Table of Contents
Zinc Oxide based Composites
Zinc phosphate cement is one of the oldest and most reliable dental materials. It has been used for at least two hundred years. It is still used for cementing cast metal crowns and onlays. It is made by mixing a strong solution (37%) of phosphoric acid with zinc oxide powder. The zinc oxide powder partially dissolves in the acid creating zinc phosphate which when dry is a very hard, waterproof matrix which bonds unreacted zinc oxide particles together. Mixing and cementing with this material is something of an art since it must be mixed slowly or else it will harden too quickly, and the work must be kept dry until the cement is set or else it will dissolve in saliva or water. Once set, it is still one of the most reliable and most durable cements for luting (cementing) cast metal crowns and onlays on teeth. It is also used to cement posts in teeth and was used until quite recently as a base under amalgam fillings. (A base is a layer of material placed under a filling to protect the nerve from hot and cold while the overlying filling is in service. Some bases can also be useful as a method of desensitizing the nerve.)
Zinc oxide has an added benefit since the acidity of the phosphoric acid etches the enamel on the tooth creating the irregular surface seen in the micrograph above. The cement flows into these irregularities to create a tight mechanical seal with the tooth itself. It also flows into irregularities in the structure of the casting to form a “lock and key” type of bond between the tooth and casting thus locking it in place. With the advent of newer cements with a quicker working time and less demanding technique, zinc phosphate is used less and less today. Note that zinc oxide is an opaque white powder. While it can be manufactured to be any color, the set material remains perfectly opaque. For this reason, and the fact that it lacks wear resistance, zinc oxide is not esthetic or tough enough to be used as a “tooth colored” filling restorative.
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.
Zinc oxide mixed with Eugenol is both a cement and a restorative material. As a cement it is most frequently used as a temporary cement, or to cement stainless steel crowns on deciduous teeth. When used as a cement, it is valued for its sedative properties and its ability to temporarily lute ill fitting crowns under which a lot of bulk is needed to take up the extra space between the tooth and the restoration.
Zinc Oxide and Eugenol is a traditional form of temporary filling used either by itself, or in conjunction with an acrylic powder (IRM). Temporaries are the least expensive (and most temporary) way to fill a tooth. Temporary fillings can be done quickly, because they are usually inserted without any of the time consuming rituals associated with a permanent filling. The patient is anesthetized, the decay removed and the temporary filling is mixed and inserted, generally simply by pushing it into the cavity preparation with a gloved finger. The patient bites into it while it is still soft in order to adjust the bite, and the patient leaves the office without even waiting for a final set on the material. In a phrase, a temporary is “fast and cheap’.
Zoe fillings are “sedative” fillings. This means that they tend to soothe an inflamed dental pulp, and may make the difference between the tooth needing a root cana
l (or an extraction), or simply filling the tooth later on, after the nerve has calmed down. Sometimes a sedative filling is the best course to relieve pain.
Temporary fillings are made of two major components: eugenol, a component of oil of clove , which has been used for centuries to relieve toothaches, and Zinc Oxide which is the ingredient that makes Desitin diaper rash ointment white. Zinc oxide is an excellent disinfectant. The oil and oxide mix together to make a stiff paste that eventually hardens into a waterproof substance which soothes the pulp and kills germs while protecting the cavity like a hard band aid. When used as a temporary filling material or cement, this material is called “zinc oxide and eugenol“, or ZOE for short.
Zinc Oxide and Eugenol (ZOE) is not very durable, and it wears away after just a few weeks, but it works to relieve pain, calm the nerve and protect the tooth until an appointment can be made to get it filled permanently. During the Vietnam war, the US Army invented a more durable form of ZOE called Intermediate Restorative Material (IRM) which is fortified with plastic powder. (It originally came in red, white or blue colors.) IRM is used almost universally in dental offices throughout the world for temporary fillings. The increase in durability allows the temporary to last three to 6 months (sometimes even longer).