Chapter 14 Materials used in temporization
It is widely acknowledged that good temporization is one of the most important factors in the fabrication of good quality, definitive indirect restorations. While there is some truth in the urban myth that temporary restorations should never be made too good as the patient may not return to have the definitive restorations fitted, it is essential that good-quality temporary cover is provided. Attention to detail during their construction facilitates the subsequent stages of the procedure, so producing a better definitive restoration. Temporary crowns protect the oral tissues and allow the patient to function while maintaining their appearance until the definitive restoration can be fitted. This chapter discusses the various materials used to provide temporary coverage.
The Biological Importance of Good Temporization
• Some 30 000 to 70 000 dentinal tubules are opened per square millimetre during tooth preparation. The passage of oral bacteria into the freshly cut dentine must be minimized as this will cause pulpal inflammation, which manifests as sensitivity if the dentine is exposed to stimuli from the oral environment such as hot, cold and sweet.
• It also allows the area to be cleaned more effectively and thus maintains a state of gingival health. This decreases the incidence of gingival inflammation and bleeding at the time of fitting the definitive restoration, which is particularly important when the restoration is being bonded using a hydrophobic resin-based composite material.
• A good provisional restoration will maintain occlusal and approximal contacts so preventing over-eruption and tilting of the prepared tooth, or adjacent and opposing teeth. This minimizes the need for occlusal or approximal adjustment of the definitive restoration at the fit appointment.
The Benefits of Quality Temporization
Benefits for the dentist
• The temporary restoration should be made prior to impression taking for the definitive restoration. When the occlusion has been verified and the temporary deemed satisfactory, the clinician can measure the width of the occlusal part of the restoration using an Iwannson gauge (Figure 14.1). This distance denotes the interocclusal clearance. If this is deemed to be insufficient, the dentist can adjust the preparation to create the required clearance prior to taking the working impression.
• In more complicated cases temporary restorations may provide the clinician with invaluable information for use in diagnosis and treatment planning, such as the construction of a new occlusal scheme or when an aesthetic case is being planned.
Benefits to the patient
It is also important from the patient’s perspective that their appearance is maintained and they can continue to function effectively and comfortably. In cases involving a number of teeth, the position and shape of the temporaries will also allow the patient to evaluate their appearance. If this is satisfactory, these features can be incorporated into the definitive restorations.
Box 14.1 The purpose of temporization
The Temporization ‘Balance’
A temporary restoration, be it an inlay, onlay, crown or bridge, must be effectively retained for the period of time between fitting and placement of the definitive restoration. Equally it must be removed easily at the fit appointment without damage to the preparation. A balance between good retention for this period and ease of removal must be established. Generally speaking, the most satisfactory combination is that of a well-prepared (mechanically retentive) preparation with a well-constructed temporary restoration grouted by a soft luting cement (see Chapter 12).
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:
• Acrylic or ceramic bonded to metal – non-precious metal alloys (Chapter 21) are normally chosen on grounds of lower cost.
Acrylate: the acrylate ion is the ion of acrylic acid and these are a group of materials that are derived from acrylic acid, that is, they are the salts and esters of acrylic acid. They contain vinyl groups, that is, two carbon atoms double bonded to each other, directly attached to the carbonyl carbon. Acrylates easily form polymers because the double bonds are very reactive. Acrylates and methacrylates (the salts and esters of methacrylic acid) are common monomers in polymer plastics, forming the acrylate polymers
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).
A crown form of the approximate size is selected by the clinician. It is then adjusted using an acrylic bur to refine its size and shape to closely match that of the preparation. Next, it is refined with another material, which is generally an acrylate (methyl, ethyl or butyl acrylate), although the dimethacrylates and light-cured temporary materials have also been advocated for this purpose. Acrylate-based materials are more suitable than the more elastic materials since it may be necessary to trim through the polycarbonate ‘shell’ to accommodate the opposing tooth so satisfying the occlusion. The acrylic inside the crown can withstand occlusal forces provided it is at least approximately 1 mm thick. However there is no chemical union between the acrylic resin and the polycarbonate crown form. The interior of the polycarbonate crown should therefore be roughened prior to refining.
Cellulose acetate crown forms
These crown forms are transparent (Figure 14.3). They are used in essentially the same manner as polycarbonate crowns: they are trimmed to size using crown shears or scissors and then packed with another material matching in shade to the surrounding teeth. Some clinicians use resin-based composite for this purpose. These crown forms have the disadvantages that:
Metal crown forms
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.
Fig. 14.5 (A) The expander used to manipulate the molar aluminium crown form to the desired shape. (B) The crown is then trimmed using crown shears so that its form approximates the margins of the preparation.
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.
|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|
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.
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).
3. Trim this crown form (using an acrylic bur for a polycarbonate crown form and crown shears for a stainless steel crown form) so that the margins approximate those of the preparation. Roughen the internal surfaces of the polycarbonate crowns.
7. At this point fully seat the temporary crown onto the moist preparation and remove the obvious excess using a probe or flat plastic to prevent it setting into the undercuts so that the crown can be removed easily later.
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.
Fig. 14.7 A Directa temporary crown filled with Trim II (Bosworth). (A) The dough stage (note the shiny surface). (B) As the reaction progresses the snap stage is arrived at a minute or so later and the surface losses its glossiness. At this point, the material is now ready to be used.
The higher methacrylates have a lower glass transition temperature than polymethylmethacrylate. This means that they are more susceptible to temperature change, which will produce softening of the set material. The reason that some materials in this family contain both the ethyl and butyl methacrylates is that the glass transition temperature of the butyl variant is 20°C whereas that of poly(ethylmethacrylate) is 66°C. A temporary restoration made entirely of poly(butylmethacrylate) would distort at mouth temperature. The combined product has an intermediate glass transition temperature and will not be affected by normal mouth temperatures. However, intake of hot foodstuffs and liquids will raise the temperature of the crown to a point where some changes in morphology can occur. The material when compared to poly(methylmethacrylate) has various advantages: