A dental liner is a protective material applied to the dentin surface inside a tooth preparation before the final filling material is placed. Its main purpose is to shield the dental pulp (the living tissue) from external irritants during and after the procedure. This thin layer helps maintain the health and long-term viability of the tooth following damage from decay or trauma. The liner minimizes the pulp’s exposure to chemical substances and microbial invasion that could lead to inflammation or sensitivity.
Distinguishing Liners from Bases and Varnishes
Liners, bases, and varnishes are distinct materials used for pulpal protection, separated primarily by their thickness and function. A dental liner is applied thinly, typically less than 0.5 millimeters thick, and is used mainly for sealing and chemical protection. Because of their low strength (e.g., calcium hydroxide), liners are restricted to small areas where the remaining dentin layer over the pulp is very thin.
In contrast, a dental base is applied much thicker, often one to two millimeters, substituting for missing dentin to provide bulk and mechanical support for the final restoration. Bases are strong enough to withstand chewing forces and condensation during the filling process. Cavity varnishes, historically used, are liquid solutions that seal dentinal tubules but have been largely replaced by modern dentin bonding agents.
Primary Functions: Chemical and Thermal Protection
The primary function of a dental liner is to act as a defensive barrier, protecting the pulp from harm originating from the restorative process and materials. Liners provide a chemical barrier preventing residual reactants (such as acidic components or monomers) from diffusing through the dentin and irritating the pulp. This protection is important with materials like composite resins, which can release substances toxic to pulpal cells.
Another function is sealing the dentinal tubules, which are microscopic channels running from the dentin surface directly to the pulp. Sealing these tubules prevents the movement of bacteria, fluids, and their by-products from the oral cavity into the tooth, a process known as microleakage. Preventing microleakage is important because it is a major cause of post-operative sensitivity and secondary decay beneath the restoration.
Liners also offer thermal insulation, though this is a secondary function compared to sealing. They protect the pulp from rapid temperature changes transmitted through large metallic restorations, such as amalgam, which conducts heat well. Even though modern restorative materials are better thermal insulators, the liner provides an initial protective layer against temperature extremes that could cause pain or damage to the pulp tissue.
Therapeutic and Regenerative Roles
Beyond passive protection, some dental liners are chosen for their active, therapeutic effects on the pulp-dentin complex. Calcium hydroxide liners, for example, are highly alkaline (pH 9 to 11). This high alkalinity neutralizes acidic bacterial by-products and stimulates the underlying odontoblast cells, encouraging the formation of reparative dentin (tertiary dentin).
This regenerative capability makes calcium hydroxide a preferred material for deep cavity preparations, especially in pulp capping procedures where the dentin covering the pulp is less than 0.5 millimeters thick. Another category, glass ionomer liners, provides a different therapeutic benefit. Glass ionomer liners chemically bond to the tooth structure and continuously release fluoride ions.
The sustained fluoride release helps prevent recurrent or secondary decay from developing at the restoration margin by enhancing remineralization of the surrounding tooth structure. Resin-modified glass ionomers combine this fluoride-releasing property with improved physical strength and better adhesion to resin-based materials. These active properties show that liners are not simply inert barriers but contribute to the biological defense and health of the restored tooth.