14: Materials used in temporization

Chapter 14 Materials used in temporization

The Benefits of Quality Temporization

Types of Temporary Restoration

Most temporary restorations are in clinical use for short periods of time, usually up to 2 weeks. These may be satisfactorily constructed using one of the directly placed temporization materials available:

Where medium- to longer-term temporization is envisaged (some months or more), consideration should be given to an indirect temporary restoration, which is constructed in the dental laboratory. This is particularly true if there is concern that flexion of the material in, for example, a longer span bridge may lead to fracture. Indirect temporary restorations have the following advantages:

Clearly there is a greater cost implication of an indirect over a direct restoration but failure to provide a satisfactory temporary prosthesis may prove to be a false economy. Laboratory temporary restorations may be constructed of:

Direct Temporary Restorations

Preformed crown forms

The first method of direct temporization involves the use of a preformed crown, which is trimmed to the margins of the preparation and refined using another material. The crown forms available are made of:

Crown forms used to construct tooth-coloured temporaries

Where aesthetics is an issue, i.e. in the anterior and premolar regions of the mouth, the clinician has a choice between a polycarbonate or a cellulose acetate crown form to use as a template to construct the temporary crown.

Polycarbonate crown forms

These temporary crown forms are made of a polymer with high impact resistance. This means they have sufficient strength to withstand occlusal forces. As the name implies the polycarbonates contain multiple carbonate groups. These may be linked by a variety of chemical groups, with the commonest one being bis-GMA. They are presented in various sizes in a tray (Figure 14.2).

Metal crown forms

Due to their appearance, metal crown forms are used in the posterior region of the mouth. Two types of metal crown are available: aluminium and stainless steel.

Aluminium crown forms

Aluminium crown forms have been used for many years as the material is easy to manipulate, and it is malleable and ductile. This makes their handling easy for the dentist as they can be bent and trimmed to shape easily. Aluminium crown forms can corrode with time as saliva can react with them. There is also a risk that if they are placed adjacent to a freshly packed amalgam or gold restoration, a galvanic cell may be established (see Chapter 6). Figure 14.4 shows some molar aluminium crown forms.

The crown form of the approximate size is selected. It is often expanded using the expander provided to fit over the prepared tooth (Figure 14.5A) and then cut to the approximate size of the preparation using a pair of crown shears (Figure 14.5B). The ability of the aluminium to be worked and shaped lends itself to this process.

This type of crown form may also be bent to shape before being refined. In exceptional circumstances if the crown form is very close fitting and retentive, it may be possible to only use a temporary cement without refining it. A number of disadvantages are apparent if the crown form is not refined:

Stainless steel crown forms

The other metal crown form available is made of stainless steel (Figure 14.6). Its main indication is in paediatric dentistry for the restoration of badly broken down deciduous molars, particularly if pulpal treatment has been performed. These crown forms are much less malleable and ductile than the aluminium crown forms, thus they are harder wearing and are less likely to deform under load. They are usually not refined but trimmed using crown shears until their fit approximates to the prepared tooth. They are placed over the buccolingual bulbosity and snapped into place. They are then luted using a glass ionomer or polycarboxylate cement. The success rate with stainless steel crowns is very high and these crowns are useful to maintain the space that may be lost when a deciduous tooth is lost prematurely.

Acrylic materials used to refine temporary crown forms

All of the aforementioned crown forms except stainless steel crowns are used in conjunction with another material to enhance the fit between their internal surface and the preparation. The closeness of the fit between the inside of the temporary restoration and a retentive preparation will ensure that the restoration is retained satisfactorily. The original refining materials were chemically cured acrylics formed by mixing methylmethacrylate monomer with polymethylmethacrylate micro-bead powder. Unfortunately there were some problems associated with these materials (Table 14.1). These shortcomings have made these materials almost obsolete and other materials have evolved in an attempt to eliminate their disadvantages. Monofunctional acrylate monomers with a higher molecular weight became available as their properties were an improvement on the original material. This family of materials is called the higher methacrylates and these are the most commonly used materials to refine a temporary crown form.

Table 14.1 Disadvantages of methylmethacrylate/polymethylmethacrylate temporary material and their clinical ramifications

Disadvantage Clinical impact
High polymerization shrinkage Unsatisfactory fit
Poor mechanical strength Breakage during function
Highly exothermic setting reaction Thermal trauma to the pulp
High level of monomer release Significant pulpal irritation
Poor wear resistance Undesirable wear during function leading to perforation or fracture of the temporary leading to occlusal instability
Poor aesthetics Unsightly restoration
Chemical interaction with eugenol Non-eugenol-containing products should be used

Higher Methacrylates

Chemically these materials are a combination of polyethylmethacrylate beads and the liquid polybutylmethacrylate (Table 14.2). The presence of a tertiary amine results in these materials tending to turn yellow after setting. This is particularly marked in sunlight as the solar ultraviolet breaks down the amine, causing a colour change.

Table 14.2 Components of a higher methacrylate material and their role in the reaction in the powder and liquid

  Chemical Role
Powder Polyethylmethacrylate Polymer
  Benzoyl peroxide Initiator
Liquid Polybutylmethacrylate Monomer
  Tertiary amine Activator

Setting reaction

Higher methacrylates set by a polymerization reaction, and, as there is no inert filler, they will exhibit significant polymerization shrinkage on curing, which is in the order of 7% linearly. It is important to try to compensate for this large amount of shrinkage clinically (see Box 14.2, p. 223).

When the powder and liquid are combined, a mass is produced which has the consistency of wet sand. At this point the monomer is starting to soften and dissolve the outer surface of the resin polymer beads. The mass then develops a glossy surface. This dough stage is convenient as it aids clinical handling. As the reaction progresses, the surface of the material loses its gloss and turns to a matt appearance (Figure 14.7). It now corresponds to the snap stage and is now ready for use. For more information on acrylics, see Chapter 23. The polymerization reaction is exothermic in nature and care must be taken when using this material on a vital preparation to prevent thermal injury to the pulp.

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 14: Materials used in temporization
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