Composites Are Being Warmed Up

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

Reduction of required light curing or polymerization time - many practitioners rush this step

Increased depth of cure - not every layer of composite is the suggested 2mm or less

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 (shown in the picture) 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 40°C (only 3°C 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 (22°C). The hardness values at 40°C were approximately double those where the composite was inserted at 5°C -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 40°C, without affecting its hardness properties
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. 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 (23°C and 54.5°C). 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 23°C to 54.5°C 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 Once the restorative is warmed to body temperature, the next 20°C does not significantly reduce the curing time, however. At 58°C, there is a major upward step in the conversion ratio, remaining constant until 68°C, 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 (54-58°C), and higher heat (68°C). 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

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 54°C. 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). Rueggeberg measured the maximum intrapulpal temperature rise from the application of a 57.2°C composite material; the observed 1.6°C increase well within the established pulpal tolerance of more than 10°C

Clinical technique for pre-warming composite

Turn Calset unit on (press control switch once). The amber LED indicates normal function

Green LED flashes to indicate composite warming 10 minutes to reach 54°C

Green LED shines steadily to indicate pre-set temperature

Heated compule is loaded into the syringe gun and applied directly to the tooth preparation

For added convenience, the top portion of the Calset unit is removable from the heater and transportable to a remote location. 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