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Resin Materials in Dentistry

1 Introduction

Resin materials are of high interest and importance in dentistry. They are used for numerous applications such as dentures, partial dentures, relining of dentures, artificial teeth, fillings, inlays, crowns, bridges, temporary restorations, sealants, luting purposes, adhesives, impressions etc.

Charles Goodyear ran the vulcanization of natural rubber the first time in 1839 and as a result the first resin material was born [64, 91-93]. This process served to manufacture denture bases in the following period of time. Although the natural rubber used for this purpose was pink-colored the esthetic appearance of the denture was very poor because of the rubber’s high opacity. Yet, this process was used till the thirties of the 20^th century. Since approx. 1870 celluloid, synthesized from nitrocellulose and camphor, was used for denture bases aside from rubber. Celluloid denture base materials entered the market in the USA under the trademarks Hecolite and Coralite [91, 94, 95]. Around 1900 phenolic resins, developed by Baekeland and, therefore, often called bakelite, with the trademarks Aldenol and Walkerit were also used as denture base materials [53, 64, 91, 95-97].

In 1934 Pierre Castan synthesized the first epoxy polymer resin (trademark: Epoxolon) in the laboratories of DeTrey Fréres Co. (today: DeTrey/Dentsply GmbH, Germany) in Switzerland while he was searching for an improved denture base material and obtained a patent in 1940 [53, 64, 91, 98]. Although they were never used for dentures since then epoxy polymers were used for products of highest quality demands. Other plastics like benzyl cellulose (trademark: Pertax), polyamides (trademark: Protenyl), polystyrenes (trademark: Polystein), polyvinyl chloride (trademark: Hekodent, Hewodent) or polyolefins (trademark: Odenta) had similar destinies. They were only used for a short period of time as denture base materials [39, 53, 64, 91, 95-97].

The crucial ascent of polymer chemistry started in the thirties of the 20^th century with Otto Röhm’s development of methyl methacrylate (also called: methacrylic acid methyl ester or MMA) from which he synthesized via polymerization polymethyl methacrylate (also called: polymethacrylic acid methyl ester or PMMA) [91, 99]. PPMA has the well-known trademark Plexiglas. In 1936 PMMA determined the great breakthrough in dental materials. The dental technician Gottfried Roth mixed milled PMMA with its monomer MMA and stirred the mixture until it becomes dough-like. Then he processed it the same way as it was commonly done to manufacture rubber dentures. He pressed the dough between two halves of a plaster mold representing the denture and boiled the whole assembly in a water bath until the dough was hardened [91]. This was the first time esthetically satisfying dentures were obtained and the method as well as the materials were patented in 1936 [91, 100]. This process was improved and optimized in the following years and decades and it is broadly used to manufacture dentures up to now. Later developments substituted MMA by numerous newly synthesized methacrylates, dimethacrylates or multifunctional methacrylates with sometimes very high molecular masses. This led to totally new high-performance composite resins that can be used for nearly almost all indications in the oral cavity.

Later MMA/PMMA composites [101-103] and newly developed high molecular mass methacrylates [104-106] were the base of modern resin-based filling materials and adhesives. R. L. Bowen laid the foundation stone for modern resin composite filling materials by his crucial research work and inventions and thus revolutionized restorative dentistry [104-111].

It was and it is still tried to use other polymers such as polycarbonates or polyacetals (also called polyoxymethylene or POM) to manufacture dentures. These products are processed via injection molding technique but they did not succeed on the market and are only occasionally used.

POM and polyaryletheretherketone (PEEK) processed via CAD/CAM are used today to manufacture denture bases or suprastructures which are usually made from metal. Very likely other polymers will be used for dental purposes in future due to the upcoming innovative processing techniques like CAD/CAM grinding or milling or 3D-printing.

Since 1955 the elastomeric polymers, the polysulfides, and in 1958 [112] the polysiloxanes (also called silicones) entered the dental market and were predominantly applied in the oral cavity for performing impressions. Today, the polysiloxanes dominate this segment. Since 1966 the polyether impression materials also took a great part of this segment [113, 114]. But polysiloxanes are also used in the dental laboratory for duplicating or embedding purposes.

2 Modern Dental Resins

Today, a very broad variety of polymeric materials and composite polymers are used in dentistry. Polymethacrylates, polyacrylates, epoxies and polysiloxanes cover the largest areas of application. It is most certain that newly developed monomers and polymers will be used to develop dental materials in future.

Still, what ever kind of resin or composite resin it is or will be they all have or will have the general composition schematically illustrated in figure 1b. Composite resins consist of very many different ingredients. The respective monomer or monomer blend, also called resin matrix, forms different polymers with different chemical and physical properties. The resin matrix is the central ingredient which contains or may contain different types of fillers, pigments, dies, stabilizers or other various additives and if necessary initiators or catalysts to create a finished product that fulfills the objectives.

The chemical and physical properties of polymers are determined by the

- type of monomers

- type of monomeric links

- alignment of the monomers (primary structure)

- spatial alignment of the monomers in the polymeric chains (secondary structure)

- spatial alignment of the chains’ secondary structures against each other (tertiary structure)

There are numerous different monomers available to create the resin matrix. Depending on the type of monomer different resins with different chemical and physical properties emerge. The chosen monomers determine the type of link by the functional groups reacting with each other and thus also the type of polyreaction as well. The type of link determines the name of the polymer. The next chapter presents various polymers, also called matrix resins, their links and the respective polyreaction.

Fig. 1b: Principal composition of composite resins.