Table of Contents
Polycarboxylate cement is a newer innovation than zinc phosphate cement. In this case, zinc oxide powder is mixed with polyacrylic acid. Sometimes the polyacrylic acid is freeze dried into a powder and mixed with the zinc oxide powder, in which case the powder is mixed with distilled water. As with zinc phosphate, the zinc oxide dissolves and creates a matrix which eventually becomes quite waterproof, and though not nearly as strong a cement as zinc phosphate, it is much easier to work with, sets much more quickly and is less irritating to the nerve of the tooth. As with zinc phosphate, the zinc oxide remains opaque and the color of this material is not easily controlled. It is rarely used as a restorative filling material. Like zinc phosphate, this cement is somewhat technique sensitive in that it too must be kept dry until it is completely set.
Silicate cement is made by mixing a powder made of Alumino-Fluoro-Silicate glass with a 37% solution of phosphoric acid. The acid partially dissolves the glass, and chemically combines with it. This creates a very hard and brittle matrix. A fluid mixture of this cement can serve the same purpose as the zinc phosphate cement described above, however, its main use in dentistry has been as a tooth colored filling material. Because the matrix is very hard, its brittleness and lack of wear resistance limits its use as a restorative in stress bearing areas. Until the advent of resin composites, silicates were the only tooth colored filling material available, and the only alternative to silver amalgam as a simple (non gold) permanent filling material. Its use was limited to front teeth, or areas of decay on non stress bearing surfaces of back teeth.
Its largest single advantage, other than its color, is that the fluoride from the glass, (which is also a component of the matrix material due to the chemical reactions involved in mixing the powder with the liquid), tends to prevent further decay around the margins of the filling. (In fact, it is a characteristic of all the formulations using an Al-Fl-Si glass/acid combination that the finished restoration continues to leach small amounts of fluoride into the surrounding tooth structure throughout its life. This is true of glass ionomer restorations as well.) The major problem with silicate cements as a restorative material is its appearance. The glass particles are prone to dislodging from the surface of the filling leaving a rough surface which is prone to staining. The brittleness of the matrix is another esthetic difficulty since it causes surface crazing and marginal chipping as the restoration ages and creating more potential places for stains to lodge.
Glass Ionomer cements and restoratives (filling materials) are a fairly recent advent in dentistry. While Silicate cements have been around for years, Glass Ionomer had to await the invention of poly-acrylic acid. The mixture of poly-acrylic acid with Alumino-Fluoro-Silicate glass causes a partial dissolving of the glass particles. The poly-acrylic acid chemically combines with the dissolved glass components and produces a hard matrix material similar to that in silicate cement. (This is essentially an acid-base reaction resulting in the formation of a “metallic polyalkenoate salt” which precipitates and begins to gel until the cement sets hard.)
The characteristics of this matrix material, however, are strikingly different than the characteristics of the matrix found in silicate cements. Unlike silicates, the matrix is reasonably translucent allowing the color of the glass particles to dominate the aesthetics. It is also much less brittle than the matrix of Silicate cement making it a bit less prone to fracturing over time. Since the filler is a glass, its aesthetics can be precisely controlled. The less brittle matrix means that the margins and surface of the restoration are less prone to chipping and crazing so there is much less staining with Glass Ionomer restorations than there is with silicates. As a restorative, glass ionomers can be used in all esthetically sensitive areas with no reservations. Of all the composite restoratives, glass ionomers are some of the most esthetic restorations available.
On the plus side:
These restorations not only look good, but they bond to tooth structure quite well. Bonding between the cement and dental hard tissues is achieved through an ionic exchange at the interface. Polyalkenoate chains enter the molecular surface of enamel and dentin, replacing phosphate ions. Calcium ions are displaced equally with the phosphate ions so as to maintain electrical equilibrium. This leads to the development of an ion-enriched layer of cement that is firmly attached to the tooth.
Glass ionomer restorations, like silicates also leach fluoride into the adjacent tooth structure throughout the life of the restoration and thus tend to reduce the likelihood of recurrent decay around the margins. For an excellent detailed technical explanation of the chemistry of glass ionomer, click on this link to the Canadian Dental Association review of glass ionomers.
Glass ionomers are very biocompatible. They are especially good as bases under composite resins. Although their use is off-label, they are gaining popularity as pulp capping agents and as permanent dressings placed over minor perforations during endodontic procedures.
Glass ionomer is a great luting agent for cementing crowns and bridges with metal and ceramic sub structures. Their chemistry allows them to bond directly with the metal substructures as well as tooth structure, and their biocompatibility produces less post operative sensitivity.
Glass Ionomer cements and restoratives exhibit much less polymerization shrinkage than any of the resin-glass composites, and thus make excellent bases for use under composite restorations. They maintain an excellent (although not perfect) marginal seal, and are not likely to debond from the pulpal floor while polymerizing.
The coefficient of thermal expansion of conventional glass ionomer cements and restoratives is close to that of dentin and enamel. Thus, glass ionomer bases are less likely to debond from the pulpal floor during service than conventional composites.
On the negative side:
The matrix material is much less hard than the matrix of silicate cement, so the restorations wear faster than silicates, and much faster than conventional composites.
They also lack fracture resistance and are much more brittle than resin-glass composites. Glass ionomers are excellent fillings on the buccal (front) surfaces of anterior teeth, but should not be used to rebuild incisal (top) edges of these teeth due to their inherent weakness.
The material is very sensitive to water contamination during placement, and poor technique on the part of the dentist (or poor cooperation on the part of the patient) can shorten the lifespan of the resulting restoration considerably. Many dentists are now using a version of glass ionomer mixed with acrylic resin known as a resin modified glass ionomer for cementing cast metal and zirconium based restorations.
The extra time necessary to mix and apply a glass ionomer base, and then allow it to set, can be a deterrent to its use by some dentists.