The evolution of resin composites for direct restorations

The 1950s were known for the postwar baby boom and the growth of suburbs. It was the decade in which televisions found their permanent home in our living rooms and hula hoops found their way to our hips. It was also the decade when advances in dental technology, technique and attitude facilitated significant advances in dental materials, including the early composite resins.

It was not, however, the decade in which resin composites entered dentistry. That happened in the U.S. in 1947, when the first direct restoration materials made from methyl methacrylate replaced the silicate cement that preceded them.¹ By 1949, clinicians were using these new resin composites.² That same year, a Swiss chemist, Oskar Hagger, developed the bonding system for acrylic resins.³

However, the new direct restoration materials had challenges with shrinkage and, as a result, high rates of marginal leakage, resulting in interfacial staining. It wasn’t until the 1950s that some discoveries helped these new materials overcome their early shortcomings.

The 1950s were an exciting time for dentistry with a lot of discoveries and innovation to the industry. In the mid- to late-‘50s, three events facilitated a new emphasis on resin composite direct restorations. These included:

• The introduction and widespread adoption of the high-speed hand drill: The late ‘40s had produced many advances in handpiece technology, but the introduction of John Borden’s air-driven Airotor handpiece with rotational speeds of up to 300,000 RPM is credited with launching the era of high-speed dentistry.⁴ The new handpieces also allowed for new preparations that could accommodate adhesive restorations.⁵

• The development of a fully reclining dental chair: Before this, chairs didn’t allow a patient to recline enough that an assistant could help with treatment. However, the Dental-EZ^® chair by John Naughton enabled a patient to fully recline during the procedure, and it quickly became the professional standard for the practice of dentistry.⁶

• The discovery of adhesive dentistry: In 1955, Dr. Michael Buonocore discovered that an acid etch on the enamel significantly increased the adhesion of resins to enamel. This discovery paved the way for the use of resins in adhesive dentistry.⁷

In addition to these innovations in equipment and materials, there was also a change in the attitude of dental professionals at this time. There was a new emphasis on biocompatibility and adhesion, which led to further

innovation in materials, including composite resins. Also, with these materials, researchers were more prone to collaborate with doctors than in the past.⁸

Following that in the early ‘60s was Dr. Rafael Bowen’s breakthrough that chemically treating silica particles prepared their surfaces to bond during curing with a substance that contained polymers to create a robust restorative material.⁹ These materials are called “coupling agents.” To summarize, Dr. Bowen coated silica and then bonded it with Bisphenol A-glycidyl methacrylate (bis-GMA), an instrumental discovery in the development of what we refer to as “dental composite” today.¹⁰

Another big step forward for composites in the late 1960s was the introduction of glass ionomer cement. Known for its high fluoride release and low solubility, this cement could bond to untreated dentine and enamel, which lead to minimal cavity preparations and more conservation of the patient’s tooth material.¹¹ Glass ionomers also improved the seals and reduced microleakage.¹²

Despite these advances and developments, it wasn’t until the 1970s that composite resins were finding their way into patients’ restorations on a widespread basis. The use of ultraviolet light curing that perfected the bond between the restorative materials and the tooth enabled better reconstruction in the anterior.¹³ In 1978, visible light-curing began, which eliminated any risks associated with UV light and increased the setting time and improved color stability.¹⁴

Other developments contributed to the increasing use of composite resins too. The ‘70s produced the first micro-filled resins (1977), which provided a more stain-resistant and better polishing restorative material than its predecessors.¹⁵ The hybrid and microfilled resins, when bonded with the acid-etch technique, were durable and esthetic. They were often used for cosmetic work from diastema closings to direct anterior veneers.¹⁶ Beginning in the late ‘60s and moving into the ‘70s, using resin composites for sealants was gaining traction in the field and proving to be effective at caries prevention in children.¹⁷

The 1980s proved to be the decade in which composite was the preferred material for direct restorations in the anterior. However, the microfills at the time didn’t have the mechanical properties most clinicians wanted for posterior restorations. Etch and rinse adhesives were also in widespread use.¹⁸ It was also the decade in which materials science introduced resin-modified glass ionomer cement, which was developed to be a lining cement, but over time proved to be an improvement over glass ionomer cement in Class II restorations.¹⁹

The early ‘90s were when significant improvements to the esthetics and mechanical properties of composite resins occurred. The introduction of microhybrids, which combined the strength of larger-particle macrofills and the polishability of the smaller-particle microfills, addressed concerns that tooth-colored fillings weren't suitable in the posterior. These “universal” composites began to fill the shelves at dental practices.

In the early 1990s, three-step total-etch adhesive systems revolutionized adhesive dentistry.²⁰ The dentin was etched with phosphoric acid, then rinsed. Next, the primer was added before the final layer of resin was laid on top. By the end of the ‘90s, however, two-step self-etch systems had come to market as well.

By the early 2000s, composite resins were challenging amalgam as the preferred materials for direct restorations, although amalgam was still widely used around the world. With the introduction of nanofills and nanohybrids, the esthetics and mechanical properties continued to improve. The particle size (which is often as small as 20 nm) allows the fillers in nanofills and nanohybrids to come together and function as larger particles, which increases the strength and wear resistance of these composites as well as enhances their polishability.²¹ By the mid-2000s, resin composites with a silorane base were introduced that presented less polymerization shrinkage.

Today, advances in composite resins have made them far superior to the composites that were first used in dentistry. With the introduction of nanotechnology and improvements in translucency, composites have improved mechanical properties and esthetic outcomes. Bonding systems have developed as well, making composite restorations last longer than their predecessors and challenging amalgam’s claim on being the longer-lasting restoration.

The dental industry and materials science have made many advances over the last 70 years. With the 2020s beginning next year, one can only wonder, what will they think of next?

References

1. Schulein, DDS, MS, Thomas M. “Significant Events in the History of Operative Dentistry.” Journal of the History of Dentistry (2005) 53:2, pp. 63-72. From Web. www.facuchard.org. 9 April 2019: < https://www.fauchard.org/system/publications/attachments/000/000/027/original/The-History-of-Operative-Dentistry.pdf>.

2. Ibid.

3. “History of Dentistry Timeline.” www.ada.org. Web. 9 April 2019. < https://www.ada.org/en/about-the-ada/ada-history-and-presidents-of-the-ada/ada-history-of-dentistry-timeline>.

4. Ibid.

5. Bayne, MS, PhD, Stephen. C. “Beginnings of the dental composite revolution.” JADA (2013) 144(8); pp. 880-884. Form Web. Jada.ada.org. Web. 9 April 2019. Web. https://jada.ada.org/article/S0002-8177(14)60507-9/fulltext.

6. Schulein, DDS, MS, Thomas M. “Significant Events in the History of Operative Dentistry.” Journal of the History of Dentistry (2005) 53:2, pp. 63-72. From Web. www.facuchard.org. 9 April 2019: < https://www.fauchard.org/system/publications/attachments/000/000/027/original/The-History-of-Operative-Dentistry.pdf>.

7. Ibid.

8/ Wilson, Alan D. “Glass-Ionomer Cement-Origins, Development and Future.” Clinical Materials. (1991) 7: 275-282 Form Web: www.ncbi.nlm.nih.gov. 9 April 2019. <https://www.ncbi.nlm.nih.gov/pubmed/10149142>.

9. Bayne, MS, PhD, Stephen. C. “Beginnings of the dental composite revolution.” JADA (2013) 144(8); pp. 880-884. Form Web. Jada.ada.org. Web. 9 April 2019. Web. https://jada.ada.org/article/S0002-8177(14)60507-9/fulltext.

10. Ibid.

11. Wilson, Alan D. “Glass-Ionomer Cement-Origins, Development and Future.” Clinical Materials. (1991) 7: 275-282 Form Web: www.ncbi.nlm.nih.gov. 9 April 2019. <https://www.ncbi.nlm.nih.gov/pubmed/10149142>.

12. Ibid.

13. Schulein, DDS, MS, Thomas M. “Significant Events in the History of Operative Dentistry.” Journal of the History of Dentistry (2005) 53:2, pp. 63-72. From Web. www.facuchard.org. 9 April 2019: < https://www.fauchard.org/system/publications/attachments/000/000/027/original/The-History-of-Operative-Dentistry.pdf>.

14. Ibid.

15. Ibid.

16. Ibid.

17. Ibid.

18. Sofan, Eshrak et al. “Classification review of dental adhesive systems: from the IV generation to the universal type.” Annali di stomatologia vol. 8,1 1-17. 3 Jul. 2017, doi:10.11138/ads/2017.8.1.001.

19. Sidhu, S. (2011), Glass-ionomer cement restorative materials: a sticky subject?. Australian Dental Journal, 56: 23-30. doi:10.1111/j.1834-7819.2010.01293.x

20. Sofan, Eshrak et al. “Classification review of dental adhesive systems: from the IV generation to the universal type.” Annali di stomatologia vol. 8,1 1-17. 3 Jul. 2017, doi:10.11138/ads/2017.8.1.001.

21. 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>.