Dental Cements: Uses and Requirements

Uses of Dental Cements

• Luting or cementation of restorations: holds indirect restorations in place like inlays, crowns, and bridges
  • Permanent cementation: long-term cementation
  • Temporary cementation: temporary restorations until permanent restorations are made
• Filling material: temporary restoration when the final filling material can’t be finished in a single visit
• Protection of the dental pulp: varnishes, liners, and bases
• Cementation of orthodontic appliances
• Gingival and periodontal packs
• Endodontic sealers (root canal sealers)

Requirements of Dental Cements

• Biocompatible: non-toxic and non-irritating
• Chemical or mechanical retention of restoration to tooth structure
• High tensile strength, fracture toughness, and fatigue strength to resist fracture
• Low solubility in neutral and acid environments to maintain marginal seal
• Radiopaque: to be able to distinguish between a luting agent and caries under a prosthesis
• Film thickness: needs to have low viscosity to fill the space between the restoration and tooth and ensure proper seating with a film thickness of 25 µm or less
• Aesthetics: optical properties should be similar to tooth structure if used to cement translucent restorations
• Has antibacterial effect

Pulp Protection

• Dental cements protect the pulp from:
  • Thermal injury
  • Chemical irritation
  • Galvanic shock
  • Mechanical forces during amalgam placement and chewing

Cavity Liners

• Thin layers of material (< 0.5 mm)
• Act as a chemical barrier between the restoration and remaining dentin after cavity preparation

Cavity Bases

• Thicker layers (1-2 mm)
• Used beneath restorations to protect the pulp from thermal, electrical, and mechanical injury

Cavity Varnish

• A protective barrier between dentin and restorative materials
• Minimizes penetration of oral fluids at the restoration-tooth interface
• Composed of resins in a volatile solvent which when applied to a cavity surface, evaporates leaving a thin film

Pulp Capping

• Placing a special material in contact with the pulp to encourage new dentin formation and promote healing
• Two types:
  • Direct pulp capping: material placed directly over the pulp
  • Indirect pulp capping: material places indirectly over the pulp, but can still induce new dentin formation

Types of Dental Cements

• Cements based on Zinc Oxide
• Cements based on Alumino Silicate Glasses
• Other cements: Resin cements, Calcium-Hydroxide cements, Cavity liners and Varnishes

Cements Based on Zinc Oxide

• Zinc Oxide powder mixed with:
  • Eugenol: forms Zinc oxide eugenol cement
  • Phosphoric acid: forms Zinc Phosphate cement
  • Polyacrylic acid: forms Zinc Polycarboxylate cement

Cements based on Alumino-Silicate Glass

• Alumino-silicate glass powder mixed with:
  • Polyacrylic acid: forms Glass ionomer cement
  • Phosphoric acid: forms Silicate cement

Zinc Oxide Eugenol Cement

• Has a sedative effect on the pulp due to the presence of eugenol and its neutral pH
• Two types:
  • Conventional zinc oxide eugenol
  • Modified zinc oxide eugenol
• Forms: Powder and liquid

Conventional Zinc Oxide Eugenol Cement

• Powder Composition:
  • Zinc oxide (ZnO): main reactant
  • Rosin: reduces brittleness of set cement
  • Zinc stearate: plasticizer
  • Zinc acetate: accelerator and improves strength of the cement
• Liquid Composition:
  • Eugenol (85%): active ingredient of clove oil
  • Olive oil (15%): makes cement more plastic during mixing and decreases brittleness after setting
• Setting reaction:
  • Acid-base reaction which begins with hydrolysis of ZnO in the presence of water to form Zn(OH)₂
  • Zn(OH)₂ reacts with eugenol to form Zn eugenolate in a chelation reaction
  • Water is essential for the reaction, so it's faster in humid environments
  • The microstructure of the cement is formed of unreacted ZnO powder particles and eugenol embedded in a matrix of Zn eugenolate
• Manipulation:
  • Powder/Liquid ratio is 3:1
  • Mix using a glass slab and stainless steel spatula
  • Most of the powder is added to the liquid at once, followed by smaller increments to adjust viscosity.
• Properties:
  • pH is about 7, has little or no effect on pulp in deep cavities
  • Has a palliative obtudent effect on the pulp due to the presence of eugenol
  • Has a bacteriostatic effect due to the presence of eugenol
  • High solubility in which leached eugenol is replaced by water
  • Poor mechanical properties (compressive strength = 15 MPa, tensile strength = 5 MPa)
  • Opaque due to the presence of unreacted zinc oxide particles
  • Bond to enamel and dentin by mechanical interlocking
  • Eugenol inhibits the setting of resins, so can not be used with resin composite restorative materials
• Uses:
  • Cavity liner in deep cavities
  • Temporary filling material
  • Temporary cementing agent
  • Surgical dressing
  • Root canal sealer

Modifications of Zinc Oxide Eugenol Cement

• Polymer reinforced cements: resin polymer is added to increase strength of the set cement
  • Composition:
    • Powder: Zinc oxide (80%), Polymethyl methacrylate (20%)

Glass Ionomer Cements

• Glass ionomer cements are used as luting agents for crowns and bridges, restorations in non-stress bearing areas, restorations in primary teeth, bases, and endodontic sealers.
• Conventional glass ionomer cements (GICs) are available in powder and liquid, anhydrous cements, and capsules.
• The powder of GICs contains calcium fluoro alumino silicate glass which consists of calcium fluoride (CaF2), sodium fluoride (NaF), silica (SiO2), and alumina (Al2O3).
• The liquid of GICs is an aqueous solution of polyacrylic acid, itaconic acid, and tartaric acid.
• The setting reaction of GICs is an acid-base reaction where the acid in the liquid attacks and dissolves the surface of glass particles, releasing calcium, aluminum, and fluoride ions.
• Sodium ions form silica gel on the surface of the powder particles.
• Calcium ions react with the carboxylic groups of the acid, forming a cross-linked carboxylic acid salt, leading to initial setting.
• Aluminum ions react with the carboxylic groups of the acid, forming a stronger cross-linked cement than calcium because aluminum is trivalent and reacts with three carboxylic groups, while calcium is divalent and reacts with two.
• Aluminum ions migrate slower than calcium ions, making the reaction of aluminum ions take longer than calcium.

Types of GICs

• Metal reinforced GICs:
  • Silver alloy admix: Made by incorporating amalgam alloy powder with the glass powder.
  • Cermet glass ionomer: Made by sintering or fusing silver particles to the glass ionomer powder.
• Resin modified GICs:
  • Contain methacrylate resins similar to those used in composite resin materials.
  • Setting reactions include the conventional acid-base reaction and free radical polymerization reaction of the resin component.
• Nanoionomer cements:
  • Combine resin-modified glass ionomer cement with nanofiller technology.
  • Fillers include:
    • Fluoroaluminosilicate glass of size less than 3 microns (average 1 micron)
    • Nanofillers (5–25 nm)
    • Nanocluster fillers (1.0–1.6 microns) derived from silica and zirconia.

Resin Cements

• Conventional composite resin cements:
  • Self-cured resin cements: Available as a two-component system to be mixed together, containing a powder and liquid or two paste system.
  • Light-cured resin cements: Available as a single paste that needs no mixing.
  • Dual-cured resin cements: Polymerize by both light and chemical curing simultaneously. Supplied as base and catalyst to be mixed before use.
• Adhesive resin cements: Self-cured resin cements that contain an adhesive promoter such as 4-methacryloxy ethyl trimellitic anhydride (4-META).

Advantages of Glass Ionomer Materials

• Chemical bonding to tooth structures
• Long-term fluoride release
• Biocompatible
• Good compressive strength
• Low solubility in oral fluids

Disadvantages of Glass Ionomer Materials

• Technique sensitive to water contamination or dehydration
• Low abrasion resistance
• Short working time and long setting time
• Brittleness (low tensile strength) and low fracture toughness

Modifications of Glass Ionomer Cements

• Modifications aimed to improve mechanical properties, sensitivity to dehydration, abrasion resistance, and optical properties.
• Metal reinforced GICs: Increase abrasion resistance, little increase in compressive strength, and reduction in solubility. Also reduce fluoride release, bond strength with tooth structures, and aesthetics. Used for core build-up materials and posterior fillings in deciduous teeth.
• Resin modified GICs: Hybrid materials that combine glass ionomer and composite resin. Improve mechanical properties, handleability, and esthetics while maintaining fluoride release.

Role of Water in Glass Ionomer Cement

• Water acts as the medium for the chemical reaction that forms the glass ionomer cement.
• During setting, water hydrates the cross-linked matrix, creating a stable gel.
• This gel strengthens the final material.
• Dryness before setting stops the reaction and causes surface cracking, weakening the cement.
• Moisture contamination before setting can dissolve the matrix, also reducing strength.
• To ensure proper setting and strength, a varnish layer should protect the cement during the setting process.

Zinc Phosphate Cement

• Water based cement, packaged as a powder and liquid system
• Powder: Zinc oxide (90%), magnesium oxide, other oxides
• Liquid: Aqueous solution of phosphoric acid (30-40% H2O), Zinc or aluminum phosphate
• Water controls ionization of the acid and influences the rate of reaction
• Additional water increases the rate of ionization, which accelerates the reaction (shortens working and setting time)
• Types: Type 1 for luting alloys restorations, Type II for bases
• Setting reaction: The surface of the zinc oxide particles reacts with phosphoric acid to give an insoluble phosphate, resulting in a heterogeneous final set mass with shrinkages
• Setting reaction is exothermic (produces heat): Zinc oxide + Phosphoric acid = Zinc phosphate + Heat
• Mixing: Carried out on a cool thick glass slab using a cement spatula to dissipate the heat of reaction. Small increments of powder are added to the liquid to prolong the working time.
• A small portion of the powder is added first to the whole liquid to neutralize the acidity and minimize heat liberation.
• Large amounts of powder can be incorporated to saturate the mix.

Polycarboxylate Cement

• Water based cement
• Composition:
  • Powder: Zinc oxide with some fluoride
  • Liquid: Polyacrylic acid (PAA) copolymer in water and tartaric acid to reduce viscosity
• Bonds with tooth structure through ionic interaction between negatively charged molecules in the cement with positively charged atoms (such as calcium) in the tooth structure.
• Carboxylic group in PAA reacts with Ca ion in tooth structures, forming an ionic bond between PAA and Ca ions in hydroxyapatite. This bond is stronger with enamel than dentin
• Setting reaction (gelation): Zinc oxide + polyacrylic acid = zinc polycaboxylate
• Manipulation: Powder is rapidly incorporated in the liquid in large quantities, mixing is done rapidly in 30 to 40 seconds
• Mix should be used while it is still glossy. Working time is 2.5 to 3.5 minutes.
• Properties:
  • Combines the strength properties of phosphate cement with the biologic acceptability of ZOE cement
  • Weaker than zinc phosphate and stronger than ZOE
  • Although polycarboxylate cement is acidic, it is not irritant as zinc phosphate cement due to the weaker nature of ployacrylic acid and lack of mobility of the polyacid chains
  • Higher solubility than zinc phosphate cement, silicophosphate and glass ionomer
  • Low irritation
  • Easy manipulation
  • Comparable film thickness with zinc phosphate
  • More difficult to mix than other cements because of its high viscosity

Copper and Silver Modified Zinc Phosphate Cements

• Similar to phosphate cements except that the powder contains copper or silver
• Considered bactericidal
• Used mainly for cementation of orthodontic appliances

Classification of Dental Cements

• Classified by the nature of the cement powder into:
  • Zinc oxide: ZOE, Zinc Phosphate, Zinc polycarboxylate cements.
  • Ion leachable glasses (alumino-silicate glass): Silicate and glass ionomer cements.