Zinc Phosphate Cement

Zinc phosphate cement has been the most widely used dental cement. It is used as a luting agent for all indirect restorations and appliances. In deep cavities, it is used as a base as a substitute of lost dentin, because the thermal conductivity is approximately equal to that of enamel and considerably less than that of amalgam and gold.

When used as a thick base, it has a low toxicity level but when used as a luting agent in a thin state, it can be quite toxic. A young tooth or deep preparations with wide open tubules are more susceptible to intense inflammatory response to zinc phosphate cement, than an older tooth, which has produced a considerable amount of sclerotic and reparative dentin that blocks the dentinal tubules and prevents acids from reaching the pulp. Zinc phosphate cement is irritating because of its low pH and the rapid penetration of its lower molecular weight phosphoric acid into the dentinal tubules and pulp tissues. The hydraulic forces, which are induced during the seating of the restoration or during functional movements, cause phosphoric acid in large quantities to be forced into the dentinal tubules. Since dentin can be penetrated by phosphoric acid to a depth of more than 1.0 mm, especially in luting procedures, insulating materials are indicated for deep preparations with narrow remaining dentin thickness. In deep preparations, a moderate to severe localized pulp damage is produced within the first three days probably because the cement has an initially low pH on setting. The pH of the set cement approximates neutrality only at 48 hours. Resolution of inflammation occurs by 5 to 8 weeks.

When zinc phosphate is used as a luting agent, there occurs an intense diffuse infiltration of neutrophils throughout the pulp tissue. The initial pH of luting mix ranges from 2.0 - 3.3, which changes to 3.0 - 4.2 after one hour. It has been established that such low pH induced vascular thrombosis and necrosis in rodent pulp when the duration of exposure is prolonged over thin dentin. The pH for zinc phosphate, polycarboxylate and glass ionomer cements rise during the first 15 minutes. The rise in pH was faster for the zinc phosphate and polycarboxylate cements but slower for the glass ionomer cements. When the cement is used as a base with a remaining dentin as 1.0 mm, there occurs lifting of the odontoblastic layer and also the inflammatory cells become countable. In case the remaining dentin thickness is 1.5 mm, there occurs only moderate reaction with segmented odontoblastic layer and some reparative dentin formation.