Zinc phosphate cement

Zinc phosphate cement

Introduction

It is a cement traditionally used for sealing crowns and bridges, but it can be used as a cavity base in certain situations. It is designed from the mixture of a powder and a liquid

1. Presentation :

Modern formulations of zinc phosphate cements consist of a powder and a liquid. These cements are divided into two groups:

– fine-grained type I (sealing);

– type II medium grain (other uses).

Each group is divided into two classes:

– class 1 fast setting;

– class 2 with normal grip.

2. Composition :

2.1. POWDER

It is made of annealed zinc oxide, alone or mixed with other powders.

The powder is annealed at a temperature above 1000 °C for several hours in order to reduce its reactivity, which allows for proper handling and setting time. Without this

so-called sintering manipulation, the setting would be too fast.

• Zinc oxide is the basic element of this powder. Depending on the type of cement, the percentage varies from 79 to 100%.
• Magnesium oxide (MgO) : increases the compressive strength of cement.
• bismuth trioxide (Bi ₂ O ₃ ) and rubidium trioxide (Rb ₂ O ₃ ), which affect setting time and physical properties.
• silica or silicon oxide (SiO ₂ )

The powder bottle should never be left open because, on contact with carbon dioxide in the air, the powder can transform into hydrated carbonate, which has the effect of modifying the reaction of

taken by the appearance of carbon dioxide, causing the formation of bubbles on the surface of the mixture.

2. LIQUID 

It is an aqueous solution of phosphoric acid (H ₃ PO ₄ ),

3. SETTING REACTION
4. CHEMISTRY OF THE PLUG

When zinc oxide powder is mixed with phosphoric acid, a solid substance is formed with a heat release that can vary from 4 to 8 °C. The reaction that took place forms primary zinc phosphate (monozinc phosphate)

ZnO + 2 H3PO4 → Zn (H2PO4)2 + H2O

2. FACTORS OF VARIATION IN SETTING TIME
3. Manufacturing process

The setting time is influenced by the manufacturing process:

– composition and sintering temperature of the powder : the higher the sintering temperature, the slower the cement sets;

– powder particle : the larger it is, the slower the setting reaction; the finer it is, the faster the reaction.

2. Handling process

Setting time is influenced by handling:

– glass plate temperature : the lower the temperature during mixing, the longer the setting time,

– mixing technique : the slower the powder is added, the longer the setting time;

– mixing time : if the mixing time is extended, the setting time is delayed.

– liquid-powder proportion : the greater the proportion of liquid, the slower the setting and the result is a fluid mixture with a more acidic pH, therefore a risk of pulp reaction on living teeth and reduced mechanical resistance.

4. Properties
5. PHYSICAL AND MECHANICAL PROPERTIES:
6. Resistance to compression or crushing  :

For most commercially available zinc phosphate cements, compressive strength varies from 100 to 104 MPa after 24 hours. The strength reaches 75% after the first hour and its maximum value approximately within 24 hours, although it may increase over 1 week.

2. Tensile strength  :

It is ten times weaker than that of compression.

3. Shear strength: 

The cement is brittle and the hardness is average since it is approximately the same as that of dentin, but much lower than that of enamel.

4. Dimensional stability:

It is good, since it is almost identical to that of dentin.

5. Solubility and disintegration  :

This is a very important factor for a luting cement. Dissolution results in a marginal hiatus around the prosthetic restoration, which facilitates bacterial infiltration and caries.

Zinc phosphate cements are very soluble in water during the first 24 hours after setting and the weight loss is 0.04 to 3.3%. A loss of 0.2% is considered acceptable.

After this period, the solubility is very reduced. The solubility depends on the powder-liquid ratio. A high ratio decreases this solubility. Once the cement has set, the solubility in water is minimal

6. Thermal conductivity  :

Thermal conductivity is low. These cements are good insulators under metal fillings. 

7. Porosity 

These cements are porous due to their crystalline structure. Being porous, these cements are soluble in oral fluids.

8. Retention

• The retention of the prosthetic restoration sealed with zinc phosphate cement is achieved by mechanical attachment of the cement to the irregularities of the surface of the dental tissues.
• Obturation retention is primarily determined by cavity geometry and not by the adhesive characteristics of zinc phosphate cements

2. BIOLOGICAL PROPERTIES

The setting reaction is very exothermic with a temperature increase of 4 to 

10°C, which would correspond to acid-base neutralization.

The effects of zinc phosphate cement on the pulp are comparable to those of glass ionomer cements; both elicit a pulp response after placement that subsides after 6 months, regardless of the type of restoration. 

1. Acidity  :

It is obvious that the presence of phosphoric acid gives these cements a fairly high acidity at the time of insertion in the mouth. Three minutes after the start of mixing, the pH is about 3.5; it increases rapidly and goes to 6, after an hour, then to around 7 (neutrality) in 1 to 2 days.

If zinc phosphate cement is to be used as a base for deep cavities, the surrounding pulp tissue must be protected from any aggression caused by the initial acidity.

The use of calcium hydroxide and protective varnishes can prevent irritation and inflammation caused by acidity.

2. Toxicity 

The danger may arise from the exothermicity of the setting reaction and from the acidity.

5. Indication 

• The major indication for zinc phosphate cements ultimately lies more in the definitive sealing in prostheses fixed on depulped teeth. 
• Although they may have been recommended as temporary coronal filling materials, they should now clearly be avoided due to their porosity and pulp toxicity (linked to the exothermic setting reaction and acidity during placement).

6. Advantages and disadvantages 

benefits	disadvantages
* Good mechanical resistance * Dental adhesion * Thermal insulation	* Porous* Very exothermic setting* Initial acidity

Conclusion

Zinc phosphate cements, used for over a century, are beginning to be replaced by more advanced and better tolerated products, especially on vital teeth . Apart from these cases, they find a great indication .

Zinc phosphate cement

  Impacted wisdom teeth may require surgery.
Zirconia crowns are durable and aesthetic.
Bleeding gums may indicate periodontitis.
Invisible orthodontic treatments are gaining popularity.
Invisible orthodontic treatments are gaining popularity.
Modern dental fillings are both durable and discreet.
Interdental brushes are ideal for narrow spaces.
Good dental hygiene reduces the risk of cardiovascular disease.
 

Zinc phosphate cement