Chapter 16 Levison’s Textbook for Dental Nurses
486
Although some temporary crowns and bridges can be constructed at the chair side, using either stock crown-forms or preoperative impressions to construct them, all other fixed prostheses are sent to a laboratory for construction by the technician.
487
•
An alginate impression of the opposing arch is taken, using the appropriate impression tray.•
An occlusal registration is often taken, especially in complicated cases, using softened wax which the patient bites into, a specific occlusal recording material such as Blu-Mousse or a face bow technique for articulation of the models at the laboratory.•
The tooth is prepared by reducing its overall dimensions by 1 mm for metallic or ceramic crowns or 1.5 mm for bonded crowns, using diamond burs which produce near-parallel sides to provide optimum retention, but without producing undercuts (Figure 16.18).•
The prepared tooth shape to be achieved is illustrated in Figure 16.19.•
To ensure accurate recording of the crown preparation margins, gingival retraction cord can be pushed into the gingival crevice and removed immediately before the impression is inserted.This is cord soaked in either adrenaline or alum, both of which cause the gingivae to retract and pull away from the tooth, thus allowing impression material to flow into the crevice created and accurately record the prepared tooth margins.
•
An elastomer impression is then taken of the working arch, using a silicone or polyether material.Figure 16.19 Crown preparations.
Figure 16.18 Tapered diamond crown preparation burs.
Chapter 16 Levison’s Textbook for Dental Nurses
488
•
When satisfactory impressions have been produced, a temporary crown is made at the chair side and cemented temporarily to the prepared tooth (see later).•
A shade of the tooth is taken by comparing the adjacent teeth to a suitable shade guide (Figure 16.20), and ensuring that any surface characteristics such as root darkening or hypomineralised spots are mimicked too – this stage may be carried out at any point in the procedure.•
All relevant details are accurately recorded on the laboratory slip, which is sent to the laboratory with the disinfected impressions and occlusal registration for construction of the permanent crown.•
A correct return date should be given, to coincide with the patient’s next appointment for fitting of the crown.Laboratories vary in the time required for the crown to be custom made, and the period may range from a few days to 2 weeks. Accurate and detailed information provided on the laboratory slip will ensure that unnecessary delays are avoided, and a professional and trusting relationship between the practice and the laboratory technician often allows for a speedier completion on occasion.
The surgery procedure for the fitting of the crown is summarised below.
•
Provide suitable personal protective equipment for the patient and all staff.•
Local anaesthesia is administered, unless the tooth is non-vital.•
At this point, some dentists may choose to apply a rubber dam to the prepared tooth, so that it is isolated from possible oral contamination.•
Removal of the temporary prosthesis, using specific crown removal instruments or a bur in the high-speed turbine to cut the temporary prosthesis off the tooth.•
Try-in of the permanent prosthesis onto the tooth (or teeth).•
The marginal fit of the crown will be checked for accuracy, along with the occlusion and the shade of the prosthesis.•
Occlusion will be checked using articulating paper – high spots will leave a coloured mark to indicate the point that needs reducing.•
Reduction is carried out using burs in the high-speed handpiece, and polishing burs or stones to smooth the area afterwards.Figure 16.20 Shade guide.
489
•
When the dentist and patient are happy with the fit, the prosthesis can be cemented into place using one of a variety of luting cements – these materials are summarised below, and discussed in detail in Chapter 15.•
If the fit is poor or the occlusion is completely incorrect, the dentist will take new impressions and bite registration and request a remake of the prosthesis.Instruments and materials required
The majority of dentists have a normal ‘conservation tray’ set up as the basic instruments required for crown preparation and fitting, and some may work under a rubber dam too. The additional equipment and materials specifically required are shown in Table 16.3.
The fixed prosthesis is permanently cemented to the prepared tooth using a luting cement.
These are adhesive to the dentine of the tooth, and are mixed to a creamy consistency so that the prosthesis can be seated fully onto the tooth before the cement sets. Types available are discussed fully in Chapter 15 and summarised in Table 16.4.
Modern types of cement tend to be provided in double syringe form with no mixing necessary, but older types (such as phosphate, polycarboxylate and glass ionomer cements) require correct proportioning and thorough mixing before use.
All can be mixed on a cool glass slab with a small spatula, by incorporating increments of powder into the relevant liquid and spatulating thoroughly until a smooth, creamy mix is produced.
Table 16.3 Crowns: additional equipment and materials
Item Function
Diamond burs
(see Figure 16.18) Tapered so that no undercuts are produced on the prepared tooth or teeth, otherwise the fixed prosthesis will not seat fully onto the tooth
Retraction cord Cord soaked in an astringent solution (adrenaline or alum) that is then packed into the gingival crevice to cause shrinkage of the gingiva away from the prepared tooth. This provides a definitive tooth margin which is reproduced in the impression and also the cast model
Impression trays (see Figures 16.10 and 16.12)
Variety of plastic or metal boxed trays, sized to fit fully over the dental arch – upper and lower styles
Also triple tray system Crown former
(see Figure 16.13) Preformed plastic or polycarbonate tooth-shaped formers, in a variety of sizes and available for each tooth shape
Beebee crown shears
(see Figure 16.21) Short beaked shears for cutting and shaping the margins of temporary crowns
Shade guide
(see Figure 16.20) Shaded teeth in holder, to determine the required shade of the prosthesis by comparing each example to the adjacent teeth and determining the best match available
Chapter 16 Levison’s Textbook for Dental Nurses
490
Temporary crowns are provided for aesthetic reasons, to prevent overeruption of the prepared tooth, and to avoid sensitivity problems in the prepared tooth while the permanent crown is being constructed. They can be hand made at the chair side on the day of crown preparation, or prefabricated types can be adjusted to fit the individual tooth.
Those hand made on the day are created as follows.
•
An alginate impression of the tooth is taken before crown preparation begins.•
A cold cure acrylic material is then mixed and placed in the impression after crown preparation, and reinserted into the mouth over the prepared tooth.•
This takes just minutes to set, and produces a temporary crown of exactly the shape of the original tooth.•
Shades are rather restricted, so colour matching is as accurate as can be expected.Temporary crowns can also be provided by mass production in various sizes, for each tooth shape.
These can be cut and trimmed at the chair side to fit any prepared tooth, using either acrylic trimming burs or ‘Beebee’ crown shears to ensure an accurate marginal fit (Figure 16.21). They are then temporarily cemented to the tooth, using a zinc oxide and eugenol temporary cement such as Temp Bond, while awaiting the permanent crown construction.
The types of permanent crown available can be summarised as follows.
•
Porcelain jacket crown (PJC) – an early type of all-porcelain crown used for anterior teeth only, to provide good aesthetics when the only other alternatives were metal crowns.•
Ceramic crown – the modern successor to PJCs, constructed of stronger ceramic materials than porcelain alone (such as zirconia), and therefore able to be used both anteriorly and posteriorly to give a more ‘tooth-like’ appearance than other crowns.Table 16.4 Types of luting cement
Type Action Mixing
Zinc phosphate Mechanically adhesive to rough inner
surface of prosthesis, and surface of tooth Glass slab and spatula Zinc polycarboxylate Chemically adhesive to tooth and inner
surface of prosthesis Glass slab and spatula
Glass ionomer Chemically adhesive to tooth and inner
surface of prosthesis Waxed pad and spatula
Polyester resin Chemically adhesive, and inert in saliva Waxed pad and spatula Self-cure resin Chemical bonding between tooth and
prosthesis Double syringe mix
Light-cure resin Light-cure bonding between tooth and
prosthesis Double syringe mix
Dual-cure resin Combination of self-cure and light-cure
bonding between tooth and prosthesis Double syringe mix
491
•
Porcelain bonded crown (PBC) – these consist of a substructure of metal for strength with a buccal or labial face of porcelain for better aesthetics than an all-metal crown (Figure 16.22);these crowns are currently popular although the porcelain can be cracked off the underlying metal in patients with a heavy bite.
•
Full gold crown (FGC) – these can be made of yellow gold (see Figure 16.14) or a mixture of pre- cious or non-precious metals to give a silvery appearance, and are the strongest of all crowns available, making them ideal for posterior teeth, especially in patients with a heavy bite.•
These can be made as full coverage crowns or three-quarter crowns which leave the buccal or labial surface of the tooth intact but cover the rest of the tooth – this gives better aesthetics while still providing adequate coverage of the tooth cusps, so providing strength to the device.•
Three-quarter crowns have tended to be superseded by bonded crowns, which provide both good aesthetics and strength in the same situations.Post crowns
As discussed in Chapter 15, when teeth die and are preserved by root filling and restoration, the remaining tooth structure often becomes brittle with time and fractures. Sometimes the fracture is so extensive that there is not enough tooth structure left to restore it without the use of additional support. This support is often achieved by the placement of a metallic post and core Figure 16.21 Beebee crown shears.
Figure 16.22 Porcelain bonded crowns.
Chapter 16 Levison’s Textbook for Dental Nurses
492
structure which is then shaped to hold a conventional crown – these restorations are called post crowns (Figure 16.23).
The metallic post and core system can be constructed from preformed posts, such as Paraposts or Dentatus posts with a core constructed at the chair side, or the prepared root forms part of the crown preparation impression, and the post and core are hand made by the technician, along with the crown.
The chairside procedure differs only in the preparation of the post hole in the root, and the chairside post and core placement or impression technique, as follows.
•
The root face margins of the fractured tooth are shaped as for a conventional crown preparation.•
The root filling material in the root canal is carefully removed to a suitable depth using Gates Glidden drills (see Figure 15.45).•
The post needs to be as long as possible to provide adequate support for the new crown, but drilling should not be so deep that there is a risk of root fracture.•
The canal is then prepared widthways, using drills specific to the type of post to be placed, so that a parallel-sided hole is produced – this will give the maximum retention for the post, once cemented.•
A prefabricated post is then either screwed into the canal (Dentatus system) or cemented into the canal (Parapost system, Composipost system) using one of the usual luting cements.Examples of the post systems are shown in Figure 16.24.
•
Alternatively, a wax post is placed in the hole and forms part of the impression to be sent to the technician for post crown construction.•
Using this technique, the post hole must then be retained as an unblocked channel while the post crown is under construction, often by the insertion of a temporary post.•
If a prefabricated post has been placed at the chair side, its top end is then used as the retainer for the core to be suitably shaped to hold the eventual crown itself.•
Suitable materials for core construction are hardened glass ionomer cements such as Vitremer.Porcelain crown
Metal core and post extending into root filled root canal
Figure 16.23 Post crown.
•
Once the impression has been taken, the core then holds the temporary crown in place while493
the final crown is under construction.
•
Otherwise the technician will construct the post and core as a single structure, and then the crown as a separate structure to be cemented onto it at the fitting appointment.Temporary crowns
Temporary crowns are placed for a limited time only while the permanent crown is being constructed, and are used for the following reasons.
•
To maintain the appearance.•
To prevent sensitivity of the prepared teeth between the preparation and fitting visits.•
To maintain the correct space between adjacent teeth so that the permanent crown fits – sometimes the adjacent teeth tend to tip into the space once the crown preparation has been carried out, as the contact points between the teeth are removed during the procedure.•
To maintain the correct occlusion between opposing teeth – the opposing tooth to the prepared tooth will have no occlusal contact after the crown preparation procedure, and may therefore tend to overerupt.Temporary crowns are made by fitting a crown form over the prepared tooth. For anterior teeth a clear plastic crown form such as an Odus pella (see Figure 15.29) may be used. It is trimmed with crown scissors (see Figure 16.21) and filled with a material which matches the teeth, such as composite. Alternatively, tough tooth-coloured polycarbonate crown forms are used, such as Directa (see Figure 16.13) and these only need trimming with slow burs.
Metal crown forms made of aluminium, nickel-chromium or stainless steel are used on posterior teeth (Figure 16.25).
Trimmed temporary crowns are cemented with a material which is adhesive but easily and cleanly removed for fitting the permanent crown, for example Temp Bond or ProTemp.
Stainless steel crown forms, cemented with glass ionomer cement, are also used as the best restoration for large cavities in deciduous molars instead of a conventional filling.
Figure 16.24 Post systems.
Chapter 16 Levison’s Textbook for Dental Nurses
494 Bridges
A bridge is a laboratory-constructed artificial device which is composed of two or more units, one of which will replace a missing tooth. Essentially, they are composed of one or more units which are each exactly the same as a single crown, but as a bridge they are all joined together to make one structure. Within that structure will be one or more units that lie over the dental ridge where a tooth is missing, while the other units sit over the prepared teeth that will hold the bridge in place. The unit replacing the missing tooth is called a pontic, the units holding the bridge in place are called retainers, and the teeth that they are cemented onto are called abutments. A conventional bridge is illustrated in Figure 16.26.
Bridges have several advantages over removable prostheses (dentures), which may also be used to replace missing teeth.
•
There is no embarrassment of a loose prosthesis falling out, as bridges are fixed to the teeth permanently.•
On the whole, their aesthetics are superior to dentures.•
They are more hygienic than dentures, because there is no involvement of any teeth except the retainers and therefore fewer stagnation areas.•
Usually only two appointments are required for their provision, while denture construction may require up to five visits.•
The materials used in their construction are better able to resist occlusal forces than the acrylic used to construct many dentures.•
The shades available can be customised in any way by the laboratory technician to mimic the patient’s other teeth, whereas those available for dentures are mass produced and unalterable.•
They solve the problem of patients with a strong gag reflex who require tooth replacement, and who usually cannot cope with a denture.•
They are also better tolerated because of the minimal amount of soft tissue coverage involved.Figure 16.25 Metal temporary crowns.
495
However, good oral hygiene control postoperatively is of paramount importance with bridges, as they produce stagnation areas unlike any others in the mouth (that is, under the pontics), and therefore require special techniques for effective cleaning to be carried out. Due to the complexity of their design and construction, as well as the cost of the materials used in their manufacture, bridges also tend to be far more expensive than dentures.
Several different types of bridges have been developed, but all designs rely on retaining teeth (abutments) to hold the bridge permanently in place, and they are joined to the missing teeth (pontics) in one structure as follows.
•
Fixed-fixed bridge where retaining teeth are involved to either side of the missing teeth, as one solid design (Figure 16.27).•
Fixed-moveable bridge where a joint is incorporated in the design to allow some degree of flexibility to the bridge (Figure 16.28).•
Cantilever bridge where the retaining tooth or teeth are to one side of the pontic only.○ Simple cantilever design where retaining teeth are those immediately to one side of the pontic only (Figure 16.29).
○ Spring cantilever design where the retaining teeth are to one side but several teeth away from the pontic (Figure 16.30).
•
Adhesive bridge where the retaining teeth undergo minimal tooth preparation and retention is provided by lingual or palatal metal wings only (Figure 16.31).The choice of which type of bridge is used depends on several factors.
•
Whether an anterior or a posterior tooth is being replaced, as the latter usually experience heavier occlusal forces, so full crown retainers are generally required.•
Like crowns, bridges can be constructed of all-metal or ceramic materials and obviously the former would not be provided anteriorly.•
Fixed-fixed bridges tend not to be used so frequently nowadays, as their inflexibility during use can cause damage to retaining teeth – their solid structure, especially with long bridge spans, allowed occlusal forces on one end of the bridge to gradually loosen the other end from the abutment tooth. While undetected, this would allow caries to seep under the retainer and eventually destroy the abutment tooth.Retainer
Abutment
Pontic
Retainer
Abutment
Figure 16.26 Bridge components.
Chapter 16 Levison’s Textbook for Dental Nurses
496
•
Wherever possible, adhesive bridges are used, as they involve minimal tooth preparation.•
If a patient has natural spaces between the teeth, only a spring cantilever design can be used so as to maintain the spaces and give good aesthetics.•
The health of the abutment teeth is of paramount importance to the success of the bridge, and if there is any cause for concern, an adhesive type of bridge is advisable so that any problems would result in its dislodgement rather than causing damage to the abutments.All types of bridge except adhesive ones rely on the retaining teeth being of full crown coverage.
Indeed, the tooth preparation is exactly the same as for a single crown, as are the instruments and impression materials used.
Dovetail joint between two parts of bridge, to allow slight flexibility Retainer
Pontic Retainer
Figure 16.28 Fixed-moveable bridge.
Pontics
Retainer teeth
Figure 16.27 Fixed-fixed bridge.