Chapter 8 The tooth-coloured restorative materials II
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.
|Component||Chemical example||Reason for inclusion|
|Filler||Fluoro-alumino-silicate glass||Imparts strength|
|Source of fluoride ions for use in the secondary reaction|
|Dimethacrylate monomer||Urethane dimethacrylate (UDMA)||Primary monomer forming the resin matrix|
|Difunctional resin||TCB resin: the reaction product of butane tetracarboxylic acid and hydroxyethyl methacrylate||Cross-linking agent in the primary reaction|
|Source of carboxyl groups for secondary reaction with glass|
|Photo-activator and initiator||Camphorquinone||Required to effect light polymerization|
|Hydrophilic monomers||Glycerol dimethacrylate||Enhances water diffusion within resin matrix|
• A difunctional resin monomer, which has both carboxyl and methacrylate groups. Another type of resin which may perform in the same way is the methacrylated polycarboxylic acid found in some resin-modified glass ionomer cements (see Chapter 10).
The principles of resin-based composite manufacture hold true for compomers in that a filler is added to decrease the amount of resin and to improve the mechanical properties and influence the material’s appearance.
The filler is a fluoro-alumino-silicate glass, which is similar to that found in a conventional glass ionomer cement, to provide a source of fluoride ions and form a salt matrix with carboxylate groups. This fine glass powder is in the sub-8 μm size range with a filler loading volume of between 42% and 67%. The glass usually contains lithium or strontium to convey radiopacity to the material.
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).
A secondary reaction occurs between the glass and the carboxylate groups of the difunctional resin. This requires the restoration to be bathed in saliva. When it becomes wet, water is taken up by the resin system. The acid side groups (hydrogen ions) start to dissolve the outer surface of the glass and release the fluoride ions, which will diffuse out of the material. A polysalt matrix is formed around the glass as in a conventional glass ionomer cement. This is an acid/base reaction that is aided by additions of small percentages of hydrophilic resin (such as glycerol dimethacrylate) to the resin mix to encourage diffusion of water within the resin and fluoride out of it. The diffusion process takes several weeks and there is a slow but steady release of fluoride />