Compomers
Compomer materials (polyacid-modified composite resins) were introduced in the early
1990s. The original manufacturer's concept was to produce a material with the handling and
aesthetics of composite, but with the fluoride-releasing properties seen in glass ionomers. In
this respect, while resin-modified glass ionomers are true glass ionomers with some
additional resin characteristics, compomers are composite resins with some glass
ionomer characteristics. The similarities between compomers and glass ionomers are
principally that they contain similar acid-decomposable FAS glasses and release fluoride,
although in compomers this is less than one-tenth of the fluoride released by glass ionomer
materials. The acid-base reaction typical of glass ionomer materials does not occur in
compomers, at least not to an extent that will lead to setting of the material in the dark.
Compomers are therefore light-cured materials.
In compomer materials, polyacrylic acid molecules are incorporated in the
resin monomer which forms the matrix. Compomers are presented as anhydrous one-
component materials, and it is considered that, when the material hydrates after being placed
in the mouth, a glass ionomer-type setting reaction takes place, leading to the small amount
of fluoride release which is seen. Early compomer materials used a self-etching primer for
tooth conditioning and bonding, but more recent materials have incorporated a phosphoric
acid etching stage prior to placement. It has been demonstrated that microleakage is reduced
when enamel margins are etched prior to restoration placement.
A wide variety of compomer materials are currently available. The resin matrix contains an
acid monomer, termed TCB resin (a bi-ester of 2-HEMA and butane tetracarboxylic acid).
This contains two acidic polycarboxylate groups and two polymerizable methacrylate groups
which enable polymerisation by light and an acid-base reaction when water is present. Water
is taken up to a maximum of 3% by weight in a period of months following placement. This
diffuses through the restoration and an acid-base reaction takes place between the strontium
fluorosilicate glass and the polycarboxylate groups of the monomer. This acid-base reaction
leads to further cross-linking of the matrix and release of a small amount of fluoride.
Compomers are not adhesive to enamel and dentine, so an intermediate bonding system must
be employed. Bonding via the systems supplied with compomer materials produces a hybrid
layer similar to that produced by dentine bonding systems, but there may be some adhesion
from an ionic bond to the inorganic part of the tooth. Bond strength measurements produce
values which are not as high as with dentine bonding systems to resin-based composite, but
retention of restorations in non-retentive cavities does not appear to be a problem. The
mechanical properties of compomers are generally inferior to resin-based composite materials
but superior to glass ionomer materials. Filler loading of a typical compomer is
approximately 70% by weight, and polymerisation shrinkage is 3-4%. A wide variety of
shades is available with many systems and, as a result, compomer materials may produce
restorations with good aesthetics. Clinical trial data on the use of compomer materials in class
III and V cavities in permanent teeth, and in the restoration of primary teeth are positive.
