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CHEMISTRY AND SETTING

Zinc phosphate cement consists of a powder and liquid which are mixed just

before use. The powder contains more than 75% of zinc oxide and up to 13% of

magnesium oxide; these powders are sintered above 1000 C, then ground into a

fine powder, and blended with radiopaque powders. The liquid containsphosphoric acid (38% to 59%), water (30% to 55%), aluminum phosphate (2% to

3%), and in some cases zinc phosphate (up to 10%). The liquid controls the pH

and the rate of the liquid-powder (acid-base) reaction. Also, the finer the particle

size, the faster the cement sets.

When mixed, phosphoric acid dissolves the zinc oxide, which reacts with the

aluminum phosphate and forms zinc

aluminophosphate gel on the remaining undissolved zinc oxide particles. The set

cement contains unreacted zinc oxide particles encased in an amorphous matrix of

zinc alumino- phosphate. Loss of water from the liquid lengthens the settingreaction, while incorporation of additional water during mixing accelerates the

reaction.

CLINICAL MANIPULATION

The reaction between zinc oxide and phosphoric acid is exo- thermic and requires

a careful mixing procedure to minimize the effect of heat generation. The zinc

oxide cement powder should be dispensed on a glass slab and divided into several

portions (Figure 14-7, A). The manufacturers instructions shoulddescribe the

amount of powder by including a mea- suring device, the number of liquid drops,

the method for dividing powder increments, the mixing time for each incre- ment,

and the total mixing time. The liquid should not be dispensed onto the slab until

the powder is dispensed and divided, and the cement is ready for use because the

water from the liquid will evaporate, as shown in Figure 14-8.

Mixing of Zinc Phosphate Cement

Mixing of this cement should be initiated by incorporating the smallest portion of

powder using a thin spatula and brisk spatulation. Most of the area of the mixingslab should be used to dissipate heat. A good rule to follow is to spatulate each

increment of powder for 15 to 20 seconds before adding another increment, and all

mixing should be completed within 1.5 to 2 minutes. After the powder is

completely incor- porated and a creamy mixture has been created, the cement is

drawn across the slab and the flat blade of the mixing spatula contacts the mixture

and is slowly drawn away. If a stringofcementcanbelifted12mm(12inch)to19mm(~34 inch) before separating from the spatula (Figure 14-7, B), the

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cement is considered fluid enough for cementing a prosthesis. If the string exceeds

19 mm, the cement is too viscous for proper seating of prostheses (Figure 14-7, C)

and another mixture should be made. After the prosthesis is coated with the

cement and is seated, it should be held under pressure until the cement sets. The

field of operation should be kept dry during the entire seating procedure.

MECHANICAL AND BIOLOGICAL PROPERTIES

Zinc phosphate luting cements have a compressive strength as high as 104 MPa, a

diametral tensile strength of 5.5 MPa, and an elastic modulus of 13 GPa (Table 14-

2), making it

relatively strong and stiff compared with other cements. Gen- erally zinc

phosphate cements have a relatively low solubility in water; however, in vivo

disintegration occurs in the pres- ence of lactic, acetic, and citric acids.

The phosphoric acid in the liquid makes the mixture quite acidic and thereforecytotoxic when a prosthesis is luted with this cement on a prepared tooth (Table

14-5). As setting occurs, the acidity is partially neutralized, but the cement

remains acidic after 24 hours, with the pH rising from about 3 to about 6. For thin

layers of dentin, a cavity liner (i.e., Ca[OH]2) is recommended to prevent an

adverse pulpal response from the pressure of luting, which forces acid into the

pulp tissue. Younger patients are especially susceptible because they have a more

open dentin tubule area, whereas older patients with sclerotic dentin will have a

more tortuous path that restricts the penetration of acid toward the pulp.

Hal 316 Anusavice 2014

Zinc phosphate

Zinc phosphate cement is one of the oldest luting cements which has been in use for long

because of advantages like, a high early strength which makes it suitable for cementation

of a prefabricated or for a cast metal post-core.12Zinc phosphate cement has been the

standard to which other luting cements are compared.2It sets by an acid-base reaction

and its physical properties are subject to variables like powder-liquid ratio, water content,

mixing temperature, etc. It has a high compressive strength and low tensile strength and

is inexpensive. It is a good choice for luting long span fixed partial dentures. It does not

chemically bond to tooth structure. The mixed cement is at a very low pH, hence, the

smear layer should be maintained to minimize penetration into dentinal tubules.13A

cavity varnish may be used to reduce the effect of low pH on the pulp. Mixing is done for

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R12http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R12http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R12http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R2http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R2http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R2http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R13http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R13http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R13http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R13http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R2http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3959149/#R12

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60 to 90 seconds on a cool, dry glass slab with the powder brought into the liquid in small

increments and is spread over a broad area thus allowing maximal powder incorporation

and keeping the viscosity low. It is placed in or on the restoration which is seated on a

clean, dry tooth with firm steady pressure that should be maintained for several minutes

to prevent pressure rebound. The initial setting reaction occurs about 5 to 9 minutes aftermixing. The excess should not be removed for at least several minutes after the initial

hardening to reduce the risk of saliva contact as zinc phosphate is very soluble in the

initial setting stage.1

12

2. EE Hill. Dental cements for definitive luting: a review and practical clinical

considerations. Dent Clin North Am. 2007;51:643658. [PubMed]

12. B Habib, JA von Fraunhofer, CF Driscoll. Comparison of two luting agents used for

the retention of cast dowel and cores. J Prosthodont. 2005;14:164

169. [PubMed]

13. EE Hill, B Rubel. Vital tooth cleaning for cementation of indirect restorations: a

review. Gen Dent. 2009;57:392395. [PubMed]

1. P Baldissara, G Comin, F Martone, R Scotti. Comparative study of the marginalleakage of six cements in fixed provisional crowns. J Prosthet Dent. 1998;80:417

422. [PubMed]

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