1-11. CHARACTERISTICS OF ZINC PHOSPHATE CEMENT
a. History. More than 100 years ago, a French architect proposed the use of zinc oxide as a stopping medium for carious teeth. Zinc phosphate cement has progressively advanced from the original wall plaster that induced its development over a century ago.
b. Clinical Uses. Zinc phosphate cement is used both as an intermediate base and as a cementing medium.
(1) Intermediate base. A thick mix of zinc phosphate cement is used as an intermediate base beneath a permanent metallic restoration. This layer of cement protects the pulp from sudden temperature changes that may be transmitted by the metallic restoration.
(2) Cementing medium. Zinc phosphate cement is used to permanently cement crowns, inlays, and fixed partial dentures upon the remaining tooth structure. It is also used to hold splints, orthodontic appliances, and other appliances in place. This cement is used to cement facings to fixed partial dentures and certain types of artificial teeth to artificial denture bases. A creamy mix of cement is used to seat the restoration or appliance completely into place. The cementing medium does not cement two objects together. Instead, the cement holds the objects together by mechanical interlocking, filling the space between the irregularities of the tooth preparation and the cemented restoration.
c. Chemical Composition.
(1) Powder. The primary ingredients of zinc phosphate cement powder are zinc oxide and magnesium oxide.
(2) Liquid. The liquid used with the powder is phosphoric acid and water in the ratio of two parts acid to one part water. The solution may also contain aluminum phosphate and zinc phosphate. The water content of the liquid is critical and must be carefully controlled by the manufacturer to provide a satisfactory setting time. Liquids exposed in open bottles will absorb moisture from the air in high humidity. The liquids will lose moisture if humidity is low. Water gain hastens setting; water loss lengthens setting time. Liquid that has been left unstoppered for a long period, or is discolored, or is the last 25 percent portion remaining in the bottle should be discarded. Since the manufacture of zinc phosphate cement is a carefully controlled process, satisfactory results can seldom be achieved by mixing the powder of one brand of cement with the liquid of another.