Crown and Bridge Temporization Part 1: Provisional Materials
Gregori M. Kurtzman, DDS
Provisional restoration materials need to fulfill the specific criteria of mechanical, esthetic, and biologic properties.1 With this in mind, the materials available fall into basic types, based on their basic chemistry, with each category having advantages and disadvantages. There is no ideal material available and selection of the material used is often a balance between the mechanical and esthetic properties, as these two properties often work against each other. Those materials that are the strongest tend to lack esthetics and conversely those that are the most esthetic tend to be weaker mechanically.1
Methacrylates, which have been available for provisional fabrication longer than the other materials addressed in this article, can be further divided in three subgroups; methyl-methacrylates, ethyl-methacrlates, and vinyl-methacrylates. These typically offer good fracture resistance and polishability, but they have higher polymerization shrinkage and typically are less esthetic then the other materials available.1
The other provisional materials available fall into the composite-resin category. These can be further subdivided into Bis-acryls, Bis-GMA, and urethane dimethacrylate resins. Bis-acryl resins offer the advantage of improved esthetics compared to methacrylates, yet they may not be suitable for multiple units when the pontic width exceeds 1 unit, as they are more brittle then the methacrylates.1 A more recent provisional group introduced are the Bis-GMA resins. These offer the advantages of fracture resistance associated with meth-acrylates and the improved esthetics associated with the Bis-acryls, permitting their use in anterior applications where esthetics is critical and longer-span pontic spaces may be required. Finally, urethane dimethacrylate resins continue the trend of enhancing the advantages of their predecessors in strength and esthetics (see Table).1
Selection of a Provisional Material
Methyl Methacrylate
One of the longest-used materials for the fabrication of provisional crown-and-bridge restorations is methyl methacrylate. This category is also referred to as polymethyl methacrylate acrylic (PMMA). The material is available as a powder and liquid formulation and is self-curing. The liquid consists of predominantly methyl methacrylate and minor amounts of other chemicals and the powder is predominantly a polymer and either dibutyl or diethyl phthalate. These materials are available in various tooth colors and also as a clear material which may be used for surgical stents or orthodontic retainers. Methyl methacrylate provisional materials may be added to by addition of more of the material; the author recommends that the best adhesion will occur to a clean surface that has been wetted by the monomer liquid.
The advantages of this type of material are good marginal fit with good transverse strength, providing a durable restoration.2 The material has good polishability, but its abrasion resistance is low and patients who exhibit parafunctional habits may show wear of the material over time.3,4 Additional disadvantages include a high volumetric shrinkage during setting, with a high exothermic reaction; the degree of heat generated by the exothermic reaction is related to the volume of material being cured.5 When this may be an issue either because of pulpal health or patient comfort, fabricating the majority of the provisional restoration extraorally on a model and relining intraorally will reduce the volume of material being cured on the teeth and lower the heat generated during the exothermic setting.6,7 Another consideration: with regard to the pulp, it has been shown that free monomer may be toxic to the pulp and this material may not be the material of choice when pulpal health is questionable or the material will be in close approximation to the pulp.8 Also, these materials may be objectionable to the patient because of odor, and there have been some reported tissue reactions to the monomer.9
Ethyl Methacrylate
Ethyl methacrylates were formulated as an alterative to the methyl methacrylates and to overcome some of their disadvantages. These are also referred to as polyethyl methacrylates acrylics (PEMA). This group of provisional material is also available as a powder and liquid formulation and is self-curing. The liquid consists of predominantly ethyl methacrylate and other chemicals and the powder is predominantly a polymer and benzoyl peroxide. These materials are available in various tooth colors. As with methyl methacrylates, ethyl methacrylate provisional materials may be added to by addition of more of the same material, and the author suggests that the best adhesion will occur to a clean surface that has been wetted by the monomer liquid.
This group of provisional materials overcomes some of the disadvantages of the methyl methacrylates but does not provide some of the advantages of that group. As with the methyl group, ethyl methacrylates have good polishability and offer good stain resistance.3,4 However, they may be better suited than methyl methacrylates when the provisional needs to remain intraorally for longer periods of time.10 Additionally, they have a much lower exothermic setting reaction and will be kinder to the pulp and more comfortable to the patient when larger volumes are being cured intraorally.6 Another benefit of the ethyls over the methyls is they have much lower setting shrinkage.5 Unfortunately, the surface hardness and fracture toughness of these materials and their transverse strength is lower then their methyl cousins and durability in high-stress areas or long pontic segments will be lower.2
Vinylethyl Methacrylate
Another alternative to the methyl metha- crylates are the vinylethyl methacrylates. These are also referred to as PEMA acrylics, as they fall into the same broad group as the ethyl methacrylates. This group of provisional material is also available as a powder and liquid formulation and is self-curing. The liquid consists of predominantly vinylethyl methacrylate and other chemicals and the powder is predominantly a polymer and benzoyl peroxide. These materials are available in various tooth colors. Vinylethyl methacrylate provisional materials, as with the methyl and ethyl versions of methacrylates, may be added to by addition of more of the same material and, in the author’s experience, the best adhesion will occur to a clean surface that has been wetted by the monomer liquid.
Using Powder-liquid Materials
When mixing these types of materials it is important that all of the powder is wet by the liquid, so the user should make sure sufficient liquid is used. The strength of the resulting acrylic is not dependent on the powder/liquid ratio, and increasing the amount of liquid used ensures wetting of all of the powder particles.
These materials set in a 5- to 6-minute period. Set time can be accelerated extraorally by immersion in hot water. Should the clinician wish to slow the setting reaction, this can be accomplished by chilling the monomer liquid before use and/or mixing in a chilled glass dish.
It is advisable to add the powder to the liquid when mixing these materials to decrease any air entrapment in the mixed material. When adding the powder, it should be sprinkled over the liquid and not poured into the mixing dish. Adding the powder while holding the mixing dish on a vibrator will also liberate any air, leading to a denser final mix. The author also recommends that minimal spatulation be performed, as over-spatulation introduces air into the mixture leading to voids and a weaker final set material.
Bis-Acryl Composite Resins
Bis-acryl resins were introduced with an aim to overcome the negatives of the methacrylates. This group of provisional materials was a foray into paste�paste materials. These are dispensed from preloaded syringes or cartridges and mixed through an automix tip. This provides consistent mixtures with no air incorporation into the final mix. Bis-acryl resins have very low exothermic setting reactions, unlike the methacrylates, and are kinder to the underlying pulpal tissue.11 Additionally, they have low shrinkage, providing good marginal fit with good transverse strength and abrasion resistance.12,13 However, these materials are more brittle then the methacrylates and, because of the thick oxygen-inhibited layer present on the surface upon setting, they are less stain-resistant than the material previously discussed.14 Also, because of dissimilar chemistry, flowable composite or dentin adhesives do not readily bond to the Bis-acryl material, making repair or modification difficult.15 Another negative to these materials is the limited shades available. Some manufacturers have introduced color modification materials to match these materials. The Bis-acryls are more costly than the methacrylates, but may be better indicated for anterior temporaries than the methacrylates because of their improved esthetics.
Bis-GMA Composite Resins
Bis-GMA provisional resins are a further extension of an attempt to eliminate the problems associated with both methacrylates and Bis-acryl materials. They provide good marginal fit, as did the Bis-acryls, and even lower shrinkage and exothermic setting.1 Upon setting, a thin oxygen- inhibited layer is present and they show good polishability.1 These materials are less brittle than the Bis-acryls and show good fracture resistance in pontic areas.1 As their chemistry is similar to the traditional composites used for restorations, these can be repaired with flowable composites.1 When larger portions are to be added, coating the surface of the Bis-GMA provisional with a dentin adhesive and light-curing permits hybrid composites to be used for reshaping the provisional restoration.1 As with the Bis-acryl materials, these are more expensive than the methacrylates.
Currently, only one syringeable Bis-GMA material is available for use on the market. TempSpan® (Pentron Clinical Technologies, Wallingford, CT) is available in a multitude of shades with good long-term color stability. As with the Bis-acryl materials, this material is provided in an automix cartridge and is a dual-cure resin. Use of a clear material made from a vacuform sheet or translucent polyvinylsiloxane (PVS) allows the material to be light-cured, accelerating the setting time.
A more recent introduction in this category is the Protemp™ crown (3M ESPE, St. Paul, MN). This preformed Bis- GMA provisional crown is available for provisionalization of single-unit posterior and cuspid teeth. It comes pre-shaped as an adaptable putty that is trimmed marginally before adapting to the preparation. After adaption to the preparation, it is light-cured, then removed and finished. As with TempSpan, should changes be needed to the margins or contour, a flowable composite may be added to the Protemp Crown and light-cured. Currently, this is available only in a single shade.
Urethane Dimethacrylate Resins
The last category of provisionalization material is fabricated from the resin urethane dimethacrylate. This group has two materials that are similar in their chemistry but very different in how they are handled. These consist of Revotec LC (GC America, Alsip, IL) and Radica® from DENTSPLY Ceramco (York, PA).
Revotec is available as a moldable putty that is packaged in a sausage-like shape. A piece is cut off, adapted to the preparation, and light-cured. This offers the ad-vantage of being able to use this material for provisionalization of multiple adjacent units and bridges. This material offers good marginal fit and polishability, with very low shrinkage and no exothermic setting reaction. No oxygen-inhibited layer is present upon setting and it can be repaired with flowable and hybrid composites. It offers good transverse strength and is relatively abrasion resistant.16 The negatives are this material is only available in a single shade and it is not as esthetic as the Bis-acryl and Bis-GMA materials.
Radica is the newest entry into the provisional material market. It is unique in how it is handled compared to the other provisional materials already discussed. This material is provided as a thermoplastic material in syringes that is light-cured to complete setting. The drawback to this material is it requires a special unit to condition the syringes and cure the material. It can be used in the dental office extraorally if the units are available, but is designed to be used in the laboratory. In the author’s experience, this material provides the highest transverse strength and abrasion resistance of the provisional materials and is well-suited for long-term provisionalization, such as with immediate-load implant prosthesis. No oxygen-inhibited layer is present after curing and it offers very good polishability. Natural esthetics are achievable through layering of the material and there is a wide shade selection available.
The technique involves a model of the pretreatment arch, which is modified to shape the teeth to be treated to ideal contours and denture teeth are placed in the edentulous areas. A PVS stent is fabricated over the model. The teeth to be treated are lightly prepared on the model and a syringe of the appropriate shade is warmed in the conditioning unit. A syringe of enamel shade is expressed into the incisal and occlusal aspects inside the PVS stent. This is followed by the dentin shade and then it is placed onto the prepared model and allowed to harden as it cools to room temperature. The stent is removed and is then carvable like wax. Any flash is removed with wax carvers and additional material may be added. When the desired morphology is achieved, the material is cured in either an Eclipse® unit or an Enterra™ unit (both DENTSPLY Prosthetics, York, PA). The restoration is then relined chairside with any of the methacrylates, Bis-acryl, or Bis-GMA provisional materials previously discussed. Radica can also be added to or repaired with flowable or hybrid composites.
Conclusion
Provisionalization is an integral part of fixed prosthetic treatment. Success with this phase of treatment needs to take into consideration the biologic, mechanical, and esthetic requirements of provisionalization. Multiple types of provisional materials are available and, depending on the provisionalization requirements for that particular patient, some materials are better suited then others. Those materials used chairside sometimes trade strength for lower esthetic values, or those that are highly esthetic are structurally weaker. Selection should be based on how long the provisional will need to be in function, how esthetic it needs to be, and the parafunctional habits of the patient.
References
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About the Author
Gregori M. Kurtzman, DDS
Private Practice
Silver Spring, Maryland
