Ceramics

Dental Ceramics

Ceramic materials have the ability to emulate natural teeth, and they are some of the oldest dental materials, going back to 1792, when complete dentures were made from them. In 1996 they are used to create inlays, veneers, and crowns, as facings on metal substrates, and even as bridges, which can be made completely from high-strength ceramics. Restorations in ceramics are generally made by building up the correct aesthetic combinations of prefired, pigmented particles, and then re-firing under vacuum to sinter them together and eliminate voids.

Defeat of Brittle Nature of Ceramics

The developers are aware of their inherent brittle nature and have discovered many ways of interfering with the propagation of cracks within them. To this end, dispersion strengthening with alumina was the first approach. However, because of the opaque nature of the alumina, it is limited to the inner most structure of a crown, known to the dentist as a core. The cracks, which lead to catastrophic failure, nucleate at the internal interface between the prepared tooth and the ceramic crown. A high strength core can prevent the growth of these cracks and the strongest cores are currently made from either alumina. The toughest all-ceramic core produced so far actually infills any cracks in a high alumina base with molten glass during a firing stage in its production.

Glass Ceramics:

Glass ceramics are also used in several CAD/CAM applications in dentistry. In one of these a restoration is designed on a video image of a prepared tooth. It is then machined from a pre-fired block of glass ceramic. All of this takes place in front of the patient. As with all types of ceramic restoration, the machined unit is then coated with a silane bonding agent and cemented to the tooth with a resin-based cement. The tooth itself is also coated with an enamel/dentine bonding agent.

Cast Metal

Dentists have been replacing the damaged crowns of molar teeth with gold alloys. These have been cast by the lost-wax process. In this process, a wax crown is invested in a wet silica-gypsum mixture. Once this has hardened, the wax is burned away and molten gold-copper-silver-palladium-platinum-zinc alloy is cast under pressure into the space left behind. Some of the alloys can be heat treated to form super-lattices and increase their strength. This makes them suitable for the construction of dental bridges, which replace a missing tooth either by cantilevering an artificial tooth from an adjacent tooth, or by suspending it between two such teeth. In either case the supporting teeth will have been cut down to accommodate a close fitting casting, which is cemented into place.

Bonded Restorations

Alloy porcelain combinations, known to the dentist as bonded restorations have been available. These porcelain covered metal castings combine the strength of a metallic superstructure with the aesthetic appearance of dental porcelain, creating the illusion that the restorations are real teeth. Alloys have been developed to which dental porcelains form durable retentive bonds, and many of these are now based on nickel-chromium. These metal frameworks are so rigid that they can be bonded via composites to the backs of acid etched teeth, thus eliminating the need for cutting down sound teeth, figure 1. Just as etching dental enamel creates retentive ‘chasms’, these nickel-chromium alloys can be electrolytically etched to produce features that allow the formation of mechanical bonds with resin-based composite cements.

Glass ionomers Cements

First aesthetic, tooth-coloured filling materials appeared in 1930s. These were the silicate cements, which were formed when phosphoric acid displaced metal ions from a glass made from alumina, silica and several other metal oxides and fluorides. They set when aluminium phosphate was precipitated between the glass particles. These cements were used by dentists for half a century to fill cavities in front teeth, for not only did they match the colour and translucency of enamel and dentine, but they also acted as a source of fluoride. It was unusual to see dental decay recurring in any tooth they were used to fill. Similar cements also form when variations on this type of glass are exposed to polymeric acids which possess carboxylate groups. The acids displace metallic ions from the glasses and these cross-link the polymeric acid chains causing the cement to set. The acids also undergo ion exchange reactions with the apatite (calcium phosphate) crystals, which form part of both dentine and dental enamel. These glass ionomer cements, as they are known, thus form direct chemical bonds to teeth, without the need for the primers described above.
The basic cements lack the strength and resistance to wear that the dental composites have, and recent research has come up with resin-modified versions. These possess not only the carboxylate groups needed to form bonds to teeth, but also the light-curable dimethacrylate components present in the composite resins. Their durability is thus considerably enhanced.

Composite Fillings

These filling materials have reached a high degree of sophistication since their appearance on the dental scene in the early 1960s. A modern dental composite consists of a paste created by combining a mixture of dimethacrylate monomers and cross-linking agents with up to 80% by weight of silane coated, ceramic particles, whose sizes range from 0.04-4 microns. This composite paste is packed into a dental cavity and the dentist exposes it for about 30 seconds to intense visible blue light. The light activates a chemical initiator within the composite and the resins undergo free radical addition polymerisation via their vinyl groups, turning the paste into a durable, solid filling.

Composite Fillings Disadvantages:

These fillings have similar strengths to amalgam but they tend to wear away more rapidly. They also shrink as they polymerise, and efforts have to be made to prevent gaps forming between the composite and the tooth. Incremental packing and curing helps, but the dentist uses other techniques and other materials to help form a seal.

Bond between Resins and Teeth:

If enamel is present, its prismatic structure of apatite can be etched with phosphoric acid to produce mini chasms, into which the resin matrix material of a composite will flow. When this sets, it results in strongly retentive mechanical bonds. However, materials scientists have spent many hours seeking to produce a bond to the dentine, which exists below the layer of protective, inorganic enamel. Dentine is a wet, porous and sensitive combination of organic and inorganic materials, and current approaches to bond formation involve the use of primers containing bifunctional compounds. These have hydrophilic molecules at one end and hydrophobic ones at the other. The hydrophilic ends infiltrate the wet dentine and the hydrophobic ends form links with the resins in the composite, and so the composite is bonded to the dentine.

Amalgam Fillings

Silver Amalgams:

Silver amalgam has been used to fill the cavities made by dentists during the removal of dental decay from teeth. When pure silver is mixed with mercury it produces a paste of slowly forming intermetallic compounds. When this is packed into the cavity at body temperature, the intermetallic compounds interlock and the amalgam hardens. However, setting is accompanied by a considerable expansion, and 100 years ago it was discovered that this can be controlled by adding tin to the silver. Unfortunately, this produces corrodible tin-mercury intermetallic phases, and their loss can cause breakdown of the filling. By adding copper, the tin-mercury phase is eliminated and modern dental amalgams are made by mixing silver-tin-copper alloy powder with mercury. This results in fillings that resist both the mechanical and chemical onslaughts within the mouth for many years. Although the amount of mercury lost from such fillings is like the contamination of a drink by a drowning midge, those determined to deny the benefits of having usefully restored teeth have over emphasised the risk, and this has generated a search for an alternative, metallic, mercury-free, filling material.