Introduction
Glass-ionomer cements, also called polyalkenoate cements, combine an acid-soluble glass powder with polymeric acids and set by ion exchange. A. Wilson and B. Kent developed them and patented the idea in 1969, and the first product, ASPA, followed in 1975. Today's products can be sorted in three ways: by clinical use, by composition and by how they are supplied.
Types by clinical use
The consistency of the mix affects the pulp. A thick mix has more powder and less acid, so type II, the thickest, is the mildest. Type III comes next, and type I, the thinnest, has the strongest effect, although that effect is still weak.
Types by composition
Conventional cements
The powder is a calcium-aluminosilicate glass that dissolves in acid, with about 20% fluorides. The liquid is about 40% polyalkenoate acids in water. They set by the acid-base reaction alone, and this makes them vulnerable for the first 24 hours: water washes out reacting ions and soluble calcium chains, while drying stops the ion exchange. A moisture-proof varnish protects the surface straight after initial hardening.
In water-mixed versions, cold-dried acids are already in the powder, which is mixed with distilled water. These keep for longer on the shelf.
Fast-setting cements
Most of the excess calcium is removed from the surface of the glass particles. Fewer soluble calcium chains form, and the strong, insoluble aluminum chains build up sooner. The cement is less sensitive to moisture just after setting and to drying over the next two weeks, helped by an improved light-cured varnish.
Resin-modified (hybrid) cements
About 20% of the liquid is HEMA, a water-loving monomer, along with an initiator. Its polymer forms a waterproof network that holds the water balance while the acid-base reaction matures. Only about 5 to 7% of the set cement is resin, and a hybrid filling cement reaches a compressive strength of about 270 MPa, close to dentin.
- Self-cured: an amine-peroxide system starts the polymerization.
- Light-cured: camphorquinone reacts to blue light at about 460 nm.
- Triple-cured: a light-cured cement with an added chemical initiator, for places the light cannot reach.
A hybrid cement has two setting reactions: the acid-base reaction and the polymerization of HEMA.
Metal-modified cements
Silver is added to improve wear resistance. In an admix cement, spherical silver alloy particles are mixed into the powder. In a cermet cement, silver is sintered into the glass. Both are opaque, so they look poor.