Dental Cements: Types and Requirements

Dental Cements Overview

• Dental cements are hard, brittle substances composed of a powder and a liquid
• When mixed thinly, cements can hold gold crowns on prepared teeth
• When mixed thickly, cements serve as temporary fillings or thermal insulation for amalgam-restored teeth
• Some cements are suited for endodontics, periodontics, oral surgery, and orthodontics

Requirements of Dental Cements

• Should be nontoxic and non-irritating to the dental pulp
• Should be insoluble in saliva and oral fluids
• As an insulating base, it must be strong enough for filling material condensation, and provide electrical, thermal, and chemical insulation to the pulp
• Should be obtundent (pain-reducing) and bacteriostatic in cavities with residual caries
• When used for cementation, it needs to have a low film thickness
• Should adhere to both enamel and dentin

Classification of Dental Cements (by Chemical Ingredients)

• Zinc oxide based cements
• Include: Zinc oxide eugenol, Zinc phosphate, and Zinc polycarboxylate
• Alumino silicate glass based cements
• Include: Silicate cement and Glass ionomer cement

Zinc Oxide Eugenol (ZnO/E) Cements

• Unmodified ZnO/E Cement Components:
• Powder: Zinc oxide, magnesium oxide (to maintain white color), and zinc acetate accelerator
• Liquid: Eugenol with 15% olive oil and acetic acid accelerator; water is essential for the setting reaction
• The main disadvantage is low strength, which additives can improve

Modified ZnO/E Cements

• Resin Bonded ZnO/E Cement
• Powder: ZnO with 20% polymethyl methacrylate
• Liquid: Eugenol with 10% polystyrene dissolved
• Ethoxy-Benzoic Acid (EBA) Cement
• Powder: ZnO with 30% alumina
• Liquid: 37.5% eugenol with 62.5% EBA

Setting Reaction of Zinc Oxide Eugenol Cement

• Zinc oxide and eugenol undergo a chelation reaction in the presence of water
• This creates an amorphous zinc eugenolate matrix that binds unreacted ZnO particles and releases free eugenol

Zinc Oxide Eugenol Cement Properties

• Neutral pH of 7; provides a pain-killing effect, suitable for deep cavities, and offers thermal, electrical, and chemical protection to the pulp
• Highest solubility of dental cements
• Compressive strength varies by type: 5 MPa for unmodified, 38 MPa for resin-bonded, and 90 MPa for EBA cements
• Opaque due to the presence of unreacted ZnO particles
• Does not form chemical bonds but bonds through mechanical interlocking
• Is contraindicated under composite fillings because it causes discoloration and inhibits polymerization
• Is mixed on a glass slab using a stainless steel spatula, adding powder to liquid until desired consistency is achieved

Zinc phosphate cement

• One of the oldest dental cement types for dentistry

Composition of Zinc phosphate cement

• Powder: Zinc oxide and magnesium oxide to maintain the white color
• Liquid: 40% phosphoric acid, 40% water to control acid ionization, and 20% zinc and aluminum phosphate buffers

Setting reaction of Zinc phosphate cement

• The reaction between zinc oxide and phosphoric acid creates a zinc phosphate matrix that binds unreacted ZnO particles and releases heat

Properties of Zinc phosphate cement

• Has a pH of 1.5 to 3.5, making it highly acidic and irritating to pulp, requiring a sub-base
• Least soluble dental cement
• Compressive strength is about 100 MPa, while tensile strength is about 5 MPa
• Film thickness is very low (15 microns), providing an advantage
• The set cement is opaque
• Does not form chemical bonds but retains restorations by mechanical interlocking of the set material with the surface roughness of the tooth and restoration
• Mixed on a thick, cool, dry glass slab with a stainless steel spatula, using a powder-to-liquid ratio of 2.5:1 for luting and 3.5:1 for cavity lining

Zinc Polycarboxylate Cement

• Composition
• Powder: Zinc oxide and magnesium oxide.
• Liquid: 40% aqueous polycarboxylic acid and sodium hydroxide to adjust the pH and viscosity.
• Setting Reaction
• Zinc oxide reacts with polycarboxylic acid, forming a zinc polyacrylate matrix that holds unreacted zinc oxide particles.

Zinc Polycarboxylate Cement Properties

• Biological Properties: Exhibits very little irritant effect on the pulp, due to polycarboxylic acid being a weak acid with a large molecular size, and rapid pH neutralization.
• Solubility: More soluble than zinc phosphate cement in acids.
• Strength: Compressive strength is 90 MPa and tensile strength is 15 MPa.
• Film Thickness: Has a film thickness of 25 to 40 microns.
• Opacity: More opaque due to the presence of unreacted ZnO particles.
• Adhesion: Bonds chemically to both enamel and dentin

Additional Information on Zinc Polycarboxylate Cement

• Bond strength to enamel is 8 MN/m² and to dentin is 4 MN/m².
• Adhesion to enamel is stronger due to greater calcium content (96%) than in dentin (70%).
• For optimum adhesion, enamel and dentin surfaces should be clean, smooth, and dry.
• Manipulation:
• The powder-to-liquid ratio of 1:1 (luting agent) or 2:1 (base) is mixed on waxed paper or a glass slab using a plastic spatula.
• Mix over a small area to avoid gel destruction.

Glass Ionomer Cements

• Applications
• Type 1: Luting (cement) agent.
• Type 2: Restorative filling material.
• Type 3: Base.
• Type 4: Pit and fissure sealants.

Glass ionomer cement composition and reaction

• Cement powder is finely ground aluminosilicate glass
• Viscous liquid is a polycarboxylate copolymer in water
• Components form a gel matrix surrounding partially reacted glass powder particles

Glass ionomer cement properties

• Compressive Strength: 150-250 MPa after 24 hours; strength improves when protected from moisture during the first 24 hours after filling
• Tensile Strength: 15-25 MPa; brittle material
• Bond Strength: Bonds chemically to the tooth structure via carboxylic groups (COOH) of polyacrylic acid and calcium of tooth structure
• Bond strength is lower than polycarboxylate cements due to moisture sensitivity during setting; surface must be treated with a conditioner
• Optical Properties: Translucent, suitable for anterior restoration in low stress-bearing areas
• Powder liquid form is mixed using a plastic spatula on special paper
• Pre-proportioned capsules can be mechanically mixed and applied
• The mix will have a shiny surface and stringiness
• Freshly placed cement should be coated to protect from moisture contamination

Cavity Varnishes

• Resin dissolved in a volatile solvent (acetone, chloroform, or ether)
• Application leaves a thin resinous film after solvent evaporation

Cavity Varnishes Uses

• To seal exposed dentinal tubules, protecting the pulp from chemical irritation from filling materials
• To reduce marginal leakage around fillings, especially amalgam
• To cover glass ionomer restorations to protect from dehydration or disintegration

Calcium Hydroxide

• Simple forms contain aqueous suspensions of calcium hydroxide

Calcium Hydroxide Properties

• Alkaline (pH = 12), harmful to microorganisms in carious dentin
• Stimulates secondary dentin formation if it contacts the pulp
• Because it is weak, use as sub-base under a stronger base

Calcium Hydroxide Uses

• Serve as Sub-base for zinc phosphate in deep cavities.
• Material of choice for pulp capping when there is microscopic pulp exposure