**10.1.2. Dental phosphate cement**

A variety of cements are used in modern clinical dentistry, such as glass ionomers, zinc phosphate and zinc polycarboxylate [38],[39]. Dental zinc phosphate cement is primarily used forthe cementation ofindirectrestorations, such as crown and bridges. It has the longestrecord of any cement, approximate 100 years, and has remained popular throughout this time. Zinc phosphate cements are also considered the strongest among the dental cements. However, it is also applied for temporary fillings, cavity bases and buildings of teeth beneath crowns. Zinc phosphate cement is primarily in contact with the pulp-dentin system and in certain cases (e.g. temporary fillings) with the gingiva. A variety of cementing materials are currently used as the bases and luting5 agents, but zinc phosphate cement has been used for many decades. Phosphoric acid-based cements originated from OSTERMANN´S formula from 1832, which was composed of calcium oxide and anhydrous phosphoric acid. In 1902, FLECK established a formula that is similar to that being in use today [40],[41],[42],[43],[44].

The powder is mainly a mixture of zinc oxide and up to 13% magnesium oxide. The liquid is an aqueous solution of phosphoric acid containing 38 – 59% H3PO4, 30 – 55% water and 0 – 10% zinc. Aluminum is essential to the cement-forming reaction, and zinc moderates the reaction between powder and liquid, allowing adequate working time and sufficient quanti‐ ty of powder to be added for optimum properties of cement. When the powder is mixed with liquid, phosphoric acid attacks the surface of particles, dissolving zinc oxide, which releases zinc ions into the solution. Aluminum in the liquid reacts with phosphoric acid to form zinc aluminophosphate gel on remaining portion of particles. Thus, the cement reveals a cored structure consisting primarily of non-reacted zinc oxide particle core embedded in a cohe‐ sive amorphous matrix of zinc aluminophosphate (glasslike phosphate). Aluminum phos‐

<sup>5</sup> The word 'luting' implies the use of a molded or moldable article to seal a space or to cement two components together [44].

phate, in addition to its role as a retarder, contributes to the increase in mechanical hardness of cement [40],[45].

Setting reactions and resultant structure of zinc phosphate cements are largely based on the formation of hopeite (Zn3(PO4)2·4H2O) and/or zinc phosphate hydrate (Zn2P2O7·3H2O) when using orthophosphoric acid (OPA) cement-forming liquids. OPA solutions buffered with aluminum and zinc ion produced better mechanical properties than non-buffered OPA solutions because of the formation of hopeite and amorphous phase. The development of crystalline forms of phosphate hydrates of zinc and magnesium was retarded and/or prevent‐ ed by the incorporation of aluminum and zinc ion in the cement-forming liquid [43].

This mechanism is similar to the cement-forming reaction described by WILSON [43 ],[46 ] in a dental silicate cement (**Fig. 5**). When H+ ions attack the glass powder, Al3+, Ca2+, Na+ and F<sup>−</sup> are liberated from the glass, leaving behind an ion-depleted layer of silicate gel at the surface of glass particles. Liberated ions migrate and react with H2PO4 − , and salts precipitate. The principal reaction is the formation of an insoluble aluminum phosphate, the gel matrix. Associated side reactions are the precipitation of calcium fluoride and the formation of soluble sodium dihydrogen phosphate.

**Fig. 5.** Setting reaction of dental silicate cement [43].
