Cements consist of a powder and a liquid which are mixed together. A paste-like mass forms, which, as a result of a hydrating reaction, gels and hardens and remains permanently solid.
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Applications for which cements can be used
- base fillings
- temporary fillings
- for the fixing of inlays, crowns, bridges, etc.
Commonly used cavity base cements include: zinc oxide phosphate cement (phosphate cement for short), glass ionomer cements, zinc oxide eugenol cements, ethoxy benzoic acid cements (EBA cements), carboxylate cements and calcium hydroxide cements on the basis of salicylate ester.
Zinc oxide phosphate cement (PHC)
Zinc oxide phosphate cement is also referred to as zinc phosphate cement or simply as phosphate cement. It is a much used material and has so far proven itself admirably as a cavity base cement and for the setting of cast restorations.
The powder-to-liquid ratio
The liquid consists of ortho phosphoric acid (45-64%), which is added to calcium, manganese, magnesium and zinc oxides in order to reduce the curing time.
The correct degree of dilution of the liquid is very important for the quality of the cement and must therefore be adhered to. The powder-to-liquid ratio not only influences the consistency but also the mechanical strength, the solubility and the film thickness of the cement.
The liquid is highly hygroscopic (attracts water) and therefore very sensitive to humidity.
An increase in the amount of water in the liquid accelerates the curing time and reduces the workable period.
On the other hand a decrease in the amount of water in the liquid lengthens the curing time and reduces the mechanical strength.
Therefore the bottle must be resealed immediately after liquid has been removed, and the liquid should be added to the mixing plate only when everything has been prepared for mixing. Contact with humidity in the air should be as short as possible. As soon as the liquid becomes cloudy or sediment begins to form, it should no longer be used.
The powder consists primarily of zinc oxide (80 % – 90 %) and may, depending on the brand, contain various additional substances (MnO, Al2O3, CaO).
The powder bottle, too, must be resealed immediately after use, as the powder can absorb CO2from the air; the cement will become porous as a result and permeable to micro-organisms and water. In addition, the mechanical strength of the cement will be reduced.
The minimum powder-to-liquid ratio prescribed by the manufacturer must always be observed.
More powder will only be advantageous; less powder will lead to deterioration in the overall quality of the cement. The use of measuring aids is advisable.
Product examples: Harvard cement, phosphate cement, Tenet
Working with zinc oxide phosphate cement
- Keep spatula, glass mixing plate, cement powder and liquid in the fridge (5° to 10° C). Remove only shortly before use!
- Use as long and as wide a spatula as possible (e.g. Aesculap DF 163 or DF 165)
- Use as large and thick (with roughened surface) a glass mixing plate as possible (large, thick mixing plates disperse the reaction heat of the cement better than small, thin ones)
- Rub the surface of the plate dry with a clean cloth before mixing (beware of dampness due to condensation)
- Drip the liquid onto the plate;
reseal the bottle immediately!
- Extract a more than sufficient amount of powder with the measuring spoon and keep it in small portions not too close to the liquid;
immediately reseal the powder bottle!
- Mix a small portion of powder into the liquid and spread it out thinly.
- After the first portion, wait approximately 15 to 30 seconds (liquid phase) before all the other allotments of powder are added in small portions.
- Mix the powder little by little in small portions into the liquid.
- Spread the mixed material out thinly so that the reaction heat can dissipate more easily.
- Never mix in too much powder at a time, as this will accelerate the curing time. This will lead to the dentist having too little time to apply the cement.
- The material should never be mixed up too quickly, as this too will accelerate the curing time.
- The powder should always be mixed into the liquid, never vice versa! Liquid should never be added to a cement mix that has become too stiff.
- The desired consistency should never be achieved by waiting for a thin mixture to 'thicken'.
- The cooler the cement is, the slower it will cure: at room temperature, the working time is only 1 minute 20 seconds; at -4° C, on the other hand, it is 4 minutes.
- Once the cement has been introduced into the cavity, it will cure quickly as a result of body temperature (which is actually to be desired) so that work can continue quickly.
PHC has the right cementing consistency if it is runny but still sticks to the spatula and can be lifted approximately 1 cm vertically off of the surface of the mixing plate before the film separates.
Cavity base cement consistency
If the PHC is rubbery, no longer sticks to the spatula and allows itself to be rolled up into a ball on the glass plate without a problem then it has the right consistency to be used as cavity base cement. It can be inserted into the cavity without sticking to the spatula.
Thickly mixed PHC offers a number of advantages: increased resistance to fracture, better adhesion to dentine as well as a lesser amount of free acid and therefore less irritation of the pulp in deep cavities. The acid can lead to temporary toothache in the patient. When powder-rich mixtures are used (little liquid, as much powder as possible), these irritations do not occur.
Zinc phosphate cements react acerbically once mixed; neutralisation can only be expected after a number of hours.