Runny or not?

When designing resin composite materials, manufacturers have to balance, among other things, the degree of filler loading, filler particle size and resin viscosity, given that these are factors in the viscosity/runniness of the material, although viscosity-modifying resins can also be added. Given that a more and more frequently asked question (FAQ) at my presentations on posterior composite is – ‘is it sinful to heat composite before placement of a restoration, and/or, should I be buying a composite heater’, it is my feeling that the practice of heating composite is increasing. Here I must make full disclosure and acquaint readers with my personal view on viscosity of resin composites – I like them to maintain their shape, ie be sculptable, and by this I mean that I like them to stay exactly where they are placed after shaping. Why? For posterior composite restorations, if at all possible, I look at the shape of the (sound) tooth or the previous restoration and, if that has been functioning well from an occlusion viewpoint, I then try to remember its shape and replicate that in my final restoration. This is important, because if the restoration initially is overcontoured/too high/too bulbous, then it will take (patient's and clinician's) time shaping the restoration to a correct shape with a turbine and/or discs. Not good for either the patient or the clinician. For treatment of tooth wear with additive composite at an increased occlusal vertical dimension, use of a sculptable composite, which stays where it is placed, facilitates freehand placement of the restorations, without the need for a putty matrix. Martin Kelleher is obviously also in this camp, given that a technique tip that he provided in the second of his two excellent McNamara's Fallacies papers on the restoration of worn teeth¹ was to keep the resin composite material in the fridge until the clinician is ready to use it, in order to make it more sculptable!

The follow up question to the FAQ on heating composites often is – does heating influence the properties of the material? The literature is replete with in vitro experiments that demonstrate that increasing temperature of the resin composite does lead to a decrease in the viscosity of the material, and, in the work of Loumprinis and co-workers,² the decrease varied from 30% to 82% in different materials (and between 40% and 90% in a different study³), although there was no difference in viscosity for the flowable composites that they heated. I assume that clinicians who heat their composites do that to achieve runniness, but, for sure, they cannot be pleased by a related finding in this paper, namely, that the stickiness of the materials generally increased after heating. This is surely one of the biggest bugbears for clinicians who may then end up purchasing non-stick coated instruments in order to overcome the problem, or dip the placement instrument in resin to stop it sticking – not a good idea – because by doing so, the surface of the composite is diluted with the resin and this may adversely affect its physical properties. My view is that, if the composite material sticks to your instruments, one should change one's composite, there being plenty of resin composite materials around that do not stick to instruments. But, on the other hand, the clinician who heats the composite to make it less viscous will have to accept the stickiness as a necessary evil! In that regard, for composites at room temperature, there is a delicate balance, given that, while the resin composite material should not stick to the packing instrument, conversely, the material should be sticky enough to adhere to the bonding agent in the prepared cavity. On the subject of stickiness, Ertl and co-workers⁴ determined that materials with reduced filler loading, which correspondingly results in a greater fraction of resin matrix, showed higher stickiness.

Another problem in clinical dentistry when using dental composites (but possibly any material?) are porosities and voids in restorations. Opdam et al⁵ considered that the risk of voids and porosities increased when the material sticks to filling instruments because air will be entrapped. Indeed, these authors stated that ‘restoring minimal preparations in the absence of porosities and voids was very difficult to achieve’. They also found that placing a layer of flowable composite that was left uncured, directly followed by injecting a medium-viscous composite, was the technique that resulted in the most homogeneous restoration, something that they subsequently termed that the ‘snowplough technique’. Furthermore, Tyas et al⁶ pointed out that a marginal opening appears when the material sticks to the condenser, ie stickiness of dental materials is not a good idea.

A further advantage of pre-heating composites became apparent in the results of in vitro work by Wagner and colleagues.⁷ In extracted third molars prepared for 10 Class II cavities (with the cervical margins on cementum) per group and resin composite restorations placed using four different materials/techniques, their results indicated that pre-heating the composite resulted in significantly less microleakage at the cervical margin, although delaying light curing by 15 seconds was considered ‘counterproductive’ as this led to increased microleakage. To me, that seems to indicate that the composite heating scenario, therefore, is not as simple as it might have seemed – the clinician needs to work fast to avoid this problem!

For a complete appraisal of the runniness/stickiness issue, I consulted the paper by da Silva et al.⁸ In their appraisal, they reviewed four papers on the heating composite subject. They concluded that reducing the viscosity of the composite by warming it, allows the material to be injected into the preparation, rather than having to use hand instruments. These workers⁷ also considered that the warm composite technique allowed handling characteristics similar to those of flowable composite, while achieving the benefits of the superior mechanical properties associated with the use of heavily filled restorative composite. The reduced viscosity also facilitated improved adaptation to cavity walls and decreased gap formation, compared with room temperature. In addition, the physical properties of the resin composite were found to not be affected by pre-heating. Indeed, one study found that mechanical properties following cure were improved.

Another expressed worry has been that the higher temperature of the warmed composite might adversely affect pulpal health. However, this may not be perceived as a problem, given that composite cools quickly on placement, and the tooth acts as a heat sink, resulting in composite temperatures equivalent to body temperature.

In summary, it's a matter of personal preference! It is necessary to purchase additional equipment, but the cost is fairly low. For clinicians like myself, who prefer a stiffer, more sculptable composite, the learning curve to a new placement technique, added to the stickiness of the warmed composite, makes me feel that going to the runny side and changing my technique would make the material less easy to use in my hands. And, as ease of use has been considered to lead to better clinical results,⁸ the price, for me, seems too high. On the other hand, there appear to be clinical benefits, and da Costa et al considered the learning curve to be ‘shallow and the propensity for adverse events low’.

This brief review has indicated that there are benefits and downsides to heating composite before use. So, why the apparent trend towards runniness? I am obviously missing something! Hopefully, readers in the runny camp will write and tell me!