Bridges

Crowns are fixed extracoronal restorations of individual teeth and bridges are fixed replacements for teeth. Crowns and bridges can be supported by natural teeth or by implants but for the purposes of undergraduate clinical practice, tooth‐supported fixed restorations will only be considered here.

Crowns replace the outer part of a tooth and are primarily used to preserve the integrity of extensively restored or broken down teeth. They can be used to restore root canal treated teeth. However, when they are used to restore vital teeth, up to 20% of these may lose their vitality over the subsequent 5 years. Therefore, crown preparation should be prescribed cautiously, particularly for vital teeth in younger patients where tooth pulp size is at a maximum. The indications for crowns include:

• Protection of weakened tooth structure following root canal treatment.
• Support to weakened tooth structure following extensive coronal restorations, caries or tooth wear.
• Changes to shape and colour of teeth.
• Replacement of existing crowns.

Treatment Planning

Treatment planning is the cornerstone of successful restorative dentistry. The essential steps in treatment planning are:

• Thorough history.
• Careful examination.
• Appropriate special tests.
• Diagnosis.
• Prevention of further disease.
• Preliminary treatment – stabilisation of disease.
• Definitive treatment usually involving crowns and bridges.

For fixed prosthodontics, the standard history and examination should be augmented with specific examination of the partially dentate mouth and of teeth that may be crowned or used as bridge abutments (Table 19.1).

Any treatment plan should include prevention and treatment of existing conditions, including caries, periodontal problems and endodontic issues. The placement or replacement of fillings, posts, cores and other primary or foundation restorations must be successfully completed before commencing construction of crowns and bridges.

Procedure: Crown Preparation

Following treatment planning, radiographic examination, sensibility testing and the preoperative procedures detailed in Table 19.2, a polyvinylsiloxane (PVS) temporary crown matrix is required (Figure 19.1). In some cases, a diagnostic wax‐up and thermoplastic former may be required if it is necessary to change tooth alignment, shape and/or occlusion.

The phases of tooth preparation will depend upon personal choice and training (Table 19.3).

During any tooth preparation, excessive cutting pressure should be avoided as this can result in rapid focal heating of tooth structure, causing pulpal damage which may be irreversible. Tooth cutting should be done with controlled, light, constantly changing contact between tooth and bur, thus allowing maximum cooling effect while minimising frictional heating. Some authors decry the use of depth orientation grooves, citing the risk of overheating the base of grooves during preparation. An alternative to using depth orientation grooves is the use of polyvinylsiloxane (PVS, addition‐cured) silicone putty overcasts of the tooth/teeth to be prepared and the neighbouring teeth. A sausage of freshly mixed heavy‐body PVS putty is pressed onto the teeth to be prepared and the neighbouring teeth, covering the crowns of the teeth and adjacent gingival tissues. When set, this is removed and trimmed. During preparation, the PVS overcast is sectioned with a scalpel and the space between the overcast and prepared teeth examined. The width and contour of this space represents the amount of tooth removed.

Management of Soft Tissues

Ideally all crowns and bridges should have supragingival finish lines with emergence and contour profiles similar to those of natural teeth. Such arrangements minimise damage to the soft tissues during preparation, possibly make the use of a retraction cord unnecessary, reduce the risk of postoperative gingival recession and increase the likelihood of a good clinical outcome, including the aesthetic qualities of the completed crown. However, many teeth to be crowned have existing restorations with subgingival margins that need to be included in the preparation. Wherever possible, subgingival preparations should be confined to critical aesthetic areas and where existing restorations already have subgingival finish lines.

In the aesthetic zone, subgingival preparations should be confined to <1 mm within the gingival crevice.

When undertaking subgingival preparation, it is essential to minimise damage to the soft tissues. It is advisable to use a flat plastic instrument to protect the tissues where there is a high risk of iatrogenic damage. If the soft tissues are traumatised, it is often impossible to achieve haemostasis adequate for impression taking on the same visit. In these circumstances, it may be necessary to temporise and record the definitive impression at a subsequent visit.

Temporary Crowns (Table 19.4)

Temporary crowns serve several important functions:

• Protection of exposed dentine: this prevents sensitivity for the patient and reduces the risk of pulpal death due to exposure of prepared dentine surfaces to the oral environment.
• Preservation of relative positions of prepared and proximal teeth ensures that the definitive crown should fit with firm proximal contacts.
• Prevention of overeruption of opposing teeth – if this happens there may be no space for the definitive crown.
• Restoration of appearance.
• An intraoral assessment of the design of the bridge or crown. Frequent debonding or fractures during the provisional stage will indicate changes are needed to the design of the restoration.

Impression Taking (Table 19.5)

Traditional crown and bridgework requires the recording of typically a PVS impression. With the advent of ‘digital dentistry’, impression taking is increasingly being replaced by digital impressioning using intraoral scanners.

Crown Cementation (Table 19.6)

To be able to deal with difficulties experienced during the cementation of crowns and bridges, clinicians need to have a good working knowledge of all relevant laboratory procedures.

Bridges

Bridges comprise retainers (the part of the bridge attached to abutment tooth/teeth) and pontics (the prosthetic replacement tooth attached to the retainer or retainers).

Retainers can be:

• Conventional (the abutment is prepared for a crown and the retainer is a crown incorporated into the bridge structure).
• Minimal preparation (light preparation within enamel or no preparation) which rely primarily on resin‐based adhesive cements for retention.

Bridge design, whether conventional or minimal preparation, may be:

• Cantilever: one retainer with one pontic attached.
• Fixed–fixed: one or more pontics with retainers at each end and sometimes between pontics.
• Fixed–movable: a pontic with a conventional retainer distally and a ‘male’ component mesially fitting into a slot in the distal of the medial retainer, where the long axes of the abutments is so divergent that parallel preparations would compromise the vitality of the abutment teeth.

Minimal Preparation Bridges

There is a genuine difference of opinion relating to preparations required for minimal preparation bridges. Some clinicians will do preparations as detailed in Table 19.7, others do no preparation whatsoever, and others pick and choose, depending on the circumstances and their personal preferences. The evidence for each approach is not strong and does not favour any specific approach.

Endodontically Treated Teeth

Teeth are weakened by endodontic therapy. Endodontically treated teeth are associated with a marked increase in the failure of bridges, so these teeth should, wherever possible, be avoided in the selection of bridge abutments. Endodontically treated teeth often require crowning because of the loss of tooth tissue associated with the reason for the root treatment (caries, trauma), endodontic access and preparation of the root canals. Root canals can be used to support a coronal restoration. Posts can be either cast or directly placed. Direct posts are made of either metal or fibre. The placement of posts and cores, in particular bonded posts and cores, requires fastidious moisture control and exact compliance with manufacturer’s directions for use.

Crowns and Mandibular Incisors

Mandibular incisors are usually unsuitable for crowns because of their small size and tapering nature cervically. Therefore, every effort should be a made to avoid crowning mandibular incisors, many of which may be successfully restored using bonded composites, veneers or combinations of bonded composites and veneers.

Partial Dentures

Partial dentures are removable prostheses that replace some, but not all the teeth in a dental arch. Partially dentate patients comprise about 21% of the adult population in the UK, and are mostly 55 years of age and older. While the edentulous proportion of the adult population has fallen from 37% in 1968 to less than 10% in the UK, the proportion of partially dentate patients is increasing with increasing longevity and individuals retaining more teeth into old age.

Partial dentures are associated with poor oral hygiene and increased levels of root and coronal caries, and with increased levels of at least gingivitis, particularly in the arch of teeth in which a partial denture is worn.

Treatment Planning

To provide each patient with the most suitable partial denture(s) to address individual needs, while minimising risk to the remaining dentition, a careful considered treatment plan should be formulated before any denture treatment is undertaken.

For partial denture prosthodontics, standard history and examination procedures should be augmented by a history and examination specific to the partially dentate mouth and existing dentures (Tables 19.8–19.10).