What does filler size have to do with composites? More than you might think

Plastic and glass are two vital components of composite resin. Together, they make a versatile and irreplaceable product in dentists’ materials inventory.

Justin Chi, DDS, CDT, director of clinical technologies for Glidewell Laboratories, uses composites for many different restorations in his practice. He describes composite as a highly useful restorative material in dentistry.

“It’s tooth-like. You can fix small situations or more demanding restorations with it. Composites are pretty incredible,” Dr. Chi says.

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However, what you might not remember from dental school is how advances in composite material and, in particular, the size of the glass fillers have improved their mechanical properties and esthetics over the past 50 or so years. Since introducing composites in the 1960s, composite material manufacturers have progressively downsized the particles.

Dr. Chi says the early composites left a lot to be desired as far as appearance went.

“Composites are a mixture of resin and glass filler particles that give it strength. Macrofills were the original composites created with larger glass filler particles. They were strong but not very esthetic,” Dr. Chi says.

As the particle sizes decreased, different benefits emerged from the composite materials - and different drawbacks, Dr. Chi explains.

“Microfills had smaller filler particles. The esthetics might have been better, and you can polish them a lot easier, but the mechanical properties, particularly strength, were lacking,” Dr. Chi says.

After microfills, the materials evolved into hybrid materials, which included various sizes of filler particles. Dr. Chi says they give you the best of both worlds.

“You have the strength of the macrofill and the polishability and esthetics of the microfills with the smaller particles. The polishing compounds and wheels could create a much smoother surface,” Dr. Chi says.

Today’s composites are measured in nanometers at the atomic level, which has led to excellent benefits in patient outcomes and increased versatility of the composite uses. Let’s take a  more in-depth look at the progression of the materials and filler size and the role the improvements play in composite direct restoration outcomes.

Why composites need fillers

Composites are made of a resin matrix, filler content and a coupling agent. Fillers are made of glass or ceramic materials, which are then added to a resin medium. A coupling agent is also present, which provides a chemical bond between the filler and the resin. The combined properties of the composite elements are superior to those of the individual parts.

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Before the addition of fillers, the polymethyl methacrylate (PMMA) resin restorations dentists placed had problems. The resin shrank too much during the polymerization process, causing marginal leakage and lack of wear resistance as well as high levels of thermal expansion and water sorption.

To combat these challenges, material manufacturers added quartz to the resin. The introduction of an inert “filler” material to the resin reduced the polymerization shrinkage,  thermal expansion and water sorption while increasing the strength and other mechanical properties of the restoration.

Over the years, manufacturers have migrated from quartz to engineered glass for their filler materials. These filler materials have increased strength, hardness, and improved chemical and optical properties.

Up next: The progression of filler sizes

The progression of filler size

Spear Education explains that composite materials were designed to replace the properties of a natural, healthy tooth from a biological standpoint as well as look and work like your natural teeth. Another significant benefit of a composite is upon completion of a direct restoration, it appears as if the tooth never had decay at all - a vast improvement on the metallic look of its predecessor, silver amalgam.

The filler size of the composite influences these benefits. Research suggests particle size is a determining factor on surface roughness. The larger particles had rougher surfaces, while the smaller particles had smoother surfaces.

Surface roughness has implications for the restoration’s mechanical properties, mainly wear resistance. Roughness also affects the appearance of the restoration, tendency to discolor, the accumulation of plaque, caries resistance, inflammation of the gums and wear to opposing or adjacent teeth.

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It was clear that improving surface texture to a smoother surface was critical. Over the years, filler size has progressively decreased as a result.

One of the ways composites are categorized is filler size. The different types of composites include:

Macrofills, filler size: Range from 10-100 micrometers

Macrofills have large-sized filler particles, as the name indicates. These were the earliest composite materials and had fillers so large that you could sometimes see the individual particles with the naked eye. The large fillers produced great strength for the restoration, but they finished and polished poorly.  Improvements in the material composition have replaced macrofills, and the only time you might see them is in some dental work you’re replacing for some of your more mature patients.

Microfills, filler size: Range from 0.03 to 0.05 micrometers

Microfills have smaller particles than macrofills, although not the smallest particles.  While the esthetics were better than macrofills, microfills lacked the mechanical properties desired. In other words, they were weak and ill-suited for posterior restorations because of their lack of durability. Unlike macrofills, however, nicrofills are still used occasionally for cases today.

Hybrids, filler size: A mix of particles between 10 and 50 micrometers and sometimes smaller particles, as small as 40 nanometers

Hybrids mix filler size to take advantage of the best of both worlds to create better results for composites. Hybrids combine the excellent finishing properties of a microfill and with the strength and durability of the macrofills. Furthermore, depending on the size of the smaller fillers in the mix, the hybrid category breaks down further into microhybrids and even nanohybrids. Many composites used today fall within this category.

Nanocomposites, filler size: Less than or equal to 100 nanometers

Nanocomposites are the latest composites, with particles small enough to be expressed on the atomic scale. These tiny particles form nanoclusters or groups of smaller particles that produce functions of larger particles. Perhaps most importantly, the filler size arranges the microstructure of the material at an individual atom and molecular level, which leads to huge improvements in the physical properties of the material.

One of the biggest complaints about composites, however, is that stress placed on them in the posterior can compromise even the most robust composites. Whether from the bite stress or polymerization shrinkage, many posterior composite restorations fracture.

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However, nanocomposites are different. Part of the reason is that with the larger particle filler composites, the fillers were ground down to their size from a more substantial material, or a “top-down” process. Nanocomposites are built up from a small level, or a “bottom-up” process.

The small filler size of nanocomposites has numerous benefits to composite restorations, per research in the Annals of Dental Specialty:

• They provide an improved interface between the tooth and the composite, which results in more stability and a more even distribution of stress within the resin than with conventional composites.

• They allow nanocomposites to have higher filler content, which leads to numerous benefits, including superior strength and hardness as well as other mechanical properties.

• They lead to a smoother surface, which has higher translucency, polish retention and better shade matching as well as keeping plaque from accumulating and irritating gums.

• They let the light shine through the composite and maintain the smooth surface longer because the particles are smaller than the wavelength of visible light.

• They blend well with the surrounding teeth and permit a more significant scattering of light, which contributes to a more life-like appearance than restorations made with composites featuring larger-sized particles.

The researchers suggest that while more research might be needed to determine the clinical success of nanocomposites, they should be considered a universal composite suitable for both anterior and posterior restorations.

Dr. Chi uses nanohybrids successfully in his practice. He says he likes their handling properties. However, he cautions doctors to understand the materials they’re using and the condition they need to get the best outcome.

“Since the resin shrinks during the curing phase, the application of the composites and the layering need to be done in increments to avoid compromising the restoration, leading to leakage or post-operative sensitivity,” Dr. Chi says. “Just understand what you are using and what the recommended application of it would be and how to finish them properly to get the esthetic result you want. Applying them correctly is key. “

References

1. Krishna Ravi, Rama & Alla, Asst Prof Rama Krishna & Mohammed, Shammas & Devarhubli, Achut. (2013). Dental Composites - A Versatile Restorative Material: An Overview. Indian Journal of Dental Sciences. 5. 111-115.
2. Ibid.
3. Ibid.
4. Eric Habib, Ruili Wang, Yazi Wang, Meifang Zhu, and X. X. Zhu. “Inorganic Fillers for Dental Resin Composites: Present and Future.”ACS Biomaterials Science & Engineering 2016 2 (1), 1-11
DOI: 10.1021/acsbiomaterials.5b00401
5. Henry A. Rodríguez and Herley Casanova, “Effects of Silica Nanoparticles and Silica-Zirconia Nanoclusters on Tribological Properties of Dental Resin Composites,” Journal of Nanotechnology, vol. 2018, Article ID 7589051, 10 pages, 2018. https://doi.org/10.1155/2018/7589051.
6. Ibid.
7. Lavigne, Courtney. “Dental Composites: Types and Recommendations.” Speareducation.com. Web. 13 December 2018. Web. < http://www.speareducation.com/spear-review/2017/01/dental-composites-in-2017-what-to-look-for-and-what-to-get>.
8. Ibid.
9. Ogle, O E and N Byles. “Nanotechnology in dentistry today”  West Indian medical journal vol. 63,4 (2015): 344-8.
10. Sachdeva, S., Kapoor, P. Tamrakar AK, Noor, R. “Non-Composite Dental Resins: An Overview.” Annals of Dental Specialty. Vol. 3, 2 (2015): 52-55. From web: 3 February 2019. < https://pdfs.semanticscholar.org/714a/35db214c61155cb4e37fa9050e9f3f5c2129.pdf>.
11. Ibid: P. 54
12. Ibid: P. 54