Introduction
Dental cements are restorative materials with relatively low mechanical properties but high biological ones. They are used where strength is not the main need.
Classification
By time in the mouth
- Temporary: for short-term fillings and for fixing crowns and bridges for a short time. These are easy to remove.
- Permanent: for long-term use.
Permanent cements by use
Luting cements must form the thinnest possible layer, no more than 25 µm, for better stability and less dissolving at the crown margin.
By composition
Permanent cements are zinc phosphate, zinc polyacrylate, glass-ionomer, composite, zinc oxide eugenol and calcium hydroxide.
Zinc phosphate cement
- Powder: 85 to 90% zinc oxide and 8 to 10% magnesium oxide. Copper or silver additions give a weak bacteriostatic effect.
- Liquid: 50 to 60% phosphoric acid, buffered with aluminum and zinc phosphates, in about 30% water.
- Mixing: on a cold, dry, thick glass slab with a stainless steel spatula. Add small portions of powder over a wide area for 60 to 90 seconds. Never add liquid to the mix.
- Toxicity: pH is 1 to 2 at first and reaches 6 to 7 after 24 hours. Phosphoric acid can penetrate up to 1.5 mm into the tubules, so it can harm the pulp under thin dentin.
- Adhesion: micromechanical only, which leaves a gap of about 3 µm.
- Strength: compressive strength is about 100 MPa, but tensile strength is only 3.5 to 5.5 MPa, so it is brittle.