Cavity Liners and Bases in Restorative Dentistry

• Restorative dentistry aims to replace lost enamel and dentin due to caries or cavity preparation.
• Cavity liners and insulating bases protect the pulp and promote pulpal response when placed between dentin or pulp and restorative materials

Hypersensitivity

• Thermal, chemical, mechanical or osmotic stimuli on exposed dentinal tubules causes dentinal fluid movement
• This movement stimulates pulpal nerve receptors leading to hypersensitivity.

Microleakage

• Restorative materials, excluding chemically bonded ones, have a micro-gap at the tooth/restoration interface
• Microleakage introduces irritants to the pulp via dentinal tubules.
• Sealing the dentinal tubules before restoration is crucial to prevent this.

Cavity Basing under Amalgam Restorations

• Removing diseased dentin increases cavity depth and reduces remaining dentin thickness (Dentin Bridge)
• In such cases, dentin substitutes offer pulpal protection.

Dentin Pulp Organ Irritants in Deep Cavities

• Sealing dentinal tubules provides chemical protection against metallic ions, corrosion products, acids, and chemicals from restorative materials
• Sealing the tubules also prevents bacterial toxins, salivary ions, and other irritants from entering through microleakage spaces.
• Thermal protection is required from thermally conductive metallic restorations like amalgam and cast gold
• Electrical protection is required to prevent galvanism from dissimilar metals like amalgam and cast gold.
• Mechanical protection is required from amalgam condensation, inlay cementation, and mastication stresses.

Ideal Requirements for Intermediary Materials

• Sedative to the pulp and conductive to repair and healing.
• Compatible with the pulp-dentin organ.
• Stimulates reparative dentin formation
• Improves marginal sealing and adaptation to the cavity walls, preferably with bonding to tooth structure.
• Provides thermal and electrical insulation
• It has sufficient strength to avoid fracture or distortion from condensation or masticatory forces
• Compatible with overlaying material and does not interfere with setting of the materials
• Resistance to degradation in oral fluids.
• Has adequate workability and easy to apply

Classification of Bases and Liners

• Liners are categorized as Solution Liners, which are thin film forming materials mainly used for chemical protection by sealing dentinal tubules.
• Varnish is in liquid form composed of 10% natural gum (copal or rosin) or synthetic resin in 90% organic solvent.

Indications of Varnish

• Prevents metallic ion and corrosion product penetration into dentinal tubules under amalgam restorations.
• Prevents pulpal irritation and tooth discoloration or amalgam blues.
• Decreases the initial microleakage space with amalgam.
• Acidic base materials such as Zinc phosphate cement requires Varnish for sealing tubules when using cast gold restoration

Contraindications of Varnish

• Avoid varnish under resin composite restorations, as it prevents mechanical interlocking of the resin with the tooth structure
• Varnish is ill advised under glass ionomer, resin modified glass ionomer, and polycarboxylate cement, because it would eliminate adhesive potential and biocompatibility of these cements
• Would also hinder fluoride uptake from glass ionomer materials.

Dentin Sealer

• Resinous material that seals dentinal tubules and desensitizes dentin in non-carious lesions
• It cannot replace dentin bonding agents because it does not produce micromechanical bonding
• It is not indicated with resin composite or chemically bonded glass ionomer cements.

Bonding Systems

• Resinous systems dissolve or penetrate the smear layer to bond micromechanically to tooth substrate
• These systems seal tubules and eliminate microleakage if properly bonded.

Cement Liners

• Materials placed in medium thickness (100-500μm) to provide pulpal medication and/or chemical protection
• Two important aspects of pulpal medication are the relief of pulpal inflammation and facilitation of dentinal bridging for physiologic protection
• Calcium hydroxide achieves these properties.

Calcium Hydroxide

• Liner of calcium hydroxide dispersed in aqueous or resinous solutions that comes in two forms
• Two pastes (Chemical-cured)
• One paste (Light-cured)
• Calcium hydroxide is used for pulpal medication when the remaining dentin bridge is ≤0.5mm or in direct contact with exposed pulpal tissue
• It stimulates odontoblasts to form reparative dentin and calcific bridging at the exposure site.
• The calcium ion concentration, alkalinity, and antibacterial potential relieve pulpal inflammation
• It is used in direct pulp capping.
• It is porous and soluble, therefore, neither an electric insulator nor a chemical insulator.

Cement Bases (Dentin Substitutes)

• Materials with thick consistency applied in thick sections to substitute lost dentin and provide thermal and mechanical pulpal protection
• Types of bases:
  • Zinc phosphate cement (ZPC)
  • Zinc polycarboxylate (PCC)
  • Glass ionomer (GIC)
  • Resin modified glass ionomer (RMGI).

Zinc Phosphate Cement (ZPC)

• Supplied as a powder (mainly zinc oxide) and a liquid (aqueous solution of orthophosphoric acid).
• It has high compressive strength, making it the most rigid, tough, and durable intermediary base material.
• It provides excellent thermal and electrical insulation at 1mm or more thickness.
• ZPC is the most irritating base material because of its acidic pH.
• The pH increases to reach neutrality after 48 hours.
• It has an exothermic setting reaction that can cause thermal irritation if not properly manipulated.
• ZPC is indicated as a base under metallic restorations.
• ZPC is not recommended when the remaining dentin bridge is less than 1.5 mm without a protective liner (sub-base) because its acidity can harm the pulp-dentin organ.

Zinc Polycarboxylate Cement (PCC)

• The product is a powder consisting of primarily zinc oxide.
• The liquid consists of a 40–50% polyacrylic acid aqueous solution.
• Chemically bonds to tooth structure for improved protection and reduced microleakage
• The pH rises rapidly to 3.4 two minutes after the start of mixing
• The polyacrylic acid is weaker than phosphoric acid
• Features low diffusion mobility because of its large molecular size
• Has lower compressive strength than ZPC
• Offers proper mechanical protection and good thermal/electrical insulator at 1.5 mm thickness.
• Indicated as a base under any restorative material and as a luting cement.

Glass Ionomer Cements (GIC and RMGI)

• Composed of an acid-soluble fluoro-alumino-silicate glass powder and a liquid of polyacrylic acid (PAA).
• Setting occurs through an acid-base reaction.
• RMGI is a hybrid ionomer modified by resin monomers.
• RMGI setting happens primarily by acid-base reaction and immediate command setting light polymerization.
• Excellent sealing ability due to chemical adhesion via carboxylate ions from PAA liquid.
• Offers anticariogenic properties due to fluoride release from the powder glass component.
• Reasonably biocompatible with pulp-dentin organ and restorative materials
• It provides adequate thermal, chemical, and mechanical protection and proper sealing of dentinal tubules, therefore serving as a dentin substitute.

Advantages of RMGI over GIC

• It has a flexible working time because of command setting by light curing.
• Improved strength and wear properties as well as easy handling
• GIC has higher fluoride release, particularly during initial 24 hours (initial fluoride burst).
• GIC and RMGI are available in liner and base formulations.
• Decreasing the Powder/Liquid ratio of the cement decreases strength but increases flow and solubility. This is suitable for use as a liner or luting cement.
• In contrast, increasing the Powder/Liquid ratio increases strength and decreases flow and solubility, resulting in use as a base or temporary filling material.
• Cement liners and bases have some degree of solubility in oral fluids, therefor it is only placed on dentin and is completely removed from cavity walls and margins

Clinical Considerations for Intermediary Materials

• Clinical judgment for liner or base depends on:
• Remaining dentin thickness (RDT) because as cavity depth increases, RDT decreases which increasing the need for intermediary material before inserting permanent restoration.
• Intermediary material should be placed directly on tooth structure to benefit from its adhesive properties unless pulpal medication is essential
• Type of Restorative Material; metallic or adhesive esthetic restoration, direct or indirect.
• After shallow tooth excavation (RDT≥2mm), pulpal protection other than chemical protection (sealing) is not needed
• An amalgam restoration requires only a solution liner (varnish, dentin sealer).
• With resin composite, only the bonding system is needed.
• GIC restoration does not need any pulpal protection.
• In moderately-deep caries excavation (RDT=1-2mm), amalgam restoration requires a cement base plus a solution liner for sealing
• In contrast, with resin composite, a dentin bonding agent will provide adequate sealing without a base
• Extensive dentin loss (RDT≤0.5mm) mandates strong pulpal protection.
• Pulpal medication with calcium hydroxide is essential to induce reparative dentin formation and relieve pulpal inflammation when using any restorative materials because sealing and a strong base is required to substitute for the dentin loss.