ZINC PHOSPHATE

 ZINC PHOSPHATE CEMENT:

It is the oldest of the luting cements. Thus it has the longest clinical “track record”and serves as a standard with which newest systems can be compared.

Synonym: 

Crown and bridge cement and zinc oxy phosphate

Applications:

1. Permanent cementation of full metal crowns and bridges, Metal ceramic prosthesis and orthodontic brackets.

2. Cavity base to protect the pulp from mechanical, thermal and electrical stimuli

Composition:

Powder: 

Zinc oxide (90%)

MgO (10%)

Other oxide

Silica

Liquid:

Phosphoric acid

Water

Aluminum phosphate 

Zinc sulphate

Water controls the ionization of the acid, which in turn influences the rate of reaction. The ingredients of powder are sintered at temperatures between 10000C and 14000C into a cake that is subsequently ground into fine powder.

Setting reaction: 

When the powder is mixed with liquid, the phosphoric acid attacks the surface of the particles and releases zinc ions into the liquid. Aluminum which already forms a complex with the phosphoric acid reacts with the zinc and yields a zinc aluminophosphate gel on the remaining portion of the particles. Thus, the set cement is a cored structure consisting primarily of unreacted zinc oxide particles embedded in a cohesive amorphous matrix of zinc aluminophosphate.

Manipulation:

Armamentarium: 

Glass slab

Stiff 

Heavy mixing spatula.

The mixing slab must be thoroughly cleaned before use. The powder is added to the liquid in small portions to achieve the desired consistency. The cement liquid should be kept sealed with a stopper to prevent the changes in water content. Cloudy liquid should be discarded. Increasing the powder liquid ratio gives a more viscous mix, shorter setting time, higher strength, lower solubility, and less free acid. Use of a chilled glass slab (50C) slows down the reaction and allows incorporation of more powder giving superior properties in the set cement. Cold glass slab permits.

Working time: 

5 min it is measured as the time from the start of mixing to the maximum time at which the viscosity of the mix is still low enough to flow readily under pressure to form a thin film.

Setting time:

 2.5-8 min

The period, during which the matrix formation has reached a point at which, an external physical disturbance will not cause permanent dimensional changes. Setting time can be measured with a 1 mm diameter needle indenter at a load of 400gm a temperature of 370C, and relative humidity greater than 90%. Setting time is defined as the elapsed time from the start of mixing to the point at which needle no longer makes a complete circular indentation in the cement.

(Clinical significance: be removed from the it represents the time at which the excess zinc phosphate cement should be removed from the margins of restoration

Factors extending setting time:

1. Reducing the P/L ratio produces a thinner mixture, which increases the working and setting times of zinc phosphate cement. However, this adversely affects the physical properties, result in lower initial pH of the cement, and is therefore, not an acceptable means of extending the setting time.

2. Mixing cement in small increments and

3. If an operator prolongs the spatulation of the last increment, the matrix will be effectively destroyed as it forming. Fragmentation of matrix means an extra time is needed to rebuild the bulk of the matrix

4. Because the reaction between powder and liquid is exothermic, the most effective method of controlling the working and setting times is to regulate the temperature of the mixing slab. Cooling the slab markedly retards the chemical reaction between the powder and liquid, thereby retarding formation of the matrix. This permits incorporation of the optimum amount of powder into the liquid without the mix developing an unduly high viscosity.

5. Temperature

Slow incorporation of increments

Properties:  

principal reasons for their satisfactory performance under routine conditions are that they can be easily manipulated and set sharply to a relatively strong mass.

Mechanical properties:

Compressive strength: 

80-100 MPa after 24 hours (Note: Minimum strength for adequate retention of cements is 60 MPa)

Tensile strength:

 5-7 MPa

Modulus of elasticity (stiffness): 

13 GPa

Note: 

Although the properties are far from ideal, they are usually regarded as standard against which the newer cements may be compared

Film thickness:

25m (Note: The cements must have the ability to wet and flow into the irregularities of the tooth and restoration surface. According to ADA, acceptable film thickness is less than 25 m and at this thickness, the cement fills the irregularities between the restoration and tooth and allows most castings to seat satisfactorily.

Solubility: 

The solubility in organic acid solutions such as lactic or citric acid is 20-30 times higher. Dissolution contributes to marginal leakage around restorations and bacterial penetration. This may be facilitated by dimensional change. The cement has been found to contract about 0.5% linearly, giving rise to marginal gap at the tooth – cement and cement –restoration interface (Note: allowable solubility 0.2%)

Biologic effects: 

The freshly mixed zinc phosphate is highly acidic with a pH between 1 and 2 after mixing. Even after setting for 1 hour, the pH may be still below 4. After 24 hours, the pH is usually 6 to 7. Pain on cementation is due not only to the free acid but also to the free acid of the mix but also to osmotic movement of fluid through the dentinal tubules. Hydraulic pressure developed during seating of the restoration may also contribute to pulpal damage.

Advantages:

1. Can be mixed easily and set sharply

2. Adequate strength

Disadvantages:

1. Pulpal irritation

2. Lack of anti bacterial action

3. Brittleness

4. Lack of adhesion

5. Solubility in oral fluids