JADA LANDMARK SERIES Spotlighting articles from past ADA Journals that have achieved landmark status thanks to their lasting impact on dental care and the dental profession

Originally published January 1963, The Journal of the American Dental Association, Vol. 66, No. 1, 57-64

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nomenclature instead. In this landmark article Beginnings of the by Bowen, the term “composite” does not even ap- pear. Dental materials science was just beginning to deal with the extreme challenges of chemically connecting internal interfaces of things to make ceramic-polymer composites. In materials science, revolution the term “composite” means a physical mixture of Stephen C. Bayne, MS, PhD any phases (metal-metal, metal-ceramic, ceramic- ceramic, ceramic-polymer, polymer-metal, he true age of dental composites was polymer-polymer). Bulk properties of any compos- launched with this initial science into ite depend on volume fraction and properties of coupling agents. In the late 1950s and each phase and the characteristics of the inter- early 1960s, the word “composite” was faces connecting those phases. Without strong Tstill new to dentistry. Its predecessor, the adjec- internal interfaces, composites behave poorly. tive “reinforced,” dominated the dental materials That was the scientific backdrop for this early

880 JADA 144(8) http://jada.ada.org August 2013 JADA LANDMARK SERIES experiment creating what every- the composite. Dental composites one understands today as “dental start with a fluid monomer that composite.” This 1963 publica- forms a continuous phase and tion by Dr. Rafael Bowen was a suspends reinforcing particles of “proof of concept” that documented silica that have been coated with a chemical treatment of silica par- silane coupling agent. Bis-GMA is ticles so their surfaces could be viscous and requires dilution with intimately bonded into a mixture other monomers to create a usable with polymer during curing and mixture. Early composites were generate a strong restorative mate- self-cured and prepared as two rial. The magical coating material components to be mixed just before was tris(2-methoxyethoxy) vinyl use. Often this mixture included silane. Ray Bowen borrowed this inadvertent air incorporation that from those making glass-reinforced Dr. Rafael Bowen left pores as mechanical defects polyester laminates.1 Materials that were extremely deleterious to that chemically bridge the interfaces of phases strength. Correctly choosing most of the com- are called “coupling agents.” There are very ponents for this early experiment was quite few types of these materials. Each depends on remarkable. Since then, 50 years of continual the chemistry of the ends of the coupling agent refinement of the formula has taken place molecule being matched well to the phases on (1963-2013). This quite extraordinary revolu- either side. Even today, coupling is a problem for Table 1 many systems. Apply- Generic components in Bowen’s* original and ing the coupling agent effectively is fickle. It modern (circa 2010) resin-based composites. works best in dilution, COMPOSITE COMPOSITION of REINFORCED RESIN-BASED COMPOSITE is often pH dependent, COMPONENTS 1963 Original (Bowen*) 2010 Typical must avoid many side Coupling Agent Vinyl silane Vinyl silane reactions and needs to Matrix: Acrylic 80 percent bisphenol A-glycidyl 70 percent bis-GMA, Comonomers methacrylate (bis-GMA), 10 30 percent TEGDMA form thin films. As dem- percent methyl methacrylate, onstrated in his article, 10 percent triethylene glycol Dr. Bowen effectively dimethacrylate (TEGDMA) coated silica, bonded it Inhibitor Hydroquinone Butylated hydroxytoluene into bisphenol A-glycidyl 55 volume percent quartz, < 150 55 volume percent glass, micrometers < 0.5 m methacrylate (bis-GMA) m Initiator Camphoroquinone and and produced a material others with very encouraging Accelerator N,N-Dimethyl-amino-p-toluidene Visible light early properties, as dem- External Interfacial None Etching-priming-bonding onstrated in his plethora Bonding system of labor­atory tests. Con- * Source: Original 1963 JADA article by Bowen. trary to the journal ar- ticles of today that tend to focus on only one or two property tests, Bowen tested setting times, shrinkage, solubility and disintegration, water sorption, coefficient of thermal expansion, color stability, visual opacity, compressive strength, tensile strength, modulus of elasticity, resis- tance to indentation and toxicity. Composites generally are complex structural designs. There are so many things that need to be managed to get good results. So often, the early trials with new components are exas- perating. Table 1 is a chart of the components showing the fortuitous choices that Ray Bowen made for this particular experiment versus typi- cal components for current composites. Figure Figure 1. Schematic of dental composite and location of 1 shows the position of the coupling agent in components.

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immersion within a rich First Composite Revolution Second Composite Revolution environment of expand- ing knowledge. This was certainly true for den- tistry from 1950 to 1970. Unbonded Dentin-Bonded Composites Composites 3c, 2c, 1c In addition to dental Acid Etching and Dentin Bonding System Enamel Bonding materials research that BONDING was being conducted 1950 1960 1970 1980 1990 2000 2010 2020 at several universities, the National Bureau of Standards (NBS) (later

Nanohybrid to become the Ameri- Original Macrofill Microfill Flowables Composite Development Self-Cured Composites can Dental Association Composites Packables Midifill Foundation Paffen- Composites FILLERS Midifill Controlled barger Research Center Composites Midihybrid Shrinkage Composites at the National Institute Midihybrid Self-Cured Composites of Standards and Tech-

UV-Cured nology) was active in re- search. Dr. Bowen found

CURING VLC (QTH, PAC, Laser, LED) a lifelong home at NBS starting in the 1950s. Dentistry was convert- ing over to high-speed Figure 2. Schematic summary of the evolution of dental composite over more than 50 years. 3c: Three-component system (etch, prime, bond). 2c: Two-component system (etch, prime/bond or handpieces that allowed etch/prime, bond). 1c: One-component system (etch/prime/bond). LED: Light-emitting diode. PAC: new types of cavity prep- Plasma arc light. QTH: Quartz-tungsten-halogen. UV: . VLC: Visible light cured. aration designs for ad- hesive restorations. New Table 2 dental chair designs Comparison of the mechanical properties of the transformed patient care procedures and original composite with those of a typical new facilitated four-handed material. dentistry.6 By substan- Mechanical ULTIMATE ULTIMATE Tensile ELASTIC tially reclining a patient, Properties, according Compressive Strength, in MPa Modulus, it was much easier to † to year Strength, in MPa* in GPa access intraoral spaces 2010 Mean Composite 360 (189-463) 50 (32-64) 12 (5-25) Value (Range)‡ and to work in tandem 1963 Silica-Reinforced 157 27 11.0 with a dental assistant. Bis-GMA§¶ Composite and bonding 1963 Control, Silicate range, 157-186 5 21.4 system placements were Cement# best managed by using * MPa: Megapascal. four-handed dentistry. † GPa: Gigapascal. ‡ Sources: Biomaterials Properties Database, University of Michigan.2-4 New areas of dental ma- § Bis-GMA: Bisphenol A-glycidyl methacrylate. terials research were ¶ Source: Original 1963 JADA article by Bowen. Glass particles from compressive strength specimens were collected by means of sieves ranging from no. 100 through no. 350. Glass particles from tensile evolving such as animal strength and modulus specimens were collected by means of a no. 400 sieve. testing for biocompat- # Source: Paffenbarger and Stanford.5 ibility7 and clinical research.8 New dental tion is schematically represented in Figure 2. companies were formed because of the chemistry Along this path virtually all of the original com- involved in dental composites (for instance, 3M ponents have been investigated and most have Dental9 [now 3M ESPE, St. Paul, Minn.] com- been significantly changed. Table 22-5 reports the menced in 1964 with the market entrance of mechanical properties of the old to newer formu- Addent dental composite restorative material). lations. Compressive and tensile strengths have Old problems were being solved with new un- improved. The intermediate values of modulus derstandings of interfacial bonding challenges, of elasticity that represent less brittle materials such as matching porcelain to metal thermal have been preserved. expansion.10,11 A huge number of experiments Discovery and invention usually depend on were revealing the success behind high-copper

882 JADA 144(8) http://jada.ada.org August 2013 JADA LANDMARK SERIES dental amalgams.12 It was an exciting time for posite revolution was fully under way. restorative dentistry. Early work would soon be Where are we today? The first composite under way on both polycarboxylates13 and glass revolution is over. We are looking forward to a ionomers.14 second composite revolution and an explosion Original uses for composite restorative ma- of new ceramic materials and fabrication tech- terials involved traditional retentive cavity nologies as the mainstays for future restorative preparation designs. However, just as this ex- dentistry. In December 2012 in London, the periment featured coupling agents to produce Dental Materials Innovation Workshop took intimate internal interfaces, new procedures place, sponsored by Kings College and the Inter- called “bonding” were being devised to create national Association for Dental Research.23 This intimate external interfaces of composites with planning may have represented the beginning enamel and dentin. Dental composite was part of the end for dental , for conventional of a revolution in new restorative techniques. composites and for glass ionomers as we have Acid-etching techniques had been introduced known them. The conference was in response to earlier.15 New acrylic resin polymerization ac- a proposed treaty brokered by the United Na- celerators had been extensively explored.16,17 tions Environmental Programme to produce a Two major conferences had brought together widely embraced world treaty to discontinue the entire dental research community to discuss the use of all products based on , in- improved restorative materials and bonding to cluding amalgam. In discussions of options for enamel.18,19 Dental composite use was explored the future at the conference, the groundwork in anterior and posterior teeth (with products for potential new composite formulations was such as Adaptic [Johnson & Johnson, New discussed. A new composite would include Brunswick, N.J.] and Addent [3M Dental]). Bis- friendlier monomer systems and advanced GMA was being evaluated for dental sealants.20 nanofillers and focus on being crack tolerant. Lightly reinforced composites subsequently Coincidentally, the National Institute of Dental were evaluated as preventive resin restora- and Craniofacial Research, National Institutes tions.21,22 Thus began a 30-year search to un- of Health, posted a request in December 2012 derstand the wear behavior of these composite for applications for entirely new dental compos- materials. This landmark publication by Bowen ites that could be new amalgam replacements was really the tip of the iceberg that signaled and last much longer. Dr. Ray Bowen, who has rapidly expanding new science across a range of continually published for almost 60 years and dental restorative materials. contributed to most aspects of the first com- General dental practice in 1950 was pri- posite revolution (documented in more than 30 marily focused on procedures involving dental patents), continues to publish and seems certain amalgam, direct gold and cast gold restorations. to be involved in the second composite revolu- Direct esthetic restorations involved silicate tion as well. The length of this second compos- and polymethyl methacrylate fillings. Indirect ite revolution will depend on competition from esthetic restorations were primarily dental readily available implant systems and tissue- porcelain in areas of low or no stress. By 1980, engineered teeth but seems sure to continue sealants, composites, preventive resin restora- through the next decade or more. n tions and glass ionomers were gaining huge mo- Dr. Bayne is a professor and the chair, Department of Cariology, mentum. Porcelain-fused-to-metal restorations Restorative Sciences, and Endodontics, School of Dentistry, Univer- were highly successful. Amalgams were being sity of Michigan, 1011 N. University Ave., Room 2353, Ann Arbor, replaced in anterior tooth sites almost entirely Mich. 48109-1078, e-mail [email protected]. He also is a member of the editorial board of The Journal of the American Dental Associa- by tooth-colored materials. The great debate in tion. Address reprint requests to Dr. Bayne. modern times over dental amalgam safety was just beginning, and the emotional misinforma- Disclosure. Dr. Bayne did not report any disclosures. tion of antiamalgamists began to capture head- 1. Vanderbilt BM, Simko JP Jr. Silane coupling agents in glass- lines. There was new interest in potential alter- reinforced plastics. Mod Plast 1960;38:135. natives to dental amalgam. Glass ionomers and 2. Biomaterials Properties Database, University of Michigan. Biomaterials properties: ultimate compressive strength. www.lib. composites were both touted as the materials umich.edu/taubman-health-sciences-library/biomaterials-properties- of the future. Clinical trials of dental composite ultimate-compressive-strength-c. Accessed June 19, 2013. 3. Biomaterials Properties Database, University of Michigan. Bio- use in posterior tooth sites to replace amalgam materials properties: ultimate tensile strength. www.lib.umich.edu/ restorations were already under way. There was taubman-health-sciences-library/biomaterials-properties-ultimate- an explosion of new composite filler types, filler tensile-strength. Accessed June 19, 2013. 4. Biomaterials Properties Database, University of Michigan. combinations, new curing lights, new resin Biomaterials properties: elastic modulus. www.lib.umich.edu/ monomers and early bonding systems. The com- taubman-health-sciences-library/biomaterials-properties-elastic-

JADA 144(8) http://jada.ada.org August 2013 883 JADA LANDMARK SERIES modulus. Accessed June 19, 2013. 14. Wilson AD, Kent BE. A new translucent cement for dentistry: 5. Paffenbarger GC, Stanford JW. Zinc phosphate and silicate ce- the . Br Dent J 1972;132(4):133-135. ments. Dent Clin North Am 1958:561-566. 15. Buonocore MG. A simple method of increasing the adhesion of 6. Four-handed dentistry: double digits. UAB Magazine. Fall 2009. acrylic filling materials to enamel surfaces. J Dent Res 1955;34(6): www.uab.edu/uabmagazine/breakthroughs/healthcare/four-handed- 849-853. dentistry. Accessed May 29, 2013. 16. Brauer GM, Davenport RM, Hansen WC. Accelerating effect of 7. U.S. Council on Dental Materials and Devices. Guide to Dental amines on polymerization of methyl methacrylate. Mod Plast 1956; Materials and Devices. 8th ed. Chicago: American Dental Association; 34:153-168. 1976:200-209. 17. Bowen RL, Argentar H. Amine accelerators for methacrylate 8. Cvar JF, Ryge G. Criteria for the Clinical Evaluation of Dental resin systems. J Dent Res 1971;50(4):923-928. Restorative Materials. San Francisco: U.S. Department of Health, 18. Phillips RW, Ryge G, eds. Adhesive Restorative Dental Materi- Education, and Welfare; Public Health Service; National Institutes of als: Proceedings of a Workshop held at Indiana University Medical Health; Bureau of Health Manpower Education; Division of Dental Center, Indianapolis, Indiana, September 28 and 29, 1961. Spencer, Health; Dental Health Center. 1971;1-4.3. Public Health Service pub- Ind.: Owen Litho Service Publishers; 1961. lication 790244. Republished as: Cvar JF, Ryge G. Reprint of criteria 19. Austin RH, Wilsdorf HGF, Phillips RW, eds. Adhesive Restor- for the clinical evaluation of dental restorative materials. 1971. Clin ative Dental Materials, II: Proceedings of a Workshop held at the Oral Invest 2005;9(4):215-232. University of Virginia, Charlottesville, Virginia, December 8 and 9, 9. 3M Health Care. 3M Health Care Profile and History. 2006. 1965. Washington: U.S. Department of Health, Education, and Wel- www.3m.com/intl/kr/microbiology/3m_info/history.pdf. Accessed May fare, Public Health Service; 1966. Public Health Service publication 29, 2013. 1494. 10. Weinstein M, Katz S, Weinstein AB, inventors and assignees. 20. Roydhouse RH. Prevention of occlusal fissure caries by use of a Fused porcelain to metal teeth. U.S. patent 3,052,982. Sept. 11, 1962. sealant: a pilot study. ASDC J Dent Child 1968;35(3):253-262. 11. Weinstein M, Katz S, Weinstein AB, inventors and assignees. 21. Simonsen RJ. Preventive resin restorations (I). Quintessence Porcelain covered metal reinforced teeth. U.S. patent 3,052,983. Sept. Int Dent Dig 1978;9(1):69-76. 11, 1962. 22. Simonsen RJ. Preventive resin restoration (II). Quintessence 12. Sarkar NK, Greener EH. Absence of the 2 phase in amalgams Int Dent Dig 1978;9(2):95-102. with high copper concentrations. J Dent Res 1972;51(5):1511. 23. Rekow ED, Watson T, Fox C. Priorities: innovation, research, 13. Smith DC. A new dental cement. Br Dent J 1968;124(9): and advocacy in dental restorative materials. Adv Dent Res (in press). 381-384.

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