Compomers first appeared in the early 1990s in an attempt to combine the potential advantages of fluoride release as seen with the glass ionomer cements, with the many advantages of the resin composites. The aim was therefore to produce a resin composite which exhibited a sustained and effective release of fluoride. Previous attempts to achieve this had involved adding various fluoride salts to conventional resin composite formulations, but this proved to be less than successful. The mechanism of fluoride release relies on water being absorbed by the resin composite which then dissolves fluoride salts such as ytterbium fluoride followed by diffusion of fluoride ions out of the material. However, the nature of the resin composites meant that only the fluoride in the subsurface layers was released by this process because of the difficulty encountered by the fluoride ions in diffusing through the bulk of polymerized resin. Small amounts of fluoride were released initially but this was not sustained in the longer term. Unfortunately, another effect of these additions was to reduce the resin composite’s longevity because of degradation of the methacrylates.

The material finally developed lies closer to the resin-based composites in the continuum of materials than to the glass ionomer cements. It is primarily a resin system and is manufactured without the addition of any water. Instead an additional resin is incorporated which is difunctional.

The commonly accepted name for these materials is ‘compomer’, which was coined by Dentsply, the manufacturer of Dyract, the first material of this type (Figure 8.1). However the generic name for this material is polyacid-modified resin composite, which effectively describes their composition and, by implication, their setting mechanism.

Fig. 8.1 A buccal compomer restoration (Dyract AP, Dentsply) on tooth 16 after having been in service for 8 years.

The material is essentially a composite in nature, in that it is composed of a resin components and a filler (Table 8.1).

Table 8.1 Components of compomer materials

The setting reaction is a two-stage process.

Stage 1

When the clinician is satisfied with the material’s placement in the cavity, the initial setting reaction is initiated by light as with the composite resins. This is a free radical polymerization reaction that leads to cross-linking of the end groups on the UDMA and the methacrylate groups on the difunctional resin to form a resin polymer matrix in which the glass is trapped (Figure 8.2).

Fig. 8.2 First stage in the setting reaction of compomer materials. The initial cure is by light polymerization.

Polysalt matrix: matrix formed of a range of salts derived from the acid and base in the cement.