Increasingly, our patients are aware of, and demanding, tooth-colored dental restorative materials instead of traditional silver-mercury amalgam. Anterior composites, critical for the aesthetic smiles that are so much in vogue today, have been the standard of practice for more than a generation; for the less readily visible posterior teeth, it is estimated that more than 65% of all direct restorations are placed with composite materials, as well. (The 50% tipping point for posterior composites in North America occurred sometime around 2000.) The universal use of tooth-colored restorations has motivated patient interest in their own dental health, and their interest in quality aesthetic dentistry.
The dental progress that has made appearance-related dentistry so successful is the result of very active science-based research and development, constantly evolving innovative materials and techniques for improved patient treatment. The parameters of successful clinical practice place additional burdens of technique and dental material awareness upon the practitioner. The dentist must assume the responsibility of keeping up with the latest innovations for the patient’s benefit.
Occasionally, scientifically unsubstantiated techniques become part of the daily routine; with time and continued utilization, they assume general professional acceptance. Such is the case with the practice of storing composite materials in a refrigerator.
As the dental profession has turned increasingly to composite restorations, there has been a demand for improved clinical properties. A significant part of recent dental research has been focused specifically on eliminating practitioner concerns in the areas of material quality, ease-of-use, and finishing. The goal of both dentists and manufacturers is a restorative material that is relatively easy to place (not technique sensitive), convenient to polymerize (rapid and effective), long lasting and aesthetic. The major objectives in composite restoration include:
1. Reduction of required light curing or polymerization time (many practitioners tend to rush this step); 2. Increased depth of cure (not every layer of composite is the suggested 2mm or less);
3. Enhanced conversion (polymerization) ratio.
These past three decades, research has focused on parameters such as various light sources, curing light intensity, curing time, clinical positioning, and the effect of moisture in the restorative field. Evaluating direct composite placement under varying thermal conditions has not been a common direction for research. This is rather surprising, considering that the physical property advantages of heat-curing composites in the manufacture of extra-orally fabricated inlays and onlays have been long been established.
Instructions typically call on dentists to store their composites in a refrigerator until immediately prior to use. This is to ostensibly increase the materials’ shelf-life and clinical properties. According to the latest research, this is probably the worst possible course of action.
In fact, the warming of composites to body temperature or somewhat higher, immediately prior to placement, has been shown to improve composite properties and to reduce curing times significantly. While any practical means may be used to heat the composite syringe or compule to the desired temperature, the Calset Composite Heater (Addent Inc., Danbury CT) (Fig. 1) has been specifically designed to warm the materials to one of two scientifically predetermined levels. In evaluating the bottom hardness of composites that were cured with different light sources, varying only the composite’s Temperature at the Moment of Polymerization (TMP), Bortolotto and Krejci1 found that the insertion temperature had a significant influence on the hardness of a composite. The restorations that were inserted pre-warmed to 40C (only 3C warmer than body temperature and therefore quite comfortable even for unanaesthetized teeth) were significantly harder (Vickers scale) than the composite restorations that were inserted at room temperature (22C). The hardness values at 40C were approximately double those where the composite was inserted at 5C (the approximate temperature of a commercial refrigerator). Another very significant finding was that the curing time for a layer of composite at room temperature could be halved when it was warmed to 40C, without affecting its hardness properties. 1
Clinical significance: pre-warming restorative composite to slightly above body temperature improves the depth of cure AND reduces curing time by 50%.
Stansbury’s study of composite conversion values under various thermal conditions provides even more dramatic results. 2 A higher conversion ratio (double bond formation = polymerization) at a greater depth increases the material modulus, resulting in less flexure, and less potential for restoration fracture under loading. Three esthetic restorative materials (microfill, hybrid, packable) were compared under three different light curing modes (LED, halogen, plasma arc) at two different temperatures (23C and 54.5C). An elevated composite temperature during photopolymerization offered substantially higher immediate and final conversion values in all the tested composite materials, and with all the different curing lights. Elevating the composite temperature from 23C to 54.5C decreased the required curing time by 50-80%.
Clinical significance: pre-warming restorative composite improves the conversion rate, with a concomitant improvement in the fracture resistance, of the material AND reduces curing time by 50% or more.
Rueggeberg indicates that composite TMP significantly impacts polymerization time. 3 Once the restorative is warmed to body temperature, the next 20C does not significantly reduce the curing time, however. At 58C, there is a major upward step in the conversion ratio, remaining constant until 68C, at which point another significant increase is noted. Ideal polymerization temperatures are found at the lower end of each thermal window: body temperature (37), medium heat (54-58C), and higher heat (68C). Warmed composites exhibit no increase in polymerization when halogen curing is adjusted between 20-60 seconds.
Clinical significance: ideal restorative composite pre-warming temperature points are scientifically established AND curing times For Pre-Warmed Composite Can Be Significantly Reduced.
Littlejohn et al measured composite conversion at various TMP levels. a significant improvement in conversion was observed warming composite from room temperature to body temperature. 4
Clinical significance: composite pre-warmed to at least body temperature offers a better restoration with improved physical properties, both in the short and the long term.
Flowable resins help to achieve better marginal adaptation in large posterior restorations. This technique involves a clinical compromise, however. The flowable’s decreased filler content provides the low viscosity; this requires a larger resin component, thereby increasing polymerization shrinkage.
Visco-elastic composite resins exhibit decreased viscosity and greater flowability at higher temperatures. Rueggeburg5 demonstrated that a composite’s film thickness is reduced by 30% as it is heated to 54C. Thus, a pre-warmed micro-hybrid composite, both flowable and highly filled, placed at the gingival margins of a deep restoration eliminates the technical compromise of flowable resins.
Overheating the pulp is always a concern (iatrogenic damage can result). Rueggeberg5 measured the maximum intrapulpal temperature rise from the application of a 57.2C composite material; the observed 1.6C increase well within the established pulpal tolerance of more than 10C. 6
Clinical technique for pre-warming composite
1.
Turn Calset unit on (press control switch once (Fig. 2)). The amber LED indicates normal function.
2. Green LED flashes to indicate composite warming (10 minutes to reach 54C).
3. Green LED shines steadily to indicate pre-set temperature.
4. Heated compule is loaded into the syringe gun (Fig. 3) and applied directly to the tooth preparation.
An old amalgam restoration (Fig. 4) exhibits redecay and marginal breakdown. After rubber dam isolation and amalgam removal, the cavity preparation is photo-disinfected (Fig. 5) (Aseptim Plus, SciCan Ltd., Toronto, Canada) and isolated with a sectional matrix (Fig. 6). The preparation is treated with BeautiBond (Shofu, San Marcos CA) (Fig. 7), a single-bottle, single-step 7th generation adhesive (Fig. 8). A pre-warmed nano-hy- brid composite, Beautifil II (Shofu, San Marcos CA) (Fig. 9), is flowed into the marginal areas and light-cured (Fig. 10).
The difficulty of achieving good marginal adaptation with highly filled, high viscosity materials has been noted. 7 Since the pre-warmed nano-hybrid has the viscosity of a flowable, it conforms very accurately in the marginal areas, and adapts into the nooks and crannies of the preparation. As each composite increment is inserted and polymerized at its ideal TMP, the curing at the elevated temperature provides improved physical and mechanical properties. Subsequent increments are placed until the occlusal layer, which is shaped with the Duckhead instrument (Hu-Friedy, Chicago IL) (Fig. 11) and then light-cured. Finishing and polishing (if necessary) are completed with the single step Jazz Supreme Polishers (SSWhite, Lakewood NJ) (Fig. 12). The restoration is now completed (Fig. 13).
For added convenience, the top portion of the Calset unit is removable from the heater and transportable to a remote location (Fig. 14). The top segment acts as a heat sink that keeps the composite warm for several minutes. Neither extended warming (up to eight hours) nor repeated thermocycling of composites has any deleterious effects on the material’s properties.
Dental technicians have been placing and polymerizing composites under elevated thermal conditions in the fabrication of extra-oral composite restorations for many years.
This technique is now available for direct intraoral composite restorations, as well.
Pre-warming composites is a practical means predictably improving composite properties in dental restorations.
OH
Prof. Dr. I. Krejci 1986: Doctoral thesis “Optimizing marginal adaptation of light-cured MOD composite resin restorations”; 1992: Privatdocent thesis “Tooth-colored restorations”; 1992-1998: Head of the restorative branch at the Department of Preventive Dentistry, Periodontology and Cariology, University of Zurich, Switzerland. Since 1998: Professor and chairman, Division of Cariology and Endodontology, and since 1999: Director, Department TERBO, University of Geneva, Switzerland.
Oral Health welcomes this original article.
REFERENCES
1. Bortolotto and Krejci . The Effect of Temperature on Hardness of a Light-curing Composite. J Dent Res vol 82 (special issue A) abstract #0119, 2003
2. Trujillo and Stansbury Thermal Effects on Composite Photopolymerization Monitored by Real-time NIR. J Dent Res, vol. 82 (special issue A) abstract #0819, 2003
3. Rueggeberg. Personal Correspondence
4. Littlejohn, Greer, Puckett, and Fitchie. Curing Efficiency of a Direct Composite at Different Temperatures. J Dent Res vol 82 (special issue A) abstract #0944, 2003
5. Rueggeberg. Personal Correspondence
6. Zach L, Cohen G. Pulp response to externally applied heat. Oral Surg, Oral Med, Oral Pathol 19:515-530, 1965.
7. Opdam NJ, Roeters JJ, Joosten M, Veeke O. Porosities and voids in Class I restorations placed by six operators using a packable or syringable composite. Dental Materials 18:58-63, 2002.
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More than 65% of all direct posterior restorations are placed with composite materials
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Warming composites to body temperature or somewhat higher, immediately prior to placement, improves composite properties and reduces curing times significantly
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The curing time for a 2mm composite layer at room temperature is halved when it is warmed to 40C without affecting its hardness properties
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Elevating the composite temperature from 23C to 54.5C decreases the required curing time by 50-80%
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Pre-warmed micro-hybrid composite adapts to the gingival margins of a deep restoration eliminating the technical compromise of flowable resins
