Dental cement is a specialized adhesive material used in restorative dentistry. Its primary function is to serve as a bonding agent, securely affixing indirect restorations, such as crowns, bridges, and inlays, to the natural tooth structure. It is also used as a protective liner beneath fillings or as a temporary restorative material. The composition of dental cement determines its strength, durability, and method of adhesion.
Water-Based Cements
The traditional category of water-based cements relies on an acid-base chemical reaction that occurs when a metal oxide powder is mixed with an acidic liquid. These materials are inorganic and ionic, setting through a process that results in a hard salt matrix. They are some of the oldest materials used in dentistry, valued for their strength and biocompatibility.
Zinc Phosphate cement, introduced in the late 19th century, is composed of a powder containing zinc oxide and magnesium oxide, mixed with a liquid primarily made of phosphoric acid and water. This cement provides high compressive strength. It achieves retention through mechanical interlocking with microscopic surface irregularities, rather than a chemical bond to the tooth itself.
Polycarboxylate cement, also known as zinc polyacrylate, was a significant advancement because it was the first cement to achieve a true chemical bond to the tooth structure. The powder is mainly zinc oxide, but the liquid component is an aqueous solution of polyacrylic acid. The carboxyl groups of the polyacrylic acid chelate with the calcium ions in the tooth’s enamel and dentin.
Glass Ionomer Cements (GICs) represent a further evolution, combining the benefits of a chemical bond with a therapeutic property. GIC powder is a fluoroaluminosilicate glass, which reacts with a liquid composed of polyacrylic acid and water. During the setting reaction, ions like calcium, aluminum, and fluoride are released from the glass particles. The fluoride ions diffuse into the surrounding tooth structure, which helps protect the tooth from decay.
Resin-Based Cements
Resin-based cements are the most modern group, deriving their properties from organic polymer chemistry similar to dental composite fillings. Their composition includes a blend of methacrylate monomers, inorganic filler particles, and a chemical initiator system. Monomers, such as Bis-GMA (Bisphenol A-glycidyl methacrylate) or UDMA (Urethane dimethacrylate), form the organic resin matrix that polymerizes, or hardens.
The hardening occurs through a free-radical polymerization process, initiated by light, chemical activators, or a combination of both in dual-cure materials. Light-cure systems require exposure to a curing light to activate photoinitiators within the cement. Dual-cure cements provide a reliable set in areas where light cannot fully penetrate, using a chemical initiator to complete the polymerization once the light-cured process begins.
Conventional Resin Cements require the separate application of an adhesive system, which involves etching the tooth surface to prepare it for bonding. This etching creates microscopic porosity in the enamel and dentin, allowing the resin components to penetrate and form a strong micromechanical bond.
A simplification led to the development of Self-Adhesive Resin Cements. These materials incorporate acidic functional monomers, like phosphoric acid methacrylates, directly into the cement’s composition. This allows the cement to mildly etch and prime the tooth surface while simultaneously polymerizing, eliminating the need for a separate bonding agent and reducing clinical steps.
Functional Categories of Use
The most common function of dental cement is as a Luting Agent, used to permanently secure an indirect restoration to the prepared tooth. For this purpose, the cement must have a low viscosity when mixed, forming a thin layer—often less than 25 micrometers—to ensure the restoration fully seats. Resin cements are frequently chosen for luting due to their high strength and low solubility, though Glass Ionomer and Zinc Phosphate are also used for certain metal restorations.
Bases and Liners are materials placed underneath a final restoration to protect the dental pulp, especially in deep cavity preparations.
Liners
A liner is a very thin coating that may provide a therapeutic effect or act as a barrier against chemical irritation.
Bases
Bases are thicker layers that provide thermal insulation and mechanical support to the final restorative material. Zinc Phosphate and Glass Ionomer cements are often used as bases because they can withstand the forces of the final filling material.
Temporary Cements are formulated to be intentionally weaker and easier to remove. These are used to hold provisional crowns or bridges in place while the permanent restoration is being fabricated. Desired properties include ease of cleanup, sufficient retention to prevent restoration loss, and compatibility with the final cementation materials. They are usually based on zinc oxide formulations, sometimes with eugenol, which is known for its soothing effect on the tooth nerve.