The Science Behind Warming Composites

You use dental composites in your practice every day, so you are likely familiar with how useful and versatile they are. As substances that are more robust combined than they are separate, the chemistry of these practice workhorses is hard to match.

But do you warm them up before you place them? If science tells us anything about warming dental composite, you probably should.

Research shows the there are many benefits to pre-heating composites before placement and curing. Studies reveal that pre-heating composites improves their handling characteristics, reduces sorption and solubility, enhances marginal adaptability while reducing marginal gaps, and increases microhardness. Research also shows that it reduces the complications that can occur with dental composites, like polymerization shrinkage that can lead to shrinkage stress, in some types or composite without compromising the degree of conversion.

Here we take a closer look at the science behind warming composites and why you could be getting more performance and fewer problems if you turned up the heat on dental composites.

A Quick Review on How Composites Work

The composite process is a chemical one. Dental composites are materials made of 3 parts: dimethacrylate monomers, fillers like glass or ceramics, and lesser additives, such as coupling agent, pigment, and photoinitiators. The resin binds everything together and provides flexibility, and the fillers give it strength. The coupling agent binds the fillers and resins into a solid material where the combined elements retain their individual qualities. Other fillers like the pigment give the composite color, while photoinitiators make polymerization of the resin possible.

Polymerization is a necessary process that sometimes causes problems with composite restorations. In particular, as the resins cure, they shrink. A little shrinking is not a big problem. However, too much polymerization shrinkage can cause shrinkage stress, which can lead to marginal gaps, microleakage, fractures, and postoperative sensitivity, among other problems with your restoration.

However, this versatile material is not always simple to place. It requires more technical skill and precision than its restorative material predecessors to get consistent and predictable results. From isolation to prep to curing to polishing, you should do everything correctly to get the best possible performance. Moreover, as many a clinician will tell you, you should "always follow the manufacturer's instructions."

Heating Composites Before Placing Enhances Their Performance

Warming composites before placing has a lot of benefits to their performance in the patient's mouth. There has been a lot of research that explores why this is.

Per Spear Education Digest, heating composites changes the physical properties of the material, which has benefits. As the composites' kinetic energy increases, the degree of conversion for monomers and free radicals increases when you cure it.¹

Water weakens the physical and mechanical properties of composites, and moisture is abundant in most patients’ mouths. If moisture infiltrates the resin, it negatively affects tensile strength, modulus of elasticity, flexural strength, and wear resistance. A 2016 study in General Dentistry suggests that preheating composite resins lowered the materials’ sorption and solubility of the material when compared to composites at room temperature.¹

Moreover, per Nathaniel Lawson, DMD, PhD, Associate Professor and Director, Division of Biomaterials at the UAB School of Dentistry, heating resin composites also improves the materials’ handling. Dr Lawson says clinicians typically heat composites to 68C (154.4F) to get the desired handling properties. For the General Dentistry Study, researchers preheated the composites to 60C (140F).¹

“Composites are heated in order to increase their flow properties and allow better adaptation to cavity preparation,” Dr Lawson explains. “In this way, a conventional composite can behave almost like a flowable composite when first dispensed and then regain some of its sculpting abilities once it returns to room temperature.”

To gain access to the General Dentistry study, please click here.

Heating composites also helps improve marginal adaptation and the prevention of marginal gaps. Dental Materials published an article in 2010 that reported composites preheated to 68C had enhanced marginal adaptation.² Specifically, the marginal adaptation for heated composites was better in the axial wall.³ Journal of Clinical Exp Dentistry published a study last year that tested whether heating composites reduced the marginal gaps in the material-tooth interface in Class 2 restorations. The researchers concluded that it decreased marginal gaps at both the enamel and dentinal walls in both the materials tested (Filtek™ P60 by 3M™ and X-tra Fil by VOCO America).³

Preheating composites also increases the microhardness of the cured material. Microhardness is often the metric chosen to analyze the physical properties of restorative materials. It is associated with compressive strength, degree of conversion and wear resistance.⁴ A 2014 article from the Journal of the Canadian Dental Association suggests that preheating resin composites not only decreased the viscosity of the material, but it increased the microhardness when tested against cured composite that wasn’t preheated.⁵ The researchers went on to conclude that “the effects of preheating resin composite may allow for easier placement of restorations and greater monomer conversion.”⁶

A literature review published in the Journal of Oral Dental Health in 2018 supports these findings. Furthermore, the researchers found the literature supports the idea that heating dental composites does not affect the flexural strength and reduces polymerization shrinkage. However, they felt that “further studies with larger sample size and analogous experimental conditions are required.”⁶ Within the boundaries of the experiment, Filtek Superme Ultra resin Composite (3M), had the highest mean microhardness.⁷

To read the literature review, please click here.

Also, there do not appear to be any downsides to heating composite. Dr Lawson says there have been studies which have showed that warming composite did not negatively affect the shrinkage stress or degree of cure of the composite tested. Per research out of University of Zurich, pre-heating high-viscosity bulk-fill composite materials and conventional restorative composites before curing decreases the polymerization shrinkage and does not compromise the degree of conversion.⁸

“There are some composites for which the manufacturer has confirmed that relevant properties are not negatively impacted by heating, such as the 3M composites,” Dr Lawson says.

Dr Lawson adds that there are several convenient tools and ways to heat dental composite materials. Dental manufacturers produce devices that heat composite compules or syringes. Some composite dispensing guns have built-in heaters. Another strategy for heating composites and maximizing the time the materials are easier to handle is to use a composite dispenser with a metal tip that can also be warmed, he says.

However, special equipment isn’t necessary to heat composite resin. Research suggests that you can use hot water baths, thermocycling, microwaves, wax-warmers, hair dryers, and even your hands (although the researchers thought the hands were not a great options for consistent results).⁸ Dr Lawson says inexpensive options are to heat composite on a coffee cup warmer or on the back of an operatory light.

“It typically takes several minutes for the composite to get warm, about 5 or 6 minutes,” Dr Lawson says of the makeshift composite heaters. “The composite will cool faster, taking about two minutes to return to room temperature.”