Dental materials

12.2.7 Dental materials

12.2.7.1 Cavity fillers

Dentistry has always been very dependent on the use of biomaterials, and particularly receptive to the application of new developments in metals, ceramics, polymers and composites.

Dental amalgams have been used in cavity fillings for more than 100 years, initially with silver– mercury amalgams, later modified by tin additions to control the amount of expansion. These amalgams produced the weak, corrodible intermetallic γ 2 phase, Sn 7 Hg, and hence the modern dental amalgam now also contains copper (>12%) in order to suppress this phase. The amalgam is made by mixing silver, tin, copper alloy powder with mercury and this mixture is packed into the cavity, where it hardens to produce a strong, corrosion-resistant, biocompatible filling. There is some

evidence that even this filling may be susceptible to corrosion as a result of the Cu 6 Sn 5 (η ′ ) phase and the addition of Pd has been advocated. Attempts to replace the Hg amalgam by gallium, indium, silver, tin and copper pastes have not yet been completely successful.

Alternative resin-based composite filling materials have been continuously developed since they were first introduced in the 1960s. These composite fillings have a strength similar to amalgams but poorer wear properties. The paste is created by mixing a dimethacrylate monomer with resin and adding a filler of micron-sized silane-coated ceramic particles. The paste is activated by strong light when the resin polymerizes. Bonding of the composite resin to the tooth structure employs a phosphoric acid etch of the tooth enamel. This produces mini-chasms into which the resin mate- rial flows and locks to form a strong mechanical bond. This technique is not successful, however, for bonding to the dentine in the tooth cavity (Figure 12.8) and so, in the absence of enamel, den- tine bonding agents have to be used. These are primers containing bifunctional compounds with (i) hydrophilic molecules which form links with the wet dentine in the tooth cavity and (ii) hydrophobic molecules which form links with the resin in the composite.

Cavities in front teeth are usually filled with glass cements to match the color and translucency of the enamel. Silicate cements are formed when phosphoric acid displaces metal ions from an alumina–silica glass, containing metal oxides and fluorides. The cement sets when aluminum phos- phate is precipitated between the glass particles. Developments based on this basic chemistry employ polymeric acids with carboxylate groups. In this case, the metal ions displaced from the glass cross- link with the polymeric acid chain, causing the cement to set. In addition, the acids undergo an ion-exchange reaction with the calcium phosphate in the apatite of the dental material. These glass ionomer cements therefore form direct chemical bonds to the tooth material. Resin-modified versions are also available which have improved durability; these contain carboxylate groups to give a good bond to the tooth and dimethacrylate, as in the composite resin.

596 Physical Metallurgy and Advanced Materials

Caries

Dentine Enamel

Figure 12.8 Schematic diagram of a tooth.

12.2.7.2 Bridges, crowns and dentures

Missing teeth may be replaced by artificial teeth in a number of different ways. For a group of missing teeth, removal partial dentures (RPDs) may be the answer; they consist of a cast metal framework of Co–Cr or Ni–Cr alloy carrying the artificial teeth and having end clasps to retain it to good natural teeth nearby. Fixed partial dentures (FPDs or bridges) may be used for a few missing teeth. Sometimes the supporting teeth are cut down to accommodate a close-fitting artificial tooth casting, which is cemented into place. In other cases, the alloy framework carrying the artificial teeth is bonded to acid-etched teeth to avoid cutting down good teeth. The teeth are acid etched and the metal framework electrolytically etched to produce structural grooves and chasms which allow strong mechanical bonds to be formed with resin-based composite cements. In some situations, etching can be avoided when the oxides on the metal framework can be treated with bifunctional primers to form chemical links to the cement.

Over the last 20 years or so the quality of bonded restorations, i.e. porcelain-covered metal castings, has been refined to combine the impact strength of a metal substructure with the appearance of dental porcelain. These porcelains have good mechanical properties and their thermal expansion characteristics are matched to the metal in order to avoid interfacial stresses and cracking.

Dental porcelains are basically vitrified feldspar with metallic oxide pigments to simulate natural tooth enamel. They are usually supplied to the dental laboratories in the reacted and ground forms for final fabrication by the technician; this involves mixing the powder with distilled water to form a paste which is used to make the crown, then drying and firing in order to sinter and densify the crown material. Generally, firing is carried out in stages, starting with the innermost structure of the crown, followed by the body, and finally the outer glaze and surface staining. Developments have included the strengthening of the inner core material with alumina to prevent cracking and the addition of magnesia to form a magnesia–alumina spinel, which has a low shrinkage on firing. Glass-ceramics have also been used either to fabricate the crown by casting followed by heat treatment to produce crystallization, or by machining from a pre-fired block of glass-ceramic under CAM/CAD conditions.

For complete replacement dentures, the basic material, which has existed for many decades, is methyl methacrylate. Substitute materials have been limited, and most improvements and develop- ments have occurred in the processing technology. While the mechanical properties of denture base

resins are not particularly good (modulus of elasticity 3 × 10 9 Nm −2 , tensile strength ∼100 kN m −2 , elongation ∼3%), they do have suitable surface and abrasion properties, and are chemically inert, non-toxic and cheap. Improvements have been forthcoming in elastomers used for taking impressions; these now include vinyl addition silicone and polyether elastomers.

12.2.7.3 Dental implants

Dental implants have been far less developed than those associated with body implants (see hip joints, etc.). Probably the simplest forms are posts, Co–Cr, stainless steel, titanium alloy or gold

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Figure 12.9 Hydroxyapatite-coated titanium root implant (courtesy P. Marquis, Dental School, Birmingham).

alloys cemented, or even screwed, into the tooth canal after the tooth has been root treated to remove the nerve. Dental porcelain caps may then be cemented onto the root post. Ti implants have been screwed into the bone beneath extracted teeth. After some time the passive surface layer of the titanium implants becomes osseo-integrated with the bone and can be used as a strong base onto which a titanium mini structure can be fitted, complete with tooth assembly. Osseo-integration is improved by using a coating on the titanium implant such as hydroxyapatite or bioglass (Figure 12.9). Ceramic and carbon implants set into the bone have been used with sapphire single crystals and pyrolitic graphite as favored materials.